Alex:
> Hi,
>
> >> I have a few postfix-2.8.7 systems on fedora15 that connect with
> >> another postfix-2.8.7 system. I'm receiving the following messages
> >> periodically in the logs:
> >>
> >> Apr 24 16:24:43 mailrelay postfix/smtpd[8814]: timeout after DATA
> >> (9832 bytes) from mail02.example.com[68.XXX.YYY.45]
> >
> > tcpdump will show if this is an MTU problem, mis-handling of TCP
> > window scaling in a buggy firewall, or something else.
> >
> > You can set the MTU with ifconfig commands; and Postfix has
> > tcp_windowsize parameter that allows you to prevent window scaling.
>
> One of the network admins responsible for an Exchange server which
> also connects to this relay server and is also having difficulty, has
> reported the MTU is set correctly to 1500.
>
> I've done a little investigation of my own, and I'm finding what I
> believe are a lot of incorrect checksums?
The TCP Maximal Segment Size (1460) is the ethernet MTU (1500) minus
the size of the IP header (20) and TCP header (20).
tcpdump will report checksum errors in outbound packets when the
OS off-loads the checksum calculation to the network interface
hardware. tcpdump sees the checksum before it is filled in.
Wietse
