On Fri, Jun 18, 2004 at 12:48:38PM +0100, Martin W wrote:
> In a nutshell - sending messages with attachments from our mail hub to
> certain sites will consistently fail with a "connection reset by peer".
> One of these sites is BT Internet, and another Yahoo, so I'm assuming it's
> a problem at our end. Sending smaller messages (ie no attachment) works
> fine. Sending to the majority of other sites is also fine. The attachments
> only have to be around 700K to cause the failure.
Is Path MTU discovery working for you? Have you got some nasty icmp
filtering in the way that might be preventing the MTU from being
negotiated? Is your MTU 1500, or lower?
> Another bizarre observation occurred when we were trying to monitor
> traffic at the border of our network. We placed a 100M *hub* between the
> router and the external link in order to 'tap' the connection. I know this
> is unwise but, as I mentioned, we were desperate. All the time the hub was
> in place, the problem went away. We could send mail happily to all of the
> sites we had problems with previously. Heisenberg eat your heart out.
> Removing the hub, the problem came back.
Gigabit Ethernet allows MTU's up to 9000 bytes, and some setups allow
beyond this (eg 15000 bytes for most Intel NICs). Is your system trying
to use Jumboframes?
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Colm MacCárthaigh / HEAnet, Teach Brooklawn, / Innealtóir Ghréasáin
+353 1 6609040 / Bóthar Shelbourne, BÁC, IE / http://www.hea.net/
