an effective anti-spam with milter - milter manager

Install to Debian

Install to Debian — How to install milter manager to Debian GNU/Linux

About this document

This document describes how to install milter manager to Debian GNU/Linux. See Install for general install information.

Install packages

Packages for stretch, the current stable release, for buster, the current testing release are distributed.

We put the following content to /etc/apt/sources.list.d/milter-manager.list:

% curl -s https://packagecloud.io/install/repositories/milter-manager/repos/script.deb.sh | sudo bash

See also: <URL:https://packagecloud.io/milter-manager/repos/install>

For sid

NOTE: packagecloud doesn't support sid for now.

/etc/apt/sources.list.d/milter-manager.list:

deb http://downloads.sourceforge.net/project/milter-manager/debian/stable unstable main
deb-src http://downloads.sourceforge.net/project/milter-manager/debian/stable unstable main

Install

We install milter manager package:

% sudo aptitude update
% sudo aptitude -V -D -y install milter-manager

We use Postfix as MTA:

% sudo aptitude -V -D -y install postfix

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

% sudo aptitude -V -D -y install spamass-milter clamav-milter milter-greylist

Configuration

Here is a basic configuration policy.

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

We set read/write permission for 'postfix' group to UNIX domain socket because existing milter packages' configuration can be used.

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

At first, we configure spamd.

We add the following configuration to /etc/spamassassin/local.cf. This configuration is for adding headers only if spam detected.

report_safe 0

remove_header ham Status
remove_header ham Level

We change /etc/default/spamassassin like the following to enable spamd:

Before:

ENABLED=0

After:

ENABLED=1

spamd should be started:

% sudo /etc/init.d/spamassassin start

There are no changes for spamass-milter's configuration.


Configure clamav-milter

We don't need to change the default clamav-milter's configuration.


Configure milter-greylist

We change /etc/milter-greylist/greylist.conf for the following configurations:

  • 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:

racl whitelist default

After:

subnetmatch /24
greylist 10m
autowhite 1w
racl greylist default

We change /etc/default/milter-greylist to enable milter-greylist. milter-greylist uses IPv4 socket because milter-greylist's run script doesn't support changing socket's group permission:

Before:

ENABLED=0

After:

ENABLED=1
SOCKET="inet:11125@[127.0.0.1]"

milter-greylist should be started:

% sudo /etc/init.d/milter-greylist start

Configure milter-manager

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

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

The following output shows milters are detected:

...
define_milter("milter-greylist") do |milter|
  milter.connection_spec = "inet:11125@[127.0.0.1]"
  ...
  milter.enabled = true
  ...
end
..
define_milter("clamav-milter") do |milter|
  milter.connection_spec = "unix:/var/run/clamav/clamav-milter.ctl"
  ...
  milter.enabled = true
  ...
end
..
define_milter("spamass-milter") do |milter|
  milter.connection_spec = "unix:/var/spool/postfix/spamass/spamass.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 /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.

We change /etc/default/milter-manager to work with Postfix:

Before:

# For postfix, you might want these settings:
# SOCKET_GROUP=postfix
# CONNECTION_SPEC=unix:/var/spool/postfix/milter-manager/milter-manager.sock

After:

# For postfix, you might want these settings:
SOCKET_GROUP=postfix
CONNECTION_SPEC=unix:/var/spool/postfix/milter-manager/milter-manager.sock

We create a directory for milter-manager's socket:

% sudo mkdir -p /var/spool/postfix/milter-manager/

We add milter-manager user to postfix group:

% sudo adduser milter-manager postfix

milter-manager's configuration is completed. We start milter-manager:

% sudo /etc/init.d/milter-manager restart

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

% sudo -u postfix milter-test-server -s unix:/var/spool/postfix/milter-manager/milter-manager.sock

Here is a sample success output:

status: accept
elapsed-time: 0.128 seconds

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

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

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

We can add the following configuration to /etc/default/milter-manager to output verbose log to standard output:

OPTION_ARGS="--verbose --no-daemon"

We start milter-manager again:

% sudo /etc/init.d/milter-manager restart

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

OPTION_ARGS configuration in /etc/default/milter-manager should be commented out after the problem is solved to run milter-manager as daemon process. And we should restart milter-manager.


Configure Postfix

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

milter_default_action = accept
milter_protocol = 6
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 information 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 need to add the following configuration to /etc/postfix/main.cf. Note that Postfix chrooted to /var/spool/postfix/.

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

We reload Postfix configuration:

% sudo /etc/init.d/postfix reload

Postfix's milter configuration is completed.

milter-manager logs to syslog. If milter-manager works well, some logs can be shown in /var/log/mail.info. 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 socket of milter-manager. They are detected automatically. We doesn't need to take care some small mistakes like typo.

milter-manager also supports ENABLED configuration used in /etc/default/milter-greylist. If we disable a milter, we use the following steps:

% sudo /etc/init.d/milter-greylist stop
% sudo vim /etc/default/milter-greylist # ENABLED=1 => ENABLED=0

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

% sudo /etc/init.d/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 maintenance cost by introducing milter-manager if we use some milters on Debian GNU/Linux.

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 Debian (optional) .