Corrections to lwip ipv6 neighbor discovery

Merged lwip 2.0.2 stable path 1 from https://github.com/ARMmbed/lwip

Patch allows lwip to do autonomous address configuration for prefixes
that are not on-link.

ad7cf16 Add David's IPv6 improvements to CHANGELOG
af9d783 Fix (bogus) MSVC 2010 warning about uninitialized variable usage in ip6.c It's wrong because the variables are initialized during first loop iteration due to best_addr == NULL
68358d7 nd6: cull destination cache on router removal
7323fa1 nd6: some work on basic RFC 4861 compliance
10eb2ca ip6: improve source address selection
6c06ecd ip6/nd6: route using on-link prefixes, not addresses
9f1714d nd6: improve router selection
2486b41 netif: more ip6 state changes invoke status callback
519d809 nd6: fix Duplicate Address Detection
9f3c6dd nd6: check link status before sending packets
8c761a2 nd6: improve address autoconfiguration support

Author: Mika Leppänen

features/FEATURE_LWIP/lwip-interface/lwip/CHANGELOG

@@ -15,6 +15,9 @@ HISTORY
 
   ++ Bugfixes:
 
+  2017-01-11: David van Moolenbroek
+  * Lots of IPv6 related fixes and improvements
+
   2017-03-08
   * tcp: do not keep sending SYNs when getting ACKs
 

features/FEATURE_LWIP/lwip-interface/lwip/src/core/ipv6/lwip_ip6.c

@@ -72,7 +72,7 @@
  *    this is a tricky case because with multiple netifs, link-local addresses only have
  *    meaning within a particular subnet/link.
  * 3) tries to match the destination subnet to a configured address
- * 4) tries to find a router
+ * 4) tries to find a router-announced route
  * 5) tries to match the source address to the netif
  * 6) returns the default netif, if configured
  *
@@ -134,22 +134,29 @@ ip6_route(const ip6_addr_t *src, const ip6_addr_t *dest)
   }
 #endif
 
-  /* See if the destination subnet matches a configured address. */
+  /* See if the destination subnet matches a configured address. In accordance
+   * with RFC 5942, dynamically configured addresses do not have an implied
+   * local subnet, and thus should be considered /128 assignments. However, as
+   * such, the destination address may still match a local address, and so we
+   * still need to check for exact matches here. By (lwIP) policy, statically
+   * configured addresses do always have an implied local /64 subnet. */
   for (netif = netif_list; netif != NULL; netif = netif->next) {
     if (!netif_is_up(netif) || !netif_is_link_up(netif)) {
       continue;
     }
     for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
       if (ip6_addr_isvalid(netif_ip6_addr_state(netif, i)) &&
-          ip6_addr_netcmp(dest, netif_ip6_addr(netif, i))) {
+          ip6_addr_netcmp(dest, netif_ip6_addr(netif, i)) &&
+          (netif_ip6_addr_isstatic(netif, i) ||
+          ip6_addr_nethostcmp(dest, netif_ip6_addr(netif, i)))) {
         return netif;
       }
     }
   }
 
-  /* Get the netif for a suitable router. */
+  /* Get the netif for a suitable router-announced route. */
   netif = nd6_find_route(dest);
-  if ((netif != NULL) && netif_is_up(netif) && netif_is_link_up(netif)) {
+  if (netif != NULL) {
     return netif;
   }
 
@@ -194,9 +201,22 @@ ip6_route(const ip6_addr_t *src, const ip6_addr_t *dest)
 
 /**
  * @ingroup ip6
- * Select the best IPv6 source address for a given destination
- * IPv6 address. Loosely follows RFC 3484. "Strong host" behavior
- * is assumed.
+ * Select the best IPv6 source address for a given destination IPv6 address.
+ *
+ * This implementation follows RFC 6724 Sec. 5 to the following extent:
+ * - Rules 1, 2, 3: fully implemented
+ * - Rules 4, 5, 5.5: not applicable
+ * - Rule 6: not implemented
+ * - Rule 7: not applicable
+ * - Rule 8: limited to "prefer /64 subnet match over non-match"
+ *
+ * For Rule 2, we deliberately deviate from RFC 6724 Sec. 3.1 by considering
+ * ULAs to be of smaller scope than global addresses, to avoid that a preferred
+ * ULA is picked over a deprecated global address when given a global address
+ * as destination, as that would likely result in broken two-way communication.
+ *
+ * As long as temporary addresses are not supported (as used in Rule 7), a
+ * proper implementation of Rule 8 would obviate the need to implement Rule 6.
  *
  * @param netif the netif on which to send a packet
  * @param dest the destination we are trying to reach
@@ -206,73 +226,72 @@ ip6_route(const ip6_addr_t *src, const ip6_addr_t *dest)
 const ip_addr_t *
 ip6_select_source_address(struct netif *netif, const ip6_addr_t *dest)
 {
-  const ip_addr_t *src = NULL;
-  u8_t i;
-
-  /* If dest is link-local, choose a link-local source. */
-  if (ip6_addr_islinklocal(dest) || ip6_addr_ismulticast_linklocal(dest) || ip6_addr_ismulticast_iflocal(dest)) {
-    for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
-      if (ip6_addr_isvalid(netif_ip6_addr_state(netif, i)) &&
-          ip6_addr_islinklocal(netif_ip6_addr(netif, i))) {
-        return netif_ip_addr6(netif, i);
-      }
-    }
+  const ip_addr_t *best_addr;
+  const ip6_addr_t *cand_addr;
+  s8_t dest_scope, cand_scope;
+  s8_t best_scope = IP6_MULTICAST_SCOPE_RESERVED;
+  u8_t i, cand_pref, cand_bits;
+  u8_t best_pref = 0;
+  u8_t best_bits = 0;
+
+  /* Start by determining the scope of the given destination address. These
+   * tests are hopefully (roughly) in order of likeliness to match. */
+  if (ip6_addr_isglobal(dest)) {
+    dest_scope = IP6_MULTICAST_SCOPE_GLOBAL;
+  } else if (ip6_addr_islinklocal(dest) || ip6_addr_isloopback(dest)) {
+    dest_scope = IP6_MULTICAST_SCOPE_LINK_LOCAL;
+  } else if (ip6_addr_isuniquelocal(dest)) {
+    dest_scope = IP6_MULTICAST_SCOPE_ORGANIZATION_LOCAL;
+  } else if (ip6_addr_ismulticast(dest)) {
+    dest_scope = ip6_addr_multicast_scope(dest);
+  } else if (ip6_addr_issitelocal(dest)) {
+    dest_scope = IP6_MULTICAST_SCOPE_SITE_LOCAL;
+  } else {
+    /* no match, consider scope global */
+    dest_scope = IP6_MULTICAST_SCOPE_GLOBAL;
   }
 
-  /* Choose a site-local with matching prefix. */
-  if (ip6_addr_issitelocal(dest) || ip6_addr_ismulticast_sitelocal(dest)) {
-    for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
-      if (ip6_addr_isvalid(netif_ip6_addr_state(netif, i)) &&
-          ip6_addr_issitelocal(netif_ip6_addr(netif, i)) &&
-          ip6_addr_netcmp(dest, netif_ip6_addr(netif, i))) {
-        return netif_ip_addr6(netif, i);
-      }
-    }
-  }
+  best_addr = NULL;
 
-  /* Choose a unique-local with matching prefix. */
-  if (ip6_addr_isuniquelocal(dest) || ip6_addr_ismulticast_orglocal(dest)) {
-    for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
-      if (ip6_addr_isvalid(netif_ip6_addr_state(netif, i)) &&
-          ip6_addr_isuniquelocal(netif_ip6_addr(netif, i)) &&
-          ip6_addr_netcmp(dest, netif_ip6_addr(netif, i))) {
-        return netif_ip_addr6(netif, i);
-      }
+  for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
+    /* Consider only valid (= preferred and deprecated) addresses. */
+    if (!ip6_addr_isvalid(netif_ip6_addr_state(netif, i))) {
+      continue;
     }
-  }
-
-  /* Choose a global with best matching prefix. */
-  if (ip6_addr_isglobal(dest) || ip6_addr_ismulticast_global(dest)) {
-    for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
-      if (ip6_addr_isvalid(netif_ip6_addr_state(netif, i)) &&
-          ip6_addr_isglobal(netif_ip6_addr(netif, i))) {
-        if (src == NULL) {
-          src = netif_ip_addr6(netif, i);
-        }
-        else {
-          /* Replace src only if we find a prefix match. */
-          /* @todo find longest matching prefix. */
-          if ((!(ip6_addr_netcmp(ip_2_ip6(src), dest))) &&
-              ip6_addr_netcmp(netif_ip6_addr(netif, i), dest)) {
-            src = netif_ip_addr6(netif, i);
-          }
-        }
-      }
+    /* Determine the scope of this candidate address. Same ordering idea. */
+    cand_addr = netif_ip6_addr(netif, i);
+    if (ip6_addr_isglobal(cand_addr)) {
+      cand_scope = IP6_MULTICAST_SCOPE_GLOBAL;
+    } else if (ip6_addr_islinklocal(cand_addr)) {
+      cand_scope = IP6_MULTICAST_SCOPE_LINK_LOCAL;
+    } else if (ip6_addr_isuniquelocal(cand_addr)) {
+      cand_scope = IP6_MULTICAST_SCOPE_ORGANIZATION_LOCAL;
+    } else if (ip6_addr_issitelocal(cand_addr)) {
+      cand_scope = IP6_MULTICAST_SCOPE_SITE_LOCAL;
+    } else {
+      /* no match, treat as low-priority global scope */
+      cand_scope = IP6_MULTICAST_SCOPE_RESERVEDF;
     }
-    if (src != NULL) {
-      return src;
+    cand_pref = ip6_addr_ispreferred(netif_ip6_addr_state(netif, i));
+    /* @todo compute the actual common bits, for longest matching prefix. */
+    cand_bits = ip6_addr_netcmp(cand_addr, dest); /* just 1 or 0 for now */
+    if (cand_bits && ip6_addr_nethostcmp(cand_addr, dest)) {
+      return netif_ip_addr6(netif, i); /* Rule 1 */
     }
-  }
-
-  /* Last resort: see if arbitrary prefix matches. */
-  for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
-    if (ip6_addr_isvalid(netif_ip6_addr_state(netif, i)) &&
-        ip6_addr_netcmp(dest, netif_ip6_addr(netif, i))) {
-      return netif_ip_addr6(netif, i);
+    if ((best_addr == NULL) || /* no alternative yet */
+        ((cand_scope < best_scope) && (cand_scope >= dest_scope)) ||
+        ((cand_scope > best_scope) && (best_scope < dest_scope)) || /* Rule 2 */
+        ((cand_scope == best_scope) && ((cand_pref > best_pref) || /* Rule 3 */
+        ((cand_pref == best_pref) && (cand_bits > best_bits))))) { /* Rule 8 */
+      /* We found a new "winning" candidate. */
+      best_addr = netif_ip_addr6(netif, i);
+      best_scope = cand_scope;
+      best_pref = cand_pref;
+      best_bits = cand_bits;
     }
   }
 
-  return NULL;
+  return best_addr; /* may be NULL */
 }
 
 #if LWIP_IPV6_FORWARD