[CentOS] Sendmail and pmtu discovery

Ralph Angenendt ra+centos at br-online.de
Tue Oct 14 13:24:19 UTC 2008

Sean Carolan wrote:
> We have an issue with some customers who refuse to accept ICMP traffic
> to their mail servers.  It seems that they have put Mordac, preventer
> of information services in charge of their firewall policy
> (http://en.wikipedia.org/wiki/List_of_minor_characters_in_Dilbert#Mordac).

BUT ICMP IS BAD!!!!!¡¡¡¡¡

> My mail logs are showing that customers who specifically disallow ICMP
> traffic have many "Connection Reset" entries in our logs:
> Oct 14 08:00:50 mailsrv sendmail[2024]: m9ED0Yf5002021:
> to=<customername at customer.org>, delay=00:00:16, xdelay=00:00:16,
> mailer=esmtp, pri=42476, relay=mail.customer.org. [XX.XX.XX.XX],
> dsn=4.0.0, stat=Deferred: Connection reset by mail.customer.org.
> I have disabled pmtu discovery on our routers as well as on all our
> outbound mail servers.  Is there anything else I can do on our side to
> help the situation?

So you basically broke your internet connection because of stupid
customers? No, there isn't anything you can do on your side - especially
if you don't know how large their MTU is set (which you cannot discover,
as they forbid you to do so). So you can only hope that you get exactly
the same MTU as they have (and that there is nothing inbetween which has
a lower MTU). 

It is their problem. If they don't want to play by the rules, they
should have to sit out the problems they themselves created.

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