network: beef up ipv6 RA support considerably

This reworks sd-ndisc and networkd substantially to support IPv6 RA much more
comprehensively. Since the API is extended quite a bit networkd has been ported
over too, and the patch is not as straight-forward as one could wish. The
rework includes:

- Support for DNSSL, RDNSS and RA routing options in sd-ndisc and networkd. Two
  new configuration options have been added to networkd to make this
  configurable.

- sd-ndisc now exposes an sd_ndisc_router object that encapsulates a full RA
  message, and has direct, friendly acessor functions for the singleton RA
  properties, as well as an iterative interface to iterate through known and
  unsupported options. The router object may either be retrieved from the wire,
  or generated from raw data. In many ways the sd-ndisc API now matches the
  sd-lldp API, except that no implicit database of seen data is kept. (Note
  that sd-ndisc actually had a half-written, but unused implementaiton of such
  a store, which is removed now.)

- sd-ndisc will now collect the reception timestamps of RA, which is useful to
  make sd_ndisc_router fully descriptive of what it covers.

Fixes: #1079

1 files changed, 55 insertions, 18 deletions

diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index 821e22aff8..487bb2ab3f 100644
--- a/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@ -528,23 +528,19 @@
         </varlistentry>
         <varlistentry>
           <term><varname>IPv6AcceptRouterAdvertisements=</varname></term>
-          <listitem><para>Force the setting of the <filename>accept_ra</filename>
-          (router advertisements) setting for the interface.
-          When unset, the kernel default is used, and router
-          advertisements are accepted only when local forwarding
-          is disabled for that interface.
-          When router advertisements are accepted, they will
-          trigger the start of the DHCPv6 client if the relevant
-          flags are passed, or if no routers are found on the link.
-          Takes a boolean. If true, router advertisements are
-          accepted, when false, router advertisements are ignored,
-          independently of the local forwarding state.</para>
-
-          <para>See
-          <ulink url="https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt">ip-sysctl.txt</ulink>
-          in the kernel documentation, but note that systemd's
-          setting of <constant>1</constant> corresponds to
-          kernel's setting of <constant>2</constant>.</para>
+          <listitem><para>Enable or disable IPv6 Router Advertisement (RA) reception support for the interface.  Takes
+          a boolean parameter. If true, RAs are accepted; if false, RAs are ignored, independently of the local
+          forwarding state. When not set, the kernel default is used, and RAs are accepted only when local forwarding
+          is disabled for that interface.  When RAs are accepted, they may trigger the start of the DHCPv6 client if
+          the relevant flags are set in the RA data, or if no routers are found on the link.</para>
+
+          <para>Further settings for the IPv6 RA support may be configured in the
+          <literal>[IPv6AcceptRouterAdvertisements]</literal> section, see below.</para>
+
+          <para>Also see <ulink
+          url="https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt">ip-sysctl.txt</ulink> in the kernel
+          documentation regarding <literal>accept_ra</literal>, but note that systemd's setting of
+          <constant>1</constant> (i.e. true) corresponds to kernel's setting of <constant>2</constant>.</para>
         </listitem>
         </varlistentry>
         <varlistentry>
@@ -799,7 +795,7 @@
             false.</para>
 
             <para>It is recommended to enable this option only on trusted networks, as setting this affects resolution
-            of all host names, in particular to single-label names. It is generally safer to use the supplied domain
+            of all host names, in particular of single-label names. It is generally safer to use the supplied domain
             only as routing domain, rather than as search domain, in order to not have it affect local resolution of
             single-label names.</para>
 
@@ -899,6 +895,47 @@
     </refsect1>
 
   <refsect1>
+    <title>[IPv6AcceptRouterAdvertisements] Section Options</title>
+      <para>The <literal>[IPv6AcceptRouterAdvertisements]</literal> section configures the IPv6 Router Advertisement
+      (RA) client, if it is enabled with the <varname>IPv6AcceptRouterAdvertisements=</varname> setting described
+      above:</para>
+
+      <variablelist class='network-directives'>
+        <varlistentry>
+          <term><varname>UseDNS=</varname></term>
+          <listitem>
+            <para>When true (the default), the DNS servers received in the Router Advertisement will be used and take
+            precedence over any statically configured ones.</para>
+
+            <para>This corresponds to the <option>nameserver</option> option in <citerefentry
+            project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><varname>UseDomains=</varname></term>
+          <listitem>
+            <para>Takes a boolean argument, or the special value <literal>route</literal>. When true, the domain name
+            received via IPv6 Router Advertisement (RA) will be used as DNS search domain over this link, similar to
+            the effect of the <option>Domains=</option> setting. If set to <literal>route</literal>, the domain name
+            received via IPv6 RA will be used for routing DNS queries only, but not for searching, similar to the
+            effect of the <option>Domains=</option> setting when the argument is prefixed with
+            <literal>~</literal>. Defaults to false.</para>
+
+            <para>It is recommended to enable this option only on trusted networks, as setting this affects resolution
+            of all host names, in particular of single-label names. It is generally safer to use the supplied domain
+            only as routing domain, rather than as search domain, in order to not have it affect local resolution of
+            single-label names.</para>
+
+            <para>When set to true, this setting corresponds to the <option>domain</option> option in <citerefentry
+            project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+          </listitem>
+        </varlistentry>
+      </variablelist>
+  </refsect1>
+
+
+  <refsect1>
     <title>[DHCPServer] Section Options</title>
     <para>The <literal>[DHCPServer]</literal> section contains
     settings for the DHCP server, if enabled via the