On 4 Sep 2014, at 17:03, Benny Kjær Nielsen wrote:
On 4 Sep 2014, at 22:18, John Grasty wrote:
Every once in a while a message will timeout and not get sent. The
activity log shows everything down to the closing . , but the mail
server never seems to receive the .
It's not a known issue. Could you send me an example of such a log?
Off list if you like.
This has been seen historically in situations where a stupid
"transparent" proxy (notably some versions of Cisco's PIX/ASA "smtp
fixup" malware) is mangling SMTP interactions at the application layer.
The common error is failure to anticipate the possibility of the
terminating <cr><lf>.<cr><lf> sequence being split between 2 TCP
segments and/or read() calls (or their logical equivalents), resulting
in the proxy not seeing the whole termination sequence at once. Some
SMTP client implementations have adapted to this careless coding by
making sure that the message itself is pushed in one segment and the
terminating sequence in a segment of its own. This assures that the full
sequence isn't split between TCP segments and makes it very unlikely
(but not quite impossible) that it will not be returned intact in a
single read().
That tactic also slightly mitigates a similar problem when a sending
system uses "Path MTU Discovery" and sets the "DONT FRAGMENT" flag on
packets, while some intermediary is dropping ICMP "MUST FRAGMENT"
replies of some distant device that can't deal with large packets. That
problem usually hits every message over 1500 bytes to a particular SMTP
server, instead of the "every once in a while" occurrence of the split
termination sequence problem.
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