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Re: MTU and Postfix

On Sun, 04 Sep 2011 09:14:45 -0300, D G Teed wrote:

> On Wed, Aug 31, 2011 at 1:09 PM, Camaleón <noelamac@gmail.com> wrote:
>> I've been busy on these days trying to solve a problem with Postfix
>> that drove me nuts.
>> Sporadically (let's say one in hundred e-mails) my Postfix had problems
>> for delivering messages with ~3 MiB of attachment to some e-mail hosts.
>> DSN service returned the final notice of delivery to the user and logs
>> displayed an error like "timed out while sending message body".


>> Googling around I found some posts and articles¹ pointing to the MTU
>> value (my bonded interface was set by default to 1500) and as I had
>> nothing to lose, I changed this and lowered to 1400.


> You might want to try the postfix mailing list and see if they have any
> ideas.
> Be prepared for cold, hard, terse answers.  They don't chat much - busy
> I guess.


Should I couldn't solve the problem, sure, I would had to post in there.

The fact is I like Postfix so much, I find it very flexible and easy to 
deal with but in specific case, I didn't think the app was the culprit 
but as Osamu has pointed out, a router or a filter (firewall) in between 
our hosts making noise.
> Was the previous MTU of 1500, a value you had set, or the default when
> queried?

1500 was the default MTU value for the bonded interfaces. I thinks is the 
system default for ethernet devices nowadays.
> I'm wondering because of a recent experience I had tweaking MTU.  I
> wanted to try jumbo frames to improve samba throughput on large video
> files.  With MTU set to 9000 on Linux and Windows, throughput increased
> about 8 times.  But it caused problems with the web service on Linux,
> which was running a domain under dyndns.  I set the MTU on Linux back to
> unspecified, but left the jumbo frame active on Windows side.  The
> performance was still very good in large samba file transfers.  I might
> remember this wrong, but it seemed it was worse performance when Linux
> side specified 1500, so I left it as unspecified and it has worked well.
> I still get high transfer speeds in samba with unspecified MTU on Linux
> but jumbo of 9000 MTU on Windows side.

That was exactly the kind of problems I would like to avoid :-) But as 
you said, lower values for MTU are not prone to errors or conflicts but 
higher ones (like jumbo frames). I hope performance is not badly 
penalized for having a low value, though...
> BTW, 1492 is a common MTU seen in FAQs.  You might get just as good with
> that.

MTU is now set at 1400. The host I was trying to contact seemed to be 
fine with that setting so I kept it so.



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