netdev - [PATCH v2 1/2] selftests: net: add test for broadcast destination

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Message-ID: <20250823121336.18492-2-bacs@librecast.net>
Date: Sat, 23 Aug 2025 12:13:36 +0000
From: Brett A C Sheffield <bacs@...recast.net>
To: regressions@...ts.linux.dev
Cc: netdev@...r.kernel.org,
	stable@...r.kernel.org,
	davem@...emloft.net,
	dsahern@...nel.org,
	oscmaes92@...il.com,
	kuba@...nel.org,
	Brett A C Sheffield <bacs@...recast.net>
Subject: [PATCH v2 1/2] selftests: net: add test for broadcast destination

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

This test uses the tcpdump and socat programs.

Send a UDP broadcast packet to UDP port 9 (DISCARD), capture this
with tcpdump and ensure that all bits of the 6 octet ethernet destination
are correctly set.

Cc: stable@...r.kernel.org
Signed-off-by: Brett A C Sheffield <bacs@...recast.net>
Link: https://lore.kernel.org/regressions/20250822165231.4353-4-bacs@librecast.net
---
 tools/testing/selftests/net/Makefile          |  1 +
 .../selftests/net/broadcast_ether_dst.sh      | 38 +++++++++++++++++++
 2 files changed, 39 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..de6abe3513b6
--- /dev/null
+++ b/tools/testing/selftests/net/broadcast_ether_dst.sh
@@ -0,0 +1,38 @@
+#!/bin/bash -eu
+# SPDX-License-Identifier: GPL-2.0
+#
+# Author: Brett A C Sheffield <bacs@...recast.net>
+#
+# Ensure destination ethernet field is correctly set for
+# broadcast packets
+
+if ! which tcpdump > /dev/null 2>&1; then
+        echo "No tcpdump found. Required for this test."
+        exit $ERR
+fi
+
+CAPFILE=$(mktemp -u cap.XXXXXXXXXX)
+
+# start tcpdump listening on udp port 9
+# tcpdump will exit after receiving a single packet
+# timeout will kill tcpdump if it is still running after 2s
+timeout 2s tcpdump -c 1 -w ${CAPFILE} udp port 9 > /dev/null 2>&1 &
+PID=$!
+sleep 0.1 # let tcpdump wake up
+
+echo "Testing ethernet broadcast destination"
+
+# send broadcast UDP packet to port 9 (DISCARD)
+echo "Alonso is a good boy" | socat - udp-datagram:255.255.255.255:9,broadcast
+
+# wait for tcpdump for exit after receiving packet
+wait $PID
+
+# compare ethernet destination field to ff:ff:ff:ff:ff:ff
+# pcap has a 24 octet header + 16 octet header for each packet
+# ethernet destination is the first field in the packet
+printf '\xff\xff\xff\xff\xff\xff'| cmp -i40:0 -n6 ${CAPFILE} > /dev/null 2>&1
+RESULT=$?
+
+rm -f "${CAPFILE}"
+exit $RESULT

base-commit: 01b9128c5db1b470575d07b05b67ffa3cb02ebf1
-- 
2.49.1