[sclug] Bizarre IMAP problem

[Darren Davison]

On Wed, Aug 24, 2005 at 10:16:56PM +0100, Roland Turner (SCLUG) wrote:

> 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.

Thanks Raz.

I connected only a few minutes ago to my wireless LAN at home, made the VPN
connection to the office and was able to retrieve all the mail via IMAP
without any problem from that Exchange box.

Having read your mail, I checked the MTU on my wlan0 interface and found it
set to 1356.  So I downed the wireless interface and stuck a cat5 cable
in the back and sure enough I had the same problem as I had from the office 
earlier.  Unfortunately however, reconfiguring the mtu on eth0 down to 1200 
made no difference.  After making the VPN connection, it had changed again, 
down to 1056 (consistent with the wireless mtu drop) and still the IMAP issue 
persisted.

Not sure what we're left with now.  Network driver?  I certainly haven't 
noticed any general networking issues from the wired interface.

Thanks again for your reply,

-- 

Darren Davison
Public Key: 0xDD356B0D
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