IPv6 use interface when sending to link-local dest

include/tins/packet_sender.h

@@ -65,7 +65,9 @@ class PDU;
  * - Those that don't contain a link layer PDU. In this case, the 
  * kernel will be responsible for picking the appropriate network interface
  * based on the destination address.
- *
+ *  - Exception: <a href="https://datatracker.ietf.org/doc/html/rfc2553#section-3.3">RFC2553</a> 
+ * requires IPv6 link-scope address have a interface defined.
+ *  .
  * \par Note for Windows users:
  * Sending layer 3 PDUs (without a link layer protocol) is very restricted
  * on Windows (<a href="https://msdn.microsoft.com/en-us/library/windows/desktop/ms740548(v=vs.85).aspx">link</a>).

src/ipv6.cpp

@@ -366,11 +366,16 @@ void IPv6::write_serialization(uint8_t* buffer, uint32_t total_sz) {
 }
 
 #ifndef BSD
-void IPv6::send(PacketSender& sender, const NetworkInterface &) {
+void IPv6::send(PacketSender& sender, const NetworkInterface& interface) {
     sockaddr_in6 link_addr;
     const PacketSender::SocketType type = PacketSender::IPV6_SOCKET;
     link_addr.sin6_family = AF_INET6;
     link_addr.sin6_port = 0;
+    // Required to set sin6_scope_id to interface index as stated in RFC2553.
+    // https://datatracker.ietf.org/doc/html/rfc2553#section-3.3
+    if (IPv6Address(header_.dst_addr).is_local_unicast()) {
+        link_addr.sin6_scope_id = interface.id();
+    }
     memcpy((uint8_t*)&link_addr.sin6_addr, header_.dst_addr, address_type::address_size);
     sender.send_l3(*this, (struct sockaddr*)&link_addr, sizeof(link_addr), type);
 }