Linux SSO for multiple windows domains

Solution 1:

I've just hit the same issue as you, and thanks to this thread here, I've found the answer!

After turning on a second (trusted) realm in my mod_auth_kerb settings, and putting the right stuff in the keytab, if I tried to sign on with a user from the second domain I was getting errors in the httpd log like:

[auth_kerb:notice] [pid 1234] [client X.X.X.X:12345] krb5_aname_to_localname() found no mapping for principal [email protected]

The good news is, I've solved it! Details below....

Firstly, in your Apache HTTPD config, you want something like:

# Use this one for both Examples and Branches together
KrbAuthRealms EXAMPLE.COM BRANCHES.EXAMPLE.COM

# Strip the realm from the username
KrbLocalUserMapping On

That tells mod_auth_kerb to accept users from either the main domain realm, or the branches one, and strip off the realm from the username. That means that [email protected] goes to admin, while [email protected] goes to guest

Next, assuming the MIT kerberos, you need to edit your /etc/krb5.conf file, and tell that how to map principals into usernames. For various historic reasons, this isn't done in a libdefaults section as you might expect. It's also not done on a per-realm section either, which caught me out. Instead, it's done with auth_to_local entries in the [realm] section of the default realm.

By default, the krb5_aname_to_localname() libkrb5 function will remove the realm from the default realm, and leave it there otherwise. So, we have to add an entry to tell it to strip the realm from the branches realm as well. (More complex rules are also possible, see the krb5.conf man page for more)

So, we'd want our config to be something like this:

[libdefaults]
  default_realm = EXAMPLE.COM

[realms]
   EXAMPLE.COM = {
     kdc = dc.example.com
     admin_server = dc.example.com
     auth_to_local = RULE:[1:$1@$0](^.*@BRANCHES\.EXAMPLE\.COM)s/@.*//
     auth_to_local = DEFAULT
   }
   BRANCHES.EXAMPLE.COM = {
     kdc = dc.branches.example.com
     admin_server = dc.branches.example.com
   }

Note how the BRANCHES.EXAMPLE.COM mapping rule doesn't live in its realm, but in the main EXAMPLE.COM realm, which is the default realm.

Also, since it is a production server which I cannot arbitrarily restart, what services do i need to restart after changing things in the krb5.conf?

Just the Apache HTTPD service needs restarting after changes

Linux SSO for multiple windows domains

Solution 1:

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