denying spam hosts ssh access - good idea?

[Matthew Seaman]

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...

	Cheers,

	Matthew

-- 
Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                  Kent, CT11 9PW

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