Fortunately there is a tool for that. The tool? Spamassassin. Spamassassin is a very versatile SPAM tool that is part of the Apache Foundation. Spamassassin uses numerous means to detect SPAM including: DNS and Checksum based SPAM detection as well as Bayesian filtering, external programs, black lists, and online databases. These tools together make for a fairly powerful detection system.

In this article you are going to see how to install and configure Spamassassin to work in conjunction with Postfix to further enhance your email server.

Installing Spamassassin

Obviously the first thing you need to do is install Spamassassin. You will find Spamassassin in the Ubuntu repositories. And since this entire series has been laid on top of a Ubuntu Server installation, that is quite convenient. So, to install Spamassassin, open up your terminal window and issue the following command:

sudo apt-get install spamassassin

There may or may not be some dependencies to install in order for the Spamassassin  installation to complete. Go ahead and OK those. Once this installation is complete you are ready to start configuring.

Configuration

Before we actually get to the configuration it is important to understand the SPAM scoring system. With Spamassassin, messages are tagged as SPAM only when they have enough SPAM-matching characteristics (according to a scoring level). The scoring level is 0-5, however it’s not as simple as saying a 0 means it is 0% SPAM. The system is set up so that every characteristic can add to the overall score. For example a message tested to find a base64 attachment does not have a file name filtered with both bayes+net will add 0.224 to the over all score of the message. When all of the characteristic scores are added up, if they exceed the default score you have set in the configuration file, that message is considered SPAM.

Now that you have a basic understand of how the scoring system works. Let’s start configuring Spamassassin.

The main configuration file is /etc/spamassassin/local.cf. The first option you want to configure is the default score. Look for the line:

# required_score 5.0

The first thing you want to do is uncomment that line (by removing the “#” character) and then changing the score. A score of 5 is pretty high and sure to be SPAM. Understand the more you lower that score the likely you are of missing message messages that are tagged false-positives. A score of 3.5 is a fairly reliable score that will catch a lot of SPAM but not a lot of false positives.

Above this line are a couple of other options that are important. The first is the option to set the option:

report_safe

To 0. This option can be set to either 0 or 1. A zero means that if a message is found to be SPAM the message will not be deleted, but instead the subject line will be rewritten to include a message marking it as SPAM.  This is handy to prevent users from losing important messages to false positives. This also allows you to set a lower score threshold.

To do this first look for the line:

# report_safe 1

Uncomment this line by removing the “#” character and then change the “1″ to “0″ (no quotes).

The next step is to uncomment the line:

# rewrite_header Subject *****SPAM*****

Now you can alter the “*****SPAM*****” section of this line to reflect what you’d prefer it to say. Just make sure it is clear to your users that a message with this rewritten subject line is most likely SPAM.

Now restart the Spamassassin daemon with the command:

sudo /etc/init.d/spamassassin restart

Configure Postfix

The last step is to set up Postfix to use Spamassassin. To do this open up the file /etc/postfix/master.cf and look for the line:

smtp     inet    n   –   –   –   –   smtpd

You need to alter this line to look like:

smtp      inet   n   -   -   -   -   smtpd -o content_filter=spamassassin

Finally, at the end of this file add the following:

spamassassin
unix - n n - - pipe
flags=R
user=spamd
argv=/usr/bin/spamc
-e /usr/sbin/sendmail
-oi -f ${sender} ${recipient}

Now all you need to do is restart Postfix with the command:

sudo /etc/init.d/postfix restart

Your mail server should now be scoring incoming message as SPAM or HAM.

Final thoughts

The mail server is a tricky beast. You have to ensure that users are getting their mail, but you have to make sure they aren’t receive SPAM or viruses. After completing this series of articles, you should have a pretty solid server running that will send out mail that is safe for users eyes.

One of the best anti-virus systems for a Postfix server is ClamAV. This anti-virus tool kit is open sourced and can be used on all UNIX-like operating systems. It’s easy to install and effective. In this article we will be following our series started way back in the Installing Ubuntu Server 9.04 article. Of course we will be installing ClamAV on a Ubuntu server running LAMP and Postfix. With that in mind, let’s get busy!

Installation

The first thing to take care of is the installation of ClamAV. There are a number of tools you will need to install. Open up a terminal window and issue the command:

sudo apt-get install clamav clamav-freshclam clamsmtp

The above command should also pick up all of the necessary dependencies. The installation will also start the clamav daemon. You will restart that momentarily

Configuration

Once installed you have some configurations to take care of. There are three files you are going to have to edit:

• /etc/clamsmtpd.conf
• /etc/postfix/main.cf
• /etc/postfix/master.cf

The first file to configure is the clamsmtpd.conf file. The configuration in this file is simple. Look for the lines:

OutAddress: 10025

127.0.0.1:10026

Change them to:

OutAddress: 10026

127.0.0.1:10025

That’s it for the clamsmtpd.conf file. Now let’s move on to the heavier configurations.

Open up the /etc/postfix/main.cf file. Scroll down to the bottom of this file and add the following:

content_filter = scan:127.0.0.1:10025

receive_override_options = no_address_mappings

Save that file and now move on over to the /etc/postfix/master.cf file. Again, scroll down to the bottom of this file and add the following:

# AV scan filter (used by content_filter)
scan unix - - n - 16 smtp
-o smtp_send_xforward_command=yes
# For injecting mail back into postfix from the filter
127.0.0.1:10026 inet n - n - 16 smtpd
-o content_filter=
-o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
-o smtpd_helo_restrictions=
-o smtpd_client_restrictions=
-o smtpd_sender_restrictions=
-o smtpd_recipient_restrictions=permit_mynetworks,reject
-o mynetworks_style=host
-o smtpd_authorized_xforward_hosts=127.0.0.0/8

Save that file.

Restarting

The first thing you need to do is restart Postfix with the command:

sudo /etc/init.d/postfix restart

Once that has restarted you need to restart clamsmtpd with the command:

sudo /etc/init.d/clamsmtpd restart

Now, if nothing has gone horribly wrong, you should have a virus protected Postfix mail server.

Updating signatures

You should never go without updating your virus signatures. This is critical for keeping your mail server virus-free as new viruses are created or old viruses mutate. Fortunately ClamAV has its own tool for this. You will need to go back to that terminal window and issue the command:

sudo freshclam

Which will update the signatures.

You might even add the freshclam command into the root users crontab for regular signature updates.

Final thoughts

Your Postfix mail server is getting better and stronger each day. Adding anti-virus is a critical step in the grand scheme of Postfix things. In our next addition to the Postfix series, we will add Spamassassin for anti-spam measures.

In this article you are going to learn how to install Postfix on your already running Ubuntu Server 9.04. I am going to assume you have it networked and have a domain registered that you want to use. At the end of this article you should have a working mail server, ready for use.

NOTE: For the purposes of this article, I will use the fake domain “www.ubuntumail.net”. You will want to use your domain in place of that.

The first step is to install the software necessary. Since you already have LAMP up and running all you will need to do is install Postfix and it’s dependencies. To do this open up a terminal and issue the following command:

sudo apt-get install postfix

The installation will not only install Postfix, but will start the Postfix daemon for you. You will most likely want to test this installation by running the old telnet test like so:

telnet localhost 25

You should see something like:
Trying 127.0.0.1...
Connected to www.ubuntumail.net.
Escape character is '^]'.
220 localhost.localdomain ESMTP Postfix (Ubuntu)

If you do, success! Now you are ready to take care of the configuration steps.

Configuration

There is only one file you need to deal with in Postfix. That file is /etc/postfix/main.cf. There really isn’t much to configuring this file (for a basic installation – which is all we are dealing with right now).

To set your Postfix installation up for your domain you will want to open that file for editing like so:

sudo nano /etc/postfix/main.cf

The above command opens main.cf in the Nano editor. What you need to look for is this section:

myhostname = ubuntumail
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = ubuntumail.net, ubuntumail, localhost.localdomain, localhost
relayhost =
#mynetworks = 127.0.0.0/8
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all

As you can see, I have already configured the above section to work with our sample domain. You will need to go through the above section and insert your domain where you see ubuntumail.net. You will want to pay close attention to the mydestination line. As you can see that line contains:

ubuntumail.net, ubuntumail, localhost.localdomain

This helps to avoid mailloops in Postfix.

Some people also like to use the mydomain parameter, but since mydomain is taken from $myhostname by removing the first part (unless that would cause the domain to be a top-level domain) it can be redundant. If, however, your mail server serves your entire domain, you will need to use the mydomain parameter. For that you would insert:

mydomain = ubuntumail.net

at the top of the section shown above.

Once you have that configuration saved, restart Postfix with the command:

sudo /etc/init.d/postfix reload

To reload the mail server.

Users

As this is a Linux box, you will want to make sure you have a user created for every email address you need to serve up. So create your users with your favorite user creation tool. Since you are most likely installing on a Ubuntu Server (sans GUI), you will be using the useradd command. You can accomplish this with the command:

sudo useradd -m USERNAME

Where USERNAME is the actual username you want to use. Next you will need to give the user a password with the command:

sudo passwd USERNAME

Where USERNAME is the actual username you want to use. You will be prompted to enter the password twice.

After you have entered the users, you can then test the mail server by sending an email from an external source to see if it arrives. If there is a problem make sure the first place you look is /var/log/mail.err.

Final thoughts

At this point you should have a basic, working mail server. Postfix does make this task much more simple than did Sendmail. In my next article I will cover adding Spamassassin to your Postfix mail server to keep SPAM from getting in.

MTAs (Mail Transfer Agents) can be a tricky lot to set up. Add in to the mix something like relaying and you have all the ingredients for an installation NOT working. Fortunately Postfix isn’t nearly as complicated as Sendmail, so relaying isn’t something you need guru-level access to solve. In this tutorial I am going to show you how to set up Postfix for easy relaying. You can always add this to the Ubuntu Server series that started with the article “Installing Ubuntu Server 9.04“.

Installing Postfix

Just in case you haven’t already, you can install Postfix on your already running Ubuntu Server install with the command:

sudo apt-get install postfix

But I am going to assume you already have Postfix up and running already. And if you’re looking into relaying with Postfix, your problem is that you can receive mail, you just can’t send it. Let’s fix that problem.

Configure Postfix

The file you need to first take care of is /etc/postfix/main.cf. You will need sudo access so open this file with the command:

sudo nano /etc/postfix/main.cf

Now add this to the bottom of that file:

# SMTP Authentication
smtp_sasl_auth_enable = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination smtp_sasl_password_maps = hash:/etc/postfix/sasl/sasl_passwd

NOTE: The portion starting with “smtpd_recipient_restrictions” and ending with “reject_unauth_destination” is actually one line.

NOTE: The portion starting with “reject_unauth” and ending with “sasl_passwd” is also only one line.

NOTE: The above assumes you are using Dovecot as your IMAP server.

The last line of the above should give you a hint as to what is next. Yes, you have to create a file, sasl_passwd, for Postfix to use as a password file. This file will be located in /etc/postfix/sasl/ and will look like the following:

address.for.relaying   username:password

Where:

• address.for.relaying is the actual mail server you will use for relaying.
• username is the username for authentication on the relaying mail server.
• password is the password for authentication on the relaying mail server.

The next step is to create the the sasl_passwd DB file so that Postfix has something it can read. To do this, issue the command:

sudo postmap hash:/etc/postfix/sasl/sasl_passwd

You should now see a new file in the /etc/postfix/sasl directory called sasl_passwd.db.

One file step before restarting Postfix. Because the password file is plaintext it should be secured so that it can not be read by just any user. Say, for instance, you want Postfix to only be read by the root user. For this issue the command:

sudo chown root:root /etc/postfix/sasl/sasl_passwd && chmod 600 /etc/postfix/sasl/sasl_passwd

Do the same thing with the sasl_passwd.db file like so:

sudo chown root:root /etc/postfix/sasl/sasl_passwd.db && chmod 600 /etc/postfix/sasl/sasl_passwd.db

NOTE: If you have need for a different user, you can chown the file to whatever user you want…just make sure it is a secure user.

You’re finished. All that is left is to restart Postfix with the command:

sudo /etc/init.d/postfix restart

Open up your mail client and text to see if relaying is now working for you.

Final thoughts

Gone are the days of the overly complicated (and insecure) Sendmail configurations. Setting up relaying is simple when you are using Postfix.