From 0e40f4c9593ba2c7c30150ed669da97bd581c0cd Mon Sep 17 00:00:00 2001 From: Jason Baron Date: Tue, 17 Jan 2017 13:37:19 -0500 Subject: [PATCH] tcp: accept RST for rcv_nxt - 1 after receiving a FIN Using a Mac OSX box as a client connecting to a Linux server, we have found that when certain applications (such as 'ab'), are abruptly terminated (via ^C), a FIN is sent followed by a RST packet on tcp connections. The FIN is accepted by the Linux stack but the RST is sent with the same sequence number as the FIN, and Linux responds with a challenge ACK per RFC 5961. The OSX client then sometimes (they are rate-limited) does not reply with any RST as would be expected on a closed socket. This results in sockets accumulating on the Linux server left mostly in the CLOSE_WAIT state, although LAST_ACK and CLOSING are also possible. This sequence of events can tie up a lot of resources on the Linux server since there may be a lot of data in write buffers at the time of the RST. Accepting a RST equal to rcv_nxt - 1, after we have already successfully processed a FIN, has made a significant difference for us in practice, by freeing up unneeded resources in a more expedient fashion. A packetdrill test demonstrating the behavior: // testing mac osx rst behavior // Establish a connection 0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 0.000 bind(3, ..., ...) = 0 0.000 listen(3, 1) = 0 0.100 < S 0:0(0) win 32768 0.100 > S. 0:0(0) ack 1 0.200 < . 1:1(0) ack 1 win 32768 0.200 accept(3, ..., ...) = 4 // Client closes the connection 0.300 < F. 1:1(0) ack 1 win 32768 // now send rst with same sequence 0.300 < R. 1:1(0) ack 1 win 32768 // make sure we are in TCP_CLOSE 0.400 %{ assert tcpi_state == 7 }% Signed-off-by: Jason Baron Cc: Eric Dumazet Acked-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 28 +++++++++++++++++++++++++--- 1 file changed, 25 insertions(+), 3 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 1a34e92..bfa165c 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -5199,6 +5199,23 @@ static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen) return err; } +/* Accept RST for rcv_nxt - 1 after a FIN. + * When tcp connections are abruptly terminated from Mac OSX (via ^C), a + * FIN is sent followed by a RST packet. The RST is sent with the same + * sequence number as the FIN, and thus according to RFC 5961 a challenge + * ACK should be sent. However, Mac OSX rate limits replies to challenge + * ACKs on the closed socket. In addition middleboxes can drop either the + * challenge ACK or a subsequent RST. + */ +static bool tcp_reset_check(const struct sock *sk, const struct sk_buff *skb) +{ + struct tcp_sock *tp = tcp_sk(sk); + + return unlikely(TCP_SKB_CB(skb)->seq == (tp->rcv_nxt - 1) && + (1 << sk->sk_state) & (TCPF_CLOSE_WAIT | TCPF_LAST_ACK | + TCPF_CLOSING)); +} + /* Does PAWS and seqno based validation of an incoming segment, flags will * play significant role here. */ @@ -5237,20 +5254,25 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb, LINUX_MIB_TCPACKSKIPPEDSEQ, &tp->last_oow_ack_time)) tcp_send_dupack(sk, skb); + } else if (tcp_reset_check(sk, skb)) { + tcp_reset(sk); } goto discard; } /* Step 2: check RST bit */ if (th->rst) { - /* RFC 5961 3.2 (extend to match against SACK too if available): - * If seq num matches RCV.NXT or the right-most SACK block, + /* RFC 5961 3.2 (extend to match against (RCV.NXT - 1) after a + * FIN and SACK too if available): + * If seq num matches RCV.NXT or (RCV.NXT - 1) after a FIN, or + * the right-most SACK block, * then * RESET the connection * else * Send a challenge ACK */ - if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) { + if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt || + tcp_reset_check(sk, skb)) { rst_seq_match = true; } else if (tcp_is_sack(tp) && tp->rx_opt.num_sacks > 0) { struct tcp_sack_block *sp = &tp->selective_acks[0]; -- 2.7.4