Jakob Hirsch wrote:
> Hi,
Hi Jakob. Thanks for the quick answer!
> could be
> - the notorious MTU issue, caused by dumb ICMP filtering (on your side
> or the remote, but then they'd have this problem with most sites). Try
> to reduce your MTU and see if it helps (e.g. on Linux "ifconfig eth0 mtu
> 1400" or "ip link set eth0 mtu 1400")
> - broken firewall that cannot handle tcp window scaling, see
> http://kerneltrap.org/node/6723, if you're using Linux. Try setting
> /proc/sys/net/ipv4/tcp_window_scaling to 0
(I'm assuming these are settings on the sending mailserver, not my
router -- the router is a cheap consumer grade Netgear thing, and not
overly configurable in this regard, although the MTU is -- currently,
it's set at 1458)
With eth0's MTU set to 1200, and tcp_window_scaling to 0, I still see
exactly the same issue, so I guess I can at least rule these issues out.
> - some other network related problem. Try to capture the sesssion with
> tcpdump and analyze the dump with wireshark.
I'm looking at the dump in wireshark right now, and I must admit I'm not
entirely sure what I'm seeing. About 3.3 seconds in, I start to see lots
of packets marked as "TCP Dup Ack" or "TCP Retransmission", but my
understanding of TCP goes no further than knowing roughly what SYN and
ACK are, and certainly doesn't extend to knowing how duplicate ACKs work...
I'm beginning to suspect that this is less of an exim issue and more of
a "someone, somewhere, has a broken network, and it could well be me"
issue :-/
Cheers,
James
