Postfix Basic Configuration

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Introduction

Postfix has several hundred configuration parameters that are controlled via
the main.cf file. Fortunately, all parameters have sensible default values. In
many cases, you need to configure only two or three parameters before you can
start to play with the mail system. Here's a quick introduction to the syntax:

  * Postfix configuration files

The text below assumes that you already have Postfix installed on the system,
either by compiling the source code yourself (as described in the INSTALL file)
or by installing an already compiled version.

This document covers basic Postfix configuration. Information about how to
configure Postfix for specific applications such as mailhub, firewall or dial-
up client can be found in the STANDARD_CONFIGURATION_README file. But don't go
there until you already have covered the material presented below.

The first parameters of interest specify the machine's identity and role in the
network.

  * What domain name to use in outbound mail

  * What domains to receive mail for

  * What clients to relay mail from

  * What destinations to relay mail to

  * What delivery method: direct or indirect

The default values for many other configuration parameters are derived from
just these.

The next parameter of interest controls the amount of mail sent to the local
postmaster:

  * What trouble to report to the postmaster

Be sure to set the following correctly if you're behind a proxy or network
address translator, and you are running a backup MX host for some other domain:

  * Proxy/NAT external network addresses

Postfix daemon processes run in the background, and log problems and normal
activity to the syslog daemon. Here are a few things that you need to be aware
of:

  * What you need to know about Postfix logging

If your machine has unusual security requirements you may want to run Postfix
daemon processes inside a chroot environment.

  * Running Postfix daemon processes chrooted

If you run Postfix on a virtual network interface, or if your machine runs
other mailers on virtual interfaces, you'll have to look at the other
parameters listed here as well:

  * My own hostname

  * My own domain name

  * My own network addresses

Postfix configuration files

By default, Postfix configuration files are in /etc/postfix. The two most
important files are main.cf and master.cf; these files must be owned by root.
Giving someone else write permission to main.cf or master.cf (or to their
parent directories) means giving root privileges to that person.

In /etc/postfix/main.cf you will have to set up a minimal number of
configuration parameters. Postfix configuration parameters resemble shell
variables, with two important differences: the first one is that Postfix does
not know about quotes like the UNIX shell does.

You specify a configuration parameter as:

    /etc/postfix/main.cf:
        parameter = value

and you use it by putting a "$" character in front of its name:

    /etc/postfix/main.cf:
        other_parameter = $parameter

You can use $parameter before it is given a value (that is the second main
difference with UNIX shell variables). The Postfix configuration language uses
lazy evaluation, and does not look at a parameter value until it is needed at
runtime.

Postfix uses database files for access control, address rewriting and other
purposes. The DATABASE_README file gives an introduction to how Postfix works
with Berkeley DB, LDAP or SQL and other types. Here is a common example of how
Postfix invokes a database:

    /etc/postfix/main.cf:
        virtual_alias_maps = hash:/etc/postfix/virtual

Whenever you make a change to the main.cf or master.cf file, execute the
following command as root in order to refresh a running mail system:

    # postfix reload

What domain name to use in outbound mail

The myorigin parameter specifies the domain that appears in mail that is posted
on this machine. The default is to use the local machine name, $myhostname,
which defaults to the name of the machine. Unless you are running a really
small site, you probably want to change that into $mydomain, which defaults to
the parent domain of the machine name.

For the sake of consistency between sender and recipient addresses, myorigin
also specifies the domain name that is appended to an unqualified recipient
address.

Examples (specify only one of the following):

    /etc/postfix/main.cf:
        myorigin = $myhostname (default: send mail as "user@$myhostname")
        myorigin = $mydomain   (probably desirable: "user@$mydomain")

What domains to receive mail for

The mydestination parameter specifies what domains this machine will deliver
locally, instead of forwarding to another machine. The default is to receive
mail for the machine itself. See the VIRTUAL_README file for how to configure
Postfix for hosted domains.

You can specify zero or more domain names, "/file/name" patterns and/or "type:
table" lookup tables (such as hash:, btree:, nis:, ldap:, or mysql:), separated
by whitespace and/or commas. A "/file/name" pattern is replaced by its
contents; "type:table" requests that a table lookup is done and merely tests
for existence: the lookup result is ignored.

IMPORTANT: If your machine is a mail server for its entire domain, you must
list $mydomain as well.

Example 1: default setting.

    /etc/postfix/main.cf:
        mydestination = $myhostname localhost.$mydomain localhost

Example 2: domain-wide mail server.

    /etc/postfix/main.cf:
        mydestination = $myhostname localhost.$mydomain localhost $mydomain

Example 3: host with multiple DNS A records.

    /etc/postfix/main.cf:
        mydestination = $myhostname localhost.$mydomain localhost
            www.$mydomain ftp.$mydomain

Caution: in order to avoid mail delivery loops, you must list all hostnames of
the machine, including $myhostname, and localhost.$mydomain.

What clients to relay mail from

By default, Postfix will forward mail from clients in authorized network blocks
to any destination. Authorized networks are defined with the mynetworks
configuration parameter. The default is to authorize all clients in the IP
subnetworks that the local machine is attached to.

IMPORTANT: If your machine is connected to a wide area network then your
default mynetworks setting may be too friendly.

Examples (specify only one of the following):

    /etc/postfix/main.cf:
        mynetworks_style = subnet  (default: authorize subnetworks)
        mynetworks_style = host    (safe: authorize local machine only)
        mynetworks = 127.0.0.0/8   (safe: authorize local machine only)
        mynetworks = 127.0.0.0/8 168.100.189.2/32 (authorize local machine)

You can specify the trusted networks in the main.cf file, or you can let
Postfix do the work for you. The default is to let Postfix do the work. The
result depends on the mynetworks_style parameter value.

  * Specify "mynetworks_style = host" when Postfix should forward mail from
    only the local machine.

  * Specify "mynetworks_style = subnet" (the default) when Postfix should
    forward mail from SMTP clients in the same IP subnetworks as the local
    machine. On Linux, this works correctly only with interfaces specified with
    the "ifconfig" command.

  * Specify "mynetworks_style = class" when Postfix should forward mail from
    SMTP clients in the same IP class A/B/C networks as the local machine.
    Don't do this with a dialup site - it would cause Postfix to "trust" your
    entire provider's network. Instead, specify an explicit mynetworks list by
    hand, as described below.

Alternatively, you can specify the mynetworks list by hand, in which case
Postfix ignores the mynetworks_style setting. To specify the list of trusted
networks by hand, specify network blocks in CIDR (network/mask) notation, for
example:

    /etc/postfix/main.cf:
        mynetworks = 168.100.189.0/28, 127.0.0.0/8

You can also specify the absolute pathname of a pattern file instead of listing
the patterns in the main.cf file.

What destinations to relay mail to

By default, Postfix will forward mail from strangers (clients outside
authorized networks) to authorized remote destinations only. Authorized remote
destinations are defined with the relay_domains configuration parameter. The
default is to authorize all domains (and subdomains) of the domains listed with
the mydestination parameter.

Examples (specify only one of the following):

    /etc/postfix/main.cf:
        relay_domains = $mydestination (default)
        relay_domains =           (safe: never forward mail from strangers)
        relay_domains = $mydomain (forward mail to my domain and subdomains)

What delivery method: direct or indirect

By default, Postfix tries to deliver mail directly to the Internet. Depending
on your local conditions this may not be possible or desirable. For example,
your system may be turned off outside office hours, it may be behind a
firewall, or it may be connected via a provider who does not allow direct mail
to the Internet. In those cases you need to configure Postfix to deliver mail
indirectly via a relay host.

Examples (specify only one of the following):

    /etc/postfix/main.cf:
        relayhost =                   (default: direct delivery to Internet)
        relayhost = $mydomain         (deliver via local mailhub)
        relayhost = [mail.$mydomain]  (deliver via local mailhub)
        relayhost = [mail.isp.tld]    (deliver via provider mailhub)

The form enclosed with [] eliminates DNS MX lookups. Don't worry if you don't
know what that means. Just be sure to specify the [] around the mailhub
hostname that your ISP gave to you, otherwise mail may be mis-delivered.

The STANDARD_CONFIGURATION_README file has more hints and tips for firewalled
and/or dial-up networks.

What trouble to report to the postmaster

You should set up a postmaster alias in the aliases(5) table that directs mail
to a human person. The postmaster address is required to exist, so that people
can report mail delivery problems. While you're updating the aliases(5) table,
be sure to direct mail for the super-user to a human person too.

    /etc/aliases:
        postmaster: you
        root: you

Execute the command "newaliases" after changing the aliases file. Instead of /
etc/aliases, your alias file may be located elsewhere. Use the command
"postconf alias_maps" to find out.

The Postfix system reports problems to the postmaster alias. You may not be
interested in all types of trouble reports, so this reporting mechanism is
configurable. The default is to report only serious problems (resource,
software) to postmaster:

Default setting:

    /etc/postfix/main.cf:
        notify_classes = resource, software

The meaning of the classes is as follows:

    bounce
        Inform the postmaster of undeliverable mail. Either send the postmaster
        a copy of undeliverable mail that is returned to the sender, or send a
        transcript of the SMTP session when Postfix rejected mail. For privacy
        reasons, the postmaster copy of undeliverable mail is truncated after
        the original message headers. This implies "2bounce" (see below). See
        also the luser_relay feature. The notification is sent to the address
        specified with the bounce_notice_recipient configuration parameter
        (default: postmaster).
    2bounce
        When Postfix is unable to return undeliverable mail to the sender, send
        it to the postmaster instead (without truncating the message after the
        primary headers). The notification is sent to the address specified
        with the 2bounce_notice_recipient configuration parameter (default:
        postmaster).
    delay
        Inform the postmaster of delayed mail. In this case, the postmaster
        receives message headers only. The notification is sent to the address
        specified with the delay_notice_recipient configuration parameter
        (default: postmaster).
    policy
        Inform the postmaster of client requests that were rejected because of
        (UCE) policy restrictions. The postmaster receives a transcript of the
        SMTP session. The notification is sent to the address specified with
        the error_notice_recipient configuration parameter (default:
        postmaster).
    protocol
        Inform the postmaster of protocol errors (client or server side) or
        attempts by a client to execute unimplemented commands. The postmaster
        receives a transcript of the SMTP session. The notification is sent to
        the address specified with the error_notice_recipient configuration
        parameter (default: postmaster).
    resource
        Inform the postmaster of mail not delivered due to resource problems
        (for example, queue file write errors). The notification is sent to the
        address specified with the error_notice_recipient configuration
        parameter (default: postmaster).
    software
        Inform the postmaster of mail not delivered due to software problems.
        The notification is sent to the address specified with the
        error_notice_recipient configuration parameter (default: postmaster).

Proxy/NAT external network addresses

Some mail servers are connected to the Internet via a network address
translator (NAT) or proxy. This means that systems on the Internet connect to
the address of the NAT or proxy, instead of connecting to the network address
of the mail server. The NAT or proxy forwards the connection to the network
address of the mail server, but Postfix does not know this.

If you run a Postfix server behind a proxy or NAT, you need to configure the
proxy_interfaces parameter and specify all the external proxy or NAT addresses
that Postfix receives mail on. You may specify symbolic hostnames instead of
network addresses.

IMPORTANT: You must specify your proxy/NAT external addresses when your system
is a backup MX host for other domains, otherwise mail delivery loops will
happen when the primary MX host is down.

Example: host behind NAT box running a backup MX host.

    /etc/postfix/main.cf:
        proxy_interfaces = 1.2.3.4 (the proxy/NAT external network address)

What you need to know about Postfix logging

Postfix daemon processes run in the background, and log problems and normal
activity to the syslog daemon. The syslogd process sorts events by class and
severity, and appends them to logfiles. The logging classes, levels and logfile
names are usually specified in /etc/syslog.conf. At the very least you need
something like:

    /etc/syslog.conf:
        mail.err                                    /dev/console
        mail.debug                                  /var/log/maillog

After changing the syslog.conf file, send a "HUP" signal to the syslogd
process.

IMPORTANT: many syslogd implementations will not create files. You must create
files before (re)starting syslogd.

IMPORTANT: on Linux you need to put a "-" character before the pathname, e.g.,
-/var/log/maillog, otherwise the syslogd process will use more system resources
than Postfix.

Hopefully, the number of problems will be small, but it is a good idea to run
every night before the syslog files are rotated:

    # postfix check
    # egrep '(reject|warning|error|fatal|panic):' /some/log/file

  * The first line (postfix check) causes Postfix to report file permission/
    ownership discrepancies.

  * The second line looks for problem reports from the mail software, and
    reports how effective the relay and junk mail access blocks are. This may
    produce a lot of output. You will want to apply some postprocessing to
    eliminate uninteresting information.

The DEBUG_README document describes the meaning of the "warning" etc. labels in
Postfix logging.

Running Postfix daemon processes chrooted

Postfix daemon processes can be configured (via the master.cf file) to run in a
chroot jail. The processes run at a fixed low privilege and with file system
access limited to the Postfix queue directories (/var/spool/postfix). This
provides a significant barrier against intrusion. The barrier is not
impenetrable (chroot limits file system access only), but every little bit
helps.

With the exception of Postfix daemons that deliver mail locally and/or that
execute non-Postfix commands, every Postfix daemon can run chrooted.

Sites with high security requirements should consider to chroot all daemons
that talk to the network: the smtp(8) and smtpd(8) processes, and perhaps also
the lmtp(8) client. The author's own porcupine.org mail server runs all daemons
chrooted that can be chrooted.

The default /etc/postfix/master.cf file specifies that no Postfix daemon runs
chrooted. In order to enable chroot operation, edit the file /etc/postfix/
master.cf, and follow instructions in the file. When you're finished, execute
"postfix reload" to make the change effective.

Note that a chrooted daemon resolves all filenames relative to the Postfix
queue directory (/var/spool/postfix). For successful use of a chroot jail, most
UNIX systems require you to bring in some files or device nodes. The examples/
chroot-setup directory in the source code distribution has a collection of
scripts that help you set up Postfix chroot environments on different operating
systems.

Additionally, you almost certainly need to configure syslogd so that it listens
on a socket inside the Postfix queue directory. Examples of syslogd command
line options that achieve this for specific systems:

FreeBSD: syslogd -l /var/spool/postfix/var/run/log

Linux, OpenBSD: syslogd -a /var/spool/postfix/dev/log

My own hostname

The myhostname parameter specifies the fully-qualified domain name of the
machine running the Postfix system. $myhostname appears as the default value in
many other Postfix configuration parameters.

By default, myhostname is set to the local machine name. If your local machine
name is not in fully-qualified domain name form, or if you run Postfix on a
virtual interface, you will have to specify the fully-qualified domain name
that the mail system should use.

Alternatively, if you specify mydomain in main.cf, then Postfix will use its
value to generate a fully-qualified default value for the myhostname parameter.

Examples (specify only one of the following):

    /etc/postfix/main.cf:
        myhostname = host.local.domain (machine name is not FQDN)
        myhostname = host.virtual.domain (virtual interface)
        myhostname = virtual.domain (virtual interface)

My own domain name

The mydomain parameter specifies the parent domain of $myhostname. By default,
it is derived from $myhostname by stripping off the first part (unless the
result would be a top-level domain).

Conversely, if you specify mydomain in main.cf, then Postfix will use its value
to generate a fully-qualified default value for the myhostname parameter.

Examples (specify only one of the following):

    /etc/postfix/main.cf:
        mydomain = local.domain
        mydomain = virtual.domain (virtual interface)

My own network addresses

The inet_interfaces parameter specifies all network interface addresses that
the Postfix system should listen on; mail addressed to "user@[network address]"
will be delivered locally, as if it is addressed to a domain listed in
$mydestination.

You can override the inet_interfaces setting in the Postfix master.cf file by
prepending an IP address to a server name.

The default is to listen on all active interfaces. If you run mailers on
virtual interfaces, you will have to specify what interfaces to listen on.

IMPORTANT: If you run MTAs on virtual interfaces you must specify explicit
inet_interfaces values for the MTA that receives mail for the machine itself:
this MTA should never listen on the virtual interfaces or you would have a
mailer loop when a virtual MTA is down.

Example: default setting.

    /etc/postfix/main.cf:
        inet_interfaces = all

Example: host running one or more virtual mailers. For each Postfix instance,
specify only one of the following.

    /etc/postfix/main.cf:
        inet_interfaces = virtual.host.tld         (virtual Postfix)
        inet_interfaces = $myhostname localhost... (non-virtual Postfix)

Note: you need to stop and start Postfix after changing this parameter.

