I've been considering deploying mod_auth_kerb on our internal web servers to enable SSO. The one obvious problem I can see is that it's an all-or-nothing approach, either all your domain users can access a site or not.

Is it possible to combine mod_auth_kerb with something like mod_authnz_ldap to check for group membership in a particular group in LDAP? I'm guessing the KrbAuthoritative option would have something to do with this?

Also, as I understand it, the module sets the username to be username@REALM after authentication, but of course in the directory the users are stored as the username only. Furthermore, some internal sites we run such as trac already have a user profile linked to each username. Is there a way to resolve this, perhaps by stripping off the realm bit after authentication somehow?

  • Just a question regarding implementation, are you using a Windows ADS for the kerberos realm or some other implementation? Commented Jul 6, 2009 at 14:17
  • Apple's OpenDirectory which comes with MIT Kerberos v5 Commented Jul 6, 2009 at 20:33
  • Okay... Haven't worked with Apple's OpenDirectory before. I was able to get Apache to authenticate using NTLM against Windows ADS using their workstation credentials and then restricting to specific groups. Commented Jul 7, 2009 at 16:54
  • Without stripping realm from username, you can use an alternate attribute in LDAP query to search for user entity, for instance the "userPrincipalName" attribute in Ms ActiveDirectory. Commented Oct 29, 2013 at 12:43

3 Answers 3


It is now possible in mod_auth_kerb 5.4 to strip the realm from REMOTE_USER with the following config directive:

KrbLocalUserMapping On

  • Wow, looks like this was released in 2008, but no mention of it (the version or the parameter) on their website. Commented Nov 21, 2009 at 1:41

It's the whole point of the authn/authz separation in 2.2 that you can authenticate with one mechanism, and authorize with another. Authentication provides you with a setting of REMOTE_USER, which you then can use authz_ldap against. In addition, authn_ldap searches then for a user (converting the REMOTE_USER to a DN if found, using search criteria you have to specify - e.g. searching for CN). Then, when a DN has been found, you can specify requirements on the LDAP object. E.g. if all users accessing a resource must be in the same OU, you specify

require ldap-dn ou=Managers, o=The Company

  • Is it possible to modify the REMOTE_USER variable before it's passed to the authorization stage? For example, to strip off the REALM portion of the Kerberos username for lookup in an LDAP database? Commented Jul 16, 2009 at 3:33
  • Not by means of configuration. However, it is relatively easy to do so in the source code of the Apache module. Look for assignments to request->user and adjust them; then rebuild the module with apxs2 -c. OTOH, it might be easier to put the Kerberos names into LDAP, under a separate attribute, and have the ldap module search the user by that attribute. Commented Jul 16, 2009 at 18:32

Debian stable now ships with version 5.4 of mod_auth_kerb.

If you're stuck with an older version, this page explains how mod_map_user can be used in combination with mod_auth_kerb and mod_authnz_ldap.

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