[nrf fromlist] net: ipv6: routing: do not loop back already routed packets

If the packet was routed between interfaces by IPv6 module it should
not be looped back but has to be passed to the destination interface
instead.

Upstream PR: zephyrproject-rtos/zephyr#76150

Signed-off-by: Konrad Derda <[email protected]>

Author: kderda

subsys/net/ip/net_core.c

@@ -255,12 +255,13 @@ static inline int check_ip(struct net_pkt *pkt)
 		}
 
 		/* If the destination address is our own, then route it
-		 * back to us.
+		 * back to us (if it is not already forwarded).
 		 */
-		if (net_ipv6_is_addr_loopback(
+		if ((net_ipv6_is_addr_loopback(
 				(struct in6_addr *)NET_IPV6_HDR(pkt)->dst) ||
 		    net_ipv6_is_my_addr(
-				(struct in6_addr *)NET_IPV6_HDR(pkt)->dst)) {
+				(struct in6_addr *)NET_IPV6_HDR(pkt)->dst)) &&
+		    !net_pkt_forwarding(pkt)) {
 			struct in6_addr addr;
 
 			/* Swap the addresses so that in receiving side