gnu: Add pam-krb5 service.

* doc/guix.texi (Kerberos Services): New node.
* gnu/services/kerberos.scm: New file.
* gnu/local.mk: Add it.

doc/guix.texi

@@ -221,6 +221,7 @@ Services
 * Desktop Services::            D-Bus and desktop services.
 * Database Services::           SQL databases.
 * Mail Services::               IMAP, POP3, SMTP, and all that.
+* Kerberos Services::           Kerberos services.
 * Web Services::                Web servers.
 * Network File System::         NFS related services.
 * Miscellaneous Services::      Other services.
@@ -7701,6 +7702,7 @@ declaration.
 * Desktop Services::            D-Bus and desktop services.
 * Database Services::           SQL databases.
 * Mail Services::               IMAP, POP3, SMTP, and all that.
+* Kerberos Services::           Kerberos services.
 * Web Services::                Web servers.
 * Network File System::         NFS related services.
 * Miscellaneous Services::      Other services.
@@ -10405,6 +10407,40 @@ could instantiate a dovecot service like this:
                   (string "")))
 @end example
 
+
+@node Kerberos Services
+@subsubsection Kerberos Services
+@cindex Kerberos
+
+The @code{(gnu services Kerberos)} module provides services relating to
+the authentication protocol @dfn{Kerberos}.
+
+@subsubheading PAM krb5 Service
+@cindex pam-krb5
+
+The pam-krb5 service allows for login authentication and password
+management via Kerberos.
+You will need this service if you want PAM enabled applications to authenticate
+users using Kerberos.
+
+@defvr {Scheme Variable} pam-krb5-service-type
+A service type for the Kerberos 5 PAM module.
+@end defvr
+
+@deftp {Data Type} pam-krb5-configuration
+Data type representing the configuration of the Kerberos 5 PAM module
+This type has the following parameters:
+@table @asis
+@item @code{pam-krb5} (default: @code{pam-krb5})
+The pam-krb5 package to use.
+
+@item @code{minimum-uid} (default: @code{1000})
+The smallest user ID for which Kerberos authentications should be attempted.
+Local accounts with lower values will silently fail to authenticate.
+@end table
+@end deftp
+
+
 @node Web Services
 @subsubsection Web Services
 
@@ -10538,7 +10574,7 @@ The @dfn{global security system} (GSS) daemon provides strong security for RPC
 based protocols.
 Before exchanging RPC requests an RPC client must establish a security
 context.  Typically this is done using the Kerberos command @command{kinit}
-or automatically at login time using PAM services.
+or automatically at login time using PAM services (@pxref{Kerberos Services}).
 
 @defvr {Scheme Variable} gss-service-type
 A service type for the Global Security System (GSS) daemon.

gnu/local.mk

@@ -400,6 +400,7 @@ GNU_SYSTEM_MODULES =				\
   %D%/services/dbus.scm				\
   %D%/services/desktop.scm			\
   %D%/services/dict.scm				\
+  %D%/services/kerberos.scm			\
   %D%/services/lirc.scm				\
   %D%/services/mail.scm				\
   %D%/services/mcron.scm			\

gnu/services/kerberos.scm

@@ -0,0 +1,67 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2016 John Darrington <jmd@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu services kerberos)
+  #:use-module (gnu packages admin)
+  #:use-module (gnu services)
+  #:use-module (gnu system pam)
+  #:use-module (guix gexp)
+  #:use-module (guix records)
+  #:export (pam-krb5-configuration
+            pam-krb5-configuration?
+            pam-krb5-service-type))
+
+(define-record-type* <pam-krb5-configuration>
+  pam-krb5-configuration  make-pam-krb5-configuration
+  pam-krb5-configuration?
+  (pam-krb5               pam-krb5-configuration-pam-krb5
+                          (default pam-krb5))
+  (minimum-uid            pam-krb5-configuration-minimum-uid
+                          (default 1000)))
+
+(define (pam-krb5-pam-service config)
+  "Return a PAM service for Kerberos authentication."
+  (lambda (pam)
+    (define pam-krb5-module
+      #~(string-append #$(pam-krb5-configuration-pam-krb5 config) "/lib/security/pam_krb5.so"))
+
+    (let ((pam-krb5-sufficient
+           (pam-entry
+            (control "sufficient")
+            (module pam-krb5-module)
+            (arguments (list
+                        (format #f "minimum_uid=~a"
+                                (pam-krb5-configuration-minimum-uid config)))))))
+      (pam-service
+       (inherit pam)
+       (auth (cons* pam-krb5-sufficient
+                    (pam-service-auth pam)))
+       (session (cons* pam-krb5-sufficient
+                       (pam-service-session pam)))
+       (account (cons* pam-krb5-sufficient
+                       (pam-service-account pam)))))))
+
+(define (pam-krb5-pam-services config)
+  (list (pam-krb5-pam-service config)))
+
+(define pam-krb5-service-type
+  (service-type (name 'pam-krb5)
+                (extensions
+                 (list
+                  (service-extension pam-root-service-type
+                                     pam-krb5-pam-services)))))