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Database administration

A Kerberos database contains all of a realm's Kerberos principals, their passwords, and other administrative information about each principal. For the most part, you will use the kdb5_util(8) program to manipulate the Kerberos database as a whole, and the kadmin(1) program to make changes to the entries in the database. (One notable exception is that users will use the kpasswd(1) program to change their own passwords.) The kadmin program has its own command-line interface, to which you type the database administrating commands.

kdb5_util(8) provides a means to create, delete, load, or dump a Kerberos database. It also contains commands to roll over the database master key, and to stash a copy of the key so that the kadmind(8) and krb5kdc(8) daemons can use the database without manual input.

kadmin(1) provides for the maintenance of Kerberos principals, password policies, and service key tables (keytabs). Normally it operates as a network client using Kerberos authentication to communicate with kadmind(8), but there is also a variant, named kadmin.local, which directly accesses the Kerberos database on the local filesystem (or through LDAP). kadmin.local is necessary to set up enough of the database to be able to use the remote version.

kadmin can authenticate to the admin server using the service principal kadmin/HOST (where HOST is the hostname of the admin server) or kadmin/admin. If the credentials cache contains a ticket for either service principal and the -c ccache option is specified, that ticket is used to authenticate to KADM5. Otherwise, the -p and -k options are used to specify the client Kerberos principal name used to authenticate. Once kadmin has determined the principal name, it requests a kadmin/admin Kerberos service ticket from the KDC, and uses that service ticket to authenticate to KADM5.

See kadmin(1) for the available kadmin and kadmin.local commands and options.

kadmin options

You can invoke kadmin(1) or kadmin.local with any of the following options:

OPTIONS

Date Format

For the supported date-time formats see getdate section in datetime.

Principals

Each entry in the Kerberos database contains a Kerberos principal and the attributes and policies associated with that principal.

Adding, modifying and deleting principals

To add a principal to the database, use the kadmin(1) add_principal command.

To modify attributes of a principal, use the kadmin modify_principal command.

To delete a principal, use the kadmin delete_principal command.

Examples

If you want to create a principal which is contained by a LDAP object, all you need to do is:

kadmin: addprinc -x dn=cn=jennifer,dc=example,dc=com jennifer
No policy specified for "jennifer@ATHENA.MIT.EDU";
defaulting to no policy.
Enter password for principal jennifer@ATHENA.MIT.EDU:  <= Type the password.
Re-enter password for principal jennifer@ATHENA.MIT.EDU:  <=Type it again.
Principal "jennifer@ATHENA.MIT.EDU" created.
kadmin:

If you want to create a principal under a specific LDAP container and link to an existing LDAP object, all you need to do is:

kadmin: addprinc -x containerdn=dc=example,dc=com -x linkdn=cn=david,dc=example,dc=com david
No policy specified for "david@ATHENA.MIT.EDU";
defaulting to no policy.
Enter password for principal david@ATHENA.MIT.EDU:  <= Type the password.
Re-enter password for principal david@ATHENA.MIT.EDU:  <=Type it again.
Principal "david@ATHENA.MIT.EDU" created.
kadmin:

If you want to associate a ticket policy to a principal, all you need to do is:

kadmin: modprinc -x tktpolicy=userpolicy david
Principal "david@ATHENA.MIT.EDU" modified.
kadmin:

If, on the other hand, you want to set up an account that expires on January 1, 2000, that uses a policy called "stduser", with a temporary password (which you want the user to change immediately), you would type the following:

kadmin: addprinc david -expire "1/1/2000 12:01am EST" -policy stduser +needchange
Enter password for principal david@ATHENA.MIT.EDU:  <= Type the password.
Re-enter password for principal
david@ATHENA.MIT.EDU:  <= Type it again.
Principal "david@ATHENA.MIT.EDU" created.
kadmin:

If you want to delete a principal:

kadmin: delprinc jennifer
Are you sure you want to delete the principal
"jennifer@ATHENA.MIT.EDU"? (yes/no): yes
Principal "jennifer@ATHENA.MIT.EDU" deleted.
Make sure that you have removed this principal from
all ACLs before reusing.
kadmin:

Retrieving information about a principal

To retrieve a listing of the attributes and/or policies associated with a principal, use the kadmin(1) get_principal command.

To generate a listing of principals, use the kadmin list_principals command.

Changing passwords

To change a principal's password use the kadmin(1) change_password command.

Note

Password changes through kadmin are subject to the same password policies as would apply to password changes through kpasswd(1).

Policies

A policy is a set of rules governing passwords. Policies can dictate minimum and maximum password lifetimes, minimum number of characters and character classes a password must contain, and the number of old passwords kept in the database.

Adding, modifying and deleting policies

To add a new policy, use the kadmin(1) add_policy command.

To modify attributes of a principal, use the kadmin modify_policy command.

To delete a policy, use the kadmin delete_policy command.

Note

You must cancel the policy from all principals before deleting it. The delete_policy command will fail if the policy is in use by any principals.

Retrieving policies

To retrieve a policy, use the kadmin(1) get_policy command.

You can retrieve the list of policies with the kadmin list_policies command.

Policies and principals

Policies can be applied to principals as they are created by using the -policy flag to add_principal. Existing principals can be modified by using the -policy or -clearpolicy flag to modify_principal.

Updating the history key

If a policy specifies a number of old keys kept of two or more, the stored old keys are encrypted in a history key, which is found in the key data of the kadmin/history principal.

Currently there is no support for proper rollover of the history key, but you can change the history key (for example, to use a better encryption type) at the cost of invalidating currently stored old keys. To change the history key, run:

kadmin: change_password -randkey kadmin/history

This command will fail if you specify the -keepold flag. Only one new history key will be created, even if you specify multiple key/salt combinations.

In the future, we plan to migrate towards encrypting old keys in the master key instead of the history key, and implementing proper rollover support for stored old keys.

Privileges

Administrative privileges for the Kerberos database are stored in the file kadm5.acl(5).

Note

A common use of an admin instance is so you can grant separate permissions (such as administrator access to the Kerberos database) to a separate Kerberos principal. For example, the user joeadmin might have a principal for his administrative use, called joeadmin/admin. This way, joeadmin would obtain joeadmin/admin tickets only when he actually needs to use those permissions.

Operations on the Kerberos database

The kdb5_util(8) command is the primary tool for administrating the Kerberos database.

OPTIONS

Dumping a Kerberos database to a file

To dump a Kerberos database into a file, use the kdb5_util(8) dump command on one of the KDCs.

Examples

shell% kdb5_util dump dumpfile
shell%

shell% kbd5_util dump -verbose dumpfile
kadmin/admin@ATHENA.MIT.EDU
krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
kadmin/history@ATHENA.MIT.EDU
K/M@ATHENA.MIT.EDU
kadmin/changepw@ATHENA.MIT.EDU
shell%

If you specify which principals to dump, you must use the full principal, as in the following example:

shell% kdb5_util dump -verbose dumpfile K/M@ATHENA.MIT.EDU kadmin/admin@ATHENA.MIT.EDU
kadmin/admin@ATHENA.MIT.EDU
K/M@ATHENA.MIT.EDU
shell%

Otherwise, the principals will not match those in the database and will not be dumped:

shell% kdb5_util dump -verbose dumpfile K/M kadmin/admin
shell%

If you do not specify a dump file, kdb5_util will dump the database to the standard output.

Restoring a Kerberos database from a dump file

To restore a Kerberos database dump from a file, use the kdb5_util(8) load command on one of the KDCs.

Examples

To dump a single principal and later load it, updating the database:

shell% kdb5_util dump dumpfile principal@REALM
shell%

shell% kdb5_util load -update dumpfile
shell%

Note

If the database file exists, and the -update flag was not given, kdb5_util will overwrite the existing database.

Note

Using kdb5_util to dump and reload the principal database is only necessary when upgrading from versions of krb5 prior to 1.2.0---newer versions will use the existing database as-is.

Creating a stash file

A stash file allows a KDC to authenticate itself to the database utilities, such as kadmind(8), krb5kdc(8), and kdb5_util(8).

To create a stash file, use the kdb5_util(8) stash command.

Example

shell% kdb5_util stash kdb5_util: Cannot find/read stored master key while reading master key kdb5_util: Warning: proceeding without master key Enter KDC database master key: <= Type the KDC database master password. shell%

If you do not specify a stash file, kdb5_util will stash the key in the file specified in your kdc.conf(5) file.

Creating and destroying a Kerberos database

If you need to create a new Kerberos database, use the kdb5_util(8) create command.

If you need to destroy the current Kerberos database, use the kdb5_util(8) destroy command.

Examples

shell% kdb5_util -r ATHENA.MIT.EDU create -s
Loading random data
Initializing database '/usr/local/var/krb5kdc/principal' for realm 'ATHENA.MIT.EDU',
master key name 'K/M@ATHENA.MIT.EDU'
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.
Enter KDC database master key:  <= Type the master password.
Re-enter KDC database master key to verify:  <= Type it again.
shell%

shell% kdb5_util -r ATHENA.MIT.EDU destroy
Deleting KDC database stored in '/usr/local/var/krb5kdc/principal', are you sure?
(type 'yes' to confirm)?  <= yes
OK, deleting database '/usr/local/var/krb5kdc/principal'...
** Database '/usr/local/var/krb5kdc/principal' destroyed.
shell%

Updating the master key

Starting with release 1.7, kdb5_util(8) allows the master key to be changed using a rollover process, with minimal loss of availability. To roll over the master key, follow these steps:

  1. On the master KDC, run kdb5_util list_mkeys to view the current master key version number (KVNO). If you have never rolled over the master key before, this will likely be version 1:

    $ kdb5_util list_mkeys
    Master keys for Principal: K/M@KRBTEST.COM
    KVNO: 1, Enctype: aes256-cts-hmac-sha384-192, Active on: Thu Jan 01 00:00:00 UTC 1970 *
  2. On the master KDC, run kdb5_util use_mkey 1 to ensure that a master key activation list is present in the database. This step is unnecessary in release 1.11.4 or later, or if the database was initially created with release 1.7 or later.

  3. On the master KDC, run kdb5_util add_mkey -s to create a new master key and write it to the stash file. Enter a secure password when prompted. If this is the first time you are changing the master key, the new key will have version 2. The new master key will not be used until you make it active.

  4. Propagate the database to all replica KDCs, either manually or by waiting until the next scheduled propagation. If you do not have any replica KDCs, you can skip this and the next step.

  5. On each replica KDC, run kdb5_util list_mkeys to verify that the new master key is present, and then kdb5_util stash to write the new master key to the replica KDC's stash file.

  6. On the master KDC, run kdb5_util use_mkey 2 to begin using the new master key. Replace 2 with the version of the new master key, as appropriate. You can optionally specify a date for the new master key to become active; by default, it will become active immediately. Prior to release 1.12, kadmind(8) must be restarted for this change to take full effect.

  7. On the master KDC, run kdb5_util update_princ_encryption. This command will iterate over the database and re-encrypt all keys in the new master key. If the database is large and uses DB2, the master KDC will become unavailable while this command runs, but clients should fail over to replica KDCs (if any are present) during this time period. In release 1.13 and later, you can instead run kdb5_util -x unlockiter update_princ_encryption to use unlocked iteration; this variant will take longer, but will keep the database available to the KDC and kadmind while it runs.

  8. Wait until the above changes have propagated to all replica KDCs and until all running KDC and kadmind processes have serviced requests using updated principal entries.

  9. On the master KDC, run kdb5_util purge_mkeys to clean up the old master key.

Operations on the LDAP database

The kdb5_ldap_util(8) is the primary tool for administrating the Kerberos LDAP database. It allows an administrator to manage realms, Kerberos services (KDC and Admin Server) and ticket policies.

OPTIONS

Creating a Kerberos realm

If you need to create a new realm, use the kdb5_ldap_util(8) create command as follows.

Modifying a Kerberos realm

If you need to modify a realm, use the kdb5_ldap_util(8) modify command as follows.

Destroying a Kerberos realm

If you need to destroy a Kerberos realm, use the kdb5_ldap_util(8) destroy command as follows.

Retrieving information about a Kerberos realm

If you need to display the attributes of a realm, use the kdb5_ldap_util(8) view command as follows.

Listing available Kerberos realms

If you need to display the list of the realms, use the kdb5_ldap_util(8) list command as follows.

Stashing service object's password

The kdb5_ldap_util(8) stashsrvpw command allows an administrator to store the password of service object in a file. The KDC and Administration server uses this password to authenticate to the LDAP server.

Ticket Policy operations

Creating a Ticket Policy

To create a new ticket policy in directory , use the kdb5_ldap_util(8) create_policy command. Ticket policy objects are created under the realm container.

Modifying a Ticket Policy

To modify a ticket policy in directory, use the kdb5_ldap_util(8) modify_policy command.

Retrieving Information About a Ticket Policy

To display the attributes of a ticket policy, use the kdb5_ldap_util(8) view_policy command.

Destroying a Ticket Policy

To destroy an existing ticket policy, use the kdb5_ldap_util(8) destroy_policy command.

Listing available Ticket Policies

To list the name of ticket policies in a realm, use the kdb5_ldap_util(8) list_policy command.

Cross-realm authentication

In order for a KDC in one realm to authenticate Kerberos users in a different realm, it must share a key with the KDC in the other realm. In both databases, there must be krbtgt service principals for both realms. For example, if you need to do cross-realm authentication between the realms ATHENA.MIT.EDU and EXAMPLE.COM, you would need to add the principals krbtgt/EXAMPLE.COM@ATHENA.MIT.EDU and krbtgt/ATHENA.MIT.EDU@EXAMPLE.COM to both databases. These principals must all have the same passwords, key version numbers, and encryption types; this may require explicitly setting the key version number with the -kvno option.

In the ATHENA.MIT.EDU and EXAMPLE.COM cross-realm case, the administrators would run the following commands on the KDCs in both realms:

shell%: kadmin.local -e "aes256-cts:normal"
kadmin: addprinc -requires_preauth krbtgt/ATHENA.MIT.EDU@EXAMPLE.COM
Enter password for principal krbtgt/ATHENA.MIT.EDU@EXAMPLE.COM:
Re-enter password for principal krbtgt/ATHENA.MIT.EDU@EXAMPLE.COM:
kadmin: addprinc -requires_preauth krbtgt/EXAMPLE.COM@ATHENA.MIT.EDU
Enter password for principal krbtgt/EXAMPLE.COM@ATHENA.MIT.EDU:
Enter password for principal krbtgt/EXAMPLE.COM@ATHENA.MIT.EDU:
kadmin:

Note

Even if most principals in a realm are generally created with the requires_preauth flag enabled, this flag is not desirable on cross-realm authentication keys because doing so makes it impossible to disable preauthentication on a service-by-service basis. Disabling it as in the example above is recommended.

Note

It is very important that these principals have good passwords. MIT recommends that TGT principal passwords be at least 26 characters of random ASCII text.

Changing the krbtgt key

A Kerberos Ticket Granting Ticket (TGT) is a service ticket for the principal krbtgt/REALM. The key for this principal is created when the Kerberos database is initialized and need not be changed. However, it will only have the encryption types supported by the KDC at the time of the initial database creation. To allow use of newer encryption types for the TGT, this key has to be changed.

Changing this key using the normal kadmin(1) change_password command would invalidate any previously issued TGTs. Therefore, when changing this key, normally one should use the -keepold flag to change_password to retain the previous key in the database as well as the new key. For example:

kadmin: change_password -randkey -keepold krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU

Warning

After issuing this command, the old key is still valid and is still vulnerable to (for instance) brute force attacks. To completely retire an old key or encryption type, run the kadmin purgekeys command to delete keys with older kvnos, ideally first making sure that all tickets issued with the old keys have expired.

Only the first krbtgt key of the newest key version is used to encrypt ticket-granting tickets. However, the set of encryption types present in the krbtgt keys is used by default to determine the session key types supported by the krbtgt service (see session_key_selection). Because non-MIT Kerberos clients sometimes send a limited set of encryption types when making AS requests, it can be important for the krbtgt service to support multiple encryption types. This can be accomplished by giving the krbtgt principal multiple keys, which is usually as simple as not specifying any -e option when changing the krbtgt key, or by setting the session_enctypes string attribute on the krbtgt principal (see set_string).

Due to a bug in releases 1.8 through 1.13, renewed and forwarded tickets may not work if the original ticket was obtained prior to a krbtgt key change and the modified ticket is obtained afterwards. Upgrading the KDC to release 1.14 or later will correct this bug.

Incremental database propagation

Overview

At some very large sites, dumping and transmitting the database can take more time than is desirable for changes to propagate from the master KDC to the replica KDCs. The incremental propagation support added in the 1.7 release is intended to address this.

With incremental propagation enabled, all programs on the master KDC that change the database also write information about the changes to an "update log" file, maintained as a circular buffer of a certain size. A process on each replica KDC connects to a service on the master KDC (currently implemented in the kadmind(8) server) and periodically requests the changes that have been made since the last check. By default, this check is done every two minutes.

Incremental propagation uses the following entries in the per-realm data in the KDC config file (See kdc.conf(5)):

iprop_enable boolean If true, then incremental propagation is enabled, and (as noted below) normal kprop propagation is disabled. The default is false.
iprop_master_ulogsize integer Indicates the number of entries that should be retained in the update log. The default is 1000; the maximum number is 2500.
iprop_replica_poll time interval Indicates how often the replica should poll the master KDC for changes to the database. The default is two minutes.
iprop_port integer Specifies the port number to be used for incremental propagation. This is required in both master and replica configuration files.
iprop_resync_timeout integer Specifies the number of seconds to wait for a full propagation to complete. This is optional on replica configurations. Defaults to 300 seconds (5 minutes).
iprop_logfile file name Specifies where the update log file for the realm database is to be stored. The default is to use the database_name entry from the realms section of the config file kdc.conf(5), with .ulog appended. (NOTE: If database_name isn't specified in the realms section, perhaps because the LDAP database back end is being used, or the file name is specified in the dbmodules section, then the hard-coded default for database_name is used. Determination of the iprop_logfile default value will not use values from the dbmodules section.)

Both master and replica sides must have a principal named kiprop/hostname (where hostname is the lowercase, fully-qualified, canonical name for the host) registered in the Kerberos database, and have keys for that principal stored in the default keytab file (). In release 1.13, the kiprop/hostname principal is created automatically for the master KDC, but it must still be created for replica KDCs.

On the master KDC side, the kiprop/hostname principal must be listed in the kadmind ACL file kadm5.acl(5), and given the p privilege (see privileges).

On the replica KDC side, kpropd(8) should be run. When incremental propagation is enabled, it will connect to the kadmind on the master KDC and start requesting updates.

The normal kprop mechanism is disabled by the incremental propagation support. However, if the replica has been unable to fetch changes from the master KDC for too long (network problems, perhaps), the log on the master may wrap around and overwrite some of the updates that the replica has not yet retrieved. In this case, the replica will instruct the master KDC to dump the current database out to a file and invoke a one-time kprop propagation, with special options to also convey the point in the update log at which the replica should resume fetching incremental updates. Thus, all the keytab and ACL setup previously described for kprop propagation is still needed.

If an environment has a large number of replicas, it may be desirable to arrange them in a hierarchy instead of having the master serve updates to every replica. To do this, run kadmind -proponly on each intermediate replica, and kpropd -A upstreamhostname on downstream replicas to direct each one to the appropriate upstream replica.

There are several known restrictions in the current implementation:

Sun/MIT incremental propagation differences

Sun donated the original code for supporting incremental database propagation to MIT. Some changes have been made in the MIT source tree that will be visible to administrators. (These notes are based on Sun's patches. Changes to Sun's implementation since then may not be reflected here.)

The Sun config file support looks for sunw_dbprop_enable, sunw_dbprop_master_ulogsize, and sunw_dbprop_slave_poll.

The incremental propagation service is implemented as an ONC RPC service. In the Sun implementation, the service is registered with rpcbind (also known as portmapper) and the client looks up the port number to contact. In the MIT implementation, where interaction with some modern versions of rpcbind doesn't always work well, the port number must be specified in the config file on both the master and replica sides.

The Sun implementation hard-codes pathnames in /var/krb5 for the update log and the per-replica kprop dump files. In the MIT implementation, the pathname for the update log is specified in the config file, and the per-replica dump files are stored in /replica_datatrans_hostname.