an effective anti-spam with milter - milter manager

Install to FreeBSD

Install to FreeBSD — How to install milter manager to FreeBSD

About this document

This document describes how to install milter manager to FreeBSD. See Install for general install information.

This document assumes that FreeBSD 10.0-RELEASE is used.

Install packages

To install the following packages, related packages are also installed:

We use Postfix as MTA:

% sudo pkg install --yes postfix

We use spamass-milter, clamav-milter and milter-greylist as milters:

% sudo pkg install --yes spamass-milter mitler-greylist clamav-milter

Install milter-manager.

% sudo pkg install --yes milter-manager

Configuration

Here is a basic configuration policy.

We use UNIX domain socket for accepting connection from MTA because security and speed.

We use general user for milter's effective user. This is also for security. 'mail' group has permission of read/write UNIX domain socket. 'postfix' user is joined to 'mail' group.

milter-greylist should be applied only if S25R condition is matched to reduce needless delivery delay. But the configuration is automatically done by milter-manager. We need to do nothing for it.

Configure spamass-milter

First, we configure spamd.

We create /usr/local/etc/mail/spamassassin/local.cf with the following configuration. This configuration is for adding headers only if spam detected.

remove_header ham Status
remove_header ham Level

We need to append the following to /etc/rc.conf to enable spamd:

spamd_enable=YES

If our SMTP server has many concurrent connections, we should increase max concurrent connections. It is 5 by default. It's a good first value that about 1/3 of the max SMTP connections. e.g. about 30 for about 100 connections SMTP server:

spamd_flags="-c --max-children=30 "

We can adjust apposite value after operation. We can see milter manager's statistics graphs at the time.

Update Spamassassin's rule file and start spamd:

% sudo sa-update
% sudo /usr/sbin/service sa-spamd start

Next, we configure spamass-milter. We run spamass-milter with 'spamd' user and 'spamd' group.

spamass-milter creates a socket file as /var/run/spamass-milter.sock by default. But a general user can't create a new file in /var/run/. We create /var/run/spamass-milter/ directory owned by 'spamd' user. spamass-milter creates a socket file in the directory:

% sudo mkdir /var/run/spamass-milter/
% sudo /usr/sbin/chown spamd:spamd /var/run/spamass-milter

We add the following to /etc/rc.conf:

spamass_milter_enable="YES"
spamass_milter_user="spamd"
spamass_milter_group="spamd"
spamass_milter_socket="/var/run/spamass-milter/spamass-milter.sock"
spamass_milter_socket_owner="spamd"
spamass_milter_socket_group="mail"
spamass_milter_socket_mode="660"
spamass_milter_localflags="-u spamd -- -u spamd"

spamass-milter should be started:

% sudo /usr/sbin/service spamass-milter start

Configure clamav-milter

First, we configure ClamAV.

We add the following to /etc/rc.conf to enable clamd and freshclam:

clamav_clamd_enable="YES"
clamav_freshclam_enable="YES"

Get the latest definition files before run clamd:

% sudo /usr/local/bin/freshcram

clamd and freshclam should be started:

% sudo /usr/sbin/service clamav-clamd start
% sudo /usr/sbin/service clamav-freshclam start

clamav-milter is ran as 'clamav' user and 'clamav' group by default. We use the configuration because 'clamav' user is general user. We set group read/write permission of socket.

We add the following to /etc/rc.conf:

clamav_milter_enable="YES"
clamav_milter_socket_mode="660"
clamav_milter_socket_group="mail"

We may need to configure /usr/local/etc/clamav-milter.conf. e.g.:

/usr/local/etc/clamav-milter.conf

Before:

#OnInfected Quarantine
#AddHeader Replace
#LogSyslog yes
#LogFacility LOG_MAIL
#LogInfected Basic

After:

OnInfected Reject
AddHeader Replace
LogSyslog yes
LogFacility LOG_MAIL
LogInfected Full

Here are explanations of the above configurations:

OnInfected Reject

Rejects infected mails. The default value is Quarantine. It puts infected mails into Postfix's hold queue. If we don't want to confirm hold queue periodically, Reject is a good way for easy maintenance.

AddHeader Replace

Replaces X-Virus-Scanned header even if it's existed.

LogSyslog yes

Logs to syslog.

LogFacility LOG_MAIL

Logs to syslog with LOG_MAIL facility. /var/log/maillog is the default LOG_MAIL log file.

LogInfected Full

Logs verbosity on finding infected mails.

clamav-milter should be started:

% sudo /usr/sbin/service clamav-milter start

Configure milter-greylist

We run milter-greylist as 'mailnull' user and 'mailnull' group. 'mailnull' user is the default configuration and it is unused user on Postfix environment.

We copy /usr/local/etc/mail/greylist.conf.sample to /usr/local/etc/mail/greylist.conf and change it for the following configurations:

  • change the effective group to "mail" group from "mailnull" group.

  • add write permission for the socket file to "mail" group.

  • use the leading 24bits for IP address match to avoid Greylist adverse effect for sender uses some MTA case.

  • decrease retransmit check time to 10 minutes from 30 minutes (default value) to avoid Greylist adverse effect.

  • increase auto whitelist period to a week from 1 day (default value) to avoid Greylist adverse effect.

  • use Greylist by default.

The configuration relaxes Greylist check to avoid Greylist adverse effect. It increases received spam mails but we should give priority to avoid false positive rather than false negative. We should not consider that we blocks all spam mails by Greylist. We can blocks spam mails that isn't blocked by Greylist by other anti-spam technique such as SpamAssassin. milter manager helps constructing mail system that combines some anti-spam techniques.

Before:

socket "/var/milter-greylist/milter-greylist.sock"
user "mailnull:mailnull"
racl whitelist default

After:

socket "/var/milter-greylist/milter-greylist.sock" 660
user "mailnull:mail"
subnetmatch /24
greylist 10m
autowhite 1w
racl greylist default

We add the following to /etc/rc.conf:

miltergreylist_enable="YES"
miltergreylist_runas="mailnull:mail"

milter-greylist should be started:

% sudo /usr/sbin/service milter-greylist start

Configure milter-manager

We create 'milter-manager' user because we run milter-manager as 'milter-manager' user:

% sudo /usr/sbin/pw groupadd milter-manager
% sudo /usr/sbin/pw useradd milter-manager -g milter-manager -G mail -m

milter-manager detects milters that installed in system. We can confirm spamass-milter, clamav-milter and milter-greylist are detected:

% sudo /usr/local/sbin/milter-manager -u milter-manager --show-config

The following output shows milters are detected:

...
define_milter("milter-greylist") do |milter|
  milter.connection_spec = "unix:/var/milter-greylist/milter-greylist.sock"
  ...
  milter.enabled = true
  ...
end
..
define_milter("clamav-milter") do |milter|
  milter.connection_spec = "unix:/var/run/clamav/clmilter.sock"
  ...
  milter.enabled = true
  ...
end
..
define_milter("spamass-milter") do |milter|
  milter.connection_spec = "unix:/var/run/spamass-milter/spamass-milter.sock"
  ...
  milter.enabled = true
  ...
end
..

We should confirm that milter's name, socket path and 'enabled = true'. If the values are unexpected, we need to change /usr/local/etc/milter-manager/milter-manager.conf. See Configuration for details of milter-manager.conf.

But if we can, we want to use milter manager without editing miter-manager.conf. If you report your environment to the milter manager project, the milter manager project may improve detect method.

milter-manager creates socket file as /var/run/milter-manager/milter-manager.sock by default on FreeBSD. We need to create /var/run/milter-manager directory before running milter-manager:

% sudo mkdir -p /var/run/milter-manager
% sudo /usr/sbin/chown -R milter-manager:milter-manager /var/run/milter-manager

milter-manager's configuration is completed. We start to setup running milter-manager.

We add the following to /etc/rc.conf to enable milter-manager:

miltermanager_enable="YES"

milter-manager should be started:

% sudo /usr/sbin/service milter-manager start

/usr/local/bin/milter-test-server is usuful to confirm milter-manager was ran:

% sudo -u mailnull milter-test-server -s unix:/var/run/milter-manager/milter-manager.sock

Here is a sample success output:

status: pass
elapsed-time: 0.128 seconds

If milter-manager fails to run, the following message will be shown:

Failed to connect to unix:/var/run/milter-manager/milter-manager.sock: No such file or directory

In this case, we can use log to solve the problem. milter-manager is verbosily if --verbose option is specified. milter-manager outputs logs to standard output if milter-manager isn't daemon process.

We add the following to /etc/rc.conf to output verbose log to standard output:

miltermanager_debug="YES"

milter-manager should be started:

% sudo /usr/sbin/service milter-manager restart

Some logs are output if there is a problem. Running milter-manager can be exitted by Ctrl+c.

milter_manager_debug configuration in /etc/rc.conf should be commented out after the problem is solved to run milter-manager as daemon process. And milter-manager should restarted.


Configure Postfix

We add 'postfix' user to 'mail' group:

% sudo /usr/sbin/pw groupmod mail -m postfix

We start milter's configuration.

We add the following milter configuration to /usr/local/etc/postfix/main.cf:

milter_protocol = 6
milter_default_action = accept
milter_mail_macros = {auth_author} {auth_type} {auth_authen}

Here are descriptions of the configuration.

milter_protocol = 6

Postfix uses milter protocol version 6.

milter_default_action = accept

Postfix accepts a mail if Postfix can't connect to milter. It's useful configuration for not stopping mail server function if milter has some problems. But it causes some problems that spam mails and virus mails may be delivered until milter is recovered.

If you can recover milter, 'tempfail' will be better choice rather than 'accept'. Default is 'tempfail'.

milter_mail_macros = {auth_author} {auth_type} {auth_authen}

Postfix passes SMTP Auth related infomation to milter. Some milters like milter-greylist use it.

We need to register milter-manager to Postfix. It's important that spamass-milter, clamav-milter, milter-greylist aren't needed to be registered because they are used via milter-manager.

We add the following to /usr/local/etc/postfix/main.cf:

smtpd_milters = unix:/var/run/milter-manager/milter-manager.sock

Postfix should reload its configuration:

% sudo /usr/sbin/service postfix reload

Postfix's milter configuration is completed.

milter-manager logs to syslog. If milter-manager works well, some logs can be showen in /var/log/maillog. We need to sent a test mail for confirming.

Conclusion

There are many configurations to work milter and Postfix together. They can be reduced by introducing milter-manager.

Without milter-manager, we need to specify sockets of spamass-milter, clamav-milter and milter-greylist to smtpd_milters. With milter-manager, we doesn't need to specify sockets of them, just specify a coket of milter-manager. They are detected automatically. We doesn't need to take care some small mistakes like typo.

milter-manager also supports xxx_enabled="NO" configuration used in /etc/rc.conf. If we disable a milter, we use the following steps:

% sudo /usr/sbin/service XXX stop
% sudo vim /etc/rc.conf # XXX_enabled="YES" => XXX_enabled="NO"

We need to reload milter-manager after we disable a milter.

% sudo /usr/sbin/service milter-manager reload

milter-manager detects a milter is disabled and doesn't use it. We doesn't need to change Postfix's main.cf.

We can reduce maintainance cost by introducing milter-manager if we use some milters on FreeBSD.

milter manager also provides tools to help operation. Installing them is optional but we can reduce operation cost too. If we also install them, we will go to Install to FreeBSD (optional) .