9f7e94b4
From: rick.jones2@hp.com
David Schwartz wrote:
> On May 25, 4:09 am, adirtymindisajoyforever
> wrote:
> > I want to check the connectivity between two machines, say A and B.
> > I don't want to install scripts or whatever on A or B, but want to
> > test
> > thsi from a third machine C.
> That's impossible in principle.
> > Did I understand it wrong? Is there another solution for this?
> If you can't ask A if it can reach B, or vice-versa, there is no
> solution. There would have to be something installed on A and/or B
> to make this happen. You may be able to find something that just
> happens to be there and make it work, but otherwise, you're out of
> luck.
I think it would be more accurate to say it is impossible in practice.
Long ago and far away, when the Internet was still the ARPAnet and a
chummy place, and things like firewalls were but a gleam in the eye of
those bent upon poking their fingers in the eye of end-to-end
principles :) there was the source routing option of IPv4 coupled
perhaps with the record route option. At that time it was possible to
send an IP datagram from C to A telling it to go via B. So, the
datagram would go from C to B and thence to A. If this datagram
happened to be an ICMP echo request, the response would then go from A
to C (not necessarily passing through B unless, perhaps, the ICMP
software repeated the source routing). Lather, rinse, repeat to check
going from A to B. The same could be done with traceroute and/or
record route and you would see (at least part of) the path between A
and B.
But then the Internet was born, and source routing of IPv4 datagrams was
branded a security threat.
rick jones
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