From d0214bffe630e7d03aa5264d697644619724f92e Mon Sep 17 00:00:00 2001
From: Jonathan Hui <jonhui@google.com>
Date: Wed, 14 Aug 2019 12:33:36 -0700
Subject: [PATCH] [ip6] fix default source address selection (#4082)

Thread defines a mesh-local scope for IPv6 unicast addresses. However,
the mesh-local scope can only be inferred by inclusion of the
mesh-local prefix.

This commit updates the IPv6 source address selection to infer the
destination address scope based on a prefix match with currently
assigned addresses.
---
 src/core/net/ip6.cpp | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/src/core/net/ip6.cpp b/src/core/net/ip6.cpp
index 2c6f304d5..894bd7e8c 100644
--- a/src/core/net/ip6.cpp
+++ b/src/core/net/ip6.cpp
@@ -1026,6 +1026,10 @@ const NetifUnicastAddress *Ip6::SelectSourceAddress(MessageInfo &aMessageInfo)
                 rvalAddr          = addr;
                 rvalPrefixMatched = candidatePrefixMatched;
             }
+            else
+            {
+                continue;
+            }
         }
         else if (addr->GetScope() > rvalAddr->GetScope())
         {
@@ -1034,6 +1038,10 @@ const NetifUnicastAddress *Ip6::SelectSourceAddress(MessageInfo &aMessageInfo)
                 rvalAddr          = addr;
                 rvalPrefixMatched = candidatePrefixMatched;
             }
+            else
+            {
+                continue;
+            }
         }
         else if ((rvalAddr->GetScope() == Address::kRealmLocalScope) && (addr->GetScope() == Address::kRealmLocalScope))
         {
@@ -1043,6 +1051,10 @@ const NetifUnicastAddress *Ip6::SelectSourceAddress(MessageInfo &aMessageInfo)
                 rvalAddr          = addr;
                 rvalPrefixMatched = candidatePrefixMatched;
             }
+            else
+            {
+                continue;
+            }
         }
         else if (addr->mPreferred && !rvalAddr->mPreferred)
         {
@@ -1058,6 +1070,16 @@ const NetifUnicastAddress *Ip6::SelectSourceAddress(MessageInfo &aMessageInfo)
             rvalAddr          = addr;
             rvalPrefixMatched = candidatePrefixMatched;
         }
+        else
+        {
+            continue;
+        }
+
+        // infer destination scope based on prefix match
+        if (rvalPrefixMatched >= rvalAddr->mPrefixLength)
+        {
+            destinationScope = rvalAddr->GetScope();
+        }
     }
 
 exit: