From: Felix Fietkau <nbd@nbd.name>
Subject: netfilter: optional tcp window check

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
 net/netfilter/nf_conntrack_proto_tcp.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -34,6 +34,9 @@
 #include <net/netfilter/ipv4/nf_conntrack_ipv4.h>
 #include <net/netfilter/ipv6/nf_conntrack_ipv6.h>
 
+/* Do not check the TCP window for incoming packets  */
+static int nf_ct_tcp_no_window_check __read_mostly = 1;
+
 /* "Be conservative in what you do,
     be liberal in what you accept from others."
     If it's non-zero, we mark only out of window RST segments as INVALID. */
@@ -484,6 +487,9 @@ static bool tcp_in_window(const struct n
 	s32 receiver_offset;
 	bool res, in_recv_win;
 
+	if (nf_ct_tcp_no_window_check)
+		return true;
+
 	/*
 	 * Get the required data from the packet.
 	 */
@@ -1059,7 +1065,7 @@ static int tcp_packet(struct nf_conn *ct
 		 IP_CT_TCP_FLAG_DATA_UNACKNOWLEDGED &&
 		 timeouts[new_state] > timeouts[TCP_CONNTRACK_UNACK])
 		timeout = timeouts[TCP_CONNTRACK_UNACK];
-	else if (ct->proto.tcp.last_win == 0 &&
+	else if (!nf_ct_tcp_no_window_check && ct->proto.tcp.last_win == 0 &&
 		 timeouts[new_state] > timeouts[TCP_CONNTRACK_RETRANS])
 		timeout = timeouts[TCP_CONNTRACK_RETRANS];
 	else
@@ -1508,6 +1514,13 @@ static struct ctl_table tcp_sysctl_table
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname       = "nf_conntrack_tcp_no_window_check",
+		.data           = &nf_ct_tcp_no_window_check,
+		.maxlen         = sizeof(unsigned int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec,
+	},
 	{ }
 };
 #endif /* CONFIG_SYSCTL */