network: ndisc - Honour CurHopLimit

RFC4861 Neighbor Discovery – Sections 4.2 and 6.3.4

From section 4.2. Router Advertisement Message Format:

Cur Hop Limit        8-bit unsigned integer.  The default value that
                     should be placed in the Hop Count field of the IP
                     header for outgoing IP packets.  A value of zero
                     means unspecified (by this router).

man/systemd.network.xml

@@ -2628,6 +2628,14 @@ Token=prefixstable:2002:da8:1::</programlisting></para>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>UseHopLimit=</varname></term>
+        <listitem>
+          <para>Takes a boolean. When true, the hop limit received in the Router Advertisement will be set to routes
+          configured based on the advertisement. See also <varname>IPv6HopLimit=</varname>. Defaults to true.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>UseGateway=</varname></term>
         <listitem>

src/network/networkd-ndisc.c

@@ -168,6 +168,7 @@ static void ndisc_set_route_priority(Link *link, Route *route) {
 static int ndisc_request_route(Route *in, Link *link, sd_ndisc_router *rt) {
         _cleanup_(route_freep) Route *route = in;
         struct in6_addr router;
+        uint8_t hop_limit = 0;
         uint32_t mtu = 0;
         bool is_new;
         int r;
@@ -187,6 +188,12 @@ static int ndisc_request_route(Route *in, Link *link, sd_ndisc_router *rt) {
                         return log_link_warning_errno(link, r, "Failed to get default router MTU from RA: %m");
         }
 
+       if (link->network->ipv6_accept_ra_use_hop_limit) {
+                r = sd_ndisc_router_get_hop_limit(rt, &hop_limit);
+                if (r < 0 && r != -ENODATA)
+                        return log_link_warning_errno(link, r, "Failed to get default router hop limit from RA: %m");
+        }
+
         route->source = NETWORK_CONFIG_SOURCE_NDISC;
         route->provider.in6 = router;
         if (!route->table_set)
@@ -199,6 +206,9 @@ static int ndisc_request_route(Route *in, Link *link, sd_ndisc_router *rt) {
         if (route->mtu == 0)
                 route->mtu = mtu;
 
+        if (route->hop_limit == 0)
+                route->hop_limit = hop_limit;
+
         is_new = route_get(NULL, link, route, NULL) < 0;
 
         r = link_request_route(link, TAKE_PTR(route), true, &link->ndisc_messages,

src/network/networkd-network-gperf.gperf

@@ -283,6 +283,7 @@ IPv6AcceptRA.UseOnLinkPrefix,                config_parse_bool,
 IPv6AcceptRA.UseDNS,                         config_parse_bool,                                        0,                             offsetof(Network, ipv6_accept_ra_use_dns)
 IPv6AcceptRA.UseDomains,                     config_parse_ipv6_accept_ra_use_domains,                  0,                             offsetof(Network, ipv6_accept_ra_use_domains)
 IPv6AcceptRA.UseMTU,                         config_parse_bool,                                        0,                             offsetof(Network, ipv6_accept_ra_use_mtu)
+IPv6AcceptRA.UseHopLimit,                    config_parse_bool,                                        0,                             offsetof(Network, ipv6_accept_ra_use_hop_limit)
 IPv6AcceptRA.DHCPv6Client,                   config_parse_ipv6_accept_ra_start_dhcp6_client,           0,                             offsetof(Network, ipv6_accept_ra_start_dhcp6_client)
 IPv6AcceptRA.RouteTable,                     config_parse_dhcp_or_ra_route_table,                      AF_INET6,                      0
 IPv6AcceptRA.RouteMetric,                    config_parse_ipv6_accept_ra_route_metric,                 0,                             0

src/network/networkd-network.c

@@ -484,6 +484,7 @@ int network_load_one(Manager *manager, OrderedHashmap **networks, const char *fi
                 .ipv6_accept_ra_use_autonomous_prefix = true,
                 .ipv6_accept_ra_use_onlink_prefix = true,
                 .ipv6_accept_ra_use_mtu = true,
+                .ipv6_accept_ra_use_hop_limit = true,
                 .ipv6_accept_ra_route_table = RT_TABLE_MAIN,
                 .ipv6_accept_ra_route_metric_high = IPV6RA_ROUTE_METRIC_HIGH,
                 .ipv6_accept_ra_route_metric_medium = IPV6RA_ROUTE_METRIC_MEDIUM,

src/network/networkd-network.h

@@ -319,6 +319,7 @@ struct Network {
         bool ipv6_accept_ra_use_autonomous_prefix;
         bool ipv6_accept_ra_use_onlink_prefix;
         bool ipv6_accept_ra_use_mtu;
+        bool ipv6_accept_ra_use_hop_limit;
         bool ipv6_accept_ra_quickack;
         bool ipv6_accept_ra_use_captive_portal;
         bool active_slave;

src/network/networkd-route.c

@@ -1241,6 +1241,12 @@ static int route_configure(const Route *route, uint32_t lifetime_sec, Link *link
                         return r;
         }
 
+        if (route->hop_limit > 0) {
+                r = sd_netlink_message_append_u32(m, RTAX_HOPLIMIT, route->hop_limit);
+                if (r < 0)
+                        return r;
+        }
+
         r = sd_netlink_message_close_container(m);
         if (r < 0)
                 return r;

src/network/networkd-route.h

@@ -50,6 +50,7 @@ struct Route {
         uint32_t initcwnd;
         uint32_t initrwnd;
         uint32_t advmss;
+        uint32_t hop_limit;
         char *tcp_congestion_control_algo;
         unsigned char pref;
         unsigned flags;