services: Add urandom-seed-service.

Fixes <http://bugs.gnu.org/23605>.

* gnu/services/base.scm (urandom-seed-service): New procedure.
(%random-seed-file, urandom-seed-service-type): New variables.
(%urandom-seed-shepherd-service): New procedure.
(%base-services): Call 'urandom-seed-service'.
* doc/guix.texi (Base Services): Document it.

doc/guix.texi

@@ -7355,6 +7355,17 @@ Return a service that runs the Guix build daemon according to
 Run @var{udev}, which populates the @file{/dev} directory dynamically.
 @end deffn
 
+@deffn {Scheme Procedure} urandom-seed-service @var{#f}
+Save some entropy in @var{%random-seed-file} to seed @file{/dev/urandom}
+when rebooting.
+@end deffn
+
+@defvr {Scheme Variable} %random-seed-file
+This is the name of the file where some random bytes are saved by
+@var{urandom-seed-service} to seed @file{/dev/urandom} when rebooting.
+It defaults to @file{/var/lib/random-seed}.
+@end defvr
+
 @deffn {Scheme Procedure} console-keymap-service @var{files} ...
 @cindex keyboard layout
 Return a service to load console keymaps from @var{files} using

gnu/services/base.scm

@@ -3,6 +3,7 @@
 ;;; Copyright © 2015, 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
+;;; Copyright © 2016 Leo Famulari <leo@famulari.name>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -93,6 +94,8 @@ (define-module (gnu services base)
             gpm-service-type
             gpm-service
 
+            urandom-seed-service
+
             %base-services))
 
 ;;; Commentary:
@@ -420,6 +423,63 @@ (define* (user-processes-service file-systems #:key (grace-delay 4))
   (service user-processes-service-type
            (list (filter file-system-mount? file-systems) grace-delay)))
 
+
+;;;
+;;; Preserve entropy to seed /dev/urandom on boot.
+;;;
+
+(define %random-seed-file
+  "/var/lib/random-seed")
+
+(define %urandom-seed-activation
+  ;; Activation gexp for the urandom seed
+  #~(begin
+      (use-modules (guix build utils))
+
+      (mkdir-p (dirname #$%random-seed-file))
+      (close-port (open-file #$%random-seed-file "a0b"))
+      (chmod #$%random-seed-file #o600)))
+
+(define (urandom-seed-shepherd-service _)
+  "Return a shepherd service for the /dev/urandom seed."
+  (list (shepherd-service
+         (documentation "Preserve entropy across reboots for /dev/urandom.")
+         (provision '(urandom-seed))
+         (requirement '(user-processes))
+         (start #~(lambda _
+                    ;; On boot, write random seed into /dev/urandom.
+                    (when (file-exists? #$%random-seed-file)
+                      (call-with-input-file #$%random-seed-file
+                        (lambda (seed)
+                          (call-with-output-file "/dev/urandom"
+                            (lambda (urandom)
+                              (dump-port seed urandom))))))
+                    #t))
+         (stop #~(lambda _
+                   ;; During shutdown, write from /dev/urandom into random seed.
+                   (let ((buf (make-bytevector 512)))
+                     (call-with-input-file "/dev/urandom"
+                       (lambda (urandom)
+                         (get-bytevector-n! urandom buf 0 512)
+                         (call-with-output-file #$%random-seed-file
+                           (lambda (seed)
+                             (put-bytevector seed buf)))
+                         #t)))))
+         (modules `((rnrs bytevectors)
+                    (rnrs io ports)
+                    ,@%default-modules)))))
+
+(define urandom-seed-service-type
+  (service-type (name 'urandom-seed)
+                (extensions
+                 (list (service-extension shepherd-root-service-type
+                                          urandom-seed-shepherd-service)
+                       (service-extension activation-service-type
+                                          (const %urandom-seed-activation))))))
+
+(define (urandom-seed-service)
+  (service urandom-seed-service-type #f))
+
 
 ;;;
 ;;; System-wide environment variables.
@@ -1200,7 +1260,6 @@ (define (swap-service device)
   "Return a service that uses @var{device} as a swap device."
   (service swap-service-type device))
 
-
 (define-record-type* <gpm-configuration>
   gpm-configuration make-gpm-configuration gpm-configuration?
   (gpm      gpm-configuration-gpm)                ;package
@@ -1281,6 +1340,7 @@ (define %base-services
           (static-networking-service "lo" "127.0.0.1"
                                      #:provision '(loopback))
           (syslog-service)
+          (urandom-seed-service)
           (guix-service)
           (nscd-service)