Dear Patrick,
Does anyone know of an easy way to scan for issues with path mtu
discovery along a hop path? E.g. if you think someone is ICMP
black-holing along a route, or even on the endpoint host, could you use
some obscure nmap flag to find out for sure, and also to identify the
offending hop/router/host? What tool would you use to test for this,
and how would you do such a test? Is there any probing tool that does
checks like this automatically?
Seems to me this happens often enough that someone has probably already
figured it out, so I am trying not to reinvent the wheel. All I can
think of would be to handcraft packets of steadily increasing sizes and
look for replies from each hop on the route (which would be laborious at
best). Google has not been kind to my researches so far.
If you have a cisco router:
ping
Protocol [ip]:
Target IP address: x.x.x.x
Repeat count [5]:
Datagram size [100]: 1500
Timeout in seconds [2]: 1
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]: yes
Validate reply data? [no]: yes
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]: y
Sweep min size [36]:
Sweep max size [18024]: 1500
Sweep interval [1]:
Kind regards,
Ingo Flaschberger
