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:
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.