Mail Servers

[Neal Morgan]

> Tuesday, May 11, 2010 2:32 PM
>
> 
> ok, slightly off topic but our mail server is suffering transferis 
> interuptis only with comcast servers over the past few weeks.
>
> running postfix  + mailscanner on ubuntu linux
>  
> reasonably current on everything, this is something that just started,

> appears to possibly be my firewall doing the interupting but I'm not 
> convinced.  my logs say lost connection after Data.
>  
> I know this is pretty general but wondering if anyone has run into
something similar ?
>  
>  
> much thanks
>  
> -Ron
 
 
Ron:

We've seen a couple of things in the last 2 years like this.  One was
related to a kernel issue where tcp window scaling was getting
corrupted.  I believe we initially got around it by setting some boot
time kernel parameters - but ultimately the kernel source got fixed.  

A second similar issue happened with SMTP traffic that was tunneled - a
router near a Cox SMTP server was unwilling to negotiate a smaller MTU
(required for traffic traversing the tunnel).  This one was odd because
it would only happen during daytime hours.  We eventually got a hold of
a Cox CCIE and found that when their routers were super busy they would
ignore the ICMP messages related to MTU negotiation.  Since the CCIE
couldn't/wouldn't fix it on his end, we ended setting up an EOIP bridge
inside the tunnel to trick our router into believing it could handle a
normal MTU.  Ugly, but effective...

In both cases, we had to do some packet captures and scour through with
Wireshark.

I hope this helps...


Neal Morgan