forward-wrong-port.sh

#! /bin/bash

BOOT_TIMEOUT=55

forward_wrong_port() {
  sleep $BOOT_TIMEOUT 
  local ports="$(ss -lt | awk '/LISTEN/ { print $4 }' | grep "*:[0-9]*")"
  if echo "$ports" | grep -v "*:$PORT$"; then
    local user_port="$(echo "$ports" | sed 's/\*://' | head -n 1)"
    socat TCP-LISTEN:$PORT,fork TCP:0.0.0.0:$user_port
  fi
}

forward_wrong_port &

warn-wrong-port.sh

#! /bin/bash

BOOT_TIMEOUT=60

warn_wrong_port() {
  local end=$((SECONDS+BOOT_TIMEOUT))

  while [ $SECONDS -lt $end ]; do
    local ports="$(ss -lt | awk '/LISTEN/ { print $4 }' | grep "*:[0-9]*" | sed 's/\*://')"
    for port in $ports; do
      if [ "$port" != "$PORT" ]; then
        echo "WARNING: Your app is binding to $port instead of the value defined by the \$PORT environment variable. You may recevie an R10 error."
      fi
    done
    sleep 10
  done
}

warn_wrong_port &