Re: easy way to scan for issues with path mtu discovery?

[Ingo Flaschberger <if () xip at>]

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