We have an app which is trying to send mail to an SMTP server via a direct TCP connection. It's really slow. To emulate this we try connecting via telnet:
telnet 10.44.0.1 25
the initial response:
Trying 10.44.0.1...
Connected to myserver.mydomain.com
Escape character is '^]'.
is really fast but there is a 2 or 3 second delay before we get the:
220 myserver.mydomain.com ESMTP Server (Microsoft Exchange Internet Mail Service 5.5.2653.13) ready
which allows us to do the sending. (OK. OK. real apologies that this is an MS-Exchange server but I can assure you that that isn't the fault and if you look carefully you will see version 5.5 which is a hint that we are about to migrate to a more sensible mail server from a different supplier...)
If we telnet to pop:
telnet 10.44.0.1 110
the response:
Trying 10.44.0.1...
Connected to myserver.mydomain.com.
Escape character is '^]'.
+OK Microsoft Exchange POP3 server version 5.5.2657.74 ready
is instantaneous.
Now we have multiple boxes that we can test this with. RedHat 7.3 doesn't have this problem (ie we get an instantaeous complete response on port 25) but AS2.1 to ES4 does.
We are not running identd. Our hosts entries are correct. We aren't running iptables.
We don't get this problem on Debian boxes. We don't get this problem on Windows boxes (sorry!) or Macs.
Any suggestions?