We didn't hear back from Tony whether indiscriminate ICMP blocking was
the problem (although I bet it was).

Here's something I picked up on another list today, from the very clever
Matthew Luckie. I think it would be of value in any situation like this:

> http://www.wand.net.nz/scamper/
>
> given an IP address, scamper will do a traceroute towards it to
> establish
> the forward path and an address at the end of the network that will
> terminate probes. then it will try ICMP based PMTUD starting with the
> interface's MTU size.
>
> If it does not get a response, it tries smaller packet sizes until it
> has
> inferred the largest packet that will get some response from the path.
> Then it does a TTL limited search to infer the hop[s] possibly
> responsible for not sending an ICMP Fragmentation Required message.
> It
> marks the mtu annotations for the hop[s] with an asterisk.
>
> For example, here's a sanitised example:
>
> traceroute from 199.109.33.1 to XXX
> 1 199.109.33.254 0.744 ms [mtu: 4470]
> 2 XXX 14.041 ms [mtu: 4470]
> 3 XXX 26.454 ms [mtu: 4470]
> 4 XXX 22.627 ms [mtu: 4470]
> 5 XXX 35.079 ms [mtu: 4470]
> 6 XXX 38.690 ms [mtu: 4470]
> 7 XXX 47.683 ms [mtu: 4470]
> 8 XXX 59.337 ms [mtu: 4470]
> 9 XXX 63.797 ms [mtu: 4470]
> 10 XXX 61.082 ms [*mtu: 1514]
> 11 XXX 60.909 ms [mtu: 1500]
> 12 XXX 61.541 ms [mtu: 1500]
>
> Note that the path between hop 9 and 10 probably has a L2 disagreement
> on
> the media's MTU size as the hop will happily generate other ICMP
> types.
>
> ./scamper -4mi <IP addresses> is the basic usage of scamper for this
> purpose.

-- 
Stephen Judd
stephen@vital.org.nz
http://vital.org.nz
-- 
This message is part of the NZ ADSL mailing list. 
see http://unixathome.org/adsl/ for archives, FAQ, 
and various documents. 
To unsubscribe: send mail to majordomo@lists.unixathome.org 
with "unsubscribe adsl" in the body of the message