Kerberos Preferences on Mac OS X 10.4 and 10.5 Documentation

This web page discusses the edu.mit.Kerberos (Kerberos configuration) file: what's in it, where it goes, and how to configure it for distribution at your site.

The information on this page applies to Mac OS X 10.4 and 10.5 only. For links to preferences documentation for other Mac OS versions, go here.

The edu.mit.Kerberos File
Setting up a Configuration File Quick Guide
edu.mit.Kerberos File Locations
Kerberos Configuration Information
Kerberos Configuration File Format
DNS Configuration

The edu.mit.Kerberos File

The edu.mit.Kerberos file is where the Kerberos v4 and v5 configuration information is stored on Mac OS X. Formerly the Kerberos Login Library and Kerberos management application preferences were stored in it, but they now have their own preference files: edu.mit.Kerberos.KerberosLogin.plist and edu.mit.Kerberos.KerberosApp.plist.

The edu.mit.Kerberos file stores this information in its data fork, which contains the realm and server configuration information (the info that would be found in the krb5.conf file on Unix). See the Kerberos Configuration File Format section for more information.

On some systems there may up to three configuration files - two edu.mit.Kerberos files in the "system" and "user" locations, and KfM now accepts the standard Unix location and name of /etc/krb5.conf for the configuration file as well. Some settings in the edu.mit.Kerberos.KerberosLogin.plist file can override settings in the edu.mit.Kerberos as well. See the edu.mit.Kerberos File Locations section for more information about why this is so.

Setting up a Configuration File Quick Guide

We recommend that you read this entire page. However, if you are in a hurry to get Kerberos for Macintosh up and working:

You need to create an edu.mit.Kerberos file in the /Library/Preferences directory which contains the realm and server configuration information for your site, although:

if your site supports DNS configuration of Kerberos realms, you may not need a configuration file, or at least not a complete one - see the About DNS Configuration section;
if you upgraded from a previous version of Mac OS X which was using Kerberos successfully, you probably already have a properly configured file and no changes are necessary;
if you've run the Mac OS X Kerberos Extras installer, you will already have a file in the correct place, but which contains MIT configuration information (which is provided as a guideline);
if you have a functioning Mac OS 9.x Kerberos installation, you can simply copy the Kerberos Preferences file from the Kerberos folder in Application Support from your Mac OS 9 volume to the /Library/Preferences on your Mac OS X volume, and rename it to edu.mit.Kerberos.

If you do not have an edu.mit.Kerberos file:

Launch the Kerberos application (/System/Library/CoreServices/Kerberos).
Choose Edit Realms... from the Edit menu.
Use the edit realms dialog to enter information about your site's realm. See the Kerberos Configuration section for information on what the various fields mean.

Note - while there may also be an edu.mit.Kerberos file in your /Users/username/Library/Preferences directory, you should place your configuration information in the /Library/Preferences location. (See edu.mit.Kerberos File Locations for more details.)

edu.mit.Kerberos File Locations

Kerberos for Macintosh supports and looks for its configuration file in three locations - two are standard locations and the third for Unix compatibility:

/Library/Preferences/edu.mit.Kerberos - the standard "system" location that contains the configuration to be used by all users of the computer,
/Users/username/Library/Preferences/edu.mit.Kerberos - the standard "user" location containing additional configuration for an individual user,
/etc/krb5.conf - the Unix compatibility location. Any configuration file in this location will also apply to all users of the computer.

The typical case is to have the Kerberos configuration information in the standard system configuration file, and no user configuration file or Unix compatibility file.

However there may be circumstances where a user wants to have additional realm and server information not shared with other users on the same machine. You can add any additional realm and server configuration information to the user configuration file, and KfM will meld the two sets of information together. You should avoid duplicate realm entries - if you have the same entry with different information in different configuration files, the behavior is not defined and you may get unexpected results.

If the user wants to have additional items in the [libdefaults] section, it's important to be aware of the order in which KfM reads the configuration files, because in case of conflicting [libdefaults] entries, the entry read first is the one that KfM will use (this is different from the situation with realm entries, which are merged). KfM first reads the configuration file in the user location, then the one in the system location, and finally the Unix compatibility location.

Similarly, if there is a configuration file in the Unix compatibility location, KfM will attempt to meld those the information in it together with any other configuration files present, with behavior as described above.

Having just a user configuration file and no system configuration file is not a supported setup. For instance, getting Kerberos tickets at login time will not work if you only have a user configuration file. The Mac OS X login window will not read the user configuration file.

Note: some settings in the edu.mit.Kerberos.KerberosLogin.plist, the Kerberos Login Library preferences file, can effectively override settings in the edu.mit.Kerberos file. These settings can be modified using the Kerberos GUI management application /System/Library/Coreservices/Kerberos .

Generally, site settings go in the /Library/Preferences/edu.mit.Kerberos file, and user settings will go into ~/Library/Preferences/edu.mit.Kerberos.KerberosLogin.plist (via changing settings in Kerberos.app). The Kerberos Login preferences exist so that the user can change their ticket management preferences without changing those preferences for every user on the machine. One user might always want addressless tickets, but another user might not.

In addition, there are some options which cannot be set with the [libdefaults] section of the edu.mit.Kerberos file. For instance, there is no edu.mit.Kerberos file preference to set the default ticket lifetime - despite config files which claim there is a "ticket_lifetime" tag, no code actually looks for it.

Kerberos Configuration Information

A Kerberos configuration is made up of a list of realms and a list of domain->realm mappings.

Each realm entry contains a list of servers and a default domain for the realm. Each type of server has a different purpose. "kdc" servers are used to obtain tickets. "admin" servers are used to perform administrator operations, such as running kadmin. At most sites there will only be one admin server per Kerberos realm. "kpasswd" servers are used to change your password, although the admin server will be used if no kpasswd server is listed. "krb524" servers are used to get v4 tickets from v5 tickets and are only used by v5 realms.

If the realm and site DNS domain are different, there will also be domain to realm mappings. For instance, if you have a domain-realm mapping ".mydepartment.mysite.com = MYSITE.COM" and try to contact a server such as "myprinter.mydepartment.mysite.com", Kerberos will know to contact the realm "MYSITE.COM" rather than the default, "MYDEPARTMENT.MYSITE.COM".

The Kerberos Configuration File Format

The Kerberos v4 and v5 configurations are stored in the data fork of edu.mit.Kerberos.

This text is similar to that of krb5.conf on Unix machines or krb5.ini on Windows machines. The configuration tells Kerberos for Macintosh what realms exist, what Kerberos versions are supported by them, and where to find the servers. You should edit this file for your site by opening the edu.mit.Kerberos file in a text editor that will save the file as pure text again, ie: BBEdit, emacs, or CodeWarrior; but not TextEdit (unless you use the "Make Plain Text" command) or Microsoft Word.

Once you are done editing the edu.mit.Kerberos file, you should log out, and then you may want to use the Edit Favorite Realms feature of the Kerberos management application to add your realms to the pop-up menu in the Login dialog.

Here is an example Kerberos configuration:

	[libdefaults]
		default_realm = ATHENA.MIT.EDU
		noaddresses = TRUE

	[realms]
	        ATHENA.MIT.EDU = {
	                kdc = kerberos.mit.edu.:88
	                kdc = kerberos-1.mit.edu.:88
	                kdc = kerberos-2.mit.edu.:88
	                admin_server = kerberos.mit.edu.
	                default_domain = mit.edu
	        }
	        MEDIA-LAB.MIT.EDU = {
	                kdc = kerberos.media.mit.edu.
	                admin_server = kerberos.media.mit.edu.
	        }

	[domain_realm]
		.mit.edu = ATHENA.MIT.EDU
		mit.edu = ATHENA.MIT.EDU
		.media.mit.edu = MEDIA-LAB.MIT.EDU
		media.mit.edu = MEDIA-LAB.MIT.EDU

	[v4 realms]
	        ATHENA.MIT.EDU = {
	                kdc = kerberos.mit.edu.
	                kdc = kerberos-1.mit.edu.
	                kdc = kerberos-2.mit.edu.
	                admin_server = kerberos.mit.edu.
	                default_domain = mit.edu
	        }
	        UMICH.EDU = {
	                kdc = kerberos.umich.edu.
	                admin_server = kerberos.umich.edu.
	                default_domain = umich.edu
	        }

	[v4 domain_realm]
		.mit.edu = ATHENA.MIT.EDU
		mit.edu = ATHENA.MIT.EDU
		.umich.edu = UMICH.EDU
		umich.edu = UMICH.EDU

The [libdefaults] section describes what the default behavior of the Kerberos libraries should be. You should always fill in the default realm. If you have Kerberos v5 at your site, you should also copy any other [libdefaults] from your site's krb5.conf or krb5.ini.

Kerberos for Macintosh 5.5 now honors ticket_lifetime entries in [libdefaults] . However, if you have set a ticket lifetime default in the GUI Kerberos management application preferences, it will override this value.

The [realms] and [domain_realm] sections refer to Kerberos v5 realms. If your site is v4-only you should omit these sections. Otherwise just copy these sections from your site's krb5.conf or krb5.ini.

The [v4 realms] and [v4 domain_realm] sections refer to Kerberos v4 realms. If your site is v5-only you should omit these sections. Otherwise you will need to create entries for each of the Kerberos v4 realms at your site. You should not specify a string_to_key_type for v4 realms anymore, because that information will be ignored - KfM will automatically determine the correct one to use.

DNS Configuration

Some sites have configured their DNS servers to provide information about local Kerberos realm configuration, such that users need only a minimum configuration file and instead can get the rest of the Kerberos configuration information over the network. For more information about DNS, see the Using DNS section of the Kerberos V5 System Administrator's Guide.

You should always have a configuration file that has a [libdefaults] section with a default_realm specified. Otherwise, getting Kerberos tickets at login time may fail.

If your Kerberos realm is named the same as your domain name, e.g. your domain name = foo.bar.edu and your Kerberos realm = FOO.BAR.EDU, you do not need any more information in your local configuration file, assuming all the realms you need to access have DNS records.

Otherwise, you also need a [domain_realm] section, mapping your domain to the appropriate realms. You can omit the [realms] sections of the configuration file.

DNS configuration of realms only applies to Kerberos v5, so unless your site does krb524 on the server, you will need to include v4 information in a local configuration file.

If you want to disable DNS lookup of Kerberos realms on your Macintosh, uncheck the "Configure additional realms automatically using DNS" checkbox in the Kerberos application's edit realms dialog or add the line:

dns_fallback = no

to the [libdefaults] section of your Kerberos configuration file.