Supply zero preference in advertise and reply messages

src/option.c

@@ -580,7 +580,7 @@ char *option_string(int prot, unsigned int opt, unsigned char *val, int opt_len,
 	break;
       }
 
-  if (buf && (!ot[o].name || nodecode))
+  if (opt_len != 0 && buf && (!ot[o].name || nodecode))
     {
       int trunc  = 0;
       if (opt_len > 13)

src/rfc3315.c

@@ -764,6 +764,10 @@ static int dhcp6_no_relay(int msg_type, struct in6_addr *link_address, struct dh
 		    
 		    end_opt6(o);
 		    	
+		    o = new_opt6(OPTION6_PREFERENCE);
+		    put_opt6_char(0);
+		    end_opt6(o);
+
 		    break;
 		  }
 	      }