The following commit has been merged in the linux branch: commit 6d01a026b7d3009a418326bdcf313503a314f1ea Author: Willy Tarreau w@1wt.eu Date: Tue Oct 13 00:27:40 2009 -0700
tcp: fix tcp_defer_accept to consider the timeout
I was trying to use TCP_DEFER_ACCEPT and noticed that if the client does not talk, the connection is never accepted and remains in SYN_RECV state until the retransmits expire, where it finally is deleted. This is bad when some firewall such as netfilter sits between the client and the server because the firewall sees the connection in ESTABLISHED state while the server will finally silently drop it without sending an RST.
This behaviour contradicts the man page which says it should wait only for some time :
TCP_DEFER_ACCEPT (since Linux 2.4) Allows a listener to be awakened only when data arrives on the socket. Takes an integer value (seconds), this can bound the maximum number of attempts TCP will make to complete the connection. This option should not be used in code intended to be portable.
Also, looking at ipv4/tcp.c, a retransmit counter is correctly computed :
case TCP_DEFER_ACCEPT: icsk->icsk_accept_queue.rskq_defer_accept = 0; if (val > 0) { /* Translate value in seconds to number of * retransmits */ while (icsk->icsk_accept_queue.rskq_defer_accept < 32 && val > ((TCP_TIMEOUT_INIT / HZ) << icsk->icsk_accept_queue.rskq_defer_accept)) icsk->icsk_accept_queue.rskq_defer_accept++; icsk->icsk_accept_queue.rskq_defer_accept++; } break;
==> rskq_defer_accept is used as a counter of retransmits.
But in tcp_minisocks.c, this counter is only checked. And in fact, I have found no location which updates it. So I think that what was intended was to decrease it in tcp_minisocks whenever it is checked, which the trivial patch below does.
Signed-off-by: Willy Tarreau w@1wt.eu Signed-off-by: David S. Miller davem@davemloft.net
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index 624c3c9..e320afe 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -644,6 +644,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */ if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { + inet_csk(sk)->icsk_accept_queue.rskq_defer_accept--; inet_rsk(req)->acked = 1; return NULL; }