[CentOS] Sendmail and pmtu discovery

Tue Oct 14 15:01:05 UTC 2008
Paul Bijnens <Paul.Bijnens at xplanation.com>

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
Technologielaan 21 bus 2, B-3001 Leuven, BELGIUM    Fax  +32 16 397.512
http://www.xplanation.com/          email:  Paul.Bijnens at xplanation.com
***********************************************************************
* I think I've got the hang of it now:  exit, ^D, ^C, ^\, ^Z, ^Q, ^^, *
* F6, quit, ZZ, :q, :q!, M-Z, ^X^C, logoff, logout, close, bye, /bye, *
* stop, end, F3, ~., ^]c, +++ ATH, disconnect, halt,  abort,  hangup, *
* PF4, F20, ^X^X, :D::D, KJOB, F14-f-e, F8-e,  kill -1 $$,  shutdown, *
* init 0, kill -9 1, Alt-F4, Ctrl-Alt-Del, AltGr-NumLock, Stop-A, ... *
* ...  "Are you sure?"  ...   YES   ...   Phew ...   I'm out          *
***********************************************************************