Re: system/1878: MTU problem when sending mail using postfix

[Damien Miller <djm@mindrot.org>]

On Fri, 15 Jun 2001, Angelos D. Keromytis wrote:

> The following reply was made to PR system/1878; it has been noted by GNATS.
>
> From: "Angelos D. Keromytis" <angelos@coredump.cis.upenn.edu>
> To: bernat@free.fr
> Cc: gnats@openbsd.org
> Subject: Re: system/1878: MTU problem when sending mail using postfix
> Date: Fri, 15 Jun 2001 08:18:36 -0400
>
>  Fragmented packets received by the OpenBSD box means the *peer* doesn't
>  do PMTU discovery. What you're describing sounds like someone upstream
>  is either filtering or not generating ICMP messages for too-large TCP
>  packets. If this doesn't happen with most/all destinations, it probably
>  means the remote mail server (or its network) is misconfigured.

I've seen this behaviour too when using PPPoE ADSL using /usr/sbin/ppp and
the tcpmssfixup option.

When postfix sends a:

[message body]
.

at the end of a SMTP conversation, the status reply to the '.' is
getting lost. I don't know why this should be the case, because it
should be way less than MTU, but turning off PMTU discovery certainly
helped me.

>  Try ftp'ing a big file to and from that host if possible, as well as some
>  other host in another network if possible.

In the case I looked at, I had no problems with bulk data transfer.

-d

-- 
| Damien Miller <djm@mindrot.org> \ ``E-mail attachments are the poor man's
| http://www.mindrot.org          /   distributed filesystem'' - Dan Geer