Work around possible Linux bug with VRF interfaces and DHCPv6.

The scope_id in the source address of recieved packets gets set
to the index of the VRF interface, not the slave. Fortunately,
the interface index returned by packetinfo is correct so we use
instead.

Thanks to Luci Stanescu <luci@safebits.tech> for characterising this.

Ref: https://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2023q4/017276.html

src/dhcp6.c

@@ -118,11 +118,6 @@ void dhcp6_packet(time_t now)
   if ((sz = recv_dhcp_packet(daemon->dhcp6fd, &msg)) == -1)
     return;
   
-#ifdef HAVE_DUMPFILE
-  dump_packet_udp(DUMP_DHCPV6, (void *)daemon->dhcp_packet.iov_base, sz,
-		  (union mysockaddr *)&from, NULL, daemon->dhcp6fd);
-#endif
-  
   for (cmptr = CMSG_FIRSTHDR(&msg); cmptr; cmptr = CMSG_NXTHDR(&msg, cmptr))
     if (cmptr->cmsg_level == IPPROTO_IPV6 && cmptr->cmsg_type == daemon->v6pktinfo)
       {
@@ -138,6 +133,34 @@ void dhcp6_packet(time_t now)
 
   if (!indextoname(daemon->dhcp6fd, if_index, ifr.ifr_name))
     return;
+  
+#ifdef HAVE_LINUX_NETWORK
+  /* This works around a possible Linux kernel bug when using interfaces
+     enslaved to a VRF. The scope_id in the source address gets set
+     to the index of the VRF interface, not the slave. Fortunately,
+     the interface index returned by packetinfo is correct so we use
+     that instead. Log this once, so if it triggers in other circumstances
+     we've not anticipated and breaks things, we get some clues. */
+  if (from.sin6_scope_id != if_index)
+    {
+      static int logged = 0;
+      
+      if (!logged)
+	{
+	  my_syslog(MS_DHCP | LOG_WARNING,
+		    _("Working around kernel bug: faulty source address scope for VRF slave %s"),
+		    ifr.ifr_name);
+	  logged = 1;
+	}
+      
+      from.sin6_scope_id = if_index;
+    }
+#endif
+
+#ifdef HAVE_DUMPFILE
+  dump_packet_udp(DUMP_DHCPV6, (void *)daemon->dhcp_packet.iov_base, sz,
+		  (union mysockaddr *)&from, NULL, daemon->dhcp6fd);
+#endif
 
   if (relay_reply6(&from, sz, ifr.ifr_name))
     {