Hello,
Alex Tang a écrit :
I've been having a problem on a machine that does a high volume of
sendmail traffic. The machine gets approx 50,000 connections per hour
to port 25.
The machine was upgraded from a RHEL3 based system (kernel rpm
2.4.21-47.EL and iptables rpm 1.2.8-12.3) to a RHEL4 based system
(kernel rpm 2.6.9-55.EL and iptables rpm 1.2.11-3.1.RHEL4).
You should really consider upgrading to something more recent, because
kernel 2.6.9 and iptables 1.2.11 are *badly* outdated.
Since the upgrade has occurred, the conntrack table fills up relatively
fast (within one day). The max size is 65536 (as per
/proc/sys/net/ipv4/netfilter/ip_conntrack_max).
I've been searching through the archives, faq, etc and have found the
usual standard answer is to increase the ip_conntrack_max. However, I'm
concerned for a couple of reasons that this may not be the proper answer.
In particular, i have another machine which is still running the RHEL3
(kernel 2.4.21-47.EL/iptables 1.2.8-12.3), that gets more connections
per hour (80,000 vs. 50,000), and there are only about 9000 entries in
the ip_conntrack table on that machine.
The problem with the conntrack table filling up fast started as soon as
we did the upgrade.
Also, on the machine that is currently experiencing problems, most (98%)
of the connections are in the ESTABLISHED state, however the majority of
these connections are not seen when doing a "netstat".
I admit that I do not fully understand the details of the iptables
implementation, but it seems that the connection close is not being
"seen" by the conntrack code and connections that have already gone away
are still in the ip_conntrack table, and we have to wait for these
connections to "timeout" before they are expired from the conntrack table.
And the default timeout for established TCP connections is 5 days, so it
takes a looong time.
A noticeable difference between kernel 2.6.9 and earlier versions is the
TCP window tracking, which was added in 2.6.9. It adds TCP sequence
number checking to the connection tracking, so any TCP packet with an
out-of-window sequence number is tagged INVALID. If the TCP window
tracking considers that the FIN packet sequence numbers are
out-of-window for whatever reason, this may be the reason why the 2.6.9
kernel connection tracking keeps old connexions. You may add LOG
iptables rules to track TCP FIN packets states.
I could of course, increase the max size of the table, or decrease the
/proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_timeout_established, but
it seems that would only mask the problem, not actually fix it.
I agree. You could also decrease the value of
/proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_timeout_established to
something much shorter than 5 days but again this would only mask the
problem.
Try to set /proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_be_liberal to
1. This setting makes TCP window tracking more liberal, so only
out-of-window TCP RST packets are tagged INVALID.
Note that later kernel versions provided some bugfixes for the TCP
window tracking that may fix this issue.
