Date: Mon, 11 Jan 2010 15:25:04 +0000 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Anton Shterenlikht <mexas@bristol.ac.uk> Cc: freebsd-questions@freebsd.org Subject: Re: denying spam hosts ssh access - good idea? Message-ID: <4B4B42D0.9070101@infracaninophile.co.uk> In-Reply-To: <20100111140105.GI61025@mech-cluster241.men.bris.ac.uk> References: <20100111140105.GI61025@mech-cluster241.men.bris.ac.uk>
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This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig4EB8A6869B9CFAD1ADF4B205 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable Anton Shterenlikht wrote: > I'm thinking of denying ssh access to host from which > I get brute force ssh attacks. >=20 > HOwever, I see in /etc/hosts.allow: >=20 > # 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 >=20 > 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 poo= r relation to using a real firewall -- it has no access to the lower leve= l bits of the networking code, so has to allow a full tcp connection setup befor= e it can block anything. Some daemons allow quite a lot of interaction with t= he remote site when using hosts.allow functionality -- eg. sendmail will apparently go through all of the stages of accepting an incoming e-mail f= rom a denied host, right up to the 'MAIL FROM...' section of the SMTP transac= tion where it will respond with a 500 permanent failure error code. [admitted= ly=20 this does have the benefit that the other side will then immediately give= up=20 trying to send the message if it's playing by the RFC rules. (Most spam-b= ots=20 don't, of course.) Otherwise, you'ld get the remote side retrying the me= ssage=20 several times an hour over the next 5 days before it timed out and gave u= p. > 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 interestin= g things to happen from their shell initialization files... Cheers, Matthew --=20 Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard Flat 3 PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate Kent, CT11 9PW --------------enig4EB8A6869B9CFAD1ADF4B205 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.14 (FreeBSD) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEAREIAAYFAktLQtYACgkQ8Mjk52CukIwtfwCfTn2hvHQST3hiUmskvrpwAcG9 +R4AnRLqHVUgG8H2j1bAU1Oromv6tKvq =Qi7V -----END PGP SIGNATURE----- --------------enig4EB8A6869B9CFAD1ADF4B205--
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