[PATCH] tcp_v4_send_reset: binding oif to iif in no sock case

[Shawn Lu <shawn.lu@ericsson.com>]

Binding RST packet outgoing interface to incomming interface
for tcp v4 when there is no socket associate with it.
This has few benefits:
1. tcp_v6_send_reset already did that.

2. This helps tcp connect with SO_BINDTODEVICE set. When connection
is lost, we still able to sending out RST using same interface.

3. we are send reply, it is most likely to be succeed if iif
is used

Signed-off-by: Shawn Lu <shawn.lu@ericsson.com>
---
V2: amend as Eric Dumazet suggested
 net/ipv4/tcp_ipv4.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 90e4793..f2fde8d 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -676,6 +676,12 @@ static void tcp_v4_send_reset(struct sock *sk, struct sk_buff *skb)
 				      arg.iov[0].iov_len, IPPROTO_TCP, 0);
 	arg.csumoffset = offsetof(struct tcphdr, check) / 2;
 	arg.flags = (sk && inet_sk(sk)->transparent) ? IP_REPLY_ARG_NOSRCCHECK : 0;
+	/*
+	 * When socket is gone, all binding information is lost.
+	 * routing might fail in this case. using iif for oif to
+	 * make sure we can deliver it
+	 */
+	arg.bound_dev_if = sk ? sk->sk_bound_dev_if : inet_iif(skb);
 
 	net = dev_net(skb_dst(skb)->dev);
 	arg.tos = ip_hdr(skb)->tos;
-- 
1.7.0.4