tap: Fill the IPv6 flow label field to represent flow association

This isn't optional: TCP streams must carry a unique, hard-to-guess,
non-zero label for each direction. Linux, probably among others,
will otherwise refuse to associate packets in a given stream to the
same connection.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

dhcpv6.c

@@ -520,7 +520,7 @@ int dhcpv6(struct ctx *c, struct ethhdr *eh, size_t len)
 			resp_not_on_link.hdr.xid = mh->xid;
 
 			tap_ip_send(c, &c->gw6, IPPROTO_UDP,
-				    (char *)&resp_not_on_link, n);
+				    (char *)&resp_not_on_link, n, mh->xid);
 
 			return 1;
 		}
@@ -569,7 +569,7 @@ int dhcpv6(struct ctx *c, struct ethhdr *eh, size_t len)
 
 	resp.hdr.xid = mh->xid;
 
-	tap_ip_send(c, &c->gw6, IPPROTO_UDP, (char *)&resp, n);
+	tap_ip_send(c, &c->gw6, IPPROTO_UDP, (char *)&resp, n, mh->xid);
 	c->addr6_seen = c->addr6;
 
 	return 1;

icmp.c

@@ -98,7 +98,7 @@ void icmp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t events,
 			icmp_id_map[V6][id].seq = seq;
 		}
 
-		tap_ip_send(c, &sr6->sin6_addr, IPPROTO_ICMPV6, buf, n);
+		tap_ip_send(c, &sr6->sin6_addr, IPPROTO_ICMPV6, buf, n, 0);
 	} else {
 		struct sockaddr_in *sr4 = (struct sockaddr_in *)&sr;
 		struct icmphdr *ih = (struct icmphdr *)buf;
@@ -115,7 +115,7 @@ void icmp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t events,
 
 		memcpy(&a6.s6_addr[12], &sr4->sin_addr, sizeof(sr4->sin_addr));
 
-		tap_ip_send(c, &a6, IPPROTO_ICMP, buf, n);
+		tap_ip_send(c, &a6, IPPROTO_ICMP, buf, n, 0);
 	}
 }
 

tap.c

@@ -91,9 +91,10 @@ int tap_send(struct ctx *c, void *data, size_t len, int vnet_pre)
  * @proto:	L4 protocol number
  * @in:		Payload
  * @len:	L4 payload length
+ * @flow:	Flow label for TCP over IPv6
  */
 void tap_ip_send(struct ctx *c, struct in6_addr *src, uint8_t proto,
-		 char *in, size_t len)
+		 char *in, size_t len, uint32_t flow)
 {
 	char buf[USHRT_MAX];
 	char *pkt = buf + 4;
@@ -161,21 +162,30 @@ void tap_ip_send(struct ctx *c, struct in6_addr *src, uint8_t proto,
 			struct tcphdr *th = (struct tcphdr *)(ip6h + 1);
 
 			th->check = 0;
-			th->check = csum_ip4(ip6h, len + sizeof(*ip6h));
+			th->check = csum_unaligned(ip6h, len + sizeof(*ip6h),
+						   0);
 		} else if (proto == IPPROTO_UDP) {
 			struct udphdr *uh = (struct udphdr *)(ip6h + 1);
 
 			uh->check = 0;
-			uh->check = csum_ip4(ip6h, len + sizeof(*ip6h));
+			uh->check = csum_unaligned(ip6h, len + sizeof(*ip6h),
+						   0);
 		} else if (proto == IPPROTO_ICMPV6) {
 			struct icmp6hdr *ih = (struct icmp6hdr *)(ip6h + 1);
 
 			ih->icmp6_cksum = 0;
-			ih->icmp6_cksum = csum_ip4(ip6h, len + sizeof(*ip6h));
+			ih->icmp6_cksum = csum_unaligned(ip6h,
+							 len + sizeof(*ip6h),
+							 0);
 		}
 		ip6h->version = 6;
 		ip6h->nexthdr = proto;
 		ip6h->hop_limit = 255;
+		if (flow) {
+			ip6h->flow_lbl[0] = (flow >> 16) & 0xf;
+			ip6h->flow_lbl[1] = (flow >> 8) & 0xff;
+			ip6h->flow_lbl[2] = (flow >> 0) & 0xff;
+		}
 
 		tap_send(c, buf, len + sizeof(*ip6h) + sizeof(*eh), 1);
 	}

tap.h

@@ -1,5 +1,5 @@
 void tap_ip_send(struct ctx *c, struct in6_addr *src, uint8_t proto,
-		 char *in, size_t len);
+		 char *in, size_t len, uint32_t flow);
 int tap_send(struct ctx *c, void *data, size_t len, int vnet_pre);
 void tap_handler(struct ctx *c, uint32_t events, struct timespec *now);
 void tap_sock_init(struct ctx *c);

tcp.c

@@ -469,6 +469,7 @@ struct tcp_tap_conn {
 	uint32_t seq_from_tap;
 	uint32_t seq_ack_to_tap;
 	uint32_t seq_init_from_tap;
+	uint32_t seq_init_to_tap;
 	uint64_t tcpi_acked_last;
 
 	int ws_allowed;
@@ -934,7 +935,8 @@ static int tcp_send_to_tap(struct ctx *c, struct tcp_tap_conn *conn,
 
 	memcpy(data, in, len);
 
-	tap_ip_send(c, &conn->a.a6, IPPROTO_TCP, buf, th->doff * 4 + len);
+	tap_ip_send(c, &conn->a.a6, IPPROTO_TCP, buf, th->doff * 4 + len,
+		    conn->seq_init_to_tap);
 
 	return 0;
 }
@@ -1116,6 +1118,7 @@ static void tcp_conn_from_tap(struct ctx *c, int af, void *addr,
 	conn->seq_ack_to_tap = conn->seq_from_tap;
 
 	conn->seq_to_tap = tcp_seq_init(c, af, addr, th->dest, th->source, now);
+	conn->seq_init_to_tap = conn->seq_to_tap;
 	conn->seq_ack_from_tap = conn->seq_to_tap + 1;
 
 	tcp_hash_insert(c, conn, af, addr);
@@ -1828,6 +1831,7 @@ static void tcp_conn_from_sock(struct ctx *c, union epoll_ref ref,
 						conn->sock_port,
 						conn->tap_port,
 						now);
+		conn->seq_init_to_tap = conn->seq_to_tap;
 
 		tcp_hash_insert(c, conn, AF_INET6, &sa6->sin6_addr);
 	} else {
@@ -1850,6 +1854,7 @@ static void tcp_conn_from_sock(struct ctx *c, union epoll_ref ref,
 						conn->sock_port,
 						conn->tap_port,
 						now);
+		conn->seq_init_to_tap = conn->seq_to_tap;
 
 		tcp_hash_insert(c, conn, AF_INET, &sa4->sin_addr);
 	}