[CentOS] Sendmail and pmtu discovery

[Paul Bijnens]

On 2008-10-14 16:31, Kai Schaetzl wrote:
> Sean Carolan wrote on Tue, 14 Oct 2008 08:13:34 -0500:
> 
>> My mail logs are showing that customers who specifically disallow ICMP
>> traffic have many "Connection Reset" entries in our logs:
> 
> Could somebody explain why ICMP might play a role in mail delivery?

Any host may reply to a IP-datagram (tcp included) with e.g. ICMP type 3,
code 4 "datagram too large" and indicating the maximum size in the ICMP reply.

Disallowing these ICMP packets can result in a TCP handshake that
succeeds, but hangs when the next packets with real data are blocked.

http://en.wikipedia.org/wiki/PMTUD



-- 
Paul Bijnens, xplanation Technology Services        Tel  +32 16 397.511
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