From: Jason Xing kernelxing@tencent.com
[ Upstream commit 0d9e5df4a257afc3a471a82961ace9a22b88295a ]
We found that one close-wait socket was reset by the other side due to a new connection reusing the same port which is beyond our expectation, so we have to investigate the underlying reason.
The following experiment is conducted in the test environment. We limit the port range from 40000 to 40010 and delay the time to close() after receiving a fin from the active close side, which can help us easily reproduce like what happened in production.
Here are three connections captured by tcpdump: 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965525191 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 2769915070 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1 127.0.0.1.40002 > 127.0.0.1.9999: Flags [F.], seq 1, ack 1 // a few seconds later, within 60 seconds 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730 127.0.0.1.9999 > 127.0.0.1.40002: Flags [.], ack 2 127.0.0.1.40002 > 127.0.0.1.9999: Flags [R], seq 2965525193 // later, very quickly 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 3120990805 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
As we can see, the first flow is reset because: 1) client starts a new connection, I mean, the second one 2) client tries to find a suitable port which is a timewait socket (its state is timewait, substate is fin_wait2) 3) client occupies that timewait port to send a SYN 4) server finds a corresponding close-wait socket in ehash table, then replies with a challenge ack 5) client sends an RST to terminate this old close-wait socket.
I don't think the port selection algo can choose a FIN_WAIT2 socket when we turn on tcp_tw_reuse because on the server side there remain unread data. In some cases, if one side haven't call close() yet, we should not consider it as expendable and treat it at will.
Even though, sometimes, the server isn't able to call close() as soon as possible like what we expect, it can not be terminated easily, especially due to a second unrelated connection happening.
After this patch, we can see the expected failure if we start a connection when all the ports are occupied in fin_wait2 state: "Ncat: Cannot assign requested address."
Reported-by: Jade Dong jadedong@tencent.com Signed-off-by: Jason Xing kernelxing@tencent.com Reviewed-by: Eric Dumazet edumazet@google.com Link: https://patch.msgid.link/20240823001152.31004-1-kerneljasonxing@gmail.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_ipv4.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 96d235bcf5cb2..df3ddf31f8e67 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -116,6 +116,9 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp) const struct tcp_timewait_sock *tcptw = tcp_twsk(sktw); struct tcp_sock *tp = tcp_sk(sk);
+ if (tw->tw_substate == TCP_FIN_WAIT2) + reuse = 0; + if (reuse == 2) { /* Still does not detect *everything* that goes through * lo, since we require a loopback src or dst address