Friday, February 12, 2010

Postfix Virtual Domain Hosting Howto

Category: — SkyHi @ Friday, February 12, 2010

Purpose of this document

This document requires Postfix version 2.0 or later.

This document gives an overview of how Postfix can be used for hosting multiple Internet domains, both for final delivery on the machine itself and for the purpose of forwarding to destinations elsewhere.

The text not only describes delivery mechanisms that are built into Postfix, but also gives pointers for using non-Postfix mail delivery software.

The following topics are covered:

Canonical versus hosted versus other domains

Most Postfix systems are final destination for only a few domain names. These include the hostnames and [the IP addresses] of the machine that Postfix runs on, and sometimes also include the parent domain of the hostname. The remainder of this document will refer to these domains as the canonical domains. They are usually implemented with the Postfix local domain address class, as defined in the ADDRESS_CLASS_README file.

Besides the canonical domains, Postfix can be configured to be final destination for any number of additional domains. These domains are called hosted, because they are not directly associated with the name of the machine itself. Hosted domains are usually implemented with the virtual alias domain address class and/or with the virtual mailbox domain address class, as defined in the ADDRESS_CLASS_README file.

But wait! There is more. Postfix can be configured as a backup MX host for other domains. In this case Postfix is not the final destination for those domains. It merely queues the mail when the primary MX host is down, and forwards the mail when the primary MX host becomes available. This function is implemented with the relay domain address class, as defined in the ADDRESS_CLASS_README file.

Finally, Postfix can be configured as a transit host for sending mail across the internet. Obviously, Postfix is not final destination for such mail. This function is available only for authorized clients and/or users, and is implemented by the default domain address class, as defined in the ADDRESS_CLASS_README file.

Local files versus network databases

The examples in this text use table lookups from local files such as DBM or Berkeley DB. These are easy to debug with the postmap command:

Example: postmap -q info@example.com hash:/etc/postfix/virtual

See the documentation in LDAP_README, MYSQL_README and PGSQL_README for how to replace local files by databases. The reader is strongly advised to make the system work with local files before migrating to network databases, and to use the postmap command to verify that network database lookups produce the exact same results as local file lookup.

Example: postmap -q info@example.com ldap:/etc/postfix/virtual.cf

As simple as can be: shared domains, UNIX system accounts

The simplest method to host an additional domain is to add the domain name to the domains listed in the Postfix mydestination configuration parameter, and to add the user names to the UNIX password file.

This approach makes no distinction between canonical and hosted domains. Each username can receive mail in every domain.

In the examples we will use "example.com" as the domain that is being hosted on the local Postfix machine. 

/etc/postfix/main.cf:
mydestination = $myhostname localhost.$mydomain ... example.com , /etc/postfix/local-host-names

The limitations of this approach are:

  • A total lack of separation: mail for info@my.host.name is delivered to the same UNIX system account as mail for info@example.com.
  • With users in the UNIX password file, administration of large numbers of users becomes inconvenient.

The examples that follow provide solutions for both limitations.


[root@postfixpam ~]# grep mydestination /etc/postfix/main.cf
# The mydestination parameter specifies the list of domains that this
#mydestination = $myhostname, localhost.$mydomain, localhost
#mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
#mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain,
#mydestination = $myhostname, localhost.$mydomain, localhost
#mydestination = $virtual_alias_domains
mydestination = $myhostname, localhost.$mydomain, localhost,/etc/postfix/local-host-names
# to $mydestination, $inet_interfaces or $proxy_interfaces.
# - You define $mydestination domain recipients in files other than
# For example, you define $mydestination domain recipients in
# response code when a recipient domain matches $mydestination or
# The default relay_domains value is $mydestination.
# - destinations that match $mydestination
#relay_domains = $mydestination
# for unknown recipients. By default, mail for unknown@$mydestination,
[root@postfixpam ~]# grep virtual_alias /etc/postfix/main.cf
#mydestination = $virtual_alias_domains
# /etc/passwd, /etc/aliases, or the $virtual_alias_maps files.
# - destinations that match $virtual_alias_domains,
#virtual_alias_domains = /etc/postfix/local-host-names
virtual_alias_maps = hash:/etc/postfix/virtusertable, hash:/etc/postfix/aliases, hash:/etc/postfix/virtual


Postfix virtual ALIAS example: separate domains, UNIX system accounts

With the approach described in this section, every hosted domain can have its own info etc. email address. However, it still uses UNIX system accounts for local mailbox deliveries.

With virtual alias domains, each hosted address is aliased to a local UNIX system account or to a remote address. The example below shows how to use this mechanism for the example.com domain. 

 1 /etc/postfix/main.cf:
2     virtual_alias_domains = example.com ...other hosted domains...
3     virtual_alias_maps = hash:/etc/postfix/virtual
4
5 /etc/postfix/virtual:
6     postmaster@example.com postmaster
7     info@example.com       joe
8     sales@example.com      jane
9     # Uncomment entry below to implement a catch-all address
10     # @example.com         jim
11     ...virtual aliases for more domains...

Notes:

  • Line 2: the virtual_alias_domains setting tells Postfix that example.com is a so-called virtual alias domain. If you omit this setting then Postfix will reject mail (relay access denied) or will not be able to deliver it (mail for example.com loops back to myself).

    NEVER list a virtual alias domain name as a mydestination domain!

  • Lines 3-8: the /etc/postfix/virtual file contains the virtual aliases. With the example above, mail for postmaster@example.com goes to the local postmaster, while mail for info@example.com goes to the UNIX account joe, and mail for sales@example.com goes to the UNIX account jane. Mail for all other addresses in example.com is rejected with the error message "User unknown".

  • Line 10: the commented out entry (text after #) shows how one would implement a catch-all virtual alias that receives mail for every example.com address not listed in the virtual alias file. This is not without risk. Spammers nowadays try to send mail from (or mail to) every possible name that they can think of. A catch-all mailbox is likely to receive many spam messages, and many bounces for spam messages that were sent in the name of anything@example.com.

Execute the command "postmap /etc/postfix/virtual" after changing the virtual file, and execute the command "postfix reload" after changing the main.cf file.

Note: virtual aliases can resolve to a local address or to a remote address, or both. They don't have to resolve to UNIX system accounts on your machine.

More details about the virtual alias file are given in the virtual(5) manual page, including multiple addresses on the right-hand side.

Virtual aliasing solves one problem: it allows each domain to have its own info mail address. But there still is one drawback: each virtual address is aliased to a UNIX system account. As you add more virtual addresses you also add more UNIX system accounts. The next section eliminates this problem.

Postfix virtual MAILBOX example: separate domains, non-UNIX accounts

As a system hosts more and more domains and users, it becomes less desirable to give every user their own UNIX system account.

With the Postfix virtual(8) mailbox delivery agent, every recipient address can have its own virtual mailbox. Unlike virtual alias domains, virtual mailbox domains do not need the clumsy translation from each recipient addresses into a different address, and owners of a virtual mailbox address do not need to have a UNIX system account.

The Postfix virtual(8) mailbox delivery agent looks up the user mailbox pathname, uid and gid via separate tables that are searched with the recipient's mail address. Maildir style delivery is turned on by terminating the mailbox pathname with "/".

If you find the idea of multiple tables bothersome, remember that you can migrate the information (once it works), to an SQL database. If you take that route, be sure to review the "local files versus databases" section at the top of this document.

Here is an example of a virtual mailbox domain "example.com": 

 1 /etc/postfix/main.cf:
2     virtual_mailbox_domains = example.com ...more domains...
3     virtual_mailbox_base = /var/mail/vhosts
4     virtual_mailbox_maps = hash:/etc/postfix/vmailbox
5     virtual_minimum_uid = 100
6     virtual_uid_maps = static:5000
7     virtual_gid_maps = static:5000
8     virtual_alias_maps = hash:/etc/postfix/virtual
9
10 /etc/postfix/vmailbox:
11     info@example.com    example.com/info
12     sales@example.com   example.com/sales/
13     # Comment out the entry below to implement a catch-all.
14     # @example.com      example.com/catchall
15     ...virtual mailboxes for more domains...
16
17 /etc/postfix/virtual:
18     postmaster@example.com postmaster

Notes:

  • Line 2: The virtual_mailbox_domains setting tells Postfix that example.com is a so-called virtual mailbox domain. If you omit this setting then Postfix will reject mail (relay access denied) or will not be able to deliver it (mail for example.com loops back to myself).

    NEVER list a virtual MAILBOX domain name as a mydestination domain!

    NEVER list a virtual MAILBOX domain name as a virtual ALIAS domain!

  • Line 3: The virtual_mailbox_base parameter specifies a prefix for all virtual mailbox pathnames. This is a safety mechanism in case someone makes a mistake. It prevents mail from being delivered all over the file system.

  • Lines 4, 10-15: The virtual_mailbox_maps parameter specifies the lookup table with mailbox (or maildir) pathnames, indexed by the virtual mail address. In this example, mail for info@example.com goes to the mailbox at /var/mail/vhosts/example.com/info while mail for sales@example.com goes to the maildir located at /var/mail/vhosts/example.com/sales/.

  • Line 5: The virtual_minimum_uid specifies a lower bound on the mailbox or maildir owner's UID. This is a safety mechanism in case someone makes a mistake. It prevents mail from being written to sensitive files.

  • Lines 6, 7: The virtual_uid_maps and virtual_gid_maps parameters specify that all the virtual mailboxes are owned by a fixed uid and gid 5000. If this is not what you want, specify lookup tables that are searched by the recipient's mail address.

  • Line 14: The commented out entry (text after #) shows how one would implement a catch-all virtual mailbox address. Be prepared to receive a lot of spam, as well as bounced spam that was sent in the name of anything@example.com.

    NEVER put a virtual MAILBOX wild-card in the virtual ALIAS file!!

  • Lines 8, 17, 18: As you see, it is possible to mix virtual aliases with virtual mailboxes. We use this feature to redirect mail for example.com's postmaster address to the local postmaster. You can use the same mechanism to redirect an address to a remote address.

  • Line 18: This example assumes that in main.cf, $myorigin is listed under the mydestination parameter setting. If that is not the case, specify an explicit domain name on the right-hand side of the virtual alias table entries or else mail will go to the wrong domain.

Execute the command "postmap /etc/postfix/virtual" after changing the virtual file, execute "postmap /etc/postfix/vmailbox" after changing the vmailbox file, and execute the command "postfix reload" after changing the main.cf file.

Note: mail delivery happens with the recipient's UID/GID privileges specified with virtual_uid_maps and virtual_gid_maps. Postfix 2.0 and earlier will not create mailDIRs in world-writable parent directories; you must create them in advance before you can use them. Postfix may be able to create mailBOX files by itself, depending on parent directory write permissions, but it is safer to create mailBOX files ahead of time.

More details about the virtual mailbox delivery agent are given in the virtual(8) manual page.

Non-Postfix mailbox store: separate domains, non-UNIX accounts

This is a variation on the Postfix virtual mailbox example. Again, every hosted address can have its own mailbox.

While non-Postfix software is being used for final delivery, some Postfix concepts are still needed in order to glue everything together. For additional background on this glue you may want to take a look at the virtual mailbox domain class as defined in the ADDRESS_CLASS_README file.

The text in this section describes what things should look like from Postfix's point of view. See CYRUS_README or MAILDROP_README for specific information about Cyrus or about Courier maildrop.

Here is an example for a hosted domain example.com that delivers to a non-Postfix delivery agent: 

 1 /etc/postfix/main.cf:
2     virtual_transport = ...see below...
3     virtual_mailbox_domains = example.com ...more domains...
4     virtual_mailbox_maps = hash:/etc/postfix/vmailbox
5     virtual_alias_maps = hash:/etc/postfix/virtual
6
7 /etc/postfix/vmailbox:
8     info@example.com    whatever
9     sales@example.com   whatever
10     # Comment out the entry below to implement a catch-all.
11     # Configure the mailbox store to accept all addresses.
12     # @example.com      whatever
13     ...virtual mailboxes for more domains...
14
15 /etc/postfix/virtual:
16     postmaster@example.com postmaster

Notes:

  • Line 2: With delivery to a non-Postfix mailbox store for hosted domains, the virtual_transport parameter usually specifies the Postfix LMTP client, or the name of a master.cf entry that executes non-Postfix software via the pipe delivery agent. Typical examples (use only one): 

    virtual_transport = lmtp:unix:/path/name (uses UNIX-domain socket)
    virtual_transport = lmtp:hostname:port   (uses TCP socket)
    virtual_transport = maildrop:            (uses pipe(8) to command)

    Postfix comes ready with support for LMTP. And an example maildrop delivery method is already defined in the default Postfix master.cf file. See the MAILDROP_README document for more details.

  • Line 3: The virtual_mailbox_domains setting tells Postfix that example.com is delivered via the virtual_transport that was discussed in the previous paragraph. If you omit this virtual_mailbox_domains setting then Postfix will either reject mail (relay access denied) or will not be able to deliver it (mail for example.com loops back to myself).

    NEVER list a virtual MAILBOX domain name as a mydestination domain!

    NEVER list a virtual MAILBOX domain name as a virtual ALIAS domain!

  • Lines 4, 7-13: The virtual_mailbox_maps parameter specifies the lookup table with all valid recipient addresses. The lookup result value is ignored by Postfix. In the above example, info@example.com and sales@example.com are listed as valid addresses; other mail for example.com is rejected with "User unknown" by the Postfix SMTP server. It's left up to the non-Postfix delivery agent to reject non-existent recipients from local submission or from local alias expansion. If you intend to use LDAP, MySQL or PgSQL instead of local files, be sure to review the "local files versus databases" section at the top of this document!

  • Line 12: The commented out entry (text after #) shows how one would inform Postfix of the existence of a catch-all address. Again, the lookup result is ignored by Postfix.

    NEVER put a virtual MAILBOX wild-card in the virtual ALIAS file!!

    Note: if you specify a wildcard in virtual_mailbox_maps, then you still need to configure the non-Postfix mailbox store to receive mail for any address in that domain.

  • Lines 5, 15, 16: As you see above, it is possible to mix virtual aliases with virtual mailboxes. We use this feature to redirect mail for example.com's postmaster address to the local postmaster. You can use the same mechanism to redirect any addresses to a local or remote address.

  • Line 16: This example assumes that in main.cf, $myorigin is listed under the mydestination parameter setting. If that is not the case, specify an explicit domain name on the right-hand side of the virtual alias table entries or else mail will go to the wrong domain.

Execute the command "postmap /etc/postfix/virtual" after changing the virtual file, execute "postmap /etc/postfix/vmailbox" after changing the vmailbox file, and execute the command "postfix reload" after changing the main.cf file.

Mail forwarding domains

Some providers host domains that have no (or only a few) local mailboxes. The main purpose of these domains is to forward mail elsewhere. The following example shows how to set up example.com as a mail forwarding domain: 

 1 /etc/postfix/main.cf:
2     virtual_alias_domains = example.com ...other hosted domains...
3     virtual_alias_maps = hash:/etc/postfix/virtual
4
5 /etc/postfix/virtual:
6     postmaster@example.com postmaster
7     joe@example.com        joe@somewhere
8     jane@example.com       jane@somewhere-else
9     # Uncomment entry below to implement a catch-all address
10     # @example.com         jim@yet-another-site
11     ...virtual aliases for more domains...

Notes:

  • Line 2: The virtual_alias_domains setting tells Postfix that example.com is a so-called virtual alias domain. If you omit this setting then Postfix will reject mail (relay access denied) or will not be able to deliver it (mail for example.com loops back to myself).

    NEVER list a virtual alias domain name as a mydestination domain!

  • Lines 3-11: The /etc/postfix/virtual file contains the virtual aliases. With the example above, mail for postmaster@example.com goes to the local postmaster, while mail for joe@example.com goes to the remote address joe@somewhere, and mail for jane@example.com goes to the remote address jane@somewhere-else. Mail for all other addresses in example.com is rejected with the error message "User unknown".

  • Line 10: The commented out entry (text after #) shows how one would implement a catch-all virtual alias that receives mail for every example.com address not listed in the virtual alias file. This is not without risk. Spammers nowadays try to send mail from (or mail to) every possible name that they can think of. A catch-all mailbox is likely to receive many spam messages, and many bounces for spam messages that were sent in the name of anything@example.com.

Execute the command "postmap /etc/postfix/virtual" after changing the virtual file, and execute the command "postfix reload" after changing the main.cf file.

More details about the virtual alias file are given in the virtual(5) manual page, including multiple addresses on the right-hand side.

Mailing lists

The examples that were given above already show how to direct mail for virtual postmaster addresses to a local postmaster. You can use the same method to direct mail for any address to a local or remote address.

There is one major limitation: virtual aliases and virtual mailboxes can't directly deliver to mailing list managers such as majordomo. The solution is to set up virtual aliases that direct virtual addresses to the local delivery agent: 

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

/etc/postfix/virtual:
listname-request@example.com listname-request
listname@example.com         listname
owner-listname@example.com   owner-listname

/etc/aliases:
listname: "|/some/where/majordomo/wrapper ..."
owner-listname: ...
listname-request: ...

This example assumes that in main.cf, $myorigin is listed under the mydestination parameter setting. If that is not the case, specify an explicit domain name on the right-hand side of the virtual alias table entries or else mail will go to the wrong domain.

More information about the Postfix local delivery agent can be found in the local(8) manual page.

Why does this example use a clumsy virtual alias instead of a more elegant transport mapping? The reason is that mail for the virtual mailing list would be rejected with "User unknown". In order to make the transport mapping work one would still need a bunch of virtual alias or virtual mailbox table entries.

  • In case of a virtual alias domain, there would need to be one identity mapping from each mailing list address to itself.
  • In case of a virtual mailbox domain, there would need to be a dummy mailbox for each mailing list address.

Autoreplies

In order to set up an autoreply for virtual recipients while still delivering mail as normal, set up a rule in a virtual alias table: 

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

/etc/postfix/virtual:
user@domain.tld user@domain.tld, user@domain.tld@autoreply.mydomain.tld

This delivers mail to the recipient, and sends a copy of the mail to the address that produces automatic replies. The address can be serviced on a different machine, or it can be serviced locally by setting up a transport map entry that pipes all mail for autoreply.mydomain.tld into some script that sends an automatic reply back to the sender.

DO NOT list autoreply.mydomain.tld in mydestination! 

/etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/transport

/etc/postfix/transport:
autoreply.mydomain.tld  autoreply:

/etc/postfix/master.cf:
# =============================================================
# service type  private unpriv  chroot  wakeup  maxproc command
#               (yes)   (yes)   (yes)   (never) (100)
# =============================================================
autoreply unix  -       n       n       -       -       pipe
 flags= user=nobody argv=/path/to/autoreply $sender $mailbox

This invokes /path/to/autoreply with the sender address and the user@domain.tld recipient address on the command line.

For more information, see the pipe(8) manual page, and the comments in the Postfix master.cf file.


REFERENCE

http://www.postfix.org/VIRTUAL_README.html