[PATCH next] netfilter: conntrack: avoid large timeout for mid-stream pickup

[Florian Westphal <fw@strlen.de>]

When loose tracking is enabled (default), non-syn packets cause
creation of new conntracks in established state with default timeout for
established state (5 days).  This causes the table to fill up with UNREPLIED
when the 'new ack' packet happened to be the last-ack of a previous,
already timed-out connection.

Consider:

A 192.168.x.52792 > 10.184.y.80: F, 426:426(0) ack 9237 win 255
B 10.184.y.80 > 192.168.x.52792: ., ack 427 win 123
<61 second pause>
C 10.184.y.80 > 192.168.x.52792: F, 9237:9237(0) ack 427 win 123
D 192.168.x.52792 > 10.184.y.80: ., ack 9238 win 255

B moves conntrack to CLOSE_WAIT and will kill it after 60 second timeout,
C is ignored (FIN set), but last packet (D) causes new ct with 5-days timeout.

Use UNACK timeout (5 minutes) instead to get rid of these entries sooner
when in ESTABLISHED state without having seen traffic in both directions.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 Warning; this is a bit different from the initial proposal in
 that it no longer bothers with old_state (the branch is taken
 for !SEEN_REPLY only).

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 4d4d8f1..7dcc376 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -1043,6 +1043,12 @@ static int tcp_packet(struct nf_conn *ct,
 			nf_ct_kill_acct(ct, ctinfo, skb);
 			return NF_ACCEPT;
 		}
+		/* ESTABLISHED without SEEN_REPLY, i.e. mid-connection
+		 * pickup with loose=1. Avoid large ESTABLISHED timeout.
+		 */
+		if (new_state == TCP_CONNTRACK_ESTABLISHED &&
+		    timeout > timeouts[TCP_CONNTRACK_UNACK])
+			timeout = timeouts[TCP_CONNTRACK_UNACK];
 	} else if (!test_bit(IPS_ASSURED_BIT, &ct->status)
 		   && (old_state == TCP_CONNTRACK_SYN_RECV
 		       || old_state == TCP_CONNTRACK_ESTABLISHED)
-- 
1.7.8.6