network/nexthop: introduce ManageForeignNextHops= boolean setting

Closes #29034.

man/networkd.conf.xml

@@ -90,6 +90,17 @@
         <xi:include href="version-info.xml" xpointer="v246"/></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>ManageForeignNextHops=</varname></term>
+        <listitem><para>A boolean. When true, <command>systemd-networkd</command> will remove nexthops
+        that are not configured in .network files (except for routes with protocol
+        <literal>kernel</literal>). When false, it will
+        not remove any foreign nexthops, keeping them even if they are not configured in a .network file.
+        Defaults to yes.</para>
+
+        <xi:include href="version-info.xml" xpointer="v256"/></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>RouteTable=</varname></term>
         <listitem><para>Defines the route table name. Takes a whitespace-separated list of the pairs of

man/systemd.network.xml

@@ -1715,8 +1715,10 @@ NFTSet=prefix:netdev:filter:eth_ipv4_prefix</programlisting>
       <varlistentry>
         <term><varname>Id=</varname></term>
         <listitem>
-          <para>The id of the next hop. Takes an integer in the range 1…4294967295. If unspecified,
-          then automatically chosen by kernel.</para>
+          <para>The id of the next hop. Takes an integer in the range 1…4294967295.
+          This is mandatory if <varname>ManageForeignNextHops=no</varname> is specified in
+          <citerefentry><refentrytitle>networkd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+          Otherwise, if unspecified, an unused ID will be automatically picked.</para>
 
           <xi:include href="version-info.xml" xpointer="v244"/>
         </listitem>

src/network/networkd-gperf.gperf

@@ -25,6 +25,7 @@ Network.SpeedMeter,                      config_parse_bool,
 Network.SpeedMeterIntervalSec,           config_parse_sec,                       0,          offsetof(Manager, speed_meter_interval_usec)
 Network.ManageForeignRoutingPolicyRules, config_parse_bool,                      0,          offsetof(Manager, manage_foreign_rules)
 Network.ManageForeignRoutes,             config_parse_bool,                      0,          offsetof(Manager, manage_foreign_routes)
+Network.ManageForeignNextHops,           config_parse_bool,                      0,          offsetof(Manager, manage_foreign_nexthops)
 Network.RouteTable,                      config_parse_route_table_names,         0,          0
 Network.IPv6PrivacyExtensions,           config_parse_ipv6_privacy_extensions,   0,          offsetof(Manager, ipv6_privacy_extensions)
 DHCPv4.DUIDType,                         config_parse_duid_type,                 0,          offsetof(Manager, dhcp_duid)

src/network/networkd-manager.c

@@ -591,6 +591,7 @@ int manager_new(Manager **ret, bool test_mode) {
                 .online_state = _LINK_ONLINE_STATE_INVALID,
                 .manage_foreign_routes = true,
                 .manage_foreign_rules = true,
+                .manage_foreign_nexthops = true,
                 .ethtool_fd = -EBADF,
                 .dhcp_duid.type = DUID_TYPE_EN,
                 .dhcp6_duid.type = DUID_TYPE_EN,
@@ -867,6 +868,9 @@ static int manager_enumerate_nexthop(Manager *m) {
         assert(m);
         assert(m->rtnl);
 
+        if (!m->manage_foreign_nexthops)
+                return 0;
+
         r = sd_rtnl_message_new_nexthop(m->rtnl, &req, RTM_GETNEXTHOP, 0, 0);
         if (r < 0)
                 return r;

src/network/networkd-manager.h

@@ -38,6 +38,7 @@ struct Manager {
         bool restarting;
         bool manage_foreign_routes;
         bool manage_foreign_rules;
+        bool manage_foreign_nexthops;
 
         Set *dirty_links;
         Set *new_wlan_ifindices;

src/network/networkd-nexthop.c

@@ -318,6 +318,10 @@ static int nexthop_acquire_id(Manager *manager, NextHop *nexthop) {
         if (nexthop->id > 0)
                 return 0;
 
+        /* If ManageForeignNextHops=no, nexthop with id == 0 should be already filtered by
+         * nexthop_section_verify(). */
+        assert(manager->manage_foreign_nexthops);
+
         /* Find the lowest unused ID. */
 
         ORDERED_HASHMAP_FOREACH(network, manager->networks) {
@@ -988,6 +992,13 @@ static int nexthop_section_verify(NextHop *nh) {
         if (section_is_invalid(nh->section))
                 return -EINVAL;
 
+        if (!nh->network->manager->manage_foreign_nexthops && nh->id == 0)
+                return log_warning_errno(SYNTHETIC_ERRNO(EINVAL),
+                                         "%s: [NextHop] section without specifying Id= is not supported "
+                                         "if ManageForeignNextHops=no is set in networkd.conf. "
+                                         "Ignoring [NextHop] section from line %u.",
+                                         nh->section->filename, nh->section->line);
+
         if (!hashmap_isempty(nh->group)) {
                 if (in_addr_is_set(nh->family, &nh->gw))
                         return log_warning_errno(SYNTHETIC_ERRNO(EINVAL),

src/network/networkd.conf

@@ -21,6 +21,7 @@
 #SpeedMeterIntervalSec=10sec
 #ManageForeignRoutingPolicyRules=yes
 #ManageForeignRoutes=yes
+#ManageForeignNextHops=yes
 #RouteTable=
 #IPv6PrivacyExtensions=no