[Dovecot] login processes from attacks staying for hours

[Jack Stewart]

Hi,

We you run 'netstat -tan' (or equivalent), what state are the packets 
in? If it is just a bunch of processes with no active connections then 
it should not be a big deal.

We've seen something on our SMTP servers that sounds similar (our IMAP 
servers haven't been hit yet). The problem is there is badly written 
spammer/hacker software that do not close connections correctly. We wind 
up with a number of useless connections, many of them in CLOSE_WAIT or 
FIN_WAIT* states.

TCP/IP kernel tuning is our solution to close connection states quicker. 
I don't know Centros. In Solaris the ndd parameters, which have stupid 
defaults, are tcp_time_wait_interval, tcp_fin_wait_2_flush_interval, 
tcp_ip_abort_interval and tcp_keepalive_interval. Redhat seems a bit 
better but the tcp_keepalive_time of 2 hours is a little high for my 
liking. I'ld pay attention to the connection state that seems to be the 
problem.

An strict iptables approach doesn't address the tcp teardown issue. Even 
with drops via iptables there still will be connections waiting to close.

Our servers are front-ended by load balancers and there is also a router 
at the border. This is where we block ip addresses, if we need to.

Hope this helps.

---Jack


Kai Schaetzl wrote:
> I'm in the process of rolling out new setups with dovecot on CentOS 5.2 
> and I notice that dovecot doesn't handle the brute-force attacks too nice.
> I reduced the limit a bit to some reasonable looking value:
> login_max_processes_count = 32
> to stop them earlier and the number of processes stops at that figure when 
> an attack happens.
> However, it stays at this count for hours although the attack is already 
> over since long. For instance, my monitoring alerts me at the moment when 
> the process count for pop3-login goes over 20 processes. This happened on 
> three machines at 2 am with a brute-force attack from the same source that 
> didn't last longer than a minute or so. However, the process count dropped 
> only at 7am under 20 on two machines and on the third machine it was still 
> over 20 when I was in the office at 9 am and finally killed them.
> As these machines are all not in production yet, there weren't any other 
> logins and the single brute-force ended within one minute according to the 
> logs (obviously when pop3-logins hit the limit).
> Shouldn't these processes go down to login_processes_count (3) within a 
> few minutes? An strace shows that they are mostly doing gettimeofday 
> lookups (=sleeping).
> This is the default dovecot (1.07) coming with CentOS 5.2.
> I've been running only one other instance of dovecot in production 
> (0.99.11) on CentOS 4.6 so far and I don't know which behavior that 
> displayed in the past as I just recognize that I accidentally ommitted it 
> from monitoring. :-(
> 
> I had this mailing list searched for "brute-force" to see how others 
> handle this and what dovecot provides to stop these attacks. I have found 
> not many threads about this. There is one with a bit more information: 
> "Delay on failed pw attempts" from January 1. Unfortunately, this 
> functionality is only in a later version of dovecot and it's not clear if 
> it was implemented or not or if it would be helpful. Was it implemented?
> 
> This thread also mentions fail2ban which may be one way to go, although I 
> don't like this log parsing approach too much. Does anyone use iptables 
> for rate-limiting per IP on the pop/imap ports to prevent brute-force 
> attacks?
> 
> 
> 
> Kai
>