check_mtu.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
'''
check_mtu.py -- Check link MTU by sending UDP packets with increasing payload size

positional arguments:
  host                  host to send packet to. If omitted, defaults to
                        www.google.com

optional arguments:
  -h, --help            show this help message and exit
  -p PORT, --port PORT  port to send packet to. If omitted, defaults to 80
  -l PAYLOADS, --payloads PAYLOADS
                        range of payloads to test. If omitted, defaults to
                        1420-1480
  -v, --verbose         print the result of each test

@author:     Hervé Le Roy
@contact:    hleroy@hleroy.com
'''

import socket
from argparse import ArgumentParser


def payload_range(string):
    val_list = string.split('-')
    return range(int(val_list[0]), int(val_list[1]) + 1)


def main():
    "Check link MTU by sending UDP packets with increasing payload size"

    # Defaults
    DEFAULT_HOST = "www.google.com"
    DEFAULT_PORT = 80
    DEFAULT_PAYLOADS = "1420-1480"

    # Setup argument parser
    parser = ArgumentParser(
        description="Check link MTU by sending UDP packets with increasing payload size")
    parser.add_argument("host", nargs="?", default=DEFAULT_HOST,
                        help="host to send packet to. If omitted, defaults to www.google.com")
    parser.add_argument("-p", "--port", type=int, default=DEFAULT_PORT,
                        help="port to send packet to. If omitted, defaults to 80")
    parser.add_argument('-l', '--payloads', type=payload_range, default=DEFAULT_PAYLOADS,
                        help="range of payloads to test. If omitted, defaults to 1420-1480")
    parser.add_argument('-v', '--verbose', action='store_true',
                        help="print the result of each test")
    args = parser.parse_args()

    # Initialize max payloads
    max_payload = 0

    # Open socket
    IP_MTU_DISCOVER = 10  # MTU discover
    IP_PMTUDISC_DO = 2    # Always set DF (don't fragment) bit
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

    for payload in args.payloads:

        s.connect((args.host, args.port))
        s.setsockopt(socket.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DO)
        try:
            s.send(b'#' * payload)
        except socket.error:
            if args.verbose:
                print('Failed to sent UDP packet with a payload of {} bytes'.format(
                    payload))
        else:
            max_payload = payload
            if args.verbose:
                print('Succesfully sent UDP packet with a payload of {} bytes'.format(
                    payload))

    if max_payload == 0:
        print('No packet went through. Try decreasing payload size.')
        exit()

    if max_payload == args.payloads[-1]:
        print('All packets went through. Try increasing payload size.')
        exit()

    # MTU = max_payload + 20 (IP header) + 8 (UDP header)
    print('Path MTU to {} is {}'.format(args.host, max_payload + 28))


if __name__ == '__main__':
    main()