ACL: Access Control List (acl) Plugin

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

Dovecot supports both administrator-configured ACL files and the IMAP ACL extension (see imap-acl plugin, 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.

Settings (dovecot.conf)

acl

Default	[None]
Value	string

The ACL driver to use. This setting is REQUIRED - if empty, the acl plugin is disabled.

The format is:

backend[:option[:option...]]

Currently, there is a single backend available: vfile. This backend supports two ways of defining the ACL configuration:

• global: ACL rules are applied to all users.

• per-mailbox: Each mailbox has separate ACL rules. They are stored in a dovecot-acl file in each mailbox (or CONTROL) directory. This is the default.

This backend has the following options:

Name	Description
<global_path>	If a path is defined, this is the location of the global ACL configuration file.
cache_secs	The interval, in seconds, for running stat() on the ACL file to check for changes. DEFAULT: 30

Example:

plugin {
  # Per-user ACL:
  acl = vfile

  # Global ACL; check for changes every minute
  #acl = vfile:/etc/dovecot/dovecot-acl:cache_secs=60
}

acl_defaults_from_inbox

Default	no
Value	boolean

If enabled, 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.

acl_globals_only

Default	no
Value	boolean

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.

acl_groups

Default	[None]
Value	string

A comma-separated string which contains all the groups the user belongs to.

A user's UNIX groups have no effect on ACLs (you can enable them by using a special post-login scripting.

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_ignore_namespace

Default	[None]
Value	string

Ignore ACLs entirely for the given namespace.

You can define multiple namespaces by appending an increasing number to the setting name.

Example:

plugin {
  acl_ignore_namespace = virtual/
  # Ignore shared/ and all its (autocreated) child namespaces
  acl_ignore_namespace2 = shared/*
}

acl_shared_dict

Default	[None]
Value	string
See Also	dictionary

A shared mailbox dictionary that defines which users may LIST mailboxes shared by other users.

See shared mailbox listing for further details on the contents of the dictionary entries.

Example:

plugin {
  acl_shared_dict = file:/var/lib/dovecot/shared-mailboxes
}

acl_user

Default	[None]
Value	string
See Also	ACL master users

See auth_master_user_separator for the format of this setting.

master_user

Default	[None]
Value	string
See Also	acl_user

TODO

Configuration

Groups

The acl_groups setting can be dynamically set via userdb extra fields.

IMAP ACLs

To enable the IMAP ACL commands, you must load the imap_acl plugin. This plugin should only be loaded inside a protocol imap {} block.

Sample Configuration

# Enable internal ACL support
mail_plugins = acl

# Enable the IMAP ACL commands
protocol imap {
  mail_plugins = $mail_plugins imap_acl
}

plugin {
  acl = vfile

  # 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.
  #acl_globals_only = yes

  # Namespace prefix to ignore. Use counter to ignore multiple, e.g.
  #   acl_ignore_namespace2
  #acl_ignore_namespace =

  # Dict for mapping which users have shared mailboxes to each other.
  #acl_shared_dict =
}

Master Users

Master users have their own ACLs. They're not the mailbox owners, so by default they have no permissions to any of the mailboxes. See ACL master users for more information.

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 (e.g., ~/Maildir, ~/Maildir/.folder/).
• mbox: Control directory. You should explicitly specify :CONTROL=<path>mail_location.
• dbox: dbox's mail directory (e.g., ~/dbox/INBOX/dbox-Mails/).

ACL File Format

The ACL files 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)
• Negative rights can be given by prepending the identifier with -

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=foo rw
group-override=tempdisabled

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

Negative rights can be used to remove rights. For example a user may be given full rights to all mailboxes, except some of the rights removed from some specific mailboxes.

Supported ACLs

ID	Type	Description
l	lookup	Mailbox is visible in mailbox list. Mailbox can be subscribed to.
r	read	Mailbox can be opened for reading.
w	write	Message flags and keywords can be changed, except \Seen and \Deleted
s	write-seen	\Seen flag can be changed
t	write-deleted	\Deleted flag can be changed
i	insert	Messages can be written or copied to the mailbox
p	post	Messages can be posted to the mailbox by LDA, e.g. from Sieve
e	expunge	Messages can be expunged
k	create	Mailboxes can be created (or renamed) directly under this mailbox (but not necessarily under its children, see ACL Inheritance) (renaming also requires delete rights)
x	delete	Mailbox can be deleted
a	admin	Administration rights to the mailbox (currently: ability to change ACLs for mailbox)

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

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

Note

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, foo has list-read privileges:

owner lrwstipekxa
user=foo 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

Allow a masteruser full access to all mailboxes, except no access to INBOX:

* user=masteruser lrwstipekxa
INBOX -user=masteruser lrwstipekxa

ACL Inheritance and Default ACLs

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.

There are default ACLs though:

• In private namespace, the owner has all ACL rights for mailboxes in the namespace.
• In shared and public namespaces, there are no ACL rights by default.
• However, optionally the default ACLs can be taken from the INBOX for private and shared namespaces. See acl_defaults_from_inbox.

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. See ACL master users 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

Global ACL file path is specified as a parameter to vfile backend in acl, /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 that do the shell-string matching, not stopping at any boundaries.

Example:

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

The first line shares every mailbox of every user to the user foo with a limited set of rights, and the last line shares every folder below Public of every user to the user bar.

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.

Dictionaries

In order for an ACL to be fully useful, it has to be communicated to IMAP clients. For example, if you use ACL to share a mailbox to another user, the client has to be explicitly told to check out the other user's mailbox too, as that one is shared.

Placing the ACL file makes the ACL effective, but Dovecot doesn't take care of the user to shared mailboxes mapping out of the box, and as a result, it won't publish shared mailboxes to clients if this is not set up. You have to configure this manually by defining an appropriate dictionary to store the map using acl_shared_dict.

plugin {
  acl_shared_dict = file:/var/lib/dovecot/dovecot-acl.db
}