On many systems, Kerberos is built into the login program, and you get tickets automatically when you log in. Other programs, such as ssh, can forward copies of your tickets to a remote host. Most of these programs also automatically destroy your tickets when they exit. However, MIT recommends that you explicitly destroy your Kerberos tickets when you are through with them, just to be sure. One way to help ensure that this happens is to add the kdestroy command to your .logout file. Additionally, if you are going to be away from your machine and are concerned about an intruder using your permissions, it is safest to either destroy all copies of your tickets, or use a screensaver that locks the screen.
Kerberos ticket properties¶
There are various properties that Kerberos tickets can have:
If a ticket is forwardable, then the KDC can issue a new ticket (with a different network address, if necessary) based on the forwardable ticket. This allows for authentication forwarding without requiring a password to be typed in again. For example, if a user with a forwardable TGT logs into a remote system, the KDC could issue a new TGT for that user with the network address of the remote system, allowing authentication on that host to work as though the user were logged in locally.
When the KDC creates a new ticket based on a forwardable ticket, it sets the forwarded flag on that new ticket. Any tickets that are created based on a ticket with the forwarded flag set will also have their forwarded flags set.
A proxiable ticket is similar to a forwardable ticket in that it allows a service to take on the identity of the client. Unlike a forwardable ticket, however, a proxiable ticket is only issued for specific services. In other words, a ticket-granting ticket cannot be issued based on a ticket that is proxiable but not forwardable.
A proxy ticket is one that was issued based on a proxiable ticket.
A postdated ticket is issued with the invalid flag set. After the starting time listed on the ticket, it can be presented to the KDC to obtain valid tickets.
Ticket-granting tickets with the postdateable flag set can be used to obtain postdated service tickets.
Renewable tickets can be used to obtain new session keys without the user entering their password again. A renewable ticket has two expiration times. The first is the time at which this particular ticket expires. The second is the latest possible expiration time for any ticket issued based on this renewable ticket.
A ticket with the initial flag set was issued based on the authentication protocol, and not on a ticket-granting ticket. Application servers that wish to ensure that the user’s key has been recently presented for verification could specify that this flag must be set to accept the ticket.
An invalid ticket must be rejected by application servers. Postdated tickets are usually issued with this flag set, and must be validated by the KDC before they can be used.
A preauthenticated ticket is one that was only issued after the client requesting the ticket had authenticated itself to the KDC.
The hardware authentication flag is set on a ticket which required the use of hardware for authentication. The hardware is expected to be possessed only by the client which requested the tickets.
If a ticket has the transit policy checked flag set, then the KDC that issued this ticket implements the transited-realm check policy and checked the transited-realms list on the ticket. The transited-realms list contains a list of all intermediate realms between the realm of the KDC that issued the first ticket and that of the one that issued the current ticket. If this flag is not set, then the application server must check the transited realms itself or else reject the ticket.
The okay as delegate flag indicates that the server specified in the ticket is suitable as a delegate as determined by the policy of that realm. Some client applications may use this flag to decide whether to forward tickets to a remote host, although many applications do not honor it.
An anonymous ticket is one in which the named principal is a generic principal for that realm; it does not actually specify the individual that will be using the ticket. This ticket is meant only to securely distribute a session key.
Obtaining tickets with kinit¶
If your site has integrated Kerberos V5 with the login system, you will get Kerberos tickets automatically when you log in. Otherwise, you may need to explicitly obtain your Kerberos tickets, using the kinit program. Similarly, if your Kerberos tickets expire, use the kinit program to obtain new ones.
To use the kinit program, simply type kinit and then type your password at the prompt. For example, Jennifer (whose username is jennifer) works for Bleep, Inc. (a fictitious company with the domain name mit.edu and the Kerberos realm ATHENA.MIT.EDU). She would type:
shell% kinit Password for jennifer@ATHENA.MIT.EDU: <-- [Type jennifer's password here.] shell%
If you type your password incorrectly, kinit will give you the following error message:
shell% kinit Password for jennifer@ATHENA.MIT.EDU: <-- [Type the wrong password here.] kinit: Password incorrect shell%
and you won’t get Kerberos tickets.
By default, kinit assumes you want tickets for your own username in your default realm. Suppose Jennifer’s friend David is visiting, and he wants to borrow a window to check his mail. David needs to get tickets for himself in his own realm, EXAMPLE.COM. He would type:
shell% kinit david@EXAMPLE.COM Password for david@EXAMPLE.COM: <-- [Type david's password here.] shell%
David would then have tickets which he could use to log onto his own machine. Note that he typed his password locally on Jennifer’s machine, but it never went over the network. Kerberos on the local host performed the authentication to the KDC in the other realm.
If you want to be able to forward your tickets to another host, you need to request forwardable tickets. You do this by specifying the -f option:
shell% kinit -f Password for jennifer@ATHENA.MIT.EDU: <-- [Type your password here.] shell%
Normally, your tickets are good for your system’s default ticket lifetime, which is ten hours on many systems. You can specify a different ticket lifetime with the -l option. Add the letter s to the value for seconds, m for minutes, h for hours, or d for days. For example, to obtain forwardable tickets for david@EXAMPLE.COM that would be good for three hours, you would type:
shell% kinit -f -l 3h david@EXAMPLE.COM Password for david@EXAMPLE.COM: <-- [Type david's password here.] shell%
You cannot mix units; specifying a lifetime of 3h30m would result in an error. Note also that most systems specify a maximum ticket lifetime. If you request a longer ticket lifetime, it will be automatically truncated to the maximum lifetime.
Viewing tickets with klist¶
The klist command shows your tickets. When you first obtain tickets, you will have only the ticket-granting ticket. The listing would look like this:
shell% klist Ticket cache: /tmp/krb5cc_ttypa Default principal: jennifer@ATHENA.MIT.EDU Valid starting Expires Service principal 06/07/04 19:49:21 06/08/04 05:49:19 krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU shell%
The ticket cache is the location of your ticket file. In the above example, this file is named /tmp/krb5cc_ttypa. The default principal is your Kerberos principal.
The “valid starting” and “expires” fields describe the period of time during which the ticket is valid. The “service principal” describes each ticket. The ticket-granting ticket has a first component krbtgt, and a second component which is the realm name.
Now, if jennifer connected to the machine daffodil.mit.edu, and then typed “klist” again, she would have gotten the following result:
shell% klist Ticket cache: /tmp/krb5cc_ttypa Default principal: jennifer@ATHENA.MIT.EDU Valid starting Expires Service principal 06/07/04 19:49:21 06/08/04 05:49:19 krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU 06/07/04 20:22:30 06/08/04 05:49:19 host/daffodil.mit.edu@ATHENA.MIT.EDU shell%
Here’s what happened: when jennifer used ssh to connect to the host daffodil.mit.edu, the ssh program presented her ticket-granting ticket to the KDC and requested a host ticket for the host daffodil.mit.edu. The KDC sent the host ticket, which ssh then presented to the host daffodil.mit.edu, and she was allowed to log in without typing her password.
Suppose your Kerberos tickets allow you to log into a host in another domain, such as trillium.example.com, which is also in another Kerberos realm, EXAMPLE.COM. If you ssh to this host, you will receive a ticket-granting ticket for the realm EXAMPLE.COM, plus the new host ticket for trillium.example.com. klist will now show:
shell% klist Ticket cache: /tmp/krb5cc_ttypa Default principal: jennifer@ATHENA.MIT.EDU Valid starting Expires Service principal 06/07/04 19:49:21 06/08/04 05:49:19 krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU 06/07/04 20:22:30 06/08/04 05:49:19 host/daffodil.mit.edu@ATHENA.MIT.EDU 06/07/04 20:24:18 06/08/04 05:49:19 krbtgt/EXAMPLE.COM@ATHENA.MIT.EDU 06/07/04 20:24:18 06/08/04 05:49:19 host/trillium.example.com@EXAMPLE.COM shell%
Depending on your host’s and realm’s configuration, you may also see a ticket with the service principal host/trillium.example.com@. If so, this means that your host did not know what realm trillium.example.com is in, so it asked the ATHENA.MIT.EDU KDC for a referral. The next time you connect to trillium.example.com, the odd-looking entry will be used to avoid needing to ask for a referral again.
You can use the -f option to view the flags that apply to your tickets. The flags are:
|T||Transit policy checked|
|O||Okay as delegate|
Here is a sample listing. In this example, the user jennifer obtained her initial tickets (I), which are forwardable (F) and postdated (d) but not yet validated (i):
shell% klist -f Ticket cache: /tmp/krb5cc_320 Default principal: jennifer@ATHENA.MIT.EDU Valid starting Expires Service principal 31/07/05 19:06:25 31/07/05 19:16:25 krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU Flags: FdiI shell%
In the following example, the user david‘s tickets were forwarded (f) to this host from another host. The tickets are reforwardable (F):
shell% klist -f Ticket cache: /tmp/krb5cc_p11795 Default principal: david@EXAMPLE.COM Valid starting Expires Service principal 07/31/05 11:52:29 07/31/05 21:11:23 krbtgt/EXAMPLE.COM@EXAMPLE.COM Flags: Ff 07/31/05 12:03:48 07/31/05 21:11:23 host/trillium.example.com@EXAMPLE.COM Flags: Ff shell%
Destroying tickets with kdestroy¶
Your Kerberos tickets are proof that you are indeed yourself, and tickets could be stolen if someone gains access to a computer where they are stored. If this happens, the person who has them can masquerade as you until they expire. For this reason, you should destroy your Kerberos tickets when you are away from your computer.
Destroying your tickets is easy. Simply type kdestroy:
shell% kdestroy shell%
If kdestroy fails to destroy your tickets, it will beep and give an error message. For example, if kdestroy can’t find any tickets to destroy, it will give the following message:
shell% kdestroy kdestroy: No credentials cache file found while destroying cache shell%