[CentOS] iptables question

Meenoo Shivdasani meenoo at gmail.com
Tue Oct 20 17:47:56 UTC 2009


> conversation.  The question is:  why are all of these remote servers
> trying to make connections back to me on high-numbered ports?  Should I
> be allowing these connections somehow?

The remote server probably thinks that it's still supposed to be
making connections back to you -- a couple of the lines you posted
showed FIN flags indicating that the TCP connection was being shut
down.  At that point, the mail message has already been sent.

If you get REJECT messages for all SMTP connections, look at your
iptables rules and see if you have a specific rule for smtp that only
permits NEW conns.

One possibility is that iptables no longer thinks that the connection
is active -- possibly the connection tracking database has already
pushed that connection out.  You can check your conntrack max value
with the command

cat /proc/sys/net/ipv4/ip_conntrack_max

Yet another possibility is that these are duplicated packets (for
whatever reason) and the connection has already been closed out.

M


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