November 30, 2015

The Penguin's Practical Network Troubleshooting Guide, Part 2 - page 2

Tracking Down Network Congestion

  • June 13, 2006
  • By Carla Schroder

What if mtr shows a clear path to your server, but performance is still bad? Then it's time to fire up tcpdump to see what the heck is going on. (See Resources for tcpdump howtos.)

Another useful tool in your server troubleshooting kit is telnet. No, really! With telnet you can query mail, Web, and FTP servers directly and actually capture useful messages that you don't see with ordinary clients. A related command is openssl s_client, part of the OpenSSL package, for testing TLS/SSL-enabled servers.

This is how to telnet to your SMTP server and send a test message. The lines in bold are commands that you type:

$ telnet 25
Connected to
Escape character is '^]'. ESMTP Exim 4.52 #1 Wed, 03 May 2006 19:33:34 -0400
220-We do not authorize the use of this system to transport unsolicited,

220 and/or bulk e-mail.
ehlo yourserver
250-SIZE 10240000

mail from:
250 Ok
rcpt to:
250 Ok
354 End data with .

Date: Wed 03, 2006
From: foober
Message-ID: six
Subject: telnet test
Hi Carla,
Did you get this?
250 Ok: queued as 6069F2290C

221 Bye
Connection closed by foreign host.

This shows a number of interesting things: a successful SMTP session, all the response codes, and how easy it is to spoof mail headers. See RFC 821 for an explanation of all response and error codes.

What about SSL/TLS encrypted mail sessions? No problem, just use the openssl s_client command:

$ openssl s_client -connect 

It will connect, then spit out trainloads of SSL certificate information. When it gets to

+OK POP3 host6 [cppop 20.0] at []

it is ready to accept commands. Some typical session commands are:

user [username]
pass [password]

list (lists the number of messages)
retr 1 (display message number 1)

You may use the same commands for an unencrypted POP3 session, telnet 110.

Test your non-SSL IMAP server like this:

$ telnet 143

Then use these commands to login and read mail:

login [username] [password]
examine inbox

Test your SSL-enable IMAP server with this command:

$ openssl s_client -connect
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