services: dnsmasq: Support the --address flag.

Introduce a new `addresses' field that translates to passing `--address='
multiple times to dnsmasq.

* gnu/services/dns.scm (<dnsmasq-configuration>): Add an addresses field.
(dnsmasq-shepherd-service): Match the addresses field and translate it to
multiple '--address=' flags.
* doc/guix.texi (DNS Services): Document it.

Signed-off-by: 宋文武 <iyzsong@member.fsf.org>

doc/guix.texi

@@ -78,6 +78,7 @@ Copyright @copyright{} 2020 Jack Hill@*
 Copyright @copyright{} 2020 Naga Malleswari@*
 Copyright @copyright{} 2020 Brice Waegeneire@*
 Copyright @copyright{} 2020 R Veera Kumar@*
+Copyright @copyright{} 2020 Pierre Langlois@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -21874,6 +21875,25 @@ When true, don't read @var{resolv-file}.
 @item @code{servers} (default: @code{'()})
 Specify IP address of upstream servers directly.
 
+@item @code{addresses} (default: @code{'()})
+For each entry, specify an IP address to return for any host in the
+given domains.  Queries in the domains are never forwarded and always
+replied to with the specified IP address.
+
+This is useful for redirecting hosts locally, for example:
+
+@lisp
+(service dnsmasq-service-type
+         (dnsmasq-configuration
+           (addresses
+             '(; Redirect to a local web-server.
+               "/example.org/127.0.0.1"
+               ; Redirect subdomain to a specific IP.
+               "/subdomain.example.org/192.168.1.42"))))
+@end lisp
+
+Note that rules in @file{/etc/hosts} take precedence over this.
+
 @item @code{cache-size} (default: @code{150})
 Set the size of dnsmasq's cache.  Setting the cache size to zero
 disables caching.

gnu/services/dns.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2018 Oleg Pykhalov <go.wigust@gmail.com>
+;;; Copyright © 2020 Pierre Langlois <pierre.langlois@gmx.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -748,6 +749,8 @@ (define-record-type* <dnsmasq-configuration>
                     (default #f))       ;boolean
   (servers          dnsmasq-configuration-servers
                     (default '()))      ;list of string
+  (addresses        dnsmasq-configuration-addresses
+                    (default '()))      ;list of string
   (cache-size       dnsmasq-configuration-cache-size
                     (default 150))      ;integer
   (negative-cache?  dnsmasq-configuration-negative-cache?
@@ -759,7 +762,7 @@ (define dnsmasq-shepherd-service
                                 no-hosts?
                                 port local-service? listen-addresses
                                 resolv-file no-resolv? servers
-                                cache-size negative-cache?)
+                                addresses cache-size negative-cache?)
      (shepherd-service
       (provision '(dnsmasq))
       (requirement '(networking))
@@ -783,6 +786,8 @@ (define dnsmasq-shepherd-service
                          '())
                   #$@(map (cut format #f "--server=~a" <>)
                           servers)
+                  #$@(map (cut format #f "--address=~a" <>)
+                          addresses)
                   #$(format #f "--cache-size=~a" cache-size)
                   #$@(if negative-cache?
                          '()