From cceea73c3f32f433f3d6b5c8212a8ca1f06cddab Mon Sep 17 00:00:00 2001
From: Joel Cunningham <joel.cunningham@me.com>
Date: Fri, 9 Dec 2016 18:10:24 -0600
Subject: [PATCH] bug #49631: handle zero-window probe and refused_data

This commit adds support for responding to a zero-window probe when
the refused_data pointer is set

A zero-window probe is a data segment received when rcv_ann_wnd
is 0. This corrects a standards violation where LwIP would not
respond to a zero-window probe with its current ACK value (RCV.NXT)
when it has refused data, thus leading to the probing TCP closing
out the connection
---
 src/core/tcp_in.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
index 3d3ae33a..fbbdc729 100644
--- a/src/core/tcp_in.c
+++ b/src/core/tcp_in.c
@@ -358,6 +358,11 @@ tcp_input(struct pbuf *p, struct netif *inp)
         ((pcb->refused_data != NULL) && (tcplen > 0))) {
         /* pcb has been aborted or refused data is still refused and the new
            segment contains data */
+        if (pcb->rcv_ann_wnd == 0) {
+          /* this is a zero-window probe, we respond to it with current RCV.NXT
+          and drop the data segment */
+          tcp_send_empty_ack(pcb);
+        }
         TCP_STATS_INC(tcp.drop);
         MIB2_STATS_INC(mib2.tcpinerrs);
         goto aborted;