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
