Debian Administration

Handling mail for multiple virtual domains with exim4

Many mail servers need to handle mail for multiple virtual domains. This can easily be achieved with Sendmail, postfix, or qmail. Here we'll cover how it's done with exim4 - the default mailserver for Sarge.

This is an update to a previous article here: Handling mail for multiple domains with exim3.

The upgrade from Exim3, the default mailserver for Woody, to Exim4, the default mail server for Sarge is a fairly painless process.

We covered the upgrade process in upgrading from Woody to Sarge: Part 3 - Exim4, but just to recap:

• There are two main exim4 packages to choose from:
  • exim4-daemon-light - A basic and functional server.
  • exim4-daemon-heavy - A more complex server with advanced features.
• When using the exim4 package you have two choices:
  • Use a single monolithic configuration file (As exim3 used).
  • Using a split configuration system with a lot of small files.

Because I'm going to want to scan incoming mail for viruses as they are delivered, rather than filter them out myself once I've accepted them I will install the heavy version of the server:

root@lappy:~# apt-get install exim4-daemon-heavy 
Reading Package Lists... Done
Building Dependency Tree... Done
The following extra packages will be installed:
  exim4-base exim4-config
Suggested packages:
  eximon4 exim4-doc-html exim4-doc-info
The following NEW packages will be installed:
  exim4-base exim4-config exim4-daemon-heavy
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 1033kB/1450kB of archives.
After unpacking 1155kB of additional disk space will be used.
Do you want to continue? [Y/n] 

Once this has downloaded you will start the debconf configuration process.

The first question asked is 'Do you wish to use a single monolithic configuration file, or split files?' Here I chose the split file approach, to make future upgrades more simple.

Once that was done you're left to choose the type of configuration you wish to use - I chose "an internet site", because I want to send and receive mail directly. This choice does affect the subsequent questions you'll have.

Choosing the mail name should be obvious, as should choosing which addresses you wish the server to listen upon.

Once that's done the basic setup is complete - but we've still got to handle the virtual domain setup.

If you've just installed the server and you followed the debconf setup I did then you'll have a server which will send and receive mail for a single domain.

The way that I've chosen to handle virtual domain mails, (which was used in the exim3 article) is to have a text file for each domain which contains a mapping between the email address and a local user.

For example assume you wish to handle mail for three domains:

• example.net
• example.org
• example.com

Create three text files:

• /etc/exim4/virtual/example.net
• /etc/exim4/virtual/example.org
• /etc/exim4/virtual/example.com

Each file will contain lines of the form:

address : username@localhost

Where "address" is the part of the email address to the left of the domain name, and "username" is the account name on the local system which should recieve that mail.

To make it more clear assume:

• Alice gets all mail sent to example.net.
• Bob gets all mail sent to example.org.
• Eve has a mail account at example.com, as does Trent.

This gives /etc/exim4/virtual/example.com:

eve : eve@localhost
trent : trent@localhost

For Bob who has a "catchall" address setup for example.org - /etc/exim4/virtual/example.org:

* : bob@localhost

And similarly for Alice - /etc/exim4/virtual/example.net:

* : alice@localhost

You can also drop all mail for a user by using:

eve : :blackhole:

Or to generate a bounce with some text added so that the sender will know why they got it:

sharon : :fail: She no longer lives here.

Now we just need to make exim4 read these files to know what to do with the incoming mail, a simple enough job.

First of all we need to update the list of domains we handle mail for by editing the file /etc/exim4/conf.d/main/01_exim4-config_listmacrosdefs, change the current "local_domains" with this one:

domainlist local_domains = @:localhost:dsearch;/etc/exim4/virtual

This will cause the local_domains setting to include the domain list from the directory we've specified. (Which is why the text files are named after the domain name to which they apply).

Now that the list of domains has been updated we need to create a new file - edit the file /etc/exim4/conf.d/router/350_exim4-config_vdom_aliases and add the following contents to it:

vdom_aliases:
      driver = redirect
      allow_defer
      allow_fail
      domains = dsearch;/etc/exim4/virtual
      data = ${expand:${lookup{$local_part}lsearch*@{/etc/exim4/virtual/$domain}}}
      retry_use_local_part
      pipe_transport   = address_pipe
      file_transport   = address_file
      no_more

Here we tell exim how to lookup the local recipients.

Once this has been done you can restart the server and see how it works:

/etc/init.d/exim4 restart

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