path_mtu_probe.sh

#!/usr/bin/env bash

set -euo pipefail

# path_mtu_probe.sh
# Finds the largest ICMP payload that reaches a host without fragmentation.

usage() {
  cat <<'EOF'
Usage: ./path_mtu_probe.sh [options]

Options:
  --host HOST            Destination host/IP (default: 1.1.1.1)
  --min-payload N        Minimum payload size to test (default: 1200)
  --max-payload N        Maximum payload size to test (default: 1472)
  --count N              Ping count per test (default: 1)
  --timeout N            Timeout seconds per ping (default: 1)
  -h, --help             Show help

Notes:
  - Uses ping with DF bit (`-M do`) on Linux.
  - Estimated path MTU is payload + 28 bytes (IPv4 + ICMP headers).
EOF
}

HOST="1.1.1.1"
MIN_PAYLOAD=1200
MAX_PAYLOAD=1472
COUNT=1
TIMEOUT=1

while [[ $# -gt 0 ]]; do
  case "$1" in
    --host) HOST="$2"; shift 2 ;;
    --min-payload) MIN_PAYLOAD="$2"; shift 2 ;;
    --max-payload) MAX_PAYLOAD="$2"; shift 2 ;;
    --count) COUNT="$2"; shift 2 ;;
    --timeout) TIMEOUT="$2"; shift 2 ;;
    -h|--help) usage; exit 0 ;;
    *) echo "Unknown option: $1"; usage; exit 1 ;;
  esac
done

if ! command -v ping >/dev/null 2>&1; then
  echo "ping command not found"
  exit 1
fi

if [[ "$MIN_PAYLOAD" -gt "$MAX_PAYLOAD" ]]; then
  echo "Invalid range: --min-payload must be <= --max-payload"
  exit 1
fi

probe_payload() {
  local payload="$1"
  if ping -M do -s "$payload" -c "$COUNT" -W "$TIMEOUT" "$HOST" >/dev/null 2>&1; then
    return 0
  fi
  return 1
}

echo "Path MTU probe timestamp: $(date -Iseconds)"
echo "host=$HOST"
echo "payload_range=${MIN_PAYLOAD}-${MAX_PAYLOAD}"
echo

low="$MIN_PAYLOAD"
high="$MAX_PAYLOAD"
best=-1

while [[ "$low" -le "$high" ]]; do
  mid=$(( (low + high) / 2 ))
  if probe_payload "$mid"; then
    best="$mid"
    low=$(( mid + 1 ))
    echo "[OK] payload=$mid"
  else
    high=$(( mid - 1 ))
    echo "[FRAG/FAIL] payload=$mid"
  fi
done

echo
if [[ "$best" -lt 0 ]]; then
  echo "No successful payload found in range ${MIN_PAYLOAD}-${MAX_PAYLOAD}."
  echo "Summary: status=FAIL"
  exit 2
fi

path_mtu=$(( best + 28 ))
echo "Largest payload without fragmentation: $best"
echo "Estimated path MTU: $path_mtu"
echo "Summary: status=OK payload=$best path_mtu=$path_mtu"
exit 0