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Postfix Documentation
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10 - Mandatory configuration file edits

Note: the material covered in this section is covered in more detail in the BASIC_CONFIGURATION_README document. The information presented below is targeted at experienced system administrators.

10.1 - 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.

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

# postfix reload

10.2 - Default domain for unqualified addresses

First of all, you must specify what domain will be appended to an unqualified address (i.e. an address without @domain.tld). The " myorigin" parameter defaults to the local hostname, but that is probably OK only for very small sites.

Some examples (use only one):

/etc/postfix/
main.cf:
    
myorigin = $
myhostname    (send mail as "[email protected]$
myhostname")
    
myorigin = $
mydomain      (send mail as "[email protected]$
mydomain")

10.3 - What domains to receive locally

Next you need to specify what mail addresses Postfix should deliver locally.

Some examples (use only one):

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

The first example is appropriate for a workstation, the second is appropriate for the mailserver for an entire domain. The third example should be used when running on a virtual host interface.

10.4 - Proxy/NAT interface addresses

The proxy_interfaces parameter specifies all network addresses that Postfix receives mail on by way of a proxy or network address translation unit. 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)

10.5 - What local clients to relay mail from

If your machine is on an open network then you must specify what client IP addresses are authorized to relay their mail through your machine into the Internet. The default setting includes all subnetworks that the machine is attached to. This may give relay permission to too many clients. My own settings are:

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

10.6 - What relay destinations to accept from strangers

If your machine is on an open network then you must also specify whether Postfix will forward mail from strangers. The default setting will forward mail to all domains (and subdomains of) what is listed in $ mydestination. This may give relay permission for too many destinations. Recommended settings (use only one):

/etc/postfix/
main.cf:
    
relay_domains =            (do not forward mail from strangers)
    
relay_domains = $
mydomain  (my domain and subdomains)
    
relay_domains = $
mydomain, other.domain.tld, ...

10.7 - Optional: configure a smart host for remote delivery

If you're behind a firewall, you should set up a relayhost. If you can, specify the organizational domain name so that Postfix can use DNS lookups, and so that it can fall back to a secondary MX host when the primary MX host is down. Otherwise just specify a hard-coded hostname.

Some examples (use only one):

/etc/postfix/
main.cf:
    
relayhost = $
mydomain
    
relayhost = [mail.$
mydomain]

The form enclosed with [] eliminates DNS MX lookups.

By default, the SMTP client will do DNS lookups even when you specify a relay host. If your machine has no access to a DNS server, turn off SMTP client DNS lookups like this:

/etc/postfix/
main.cf:
    
disable_dns_lookups = yes

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

10.8 - Create the aliases database

Postfix uses a Sendmail-compatible aliases(5) table to redirect mail for local(8) recipients. Typically, this information is kept in two files: in a text file /etc/aliases and in an indexed file /etc/aliases.db. The command "postconf alias_maps" will tell you the exact location of the text file.

First, be sure to update the text file with aliases for root, postmaster and "postfix" that forward mail to a real person. Postfix has a sample aliases file /etc/postfix/aliases that you can adapt to local conditions.

/etc/aliases:
    root: you
    postmaster: root
    postfix: root
    bin: root
    etcetera...

Note: there should be no whitespace before the ":".

Finally, build the indexed aliases file with one of the following commands:

# newaliases
# sendmail -bi
Postfix Documentation
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