[CentOS] SYD flood dropped on Sendmail (centos 4.x)

Chris Heiner cheiner at networkdesignsinc.net
Thu Nov 20 21:43:44 UTC 2008

I get complaints about "the servers asking for username and password". I
started test@ accounts all many servers to try and track it down. And it
happens to all the servers that receive a SYN Flood. I.E. the problem with
each server co insides with firewall logs. It’s a pattern every few weeks,
sometimes a few servers sometimes 2 or 3 but it always matches up with the
firewall log. I now have emails sent to me to alert of a port 110 SYD flood
so I am aware of the problem before I get a full voicemail box from
complaints. Most of the time it's in the middle of the night at 2am to 3am
and the problem is resolved by start of business day. So that would rule out
heavy usage from my users as the network reports show that it's quiet. We
have 10 MB fiber connection and all traffic is logged at many levels.

I have tried restarting POP and SMTP in the past, but rebooting seems to
work and if there isn’t a fix I will have to continue this as I have many
other networking issues to resolve. 

I just thought I would throw this problem out to the group and see if anyone
has any good ideas.

I have tracked this mail list for years and everyone is extremely

Thanks for any replies..

-----Original Message-----
From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On Behalf
Of Kai Schaetzl
Sent: Thursday, November 20, 2008 12:53 PM
To: centos at centos.org
Subject: Re: [CentOS] SYD flood dropped on Sendmail (centos 4.x)

Chris, you still didn't answer *why* you have to reboot them. What exactly 
is the symptom that makes you think you have to reboot?

I assume now that with "My firewall says it's blocked" you referred to the 
drops? (Next time say so, as this wording is really ambiguous.)

> What would you like to know about my situation? I have 6 servers running

Yeah, so you are not a home user where one could rate-limit the port ;-)


Kai Schätzl, Berlin, Germany
Get your web at Conactive Internet Services: http://www.conactive.com

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