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Postfix Documentation
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Postfix on hosts without a real Internet hostname

This section is for hosts that don't have their own Internet hostname. Typically these are systems that get a dynamic IP address via DHCP or via dialup. Postfix will let you send and receive mail just fine between accounts on a machine with a fantasy name. However, you cannot use a fantasy hostname in your email address when sending mail into the Internet, because no-one would be able to reply to your mail. In fact, more and more sites refuse mail addresses with non-existent domain names.

Note: the following information is Postfix version dependent. To find out what Postfix version you have, execute the command "postconf mail_version".

Solution 1: Postfix version 2.2 and later

Postfix 2.2 uses the generic(5) address mapping to replace local fantasy email addresses by valid Internet addresses. This mapping happens ONLY when mail leaves the machine; not when you send mail between users on the same machine.

The following example presents additional configuration. You need to combine this with basic configuration information as discussed the first half of this document.

1 /etc/postfix/
main.cf:
2     
smtp_generic_maps = hash:/etc/postfix/generic
3 
4 /etc/postfix/generic:
5     [email protected]             [email protected]
6     [email protected]             [email protected]
7     @localdomain.local                [email protected]

When mail is sent to a remote host via SMTP:

  • Line 5 replaces [email protected] by his ISP mail address,

  • Line 6 replaces [email protected] by her ISP mail address, and

  • Line 7 replaces other local addresses by his ISP account, with an address extension of +local (this example assumes that the ISP supports "+" style address extensions).

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what lookup tables Postfix supports, use the command "postconf -m".

Execute the command "postmap /etc/postfix/generic" whenever you change the generic table.

Solution 2: Postfix version 2.1 and earlier

The solution with older Postfix systems is to use valid Internet addresses where possible, and to let Postfix map valid Internet addresses to local fantasy addresses. With this, you can send mail to the Internet and to local fantasy addresses, including mail to local fantasy addresses that don't have a valid Internet address of their own.

The following example presents additional configuration. You need to combine this with basic configuration information as discussed the first half of this document.

 1 /etc/postfix/
main.cf:
 2     
myhostname = hostname.localdomain
 3     
mydomain = localdomain
 4 
 5     
canonical_maps = hash:/etc/postfix/canonical
 6 
 7     
virtual_alias_maps = hash:/etc/postfix/virtual
 8 
 9 /etc/postfix/canonical:
10     your-login-name    [email protected]
11 
12 /etc/postfix/
virtual:
13     [email protected]       your-login-name

Translation:

  • Lines 2-3: Substitute your fantasy hostname here. Do not use a domain name that is already in use by real organizations on the Internet. See RFC 2606 for examples of domain names that are guaranteed not to be owned by anyone.

  • Lines 5, 9, 10: This provides the mapping from "[email protected]" to "[email protected]". This part is required.

  • Lines 7, 12, 13: Deliver mail for "[email protected]" locally, instead of sending it to the ISP. This part is not required but is convenient.

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what lookup tables Postfix supports, use the command "postconf -m".

Execute the command "postmap /etc/postfix/canonical" whenever you change the canonical table.

Execute the command "postmap /etc/postfix/virtual" whenever you change the virtual table.

Postfix Documentation
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