tproxy: added tproxy sockopt interface in the IPV6 layer

Support for IPV6_RECVORIGDSTADDR sockopt for UDP sockets were contributed by
Harry Mason.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
index ef371aa..320bdb8 100644
--- a/net/ipv6/datagram.c
+++ b/net/ipv6/datagram.c
@@ -577,6 +577,25 @@
 		u8 *ptr = nh + opt->dst1;
 		put_cmsg(msg, SOL_IPV6, IPV6_2292DSTOPTS, (ptr[1]+1)<<3, ptr);
 	}
+	if (np->rxopt.bits.rxorigdstaddr) {
+		struct sockaddr_in6 sin6;
+		u16 *ports = (u16 *) skb_transport_header(skb);
+
+		if (skb_transport_offset(skb) + 4 <= skb->len) {
+			/* All current transport protocols have the port numbers in the
+			 * first four bytes of the transport header and this function is
+			 * written with this assumption in mind.
+			 */
+
+			sin6.sin6_family = AF_INET6;
+			ipv6_addr_copy(&sin6.sin6_addr, &ipv6_hdr(skb)->daddr);
+			sin6.sin6_port = ports[1];
+			sin6.sin6_flowinfo = 0;
+			sin6.sin6_scope_id = 0;
+
+			put_cmsg(msg, SOL_IPV6, IPV6_ORIGDSTADDR, sizeof(sin6), &sin6);
+		}
+	}
 	return 0;
 }