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Mailbox sharing between users
=============================

To enable mailbox sharing, you'll need to create a shared namespace. See
<ACL.txt> for more information about ACL-specific settings.

---%<-------------------------------------------------------------------------
# User's private mail location.
mail_location = maildir:~/Maildir

# When creating any namespaces, you must also have a private namespace:
namespace {
  type = private
  separator = /
  prefix =
  #location defaults to mail_location.
  inbox = yes
}

namespace {
  type = shared
  separator = /
  prefix = shared/%%u/
  # a) Per-user seen flags. Maildir indexes are shared. (INDEXPVT requires
v2.2+)
  location = maildir:%%h/Maildir:INDEXPVT=~/Maildir/shared/%%u
  # b) Per-user seen flags. Maildir indexes are not shared. If users have
direct filesystem level access to their mails, this is a safer option:
  #location =
maildir:%%h/Maildir:INDEX=~/Maildir/shared/%%u:INDEXPVT=~/Maildir/shared/%%u
  subscriptions = no
  list = children
}

mail_plugins = acl
protocol imap {
  mail_plugins = $mail_plugins imap_acl
}
plugin {
  acl = vfile
}
---%<-------------------------------------------------------------------------

This creates a shared/ namespace under which each user's mailboxes are. If you
have multiple domains and allow sharing between them, you might want to set
'prefix=shared/%%d/%%n/' instead (although %%u works just fine too). If you
don't, you might want to drop the domain part and instead use
'prefix=shared/%%n/'.

'list=children' specifies that if no one has shared mailboxes to the user, the
"shared" directory isn't listed by the LIST command. If you wish it to be
visible always, you can set 'list=yes'.

The 'location' setting specifies how to access other users' mailboxes. If you
use %%h, the user's home directory is asked from auth process via auth-userdb
socket. See <LDA.txt> for how to configure the socket. If the users' mailboxes
can be found using a template, it's faster not to use the %%h. For example:

---%<-------------------------------------------------------------------------
  location = maildir:/var/mail/%%d/%%n/Maildir:INDEXPVT=~/Maildir/shared/%%u
---%<-------------------------------------------------------------------------

% vs %%
-------

%var expands to the logged in user's variable, while %%var expands to the other
users' variables. For example if your name is "myself" and "someone1" and
"someone2" have shared mailboxes to you, the variables could be expanded like:

 * %u expands to "myself"
 * %%u expands to "someone1" or "someone2"
 * %h might expand to "/home/myself"
 * %%h might expand to "/home/someone1" or "/home/someone2"
 * ~/ equals %h/

Note that in e.g. mail_location setting you might need both. For example in:

---%<-------------------------------------------------------------------------
mail_location = maildir:%%h/Maildir:INDEXPVT=%h/Maildir/shared/%%u
---%<-------------------------------------------------------------------------

What it means is:

 * %%h/Maildir points to the other user's Maildir, e.g. "/home/someone1".
 * :INDEXPVT=%h/Maildir/shared/%%u points to a per-user directory under your
   own Maildir, e.g. "/home/myself/Maildir/someone1" or
   "/home/myself/Maildir/someone2". This is necessary for storing per-user seen
   flags.

dbox
----

With dbox the index files are a very important part of the mailboxes. You must
not try to :INDEX= to have copies of index files. This will only result in
mailbox corruption. (INDEXPVT can be used though.)

Filesystem permissions
----------------------

Dovecot assumes that it can access the other users' mailboxes. If you use
multiple UNIX UIDs, you may have problems setting up the permissions so that
the mailbox sharing works. Dovecot never modifies existing files' permissions.
See <SharedMailboxes.Permissions.txt> for more information.

Shared mailbox listing
----------------------

With the above configuration it's possible to open shared mailboxes if you know
their name, but they won't be visible in the mailbox list. This is because
Dovecot has no way of knowing what users have shared mailboxes to whom.
Iterating through all users and looking inside their mail directories would be
horribly inefficient for more than a couple users.

To overcome this problem Dovecot needs a dictionary, which contains the list of
users who have shared mailboxes and to whom they have shared. If the users
aren't properly listed in this dictionary, their shared mailboxes won't be
visible. Currently there's no way to automatically rebuild this dictionary, so
make sure it doesn't get lost. If it does, each user having shared mailboxes
must use the IMAP SETACL command (see below) to get the dictionary updated for
themselves.

You could use any dictionary backend, including SQL, but a simple flat file
should work pretty well too:

---%<-------------------------------------------------------------------------
plugin {
  acl_shared_dict = file:/var/lib/dovecot/db/shared-mailboxes.db
}
---%<-------------------------------------------------------------------------

The IMAP processes must be able to write to the 'db/' directory. If you're
using system users, you probably want to make it mode 0770 and group
'sharedusers' and set 'mail_access_groups=sharedusers' (or something similar).

If you use multiple domains and don't wish users to share their mailboxes to
users in other domains, you can use separate dict files for each domain:

---%<-------------------------------------------------------------------------
plugin {
  # assumes mailboxes are in /var/mail/%d/%n:
  acl_shared_dict = file:/var/mail/%d/shared-mailboxes.db
}
---%<-------------------------------------------------------------------------

Using SQL dictionary
--------------------

'dovecot.conf':

---%<-------------------------------------------------------------------------
plugin {
  acl_shared_dict = proxy::acl
}

dict {
  acl = pgsql:/etc/dovecot/dovecot-dict-sql.conf.ext
}
---%<-------------------------------------------------------------------------

See <Dict.txt> for more information, especially about permission issues.

Database tables:

---%<-------------------------------------------------------------------------
CREATE TABLE user_shares (
  from_user varchar(100) not null,
  to_user varchar(100) not null,
  dummy char(1) DEFAULT '1',    -- always '1' currently
  primary key (from_user, to_user)
);
COMMENT ON TABLE user_shares IS 'User from_user shares folders to user
to_user.';

CREATE INDEX to_user
ON user_shares (to_user); -- because we always search for to_user

CREATE TABLE anyone_shares (
  from_user varchar(100) not null,
  dummy char(1) DEFAULT '1',    -- always '1' currently
  primary key (from_user)
);
COMMENT ON TABLE anyone_shares IS 'User from_user shares folders to anyone.';
---%<-------------------------------------------------------------------------

'/etc/dovecot/dovecot-dict-sql.conf.ext':

---%<-------------------------------------------------------------------------
connect = host=localhost dbname=mails user=sqluser password=sqlpass
map {
  pattern = shared/shared-boxes/user/$to/$from
  table = user_shares
  value_field = dummy

  fields {
    from_user = $from
    to_user = $to
  }
}

map {
  pattern = shared/shared-boxes/anyone/$from
  table = anyone_shares
  value_field = dummy

  fields {
    from_user = $from
  }
}
---%<-------------------------------------------------------------------------

Mailbox sharing
---------------

You can use <doveadm acl> [Tools.Doveadm.ACL.txt] to share mailboxes or it can
be done using IMAP SETACL command. It is the only way to update the shared
mailbox list dictionary.

Below is a quick introduction to IMAP ACL commands. See RFC 4314
[http://www.ietf.org/rfc/rfc4314.txt] for more details.

 * MYRIGHTS <mailbox>: Returns the user's current rights to the mailbox.
 * GETACL <mailbox>: Returns the mailbox's all ACLs.
 * SETACL <mailbox> <id> [+|-]<rights>: Give <id> the specified rights to the
   mailbox.
 * DELETEACL <mailbox> [-]<id>: Delete <id>'s ACL from the mailbox.

<id> is one of:

 * anyone: Matches all users, including anonymous users.
 * authenticated: Like "anyone", but doesn't match anonymous users.
 * $group: Matches all users belonging to the group ($ is not part of the group
   name).
 * $!group: See group-override in <ACL.txt> (Dovecot-specific feature).
 * user: Matches the given user.

The $group syntax is not a standard, but it is mentioned in RFC 4314 examples
and is also understood by at least Cyrus IMAP. Having '-' before the identifier
specifies negative rights.

See <ACL.txt> for list of <rights>.

Sharing mailboxes to everyone
-----------------------------

By default Dovecot doesn't allow using the IMAP "anyone" or "authenticated"
identifier, because it would be an easy way to spam other users in the system.
If you wish to allow it, set:

---%<-------------------------------------------------------------------------
plugin {
  acl_anyone = allow
}
---%<-------------------------------------------------------------------------

Note that you can also do this only for some users by using the second table
"anyone_shares". Every user listed in this table shares his folders with
everyone. See also <userdb extra field> [UserDatabase.ExtraFields.txt].

IMAP ACL examples
-----------------

Let's begin with some simple example that first gives "read" and "lookup"
rights, and later adds "write-seen" right:

---%<-------------------------------------------------------------------------
1 SETACL Work user@domain rl
1 OK Setacl complete.

2 SETACL Work user@domain +s
2 OK Setacl complete.

3 GETACL Work
* ACL "Work" "user@domain" lrs "myself" lrwstipekxacd
3 OK Getacl completed.
---%<-------------------------------------------------------------------------

Let's see how negative rights work by testing it on ourself. See how we
initially have "lookup" right, but later we don't:

---%<-------------------------------------------------------------------------
1 MYRIGHTS Work
* MYRIGHTS "Work" lrwstipekxacd
1 OK Myrights completed.

2 SETACL Work -myself l
2 OK Setacl complete.

3 GETACL Work
* ACL "Work" "-myself" l "user@domain" lr "myself" lrwstipekxacd
3 OK Getacl completed.

4 myrights Work
* MYRIGHTS "Work" rwstipekxacd
4 OK Myrights completed.
---%<-------------------------------------------------------------------------

Troubleshooting
---------------

 * Make sure the % and %% variables are specified correctly in the namespace
   location.'mail_debug=yes' will help you see if Dovecot is trying to access
   correct paths.
 * 'doveadm acl debug -u user@domain shared/user/box' can be helpful in
   figuring out why a mailbox can't be accessed.

(This file was created from the wiki on 2019-06-19 12:42)