Virtual mailboxes on Debian Squeeze with Exim4 and Dovecot

You know how it is. Your little mail host has just sat there for a long while, slowly accreting a user here and a user there. They are all full users on the machine, so they have logins and everything, and exim is configured to deliver their mails to ~/Maildir.

And then some dangerous loon suddenly demands that you add mailboxes for people using a completely different domain. They're not really much to do with you, and certainly shouldn't be full-blown users on your host.

Time, then, to add virtual mailboxes to your poor little mail host.

There all sorts of HOWTOs on doing this, of various ages and usefulness. I'm adding this page to the malestrom just to document the configuration changes I did. Or rather, the configuration changes I ended up with. The details of configuration aren't original, but synthesised from too many of the HOWTOs I read to properly credit each.

I'm illustrating this by setting up a new domain example.mod with user tommy.atkins.

First I created a system user and group vmail to own all virtual mailboxes.

# adduser -system --home /var/local/vmail --group vmail

and a configuration directory.

# mkdir /etc/vmail

Configuration information for the domain will be under /etc/vmail/<domain>. If that directory doesn't exist, the domain isn't supported.

# mkdir /etc/vmail/example.mod

The general scheme is that we have mailboxes under /var/local/vmail/<domain>/<user>/Maildir. Why Maildir? Because our existing users will continue to get mail in ~/Maildir and it keeps dovecot config a little simpler.

# mkdir -p /var/local/vmail/example.mod
# chown -R vmail:vmail /var/local/vmail

Now add two configuration files. The first, aliases, is a conventional aliases file for the domain.

# cat > /etc/vmail/example.mod/aliases
postmaster: root
webmaster: root
security: root
admin: root
root: guru@example-owner.mod

squaddie: tommy.atkins
^D

The second, passwd contains the account information for the domain. There are two items on a line, username and password hash, separated by a colon. Generate the password has using the dovecotpw utility. Just to be on the safe side, we'll ensure the password file isn't world readable.

# /usr/sbin/dovecotpw -p password
{CRAM-MD5}9186d855e11eba527a7a52ca82b313e180d62234f0acc9051b527243d41e2740
# cat > /etc/vmail/example.mod/passwd
tommy.atkins:{CRAM-MD5}9186d855e11eba527a7a52ca82b313e180d62234f0acc9051b527243d41e2740
^D
# chgrp /etc/vmail/example.mod/passwd
# chmod 0640 /etc/vmail/example.mod/passwd

The next step is to configure Exim to deliver to virtual mailboxes.

The first thing to do is to add the domain to the list of local domains. How you do this depends on which of Debian's configuration scheme you are using. You need to end up with a configuration file with the domain as part of the domainlist local_domains. From here on, I'll show what I ended up with in the Exim configuration file, and leave it up to you to work out how to get it there.

Next we need a router to expand virtual domain aliases.

vmail_aliases:
  driver = redirect
  data = ${lookup{$local_part}lsearch{/etc/vmail/$domain/aliases}}
  domains = dsearch;/etc/vmail
  qualify_domain = $domain
  retry_use_local_part

That desarch;/etc/vmail will expand to a list of the files/directories under /etc/vmail. Which will be the domains to be handled. qualify_domain = $domain ensures the expanded alias, if any, has the same domain as the original, if the domain is not specified in the alias.

Once that's done, we can think about routing to a transport for delivery.

vmail_deliver:
  driver = accept
  condition = ${if eq {}{${lookup{$local_part}lsearch{/etc/vmail/$domain/passwd}}}{no}{yes}}
  domains = dsearch;/etc/vmail
  no_more
  retry_use_local_part
  transport = vmail_delivery

Here we're accepting the mail on condition that the local part of the address appears in the domain's password file. If it does, the message proceeds to the transport.

vmail_delivery:
  driver = appendfile
  envelope_to_add
  directory = /var/local/vmail/$domain/$local_part/Maildir
  maildir_format
  create_directory = true
  user = vmail
  group = vmail
  return_path_add

This transport adds the message to the user's Maildir. It runs as user and group vmail and will create any directories that don't exist.

By the way, when you add a user, it's an idea to either send them a welcome mail to check things are working properly and create their Maildir into the bargain. Otherwise, you should create the Maildir by hand, so that there is something there when they try to read their mail.

When that's done, test your handiwork:

# exim4 -bt tommy.atkins@example.mod
tommy.atkins@example.mod
router = vmail_deliver, transport = vmail_delivery

Now we need to modify the Dovecot setup to allow our user to read mail.

I'm assuming your dovecot.conf already has

mail_location = maildir:~/Maildir

in it, telling Dovecot to find your regular users mail in ~/Maildir.

We need to add the virtual domains to the

auth default {
}

section of the configuration. The first stage is to add the virtual users password files.

passdb passwd-file {
  args = username_format=%n /etc/vmail/%d/passwd
}

Here's a tip. My configuration also has a configuration for PAM

passdb pam {
}

Put the virtual users passwd-file entry before the pam section. If you don't, your virtual usernames will be tried in pam first. Confusion will abound if they succeed. And if they don't, it will look to programs like denyhosts or fail2ban as if a PAM login failed, and you may find access to the host unexpectedly blocked.

Now you need to add a userdb section telling Dovecot how to find the mailbox. You will have a

userdb passwd {
}

section serving your existing users. After that, add a catch-all for the virtual mailbox users.

userdb static {
  args = uid=vmail gid=vmail home=/var/local/vmail/%d/%n
}

This tells Dovecot to use vmail permissions to access the mailbox, and that the user home is at /var/local/main/<domain>/<user>. Dovecot will apply it's existing rule that the mail is to be found in directory Maildir under the user's home, and you are off to the races.

In practice, you'll most probably run into authentication problems. For help sorting them out, add

auth_debug = yes
auth_debug_passwords = yes

to your dovecot.conf while you try and work out what's going wrong.

Your virtual mail users will probably want to send some mail, too. So you may want to give them access to your Exim server to relay mail. But, of course, you don't want world+dog also relaying their spam through your host.

There's several ways of cracking this nut. I'll just mention that if you do it by allowing authenticated SMTP, I found it easiest to do by handing the authentication over to Dovecot.

I needed to create the Dovecot authenticator socket by adding

 socket listen {
   client {
     path = /var/run/dovecot/auth-client
     mode = 0666
   }
 }

to the

auth default {
}

section in dovecot.conf. It's commented out by default in Debian. Without other arrangements, Exim needs its mode to be 0666; comments in the Dovecot config suggest this is generally safe.

Then, in the Exim configuration, use these authenticators:

dovecot_plain:
  driver = dovecot
  public_name = PLAIN
  server_socket = /var/run/dovecot/auth-client
  #server_set_id = $auth2
  server_mail_auth_condition = false


dovecot_login:
  driver = dovecot
  public_name = LOGIN
  server_socket = /var/run/dovecot/auth-client
  #server_set_id = $auth2
  server_mail_auth_condition = false