• ACL

Access Control Lists

This page talks mainly about how ACLs work, for more general description of how shared mailboxes work, see SharedMailboxes.

Dovecot v1.0 and v1.1 supports administrator-configured ACL files. v1.2+ supports also IMAP ACL extension, which allows users to change ACLs themselves. The ACL code was written to allow multiple ACL backends, but currently Dovecot supports only virtual ACL files. Note that using ACLs doesn't grant mail processes any extra filesystem permissions that they already don't have. You must make sure that the processes have enough permissions to be able to access the mailboxes. When testing you could first try accessing shared/public mailboxes without ACL plugin even enabled.

ACLs can be enabled in dovecot.conf with:

mail_plugins = acl
protocol imap {
  mail_plugins = $mail_plugins imap_acl
}

plugin {
  # Without global ACLs:
  acl = vfile

  # With global ACL files in /etc/dovecot/dovecot-acls file (v2.2.11+):
  #acl = vfile:/etc/dovecot/dovecot-acl

  # With global ACLs in /etc/dovecot/acls/ directory (obsolete):
  #acl = vfile:/etc/dovecot/acls

  # If enabled, don't try to find dovecot-acl files from mailbox directories.
  # This reduces unnecessary disk I/O when only global ACLs are used. (v2.2.31+)
  #acl_globals_only = yes
}

ACL groups support works by returning a comma-separated acl_groups extra field from userdb, which contains all the groups the user belongs to. User's UNIX groups have no effect on ACLs (you can "enable" them by using a special post-login script).

The default ACL for mailboxes is to give the mailbox owner all permissions and other users none. Mailboxes in public namespaces don't have owners, so by default no one can access them.

ACL vfile backend

vfile backend supports per-mailbox ACLs and global ACLs.

Per-mailbox ACLs are stored in dovecot-acl named file, which exists in:

• maildir: The Maildir's mail directory (eg. ~/Maildir, ~/Maildir/.folder/)

• mbox: Control directory. You should explicitly specify :CONTROL=<path> in mail location.

• dbox: dbox's mail directory (eg. ~/dbox/INBOX/dbox-Mails/)

ACL Inheritance

Every time you create a new mailbox, it gets its ACLs from the parent mailbox. If you're creating a root-level mailbox, it uses the namespace's default ACLs. There is no actual inheritance, however: If you modify parent's ACLs, the child's ACLs stay the same. There is currently no support for ACL inheritance.

• Maildir: Namespace's default ACLs are read from "dovecot-acl" file in the namespace's mail root directory (e.g. /var/public/Maildir). Note that currently these default ACLs are used only when creating new mailboxes, they aren't used for mailboxes without ACLs.

• v2.2.2+: If plugin { acl_defaults_from_inbox=yes } , the default ACLs for private and shared namespaces (but not public namespaces) are taken from the INBOX. This means that giving somebody access to your INBOX will give them access to all your other mailboxes as well, unless the specific mailboxes' ACLs override the INBOX's.

NOTE: Currently the default ACLs are merged with the mailbox-specific ACLs. So if a default ACL gives access to "user1" and a per-mailbox ACL gives access to "user2", the "user1" still has access to that mailbox.

Global ACLs

Global ACLs can be used to apply ACLs globally to all user's specific mailboxes. They are used mainly for two purposes:

1. Removing some permissions from users' personal mailboxes. For example each user might have an "Invoices" mailbox which will be read-only.

2. Giving permissions to master user logins. Master user is not the mailbox owner, so by default it has no permissions to any of the mailboxes. See Authentication/MasterUsers#ACLs for more information.

If a mailbox has both global ACLs and the per-mailbox ACL file, both of them are read and the ACLs are merged. If there are any conflicts, the global ACL file overrides per-mailbox ACL file. This is because users can modify their own per-mailbox ACL files via IMAP ACL extension. Global ACLs can only be modified by administrator, so users shouldn't be able to override them.

Global ACL file (v2.2.11+)

Global ACL file path is specified as a parameter to vfile backend in acl setting (/etc/dovecot/dovecot-acl in the above example). The file contains otherwise the same data as regular per-mailbox dovecot-acl files, except each line is prefixed by the mailbox name pattern. The pattern may contain "*" and "?" wildcards. For example:

* user=foo lrw
Public user=bar lrwstipekxa
Public/* user=bar lrwstipekxa

Global ACL directory (obsolete)

Global ACL directory is specified as a parameter to vfile backend in acl setting (/etc/dovecot/acls/ in the above example). They are looked up using the mailbox's virtual name. For example:

• INBOX: /etc/dovecot/acls/INBOX

• archives.2007: /etc/dovecot/acls/archives.2007

• archives/2007: /etc/dovecot/acls/archives/2007

The filenames must start with namespace prefix (if it has one). For example with namespace prefix=INBOX/ containing mailbox "foo" use /etc/dovecot/acls/INBOX/foo.

There is an extra problem with mailbox formats that use '/' as the separator (e.g. mbox, dbox): For example if you have mailboxes "foo" and "foo/bar" and you wish to give ACLs to both of them, you can't create both /etc/dovecot/acls/foo and /etc/dovecot/acls/foo/bar files. The foo has to be either a directory or a file, it can't be both. To solve this problem, you can instead create a .DEFAULT file for "foo":

• foo: /etc/dovecot/acls/foo/.DEFAULT

• foo/bar: /etc/dovecot/acls/foo/bar

ACL files

The files themselves are in format:

<identifier> <ACLs> [:<named ACLs>]

Where identifier is one of:

• group-override=group name

• user=user name

• owner

• group=group name

• authenticated
• anyone (or anonymous, which is alias for anyone)

The ACLS are processed in the precedence given above, so for example if you have given read-access to a group, you can still remove that from specific users inside the group.

Group-override identifier allows you to override users' ACLs. Probably the most useful reason to do this is to temporarily disable access for some users. For example:

user=timo rw
group-override=tempdisabled

Now if timo is in tempdisabled group, he has no access to the mailbox. This wouldn't be possible with a normal group identifier, because the user=timo would override it.

The currently supported ACLs and their corresponding named ACLs are:

The ACLs are compatible with RFC 4314 (IMAP ACL extension, updated version).

Unknown ACL letters are complained about, but unknown named ACLs are ignored. Named ACLs are mostly intended for future extensions.

Note that the file is rather picky about formatting; using a tab (or multiple spaces) instead of a space character between fields may not work. If you are having problems, make sure to check for tabs, extra spaces and other unwanted characters.

Examples

Mailbox owner has all privileges, "timo" has list-read privileges:

owner lrwstipekxa
user=timo lr

Allow everyone to list and read a public mailbox (public namespace has no owner):

anyone lr

Prevent all users from deleting their Spam folder (notice no x flag)

INBOX.Spam owner lrwstipeka

List cache

dovecot-acl-list file lists all mailboxes that have "l" rights assigned. If you manually add/edit dovecot-acl files, you may need to delete the dovecot-acl-list to get the mailboxes visible.