Re: [PATCH] Fix NAT TCP sequence adjustment

[Phil Oester <kernel@linuxace.com>]

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On Mon, Apr 04, 2005 at 01:47:16PM -0700, Phil Oester wrote:
> So as it stands, only retransmits on positive sizediff are handled
> properly.  But my original check to compensate for this was flawed,
> since seq != correction_pos when entering adjust_tcp_sequence - but
> it does seem clear we cannot blindly -= offset_after.

I've become more convinced that this is the correct approach.  Anticipating
that you will too, below is an updated patch with comments added.

Phil

Signed-off-by: Phil Oester <kernel@linuxace.com>



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diff -ru linux-orig/net/ipv4/netfilter/ip_nat_helper.c linux-stable/net/ipv4/netfilter/ip_nat_helper.c
--- linux-orig/net/ipv4/netfilter/ip_nat_helper.c	2005-03-02 02:37:49.000000000 -0500
+++ linux-stable/net/ipv4/netfilter/ip_nat_helper.c	2005-04-06 00:42:48.736295496 -0400
@@ -72,15 +72,23 @@
 
 	LOCK_BH(&ip_nat_seqofs_lock);
 
-	/* SYN adjust. If it's uninitialized, or this is after last
-	 * correction, record it: we don't handle more than one
-	 * adjustment in the window, but do deal with common case of a
-	 * retransmit */
-	if (this_way->offset_before == this_way->offset_after
-	    || before(this_way->correction_pos, seq)) {
-		    this_way->correction_pos = seq;
-		    this_way->offset_before = this_way->offset_after;
-		    this_way->offset_after += sizediff;
+	/* SYN adjust. In the case of a retransmit, ip_nat_seq_adjust has
+	 * added offset_before to client's seq, so subtract it to test
+	 * for retransmit here */
+	if (seq - this_way->offset_before != this_way->correction_pos) {
+		/* If it's uninitialized, or this is after last correction,
+		 * record it. We don't handle more than one adjustment in
+		 * the window */
+		seq -= this_way->offset_after;
+		if (this_way->offset_before == this_way->offset_after
+		    || before(this_way->correction_pos, seq)) {
+			/* Since ip_nat_seq_adjust has added
+			 * offset_after, subtract it to store
+			 * the client's seq in correction_pos */
+			this_way->correction_pos = seq;
+			this_way->offset_before = this_way->offset_after;
+			this_way->offset_after += sizediff;
+		}
 	}
 	UNLOCK_BH(&ip_nat_seqofs_lock);