Hi,

I'm having the following strange problem.
I have a mailserver [2.6.18-1.2798.fc6] based on qmail. Some mail hangs on sending because it does not receive a 220 banner from the receiving mailserver.
The problem is with some mailservers.

So I thought I'll telnet into those servers to eliminate any configuration problems.

But then I telnet into those servers I get:
[root@medusa qmail]# telnet mail-in.<domain>.tld 25
Trying 62.xx.xx.xx...
Connected to mail-in.<domain>.tld (62.xx.xx.xx).
Escape character is '^]'.

And then it would sit there forever.
So I don't get a 220 header.
But when I directly issue a 'ehlo' command after connection, then after a while (about 10-20 sec) I get 220 header directly followed by a 250 welcome header.

When I run the exact same command from my other server [2.6.18-1.2798.fc6] (no qmail), I get a 220 header right away!

I'd searched the internet for 2 days now, and this didn't find a solution.

TCPDUMP reveils a small different between the two servers, but I don't know if it has something to do with my problem: (diff in bold)
Tcpdump -X
From failing server: 21:25:29.076726 IP [server].34674 > [remote].smtp: S 2639015205:2639015205(0) win 5840 <mss 1460,sackOK,timestamp 408922509 0,nop,wscale 7>

From working server: 21:11:43.346551 IP [server].53866 > [remote].smtp: S 2062653912:2062653912(0) win 5840 <mss 1460>


I hope someone had any idea!!
Thanks!
Onno.

this isn't going to be the network at all if you can happily type stuff into the server to get a response, the network must be fine. can you enable debugging on the server side processes or something? i'd certainly be looking in that direction. moved to Linux - Server.

How did you start your qmail-smtpd (I am assuming you are using this)?

Dear all,

Thanks for your thoughts.
The problem was caused by window scaling.
Solved it with: echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

Description of window scaling: http://kerneltrap.org/node/6723

So Chris, I do think it is a 'network' problem...

Thanks!

hmm, interesting... i appologise... pretty subtle though of course.

No Appologise needed. It was also the last thing I would think of...
Always fun with Linux ... ;-)