Match interface with longest prefix in IPv6

- When several interfaces share the same prefix w/ distinct prefix length

  (e.g., in 6to4, 2002::/16 on WAN and 2002:<IPv4>:<subnet>::/64 on LAN),

  the interface with shorter prefix length might be matched instead of

  the longer one, if it appears later in the netlink message.

Signed-off-by: Lung-Pin Chang <changlp@cs.nctu.edu.tw>

src/dnsmasq.h

@@ -640,6 +640,7 @@ struct dhcp_lease {
   unsigned char *extradata;
   unsigned int extradata_len, extradata_size;
   int last_interface;
+  int last_prefixlen;
 #ifdef HAVE_DHCP6
   struct in6_addr addr6;
   int iaid;

src/lease.c

@@ -379,8 +379,13 @@ static int find_interface_v6(struct in6_addr *local,  int prefix,
 
   for (lease = leases; lease; lease = lease->next)
     if ((lease->flags & (LEASE_TA | LEASE_NA)))
-      if (is_same_net6(local, &lease->addr6, prefix))
-	lease_set_interface(lease, if_index, *((time_t *)vparam));
+      if (is_same_net6(local, &lease->addr6, prefix) && prefix > lease->last_prefixlen) {
+        /* save prefix length for comparison, as we might get shorter matching
+         * prefix in upcoming netlink GETADDR responses
+         * */
+        lease->last_prefixlen = prefix;
+        lease_set_interface(lease, if_index, *((time_t *)vparam));
+      }
 
   return 1;
 }