The setuid helper of D-Bus is responsible for "service activation".
It looks for '.service' files in fixed locations, but the default locations
make no sense (see below), so replace them with /etc/dbus-1/system-services.

--- dbus-1.10.0/dbus/dbus-sysdeps-util-unix.c	2015-10-22 00:07:03.829251854 +0200
+++ dbus-1.10.0/dbus/dbus-sysdeps-util-unix.c	2015-10-22 00:07:14.893445175 +0200
@@ -1410,10 +1410,7 @@ _dbus_get_standard_system_servicedirs (D
    * be available.
    */
   static const char standard_search_path[] =
-    "/usr/local/share:"
-    "/usr/share:"
-    DBUS_DATADIR ":"
-    "/lib";
+    "/etc";
   DBusString servicedir_path;
 
   _dbus_string_init_const (&servicedir_path, standard_search_path);
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<div class='path'>path: <a href='/guix/log/?id=c2cb1160322c5b18b839b3c2ab0d53fb78793125'>root</a>/<a href='/guix/log/gnu?id=c2cb1160322c5b18b839b3c2ab0d53fb78793125'>gnu</a>/<a href='/guix/log/gnu/tests?id=c2cb1160322c5b18b839b3c2ab0d53fb78793125'>tests</a>/<a href='/guix/log/gnu/tests/networking.scm?id=c2cb1160322c5b18b839b3c2ab0d53fb78793125'>networking.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/tests/networking.scm?id=c2cb1160322c5b18b839b3c2ab0d53fb78793125&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-12-13 23:37:30 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=6601cc78bb728f19cb181b1bd3f78cccce23ec4b'>tests: static-networking: Adjust to new style.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/networking.scm (run-static-networking-test)[test]: Remove
'mkdir', 'chdir', and 'exit' calls'.  Add 'test-runner-current' call.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-13 11:49:15 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=1052ae5f03de931b52c7a638c8e4f8d8d7093af3'>Merge branch 'master' into core-updates-frozen</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-12 23:20:50 +0100'>2021-12-12</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=f73ba627abd46018da7f6f41ebd5efb99412cc54'>tests: Replace uses of deprecated 'static-networking-service'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/ganeti.scm (%ganeti-os): Replace call to
'static-networking-service' by instantiating
STATIC-NETWORKING-SERVICE-TYPE.
* gnu/tests/networking.scm (%test-static-networking)
(%openvswitch-os, %dhcpd-os): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-12 23:20:49 +0100'>2021-12-12</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=223f1b1eb3707f1d3ef91200dd616ee6c8b77db0'>services: static-networking: Change interface to mimic netlink.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/base.scm (&lt;static-networking&gt;)[interface, ip, netmask]
[gateway]: Remove.
[addresses, links, routes]: New fields.
[requirement]: Default to '(udev).
(&lt;network-address&gt;, &lt;network-link&gt;, &lt;network-route&gt;): New record types.
(ensure-no-separate-netmask, %ensure-no-separate-netmask): Remove.
(ipv6-address?, cidr-&gt;netmask, ip+netmask-&gt;cidr)
(network-set-up/hurd, network-tear-down/hurd)
(network-set-up/linux, network-tear-down/linux)
(static-networking-&gt;hurd-pfinet-options): New procedures.
(static-networking-shepherd-service): New procedure.
(static-networking-shepherd-services): Rewrite in terms of the above.
(static-networking-service): Deprecate.  Adjust to new
'static-networking' API.
(%base-services): Likewise.
* gnu/system/install.scm (%installation-services): Likewise.
* gnu/system/hurd.scm (%base-services/hurd): Likewise, and separate
'loopback' from 'networking'.
* gnu/build/hurd-boot.scm (set-hurd-device-translators): Remove
"servers/socket/2".
* gnu/tests/networking.scm (run-openvswitch-test)["networking has
started on ovs0"]: Check for 'networking instead of 'networking-ovs0,
which is no longer provided.
* doc/guix.texi (Networking Setup): Document the new interface.  Remove
documentation of 'static-networking-service'.
(Virtualization Services): Change Ganeti example to use the new
interface.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-12 23:20:49 +0100'>2021-12-12</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=33c498b9ee93429adc9e444ffe90df5b158fa0df'>tests: openvswitch: Check whether ovs0 is up.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/networking.scm (run-openvswitch-test)["ovs0 is up"]: New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-12 23:20:48 +0100'>2021-12-12</span></td><td><a href='/guix/commit/gnu/tests/networking.scm?id=72f140c25314141e98f0c02950590afe84284ca2'>tests: Add 'static-networking' test.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/networking.scm (run-static-networking-test): New procedure.
(%test-static-networking): New variable.
</span></span></td><td>Ludovic Courtès</td></tr>