From d976d94e3d902c23a9ebbc3bc7afab61694280ac Mon Sep 17 00:00:00 2001
From: Simon Kelley <simon@thekelleys.org.uk>
Date: Tue, 22 Jul 2025 22:09:49 +0100
Subject: [PATCH] Implement RFC 6221 para 4.

If the link address in a relayed DHCPv6 message is zero, don't
use it for addresss selection.
---
 src/rfc3315.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/rfc3315.c b/src/rfc3315.c
index a502bb3..90b50ce 100644
--- a/src/rfc3315.c
+++ b/src/rfc3315.c
@@ -245,7 +245,11 @@ static int dhcp6_maybe_relay(struct state *state, unsigned char *inbuff, size_t
 	      struct in6_addr align;
 	      /* the packet data is unaligned, copy to aligned storage */
 	      memcpy(&align, inbuff + 2, IN6ADDRSZ); 
-	      state->link_address = &align;
+
+
+	      /* RFC6221 para 4 */
+	      if (!IN6_IS_ADDR_UNSPECIFIED(&align))
+		state->link_address = &align;
 	      /* zero is_unicast since that is now known to refer to the 
 		 relayed packet, not the original sent by the client */
 	      if (!dhcp6_maybe_relay(state, opt6_ptr(opt, 0), opt6_len(opt), client_addr, 0, now))