netdev - [PATCH] tcp resets are misrouted

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Date:	Fri, 12 Oct 2012 18:34:17 +0400
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	netdev@...r.kernel.org, davem@...emloft.net, shawn.lu@...csson.com,
	eric.dumazet@...il.com, sol@....ru
Subject: [PATCH] tcp resets are misrouted

After commit e2446eaa.. tcp resets are always lost, when routing is asymmetric.
Yes, backing out that patch will result in misrouting of resets for dead connections
which used interface binding when were alive, but we actually cannot do anything here.
What's died that's died and correct handling normal unbound connections is obviously a priority.

Comment to comment:
> This has few benefits:
>   1. tcp_v6_send_reset already did that.

It was done to route resets for IPv6 link local addresses. It was a mistake to
do so for global addresses. The patch fixes this as well.

Actually, the problem appears to be even more serious than guaranteed loss of resets.
As reported by Sergey Soloviev <sol@....ru>, those misrouted resets create a lot of
arp traffic and huge amount of unresolved arp entires putting down to knees NAT firewalls
which use asymmetric routing.

Signed-off-by: Alexey Kuznetsov <kuznet@....inr.ac.ru>
---
 net/ipv4/tcp_ipv4.c |    7 ++++---
 net/ipv6/tcp_ipv6.c |    3 ++-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 75735c9..ef998b0 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -708,10 +708,11 @@ static void tcp_v4_send_reset(struct sock *sk, struct sk_buff *skb)
 	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
+	 * routing might fail in this case. No choice here, if we choose to force
+	 * input interface, we will misroute in case of asymmetric route.
 	 */
-	arg.bound_dev_if = sk ? sk->sk_bound_dev_if : inet_iif(skb);
+	if (sk)
+		arg.bound_dev_if = sk->sk_bound_dev_if;
 
 	net = dev_net(skb_dst(skb)->dev);
 	arg.tos = ip_hdr(skb)->tos;
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 49c8903..26175bf 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -877,7 +877,8 @@ static void tcp_v6_send_response(struct sk_buff *skb, u32 seq, u32 ack, u32 win,
 	__tcp_v6_send_check(buff, &fl6.saddr, &fl6.daddr);
 
 	fl6.flowi6_proto = IPPROTO_TCP;
-	fl6.flowi6_oif = inet6_iif(skb);
+	if (ipv6_addr_type(&fl6.daddr) & IPV6_ADDR_LINKLOCAL)
+		fl6.flowi6_oif = inet6_iif(skb);
 	fl6.fl6_dport = t1->dest;
 	fl6.fl6_sport = t1->source;
 	security_skb_classify_flow(skb, flowi6_to_flowi(&fl6));
-- 
1.7.2.3

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