Thousands of ssh probes

[Matthew Seaman]

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On 05/03/2010 15:44:39, John wrote:
> Maybe I'll have to learn how to do a VPN from FreeBSD....
> 
> One thought that occurs to me is that pf tables would provide a
> direct API without having to hit a database.
> 
> I think I really like this.  I may have to implement it for pf. 
> It should be really easy with CGI and calls to pfctl.

There's already a mechanism whereby you can connect into a PF firewall
and have it open up extra access for you, all controlled by ssh keys.

See: http://www.openbsd.org/faq/pf/authpf.html

Not only that, but you can dynamically block brute force attempts to
crack SSH passwords using just PF -- no need to scan through auth.log or
use an external database.  You need something like this in pf.conf:

table <ssh-bruteforce> persist

[...near the top of the rules section...]
block drop in log quick on $ext_if from <ssh-bruteforce>

[...later in the rules section...]
pass in on $ext_if proto tcp      \
     from any to $ext_if port ssh \
     flags S/SA keep state        \
     (max-src-conn-rate 3/30, overload <ssh-bruteforce> flush global)

This adds IPs to the ssh-bruteforce table if there are too frequent
attempts to connect from them (more than 3 within 30 seconds in this
case) and so blocks all further access.

You need to run a cron job to clear out old entries from the
ssh-bruteforce table or it will grow continually over time:

*/12 * * * *	/sbin/pfctl -t ssh-bruteforce -T expire 86400 >/dev/null 2>&1

	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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