[sclug] Bizarre IMAP problem

[Dickon Hood]

On Thu, Aug 25, 2005 at 02:20:33 +0100, Alex Butcher wrote:
: On Wed, 24 Aug 2005, Roland Turner (SCLUG) wrote:
: >Darren Davison wrote:
: >>* 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.

: Good call. I should have spotted this too, given how many times it was my
: first fix for calls logged with me in a previous life...

It was my first thought when reading this thread this morning, which is
nice to know.

: >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.

: Even better, ifconfig eth0 mtu 576, which is the largest MTU that is
: guaranteed to traverse any and all types of network without fragmentation.

PPP is guaranteed to pass that, certainly.  The problem is that the MTU
problem is at (least at) the *remote* end, not the local.  PMTUD is
carried out by both ends at the same time, independently of each other
(inbound and outbound paths may be different, so this makes sense,
although it's generally frowned upon in practice), so setting the client
MTU may not help.

You'll need to set the server MTU, or locate the buggy device.  Being
Windows, you're probably screwed...

-- 
Dickon Hood

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