Simple server that responds an HTTP 204 No Content to anything received

serve204.sh

#!/bin/sh

# Command line variables
HOST=${1:-127.0.0.1}
PORT=${2:-4422}

# Test PORT
nc -z -w 5 127.0.0.1 ${PORT}
if [ $? -eq 0 ]; then
  echo "Port ${PORT} in use, choose a different one"
  exit 1
fi

# Minimal server that always responds 204 No Content
while :; do
  DATE=$(date -u +%a,\ %d\ %b\ %Y\ %H:%M:%S\ GMT)
  RESPONSE="HTTP/1.1 204 No Content\r\nDate: ${DATE}\r\n\r\n"
  printf %b "${RESPONSE}" | nc -vl -s ${HOST} -p ${PORT}

  # Stop the server on errors
  RESULT=$?; if [ $RESULT -ne 0 ]; then exit $RESULT; fi

  # Print the sent response
  printf %b "\n\nResponse:\n${RESPONSE}"
done

Using printf instead of echo for portability.
Tested in an Alpine docker container.