Date: Mon, 11 Jan 2010 15:59:49 +0000 From: Anton Shterenlikht <mexas@bristol.ac.uk> To: Matthew Seaman <m.seaman@infracaninophile.co.uk> Cc: Anton Shterenlikht <mexas@bristol.ac.uk>, freebsd-questions@freebsd.org Subject: Re: denying spam hosts ssh access - good idea? Message-ID: <20100111155949.GA61863@mech-cluster241.men.bris.ac.uk> In-Reply-To: <4B4B42D0.9070101@infracaninophile.co.uk> References: <20100111140105.GI61025@mech-cluster241.men.bris.ac.uk> <4B4B42D0.9070101@infracaninophile.co.uk>
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On Mon, Jan 11, 2010 at 03:25:04PM +0000, Matthew Seaman wrote: > Anton Shterenlikht wrote: > > I'm thinking of denying ssh access to host from which > > I get brute force ssh attacks. > > > > HOwever, I see in /etc/hosts.allow: > > > > # Wrapping sshd(8) is not normally a good idea, but if you > > # need to do it, here's how > > #sshd : .evil.cracker.example.com : deny > > > > Why is it not a good idea? > > Probably because ssh is likely to be the only method of login access > you have to a remote server, and hosts.allow could conceivably be spoofed > into blocking your legitimate access? In any case, hosts.allow is a poor relation to using a real firewall -- it has no access to the lower level bits > of the networking code, so has to allow a full tcp connection setup before it > can block anything. Some daemons allow quite a lot of interaction with the > remote site when using hosts.allow functionality -- eg. sendmail will > apparently go through all of the stages of accepting an incoming e-mail from > a denied host, right up to the 'MAIL FROM...' section of the SMTP transaction > where it will respond with a 500 permanent failure error code. [admittedly > this does have the benefit that the other side will then immediately give up > trying to send the message if it's playing by the RFC rules. (Most spam-bots > don't, of course.) Otherwise, you'ld get the remote side retrying the message > several times an hour over the next 5 days before it timed out and gave up. > > > Also, apparently in older ssh there was DenyHosts option, > > but no longer in the current version. > > Is there a replacement for DenyHOsts? > > Or is there a good reason for such option not to be used? > > I believe you can do something like this: > > match address 192.168.23.0/24,172.16.0.0/16 > ForceCommand /usr/sbin/nologin > > but this is not foolproof, as it is run via the users' login shell > and a sufficiently cunning person can arrange for all sorts of interesting > things to happen from their shell initialization files... Matthew, this makes sense many thanks anton -- Anton Shterenlikht Room 2.6, Queen's Building Mech Eng Dept Bristol University University Walk, Bristol BS8 1TR, UK Tel: +44 (0)117 331 5944 Fax: +44 (0)117 929 4423
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