- allow the user to force the TCP mss below the fail-safe 216 with a low

interface MTU.
- break a tcp_output() -> tcp_mtudisc() -> tcp_output() infinite recursion
when the TCP mss ends up larger than the interface MTU (when the if_mtu is
smaller than the tcp header).  connections will still stall
feedback from itojun@, claudio@ and provos and testing from beck@

1 files changed, 5 insertions, 3 deletions

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index f0cffa74bb5..57c2a850dae 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: tcp_input.c,v 1.164 2004/04/20 20:05:29 markus Exp $	*/
+/*	$OpenBSD: tcp_input.c,v 1.165 2004/04/26 18:12:25 frantzen Exp $	*/
 /*	$NetBSD: tcp_input.c,v 1.23 1996/02/13 23:43:44 christos Exp $	*/
 
 /*
@@ -3068,13 +3068,15 @@ tcp_mss(tp, offer)
 	 * If we compute a larger value, return it for use in sending
 	 * a max seg size option, but don't store it for use
 	 * unless we received an offer at least that large from peer.
-	 * However, do not accept offers under 216 bytes.
+	 * However, do not accept offers under 216 bytes unless the
+	 * interface MTU is actually that low.
 	 */
 	if (offer > 0)
 		tp->t_peermss = offer;
 	if (tp->t_peermss)
 		mss = min(mss, tp->t_peermss);
-	mss = max(mss, 216);		/* sanity - at least max opt. space */
+	/* sanity - at least max opt. space */
+	mss = max(mss, min(216, ifp->if_mtu - iphlen - sizeof(struct tcphdr)));
 
 	/*
 	 * maxopd stores the maximum length of data AND options