network: prohibit to set MTUBytes= and UseMTU= simultaneously

man/systemd.network.xml

@@ -1167,6 +1167,7 @@
           <listitem>
             <para>When true, the interface maximum transmission unit
             from the DHCP server will be used on the current link.
+            If <varname>MTUBytes=</varname> is set, then this setting is ignored.
             Defaults to false.</para>
           </listitem>
         </varlistentry>

src/network/networkd-network.c

@@ -281,6 +281,12 @@ static int network_load_one(Manager *manager, const char *filename) {
         if (network->ip_masquerade)
                 network->ip_forward |= ADDRESS_FAMILY_IPV4;
 
+        if (network->mtu > 0 && network->dhcp_use_mtu) {
+                log_warning("MTUBytes= in [Link] section and UseMTU= in [DHCP] section are set in %s. "
+                            "Disabling UseMTU=.", filename);
+                network->dhcp_use_mtu = false;
+        }
+
         LIST_PREPEND(networks, manager->networks, network);
 
         r = hashmap_ensure_allocated(&manager->networks_by_name, &string_hash_ops);