North American Network Operators Group
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Re: easy way to scan for issues with path mtu discovery?

  • From: Ingo Flaschberger
  • Date: Tue Jun 24 10:40:15 2008
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