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# Cyrus IMAP

# 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 be the prefix “group:”. For example, “group:accounting”.

There are two special identifiers, “anonymous”, and “anyone”, which are explained below. The meaning of other identifiers usually depends on the authorization mechanism being used (selected by `--with-auth` at compile time, defaulting to Unix).

## `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.

## Kerberos vs. Unix Authorization

The Cyrus IMAP server comes with four authorization mechanisms, one is compatible with Unix-style (`/etc/passwd`) authorization, one for use with Kerberos 4, 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.

### Unix Authorization

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.

### Kerberos Authorization

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.

The file `/etc/krb.equiv` contains mappings between Kerberos principals. The file contains zero or more lines, each containing two fields. Any identity matching the first field of a line is changed to the second identity during canonicalization. For example, a line in `/etc/krb.equiv` of:

    bovik@REMOTE.COM bovik

will cause the identity `bovik@REMOTE.COM` to be treated as if it were the local identity `bovik`.

### Alternative Authorization

A site may wish to write their own authorization mechanism, perhaps to implement a local group mechanism. If it does so (by implementing an `auth_[whatever]` PTS module), it will dictate its own form and meaning of identifiers.