Add mtu facility to --ra-param.
diff --git a/src/radv.c b/src/radv.c
index 6cf9a66..70211c5 100644
--- a/src/radv.c
+++ b/src/radv.c
@@ -243,7 +243,7 @@
   struct dhcp_netid iface_id;
   struct dhcp_opt *opt_cfg;
   struct ra_interface *ra_param = find_iface_param(iface_name);
-  int done_dns = 0, old_prefix = 0;
+  int done_dns = 0, old_prefix = 0, mtu = 0;
   unsigned int min_pref_time;
 #ifdef HAVE_LINUX_NETWORK
   FILE *f;
@@ -399,22 +399,31 @@
       put_opt6_long(1000 * calc_interval(find_iface_param(iface_name)));
     }
 
+  /* Set the MTU from ra_param if any, an MTU of 0 mean automatic for linux, */
+  /* an MTU of -1 prevents the option from being sent. */
+  if (ra_param)
+    mtu = ra_param->mtu;
 #ifdef HAVE_LINUX_NETWORK
   /* Note that IPv6 MTU is not neccessarily the same as the IPv4 MTU
      available from SIOCGIFMTU */
-  sprintf(daemon->namebuff, "/proc/sys/net/ipv6/conf/%s/mtu", iface_name);
-  if ((f = fopen(daemon->namebuff, "r")))
+  if (mtu == 0)
     {
-      if (fgets(daemon->namebuff, MAXDNAME, f))
-	{
-	  put_opt6_char(ICMP6_OPT_MTU);
-	  put_opt6_char(1);
-	  put_opt6_short(0);
-	  put_opt6_long(atoi(daemon->namebuff));
-	}
-      fclose(f);
+      sprintf(daemon->namebuff, "/proc/sys/net/ipv6/conf/%s/mtu", iface_name);
+      if ((f = fopen(daemon->namebuff, "r")))
+        {
+          if (fgets(daemon->namebuff, MAXDNAME, f))
+            mtu = atoi(daemon->namebuff);
+          fclose(f);
+        }
     }
 #endif
+  if (mtu > 0)
+    {
+      put_opt6_char(ICMP6_OPT_MTU);
+      put_opt6_char(1);
+      put_opt6_short(0);
+      put_opt6_long(mtu);
+    }
      
   iface_enumerate(AF_LOCAL, &send_iface, add_lla);