Configure Dovecot for GSSAPI [formerly: Looking for GSSAPI config]

[Mark Foley]

After a over a year and a half struggling to get Dovecot to do either NTLM or GSSAPI
authentication with Samba4 AD/DC, I believe I've finally got it! Thanks to all those in this
list who helped: Jan Jurkus, Edgar Pettijohn, Gregory Sloop, Tom Talpey especially Aki Tuomi;
and infinite thanks to Achim Gottinger on the SambaList for his patience in working this
through with me.  Although my purpose was for Dovecot to authenticate mail clients, the
configuration settings needed were on the Samba side.  I hope a variation of these instructions
can eventually make it into:

http://wiki2.dovecot.org/Authentication/Kerberos

What is essentially missing from the wiki is how to set up the proper Service Principal Names
and the subsequent creation of a dovecot useable kerberos keytab file.  The wiki comment on
"k5principals passdb" was not helpful and largely unintelligble to me. 

Perhaps like many of you, I have switched from Microsoft SBS and Exchange to Samaba4 and
Dovecot/IMAP. The transition was completely transparent to my users, except they needed a
separate password for email authentication in the absence of NTLM or GSSAPI working with
Dovecot. A mild inconvenience, but I have been on a "quest" to fill that gap. This solution
finally takes care of that last piece.

The following describes how to create the SPNs and krb5 keytab files using Samba4 which has
its own built-in (Heimdal) kerberos. The procedures are probably similar for other facilities
such as setspn for Windows, but I've not used those so I won't attempt to discuss those
mechanism here.

You do need kerberos as the Samba built-in kerberos does not have needed commands like `klist`.

My distro (Slackware 14.1) does not come with kerberos (nor, I think, does Ubuntu), but is
easily found at:

https://slackbuilds.org/repository/14.1/network/krb5/

Ubuntu/Debian: apt-get install krb5-config libpam-krb5 krb5-user ssh-krb5 (perhaps more)

After provisioning Samba4, copy the krb5.conf template to /etc/krb5.conf.

(Note: the actual docs advise symlinking:

  ln -sf /usr/local/samba/private/krb5.conf /etc/krb5.conf

but I prefer making a copy in case I need to modify things).

I've set The /etc/krb5.conf file to world readable.  It's default contents are (and these do
not need to be changed):

[libdefaults]
        default_realm = HPRS.LOCAL
        dns_lookup_realm = false
        dns_lookup_kdc = true

where HPRS.LOCAL is my realm, of course use your own.

Now, we need a samba user in order to create the necessary SPNs (Server Principal Names):

$ samba-tool user create dovecot
New Password:
Retype Password:
User 'dovecot' created successfully

Next, add the SPN(s), and create the keytab:

$ samba-tool spn add imap/mail.hprs.local dovecot
$ samba-tool domain exportkeytab --principal imap/mail.hprs.local /etc/dovecot/dovecot.keytab

Dovecot does not do my (outgoing) SMTP serving, only (incoming) IMAP, but if it did I'd have to
create another SPN for smtp:

$ samba-tool spn add smtp/mail.hprs.local dovecot
$ samba-tool domain exportkeytab --principal smtp/mail.hprs.local /etc/dovecot/dovecot.keytab

Dovecot needs to be able to read the keytab file:

$ chgrp dovecot /etc/dovecot/dovecot.keytab
$ chmod g+r /etc/dovecot/dovecot.keytab

my new keytab:

$ klist -Kek /etc/dovecot/dovecot.keytab
Keytab name: FILE:/etc/dovecot/dovecot.keytab
KVNO Principal
---- --------------------------------------------------------------------------
   1 imap/mail.hprs.local at HPRS.LOCAL (des-cbc-crc)  (0x232616c2a4fd08f7)
   1 imap/mail.hprs.local at HPRS.LOCAL (des-cbc-md5)  (0x232616c2a4fd08f7)
   1 imap/mail.hprs.local at HPRS.LOCAL (arcfour-hmac)  (0x9dae89a221dc374a39f560833352f60f)
(and if I also created the spn for smtp I would also have these:) 
   1 smtp/mail.hprs.local at HPRS.LOCAL (des-cbc-crc)  (0x232616c2a4fd08f7)
   1 smtp/mail.hprs.local at HPRS.LOCAL (des-cbc-md5)  (0x232616c2a4fd08f7)
   1 smtp/mail.hprs.local at HPRS.LOCAL (arcfour-hmac)  (0x9dae89a221dc374a39f560833352f60f)

DOVECOT SETTINGS:

My version: 2.2.15

Of crucial importance is to build dovecot with GSSAPI! That is NOT one of the default settings. 
In the dovecot build directory:

./configure --with-gssapi=yes

Other than that serious build gotcha, settings are pretty simple.  Add the following 3
settings to 10-auth.conf:

auth_gssapi_hostname = "$ALL"
auth_krb5_keytab = /etc/dovecot/dovecot.keytab
auth_mechanisms = plain login gssapi

The auth_gssapi_hostname is supposedly not required according to some of the above-listed
commentors, but my 10-auth.conf template implies differently, so it can't hurt. 

gssapi does not require a passdb. Use whatever userdb you want. The dovecot wiki doc has some
suggestions, not of which I've tried. I use the 'driver = passwd' for my userdb for unrelated
reasons.

I couldn't get any of this working until I rebooted the Samba AD/DC-Dovecot server, but that
just may have been me not stopping/starting Samba and Dovecot in the right sequence (or,
everything happened to start working with a concurrent upgrade to Samba 4.2!). 

In my WIN7 and Ubuntu Thunderbird clients I selected gssapi/kerberos for the IMAP authenticate
method and it works!

I've even changed the test user's AD password, just to make sure.

Someone please put at least the required info on creating the SPNs and krb keytab file for
other poor schmucks like me. 

Now, if I can get NTLM figured out for the remaining Outlook users ... !

--Mark