How to Add Antivirus and Spam Protection to Postfix Mail Server with ClamAV and SpamAssassin – Part 3

In the previous two articles of this Postfix series you learned how to set up and manage the email server database through phpMyAdmin, and how to configure Postfix and Dovecot to handle incoming and outgoing mail. In addition, we explained how to set up a mail client, such as Thunderbird, for the virtual accounts we created previously.

  1. Setup Postfix Mail Server and Dovecot with MariaDB – Part 1
  2. How to Configure Postfix and Dovecot with Virtual Domain Users – Part 2
  3. Install and Configure RoundCube Webmail Client with Virtual Users in Postfix – Part 4
  4. Use Sagator, an Antivirus/Antispam Gateway to Protect Your Mail Server – Part 5

Since no email server setup can be complete without taking precautions against viruses and spam, we are going to cover that topic in the current article.

Integrate ClamAV and SpamAssassin to Protect Postfix

Integrate ClamAV and SpamAssassin to Protect Postfix

Please keep in mind that even when *nix-like operating systems are usually considered to be virus-free, chances are clients using other operating systems will also connect to your email server.

For that reason, you need to provide them with the confidence that you have taken the necessary measures to protect them to the extent possible from such threats.

Configuring SpamAssassin for Postfix

In the process of receiving email, spamassassin will stand between the outside world and the email services running on your server itself. If it finds, according to its definition rules and configuration, that an incoming message is spam, it will rewrite the subject line to clearly identify it as such. Let’s see how.

The main configuration file is /etc/mail/spamassassin/local.cf, and we should make sure the following options are available (add them if they are not present or uncomment if necessary):

local.cf
report_safe 0
required_score 8.0
rewrite_header Subject [SPAM]
  1. When report_safe is set to 0 (recommended value), incoming spam is only modified by modifying the email headers as per rewrite_header. If it is set to 1, the message will be deleted.
  2. To set the aggressivity of the spam filter, required_score must be followed by an integer or decimal number. The lesser the number, the more sensitive the filter becomes. Setting required_score to a value somewhere between 8.0 and 10.0 is recommended for a large system serving many (~100s) email accounts.

Once you’ve saved those changes, enable and start the spam filter service, and then update the spam rules:

# systemctl enable spamassassin
# systemctl start spamassassin
# sa-update

For more configuration options, you may want to refer to the documentation by running perldoc Mail::SpamAssassin::Conf in the command line.

Integrating Postfix and SpamAssassin

In order to efficiently integrate Postfix and spamassassin, we will need to create a dedicated user and group to run the spam filter daemon:

# useradd spamd -s /bin/false -d /var/log/spamassassin

Next, add the following line at the bottom of /etc/postfix/master.cf:

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

And indicate (at the top) that spamassassin will serve as content_filter:

-o content_filter=spamassassin
Integrate Postfix with SpamAssassin

Integrate Postfix with SpamAssassin

Finally, restart Postfix to apply changes:

# systemctl restart postfix

To verify that SpamAssassin is working properly and detecting incoming spam, a test known as GTUBE (Generic Test for Unsolicited Bulk Email) is provided.

To perform this test, send an email from a domain outside your network (such as Yahoo!, Hotmail, or Gmail) to an account residing in your email server. Set the Subject line to whatever you want and include the following text in the message body:

XJS*C4JDBQADN1.NSBN3*2IDNEN*GTUBE-STANDARD-ANTI-UBE-TEST-EMAIL*C.34X

For example, sending the above text in a message body from my Gmail account produces the following result:

Verify SpamAssassin Detecting Spam Mails

Verify SpamAssassin Detecting Spam Mails

And shows the corresponding notice in the logs:

# journalctl | grep spam
Monitor SpamAssassin Mail Logs

Monitor SpamAssassin Mail Logs

As you can see in the image above, this email message got a spam score of 1002.3. Additionally, you can test spamassassin right from the command line:

# spamassassin -D < /usr/share/doc/spamassassin-3.4.0/sample-spam.txt

The above command will produce some really verbose output that should include the following:

Test SpamAssassin Spam from Commandline

Test SpamAssassin Spam from Commandline

If these tests are not successful, you may want to refer to the spamassassin integrations guide.

Starting ClamAV and Update Virus Definitions

To begin, we will need to edit /etc/clamd.d/scan.conf. Uncomment the following line:

LocalSocket /var/run/clamd.scan/clamd.sock

and comment out or delete the line:

Example

Then enable and start the clamav scanner daemon:

# systemctl enable [email protected]
# systemctl start [email protected]

and don’t forget to set the antivirus_can_scan_system SELinux boolean to 1:

# setsebool -P antivirus_can_scan_system 1

At this point it is worth and well to check on the service’s status:

Start and Update ClamAV Virus Definitions

Start and Update ClamAV Virus Definitions

As you can see in the image above, our virus signatures are older than 7 days. To update them we will use a tool called freshclam that was installed as part of the clamav-update package.

The easiest way to update the virus definitions is through a cron job that executes as often as desired (once a day for example, at 1 am server time as indicated in the following example is considered enough):

00 01 * * * root /usr/share/clamav/freshclam-sleep

You can also update the virus definitions manually, but before you’ll also have to remove or comment out the following line in /etc/freshclam.conf.

Example

Now you should be able to run:

# freshclam

which will update the virus definitions as desired:

FreshClam Update ClamAV Virus Database

FreshClam Update ClamAV Virus Database

Testing ClamAV for Virus in Emails

To verify ClamAV is working properly, let’s download a test virus (which we can get from http://www.eicar.org/download/eicar.com) to the Maildir of [email protected] (which is located in /home/vmail/linuxnewz.com/tecmint/Maildir) to simulate an infected file received as a mail attachment:

# cd /home/vmail/linuxnewz.com/tecmint/Maildir
# wget http://www.eicar.org/download/eicar.com

And then scan the /home/vmail/linuxnewz.com directory recursively:

# clamscan --infected --remove --recursive /home/vmail/linuxnewz.com
ClamAV Scan For Email Virus

ClamAV Scan For Email Virus

Now, feel free to set up this scan to run through a cronjob. Create a file named /etc/cron.daily/dailyclamscan, insert the following lines:

#!/bin/bash
SCAN_DIR="/home/vmail/linuxnewz.com"
LOG_FILE="/var/log/clamav/dailyclamscan.log"
touch $LOG_FILE
/usr/bin/clamscan --infected --remove --recursive $SCAN_DIR >> $LOG_FILE

and grant execute permissions:

# chmod +x /etc/cron.daily/dailyclamscan

The above cronjob will scan the mail server directory recursively and leave a log of its operation in /var/log/clamav/dailyclamscan.log (make sure the /var/log/clamav directory exists).

Let’s see what happens when we send the eicar.com file from [email protected] to [email protected]:

Test and Find Virus in Emails

Test and Find Virus in Emails

Summary

If you followed the steps outlined in this tutorial and in the previous two articles of this series, you now have a working Postfix email server with spam and antivirus protection.

DISCLAIMER: Please note that server security is a vast subject and cannot be adequately covered in a short series like this.

For that reason, I highly encourage you to become familiar with the tools used in this series and their man pages. Although I have done my best to cover the essential concepts associated with this topic, do not assume that after going through this series you are fully qualified to set up and maintain a email server in a production environment.

This series is intended as a starting point and not as an exhaustive guide to mail server administration in Linux.

You will probably think of other ideas that can enrich this series. If so, feel free to drop us a note using the comment form below. Questions and other suggestions are appreciated as well – we look forward to hearing from you!

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Gabriel Cánepa

Gabriel Cánepa is a GNU/Linux sysadmin and web developer from Villa Mercedes, San Luis, Argentina. He works for a worldwide leading consumer product company and takes great pleasure in using FOSS tools to increase productivity in all areas of his daily work.

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12 Responses

  1. Bruce says:

    Hello,

    I have a working system except for Spamassassin. All the tests you mention work except sending from an outside domain to my server. Here’s my master.cf… How would I move messages to JUNK when it is working… Bruce

    # Postfix master process configuration file. For details on the format
    # of the file, see the master(5) manual page (command: “man 5 master”).
    #
    # Do not forget to execute “postfix reload” after editing this file.
    #
    # ==========================================================================
    # service type private unpriv chroot wakeup maxproc command + args
    # (yes) (yes) (yes) (never) (100)
    # ==========================================================================
    smtp inet n – n – – smtpd -o content_filter=spamassassin
    #smtp inet n – n – 1 postscreen
    #smtpd pass – – n – – smtpd
    #dnsblog unix – – n – 0 dnsblog
    #tlsproxy unix – – n – 0 tlsproxy
    submission inet n – n – – smtpd
    -o syslog_name=postfix/submission
    -o smtpd_tls_security_level=encrypt
    -o smtpd_sasl_auth_enable=yes
    -o smtpd_reject_unlisted_recipient=no
    # -o smtpd_client_restrictions=$mua_client_restrictions
    # -o smtpd_helo_restrictions=$mua_helo_restrictions
    # -o smtpd_sender_restrictions=$mua_sender_restrictions
    -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
    -o milter_macro_daemon_name=ORIGINATING
    smtps inet n – n – – smtpd
    -o syslog_name=postfix/smtps
    -o smtpd_tls_wrappermode=yes
    -o smtpd_sasl_auth_enable=yes
    -o smtpd_reject_unlisted_recipient=no
    # -o smtpd_client_restrictions=$mua_client_restrictions
    # -o smtpd_helo_restrictions=$mua_helo_restrictions
    # -o smtpd_sender_restrictions=$mua_sender_restrictions
    # -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
    # -o milter_macro_daemon_name=ORIGINATING
    #628 inet n – n – – qmqpd
    pickup unix n – n 60 1 pickup
    cleanup unix n – n – 0 cleanup
    qmgr unix n – n 300 1 qmgr
    #qmgr unix n – n 300 1 oqmgr
    tlsmgr unix – – n 1000? 1 tlsmgr
    rewrite unix – – n – – trivial-rewrite
    bounce unix – – n – 0 bounce
    defer unix – – n – 0 bounce
    trace unix – – n – 0 bounce
    verify unix – – n – 1 verify
    flush unix n – n 1000? 0 flush
    proxymap unix – – n – – proxymap
    proxywrite unix – – n – 1 proxymap
    smtp unix – – n – – smtp
    relay unix – – n – – smtp
    # -o smtp_helo_timeout=5 -o smtp_connect_timeout=5
    showq unix n – n – – showq
    error unix – – n – – error
    retry unix – – n – – error
    discard unix – – n – – discard
    local unix – n n – – local
    virtual unix – n n – – virtual
    lmtp unix – – n – – lmtp
    anvil unix – – n – 1 anvil
    scache unix – – n – 1 scache
    dovecot unix – n n – – pipe
    flags=DRhu user=vmail:vmail argv=/usr/libexec/dovecot/deliver -f ${sender} -d ${recipient}
    spamassassin unix – n n – – pipe flags=R user=spamd argv=/usr/bin/spamc -f -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}

  2. Sergio says:

    I am using Debian and when I try to use the systemctl enable [email protected] and the start one, shows me an error saying no such file or directory, are these commands just for Ubuntu? Or they should be working on Debian too? Thanks.

    • Gabriel A. Cánepa says:

      Hi Sergio,
      Which version of Debian are you using? Please note that systemctl is part of the systemd suite, which is available starting with Debian 8 Jessie.
      If you are sure you’re using Jessie, you can check how these services are called by doing
      systemctl | grep clam
      Hope it helps.

  3. Peter says:

    Is there a way to make all the spam automatically go to the users junk-mail folder?

    On workaround.org that function is implemented, but the guides isn’t identically so I don’t think I can use the configuration from that site.

    Some script that delete everything from all junk-folders that are older than 1 month or so would also be nice.

  4. Harmon20 says:

    Spamassassin went fine but I’m having trouble with clamav. I tried removing all (they were installed in Part 1 of this guide), including config files and backups, then installing from scratch and following these instructions precisely. Got the same results and my Google-fu is failing me. CentOS 7.2.1511 and ClamAV 0.99.1

    Everything went according to the script but the service won’t start. The output of the status query is:

    [[email protected] /]# systemctl -l status [email protected]
    [email protected] – Generic clamav scanner daemon
    Loaded: loaded (/usr/lib/systemd/system/[email protected]; enabled; vendor preset: disabled)
    Active: failed (Result: start-limit) since Wed 2016-06-01 15:32:40 EDT; 4min 19s ago
    Process: 2528 ExecStart=/usr/sbin/clamd -c /etc/clamd.d/%i.conf –nofork=yes (code=exited, status=1/FAILURE)
    Main PID: 2528 (code=exited, status=1/FAILURE)

    Jun 01 15:32:40 mx systemd[1]: [email protected]: main process exited, code=exited, status=1/FAILURE
    Jun 01 15:32:40 mx systemd[1]: Unit [email protected] entered failed state.
    Jun 01 15:32:40 mx systemd[1]: [email protected] failed.
    Jun 01 15:32:40 mx systemd[1]: [email protected] holdoff time over, scheduling restart.
    Jun 01 15:32:40 mx systemd[1]: start request repeated too quickly for [email protected]
    Jun 01 15:32:40 mx systemd[1]: Failed to start Generic clamav scanner daemon.
    Jun 01 15:32:40 mx systemd[1]: Unit [email protected] entered failed state.
    Jun 01 15:32:40 mx systemd[1]: [email protected] failed.
    [[email protected] /]#

    Any ideas what I might be missing?

    • Harmon20 says:

      additional info: In case this was a problem with me trying to manually start the service I rebooted the server and checked the service status after it came back up. Same output.

      • @Harmon20, we’re almost there, brother :). Hang in there.
        Can you paste the output of journalctl -xn RIGHT AFTER attempting to manually start clamd? We are mostly interested in lines containing the word ERROR in them.

  5. ura soul says:

    if the clamav scan only runs once a day, won’t that mean that new messages that are infected will be available for download for up to 1 day? so therefore, many of them will get through to mail clients?

    • @ura,
      What runs once a day is the cron job that will update the virus definitions. On the other hand, the antivirus service itself should be running as a daemon at all times. That is what will protect your new messages.
      Of course, it is possible -though highly unlikely- that a message (infected with a virus released in the recent past, whose signature has not yet been identified by major antivirus engines) finds its way to your mailbox. With virus and antivirus you can be as paranoid as with backups. A solid knowledge of the threats your environment faces will give you the last word as to how paranoid you need to be about it.

      • Ivan says:

        Hello Gabriel,

        First of all, let me thank you for the great series of articles. I’m following along, and so far so good :)

        regarding the above question, I thing @ura souls was referring to this part:

        “The above cronjob will scan the mail server directory recursively and leave a log of its operation in /var/log/clamav/dailyclamscan.log”

        I understood as he did. Also, I find that updating is by default, but what needs to be done is to remove this four lines:

        ### !!!!! REMOVE ME !!!!!!
        ### REMOVE ME: By default, the freshclam update is disabled to avoid
        ### REMOVE ME: network access without prior activation
        FRESHCLAM_DELAY=disabled-warn # REMOVE ME

        from vi /etc/sysconfig/freshclam.

        If you can comment this, it would be really helpful.

        All the best

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