Re: BSD 4.2 style TCP keepalives

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Wed, 06 Jan 2010 15:04:53 -0800 (PST)

> Someone with access to a system exhibiting this will probably need to
> do some diagnostics to figure out what's going on.

To make this easier to diagnose, I cooked up a hack patch that
makes Linux emit BSD 4.2 style keepalives, and indeed a quick
test shows that we do indeed not ACK these for some reason:

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 383ce23..e0db52e 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2727,6 +2727,19 @@ static int tcp_xmit_probe_skb(struct sock *sk, int urgent)
 	 * send it.
 	 */
 	tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPCB_FLAG_ACK);
+#if 1
+	/* Construct BSD 4.2 style zero-window probe with one
+	 * out-of-window garbage data byte.
+	 *
+	 * XXX this does the wrong thing when 'urgent' is true
+	 */
+	{
+		unsigned char *garbage_byte = skb_put(skb, 1);
+
+		*garbage_byte = 0xff;
+		TCP_SKB_CB(skb)->end_seq++;
+	}
+#endif
 	TCP_SKB_CB(skb)->when = tcp_time_stamp;
 	return tcp_transmit_skb(sk, skb, 0, GFP_ATOMIC);
 }