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

Les Mikesell lesmikesell at gmail.com
Thu Nov 20 20:07:03 UTC 2008

Kai Schaetzl wrote:
> Chris Heiner wrote on Thu, 20 Nov 2008 08:48:50 -0800:
>> My firewall seems to block an attack my Centos / Sendmail boxes on port 110.
> port 110 is your POP server, probably dovecot.
>> These servers require a reboot after each attack.
> Because of what?
>> My firewall says it's
>> blocked?
> I don't see this statement in your logs. How/where does it say this?
>> Do I need to patch something on sendmail? Or is my firewall not
>> doing its job (Sonicwall)? This is not the first time this has happened.
> SYN floods are not unusual, even if it is not an attack. 
> What or if you want to do something depends on your situation.

If you have a popular server you can get what appear to be syn floods 
from broken asymmetrical routing or bad firewall settings that permit 
what would ordinarily be a normal number of client connection requests 
to reach you but keep your response from getting back.  So the clients 
sit and retry, hammering you with syn's.

   Les Mikesell
    lesmikesell at gmail.com

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