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2.4.7 ksu

The Kerberos V5 ksu program replaces the standard UNIX su program. ksu first authenticates you to Kerberos. Depending on the configuration of your system, ksu may ask for your Kerberos password if authentication fails. Note that you should never type your password if you are remotely logged in using an unencrypted connection.

Once ksu has authenticated you, if your Kerberos principal appears in the target's .k5login file (see Granting Access to Your Account) or in the target's .k5users file (see below), it switches your user ID to the target user ID.

For example, david has put jennifer's Kerberos principal in his .k5login file. If jennifer uses ksu to become david, the exchange would look like this. (To differentiate between the two shells, jennifer's prompt is represented as jennifer% and david's prompt is represented as david%.)

     jennifer% ksu david
     Account david: authorization for jennifer@ATHENA.MIT.EDU successful
     Changing uid to david (3382)

Note that the new shell has a copy of jennifer's tickets. The ticket filename contains david's UID with .1 appended to it:

     david% klist
     Ticket cache: /tmp/krb5cc_3382.1
     Default principal: jennifer@ATHENA.MIT.EDU
     Valid starting      Expires             Service principal
     07/31/04 21:53:01  08/01/04 07:52:53  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
     07/31/04 21:53:39  08/01/04 07:52:53  host/

If jennifer had not appeared in david's .k5login file (and the system was configured to ask for a password), the exchange would have looked like this (assuming david has taken appropriate precautions in protecting his password):

     jennifer% ksu david
     WARNING: Your password may be exposed if you enter it here and are logged
              in remotely using an unsecure (non-encrypted) channel.
     Kerberos password for david@ATHENA.MIT.EDU:  <-  jennifer types the wrong password here.
     ksu: Password incorrect
     Authentication failed.

Now, suppose david did not want to give jennifer full access to his account, but wanted to give her permission to list his files and use the "more" command to view them. He could create a .k5users file giving her permission to run only those specific commands.

The .k5users file is like the .k5login file, except that each principal is optionally followed by a list of commands. ksu will let those principals execute only the commands listed, using the -e option. david's .k5users file might look like the following:

     jennifer@ATHENA.MIT.EDU       /bin/ls /usr/bin/more
     joeadmin@ATHENA.MIT.EDU         /bin/ls
     joeadmin/admin@ATHENA.MIT.EDU   *

The above .k5users file would let jennifer run only the commands /bin/ls and /usr/bin/more. It would let joeadmin run only the command /bin/ls if he had regular tickets, but if he had tickets for his admin instance, joeadmin/admin@ATHENA.MIT.EDU, he would be able to execute any command. The last line gives david in the realm EXAMPLE.COM permission to execute any command. (I.e., having only a Kerberos principal on a line is equivalent to giving that principal permission to execute *.) This is so that david can allow himself to execute commands when he logs in, using Kerberos, from a machine in the realm EXAMPLE.COM.

Then, when jennifer wanted to list his home directory, she would type:

     jennifer% ksu david -e ls ~david
     Authenticated jennifer@ATHENA.MIT.EDU
     Account david: authorization for jennifer@ATHENA.MIT.EDU for execution of
                    /bin/ls successful
     Changing uid to david (3382)
     Mail            News            Personal        misc            bin

If jennifer had tried to give a different command to ksu, it would have prompted for a password as with the previous example.

Note that unless the .k5users file gives the target permission to run any command, the user must use ksu with the -e command option.

The ksu options you are most likely to use are:

-n principal
specifies which Kerberos principal you want to use for ksu. (e.g., the user joeadmin might want to use his admin instance. See What is a Ticket?.)
specifies the location of your Kerberos credentials cache (ticket file).
tells ksu not to destroy your Kerberos tickets when ksu is finished.
requests forwardable tickets. (See Obtaining Tickets with kinit.) This is only applicable if ksu needs to obtain tickets.
-l lifetime
sets the ticket lifetime. (See Obtaining Tickets with kinit.) This is only applicable if ksu needs to obtain tickets.
tells ksu to copy your Kerberos tickets only if the UID you are switching is the same as the Kerberos primary (either yours or the one specified by the -n option).
tells ksu not to copy any Kerberos tickets to the new UID.
-e command
tells ksu to execute command and then exit. See the description of the .k5users file above.
-a text
(at the end of the command line) tells ksu to pass everything after -a to the target shell.

The full set of options to Kerberos V5 ksu are discussed in the Reference section of this manual. (see ksu Reference)