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From: Russ Allbery
Subject: Re: PAM vs GSSAPI?
Date: Wed, 21 Mar 2007 11:54:13 -0700
User-agent: Gnus/5.110006 (No Gnus v0.6) XEmacs/21.4.19 (linux)

Simon Josefsson <address@hidden> writes:

> Ah, right.  I recalled some GSS-API extensions for initial acquisition,
> but I guess they were never implemented widely.  It might have been a
> better approach, though.  But maybe there are other things that pam_krb5
> do which isn't possible to do via GSS-API?

Basically, what pam_krb5 has to do is:

 * Acquire initial credentials.  This is normally done using a password
   but may instead be done using a smartcard and PKINIT.  This requires
   dealing with a Kerberos prompting function and passing the prompts back
   to the user, since the Kerberos library may be doing other things at
   the same time (like forcing the user to change an expired password).

 * Verify that the initial credentials are valid by obtaining a service
   ticket for a local key and then checking that service ticket against
   the local key.  This prevents KDC spoofing.

 * Store the initial credentials into a ticket cache and chown that ticket
   cache to the user.  (Due to the way that OpenSSH works, it also has to
   be able to store them in a temporary file and read them back out

 * Verify that the user is permitted to log on to this account by calling
   krb5_kuserok, which checks the Kerberos principal against the local
   account name, a .k5login if any, and against any principal to local
   account name mappings as configured in krb5.conf.

 * Obtain password changing credentials (different than the TGT since
   password changing credentials are marked DISALLOW_TGT_BASED).

 * Change the user's password.

As near as I can tell, GSS-API doesn't provide facilities to do any of
those operations.

Russ Allbery (address@hidden)             <>

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