[ILUG] problem with win9x clients talking to linux sendmail.

[Paul Jakma]

i have a problem with win9x clients running outlook connecting to sendmail
8.9.3 on a RH6.2 box. /Occasionally/ clients get an 'error' email when
trying to send mail.

the error message says something like:

"
From: "System Administrator"

joe at acme.com: 
no transport provider was available for delivery to this recipient""
"

"System Administrator" is bogus, so it must be an Outlook generated email.

When this error occurs there is no record of a failure logged to
/var/log/maillog, ie sendmail doesn't even get the connection. Here's a
tcpdump of what i /think/ is a session that caused outlook to give the
above error:

16:08:10.046046 < 192.168.0.57.supfiledbg > 192.168.0.1.smtp: S 25462437:25462437(0) win 8192 <mss 1460,nop,wscale 0,nop,nop,timestamp 0 0,nop,nop,sackOK> (DF) [tos 0x17,ECT,CE] 
16:08:10.046118 > 192.168.0.1.smtp > 192.168.0.57.supfiledbg: S 1053669156:1053669156(0) ack 25462438 win 32120 <mss 1460,sackOK,timestamp 317431741 0,nop,wscale 0> (DF)
16:08:10.046276 < 192.168.0.57.supfiledbg > 192.168.0.1.smtp: . 1:1(0) ack 1 win 8760 <nop,nop,timestamp 233874 317431741> (DF) [tos 0x17,ECT,CE] 
16:08:40.165482 < 192.168.0.57.supfiledbg > 192.168.0.1.smtp: F 1:1(0) ack 1 win 8760 <nop,nop,timestamp 234152 317431741> (DF) [tos 0x17,ECT,CE] 
16:08:40.165527 > 192.168.0.1.smtp > 192.168.0.57.supfiledbg: . 1:1(0) ack 2 win 32120 <nop,nop,timestamp 317434753 234152> (DF)
16:09:25.090640 > 192.168.0.1.smtp > 192.168.0.57.supfiledbg: P 1:70(69) ack 2 win 32120 <nop,nop,timestamp 317439246 234152> (DF)
16:09:25.090840 < 192.168.0.57.supfiledbg > 192.168.0.1.smtp: R 25462439:25462439(0) win 0 (DF) [tos 0x17,ECT,CE] 

As i understand it: 

The windows machine tries to connect, 
Linux responds with a SYN + ack.
The windows machine acks our SYN, no data.
<30 second pause>
The windows machine terminates the connection and acks our SYN.
linux acks the win9x FIN
<no response from win9x>
linux acks the FIN again
Win9x sends a RST.

This particular client was Win95 4.0.95B.

So my question: 

why the 30 second pause? Why doesn't sendmail respond to the connection?
Has anyone seen this problem before? Is it a linux tcp stack problem? or
is it sendmail?

ta,

Paul Jakma.