System users used by Dovecot

Dovecot typically requires 3 or more system users:

  • root: Dovecot is started as root.

  • dovenull: Dovecot uses an unprivileged dovenull user for untrusted login processes.

  • dovecot: Dovecot uses an unprivileged dovecot user for internal processes.

  • auth user: Password and user database lookups are done as auth user.

  • mail user(s): Mails are accessed using yet another user. The mail user should not be dovecot user.

Using multiple users allows privilege separation, which makes it harder for attackers to compromise the whole system if a security hole is found from one component. However, if you really want to run everything under a single user, it’s possible.

Dovenull user

dovenull user is used internally for processing users’ logins. It shouldn’t have access to any files, authentication databases or anything else either. It should belong to its own private dovenull group where no one else belongs to, and which doesn’t have access to any files either (other than what Dovecot internally creates).

You can change the default dovenull user to something else from default_login_user setting.

Dovecot user

dovecot user is used internally for unprivileged Dovecot processes. It should belong to its own private dovecot group. Mail files are not accessed as dovecot user, so you shouldn’t give it access to mails.

You can change the default dovecot user to something else from default_internal_user setting.

Mail users

You can use one or more system users for accessing users’ mails. Most configurations can be placed to two categories:

  1. System users where each Dovecot user has their own system user in /etc/passwd. For system user setups you generally don’t have to worry about UIDs or GIDs, they are returned by the userdb passwd lookup.

  2. Virtual users where all Dovecot users run under a single system user. Typically you’d set this with mail_uid setting (e.g. mail_uid=vmail). Note that you most likely don’t want the userdb lookup to return any UID/GID, as they override the mail_uid setting.

However it’s possible to use a setup that is anything between these two. For example use a separate system user for each domain. See below for more information about how UIDs can be used.

UIDs

Dovecot’s User Databases (userdb) configuration calls system users UIDs. There are a few things you should know about them:

  • Although UID normally means a numeric ID (as specified by /etc/passwd), it’s anyway possible to use names as UID values and let Dovecot do the lookup (eg. uid=vmail). However depending on where you used it, it may slow down the authentication.

  • The UIDs don’t really have to exist in /etc/passwd (the kernel doesn’t care about that). For example you could decide to use UIDs 10000-59999 for 50000 virtual Dovecot users. You’ll then just have to be careful that the UIDs aren’t used unintentionally elsewhere.

  • The important thing to consider with your UID allocation policy is that if Dovecot has a security hole in its IMAP or POP3 implementation, the attacker can read mails of other people who are using the same UID. So clearly the most secure way is to allocate a different UID for each user. It can however be a bit of a pain and OSes don’t always support more than 65536 UIDs.

  • By default Dovecot allows users to log in only with UID numbers 500 and above. This check tries to make sure that no-one can ever log in as daemons or other system users. If you’re using an UID lower than 500, you’ll need to change the first_valid_uid setting.

GIDs

System groups (GIDs) work very much the same way as UIDs described above: You can use names instead of numbers for GID values, and the used GIDs don’t have to exist in /etc/group.

System groups are useful for sharing mailboxes between users that have different UIDs but belong to a same group. Currently Dovecot doesn’t try to do anything special with the groups, so if you’re not sure how you should create them, you might as well place all the users into a single group or create a separate group for each user.

If you use multiple UIDs and you wish to create shared mailboxes, setting up the groups properly may make your configuration more secure. For example if you have two teams and their mailboxes are shared only to their team members, you could create a group for each team and set the shared mailbox’s group to the team’s group and permissions to 0660, so neither team can even accidentally see each others’ shared mailboxes.

Currently Dovecot supports specifying only the primary group, but if your userdb returns system_user extra field, the non-primary groups are taken from /etc/group for that user. In a future version the whole GID list will be configurable without help from /etc/group.

It’s also possible to give all the users access to extra groups with mail_access_groups setting.

Authentication process user

Depending on passdb and userdb configuration, the lookups are done either by auth process or auth worker process. They have different default users:

service auth {
  user = $default_internal_user
}
service auth-worker {
  user = root
}

The user must have access to your password databases and user databases. It’s not used for anything else. The default is to use root, because it’s guaranteed to have access to all the password databases. If you don’t need this, you should change it to $default_internal_user.

PAM and shadow passdbs are usually configured to read /etc/shadow file. Even this doesn’t need root access if the file is readable by shadow group:

service auth-worker {
  user = $default_internal_user
  group = shadow
}