#!/bin/bash
# VPN Path MTU Discovery Script
# Tests various packet sizes to find the maximum working MTU
# Usage: ./scripts/test-mtu.sh <VPN_SERVER_IP> [WG_INTERFACE]

set -e

VPN_SERVER_IP="${1:-}"
WG_INTERFACE="${2:-wg0}"

if [ -z "$VPN_SERVER_IP" ]; then
    echo "Usage: $0 <VPN_SERVER_IP> [WG_INTERFACE]"
    echo "Example: $0 203.0.113.5 wg0"
    exit 1
fi

echo "=============================================="
echo "VPN Path MTU Discovery"
echo "Target: $VPN_SERVER_IP"
echo "WG Interface: $WG_INTERFACE"
echo "=============================================="
echo ""

# Test from largest to smallest
SIZES=(1472 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 900 800)

WORKING_MTU=0
FAILED_MTU=1500

echo "Testing packet sizes (ICMP header = 28 bytes):"
echo ""

for size in "${SIZES[@]}"; do
    echo -n "Testing ${size} bytes (MTU $((size + 28)))... "
    
    if ping -c 2 -M do -s "$size" "$VPN_SERVER_IP" >/dev/null 2>&1; then
        echo "✓ OK"
        WORKING_MTU=$((size + 28))
    else
        echo "✗ FAIL"
        if [ $FAILED_MTU -eq 1500 ]; then
            FAILED_MTU=$((size + 28))
        fi
    fi
done

echo ""
echo "=============================================="
echo "Results:"
echo "=============================================="

if [ $WORKING_MTU -gt 0 ]; then
    echo "✓ Maximum working MTU: $WORKING_MTU"
    echo ""
    echo "Recommended WireGuard MTU:"
    echo "  Conservative: 1280"
    echo "  Recommended:  1300"
    echo "  Aggressive:   $((WORKING_MTU - 80))"
    echo ""
    echo "PostUp command:"
    echo "  PostUp = ip link set dev $WG_INTERFACE mtu 1300"
else
    echo "✗ No working MTU found!"
    echo "Check:"
    echo "  1. VPN server is reachable"
    echo "  2. ICMP is not blocked"
    echo "  3. Firewall allows ping"
fi

echo "=============================================="