Manpages - kadm5.acl.5

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kadm5.acl - Kerberos ACL file


The Kerberos kadmind(8) daemon uses an Access Control List (ACL) file to manage access rights to the Kerberos database. For operations that affect principals, the ACL file also controls which principals can operate on which other principals.

The default location of the Kerberos ACL file is /var/lib/krb5kdc/kadm5.acl unless this is overridden by the acl_file variable in kdc.conf(5).


Empty lines and lines starting with the sharp sign (#) are ignored. Lines containing ACL entries have the format:


      principal  permissions  [target_principal  [restrictions] ]



#+begin_quote Line order in the ACL file is important. The first matching entry will control access for an actor principal on a target principal.


(Partially or fully qualified Kerberos principal name.) Specifies the principal whose permissions are to be set.

Each component of the name may be wildcarded using the * character.

Specifies what operations may or may not be performed by a principal matching a particular entry. This is a string of one or more of the following list of characters or their upper-case counterparts. If the character is upper-case, then the operation is disallowed. If the character is lower-case, then the operation is permitted.
a [Dis]allows the addition of principals or policies
c [Dis]allows the changing of passwords for principals
d [Dis]allows the deletion of principals or policies
e [Dis]allows the extraction of principal keys
i [Dis]allows inquiries about principals or policies
l [Dis]allows the listing of all principals or policies
m [Dis]allows the modification of principals or policies
p [Dis]allows the propagation of the principal database (used in incr_db_prop)
s [Dis]allows the explicit setting of the key for a principal
x Short for admcilsp. All privileges (except e)
* Same as x.


#+begin_quote The extract privilege is not included in the wildcard privilege; it must be explicitly assigned. This privilege allows the user to extract keys from the database, and must be handled with great care to avoid disclosure of important keys like those of the kadmin/* or krbtgt/* principals. The lockdown_keys principal attribute can be used to prevent key extraction from specific principals regardless of the granted privilege.


(Optional. Partially or fully qualified Kerberos principal name.) Specifies the principal on which permissions may be applied. Each component of the name may be wildcarded using the * character.

target_principal can also include back-references to principal, in which *number matches the corresponding wildcard in principal.


(Optional) A string of flags. Allowed restrictions are:



flag is forced to the indicated value. The permissible flags are the same as those for the default_principal_flags variable in kdc.conf(5).
policy is forced to be empty.
-policy pol
policy is forced to be pol.
(no term)
-{expire, pwexpire, maxlife, maxrenewlife} time :: (getdate string) associated value will be forced to MIN(time, requested value).


The above flags act as restrictions on any add or modify operation which is allowed due to that ACL line. #+end_quote


#+begin_quote If the kadmind ACL file is modified, the kadmind daemon needs to be restarted for changes to take effect.



Here is an example of a kadm5.acl file:


      */admin@ATHENA.MIT.EDU    *                               # line 1
      joeadmin@ATHENA.MIT.EDU   ADMCIL                          # line 2
      joeadmin/*@ATHENA.MIT.EDU i   */root@ATHENA.MIT.EDU       # line 3
      */root@ATHENA.MIT.EDU     ci  *1@ATHENA.MIT.EDU           # line 4
      */root@ATHENA.MIT.EDU     l   *                           # line 5
      sms@ATHENA.MIT.EDU        x   * -maxlife 9h -postdateable # line 6


(line 1) Any principal in the ATHENA.MIT.EDU realm with an admin instance has all administrative privileges except extracting keys.

(lines 1-3) The user joeadmin has all permissions except extracting keys with his admin instance, joeadmin/admin@ATHENA.MIT.EDU (matches line 1). He has no permissions at all with his null instance, joeadmin@ATHENA.MIT.EDU (matches line 2). His root and other non-admin, non-null instances (e.g., extra or dbadmin) have inquire permissions with any principal that has the instance root (matches line 3).

(line 4) Any root principal in ATHENA.MIT.EDU can inquire or change the password of their null instance, but not any other null instance. (Here, *1 denotes a back-reference to the component matching the first wildcard in the actor principal.)

(line 5) Any root principal in ATHENA.MIT.EDU can generate the list of principals in the database, and the list of policies in the database. This line is separate from line 4, because list permission can only be granted globally, not to specific target principals.

(line 6) Finally, the Service Management System principal sms@ATHENA.MIT.EDU has all permissions except extracting keys, but any principal that it creates or modifies will not be able to get postdateable tickets or tickets with a life of longer than 9 hours.


The ACL file can coexist with other authorization modules in release 1.16 and later, as configured in the kadm5_auth section of krb5.conf(5). The ACL file will positively authorize operations according to the rules above, but will never authoritatively deny an operation, so other modules can authorize operations in addition to those authorized by the ACL file.

To operate without an ACL file, set the acl_file variable in kdc.conf(5) to the empty string with acl_file = “”.


kdc.conf(5), kadmind(8)




1985-2021, MIT

Author: dt

Created: 2022-02-20 Sun 09:32