# Access Control Identifier (ACI)

The Access Control Identifier (ACI) part of an ACL entry specifies the user or group for which the entry applies. Group identifiers are distinguished by the prefix "group:". For example, "group:accounting".

There are two special identifiers, "anonymous", and "anyone". The meaning of other identifiers usually depends on the authorization mechanism being used.

## `anonymous` and `anyone`

With any authorization mechanism, two special identifiers are defined. The identifier `anonymous` refers to the anonymous, or unauthenticated user. The identifier `anyone` refers to all users, including the anonymous user.

Both `anonymous` and `anyone` may commonly be used with the post right `p` to allow message insertion to mailboxes.

# Authorization Mechanisms

The Cyrus IMAP server comes with four authorization mechanisms, one is compatible with Unix-style (`/etc/passwd`) authorization, one called `mboxgroups`, one for use with Kerberos 5, and one for use with an external authorization process (ptloader) which can interface with other group databases (e.g. AFS PTS groups, LDAP Groups, etc).

Note

Authentication !== Authorization

Note that authorization is not the same thing as authentication. Authentication is the act of proving who you are. Authorization is the act of determining what rights you have. Authentication is discussed in the Login Authentication part of this document.

The authorization mechanism in use is determined by the `auth_mech` imapd.conf(5) option:

> > `auth_mech:` unix

> > > The authorization mechanism to use.

> > > Allowed values: unix, pts, krb5, mboxgroups

## Unix Authorization

    auth_mech: unix

In the Unix authorization mechanism, ACIs are either a valid userid or the string `group:` followed by a group listed in `/etc/group`. Thus:

    root                Refers to the user root
    group:staff         Refers to the group staff

It is also possible to use unix groups with users authenticated through a non-/etc/passwd backend. Note that using unix groups in this way (without associated `/etc/passwd` entries) is not recommended.

Note

Cyrus requires the getgrent(3) POSIX sysctl. As such, NSS needs to be configured to have the groups available, one of which includes "files", but could also include "ldap".

NSS augmentations, such as `nss_ldap`, `pam_ldap` or `sssd` may be used to provide Cyrus access to group information via NSS.

## mboxgroups Authorization

    auth_mech: mboxgroups

The mboxgroups authorization mechanism is like the Unix mechanism, but it looks for groups stored in the mailboxes.db instead of the system groups file.

When this authorization mechanism is in use, imapd will report the capability `XUSERGROUPS`, and admins can use the IMAP commands `GETUSERGROUP`, `SETUSERGROUP`, and `UNSETUSERGROUP` for group management.

> GETUSERGROUP item

> > If item is a userid, returns the groups the user belongs to. If item is a group identifier, returns its members.

> >     C: 8 GETUSERGROUP cassandane
> >     S: * USERGROUP cassandane ("group:group c" "group:group co")
> >     S: 8 OK Completed
> >     C: 9 GETUSERGROUP "group:group co"
> >     S: * USERGROUP "group:group co" (cassandane otheruser)
> >     S: 9 OK Completed

> SETUSERGROUP userid group

> > Adds userid as a member of group

> >     C: 9 SETUSERGROUP cassandane "group:new group"
> >     S: 9 OK Completed

> UNSETUSERGROUP userid group

> > Removes userid from group

> >     C: 9 UNSETUSERGROUP cassandane "group:group c"
> >     S: 9 OK Completed

## Kerberos Authorization

    auth_mech: krb5

Using the Kerberos authorization mechanism, ACIs are of the form:

> $principal.*$instance*@*$realm*

If `$instance` is omitted, it defaults to the null string. If `$realm` is omitted, it defaults to the local realm.

## PTS Authorization

    auth_mech: pts

The PTS authorization mechanism is modular, with the module selected by the `pts_module` imapd.conf(5) option:

> > `pts_module:` afskrb

> > > The PTS module to use.

> > > Allowed values: afskrb, ldap, http

The meaning of identifiers depends on the PTS module being used.

### AFSKRB Authorization using PTS

    auth_mech: pts
    pts_module: afskrb

Document this! Probably by linking to a separate document.

### HTTP Authorization using PTS

    auth_mech: pts
    pts_module: http

Document this! Probably by linking to a separate document.

### LDAP Authorization using PTS

    auth_mech: pts
    pts_module: ldap

Document this! Probably by linking to a separate document.

### Alternative Authorization using PTS

    auth_mech: pts
    pts_module: ???

A site may wish to write their own authorization mechanism, perhaps to implement a local group mechanism. You do this by implementing a custom PTS module. The form and meaning of identifiers will be up to the implementation.