April 20, 2014

Testing Linux Mail Servers with OpenSSL

Testing POP3 TLS/SSL

  • February 15, 2011
  • By Carla Schroder
Yesterday Juliet Kemp showed us how to perform basic server tests with telnet. Today Carla Schroder shows how to test TLS/SSL-enabled POP3 and IMAP servers.
This is a followup to Juliet Kemp's excellent Troubleshooting Linux Servers with telnet article. This is adapted from my aging but still-excellent Linux Cookbook.

Testing POP3 TLS/SSL

Telnet does not support any encryption. If you are protecting your POP3 sessions with TLS/SSL then you need s_client, which is part of OpenSSL. You can query both local and remote mail servers, using your own server names of course. Commands that you type are in bold:

$ openssl s_client -connect localhost.com:995
$ openssl s_client -connect remotehost.com:995

You'll see a whole lot of lines about certificates and protocols, and eventually something like this:

+OK Dovecot ready.

Now we know we're talking to a Dovecot server. Dovecot supports both secure POP3 and secure IMAP. Now I can give Dovecot my login. Note that if you enter an invalid username it won't tell you, but will still say +OK:

+OK Dovecot ready.
user carla
pass password
+OK Logged in.

Let's see if I have any messages:

+OK 2 messages:
1 1759
2 12422

Yay, two messages for me! Use the retr command to read them:

retr 1
+OK 1759 octets
[snip headers]
Date: Tue, 15 Feb 2011 11:56:48 -0800
From: Mom
To: carla@example.com
Subject: dinner this weekend
[snip message-id]

Hello Little Carla,

We're having a little get-together this weekend and hope you can come. Bring your fabulous chicken skewers.

the Mama

To read the second message type retr 2. To delete messages type dele followed by the message number, for example dele 1. Messages are not really deleted until you type quit, so you can change your mind and un-delete with the rset command, which un-deletes all messages marked for deletion.

You may need to use the domain name on a remote server to log in, for example user carla@example.com. RFC 1939 contains a complete listing of POP3 commands.

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