Message-ID: <4E00A214.2060306@indiana.edu>
Date: Tue, 21 Jun 2011 09:52:20 -0400
From: Rob Henderson <robh@indiana.edu>
To: linux-nfs@vger.kernel.org
Subject: NFSv4 plus Kerberos with RHEL5 servers and RHEL6 clients using a
 Windows KDC (SOLVED)
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Sender: linux-nfs-owner@vger.kernel.org
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We had trouble getting kerberized nfsv4 working in our environment with RHEL5 servers and RHEL6 clients using the campus Windows 2008 AD servers as our KDC.  We've resolved the issue now but I just wanted to summarize our solution in case others are fighting the same problem.

The key issue seems related to encryption since 1) the older RHEL5 kernels only support DES for nfs and 2) it appears that limiting the keytabs to only DES causes a failure to communicate with our campus Windows KDC.  I found a number of discussions that suggested limiting the encryption types to only DES and this seems to be the common fix when using a standard MIT kerberos server.  However, in our environment we are using a Windows KDC and that solution did not work for us.  I don't know if that is a generic problem with a Windows KDC or something particular to the configuration of our campus servers.

Cutting to the chase, here is the configuration in the /etc/krb5.conf on the RHEL6 client that worked for us:

    [libdefaults]
        default_tkt_enctypes = des-cbc-crc aes256-cts-hmac-sha1-96 arcfour-hmac
        allow_weak_crypto = 1


All the gory details are below for anyone interested.

   --Rob

Disclaimer:  I am not saying this is the only and/or best solution, but it is working for us... :)

===================================================================


Keytab Generation
=================

The campus Windows AD admins generate nfs keytabs for us using the "/crypto ALL" argument to ktpass.  They also set the NO_AUTH_DATA_REQUIRED UAC flag on the associated machine accounts.  Keytabs are generated using the same procedure for both servers and clients.


RHEL5 NFS Server
================

The keytabs are installed on the RHEL5 server and look like this:

# klist -kte

Keytab name: FILE:/etc/krb5.keytab
KVNO Timestamp         Principal
---- ----------------- --------------------------------------------------------
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (DES cbc mode with CRC-32)
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (DES cbc mode with RSA-MD5) 
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (ArcFour with HMAC/md5) 
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (AES-256 CTS mode with 96-bit SHA-1 HMAC) 
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (AES-128 CTS mode with 96-bit SHA-1 HMAC) 

There is nothing special in the /etc/krb5.conf on the server side.


RHEL6 NFS Client
================

The keytabs for the client are generated in the same way as for the server and look exactly like that for the server (with the exception that the encryption types are displayed differently with the newer version of klist).  They look like this:

# klist -kte
Keytab name: WRFILE:/etc/krb5.keytab
KVNO Timestamp         Principal
---- ----------------- --------------------------------------------------------
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (des-cbc-crc) 
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (des-cbc-md5) 
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (arcfour-hmac) 
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (aes256-cts-hmac-sha1-96) 
   3 12/31/69 19:00:00 nfs/[hostname]@[REALM] (aes128-cts-hmac-sha1-96)


The following are added to the /etc/krb5.conf:

     [libdefaults]
        default_tkt_enctypes = des-cbc-crc aes256-cts-hmac-sha1-96 arcfour-hmac
        allow_weak_crypto = 1

After the mount, the machine credentials on the client look like this:

      # klist -e /tmp/krb5cc_machine_[REALM]
      Ticket cache: FILE:/tmp/krb5cc_machine_[REALM]
      Default principal: nfs/[client hostname]@[REALM]

      Valid starting     Expires            Service principal
      06/21/11 02:44:18  06/21/11 12:44:18  krbtgt/[REALM]@[REALM]
              renew until 06/28/11 02:44:18, Etype (skey, tkt): des-cbc-crc, aes256-cts-hmac-sha1-96
      06/21/11 02:44:18  06/21/11 12:44:18  nfs/[server hostname]@[REALM]
              renew until 06/28/11 02:44:18, Etype (skey, tkt): des-cbc-crc, aes256-cts-hmac-sha1-96

And, the user sees the following:

      $ klist -e
      Ticket cache: FILE:/tmp/krb5cc_[UID]_EfGvpD
      Default principal: [USERNAME]@[REALM]

      Valid starting     Expires            Service principal
      06/21/11 09:33:01  06/21/11 19:33:01  krbtgt/[REALM]@[REALM]
              renew until 06/28/11 09:33:01, Etype (skey, tkt): des-cbc-crc, aes256-cts-hmac-sha1-96 
      06/21/11 09:33:03  06/21/11 19:33:01  nfs/[server hostname]@[REALM]
             renew until 06/28/11 09:33:01, Etype (skey, tkt): des-cbc-crc, aes256-cts-hmac-sha1-96