19.14. Troubleshooting SMTP Servers

Problem

What are some tests you can run directly on your SMTP server to see if it is working correctly? You want to eliminate as many variables as you can, and talk directly to the server, if that's possible.

Solution

Good old telnet does the job. You also needthe mailx package installed, and Netstat.

First, run telnet on your SMTP server to see if you can talk to it. This example creates and sends a test message:

	$ telnet localhost 25
	Trying 127.0.0.1...
	Connected to localhost.
	Escape character is '^]'.
	220 xena.alrac.net ESMTP Postfix (Ubuntu)
	ehlo xena
	250-xena.alrac.net
	250-PIPELINING
	250-SIZE 10240000
	250-VRFY
	250-ETRN
	250-STARTTLS
	250-ENHANCEDSTATUSCODES
	250-8BITMIME
	250 DSN
	mail from: carla@testing.net
	250 2.1.0 Ok
	rcpt to: carla@xena
	250 2.1.5 Ok
	data
	354 End data with <CR><LF>.<CR><LF>
	Date: July 4, 2007
	From: testcarla
	Reply-to: testcarla@testing.net
	Message-ID: one
	Subject: SMTP testing
	Hi Carla,
	If you can read this, the SMTP server works.
	.
	250 2.0.0 Ok: queued as B2A033FBA
	quit
	221 2.0.0 Bye
	Connection closed by foreign host.

Now, run mail to read your message:

	$ mail
	Mail version 8.1.2 01/15/2001.  Type ? for help.
	"/var/mail/carla": 1 message 1 new
	>N  1 testcarla@xena.al  Sun Jul 15 10:46   17/523      SMTP testing
	& t
	Message 1:
	From carla@testing.net  Sun Jul 15 10:46:21 2007
	X-Original-To: carla@xena.alrac.net
	Date: July 4, 2007
	From: testcarla@xena.alrac.net
	Reply-to: testcarla@testing.net
	Subject: SMTP testing
	To: undisclosed-recipients:;

	Hi Carla,
	If you can read this, the SMTP server works.

	& q
	Saved 1 message in /home/carla/mbox

This shows you that your name services are working, and that the SMTP server is working. If you see this instead:

	$ telnet localhost 25
	Trying 127.0.0.1...
	telnet: Unable to connect to remote host: Connection refused

That means the server is not running, which you can confirm with Netstat:

	# netstat -pant|grep :25

If it returns nothing, your SMTP server is not running. This example shows a running Postfix server:

	# netstat -pant|grep :25
	tcp   0  127.0.0.1:25          0.0.0.0:*        LISTEN         8000/master

Once you get it running on localhost, you can test it remotely:

	terry@uberpc:~$ telnet xena 25
	Trying 192.168.1.10...
	telnet: Unable to connect to remote host: Connection refused

Netstat already showed why you can't connect remotely—Postfix is only listening on localhost. So, you need to configure it to also listen on the LAN interface, which means you need two lines like this in main.cf:

	mynetworks = 127.0.0.0/8, 192.168.1.0/24
	inet_interfaces = 127.0.0.1, 192.168.1.10

Restart Postfix, and now Netstat should report this:

	# netstat -pant|grep :25
	tcp   0  0 192.168.1.10:25  0.0.0.0:*   LISTEN    8324/master
	tcp   0  0 127.0.0.1:25    0.0.0.0:*   LISTEN    8324/master

Following along with tcpdump as you run your other tests is helpful:

	# tcpdump -pi eth0 port 25

And, always check logfiles.

Discussion

Other SMTP servers are configured differently, so you'll need the documentation for your own server.

To exit a telnet session early, hit Ctrl-], then Q.

Why use telnet? Because it can talk directly to the server and find out quickly if the server is operating correctly. Bypassing intermediaries is always a good first step.

This recipe also shows you how easy it is to spoof mail headers, and how careful you must be with access controls. The SMTP protocol is completely insecure as spammers discovered many years ago, so make sure that you are not providing SMTP services to the world. As with all services, it's a two-pronged approach: careful configuration of the server's own access controls, and careful firewalling. You should also consider using smtp-auth, which requires your SMTP users to authenticate themselves to your server.

19.14. Troubleshooting SMTP Servers

Problem

Solution

Discussion

See Also