Re: [PATCH net v4] selftests: net: add test for destination in broadcast packets

On Thu, Aug 28, 2025 at 01:42:42PM +0200, Oscar Maes wrote:

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@gmail.com Co-authored-by: Brett A C Sheffield bacs@librecast.net

...

+test_broadcast_ether_dst() {

• local rc=0
• CAPFILE=$(mktemp -u cap.XXXXXXXXXX)
• OUTPUT=$(mktemp -u out.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 "${CAPFILE}" icmp &> "${OUTPUT}" &
  

• pid=$!
• slowwait 1 grep -qs "listening" "${OUTPUT}"
• # 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}"

Hi Oscar and Brett,

I am concerned that if something goes wrong this may block forever. Also, I'm wondering if this test could make use of the tcpdump helpers provided in tools/testing/selftests/net/forwarding/lib.sh

• # 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}"

+}

...