[sclug] Bizarre IMAP problem

[Roland Turner SCLUG]

Darren Davison wrote:


> 20 OK [READ-WRITE] SELECT completed.
> 30 FETCH 1 ALL
> * 1 FETCH (FLAGS (\Seen) INTERNALDATE "17-Aug-2005 00:22:19 +0100"
> RFC822.SIZE
> 11304 ENVELOPE ("Wed, 17 Aug 2005 00:21:06 +0100" "some subject"
> (( remaining headers cut out ))
> 30 OK FETCH completed.
> 40 FETCH 1 BODY[TEXT]
> * 1 FETCH (BODY[TEXT]
>
> ..and that's it.  Always at that exact point, the connection dries up
> and


Sounds something like a PMTU problem, particularly given that there's a
VPN involved. Perhaps TCP on the host running Exchange and/or the Linux
client host has assumed a PMTU of 1500 (Ethernet's 1512 bytes - 12 bytes
of Ethernet header) when in fact it should further shrink it by the VPN's
overhead. This will tend to occur particulary when ICMP "fragmentation
required" (type 3 code 4) is being eaten by a broken firewall somewhere,
thereby breaking PMTU discovery. A quick test for this is to force the
initial segment size down on the Linux side with something like "ifconfig
eth0 mtu 1000" before any TCP connections are established. If this works,
look for firewall stupidity somewhere.


- Raz