ip_conntrack table full after upgrade from RHEL3 (2.4/1.2.8) to RHEL4U4 (2.6.9/1.2.11)

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).

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.

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.

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Pascal Hambourg
Sent: Tuesday, May 22, 2007 2:59 AM
To: [email protected]
Subject: Re: ip_conntrack table full after upgrade from RHEL3 (2.4/1.2.8) to RHEL4U4 (2.6.9/1.2.11)

Hello,

Quoted 3 lines · Alex Tang

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

Quoted 4 lines

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.

Quoted 1 line

Since the upgrade has occurred, the conntrack table fills

up relatively

Quoted 4 lines

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

Quoted 1 line

usual standard answer is to increase the ip_conntrack_max.

However, I'm

Quoted 1 line

concerned for a couple of reasons that this may not be the

proper answer.

Quoted 2 lines

In particular, i have another machine which is still

running the RHEL3

Quoted 1 line

(kernel 2.4.21-47.EL/iptables 1.2.8-12.3), that gets more

connections

Quoted 1 line

per hour (80,000 vs. 50,000), and there are only about 9000

entries in

Quoted 3 lines

the ip_conntrack table on that machine.

The problem with the conntrack table filling up fast

started as soon as

Quoted 3 lines

we did the upgrade.

Also, on the machine that is currently experiencing

problems, most (98%)

Quoted 1 line

of the connections are in the ESTABLISHED state, however

the majority of

Quoted 5 lines

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

Quoted 2 lines

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.

Quoted 1 line

I could of course, increase the max size of the table, or

decrease the

Quoted 1 line

/proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_timeout_establis hed, but

Quoted 1 line

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.