Κεφάλαιο 15. Ασφάλεια

15.1. Σύνοψη

Το κεφάλαιο αυτό παρέχει μια βασική εισαγωγή στις έννοιες της ασφάλειας συστήματος, κάποιους γενικά καλούς κανόνες, και ορισμένα προχωρημένα θέματα σχετικά με το FreeBSD. Αρκετά από τα θέματα που καλύπτονται εδώ, μπορούν να εφαρμοστούν το ίδιο καλά τόσο στο ίδιο το σύστημα, όσο και για ασφάλεια μέσω Internet. Το Internet δεν είναι πλέον ένα "φιλικό" μέρος στο οποίο καθένας θέλει να είναι ο ευγενικός σας γείτονας. Η ανάγκη ασφάλισης του συστήματος σας είναι επιτακτική για να προστατέψετε τα δεδομένα σας,την πνευματική σας ιδιοκτησία, το χρόνο σας, και πολλά περισσότερα από τα χέρια των χάκερς και των ομοίων τους.

Το FreeBSD παρέχει μια σειρά από βοηθητικά προγράμματα και μηχανισμούς για να εξασφαλίσει την ακεραιότητα και την ασφάλεια του συστήματος σας και του δικτύου.

Αφού διαβάσετε αυτό το κεφάλαιο, θα ξέρετε:

  • Βασικές έννοιες για την ασφάλεια, σε σχέση με το FreeBSD.

  • Στοιχεία σχετικά με τους διάφορους μηχανισμούς κρυπτογράφησης που είναι διαθέσιμοι στο FreeBSD, όπως το DES και το MD5.

  • Πως να ρυθμίσετε το σύστημα σας για κωδικούς μιας χρήσης.

  • Πως να ρυθμίσετε TCP Wrappers για χρήση με την inetd.

  • Πως να ρυθμίσετε τον KerberosIV σε FreeBSD εκδόσεις πριν τη 5.0.

  • Πως να ρυθμίσετε τον Kerberos5 στο FreeBSD.

  • Πως να ρυθμίσετε το IPsec και να δημιουργήσετε ένα VPN μεταξύ μηχανημάτων FreeBSD/Windows®.

  • Πως να ρυθμίσετε και να χρησιμοποιήσετε την κατά FreeBSD υλοποίηση SSH του OpenSSH

  • Τι είναι τα ACLs στο σύστημα αρχείων και πως να τα χρησιμοποιήσετε.

  • Πως να χρησιμοποιήσετε το βοηθητικό πρόγραμμα Portaudit για να ελέγξετε λογισμικό τρίτου κατασκευαστή που έχει εγκατασταθεί μέσω της συλλογής Ports.

  • Πως να χρησιμοποιήσετε τις δημοσιεύσεις security advisories του FreeBSD.

  • Θα έχετε μια ιδέα για το τι είναι το Process Accounting και πως να το ενεργοποιήσετε στο FreeBSD.

Πριν διαβάσετε αυτό το κεφάλαιο, θα πρέπει:

  • Να κατανοείτε βασικές έννοιες του FreeBSD και του Internet.

Πρόσθετα θέματα σχετικά με την ασφάλεια καλύπτονται σε ολόκληρο το βιβλίο. Για παράδειγμα, ο Υποχρεωτικός Έλεγχος Πρόσβασης συζητείται στο Υποχρεωτικός Έλεγχος Πρόσβασης και τα Internet Firewalls συζητούνται στο Firewalls.

15.2. Introduction

Security is a function that begins and ends with the system administrator. While all BSD UNIX® multi-user systems have some inherent security, the job of building and maintaining additional security mechanisms to keep those users "honest" is probably one of the single largest undertakings of the sysadmin. Machines are only as secure as you make them, and security concerns are ever competing with the human necessity for convenience. UNIX® systems, in general, are capable of running a huge number of simultaneous processes and many of these processes operate as servers - meaning that external entities can connect and talk to them. As yesterday’s mini-computers and mainframes become today’s desktops, and as computers become networked and inter-networked, security becomes an even bigger issue.

System security also pertains to dealing with various forms of attack, including attacks that attempt to crash, or otherwise make a system unusable, but do not attempt to compromise the root account ("break root"). Security concerns can be split up into several categories:

  1. Denial of service attacks.

  2. User account compromises.

  3. Root compromise through accessible servers.

  4. Root compromise via user accounts.

  5. Backdoor creation.

A denial of service attack is an action that deprives the machine of needed resources. Typically, DoS attacks are brute-force mechanisms that attempt to crash or otherwise make a machine unusable by overwhelming its servers or network stack. Some DoS attacks try to take advantage of bugs in the networking stack to crash a machine with a single packet. The latter can only be fixed by applying a bug fix to the kernel. Attacks on servers can often be fixed by properly specifying options to limit the load the servers incur on the system under adverse conditions. Brute-force network attacks are harder to deal with. A spoofed-packet attack, for example, is nearly impossible to stop, short of cutting your system off from the Internet. It may not be able to take your machine down, but it can saturate your Internet connection.

A user account compromise is even more common than a DoS attack. Many sysadmins still run standard telnetd, rlogind, rshd, and ftpd servers on their machines. These servers, by default, do not operate over encrypted connections. The result is that if you have any moderate-sized user base, one or more of your users logging into your system from a remote location (which is the most common and convenient way to login to a system) will have his or her password sniffed. The attentive system admin will analyze his remote access logs looking for suspicious source addresses even for successful logins.

One must always assume that once an attacker has access to a user account, the attacker can break root. However, the reality is that in a well secured and maintained system, access to a user account does not necessarily give the attacker access to root. The distinction is important because without access to root the attacker cannot generally hide his tracks and may, at best, be able to do nothing more than mess with the user’s files, or crash the machine. User account compromises are very common because users tend not to take the precautions that sysadmins take.

System administrators must keep in mind that there are potentially many ways to break root on a machine. The attacker may know the root password, the attacker may find a bug in a root-run server and be able to break root over a network connection to that server, or the attacker may know of a bug in a suid-root program that allows the attacker to break root once he has broken into a user’s account. If an attacker has found a way to break root on a machine, the attacker may not have a need to install a backdoor. Many of the root holes found and closed to date involve a considerable amount of work by the attacker to cleanup after himself, so most attackers install backdoors. A backdoor provides the attacker with a way to easily regain root access to the system, but it also gives the smart system administrator a convenient way to detect the intrusion. Making it impossible for an attacker to install a backdoor may actually be detrimental to your security, because it will not close off the hole the attacker found to break in the first place.

Security remedies should always be implemented with a multi-layered "onion peel" approach and can be categorized as follows:

  1. Securing root and staff accounts.

  2. Securing root-run servers and suid/sgid binaries.

  3. Securing user accounts.

  4. Securing the password file.

  5. Securing the kernel core, raw devices, and file systems.

  6. Quick detection of inappropriate changes made to the system.

  7. Paranoia.

The next section of this chapter will cover the above bullet items in greater depth.

15.3. Securing FreeBSD

Command vs. Protocol

Throughout this document, we will use bold text to refer to an application, and a monospaced font to refer to specific commands. Protocols will use a normal font. This typographical distinction is useful for instances such as ssh, since it is a protocol as well as command.

The sections that follow will cover the methods of securing your FreeBSD system that were mentioned in the last section of this chapter.

15.3.1. Securing the root Account and Staff Accounts

First off, do not bother securing staff accounts if you have not secured the root account. Most systems have a password assigned to the root account. The first thing you do is assume that the password is always compromised. This does not mean that you should remove the password. The password is almost always necessary for console access to the machine. What it does mean is that you should not make it possible to use the password outside of the console or possibly even with the su(1) command. For example, make sure that your ptys are specified as being insecure in the /etc/ttys file so that direct root logins via telnet or rlogin are disallowed. If using other login services such as sshd, make sure that direct root logins are disabled there as well. You can do this by editing your /etc/ssh/sshd_config file, and making sure that PermitRootLogin is set to NO. Consider every access method - services such as FTP often fall through the cracks. Direct root logins should only be allowed via the system console.

Of course, as a sysadmin you have to be able to get to root, so we open up a few holes. But we make sure these holes require additional password verification to operate. One way to make root accessible is to add appropriate staff accounts to the wheel group (in /etc/group). The staff members placed in the wheel group are allowed to su to root. You should never give staff members native wheel access by putting them in the wheel group in their password entry. Staff accounts should be placed in a staff group, and then added to the wheel group via the /etc/group file. Only those staff members who actually need to have root access should be placed in the wheel group. It is also possible, when using an authentication method such as Kerberos, to use Kerberos' .k5login file in the root account to allow a ksu(1) to root without having to place anyone at all in the wheel group. This may be the better solution since the wheel mechanism still allows an intruder to break root if the intruder has gotten hold of your password file and can break into a staff account. While having the wheel mechanism is better than having nothing at all, it is not necessarily the safest option.

An indirect way to secure staff accounts, and ultimately root access is to use an alternative login access method and do what is known as "starring" out the encrypted password for the staff accounts. Using the vipw(8) command, one can replace each instance of an encrypted password with a single “*” character. This command will update the /etc/master.passwd file and user/password database to disable password-authenticated logins.

A staff account entry such as:

foobar:R9DT/Fa1/LV9U:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh

Should be changed to this:

foobar:*:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh

This change will prevent normal logins from occurring, since the encrypted password will never match “*”. With this done, staff members must use another mechanism to authenticate themselves such as kerberos(1) or ssh(1) using a public/private key pair. When using something like Kerberos, one generally must secure the machines which run the Kerberos servers and your desktop workstation. When using a public/private key pair with ssh, one must generally secure the machine used to login from (typically one’s workstation). An additional layer of protection can be added to the key pair by password protecting the key pair when creating it with ssh-keygen(1). Being able to "star" out the passwords for staff accounts also guarantees that staff members can only login through secure access methods that you have set up. This forces all staff members to use secure, encrypted connections for all of their sessions, which closes an important hole used by many intruders: sniffing the network from an unrelated, less secure machine.

The more indirect security mechanisms also assume that you are logging in from a more restrictive server to a less restrictive server. For example, if your main box is running all sorts of servers, your workstation should not be running any. In order for your workstation to be reasonably secure you should run as few servers as possible, up to and including no servers at all, and you should run a password-protected screen blanker. Of course, given physical access to a workstation an attacker can break any sort of security you put on it. This is definitely a problem that you should consider, but you should also consider the fact that the vast majority of break-ins occur remotely, over a network, from people who do not have physical access to your workstation or servers.

Using something like Kerberos also gives you the ability to disable or change the password for a staff account in one place, and have it immediately affect all the machines on which the staff member may have an account. If a staff member’s account gets compromised, the ability to instantly change his password on all machines should not be underrated. With discrete passwords, changing a password on N machines can be a mess. You can also impose re-passwording restrictions with Kerberos: not only can a Kerberos ticket be made to timeout after a while, but the Kerberos system can require that the user choose a new password after a certain period of time (say, once a month).

15.3.2. Securing Root-run Servers and SUID/SGID Binaries

The prudent sysadmin only runs the servers he needs to, no more, no less. Be aware that third party servers are often the most bug-prone. For example, running an old version of imapd or popper is like giving a universal root ticket out to the entire world. Never run a server that you have not checked out carefully. Many servers do not need to be run as root. For example, the ntalk, comsat, and finger daemons can be run in special user sandboxes. A sandbox is not perfect, unless you go through a large amount of trouble, but the onion approach to security still stands: If someone is able to break in through a server running in a sandbox, they still have to break out of the sandbox. The more layers the attacker must break through, the lower the likelihood of his success. Root holes have historically been found in virtually every server ever run as root, including basic system servers. If you are running a machine through which people only login via sshd and never login via telnetd or rshd or rlogind, then turn off those services!

FreeBSD now defaults to running ntalkd, comsat, and finger in a sandbox. Another program which may be a candidate for running in a sandbox is named(8). /etc/defaults/rc.conf includes the arguments necessary to run named in a sandbox in a commented-out form. Depending on whether you are installing a new system or upgrading an existing system, the special user accounts used by these sandboxes may not be installed. The prudent sysadmin would research and implement sandboxes for servers whenever possible.

There are a number of other servers that typically do not run in sandboxes: sendmail, popper, imapd, ftpd, and others. There are alternatives to some of these, but installing them may require more work than you are willing to perform (the convenience factor strikes again). You may have to run these servers as root and rely on other mechanisms to detect break-ins that might occur through them.

The other big potential root holes in a system are the suid-root and sgid binaries installed on the system. Most of these binaries, such as rlogin, reside in /bin, /sbin, /usr/bin, or /usr/sbin. While nothing is 100% safe, the system-default suid and sgid binaries can be considered reasonably safe. Still, root holes are occasionally found in these binaries. A root hole was found in Xlib in 1998 that made xterm (which is typically suid) vulnerable. It is better to be safe than sorry and the prudent sysadmin will restrict suid binaries, that only staff should run, to a special group that only staff can access, and get rid of (chmod 000) any suid binaries that nobody uses. A server with no display generally does not need an xterm binary. Sgid binaries can be almost as dangerous. If an intruder can break an sgid-kmem binary, the intruder might be able to read /dev/kmem and thus read the encrypted password file, potentially compromising any passworded account. Alternatively an intruder who breaks group kmem can monitor keystrokes sent through ptys, including ptys used by users who login through secure methods. An intruder that breaks the tty group can write to almost any user’s tty. If a user is running a terminal program or emulator with a keyboard-simulation feature, the intruder can potentially generate a data stream that causes the user’s terminal to echo a command, which is then run as that user.

15.3.3. Securing User Accounts

User accounts are usually the most difficult to secure. While you can impose draconian access restrictions on your staff and "star" out their passwords, you may not be able to do so with any general user accounts you might have. If you do have sufficient control, then you may win out and be able to secure the user accounts properly. If not, you simply have to be more vigilant in your monitoring of those accounts. Use of ssh and Kerberos for user accounts is more problematic, due to the extra administration and technical support required, but still a very good solution compared to a encrypted password file.

15.3.4. Securing the Password File

The only sure fire way is to star out as many passwords as you can and use ssh or Kerberos for access to those accounts. Even though the encrypted password file (/etc/spwd.db) can only be read by root, it may be possible for an intruder to obtain read access to that file even if the attacker cannot obtain root-write access.

Your security scripts should always check for and report changes to the password file (see the Checking file integrity section below).

15.3.5. Securing the Kernel Core, Raw Devices, and File systems

If an attacker breaks root he can do just about anything, but there are certain conveniences. For example, most modern kernels have a packet sniffing device driver built in. Under FreeBSD it is called the bpf device. An intruder will commonly attempt to run a packet sniffer on a compromised machine. You do not need to give the intruder the capability and most systems do not have the need for the bpf device compiled in.

But even if you turn off the bpf device, you still have /dev/mem and /dev/kmem to worry about. For that matter, the intruder can still write to raw disk devices. Also, there is another kernel feature called the module loader, kldload(8). An enterprising intruder can use a KLD module to install his own bpf device, or other sniffing device, on a running kernel. To avoid these problems you have to run the kernel at a higher secure level, at least securelevel 1. The securelevel can be set with a sysctl on the kern.securelevel variable. Once you have set the securelevel to 1, write access to raw devices will be denied and special chflags flags, such as schg, will be enforced. You must also ensure that the schg flag is set on critical startup binaries, directories, and script files - everything that gets run up to the point where the securelevel is set. This might be overdoing it, and upgrading the system is much more difficult when you operate at a higher secure level. You may compromise and run the system at a higher secure level but not set the schg flag for every system file and directory under the sun. Another possibility is to simply mount / and /usr read-only. It should be noted that being too draconian in what you attempt to protect may prevent the all-important detection of an intrusion.

15.3.6. Checking File Integrity: Binaries, Configuration Files, Etc.

When it comes right down to it, you can only protect your core system configuration and control files so much before the convenience factor rears its ugly head. For example, using chflags to set the schg bit on most of the files in / and /usr is probably counterproductive, because while it may protect the files, it also closes a detection window. The last layer of your security onion is perhaps the most important - detection. The rest of your security is pretty much useless (or, worse, presents you with a false sense of security) if you cannot detect potential intrusions. Half the job of the onion is to slow down the attacker, rather than stop him, in order to be able to catch him in the act.

The best way to detect an intrusion is to look for modified, missing, or unexpected files. The best way to look for modified files is from another (often centralized) limited-access system. Writing your security scripts on the extra-secure limited-access system makes them mostly invisible to potential attackers, and this is important. In order to take maximum advantage you generally have to give the limited-access box significant access to the other machines in the business, usually either by doing a read-only NFS export of the other machines to the limited-access box, or by setting up ssh key-pairs to allow the limited-access box to ssh to the other machines. Except for its network traffic, NFS is the least visible method - allowing you to monitor the file systems on each client box virtually undetected. If your limited-access server is connected to the client boxes through a switch, the NFS method is often the better choice. If your limited-access server is connected to the client boxes through a hub, or through several layers of routing, the NFS method may be too insecure (network-wise) and using ssh may be the better choice even with the audit-trail tracks that ssh lays.

Once you have given a limited-access box at least read access to the client systems it is supposed to monitor, you must write scripts to do the actual monitoring. Given an NFS mount, you can write scripts out of simple system utilities such as find(1) and md5(1). It is best to physically md5 the client-box files at least once a day, and to test control files such as those found in /etc and /usr/local/etc even more often. When mismatches are found, relative to the base md5 information the limited-access machine knows is valid, it should scream at a sysadmin to go check it out. A good security script will also check for inappropriate suid binaries and for new or deleted files on system partitions such as / and /usr.

When using ssh rather than NFS, writing the security script is much more difficult. You essentially have to scp the scripts to the client box in order to run them, making them visible, and for safety you also need to scp the binaries (such as find) that those scripts use. The ssh client on the client box may already be compromised. All in all, using ssh may be necessary when running over insecure links, but it is also a lot harder to deal with.

A good security script will also check for changes to user and staff members access configuration files: .rhosts, .shosts, .ssh/authorized_keys and so forth, files that might fall outside the purview of the MD5 check.

If you have a huge amount of user disk space, it may take too long to run through every file on those partitions. In this case, setting mount flags to disallow suid binaries and devices on those partitions is a good idea. The nodev and nosuid options (see mount(8)) are what you want to look into. You should probably scan them anyway, at least once a week, since the object of this layer is to detect a break-in attempt, whether or not the attempt succeeds.

Process accounting (see accton(8)) is a relatively low-overhead feature of the operating system which might help as a post-break-in evaluation mechanism. It is especially useful in tracking down how an intruder has actually broken into a system, assuming the file is still intact after the break-in has occurred.

Finally, security scripts should process the log files, and the logs themselves should be generated in as secure a manner as possible - remote syslog can be very useful. An intruder will try to cover his tracks, and log files are critical to the sysadmin trying to track down the time and method of the initial break-in. One way to keep a permanent record of the log files is to run the system console to a serial port and collect the information to a secure machine monitoring the consoles.

15.3.7. Paranoia

A little paranoia never hurts. As a rule, a sysadmin can add any number of security features, as long as they do not affect convenience, and can add security features that do affect convenience with some added thought. Even more importantly, a security administrator should mix it up a bit - if you use recommendations such as those given by this document verbatim, you give away your methodologies to the prospective attacker who also has access to this document.

15.3.8. Denial of Service Attacks

This section covers Denial of Service attacks. A DoS attack is typically a packet attack. While there is not much you can do about modern spoofed packet attacks that saturate your network, you can generally limit the damage by ensuring that the attacks cannot take down your servers by:

  1. Limiting server forks.

  2. Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.).

  3. Overloading the Kernel Route Cache.

A common DoS attack scenario is attacking a forking server and making it spawning so many child processes that the host system eventually runs out of memory, file descriptors, etc. and then grinds to a halt. inetd (see inetd(8)) has several options to limit this sort of attack. It should be noted that while it is possible to prevent a machine from going down, it is not generally possible to prevent a service from being disrupted by the attack. Read the inetd manual page carefully and pay specific attention to the -c, -C, and -R options. Note that spoofed-IP attacks will circumvent the -C option to inetd, so typically a combination of options must be used. Some standalone servers have self-fork-limitation parameters.

Sendmail has its -OMaxDaemonChildren option, which tends to work much better than trying to use Sendmail’s load limiting options due to the load lag. You should specify a MaxDaemonChildren parameter, when you start sendmail; high enough to handle your expected load, but not so high that the computer cannot handle that number of Sendmail instances without falling on its face. It is also prudent to run Sendmail in queued mode (-ODeliveryMode=queued) and to run the daemon (sendmail -bd) separate from the queue-runs (sendmail -q15m). If you still want real-time delivery you can run the queue at a much lower interval, such as -q1m, but be sure to specify a reasonable MaxDaemonChildren option for that Sendmail to prevent cascade failures.

Syslogd can be attacked directly and it is strongly recommended that you use the -s option whenever possible, and the -a option otherwise.

You should also be fairly careful with connect-back services such as TCP Wrapper’s reverse-identd, which can be attacked directly. You generally do not want to use the reverse-ident feature of TCP Wrapper for this reason.

It is a very good idea to protect internal services from external access by firewalling them off at your border routers. The idea here is to prevent saturation attacks from outside your LAN, not so much to protect internal services from network-based root compromise. Always configure an exclusive firewall, i.e., "firewall everything except ports A, B, C, D, and M-Z". This way you can firewall off all of your low ports except for certain specific services such as named (if you are primary for a zone), ntalkd, sendmail, and other Internet-accessible services. If you try to configure the firewall the other way - as an inclusive or permissive firewall, there is a good chance that you will forget to "close" a couple of services, or that you will add a new internal service and forget to update the firewall. You can still open up the high-numbered port range on the firewall, to allow permissive-like operation, without compromising your low ports. Also take note that FreeBSD allows you to control the range of port numbers used for dynamic binding, via the various net.inet.ip.portrange sysctl's (sysctl -a | fgrep portrange), which can also ease the complexity of your firewall’s configuration. For example, you might use a normal first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then block off everything under 4000 in your firewall (except for certain specific Internet-accessible ports, of course).

Another common DoS attack is called a springboard attack - to attack a server in a manner that causes the server to generate responses which overloads the server, the local network, or some other machine. The most common attack of this nature is the ICMP ping broadcast attack. The attacker spoofs ping packets sent to your LAN’s broadcast address with the source IP address set to the actual machine they wish to attack. If your border routers are not configured to stomp on ping packets to broadcast addresses, your LAN winds up generating sufficient responses to the spoofed source address to saturate the victim, especially when the attacker uses the same trick on several dozen broadcast addresses over several dozen different networks at once. Broadcast attacks of over a hundred and twenty megabits have been measured. A second common springboard attack is against the ICMP error reporting system. By constructing packets that generate ICMP error responses, an attacker can saturate a server’s incoming network and cause the server to saturate its outgoing network with ICMP responses. This type of attack can also crash the server by running it out of memory, especially if the server cannot drain the ICMP responses it generates fast enough. Use the sysctl variable net.inet.icmp.icmplim to limit these attacks. The last major class of springboard attacks is related to certain internal inetd services such as the udp echo service. An attacker simply spoofs a UDP packet with the source address being server A’s echo port, and the destination address being server B’s echo port, where server A and B are both on your LAN. The two servers then bounce this one packet back and forth between each other. The attacker can overload both servers and their LANs simply by injecting a few packets in this manner. Similar problems exist with the internal chargen port. A competent sysadmin will turn off all of these inetd-internal test services.

Spoofed packet attacks may also be used to overload the kernel route cache. Refer to the net.inet.ip.rtexpire, rtminexpire, and rtmaxcache sysctl parameters. A spoofed packet attack that uses a random source IP will cause the kernel to generate a temporary cached route in the route table, viewable with netstat -rna | fgrep W3. These routes typically timeout in 1600 seconds or so. If the kernel detects that the cached route table has gotten too big it will dynamically reduce the rtexpire but will never decrease it to less than rtminexpire. There are two problems:

  1. The kernel does not react quickly enough when a lightly loaded server is suddenly attacked.

  2. The rtminexpire is not low enough for the kernel to survive a sustained attack.

If your servers are connected to the Internet via a T3 or better, it may be prudent to manually override both rtexpire and rtminexpire via sysctl(8). Never set either parameter to zero (unless you want to crash the machine). Setting both parameters to 2 seconds should be sufficient to protect the route table from attack.

15.3.9. Access Issues with Kerberos and SSH

There are a few issues with both Kerberos and ssh that need to be addressed if you intend to use them. Kerberos 5 is an excellent authentication protocol, but there are bugs in the kerberized telnet and rlogin applications that make them unsuitable for dealing with binary streams. Also, by default Kerberos does not encrypt a session unless you use the -x option. ssh encrypts everything by default.

Ssh works quite well in every respect except that it forwards encryption keys by default. What this means is that if you have a secure workstation holding keys that give you access to the rest of the system, and you ssh to an insecure machine, your keys are usable. The actual keys themselves are not exposed, but ssh installs a forwarding port for the duration of your login, and if an attacker has broken root on the insecure machine he can utilize that port to use your keys to gain access to any other machine that your keys unlock.

We recommend that you use ssh in combination with Kerberos whenever possible for staff logins. Ssh can be compiled with Kerberos support. This reduces your reliance on potentially exposed ssh keys while at the same time protecting passwords via Kerberos. Ssh keys should only be used for automated tasks from secure machines (something that Kerberos is unsuited to do). We also recommend that you either turn off key-forwarding in the ssh configuration, or that you make use of the from=IP/DOMAIN option that ssh allows in its authorized_keys file to make the key only usable to entities logging in from specific machines.

15.4. DES, MD5, and Crypt

Every user on a UNIX® system has a password associated with their account. It seems obvious that these passwords need to be known only to the user and the actual operating system. In order to keep these passwords secret, they are encrypted with what is known as a "one-way hash", that is, they can only be easily encrypted but not decrypted. In other words, what we told you a moment ago was obvious is not even true: the operating system itself does not really know the password. It only knows the encrypted form of the password. The only way to get the "plain-text" password is by a brute force search of the space of possible passwords.

Unfortunately the only secure way to encrypt passwords when UNIX® came into being was based on DES, the Data Encryption Standard. This was not such a problem for users resident in the US, but since the source code for DES could not be exported outside the US, FreeBSD had to find a way to both comply with US law and retain compatibility with all the other UNIX® variants that still used DES.

The solution was to divide up the encryption libraries so that US users could install the DES libraries and use DES but international users still had an encryption method that could be exported abroad. This is how FreeBSD came to use MD5 as its default encryption method. MD5 is believed to be more secure than DES, so installing DES is offered primarily for compatibility reasons.

15.4.1. Recognizing Your Crypt Mechanism

Currently the library supports DES, MD5 and Blowfish hash functions. By default FreeBSD uses MD5 to encrypt passwords.

It is pretty easy to identify which encryption method FreeBSD is set up to use. Examining the encrypted passwords in the /etc/master.passwd file is one way. Passwords encrypted with the MD5 hash are longer than those encrypted with the DES hash and also begin with the characters $1$. Passwords starting with $2a$ are encrypted with the Blowfish hash function. DES password strings do not have any particular identifying characteristics, but they are shorter than MD5 passwords, and are coded in a 64-character alphabet which does not include the $ character, so a relatively short string which does not begin with a dollar sign is very likely a DES password.

The password format used for new passwords is controlled by the passwd_format login capability in /etc/login.conf, which takes values of des, md5 or blf. See the login.conf(5) manual page for more information about login capabilities.

15.5. One-time Passwords

By default, FreeBSD includes support for OPIE (One-time Passwords In Everything), which uses the MD5 hash by default.

There are three different sorts of passwords which we will discuss below. The first is your usual UNIX® style or Kerberos password; we will call this a "UNIX® password". The second sort is the one-time password which is generated by the OPIE opiekey(1) program and accepted by the opiepasswd(1) program and the login prompt; we will call this a "one-time password". The final sort of password is the secret password which you give to the opiekey program (and sometimes the opiepasswd programs) which it uses to generate one-time passwords; we will call it a "secret password" or just unqualified "password".

The secret password does not have anything to do with your UNIX® password; they can be the same but this is not recommended. OPIE secret passwords are not limited to 8 characters like old UNIX® passwords, they can be as long as you like. Passwords of six or seven word long phrases are fairly common. For the most part, the OPIE system operates completely independently of the UNIX® password system.

Besides the password, there are two other pieces of data that are important to OPIE. One is what is known as the "seed" or "key", consisting of two letters and five digits. The other is what is called the "iteration count", a number between 1 and 100. OPIE creates the one-time password by concatenating the seed and the secret password, then applying the MD5 hash as many times as specified by the iteration count and turning the result into six short English words. These six English words are your one-time password. The authentication system (primarily PAM) keeps track of the last one-time password used, and the user is authenticated if the hash of the user-provided password is equal to the previous password. Because a one-way hash is used it is impossible to generate future one-time passwords if a successfully used password is captured; the iteration count is decremented after each successful login to keep the user and the login program in sync. When the iteration count gets down to 1, OPIE must be reinitialized.

There are a few programs involved in each system which we will discuss below. The opiekey program accepts an iteration count, a seed, and a secret password, and generates a one-time password or a consecutive list of one-time passwords. The opiepasswd program is used to initialize OPIE, and to change passwords, iteration counts, or seeds; it takes either a secret passphrase, or an iteration count, seed, and a one-time password. The opieinfo program will examine the relevant credentials files (/etc/opiekeys) and print out the invoking user’s current iteration count and seed.

There are four different sorts of operations we will cover. The first is using opiepasswd over a secure connection to set up one-time-passwords for the first time, or to change your password or seed. The second operation is using opiepasswd over an insecure connection, in conjunction with opiekey over a secure connection, to do the same. The third is using opiekey to log in over an insecure connection. The fourth is using opiekey to generate a number of keys which can be written down or printed out to carry with you when going to some location without secure connections to anywhere.

15.5.1. Secure Connection Initialization

To initialize OPIE for the first time, execute the opiepasswd command:

% opiepasswd -c
[grimreaper] ~ $ opiepasswd -f -c
Adding unfurl:
Only use this method from the console; NEVER from remote. If you are using
telnet, xterm, or a dial-in, type ^C now or exit with no password.
Then run opiepasswd without the -c parameter.
Using MD5 to compute responses.
Enter new secret pass phrase:
Again new secret pass phrase:
ID unfurl OTP key is 499 to4268
MOS MALL GOAT ARM AVID COED

At the Enter new secret pass phrase: or Enter secret password: prompts, you should enter a password or phrase. Remember, this is not the password that you will use to login with, this is used to generate your one-time login keys. The "ID" line gives the parameters of your particular instance: your login name, the iteration count, and seed. When logging in the system will remember these parameters and present them back to you so you do not have to remember them. The last line gives the particular one-time password which corresponds to those parameters and your secret password; if you were to re-login immediately, this one-time password is the one you would use.

15.5.2. Insecure Connection Initialization

To initialize or change your secret password over an insecure connection, you will need to already have a secure connection to some place where you can run opiekey; this might be in the form of a shell prompt on a machine you trust. You will also need to make up an iteration count (100 is probably a good value), and you may make up your own seed or use a randomly-generated one. Over on the insecure connection (to the machine you are initializing), use opiepasswd:

% opiepasswd

Updating unfurl:
You need the response from an OTP generator.
Old secret pass phrase:
        otp-md5 498 to4268 ext
        Response: GAME GAG WELT OUT DOWN CHAT
New secret pass phrase:
        otp-md5 499 to4269
        Response: LINE PAP MILK NELL BUOY TROY

ID mark OTP key is 499 gr4269
LINE PAP MILK NELL BUOY TROY

To accept the default seed press Return. Then before entering an access password, move over to your secure connection and give it the same parameters:

% opiekey 498 to4268
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT

Now switch back over to the insecure connection, and copy the one-time password generated over to the relevant program.

15.5.3. Generating a Single One-time Password

Once you have initialized OPIE and login, you will be presented with a prompt like this:

% telnet example.com
Trying 10.0.0.1...
Connected to example.com
Escape character is '^]'.

FreeBSD/i386 (example.com) (ttypa)

login: <username>
otp-md5 498 gr4269 ext
Password:

As a side note, the OPIE prompts have a useful feature (not shown here): if you press Return at the password prompt, the prompter will turn echo on, so you can see what you are typing. This can be extremely useful if you are attempting to type in a password by hand, such as from a printout.

At this point you need to generate your one-time password to answer this login prompt. This must be done on a trusted system that you can run opiekey on. (There are versions of these for DOS, Windows® and Mac OS® as well.) They need the iteration count and the seed as command line options. You can cut-and-paste these right from the login prompt on the machine that you are logging in to.

On the trusted system:

% opiekey 498 to4268
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT

Now that you have your one-time password you can continue logging in.

15.5.4. Generating Multiple One-time Passwords

Sometimes you have to go places where you do not have access to a trusted machine or secure connection. In this case, it is possible to use the opiekey command to generate a number of one-time passwords beforehand to be printed out and taken with you. For example:

% opiekey -n 5 30 zz99999
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase: <secret password>
26: JOAN BORE FOSS DES NAY QUIT
27: LATE BIAS SLAY FOLK MUCH TRIG
28: SALT TIN ANTI LOON NEAL USE
29: RIO ODIN GO BYE FURY TIC
30: GREW JIVE SAN GIRD BOIL PHI

The -n 5 requests five keys in sequence, the 30 specifies what the last iteration number should be. Note that these are printed out in reverse order of eventual use. If you are really paranoid, you might want to write the results down by hand; otherwise you can cut-and-paste into lpr. Note that each line shows both the iteration count and the one-time password; you may still find it handy to scratch off passwords as you use them.

15.5.5. Restricting Use of UNIX® Passwords

OPIE can restrict the use of UNIX® passwords based on the IP address of a login session. The relevant file is /etc/opieaccess, which is present by default. Please check opieaccess(5) for more information on this file and which security considerations you should be aware of when using it.

Here is a sample opieaccess file:

permit 192.168.0.0 255.255.0.0

This line allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use UNIX® passwords at any time.

If no rules in opieaccess are matched, the default is to deny non-OPIE logins.

15.6. TCP Wrappers

Anyone familiar with inetd(8) has probably heard of TCP Wrappers at some point. But few individuals seem to fully comprehend its usefulness in a network environment. It seems that everyone wants to install a firewall to handle network connections. While a firewall has a wide variety of uses, there are some things that a firewall not handle such as sending text back to the connection originator. The TCP software does this and much more. In the next few sections many of the TCP Wrappers features will be discussed, and, when applicable, example configuration lines will be provided.

The TCP Wrappers software extends the abilities of inetd to provide support for every server daemon under its control. Using this method it is possible to provide logging support, return messages to connections, permit a daemon to only accept internal connections, etc. While some of these features can be provided by implementing a firewall, this will add not only an extra layer of protection but go beyond the amount of control a firewall can provide.

The added functionality of TCP Wrappers should not be considered a replacement for a good firewall. TCP Wrappers can be used in conjunction with a firewall or other security enhancements though and it can serve nicely as an extra layer of protection for the system.

Since this is an extension to the configuration of inetd, the reader is expected have read the inetd configuration section.

While programs run by inetd(8) are not exactly "daemons", they have traditionally been called daemons. This is the term we will use in this section too.

15.6.1. Initial Configuration

The only requirement of using TCP Wrappers in FreeBSD is to ensure the inetd server is started from rc.conf with the -Ww option; this is the default setting. Of course, proper configuration of /etc/hosts.allow is also expected, but syslogd(8) will throw messages in the system logs in these cases.

Unlike other implementations of TCP Wrappers, the use of hosts.deny has been deprecated. All configuration options should be placed in /etc/hosts.allow.

In the simplest configuration, daemon connection policies are set to either be permitted or blocked depending on the options in /etc/hosts.allow. The default configuration in FreeBSD is to allow a connection to every daemon started with inetd. Changing this will be discussed only after the basic configuration is covered.

Basic configuration usually takes the form of daemon : address : action. Where daemon is the daemon name which inetd started. The address can be a valid hostname, an IP address or an IPv6 address enclosed in brackets ([ ]). The action field can be either allow or deny to grant or deny access appropriately. Keep in mind that configuration works off a first rule match semantic, meaning that the configuration file is scanned in ascending order for a matching rule. When a match is found the rule is applied and the search process will halt.

Several other options exist but they will be explained in a later section. A simple configuration line may easily be constructed from that information alone. For example, to allow POP3 connections via the mail/qpopper daemon, the following lines should be appended to hosts.allow:

# This line is required for POP3 connections:
qpopper : ALL : allow

After adding this line, inetd will need restarted. This can be accomplished by use of the kill(1) command, or with the restart parameter with /etc/rc.d/inetd.

15.6.2. Advanced Configuration

TCP Wrappers has advanced options too; they will allow for more control over the way connections are handled. In some cases it may be a good idea to return a comment to certain hosts or daemon connections. In other cases, perhaps a log file should be recorded or an email sent to the administrator. Other situations may require the use of a service for local connections only. This is all possible through the use of configuration options known as wildcards, expansion characters and external command execution. The next two sections are written to cover these situations.

15.6.2.1. External Commands

Suppose that a situation occurs where a connection should be denied yet a reason should be sent to the individual who attempted to establish that connection. How could it be done? That action can be made possible by using the twist option. When a connection attempt is made, twist will be called to execute a shell command or script. An example already exists in the hosts.allow file:

# The rest of the daemons are protected.
ALL : ALL \
        : severity auth.info \
        : twist /bin/echo "You are not welcome to use %d from %h."

This example shows that the message, "You are not allowed to use daemon from hostname." will be returned for any daemon not previously configured in the access file. This is extremely useful for sending a reply back to the connection initiator right after the established connection is dropped. Note that any message returned must be wrapped in quote " characters; there are no exceptions to this rule.

It may be possible to launch a denial of service attack on the server if an attacker, or group of attackers could flood these daemons with connection requests.

Another possibility is to use the spawn option in these cases. Like twist, the spawn implicitly denies the connection and may be used to run external shell commands or scripts. Unlike twist, spawn will not send a reply back to the individual who established the connection. For an example, consider the following configuration line:

# We do not allow connections from example.com:
ALL : .example.com \
	: spawn (/bin/echo %a from %h attempted to access %d >> \
	  /var/log/connections.log) \
	: deny

This will deny all connection attempts from the *.example.com domain; simultaneously logging the hostname, IP address and the daemon which they attempted to access in the /var/log/connections.log file.

Aside from the already explained substitution characters above, e.g. %a, a few others exist. See the hosts_access(5) manual page for the complete list.

15.6.2.2. Wildcard Options

Thus far the ALL example has been used continuously throughout the examples. Other options exist which could extend the functionality a bit further. For instance, ALL may be used to match every instance of either a daemon, domain or an IP address. Another wildcard available is PARANOID which may be used to match any host which provides an IP address that may be forged. In other words, paranoid may be used to define an action to be taken whenever a connection is made from an IP address that differs from its hostname. The following example may shed some more light on this discussion:

# Block possibly spoofed requests to sendmail:
sendmail : PARANOID : deny

In that example all connection requests to sendmail which have an IP address that varies from its hostname will be denied.

Using the PARANOID may severely cripple servers if the client or server has a broken DNS setup. Administrator discretion is advised.

To learn more about wildcards and their associated functionality, see the hosts_access(5) manual page.

Before any of the specific configuration lines above will work, the first configuration line should be commented out in hosts.allow. This was noted at the beginning of this section.

15.7. KerberosIV

Kerberos is a network add-on system/protocol that allows users to authenticate themselves through the services of a secure server. Services such as remote login, remote copy, secure inter-system file copying and other high-risk tasks are made considerably safer and more controllable.

The following instructions can be used as a guide on how to set up Kerberos as distributed for FreeBSD. However, you should refer to the relevant manual pages for a complete description.

15.7.1. Installing KerberosIV

Kerberos is an optional component of FreeBSD. The easiest way to install this software is by selecting the krb4 or krb5 distribution in sysinstall during the initial installation of FreeBSD. This will install the "eBones" (KerberosIV) or "Heimdal" (Kerberos5) implementation of Kerberos. These implementations are included because they are developed outside the USA/Canada and were thus available to system owners outside those countries during the era of restrictive export controls on cryptographic code from the USA.

Alternatively, the MIT implementation of Kerberos is available from the Ports Collection as security/krb5.

15.7.2. Creating the Initial Database

This is done on the Kerberos server only. First make sure that you do not have any old Kerberos databases around. You should change to the directory /etc/kerberosIV and check that only the following files are present:

d /etc/kerberosIV
# ls
README		krb.conf        krb.realms

If any additional files (such as principal.* or master_key) exist, then use the kdb_destroy command to destroy the old Kerberos database, or if Kerberos is not running, simply delete the extra files.

You should now edit the krb.conf and krb.realms files to define your Kerberos realm. In this case the realm will be EXAMPLE.COM and the server is grunt.example.com. We edit or create the krb.conf file:

# cat krb.conf
EXAMPLE.COM
EXAMPLE.COM grunt.example.com admin server
CS.BERKELEY.EDU okeeffe.berkeley.edu
ATHENA.MIT.EDU kerberos.mit.edu
ATHENA.MIT.EDU kerberos-1.mit.edu
ATHENA.MIT.EDU kerberos-2.mit.edu
ATHENA.MIT.EDU kerberos-3.mit.edu
LCS.MIT.EDU kerberos.lcs.mit.edu
TELECOM.MIT.EDU bitsy.mit.edu
ARC.NASA.GOV trident.arc.nasa.gov

In this case, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to not include them for simplicity.

The first line names the realm in which this system works. The other lines contain realm/host entries. The first item on a line is a realm, and the second is a host in that realm that is acting as a "key distribution center". The words admin server following a host’s name means that host also provides an administrative database server. For further explanation of these terms, please consult the Kerberos manual pages.

Now we have to add grunt.example.com to the EXAMPLE.COM realm and also add an entry to put all hosts in the .example.com domain in the EXAMPLE.COM realm. The krb.realms file would be updated as follows:

# cat krb.realms
grunt.example.com EXAMPLE.COM
.example.com EXAMPLE.COM
.berkeley.edu CS.BERKELEY.EDU
.MIT.EDU ATHENA.MIT.EDU
.mit.edu ATHENA.MIT.EDU

Again, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to remove them to simplify things.

The first line puts the specific system into the named realm. The rest of the lines show how to default systems of a particular subdomain to a named realm.

Now we are ready to create the database. This only needs to run on the Kerberos server (or Key Distribution Center). Issue the kdb_init command to do this:

# kdb_init
Realm name [default  ATHENA.MIT.EDU ]: EXAMPLE.COM
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.

Enter Kerberos master key:

Now we have to save the key so that servers on the local machine can pick it up. Use the kstash command to do this:

# kstash

Enter Kerberos master key:

Current Kerberos master key version is 1.

Master key entered. BEWARE!

This saves the encrypted master password in /etc/kerberosIV/master_key.

15.7.3. Making It All Run

Two principals need to be added to the database for each system that will be secured with Kerberos. Their names are kpasswd and rcmd. These two principals are made for each system, with the instance being the name of the individual system.

These daemons, kpasswd and rcmd allow other systems to change Kerberos passwords and run commands like rcp(1), rlogin(1) and rsh(1).

Now let us add these entries:

# kdb_edit
Opening database...

Enter Kerberos master key:

Current Kerberos master key version is 1.

Master key entered.  BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.

Principal name: passwd
Instance: grunt

<Not found>, Create [y] ? y

Principal: passwd, Instance: grunt, kdc_key_ver: 1
New Password:                    <---- enter RANDOM here
Verifying password

New Password: <---- enter RANDOM here

Random password [y] ? y

Principal's new key version = 1
Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
Max ticket lifetime (*5 minutes) [ 255 ] ?
Attributes [ 0 ] ?
Edit O.K.
Principal name: rcmd
Instance: grunt

<Not found>, Create [y] ?

Principal: rcmd, Instance: grunt, kdc_key_ver: 1
New Password:		<---- enter RANDOM here
Verifying password

New Password:           <---- enter RANDOM here

Random password [y] ?

Principal's new key version = 1
Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
Max ticket lifetime (*5 minutes) [ 255 ] ?
Attributes [ 0 ] ?
Edit O.K.
Principal name:         <---- null entry here will cause an exit

15.7.4. Creating the Server File

We now have to extract all the instances which define the services on each machine. For this we use the ext_srvtab command. This will create a file which must be copied or moved by secure means to each Kerberos client’s /etc directory. This file must be present on each server and client, and is crucial to the operation of Kerberos.

# ext_srvtab grunt
Enter Kerberos master key:

Current Kerberos master key version is 1.

Master key entered. BEWARE!
Generating 'grunt-new-srvtab'....

Now, this command only generates a temporary file which must be renamed to srvtab so that all the servers can pick it up. Use the mv(1) command to move it into place on the original system:

# mv grunt-new-srvtab srvtab

If the file is for a client system, and the network is not deemed safe, then copy the client-new-srvtab to removable media and transport it by secure physical means. Be sure to rename it to srvtab in the client’s /etc directory, and make sure it is mode 600:

# mv grumble-new-srvtab srvtab
# chmod 600 srvtab

15.7.5. Populating the Database

We now have to add some user entries into the database. First let us create an entry for the user jane. Use the kdb_edit command to do this:

# kdb_edit
Opening database...

Enter Kerberos master key:

Current Kerberos master key version is 1.

Master key entered.  BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.

Principal name: jane
Instance:

<Not found>, Create [y] ? y

Principal: jane, Instance: , kdc_key_ver: 1
New Password:                <---- enter a secure password here
Verifying password

New Password:                <---- re-enter the password here
Principal's new key version = 1
Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
Max ticket lifetime (*5 minutes) [ 255 ] ?
Attributes [ 0 ] ?
Edit O.K.
Principal name:		   <---- null entry here will cause an exit

15.7.6. Testing It All Out

First we have to start the Kerberos daemons. Note that if you have correctly edited your /etc/rc.conf then this will happen automatically when you reboot. This is only necessary on the Kerberos server. Kerberos clients will automatically get what they need from the /etc/kerberosIV directory.

# kerberos &
Kerberos server starting
Sleep forever on error
Log file is /var/log/kerberos.log
Current Kerberos master key version is 1.

Master key entered. BEWARE!

Current Kerberos master key version is 1
Local realm: EXAMPLE.COM
# kadmind -n &
KADM Server KADM0.0A initializing
Please do not use 'kill -9' to kill this job, use a
regular kill instead

Current Kerberos master key version is 1.

Master key entered.  BEWARE!

Now we can try using the kinit command to get a ticket for the ID jane that we created above:

% kinit jane
MIT Project Athena (grunt.example.com)
Kerberos Initialization for "jane"
Password:

Try listing the tokens using klist to see if we really have them:

% klist
Ticket file:    /tmp/tkt245
Principal:      jane@EXAMPLE.COM

  Issued           Expires          Principal
Apr 30 11:23:22  Apr 30 19:23:22  krbtgt.EXAMPLE.COM@EXAMPLE.COM

Now try changing the password using passwd(1) to check if the kpasswd daemon can get authorization to the Kerberos database:

% passwd
realm EXAMPLE.COM
Old password for jane:
New Password for jane:
Verifying password
New Password for jane:
Password changed.

15.7.7. Adding su Privileges

Kerberos allows us to give each user who needs root privileges their own separate su(1) password. We could now add an ID which is authorized to su(1) to root. This is controlled by having an instance of root associated with a principal. Using kdb_edit we can create the entry jane.root in the Kerberos database:

# kdb_edit
Opening database...

Enter Kerberos master key:

Current Kerberos master key version is 1.

Master key entered.  BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.

Principal name: jane
Instance: root

<Not found>, Create [y] ? y

Principal: jane, Instance: root, kdc_key_ver: 1
New Password:                    <---- enter a SECURE password here
Verifying password

New Password:    	 	 <---- re-enter the password here

Principal's new key version = 1
Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
Max ticket lifetime (*5 minutes) [ 255 ] ? 12 <--- Keep this short!
Attributes [ 0 ] ?
Edit O.K.
Principal name:		         <---- null entry here will cause an exit

Now try getting tokens for it to make sure it works:

# kinit jane.root
MIT Project Athena (grunt.example.com)
Kerberos Initialization for "jane.root"
Password:

Now we need to add the user to root's .klogin file:

# cat /root/.klogin
jane.root@EXAMPLE.COM

Now try doing the su(1):

% su
Password:

and take a look at what tokens we have:

# klist
Ticket file:	/tmp/tkt_root_245
Principal:      jane.root@EXAMPLE.COM

  Issued           Expires          Principal
May  2 20:43:12  May  3 04:43:12  krbtgt.EXAMPLE.COM@EXAMPLE.COM

15.7.8. Using Other Commands

In an earlier example, we created a principal called jane with an instance root. This was based on a user with the same name as the principal, and this is a Kerberos default; that a <principal>.<instance> of the form <username>. root will allow that <username> to su(1) to root if the necessary entries are in the .klogin file in root's home directory:

# cat /root/.klogin
jane.root@EXAMPLE.COM

Likewise, if a user has in their own home directory lines of the form:

% cat ~/.klogin
jane@EXAMPLE.COM
jack@EXAMPLE.COM

This allows anyone in the EXAMPLE.COM realm who has authenticated themselves as jane or jack (via kinit, see above) to access to jane's account or files on this system (grunt) via rlogin(1), rsh(1) or rcp(1).

For example, jane now logs into another system using Kerberos:

% kinit
MIT Project Athena (grunt.example.com)
Password:
% rlogin grunt
Last login: Mon May  1 21:14:47 from grumble
Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
        The Regents of the University of California.   All rights reserved.

FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995

Or jack logs into jane's account on the same machine (jane having set up the .klogin file as above, and the person in charge of Kerberos having set up principal jack with a null instance):

% kinit
% rlogin grunt -l jane
MIT Project Athena (grunt.example.com)
Password:
Last login: Mon May  1 21:16:55 from grumble
Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
        The Regents of the University of California.   All rights reserved.
FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995

15.8. Kerberos5

Every FreeBSD release beyond FreeBSD-5.1 includes support only for Kerberos5. Hence Kerberos5 is the only version included, and its configuration is similar in many aspects to that of KerberosIV. The following information only applies to Kerberos5 in post FreeBSD-5.0 releases. Users who wish to use the KerberosIV package may install the security/krb4 port.

Kerberos is a network add-on system/protocol that allows users to authenticate themselves through the services of a secure server. Services such as remote login, remote copy, secure inter-system file copying and other high-risk tasks are made considerably safer and more controllable.

Kerberos can be described as an identity-verifying proxy system. It can also be described as a trusted third-party authentication system. Kerberos provides only one function - the secure authentication of users on the network. It does not provide authorization functions (what users are allowed to do) or auditing functions (what those users did). After a client and server have used Kerberos to prove their identity, they can also encrypt all of their communications to assure privacy and data integrity as they go about their business.

Therefore it is highly recommended that Kerberos be used with other security methods which provide authorization and audit services.

The following instructions can be used as a guide on how to set up Kerberos as distributed for FreeBSD. However, you should refer to the relevant manual pages for a complete description.

For purposes of demonstrating a Kerberos installation, the various name spaces will be handled as follows:

  • The DNS domain ("zone") will be example.org.

  • The Kerberos realm will be EXAMPLE.ORG.

Please use real domain names when setting up Kerberos even if you intend to run it internally. This avoids DNS problems and assures inter-operation with other Kerberos realms.

15.8.1. History

Kerberos was created by MIT as a solution to network security problems. The Kerberos protocol uses strong cryptography so that a client can prove its identity to a server (and vice versa) across an insecure network connection.

Kerberos is both the name of a network authentication protocol and an adjective to describe programs that implement the program (Kerberos telnet, for example). The current version of the protocol is version 5, described in RFC 1510.

Several free implementations of this protocol are available, covering a wide range of operating systems. The Massachusetts Institute of Technology (MIT), where Kerberos was originally developed, continues to develop their Kerberos package. It is commonly used in the US as a cryptography product, as such it has historically been affected by US export regulations. The MITKerberos is available as a port (security/krb5). Heimdal Kerberos is another version 5 implementation, and was explicitly developed outside of the US to avoid export regulations (and is thus often included in non-commercial UNIX® variants). The Heimdal Kerberos distribution is available as a port (security/heimdal), and a minimal installation of it is included in the base FreeBSD install.

In order to reach the widest audience, these instructions assume the use of the Heimdal distribution included in FreeBSD.

15.8.2. Setting up a Heimdal KDC

The Key Distribution Center (KDC) is the centralized authentication service that Kerberos provides - it is the computer that issues Kerberos tickets. The KDC is considered "trusted" by all other computers in the Kerberos realm, and thus has heightened security concerns.

Note that while running the Kerberos server requires very few computing resources, a dedicated machine acting only as a KDC is recommended for security reasons.

To begin setting up a KDC, ensure that your /etc/rc.conf file contains the correct settings to act as a KDC (you may need to adjust paths to reflect your own system):

kerberos5_server_enable="YES"
kadmind5_server_enable="YES"

Next we will set up your Kerberos config file, /etc/krb5.conf:

[libdefaults]
    default_realm = EXAMPLE.ORG
[realms]
    EXAMPLE.ORG = {
        kdc = kerberos.example.org
        admin_server = kerberos.example.org
    }
[domain_realm]
    .example.org = EXAMPLE.ORG

Note that this /etc/krb5.conf file implies that your KDC will have the fully-qualified hostname of kerberos.example.org. You will need to add a CNAME (alias) entry to your zone file to accomplish this if your KDC has a different hostname.

For large networks with a properly configured BINDDNS server, the above example could be trimmed to:

[libdefaults]
      default_realm = EXAMPLE.ORG

With the following lines being appended to the example.org zonefile:

_kerberos._udp      IN  SRV     01 00 88 kerberos.example.org.
_kerberos._tcp      IN  SRV     01 00 88 kerberos.example.org.
_kpasswd._udp       IN  SRV     01 00 464 kerberos.example.org.
_kerberos-adm._tcp  IN  SRV     01 00 749 kerberos.example.org.
_kerberos           IN  TXT     EXAMPLE.ORG

For clients to be able to find the Kerberos services, you must have either a fully configured /etc/krb5.conf or a minimally configured /etc/krb5.confand a properly configured DNS server.

Next we will create the Kerberos database. This database contains the keys of all principals encrypted with a master password. You are not required to remember this password, it will be stored in a file (/var/heimdal/m-key). To create the master key, run kstash and enter a password.

Once the master key has been created, you can initialize the database using the kadmin program with the -l option (standing for "local"). This option instructs kadmin to modify the database files directly rather than going through the kadmind network service. This handles the chicken-and-egg problem of trying to connect to the database before it is created. Once you have the kadmin prompt, use the init command to create your realms initial database.

Lastly, while still in kadmin, create your first principal using the add command. Stick to the defaults options for the principal for now, you can always change them later with the modify command. Note that you can use the ? command at any prompt to see the available options.

A sample database creation session is shown below:

# kstash
Master key: xxxxxxxx
Verifying password - Master key: xxxxxxxx

# kadmin -l
kadmin> init EXAMPLE.ORG
Realm max ticket life [unlimited]:
kadmin> add tillman
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
Password: xxxxxxxx
Verifying password - Password: xxxxxxxx

Now it is time to start up the KDC services. Run /etc/rc.d/kerberos start and /etc/rc.d/kadmind start to bring up the services. Note that you will not have any kerberized daemons running at this point but you should be able to confirm the that the KDC is functioning by obtaining and listing a ticket for the principal (user) that you just created from the command-line of the KDC itself:

% kinit tillman
tillman@EXAMPLE.ORG's Password:

% klist
Credentials cache: FILE:/tmp/krb5cc_500
	Principal: tillman@EXAMPLE.ORG

  Issued           Expires          Principal
Aug 27 15:37:58  Aug 28 01:37:58  krbtgt/EXAMPLE.ORG@EXAMPLE.ORG

The ticket can then be revoked when you have finished:

% k5destroy

15.8.3. Kerberos enabling a server with Heimdal services

First, we need a copy of the Kerberos configuration file, /etc/krb5.conf. To do so, simply copy it over to the client computer from the KDC in a secure fashion (using network utilities, such as scp(1), or physically via a floppy disk).

Next you need a /etc/krb5.keytab file. This is the major difference between a server providing Kerberos enabled daemons and a workstation - the server must have a keytab file. This file contains the server’s host key, which allows it and the KDC to verify each others identity. It must be transmitted to the server in a secure fashion, as the security of the server can be broken if the key is made public. This explicitly means that transferring it via a clear text channel, such as FTP, is a very bad idea.

Typically, you transfer to the keytab to the server using the kadmin program. This is handy because you also need to create the host principal (the KDC end of the krb5.keytab) using kadmin.

Note that you must have already obtained a ticket and that this ticket must be allowed to use the kadmin interface in the kadmind.acl. See the section titled "Remote administration" in the Heimdal info pages (info heimdal) for details on designing access control lists. If you do not want to enable remote kadmin access, you can simply securely connect to the KDC (via local console, ssh(1) or Kerberos telnet(1)) and perform administration locally using kadmin -l.

After installing the /etc/krb5.conf file, you can use kadmin from the Kerberos server. The add --random-key command will let you add the server’s host principal, and the ext command will allow you to extract the server’s host principal to its own keytab. For example:

# kadmin
kadmin> add --random-key host/myserver.example.org
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
kadmin> ext host/myserver.example.org
kadmin> exit

Note that the ext command (short for "extract") stores the extracted key in /etc/krb5.keytab by default.

If you do not have kadmind running on the KDC (possibly for security reasons) and thus do not have access to kadmin remotely, you can add the host principal (host/myserver.EXAMPLE.ORG) directly on the KDC and then extract it to a temporary file (to avoid over-writing the /etc/krb5.keytab on the KDC) using something like this:

# kadmin
kadmin> ext --keytab=/tmp/example.keytab host/myserver.example.org
kadmin> exit

You can then securely copy the keytab to the server computer (using scp or a floppy, for example). Be sure to specify a non-default keytab name to avoid over-writing the keytab on the KDC.

At this point your server can communicate with the KDC (due to its krb5.conf file) and it can prove its own identity (due to the krb5.keytab file). It is now ready for you to enable some Kerberos services. For this example we will enable the telnet service by putting a line like this into your /etc/inetd.conf and then restarting the inetd(8) service with /etc/rc.d/inetd restart:

telnet    stream  tcp     nowait  root    /usr/libexec/telnetd  telnetd -a user

The critical bit is that the -a (for authentication) type is set to user. Consult the telnetd(8) manual page for more details.

15.8.4. Kerberos enabling a client with Heimdal

Setting up a client computer is almost trivially easy. As far as Kerberos configuration goes, you only need the Kerberos configuration file, located at /etc/krb5.conf. Simply securely copy it over to the client computer from the KDC.

Test your client computer by attempting to use kinit, klist, and kdestroy from the client to obtain, show, and then delete a ticket for the principal you created above. You should also be able to use Kerberos applications to connect to Kerberos enabled servers, though if that does not work and obtaining a ticket does the problem is likely with the server and not with the client or the KDC.

When testing an application like telnet, try using a packet sniffer (such as tcpdump(1)) to confirm that your password is not sent in the clear. Try using telnet with the -x option, which encrypts the entire data stream (similar to ssh).

Various non-core Kerberos client applications are also installed by default. This is where the "minimal" nature of the base Heimdal installation is felt: telnet is the only Kerberos enabled service.

The Heimdal port adds some of the missing client applications: Kerberos enabled versions of ftp, rsh, rcp, rlogin, and a few other less common programs. The MIT port also contains a full suite of Kerberos client applications.

15.8.5. User configuration files: .k5login and .k5users

Users within a realm typically have their Kerberos principal (such as tillman@EXAMPLE.ORG) mapped to a local user account (such as a local account named tillman). Client applications such as telnet usually do not require a user name or a principal.

Occasionally, however, you want to grant access to a local user account to someone who does not have a matching Kerberos principal. For example, tillman@EXAMPLE.ORG may need access to the local user account webdevelopers. Other principals may also need access to that local account.

The .k5login and .k5users files, placed in a users home directory, can be used similar to a powerful combination of .hosts and .rhosts, solving this problem. For example, if a .k5login with the following contents:

tillman@example.org
jdoe@example.org

Were to be placed into the home directory of the local user webdevelopers then both principals listed would have access to that account without requiring a shared password.

Reading the manual pages for these commands is recommended. Note that the ksu manual page covers .k5users.

15.8.6. Kerberos Tips, Tricks, and Troubleshooting

  • When using either the Heimdal or MITKerberos ports ensure that your PATH environment variable lists the Kerberos versions of the client applications before the system versions.

  • Do all the computers in your realm have synchronized time settings? If not, authentication may fail. Συγχρονισμός Ρολογιού Συστήματος με NTP describes how to synchronize clocks using NTP.

  • MIT and Heimdal inter-operate nicely. Except for kadmin, the protocol for which is not standardized.

  • If you change your hostname, you also need to change your host/ principal and update your keytab. This also applies to special keytab entries like the www/ principal used for Apache’s www/mod_auth_kerb.

  • All hosts in your realm must be resolvable (both forwards and reverse) in DNS (or /etc/hosts as a minimum). CNAMEs will work, but the A and PTR records must be correct and in place. The error message is not very intuitive: Kerberos5 refuses authentication because Read req failed: Key table entry not found.

  • Some operating systems that may being acting as clients to your KDC do not set the permissions for ksu to be setuid root. This means that ksu does not work, which is a good security idea but annoying. This is not a KDC error.

  • With MITKerberos, if you want to allow a principal to have a ticket life longer than the default ten hours, you must use modify_principal in kadmin to change the maxlife of both the principal in question and the krbtgt principal. Then the principal can use the -l option with kinit to request a ticket with a longer lifetime.

If you run a packet sniffer on your KDC to add in troubleshooting and then run kinit from a workstation, you will notice that your TGT is sent immediately upon running kinit - even before you type your password! The explanation is that the Kerberos server freely transmits a TGT (Ticket Granting Ticket) to any unauthorized request; however, every TGT is encrypted in a key derived from the user’s password. Therefore, when a user types their password it is not being sent to the KDC, it is being used to decrypt the TGT that kinit already obtained. If the decryption process results in a valid ticket with a valid time stamp, the user has valid Kerberos credentials. These credentials include a session key for establishing secure communications with the Kerberos server in the future, as well as the actual ticket-granting ticket, which is actually encrypted with the Kerberos server’s own key. This second layer of encryption is unknown to the user, but it is what allows the Kerberos server to verify the authenticity of each TGT.

  • If you want to use long ticket lifetimes (a week, for example) and you are using OpenSSH to connect to the machine where your ticket is stored, make sure that Kerberos TicketCleanup is set to no in your sshd_config or else your tickets will be deleted when you log out.

  • Remember that host principals can have a longer ticket lifetime as well. If your user principal has a lifetime of a week but the host you are connecting to has a lifetime of nine hours, you will have an expired host principal in your cache and the ticket cache will not work as expected.

  • When setting up a krb5.dict file to prevent specific bad passwords from being used (the manual page for kadmind covers this briefly), remember that it only applies to principals that have a password policy assigned to them. The krb5.dict files format is simple: one string per line. Creating a symbolic link to /usr/shared/dict/words might be useful.

15.8.7. Differences with the MIT port

The major difference between the MIT and Heimdal installs relates to the kadmin program which has a different (but equivalent) set of commands and uses a different protocol. This has a large implications if your KDC is MIT as you will not be able to use the Heimdal kadmin program to administer your KDC remotely (or vice versa, for that matter).

The client applications may also take slightly different command line options to accomplish the same tasks. Following the instructions on the MITKerberos web site (http://web.mit.edu/Kerberos/www/) is recommended. Be careful of path issues: the MIT port installs into /usr/local/ by default, and the "normal" system applications may be run instead of MIT if your PATH environment variable lists the system directories first.

With the MITsecurity/krb5 port that is provided by FreeBSD, be sure to read the /usr/local/shared/doc/krb5/README.FreeBSD file installed by the port if you want to understand why logins via telnetd and klogind behave somewhat oddly. Most importantly, correcting the "incorrect permissions on cache file" behavior requires that the login.krb5 binary be used for authentication so that it can properly change ownership for the forwarded credentials.

The rc.conf must also be modified to contain the following configuration:

kerberos5_server="/usr/local/sbin/krb5kdc"
kadmind5_server="/usr/local/sbin/kadmind"
kerberos5_server_enable="YES"
kadmind5_server_enable="YES"

This is done because the applications for MIT kerberos installs binaries in the /usr/local hierarchy.

15.8.8. Mitigating limitations found in Kerberos

15.8.8.1. Kerberos is an all-or-nothing approach

Every service enabled on the network must be modified to work with Kerberos (or be otherwise secured against network attacks) or else the users credentials could be stolen and re-used. An example of this would be Kerberos enabling all remote shells (via rsh and telnet, for example) but not converting the POP3 mail server which sends passwords in plain text.

15.8.8.2. Kerberos is intended for single-user workstations

In a multi-user environment, Kerberos is less secure. This is because it stores the tickets in the /tmp directory, which is readable by all users. If a user is sharing a computer with several other people simultaneously (i.e. multi-user), it is possible that the user’s tickets can be stolen (copied) by another user.

This can be overcome with the -c filename command-line option or (preferably) the KRB5CCNAME environment variable, but this is rarely done. In principal, storing the ticket in the users home directory and using simple file permissions can mitigate this problem.

15.8.8.3. The KDC is a single point of failure

By design, the KDC must be as secure as the master password database is contained on it. The KDC should have absolutely no other services running on it and should be physically secured. The danger is high because Kerberos stores all passwords encrypted with the same key (the "master" key), which in turn is stored as a file on the KDC.

As a side note, a compromised master key is not quite as bad as one might normally fear. The master key is only used to encrypt the Kerberos database and as a seed for the random number generator. As long as access to your KDC is secure, an attacker cannot do much with the master key.

Additionally, if the KDC is unavailable (perhaps due to a denial of service attack or network problems) the network services are unusable as authentication can not be performed, a recipe for a denial-of-service attack. This can alleviated with multiple KDCs (a single master and one or more slaves) and with careful implementation of secondary or fall-back authentication (PAM is excellent for this).

15.8.8.4. Kerberos Shortcomings

Kerberos allows users, hosts and services to authenticate between themselves. It does not have a mechanism to authenticate the KDC to the users, hosts or services. This means that a trojanned kinit (for example) could record all user names and passwords. Something like security/tripwire or other file system integrity checking tools can alleviate this.

15.9. OpenSSL

One feature that many users overlook is the OpenSSL toolkit included in FreeBSD. OpenSSL provides an encryption transport layer on top of the normal communications layer; thus allowing it to be intertwined with many network applications and services.

Some uses of OpenSSL may include encrypted authentication of mail clients, web based transactions such as credit card payments and more. Many ports such as www/apache13-ssl, and mail/sylpheed-claws will offer compilation support for building with OpenSSL.

In most cases the Ports Collection will attempt to build the security/openssl port unless the WITH_OPENSSL_BASE make variable is explicitly set to "yes".

The version of OpenSSL included in FreeBSD supports Secure Sockets Layer v2/v3 (SSLv2/SSLv3), Transport Layer Security v1 (TLSv1) network security protocols and can be used as a general cryptographic library.

While OpenSSL supports the IDEA algorithm, it is disabled by default due to United States patents. To use it, the license should be reviewed and, if the restrictions are acceptable, the MAKE_IDEA variable must be set in make.conf.

One of the most common uses of OpenSSL is to provide certificates for use with software applications. These certificates ensure that the credentials of the company or individual are valid and not fraudulent. If the certificate in question has not been verified by one of the several "Certificate Authorities", or CAs, a warning is usually produced. A Certificate Authority is a company, such as VeriSign, which will sign certificates in order to validate credentials of individuals or companies. This process has a cost associated with it and is definitely not a requirement for using certificates; however, it can put some of the more paranoid users at ease.

15.9.1. Generating Certificates

To generate a certificate, the following command is available:

# openssl req -new -nodes -out req.pem -keyout cert.pem
Generating a 1024 bit RSA private key
................++++++
.......................................++++++
writing new private key to 'cert.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:PA
Locality Name (eg, city) []:Pittsburgh
Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Company
Organizational Unit Name (eg, section) []:Systems Administrator
Common Name (eg, YOUR name) []:localhost.example.org
Email Address []:trhodes@FreeBSD.org

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:SOME PASSWORD
An optional company name []:Another Name

Notice the response directly after the "Common Name" prompt shows a domain name. This prompt requires a server name to be entered for verification purposes; placing anything but a domain name would yield a useless certificate. Other options, for instance expire time, alternate encryption algorithms, etc. are available. A complete list may be obtained by viewing the openssl(1) manual page.

Two files should now exist in the directory in which the aforementioned command was issued. The certificate request, req.pem, may be sent to a certificate authority who will validate the credentials that you entered, sign the request and return the certificate to you. The second file created will be named cert.pem and is the private key for the certificate and should be protected at all costs; if this falls in the hands of others it can be used to impersonate you (or your server).

In cases where a signature from a CA is not required, a self signed certificate can be created. First, generate the RSA key:

# openssl dsaparam -rand -genkey -out myRSA.key 1024

Next, generate the CA key:

# openssl gendsa -des3 -out myca.key myRSA.key

Use this key to create the certificate:

# openssl req -new -x509 -days 365 -key myca.key -out new.crt

Two new files should appear in the directory: a certificate authority signature file, myca.key and the certificate itself, new.crt. These should be placed in a directory, preferably under /etc, which is readable only by root. Permissions of 0700 should be fine for this and they can be set with the chmod utility.

15.9.2. Using Certificates, an Example

So what can these files do? A good use would be to encrypt connections to the SendmailMTA. This would dissolve the use of clear text authentication for users who send mail via the local MTA.

This is not the best use in the world as some MUAs will present the user with an error if they have not installed the certificate locally. Refer to the documentation included with the software for more information on certificate installation.

The following lines should be placed inside the local .mc file:

dnl SSL Options
define(`confCACERT_PATH',`/etc/certs')dnl
define(`confCACERT',`/etc/certs/new.crt')dnl
define(`confSERVER_CERT',`/etc/certs/new.crt')dnl
define(`confSERVER_KEY',`/etc/certs/myca.key')dnl
define(`confTLS_SRV_OPTIONS', `V')dnl

Where /etc/certs/ is the directory to be used for storing the certificate and key files locally. The last few requirements are a rebuild of the local .cf file. This is easily achieved by typing makeinstall within the /etc/mail directory. Follow that up with makerestart which should start the Sendmail daemon.

If all went well there will be no error messages in the /var/log/maillog file and Sendmail will show up in the process list.

For a simple test, simply connect to the mail server using the telnet(1) utility:

# telnet example.com 25
Trying 192.0.34.166...
Connected to example.com.
Escape character is '^]'.
220 example.com ESMTP Sendmail 8.12.10/8.12.10; Tue, 31 Aug 2004 03:41:22 -0400 (EDT)
ehlo example.com
250-example.com Hello example.com [192.0.34.166], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH LOGIN PLAIN
250-STARTTLS
250-DELIVERBY
250 HELP
quit
221 2.0.0 example.com closing connection
Connection closed by foreign host.

If the "STARTTLS" line appears in the output then everything is working correctly.

15.10. VPN over IPsec

Creating a VPN between two networks, separated by the Internet, using FreeBSD gateways.

15.10.1. Understanding IPsec

This section will guide you through the process of setting up IPsec, and to use it in an environment which consists of FreeBSD and Microsoft® Windows® 2000/XP machines, to make them communicate securely. In order to set up IPsec, it is necessary that you are familiar with the concepts of building a custom kernel (see Ρυθμίζοντας τον Πυρήνα του FreeBSD).

IPsec is a protocol which sits on top of the Internet Protocol (IP) layer. It allows two or more hosts to communicate in a secure manner (hence the name). The FreeBSD IPsec "network stack" is based on the KAME implementation, which has support for both protocol families, IPv4 and IPv6.

FreeBSD contains a "hardware accelerated" IPsec stack, known as "Fast IPsec", that was obtained from OpenBSD. It employs cryptographic hardware (whenever possible) via the crypto(4) subsystem to optimize the performance of IPsec. This subsystem is new, and does not support all the features that are available in the KAME version of IPsec. However, in order to enable hardware-accelerated IPsec, the following kernel option has to be added to your kernel configuration file:

options	  FAST_IPSEC  # new IPsec (cannot define w/ IPSEC)

Note, that it is not currently possible to use the "Fast IPsec" subsystem in lieu of the KAME implementation of IPsec. Consult the manual page for more information.

To let firewalls properly track state for gif(4) tunnels too, you have to enable the IPSEC_FILTERGIF in your kernel configuration:

options   IPSEC_FILTERGIF  #filter ipsec packets from a tunnel

IPsec consists of two sub-protocols:

  • Encapsulated Security Payload (ESP), protects the IP packet data from third party interference, by encrypting the contents using symmetric cryptography algorithms (like Blowfish, 3DES).

  • Authentication Header (AH), protects the IP packet header from third party interference and spoofing, by computing a cryptographic checksum and hashing the IP packet header fields with a secure hashing function. This is then followed by an additional header that contains the hash, to allow the information in the packet to be authenticated.

ESP and AH can either be used together or separately, depending on the environment.

IPsec can either be used to directly encrypt the traffic between two hosts (known as Transport Mode); or to build "virtual tunnels" between two subnets, which could be used for secure communication between two corporate networks (known as Tunnel Mode). The latter is more commonly known as a Virtual Private Network (VPN). The ipsec(4) manual page should be consulted for detailed information on the IPsec subsystem in FreeBSD.

To add IPsec support to your kernel, add the following options to your kernel configuration file:

options   IPSEC        #IP security
options   IPSEC_ESP    #IP security (crypto; define w/ IPSEC)

If IPsec debugging support is desired, the following kernel option should also be added:

options   IPSEC_DEBUG  #debug for IP security

15.10.2. The Problem

There is no standard for what constitutes a VPN. VPNs can be implemented using a number of different technologies, each of which have their own strengths and weaknesses. This section presents a scenario, and the strategies used for implementing a VPN for this scenario.

15.10.3. The Scenario: Two networks, connected to the Internet, to behave as one

The premise is as follows:

  • You have at least two sites

  • Both sites are using IP internally

  • Both sites are connected to the Internet, through a gateway that is running FreeBSD.

  • The gateway on each network has at least one public IP address.

  • The internal addresses of the two networks can be public or private IP addresses, it does not matter. You can be running NAT on the gateway machine if necessary.

  • The internal IP addresses of the two networks do not collide. While I expect it is theoretically possible to use a combination of VPN technology and NAT to get this to work, I expect it to be a configuration nightmare.

If you find that you are trying to connect two networks, both of which, internally, use the same private IP address range (e.g. both of them use 192.168.1.x), then one of the networks will have to be renumbered.

The network topology might look something like this:

ipsec network

Notice the two public IP addresses. I will use the letters to refer to them in the rest of this article. Anywhere you see those letters in this article, replace them with your own public IP addresses. Note also that internally, the two gateway machines have .1 IP addresses, and that the two networks have different private IP addresses (192.168.1.x and 192.168.2.x respectively). All the machines on the private networks have been configured to use the .1 machine as their default gateway.

The intention is that, from a network point of view, each network should view the machines on the other network as though they were directly attached the same router — albeit a slightly slow router with an occasional tendency to drop packets.

This means that (for example), machine 192.168.1.20 should be able to run

ping 192.168.2.34

and have it work, transparently. Windows® machines should be able to see the machines on the other network, browse file shares, and so on, in exactly the same way that they can browse machines on the local network.

And the whole thing has to be secure. This means that traffic between the two networks has to be encrypted.

Creating a VPN between these two networks is a multi-step process. The stages are as follows:

  1. Create a "virtual" network link between the two networks, across the Internet. Test it, using tools like ping(8), to make sure it works.

  2. Apply security policies to ensure that traffic between the two networks is transparently encrypted and decrypted as necessary. Test this, using tools like tcpdump(1), to ensure that traffic is encrypted.

  3. Configure additional software on the FreeBSD gateways, to allow Windows® machines to see one another across the VPN.

Suppose that you were logged in to the gateway machine on network #1 (with public IP address A.B.C.D, private IP address 192.168.1.1), and you ran ping 192.168.2.1, which is the private address of the machine with IP address W.X.Y.Z. What needs to happen in order for this to work?

  1. The gateway machine needs to know how to reach 192.168.2.1. In other words, it needs to have a route to 192.168.2.1.

  2. Private IP addresses, such as those in the 192.168.x range are not supposed to appear on the Internet at large. Instead, each packet you send to 192.168.2.1 will need to be wrapped up inside another packet. This packet will need to appear to be from A.B.C.D, and it will have to be sent to W.X.Y.Z. This process is called encapsulation.

  3. Once this packet arrives at W.X.Y.Z it will need to "unencapsulated", and delivered to 192.168.2.1.

You can think of this as requiring a "tunnel" between the two networks. The two "tunnel mouths" are the IP addresses A.B.C.D and W.X.Y.Z, and the tunnel must be told the addresses of the private IP addresses that will be allowed to pass through it. The tunnel is used to transfer traffic with private IP addresses across the public Internet.

This tunnel is created by using the generic interface, or gif devices on FreeBSD. As you can imagine, the gif interface on each gateway host must be configured with four IP addresses; two for the public IP addresses, and two for the private IP addresses.

Support for the gif device must be compiled in to the FreeBSD kernel on both machines. You can do this by adding the line:

device gif

to the kernel configuration files on both machines, and then compile, install, and reboot as normal.

Configuring the tunnel is a two step process. First the tunnel must be told what the outside (or public) IP addresses are, using ifconfig(8). Then the private IP addresses must be configured using ifconfig(8).

On the gateway machine on network #1 you would run the following commands to configure the tunnel.

# ifconfig gif0 create
# ifconfig gif0 tunnel A.B.C.D W.X.Y.Z
# ifconfig gif0 inet 192.168.1.1 192.168.2.1 netmask 0xffffffff

On the other gateway machine you run the same commands, but with the order of the IP addresses reversed.

# ifconfig gif0 create
# ifconfig gif0 tunnel W.X.Y.Z A.B.C.D
# ifconfig gif0 inet 192.168.2.1 192.168.1.1 netmask 0xffffffff

You can then run:

ifconfig gif0

to see the configuration. For example, on the network #1 gateway, you would see this:

# ifconfig gif0
gif0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1280
        tunnel inet A.B.C.D --> W.X.Y.Z
        inet 192.168.1.1 --> 192.168.2.1 netmask 0xffffffff

As you can see, a tunnel has been created between the physical addresses A.B.C.D and W.X.Y.Z, and the traffic allowed through the tunnel is that between 192.168.1.1 and 192.168.2.1.

This will also have added an entry to the routing table on both machines, which you can examine with the command netstat -rn. This output is from the gateway host on network #1.

# netstat -rn
Routing tables

Internet:
Destination      Gateway       Flags    Refs    Use    Netif  Expire
...
192.168.2.1      192.168.1.1   UH        0        0    gif0
...

As the "Flags" value indicates, this is a host route, which means that each gateway knows how to reach the other gateway, but they do not know how to reach the rest of their respective networks. That problem will be fixed shortly.

It is likely that you are running a firewall on both machines. This will need to be circumvented for your VPN traffic. You might want to allow all traffic between both networks, or you might want to include firewall rules that protect both ends of the VPN from one another.

It greatly simplifies testing if you configure the firewall to allow all traffic through the VPN. You can always tighten things up later. If you are using ipfw(8) on the gateway machines then a command like

ipfw add 1 allow ip from any to any via gif0

will allow all traffic between the two end points of the VPN, without affecting your other firewall rules. Obviously you will need to run this command on both gateway hosts.

This is sufficient to allow each gateway machine to ping the other. On 192.168.1.1, you should be able to run

ping 192.168.2.1

and get a response, and you should be able to do the same thing on the other gateway machine.

However, you will not be able to reach internal machines on either network yet. This is because of the routing — although the gateway machines know how to reach one another, they do not know how to reach the network behind each one.

To solve this problem you must add a static route on each gateway machine. The command to do this on the first gateway would be:

route add 192.168.2.0 192.168.2.1 netmask 0xffffff00

This says "In order to reach the hosts on the network 192.168.2.0, send the packets to the host 192.168.2.1". You will need to run a similar command on the other gateway, but with the 192.168.1.x addresses instead.

IP traffic from hosts on one network will now be able to reach hosts on the other network.

That has now created two thirds of a VPN between the two networks, in as much as it is "virtual" and it is a "network". It is not private yet. You can test this using ping(8) and tcpdump(1). Log in to the gateway host and run

tcpdump dst host 192.168.2.1

In another log in session on the same host run

ping 192.168.2.1

You will see output that looks something like this:

16:10:24.018080 192.168.1.1  192.168.2.1: icmp: echo request
16:10:24.018109 192.168.1.1  192.168.2.1: icmp: echo reply
16:10:25.018814 192.168.1.1  192.168.2.1: icmp: echo request
16:10:25.018847 192.168.1.1  192.168.2.1: icmp: echo reply
16:10:26.028896 192.168.1.1  192.168.2.1: icmp: echo request
16:10:26.029112 192.168.1.1  192.168.2.1: icmp: echo reply

As you can see, the ICMP messages are going back and forth unencrypted. If you had used the -s parameter to tcpdump(1) to grab more bytes of data from the packets you would see more information.

Obviously this is unacceptable. The next section will discuss securing the link between the two networks so that all traffic is automatically encrypted.

Summary:
  • Configure both kernels with "device gif".

  • Edit /etc/rc.conf on gateway host #1 and add the following lines (replacing IP addresses as necessary).

    gif_interfaces="gif0"
    gifconfig_gif0="A.B.C.D W.X.Y.Z"
    ifconfig_gif0="inet 192.168.1.1 192.168.2.1 netmask 0xffffffff"
    static_routes="vpn"
    route_vpn="192.168.2.0 192.168.2.1 netmask 0xffffff00"
  • Edit your firewall script (/etc/rc.firewall, or similar) on both hosts, and add

    ipfw add 1 allow ip from any to any via gif0
  • Make similar changes to /etc/rc.conf on gateway host #2, reversing the order of IP addresses.

To secure the link we will be using IPsec. IPsec provides a mechanism for two hosts to agree on an encryption key, and to then use this key in order to encrypt data between the two hosts.

The are two areas of configuration to be considered here.

  1. There must be a mechanism for two hosts to agree on the encryption mechanism to use. Once two hosts have agreed on this mechanism there is said to be a "security association" between them.

  2. There must be a mechanism for specifying which traffic should be encrypted. Obviously, you do not want to encrypt all your outgoing traffic — you only want to encrypt the traffic that is part of the VPN. The rules that you put in place to determine what traffic will be encrypted are called "security policies".

Security associations and security policies are both maintained by the kernel, and can be modified by userland programs. However, before you can do this you must configure the kernel to support IPsec and the Encapsulated Security Payload (ESP) protocol. This is done by configuring a kernel with:

options IPSEC
options IPSEC_ESP

and recompiling, reinstalling, and rebooting. As before you will need to do this to the kernels on both of the gateway hosts.

You have two choices when it comes to setting up security associations. You can configure them by hand between two hosts, which entails choosing the encryption algorithm, encryption keys, and so forth, or you can use daemons that implement the Internet Key Exchange protocol (IKE) to do this for you.

I recommend the latter. Apart from anything else, it is easier to set up.

Editing and displaying security policies is carried out using setkey(8). By analogy, setkey is to the kernel’s security policy tables as route(8) is to the kernel’s routing tables. setkey can also display the current security associations, and to continue the analogy further, is akin to netstat -r in that respect.

There are a number of choices for daemons to manage security associations with FreeBSD. This article will describe how to use one of these, racoon - which is available from security/ipsec-tools in the FreeBSD Ports collection.

The racoon software must be run on both gateway hosts. On each host it is configured with the IP address of the other end of the VPN, and a secret key (which you choose, and must be the same on both gateways).

The two daemons then contact one another, confirm that they are who they say they are (by using the secret key that you configured). The daemons then generate a new secret key, and use this to encrypt the traffic over the VPN. They periodically change this secret, so that even if an attacker were to crack one of the keys (which is as theoretically close to unfeasible as it gets) it will not do them much good — by the time they have cracked the key the two daemons have chosen another one.

The configuration file for racoon is stored in ${PREFIX}/etc/racoon. You should find a configuration file there, which should not need to be changed too much. The other component of racoon’s configuration, which you will need to change, is the "pre-shared key".

The default racoon configuration expects to find this in the file ${PREFIX}/etc/racoon/psk.txt. It is important to note that the pre-shared key is not the key that will be used to encrypt your traffic across the VPN link, it is simply a token that allows the key management daemons to trust one another.

psk.txt contains a line for each remote site you are dealing with. In this example, where there are two sites, each psk.txt file will contain one line (because each end of the VPN is only dealing with one other end).

On gateway host #1 this line should look like this:

W.X.Y.Z            secret

That is, the public IP address of the remote end, whitespace, and a text string that provides the secret. Obviously, you should not use "secret" as your key — the normal rules for choosing a password apply.

On gateway host #2 the line would look like this

A.B.C.D            secret

That is, the public IP address of the remote end, and the same secret key. psk.txt must be mode 0600 (i.e., only read/write to root) before racoon will run.

You must run racoon on both gateway machines. You will also need to add some firewall rules to allow the IKE traffic, which is carried over UDP to the ISAKMP (Internet Security Association Key Management Protocol) port. Again, this should be fairly early in your firewall ruleset.

ipfw add 1 allow udp from A.B.C.D to W.X.Y.Z isakmp
ipfw add 1 allow udp from W.X.Y.Z to A.B.C.D isakmp

Once racoon is running you can try pinging one gateway host from the other. The connection is still not encrypted, but racoon will then set up the security associations between the two hosts — this might take a moment, and you may see this as a short delay before the ping commands start responding.

Once the security association has been set up you can view it using setkey(8). Run

setkey -D

on either host to view the security association information.

That’s one half of the problem. The other half is setting your security policies.

To create a sensible security policy, let’s review what’s been set up so far. This discussions hold for both ends of the link.

Each IP packet that you send out has a header that contains data about the packet. The header includes the IP addresses of both the source and destination. As we already know, private IP addresses, such as the 192.168.x.y range are not supposed to appear on the public Internet. Instead, they must first be encapsulated inside another packet. This packet must have the public source and destination IP addresses substituted for the private addresses.

So if your outgoing packet started looking like this:

ipsec out pkt

Then it will be encapsulated inside another packet, looking something like this:

ipsec encap pkt

This encapsulation is carried out by the gif device. As you can see, the packet now has real IP addresses on the outside, and our original packet has been wrapped up as data inside the packet that will be put out on the Internet.

Obviously, we want all traffic between the VPNs to be encrypted. You might try putting this in to words, as:

"If a packet leaves from A.B.C.D, and it is destined for W.X.Y.Z, then encrypt it, using the necessary security associations."

"If a packet arrives from W.X.Y.Z, and it is destined for A.B.C.D, then decrypt it, using the necessary security associations."

That’s close, but not quite right. If you did this, all traffic to and from W.X.Y.Z, even traffic that was not part of the VPN, would be encrypted. That’s not quite what you want. The correct policy is as follows

"If a packet leaves from A.B.C.D, and that packet is encapsulating another packet, and it is destined for W.X.Y.Z, then encrypt it, using the necessary security associations."

"If a packet arrives from W.X.Y.Z, and that packet is encapsulating another packet, and it is destined for A.B.C.D, then decrypt it, using the necessary security associations."

A subtle change, but a necessary one.

Security policies are also set using setkey(8). setkey(8) features a configuration language for defining the policy. You can either enter configuration instructions via stdin, or you can use the -f option to specify a filename that contains configuration instructions.

The configuration on gateway host #1 (which has the public IP address A.B.C.D) to force all outbound traffic to W.X.Y.Z to be encrypted is:

spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P out ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require;

Put these commands in a file (e.g. /etc/ipsec.conf) and then run

# setkey -f /etc/ipsec.conf

spdadd tells setkey(8) that we want to add a rule to the secure policy database. The rest of this line specifies which packets will match this policy. A.B.C.D/32 and W.X.Y.Z/32 are the IP addresses and netmasks that identify the network or hosts that this policy will apply to. In this case, we want it to apply to traffic between these two hosts. ipencap tells the kernel that this policy should only apply to packets that encapsulate other packets. -P out says that this policy applies to outgoing packets, and ipsec says that the packet will be secured.

The second line specifies how this packet will be encrypted. esp is the protocol that will be used, while tunnel indicates that the packet will be further encapsulated in an IPsec packet. The repeated use of A.B.C.D and W.X.Y.Z is used to select the security association to use, and the final require mandates that packets must be encrypted if they match this rule.

This rule only matches outgoing packets. You will need a similar rule to match incoming packets.

spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P in ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require;

Note the in instead of out in this case, and the necessary reversal of the IP addresses.

The other gateway host (which has the public IP address W.X.Y.Z) will need similar rules.

spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P out ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require;
spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P in ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require;

Finally, you need to add firewall rules to allow ESP and IPENCAP packets back and forth. These rules will need to be added to both hosts.

ipfw add 1 allow esp from A.B.C.D to W.X.Y.Z
ipfw add 1 allow esp from W.X.Y.Z to A.B.C.D
ipfw add 1 allow ipencap from A.B.C.D to W.X.Y.Z
ipfw add 1 allow ipencap from W.X.Y.Z to A.B.C.D

Because the rules are symmetric you can use the same rules on each gateway host.

Outgoing packets will now look something like this:

ipsec crypt pkt

When they are received by the far end of the VPN they will first be decrypted (using the security associations that have been negotiated by racoon). Then they will enter the gif interface, which will unwrap the second layer, until you are left with the innermost packet, which can then travel in to the inner network.

You can check the security using the same ping(8) test from earlier. First, log in to the A.B.C.D gateway machine, and run:

tcpdump dst host 192.168.2.1

In another log in session on the same host run

ping 192.168.2.1

This time you should see output like the following:

XXX tcpdump output

Now, as you can see, tcpdump(1) shows the ESP packets. If you try to examine them with the -s option you will see (apparently) gibberish, because of the encryption.

Congratulations. You have just set up a VPN between two remote sites.

Summary
  • Configure both kernels with:

    options IPSEC
    options IPSEC_ESP
  • Install security/ipsec-tools. Edit ${PREFIX}/etc/racoon/psk.txt on both gateway hosts, adding an entry for the remote host’s IP address and a secret key that they both know. Make sure this file is mode 0600.

  • Add the following lines to /etc/rc.conf on each host:

    ipsec_enable="YES"
    ipsec_file="/etc/ipsec.conf"
  • Create an /etc/ipsec.conf on each host that contains the necessary spdadd lines. On gateway host #1 this would be:

    spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P out ipsec
      esp/tunnel/A.B.C.D-W.X.Y.Z/require;
    spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P in ipsec
      esp/tunnel/W.X.Y.Z-A.B.C.D/require;

    On gateway host #2 this would be:

    spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P out ipsec
      esp/tunnel/W.X.Y.Z-A.B.C.D/require;
    spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P in ipsec
      esp/tunnel/A.B.C.D-W.X.Y.Z/require;
  • Add firewall rules to allow IKE, ESP, and IPENCAP traffic to both hosts:

    ipfw add 1 allow udp from A.B.C.D to W.X.Y.Z isakmp
    ipfw add 1 allow udp from W.X.Y.Z to A.B.C.D isakmp
    ipfw add 1 allow esp from A.B.C.D to W.X.Y.Z
    ipfw add 1 allow esp from W.X.Y.Z to A.B.C.D
    ipfw add 1 allow ipencap from A.B.C.D to W.X.Y.Z
    ipfw add 1 allow ipencap from W.X.Y.Z to A.B.C.D

The previous two steps should suffice to get the VPN up and running. Machines on each network will be able to refer to one another using IP addresses, and all traffic across the link will be automatically and securely encrypted.

15.11. OpenSSH

OpenSSH is a set of network connectivity tools used to access remote machines securely. It can be used as a direct replacement for rlogin, rsh, rcp, and telnet. Additionally, TCP/IP connections can be tunneled/forwarded securely through SSH. OpenSSH encrypts all traffic to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks.

OpenSSH is maintained by the OpenBSD project, and is based upon SSH v1.2.12 with all the recent bug fixes and updates. It is compatible with both SSH protocols 1 and 2.

15.11.1. Advantages of Using OpenSSH

Normally, when using telnet(1) or rlogin(1), data is sent over the network in an clear, un-encrypted form. Network sniffers anywhere in between the client and server can steal your user/password information or data transferred in your session. OpenSSH offers a variety of authentication and encryption methods to prevent this from happening.

15.11.2. Enabling sshd

The sshd is an option presented during a Standard install of FreeBSD. To see if sshd is enabled, check the rc.conf file for:

sshd_enable="YES"

This will load sshd(8), the daemon program for OpenSSH, the next time your system initializes. Alternatively, it is possible to use /etc/rc.d/sshd rc(8) script to start OpenSSH:

/etc/rc.d/sshd start

15.11.3. SSH Client

The ssh(1) utility works similarly to rlogin(1).

# ssh user@example.com
Host key not found from the list of known hosts.
Are you sure you want to continue connecting (yes/no)? yes
Host 'example.com' added to the list of known hosts.
user@example.com's password: *******

The login will continue just as it would have if a session was created using rlogin or telnet. SSH utilizes a key fingerprint system for verifying the authenticity of the server when the client connects. The user is prompted to enter yes only when connecting for the first time. Future attempts to login are all verified against the saved fingerprint key. The SSH client will alert you if the saved fingerprint differs from the received fingerprint on future login attempts. The fingerprints are saved in ~/.ssh/known_hosts, or ~/.ssh/known_hosts2 for SSH v2 fingerprints.

By default, recent versions of the OpenSSH servers only accept SSH v2 connections. The client will use version 2 if possible and will fall back to version 1. The client can also be forced to use one or the other by passing it the -1 or -2 for version 1 or version 2, respectively. The version 1 compatibility is maintained in the client for backwards compatibility with older versions.

15.11.4. Secure Copy

The scp(1) command works similarly to rcp(1); it copies a file to or from a remote machine, except in a secure fashion.

#  scp user@example.com:/COPYRIGHT COPYRIGHT
user@example.com's password: *******
COPYRIGHT            100% |*****************************|  4735
00:00
#

Since the fingerprint was already saved for this host in the previous example, it is verified when using scp(1) here.

The arguments passed to scp(1) are similar to cp(1), with the file or files in the first argument, and the destination in the second. Since the file is fetched over the network, through SSH, one or more of the file arguments takes on the form user@host:path_to_remote_file.

15.11.5. Configuration

The system-wide configuration files for both the OpenSSH daemon and client reside within the /etc/ssh directory.

ssh_config configures the client settings, while sshd_config configures the daemon.

Additionally, the sshd_program (/usr/sbin/sshd by default), and sshd_flagsrc.conf options can provide more levels of configuration.

15.11.6. ssh-keygen

Instead of using passwords, ssh-keygen(1) can be used to generate DSA or RSA keys to authenticate a user:

% ssh-keygen -t dsa
Generating public/private dsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_dsa):
Created directory '/home/user/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user/.ssh/id_dsa.
Your public key has been saved in /home/user/.ssh/id_dsa.pub.
The key fingerprint is:
bb:48:db:f2:93:57:80:b6:aa:bc:f5:d5:ba:8f:79:17 user@host.example.com

ssh-keygen(1) will create a public and private key pair for use in authentication. The private key is stored in ~/.ssh/id_dsa or ~/.ssh/id_rsa, whereas the public key is stored in ~/.ssh/id_dsa.pub or ~/.ssh/id_rsa.pub, respectively for DSA and RSA key types. The public key must be placed in ~/.ssh/authorized_keys of the remote machine in order for the setup to work. Similarly, RSA version 1 public keys should be placed in ~/.ssh/authorized_keys.

This will allow connection to the remote machine based upon SSH keys instead of passwords.

If a passphrase is used in ssh-keygen(1), the user will be prompted for a password each time in order to use the private key. ssh-agent(1) can alleviate the strain of repeatedly entering long passphrases, and is explored in the ssh-agent and ssh-add section below.

The various options and files can be different according to the OpenSSH version you have on your system; to avoid problems you should consult the ssh-keygen(1) manual page.

15.11.7. ssh-agent and ssh-add

The ssh-agent(1) and ssh-add(1) utilities provide methods for SSH keys to be loaded into memory for use, without needing to type the passphrase each time.

The ssh-agent(1) utility will handle the authentication using the private key(s) that are loaded into it. ssh-agent(1) should be used to launch another application. At the most basic level, it could spawn a shell or at a more advanced level, a window manager.

To use ssh-agent(1) in a shell, first it will need to be spawned with a shell as an argument. Secondly, the identity needs to be added by running ssh-add(1) and providing it the passphrase for the private key. Once these steps have been completed the user will be able to ssh(1) to any host that has the corresponding public key installed. For example:

% ssh-agent csh
% ssh-add
Enter passphrase for /home/user/.ssh/id_dsa:
Identity added: /home/user/.ssh/id_dsa (/home/user/.ssh/id_dsa)
%

To use ssh-agent(1) in X11, a call to ssh-agent(1) will need to be placed in ~/.xinitrc. This will provide the ssh-agent(1) services to all programs launched in X11. An example ~/.xinitrc file might look like this:

exec ssh-agent startxfce4

This would launch ssh-agent(1), which would in turn launch XFCE, every time X11 starts. Then once that is done and X11 has been restarted so that the changes can take effect, simply run ssh-add(1) to load all of your SSH keys.

15.11.8. SSH Tunneling

OpenSSH has the ability to create a tunnel to encapsulate another protocol in an encrypted session.

The following command tells ssh(1) to create a tunnel for telnet:

% ssh -2 -N -f -L 5023:localhost:23 user@foo.example.com
%

The ssh command is used with the following options:

-2

Forces ssh to use version 2 of the protocol. (Do not use if you are working with older SSH servers)

-N

Indicates no command, or tunnel only. If omitted, ssh would initiate a normal session.

-f

Forces ssh to run in the background.

-L

Indicates a local tunnel in localport:remotehost:remoteport fashion.

user@foo.example.com

The remote SSH server.

An SSH tunnel works by creating a listen socket on localhost on the specified port. It then forwards any connection received on the local host/port via the SSH connection to the specified remote host and port.

In the example, port 5023 on localhost is being forwarded to port 23 on localhost of the remote machine. Since 23 is telnet, this would create a secure telnet session through an SSH tunnel.

This can be used to wrap any number of insecure TCP protocols such as SMTP, POP3, FTP, etc.

Παράδειγμα 1. Using SSH to Create a Secure Tunnel for SMTP
% ssh -2 -N -f -L 5025:localhost:25 user@mailserver.example.com
user@mailserver.example.com's password: *****
% telnet localhost 5025
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mailserver.example.com ESMTP

This can be used in conjunction with an ssh-keygen(1) and additional user accounts to create a more seamless/hassle-free SSH tunneling environment. Keys can be used in place of typing a password, and the tunnels can be run as a separate user.

15.11.8.1. Practical SSH Tunneling Examples

15.11.8.1.1. Secure Access of a POP3 Server

At work, there is an SSH server that accepts connections from the outside. On the same office network resides a mail server running a POP3 server. The network, or network path between your home and office may or may not be completely trustable. Because of this, you need to check your e-mail in a secure manner. The solution is to create an SSH connection to your office’s SSH server, and tunnel through to the mail server.

% ssh -2 -N -f -L 2110:mail.example.com:110 user@ssh-server.example.com
user@ssh-server.example.com's password: ******

When the tunnel is up and running, you can point your mail client to send POP3 requests to localhost port 2110. A connection here will be forwarded securely across the tunnel to mail.example.com.

15.11.8.1.2. Bypassing a Draconian Firewall

Some network administrators impose extremely draconian firewall rules, filtering not only incoming connections, but outgoing connections. You may be only given access to contact remote machines on ports 22 and 80 for SSH and web surfing.

You may wish to access another (perhaps non-work related) service, such as an Ogg Vorbis server to stream music. If this Ogg Vorbis server is streaming on some other port than 22 or 80, you will not be able to access it.

The solution is to create an SSH connection to a machine outside of your network’s firewall, and use it to tunnel to the Ogg Vorbis server.

% ssh -2 -N -f -L 8888:music.example.com:8000 user@unfirewalled-system.example.org
user@unfirewalled-system.example.org's password: *******

Your streaming client can now be pointed to localhost port 8888, which will be forwarded over to music.example.com port 8000, successfully evading the firewall.

15.11.9. The AllowUsers Users Option

It is often a good idea to limit which users can log in and from where. The AllowUsers option is a good way to accomplish this. For example, to only allow the root user to log in from 192.168.1.32, something like this would be appropriate in the /etc/ssh/sshd_config file:

AllowUsers root@192.168.1.32

To allow the user admin to log in from anywhere, just list the username by itself:

AllowUsers admin

Multiple users should be listed on the same line, like so:

AllowUsers root@192.168.1.32 admin

It is important that you list each user that needs to log in to this machine; otherwise they will be locked out.

After making changes to /etc/ssh/sshd_config you must tell sshd(8) to reload its config files, by running:

# /etc/rc.d/sshd reload

15.12. File System Access Control Lists

In conjunction with file system enhancements like snapshots, FreeBSD 5.0 and later offers the security of File System Access Control Lists (ACLs).

Access Control Lists extend the standard UNIX® permission model in a highly compatible (POSIX®.1e) way. This feature permits an administrator to make use of and take advantage of a more sophisticated security model.

To enable ACL support for UFS file systems, the following:

options UFS_ACL

must be compiled into the kernel. If this option has not been compiled in, a warning message will be displayed when attempting to mount a file system supporting ACLs. This option is included in the GENERIC kernel. ACLs rely on extended attributes being enabled on the file system. Extended attributes are natively supported in the next generation UNIX® file system, UFS2.

A higher level of administrative overhead is required to configure extended attributes on UFS1 than on UFS2. The performance of extended attributes on UFS2 is also substantially higher. As a result, UFS2 is generally recommended in preference to UFS1 for use with access control lists.

ACLs are enabled by the mount-time administrative flag, acls, which may be added to /etc/fstab. The mount-time flag can also be automatically set in a persistent manner using tunefs(8) to modify a superblock ACLs flag in the file system header. In general, it is preferred to use the superblock flag for several reasons:

  • The mount-time ACLs flag cannot be changed by a remount (mount(8) -u), only by means of a complete umount(8) and fresh mount(8). This means that ACLs cannot be enabled on the root file system after boot. It also means that you cannot change the disposition of a file system once it is in use.

  • Setting the superblock flag will cause the file system to always be mounted with ACLs enabled even if there is not an fstab entry or if the devices re-order. This prevents accidental mounting of the file system without ACLs enabled, which can result in ACLs being improperly enforced, and hence security problems.

We may change the ACLs behavior to allow the flag to be enabled without a complete fresh mount(8), but we consider it desirable to discourage accidental mounting without ACLs enabled, because you can shoot your feet quite nastily if you enable ACLs, then disable them, then re-enable them without flushing the extended attributes. In general, once you have enabled ACLs on a file system, they should not be disabled, as the resulting file protections may not be compatible with those intended by the users of the system, and re-enabling ACLs may re-attach the previous ACLs to files that have since had their permissions changed, resulting in other unpredictable behavior.

File systems with ACLs enabled will show a + (plus) sign in their permission settings when viewed. For example:

drwx------  2 robert  robert  512 Dec 27 11:54 private
drwxrwx---+ 2 robert  robert  512 Dec 23 10:57 directory1
drwxrwx---+ 2 robert  robert  512 Dec 22 10:20 directory2
drwxrwx---+ 2 robert  robert  512 Dec 27 11:57 directory3
drwxr-xr-x  2 robert  robert  512 Nov 10 11:54 public_html

Here we see that the directory1, directory2, and directory3 directories are all taking advantage of ACLs. The public_html directory is not.

15.12.1. Making Use of ACLs

The file system ACLs can be viewed by the getfacl(1) utility. For instance, to view the ACL settings on the test file, one would use the command:

% getfacl test
	#file:test
	#owner:1001
	#group:1001
	user::rw-
	group::r--
	other::r--

To change the ACL settings on this file, invoke the setfacl(1) utility. Observe:

% setfacl -k test

The -k flag will remove all of the currently defined ACLs from a file or file system. The more preferable method would be to use -b as it leaves the basic fields required for ACLs to work.

% setfacl -m u:trhodes:rwx,group:web:r--,o::--- test

In the aforementioned command, the -m option was used to modify the default ACL entries. Since there were no pre-defined entries, as they were removed by the previous command, this will restore the default options and assign the options listed. Take care to notice that if you add a user or group which does not exist on the system, an Invalid argument error will be printed to stdout.

15.13. Monitoring Third Party Security Issues

In recent years, the security world has made many improvements to how vulnerability assessment is handled. The threat of system intrusion increases as third party utilities are installed and configured for virtually any operating system available today.

Vulnerability assessment is a key factor in security, and while FreeBSD releases advisories for the base system, doing so for every third party utility is beyond the FreeBSD Project’s capability. There is a way to mitigate third party vulnerabilities and warn administrators of known security issues. A FreeBSD add on utility known as Portaudit exists solely for this purpose.

The ports-mgmt/portaudit port polls a database, updated and maintained by the FreeBSD Security Team and ports developers, for known security issues.

To begin using Portaudit, one must install it from the Ports Collection:

# cd /usr/ports/ports-mgmt/portaudit  make install clean

During the install process, the configuration files for periodic(8) will be updated, permitting Portaudit output in the daily security runs. Ensure the daily security run emails, which are sent to root's email account, are being read. No more configuration will be required here.

After installation, an administrator can update the database and view known vulnerabilities in installed packages by invoking the following command:

# portaudit -Fda

The database will automatically be updated during the periodic(8) run; thus, the previous command is completely optional. It is only required for the following examples.

To audit the third party utilities installed as part of the Ports Collection at anytime, an administrator need only run the following command:

# portaudit -a

Portaudit will produce something like this for vulnerable packages:

Affected package: cups-base-1.1.22.0_1
Type of problem: cups-base -- HPGL buffer overflow vulnerability.
Reference: http://www.FreeBSD.org/ports/portaudit/40a3bca2-6809-11d9-a9e7-0001020eed82.html

1 problem(s) in your installed packages found.

You are advised to update or deinstall the affected package(s) immediately.

By pointing a web browser to the URL shown, an administrator may obtain more information about the vulnerability in question. This will include versions affected, by FreeBSD Port version, along with other web sites which may contain security advisories.

In short, Portaudit is a powerful utility and extremely useful when coupled with the Portupgrade port.

15.14. FreeBSD Security Advisories

Like many production quality operating systems, FreeBSD publishes "Security Advisories". These advisories are usually mailed to the security lists and noted in the Errata only after the appropriate releases have been patched. This section will work to explain what an advisory is, how to understand it, and what measures to take in order to patch a system.

15.14.1. What does an advisory look like?

The FreeBSD security advisories look similar to the one below, taken from the ηλεκτρονική λίστα Ανακοινώσεων για Θέματα Ασφάλειας του FreeBSD mailing list.

=============================================================================
FreeBSD-SA-XX:XX.UTIL                                     Security Advisory
                                                          The FreeBSD Project

Topic:          denial of service due to some problem (1)

Category:       core (2)
Module:         sys (3)
Announced:      2003-09-23 (4)
Credits:        Person@EMAIL-ADDRESS (5)
Affects:        All releases of FreeBSD (6)
                FreeBSD 4-STABLE prior to the correction date
Corrected:      2003-09-23 16:42:59 UTC (RELENG_4, 4.9-PRERELEASE)
                2003-09-23 20:08:42 UTC (RELENG_5_1, 5.1-RELEASE-p6)
                2003-09-23 20:07:06 UTC (RELENG_5_0, 5.0-RELEASE-p15)
                2003-09-23 16:44:58 UTC (RELENG_4_8, 4.8-RELEASE-p8)
                2003-09-23 16:47:34 UTC (RELENG_4_7, 4.7-RELEASE-p18)
                2003-09-23 16:49:46 UTC (RELENG_4_6, 4.6-RELEASE-p21)
                2003-09-23 16:51:24 UTC (RELENG_4_5, 4.5-RELEASE-p33)
                2003-09-23 16:52:45 UTC (RELENG_4_4, 4.4-RELEASE-p43)
                2003-09-23 16:54:39 UTC (RELENG_4_3, 4.3-RELEASE-p39) (7)
CVE Name:	CVE-XXXX-XXXX (8)

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit
http://www.FreeBSD.org/security/.

I.   Background (9)

II.  Problem Description (10)

III. Impact (11)

IV.  Workaround (12)

V.   Solution (13)

VI.  Correction details (14)

VII. References (15)
1The Topic field indicates exactly what the problem is. It is basically an introduction to the current security advisory and notes the utility with the vulnerability.
2The Category refers to the affected part of the system which may be one of core, contrib, or ports. The core category means that the vulnerability affects a core component of the FreeBSD operating system. The contrib category means that the vulnerability affects software contributed to the FreeBSD Project, such as sendmail. Finally the ports category indicates that the vulnerability affects add on software available as part of the Ports Collection.
3The Module field refers to the component location, for instance sys. In this example, we see that the module, sys, is affected; therefore, this vulnerability affects a component used within the kernel.
4The Announced field reflects the date said security advisory was published, or announced to the world. This means that the security team has verified that the problem does exist and that a patch has been committed to the FreeBSD source code repository.
5The Credits field gives credit to the individual or organization who noticed the vulnerability and reported it.
6The Affects field explains which releases of FreeBSD are affected by this vulnerability. For the kernel, a quick look over the output from ident on the affected files will help in determining the revision. For ports, the version number is listed after the port name in /var/db/pkg. If the system does not sync with the FreeBSD CVS repository and rebuild daily, chances are that it is affected.
7The Corrected field indicates the date, time, time offset, and release that was corrected. Reserved for the identification information used to look up vulnerabilities in the Common Vulnerabilities Database system.
8The Background field gives information on exactly what the affected utility is. Most of the time this is why the utility exists in FreeBSD, what it is used for, and a bit of information on how the utility came to be.
9The Problem Description field explains the security hole in depth. This can include information on flawed code, or even how the utility could be maliciously used to open a security hole.
10The Impact field describes what type of impact the problem could have on a system. For example, this could be anything from a denial of service attack, to extra privileges available to users, or even giving the attacker superuser access.
11The Workaround field offers a feasible workaround to system administrators who may be incapable of upgrading the system. This may be due to time constraints, network availability, or a slew of other reasons. Regardless, security should not be taken lightly, and an affected system should either be patched or the security hole workaround should be implemented.
12The Solution field offers instructions on patching the affected system. This is a step by step tested and verified method for getting a system patched and working securely.
13The Correction Details field displays the CVS branch or release name with the periods changed to underscore characters. It also shows the revision number of the affected files within each branch.
14The References field usually offers sources of other information. This can include web URLs, books, mailing lists, and newsgroups.

15.15. Process Accounting

Process accounting is a security method in which an administrator may keep track of system resources used, their allocation among users, provide for system monitoring, and minimally track a user’s commands.

This indeed has its own positive and negative points. One of the positives is that an intrusion may be narrowed down to the point of entry. A negative is the amount of logs generated by process accounting, and the disk space they may require. This section will walk an administrator through the basics of process accounting.

15.15.1. Enable and Utilizing Process Accounting

Before making use of process accounting, it must be enabled. To do this, execute the following commands:

# touch /var/account/acct

# accton /var/account/acct

# echo 'accounting_enable="YES"' >> /etc/rc.conf

Once enabled, accounting will begin to track CPU stats, commands, etc. All accounting logs are in a non-human readable format and may be viewed using the sa(8) utility. If issued without any options, sa will print information relating to the number of per user calls, the total elapsed time in minutes, total CPU and user time in minutes, average number of I/O operations, etc.

To view information about commands being issued, one would use the lastcomm(1) utility. The lastcomm may be used to print out commands issued by users on specific ttys(5), for example:

# lastcomm ls
	trhodes ttyp1

Would print out all known usage of the ls by trhodes on the ttyp1 terminal.

Many other useful options exist and are explained in the lastcomm(1), acct(5) and sa(8) manual pages.


Last modified on: 4 Σεπτεμβρίου 2022 by Gordon Bergling