netfilter: nf_conntrack: allow server to become a client in TW handling

When a port that was used to listen for inbound connections gets closed
and reused for outgoing connections (like rsh ends up doing for stderr
flow), current we may reject the SYN/ACK packet for the new connection
because tcp_conntracks states forbirds a port to become a client while
there is still a TIME_WAIT entry in there for it.

As TCP may expire the TIME_WAIT socket in 60s and conntrack's timeout
for it is 120s, there is a ~60s window that the application can end up
opening a port that conntrack will end up blocking.

This patch fixes this by simply allowing such state transition: if we
see a SYN, in TIME_WAIT state, on REPLY direction, move it to sSS. Note
that the rest of the code already handles this situation, more
specificly in tcp_packet(), first switch clause.
Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_proto_tcp.c

@@ -213,7 +213,7 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
 	{
 /* REPLY */
 /* 	     sNO, sSS, sSR, sES, sFW, sCW, sLA, sTW, sCL, sS2	*/
-/*syn*/	   { sIV, sS2, sIV, sIV, sIV, sIV, sIV, sIV, sIV, sS2 },
+/*syn*/	   { sIV, sS2, sIV, sIV, sIV, sIV, sIV, sSS, sIV, sS2 },
 /*
  *	sNO -> sIV	Never reached.
  *	sSS -> sS2	Simultaneous open
@@ -223,7 +223,7 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
  *	sFW -> sIV
  *	sCW -> sIV
  *	sLA -> sIV
- *	sTW -> sIV	Reopened connection, but server may not do it.
+ *	sTW -> sSS	Reopened connection, but server may have switched role
  *	sCL -> sIV
  */
 /* 	     sNO, sSS, sSR, sES, sFW, sCW, sLA, sTW, sCL, sS2	*/