Date: Fri, 10 Feb 2006 09:12:15 -0500 From: "fbsd_user" <fbsd_user@a1poweruser.com> To: "Ken Stevenson" <ken@allenmyland.com> Cc: "freebsd-questions@FreeBSD. ORG" <freebsd-questions@FreeBSD.ORG> Subject: RE: sshd / ssh setup Message-ID: <MIEPLLIBMLEEABPDBIEGOEAAHNAA.fbsd_user@a1poweruser.com> In-Reply-To: <43EA92E2.8000301@allenmyland.com>
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For the archives.
Example of configuring OpenSSH
Environment description:
In this example we have a FreeBSD system which we will call the
host.
We have an Remote FreeBSD system which is located some where on the
public internet, we will call this the FBSD-client.
We also have an Remote MS/windows system which is located some
where
on the public internet, we will call this the Win-client.
OpenSSH has a few different security levels when it comes to how
the ssh login is handled. This example details the encrypted
host/client key with passphrase method.
This method gives the maximum protection possible utilizing ssh.
Host setup steps.
1. Edit /etc/rc.conf and add this statement
sshd_enable=”YES”
Make sure your firewall allows port 22 in from the public internet.
Reboot your system to activate sshd and login as root.
If this is your first time booting with sshd you will have to
create the host keys.
sshd will show you this on the first sshd boot only.
====================================================
Type a full screen full of random junk to unblock
it and remember to finish with <enter>. This will
timeout in 300 seconds, but waiting for
the timeout without typing junk may make the
entropy source deliver predictable output.
Just hit <enter> for fast+insecure startup.
====================================================
kern.random.sys.seeded: 1 -> 0
qkcir83,2jsn40pl722jjbqok ---- this is the example junk entered
Generating public/private rsa1 key pair.
Your identification has been saved in /etc/ssh/ssh_host_key.
Your public key has been saved in /etc/ssh/ssh_host_key.pub.
The key fingerprint is:
ed:5d:97:dc:49:98:36:66:fc root@domainname
Generating public/private dsa key pair.
Your identification has been saved in /etc/ssh/ssh_host_dsa_key.
Your public key has been saved in /etc/ssh/ssh_host_dsa_key.pub.
The key fingerprint is:
67:e7:90:04:0e:27:2e:d2:97:6a root@a1poweruser.com
Generating public/private rsa key pair.
Your identification has been saved in /etc/ssh/ssh_host_rsa_key.
Your public key has been saved in /etc/ssh/ssh_host_rsa_key.pub.
The key fingerprint is:
96:db:50:5c:9e:69:88:26:28:54 root@ domainname
2. If you do a “ps ax” command you will see sshd as
one of the running tasks.
3. Using adduser or pw command create a normal user account.
For this example we will use bob as the host user account name.
4. Hit alt/f2 at same time to open second session and login using
bob.
5. Run this command ssh-keygen -t rsa
Just hit enter to take default location and file name
No need to enter a pass phrase for the host user here,
just hit enter 2 times
This is what you will see
Generating public/private rsa key pair.
Enter file in which to save the key (/bob/.ssh/id_rsa):
Created directory '/bob/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /bob/.ssh/id_rsa.
Your public key has been saved in /bob/.ssh/id_rsa.pub.
The key fingerprint is:
e7:e6:8f:d3:b1:b4:08:27:09:d2 bob@domainname
6. If you want to ssh login as Host ‘root’, you have to run
step 5 above while logged in as root on the host. Also
edit /etc/ssh/sshd_config and change this statement
#PermitRootLogin no to PermitRootLogin yes
Then killall –HUP sshd
to make sshd task reread it’s sshd_config file.
FBSD-client setup steps.
1. Using adduser or pw command create a normal user account.
For this example we will use remotetom as the user account
name.
2. Login using remotetom.
3. Run this command ssh-keygen -t rsa
Just hit enter to take default location and file name
At the “Enter a passphrase prompt” [enter one and write it down,
because it will be needed for ssh login to the host].
This is what you will see
Generating public/private rsa key pair.
Enter file in which to save the key (/remotetom/.ssh/id_rsa):
Created directory '/remotetom/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /remotetom/.ssh/id_rsa.
Your public key has been saved in /remotetom/.ssh/id_rsa.pub.
The key fingerprint is:
e7:e6:8f:d3:b1:b4:08:27:09:56:de:d2 tom@domainname
4. The Public key file you just created /remotetom/.ssh/id_rsa.pub
has to be sent to the Host system. On the host system rename
it
to authorized_keys2 and put it into the home directory of the
user setup earlier. In this case ~/bob/.ssh/authorized_keys2.
5. To ssh to the host enter this ssh hostname or ssh
host-ip-address
The first time you ssh to the Host you will get these messages.
Answer yes if you are sure this first connection is with your
targeted host.
Enter exit to terminate session.
# /home/remotetom/.ssh >ssh 10.0.10.2
The authenticity of host '10.0.10.2 (10.0.10.2)' can't be
established.
DSA key fingerprint is
67:e7:90:04:0e:27:2e:d2:97:9d:7b:62:7e:c9:32:6a.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '10.0.10.2' (DSA) to the list of known
hosts.
Enter passphrase for key /remotetom/.ssh/id_rsa
This is where you enter the pass phrase you entered while running
ssh-keygen on this remote client. (remotetom)
You are now logged into the Host account bob.
Every time you ssh login again you will just get this prompt
Enter passphrase for key /remotetom/.ssh/id_rsa
7. At this point the Host user bob can login from Host console,
or using telnet from a local LAN machine, or from the public
internet
if your host firewall has port 23 open. If this user is to be
only
allowed remote ssh client access you can use vipw command to
edit the
system’s master password file and star out his encrypted
password.
This will deny login from anywhere except remotebob
bob:$1$FXF.CEpf$B2cF:1001:31::0:0:remote ssh
user#:/home/bob:/bin/csh
edit it to look like this
bob:*:1001:31::0:0:remote ssh user#:/home/bob:/bin/csh
8. At this point, if remotebob is a notebook pc he can ssh login
from any place in the world. If remotebob is always working
from
home using ip address 192.168.1.32 you can add another level
of
security by restricting his login to come from that ip
address.
Do that by adding this statement to the Host
/etc/ssh/sshd_config file
AllowUsers bob@192.168.1.32
Win-client setup steps.
1. You have to get windows putty program from
http://www.chiark.greenend.org.uk/~sgtatham/putty/
Download the current windows installer program.
Currently that’s putty-0.58-installer.exe
2. Run the installer.exe and take all the defaults.
3. start/programs/Putty/Puttygen
This will create your private and public ssh keys
At the bottom of the window select ssh2-rsa
Click on generate button.
After keys are created then enter your pass phrase.
Then click on ‘save public key’ button.
Save it to c:/programs/putty/ putty_public_rsa_key
Then click on ‘save private key’ button.
Save it to c:/programs/putty/ putty_private_rsa_key.ppk
In the window area labeled
“Public key for pasting into Open SSH authorized_key file”
Highlight that content and past it into a notepad text file
named win_authorized_keys2.txt
4. The win_authorized_keys2.txt file has to be sent to the Host
system and put in the ~/home/bob/.ssh/ directory. In this
example
both the FBSD-client & the WIN-client will both use the same
Host
user account to ssh login to.
Since the ~/home/bob/.ssh/authorized_keys2 already contains the
private key for the FBSD-client you have to append the
win_authorized_keys2.txt content to it.
cat win_authorized_keys2.txt >> cat win_authorized_keys2.txt
If bob is to only be used by WIN-client then just
cp win_authorized_keys2.txt authorized_keys2
5. start/programs/putty/putty to run it
On the bottom of the window click on “never exit”
Under protocol be sure ssh is clicked
On the left side click on ‘SSH’, then in the
‘preferred ssh protocol version’
click on ‘2 only’.
On the left side click on ‘AUTH’, remove check mark from
‘Attempt keyboard-interactive auth (SSH-2) .
On the left side click on ‘Sessions’ to return to main
login window.
Enter you host name (IE; your official registered domain
name of the Host box) or it’s IP address. Then type a name
in the saved session window and click on save button.
Now click on the open button at bottom of window to login to
the host.
A ms/dos window will open up and you will receive ‘login as’ prompt
Enter bob
Then you get
Authenticating with public key "rsa-key-20060209"
Passphrase for key "rsa-key-20060209":
At this point enter the pass phrase you entered when you ran
start/programs/Putty/Puttygen to create the rsa keys on the
client remotetom.
You are now logged into you host box using SSH. You have all
the privileges that host user bob has when you created him.
Every time you start/programs/putty/putty highlight the name
of your saved session and click on the load button. This will
restore your saved session configuration, then click open to
launch.
Conclusion;
Once you open ssh’s port 22 in your firewall it won't be long before
your auth.log starts filling up with failed login attempts.
About 99 percent of these failed login attempts are script kiddies
or robots.
Their attacks are all most totally based on indiscriminate rolling
through
a range of sequential IP address. (IE: They never use DNS to lookup
your
domain name.) You were found by plain bad luck. They run scripts
that
address the know ports listened on by those services which accept
logins.
You use this knowledge to defend against this type of attack.
The simplest defense is to change the port number ssh uses.
The /etc/services is where the port number used by SSH is defined
and where
you would change it at. Remote clients who want to access your host
system
will have to enter the alternate port number as part of the login
command.
ssh –p 77777 hostname or host-ip-address
[-p 77777 is the example alternate ssh port number you specified
in /etc/services file] or edit /etc/ssh/ssh_config on the remote
client and change this statement from this
# Port 22 to Port 77777 [to make the alternate port number
permanent]
Windows gutty also has way to change the default ssh port number
right on the login window. The technique of assigning a non-standard
port for SSH is security through obscurity and is one of the first
and easiest security measures an administrator may consider.
By no means does this provide protection from the dedicated attacker
who has targeted your IP address for some reason. This is just the
first
security level of hiding through obscurity from the majority of
script
kiddies and their robots. You will be surprised at how effective
this
really is at discouraging repeat port scan attacks and fraudulent
login attempts. You will have to open your firewall for the
alternate
port number you assign to ssh for this to work.
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