Filesystem permissions in shared mailboxes
==========================================

IMAP processes need filesystem level permissions to access shared/public
mailboxes. This means that:

 * If you use more than one <UNIX UID> [UserIds.txt] for your mail users (e.g.
   you use system users), you'll need to make sure that all users can access
   the mailboxes on filesystem level. ( <ACL plugin> [ACL.txt] won't help you
   with this.)
 * You can remove write permissions on purpose from public namespace root
   directory to prevent users from creating new mailboxes under it.

Dovecot never modifies permissions for existing mail files or directories. When
users share mailboxes between each others, the system must have been set up in
a way that filesystem permissions don't get in the way. The easiest way to do
that is to use only a single UID. Another possibility would be to use one or
more groups for all the mail files that may be shared to other users belonging
to the same group. For example if you host multiple domains, you might create a
group for each domain and allow mailbox sharing (only) between users in the
same domain.

System user UNIX groups
-----------------------

There's no requirement to use UNIX groups (i.e. typically defined in
'/etc/group') for anything. If you don't care about them, you can safely ignore
this section.

If you use <passwd> [AuthDatabase.Passwd.txt] userdb, the IMAP process has
access to all the UNIX groups defined for that user. You may use these groups
when granting filesystem permissions. If you wish to use UNIX groups defined in
'/etc/group' but don't use passwd userdb, you can still do this by returning
'system_groups_user' <userdb extra field> [UserDatabase.ExtraFields.txt], which
contains the UNIX user name whose groups are read from the group file.

You can also set up extra UNIX groups by listing them in 'mail_access_groups'
setting. To have per-user UNIX groups, return 'mail_access_groups' as userdb
extra field. The advantage of using this method is that only Dovecot mail
processes have access to the group, but nothing else, such as user's SSH
session. For example a simple way to set up shared mailbox access for all
system users is to make all mail dirs/files 0770/0660 mode and owned by group
"sharedmail" and then set 'mail_access_groups=sharedmail'. Using more fine
grained groups of course leaks less mail data in case there's a security hole
in Dovecot.

Permissions for new mailboxes
-----------------------------

When creating a new mailbox, Dovecot copies the permissions from the mailbox
root directory. For example with mboxes if you have directories:

---%<-------------------------------------------------------------------------
drwx--xr-x 8 user group 4096 2009-02-21 18:31 /home/user/mail/
drwxrwxrwx 2 user group 4096 2009-02-21 18:32 /home/user/mail/foo/
---%<-------------------------------------------------------------------------

When creating a new foo/bar/ directory, Dovecot gives it permissions:

---%<-------------------------------------------------------------------------
drwx--xr-x 2 user group 4096 2009-02-21 18:33 /home/user/mail/foo/bar/
---%<-------------------------------------------------------------------------

As you can see, the file mode was copied from mail/ directory, not mail/foo/.
The group is also preserved. If this causes problems (e.g. different users
having different groups create mailboxes, causing permission denied errors when
trying to preserve the group) you can set the setgid bit for the root
directory:

---%<-------------------------------------------------------------------------
chmod g+s /home/user/mail
---%<-------------------------------------------------------------------------

This will cause the group to be automatically copied by the OS for all created
files/directories under it, even if the user doesn't belong to the group.

Permissions for new files in mailboxes
--------------------------------------

When creating new files inside a mailbox, Dovecot copies the read/write
permissions from the mailbox's directory. For example if you have:

---%<-------------------------------------------------------------------------
drwx--xr-x 5 user group 4096 2009-02-21 18:53 /home/user/Maildir/.foo/
---%<-------------------------------------------------------------------------

Dovecot creates files under it with modes:

---%<-------------------------------------------------------------------------
drwx--xr-x 2 user group 4096 2009-02-21 18:54 cur/
drwx--xr-x 2 user group 4096 2009-02-21 18:54 new/
drwx--xr-x 2 user group 4096 2009-02-21 18:54 tmp/
-rw----r-- 1 user group  156 2009-02-21 18:54 dovecot.index.log
-rw----r-- 1 user group   17 2009-02-21 18:54 dovecot-uidlist
---%<-------------------------------------------------------------------------

Note how the g+x gets copied to directories, but for files it's simply ignored.
The group is copied the same way as explained in the previous section.

When mails are copied between Maildirs, it's usually done by hard linking. If
the source and destination directory permissions are different, Dovecot create
a new file and copies data the slow way so that it can assign the wanted
destination permissions. The source and destination permission lookups are done
only by looking at the mailbox root directories' permissions, not individual
mail files. This may become a problem if the mail files' permissions aren't as
Dovecot expects.

Permissions to new /domain/user directories
-------------------------------------------

If each user has different UIDs and you have '/var/mail/domain/user/' style
directories, you run into a bit of trouble. The problem is that the first user
who creates '/var/mail/domain/' will create it as 0700 mode, and other users
can't create their own user/ directories under it anymore. The solution is to
use a common group for the users and set '/var/mail/' directory's permissions
properly (group-suid is required):

---%<-------------------------------------------------------------------------
chgrp dovemail /var/mail
chmod 02770 /var/mail # or perhaps 03770 for extra security
---%<-------------------------------------------------------------------------

and in dovecot.conf:

---%<-------------------------------------------------------------------------
mail_location = maildir:/var/vmail/%d/%n/Maildir
mail_access_groups = dovemail
---%<-------------------------------------------------------------------------

The end result should look like this:

---%<-------------------------------------------------------------------------
drwxrwsr-x 3 user dovemail 60 Oct 24 12:04 domain.example.com/
drwx--S--- 3 user user 60 Oct 24 12:04 domain.example.com/user/
---%<-------------------------------------------------------------------------

Note that this requires that the mail_location setting is in its explicit
format with %variables. Using 'maildir:~/Maildir' won't work, because Dovecot
can't really know how far down it should copy the permissions from.

Permissions to new user home directories (v2.2+)
------------------------------------------------

When mail_location begins with '%h' or '~/', its permissions are copied from
the first existing parent directory if it has setgid-bit set. This isn't done
when the path contains any other %variables.

Mail Delivery Agent permissions
-------------------------------

When using Dovecot <LDA.txt>, it uses all the same configuration files as
IMAP/POP3, so you don't need to worry about it.

When using an external MDA to deliver to a shared mailbox, you need to make
sure that the resulting files have proper permissions. For example with
Procmail + Maildir, set 'UMASK=007' in '.procmailrc' to make the delivered mail
files group-readable. To get the file to use the proper group, set the group to
the Maildir's 'tmp/' directory and also set its setgid bit ('chmod g+s').

Dictionary files
----------------

Created dictionary files (e.g. 'acl_shared_dict = file:...') also base their
initial permissions on parent directory's permissions. After the initial
creation, the permissions are permanently preserved. So if you want to use
different permissions, just chown/chmod the file.

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