[Dovecot] limiting number of incorrect logins per connection

Ed W lists at wildgooses.com
Mon Aug 29 17:12:35 EEST 2011

Slightly advanced, but note that you CAN block in progress connections
using iptables

What is happening is that the usual top (ish) rule in your iptable
script says something like "continue if connection established", this
can be seen as a performance optimisation to avoid running your full
rulebase after the connection exists, however, it's not a limitation of
iptables.  So you have two main ways to tackle this:

1) Add your blacklist rules *before* the continue rule. One of the
fastest/simplest ways to achieve this is with the new ipset
functionality of recent kernels.  You can then have a single iptables
(block) rule, which references a dynamically updated ipset that contains
all your blacklisted IPs. Curiously ipsets can have a timeout value
which appears to cause entries to fall out after a set time period. 
Requires some changes to the "actions" in fail2ban

2) Alternatively note that you can purge specific connections from
conntrack (subsequent packets will either be treated as "invalid" or
"new" depending on whether you have loose tracking set in /proc/sys/... 
Something like "conntrack -D -n a.b.c.d" should do it?  This still
requires some tweaking to fail2ban, but the iptables rules stay the same

Just saying...

Good luck

Ed W

On 26/08/2011 13:22, Felipe Scarel wrote:
> Yeah, I had read about half of that thread, and after I sent my mail kept
> reading and stumbled upon this: "(...) using the recent module needs
> dovecotto close the connection upon authentication failure, as iptables only
> (normally) comes in to play for new connections (...)".
> So, yeah, my suggestion probably won't work.
> On Fri, Aug 26, 2011 at 09:15, Felipe Scarel <fbscarel at gmail.com> wrote:
>> Alex, I've not personally done it (so just speculating here, bear with me)
>> but you can customize Fail2Ban's actions if needed. So, if you can match the
>> attemps through some regex (and since you're seeing them in the logs, that
>> should be quite possible), then you can edit one of the 'actions' to drop
>> the connection for <ip>.

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