network: sd-radv - Allow to set the MTU option is used in Router RA

closes https://github.com/systemd/systemd/issues/33160

man/systemd.network.xml

@@ -4080,6 +4080,19 @@ ServerAddress=192.168.0.1/24</programlisting>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>MTUBytes=</varname></term>
+        <listitem>
+          <para>Specifies the size of the router advertisement MTU or one of the special
+          value <literal>auto</literal>. When suffixed with K, M, or G, the specified
+          size is parsed as Kilobytes, Megabytes, or Gigabytes, respectively, to the base of 1024.
+          When <literal>auto</literal>, is specified the MTU of the link is automatically taken as
+          router advertisement MTU. Defaults to<literal>auto</literal>.</para>
+
+          <xi:include href="version-info.xml" xpointer="v257"/>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>UplinkInterface=</varname></term>
         <listitem><para>Specifies the name or the index of the uplink interface, or one of the special

src/network/networkd-link.c

@@ -2448,6 +2448,9 @@ static int link_update_mtu(Link *link, sd_netlink_message *message) {
         }
 
         if (link->radv) {
+                if (link->have_router_mtu)
+                        return 0;
+
                 r = sd_radv_set_mtu(link->radv, link->mtu);
                 if (r < 0)
                         return log_link_debug_errno(link, r, "Could not set MTU for Router Advertisement: %m");

src/network/networkd-link.h

@@ -172,6 +172,7 @@ typedef struct Link {
         bool ndisc_configured:1;
 
         sd_radv *radv;
+        bool have_router_mtu:1;
 
         sd_dhcp6_client *dhcp6_client;
         sd_dhcp6_lease *dhcp6_lease;

src/network/networkd-network-gperf.gperf

@@ -401,6 +401,7 @@ IPv6SendRA.Managed,                          config_parse_bool,
 IPv6SendRA.OtherInformation,                 config_parse_bool,                                        0,                             offsetof(Network, router_other_information)
 IPv6SendRA.RouterPreference,                 config_parse_router_preference,                           0,                             0
 IPv6SendRA.HopLimit,                         config_parse_uint8,                                       0,                             offsetof(Network, router_hop_limit)
+IPv6SendRA.MTUBytes,                         config_parse_router_mtu,                                  AF_INET6,                      offsetof(Network, router_mtu)
 IPv6SendRA.EmitDNS,                          config_parse_bool,                                        0,                             offsetof(Network, router_emit_dns)
 IPv6SendRA.DNS,                              config_parse_radv_dns,                                    0,                             0
 IPv6SendRA.EmitDomains,                      config_parse_bool,                                        0,                             offsetof(Network, router_emit_domains)

src/network/networkd-network.h

@@ -249,6 +249,8 @@ struct Network {
         usec_t router_reachable_usec;
         usec_t router_retransmit_usec;
         uint8_t router_hop_limit;
+        uint32_t router_mtu;
+        bool router_mtu_auto;
         bool router_managed;
         bool router_other_information;
         bool router_emit_dns;

src/network/networkd-radv.c

@@ -542,6 +542,15 @@ static int radv_configure(Link *link) {
         if (r < 0)
                 return r;
 
+        if (link->network->router_mtu > 0 || link->network->router_mtu_auto) {
+                r = sd_radv_set_mtu(link->radv, link->network->router_mtu_auto ? link->mtu : link->network->router_mtu);
+                if (r < 0)
+                        return r;
+
+                if (link->network->router_mtu > 0)
+                        link->have_router_mtu = true;
+        }
+
         r = sd_radv_set_preference(link->radv, link->network->router_preference);
         if (r < 0)
                 return r;
@@ -1625,3 +1634,26 @@ int config_parse_router_home_agent_lifetime(
         *home_agent_lifetime_usec = usec;
         return 0;
 }
+
+int config_parse_router_mtu(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+        Network *network = userdata;
+
+        assert(rvalue);
+
+        if (streq(rvalue, "auto"))
+                network->router_mtu_auto = true;
+        else
+                return config_parse_mtu(unit, filename, line, section, section_line, lvalue, ltype, rvalue, data, userdata);
+
+        return 1;
+}

src/network/networkd-radv.h

@@ -76,6 +76,7 @@ CONFIG_PARSER_PROTOTYPE(config_parse_router_prefix_delegation);
 CONFIG_PARSER_PROTOTYPE(config_parse_router_lifetime);
 CONFIG_PARSER_PROTOTYPE(config_parse_router_uint32_msec_usec);
 CONFIG_PARSER_PROTOTYPE(config_parse_router_preference);
+CONFIG_PARSER_PROTOTYPE(config_parse_router_mtu);
 CONFIG_PARSER_PROTOTYPE(config_parse_prefix);
 CONFIG_PARSER_PROTOTYPE(config_parse_prefix_boolean);
 CONFIG_PARSER_PROTOTYPE(config_parse_prefix_lifetime);