netdev - [PATCH net 2/2] selftests: net: add test for destination in broadcast packets

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Message-Id: <20250825060918.4799-2-oscmaes92@gmail.com>
Date: Mon, 25 Aug 2025 08:09:18 +0200
From: Oscar Maes <oscmaes92@...il.com>
To: bacs@...recast.net,
	brett@...recast.net,
	kuba@...nel.org
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	netdev@...r.kernel.org,
	regressions@...ts.linux.dev,
	stable@...r.kernel.org,
	Oscar Maes <oscmaes92@...il.com>
Subject: [PATCH net 2/2] selftests: net: add test for destination in broadcast packets

Add test to check the broadcast ethernet destination field is set
correctly.

This test sends a broadcast ping, captures it using tcpdump and
ensures that all bits of the 6 octet ethernet destination address
are correctly set by examining the output capture file.

Signed-off-by: Oscar Maes <oscmaes92@...il.com>
---
 tools/testing/selftests/net/Makefile          |  1 +
 .../selftests/net/broadcast_ether_dst.sh      | 82 +++++++++++++++++++
 2 files changed, 83 insertions(+)
 create mode 100755 tools/testing/selftests/net/broadcast_ether_dst.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index b31a71f2b372..463642a78eea 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -116,6 +116,7 @@ TEST_GEN_FILES += skf_net_off
 TEST_GEN_FILES += tfo
 TEST_PROGS += tfo_passive.sh
 TEST_PROGS += broadcast_pmtu.sh
+TEST_PROGS += broadcast_ether_dst.sh
 TEST_PROGS += ipv6_force_forwarding.sh
 
 # YNL files, must be before "include ..lib.mk"
diff --git a/tools/testing/selftests/net/broadcast_ether_dst.sh b/tools/testing/selftests/net/broadcast_ether_dst.sh
new file mode 100755
index 000000000000..865b5c7c8c8a
--- /dev/null
+++ b/tools/testing/selftests/net/broadcast_ether_dst.sh
@@ -0,0 +1,82 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Author: Brett A C Sheffield <bacs@...recast.net>
+# Author: Oscar Maes <oscmaes92@...il.com>
+#
+# Ensure destination ethernet field is correctly set for
+# broadcast packets
+
+source lib.sh
+
+CLIENT_IP4="192.168.0.1"
+GW_IP4="192.168.0.2"
+
+setup() {
+	setup_ns CLIENT_NS SERVER_NS
+
+	ip -net "${SERVER_NS}" link add link1 type veth \
+		peer name link0 netns "${CLIENT_NS}"
+
+	ip -net "${CLIENT_NS}" link set link0 up
+	ip -net "${CLIENT_NS}" addr add "${CLIENT_IP4}"/24 dev link0
+
+	ip -net "${SERVER_NS}" link set link1 up
+
+	ip -net "${CLIENT_NS}" route add default via "${GW_IP4}"
+	ip netns exec "${CLIENT_NS}" arp -s "${GW_IP4}" 00:11:22:33:44:55
+}
+
+cleanup() {
+	rm -f "${CAPFILE}"
+	ip -net "${SERVER_NS}" link del link1
+	cleanup_ns "${CLIENT_NS}" "${SERVER_NS}"
+}
+
+test_broadcast_ether_dst() {
+	local rc=0
+	CAPFILE=$(mktemp -u cap.XXXXXXXXXX)
+
+	echo "Testing ethernet broadcast destination"
+
+	# start tcpdump listening for icmp
+	# tcpdump will exit after receiving a single packet
+	# timeout will kill tcpdump if it is still running after 2s
+	timeout 2s ip netns exec "${CLIENT_NS}" \
+		tcpdump -i link0 -c 1 -w "${CAPFILE}" icmp &> /dev/null &
+	pid=$!
+	sleep 0.1 # let tcpdump wake up
+
+	# send broadcast ping
+	ip netns exec "${CLIENT_NS}" \
+		ping -W0.01 -c1 -b 255.255.255.255 &> /dev/null
+
+	# wait for tcpdump for exit after receiving packet
+	wait "${pid}"
+
+	# compare ethernet destination field to ff:ff:ff:ff:ff:ff
+	ether_dst=$(tcpdump -r "${CAPFILE}" -tnne 2>/dev/null | \
+			awk '{sub(/,/,"",$3); print $3}')
+	if [[ "${ether_dst}" == "ff:ff:ff:ff:ff:ff" ]]; then
+		echo "[ OK ]"
+		rc="${ksft_pass}"
+	else
+		echo "[FAIL] expected dst ether addr to be ff:ff:ff:ff:ff:ff," \
+			"got ${ether_dst}"
+		rc="${ksft_fail}"
+	fi
+
+	return "${rc}"
+}
+
+if [ ! -x "$(command -v tcpdump)" ]; then
+	echo "SKIP: Could not run test without tcpdump tool"
+	exit "${ksft_skip}"
+fi
+
+trap cleanup EXIT
+
+setup
+test_broadcast_ether_dst
+
+exit $?
-- 
2.39.5