"No Response from <ip address>" FAQ entry

Rich Kennerly (rbk1@cornell.edu)
Wed, 26 Apr 1995 14:47:09 -0400


Windows users:

I'm trying to write a FAQ entry for the "No Response from <ip addr>"
problem that many people have, especially from PC's. I was not too deeply
involved in a lot of this so please tell me if I should change or amend
the following:

-Rich Kennerly - PC Windows CU-SeeMe developer. <rbk1@cornell.edu>
-----------------------------------------------------------------------------
"No Response from <ip address>" error.

Assuming that you have tested other winsock network applications so you
know you have basic network connectivity, there are several possible reasons
why you might be getting this response:

* The reflector machine is down - Try to telnet, FTP, or PING the host
address to check this. Even if you can't log in, any 'login' -type response
will indicate that the host is on the net.

* The reflector software is not currently running on the reflector host. Try
a couple of reflector sites - it is not likely that too many will be down at
the same time.

* You have a 'firewall' somewhere at 'your end' - between your local network
and the Internet. A firewall prevents traffic of certain types from
passing in and out of a network or networks. For example, a 'firewall'
may exist to reduce traffic volume by disallowing UDP packets - this will
prevent PING and NFS (Network File System) packets from passing through.
And, since CU-SeeMe uses UDP, it will not work either. It is also possible
for a firewall to disallow certain UDP port and allow others. Your network
provider can decide what type(s) of 'firewall' restrictions if any to
impose on your network.

It can be difficult to determine whether or not there is a firewall in
place on your network. If you can telnet to a reflector host (even though
you can't log on) but CU-SeeMe doesn't work then it could be a firewall.
You can contact your network provider to see if there are any restrictions
on the traffic passing in and out of your network. CU-SeeMe uses UDP
(not TCP) on UDP port 7648. Or, if you're using a PC with Winsock, it
could be:

* There is a DNS IP address problem or 'proxy' DNS is being done. Somehow,
with Winsock, it is possible for the machine to make a DNS query to the
network for its own address which is not really the actual address of the
Windows PC. CU-SeeMe uses this address in the CU-SeeMe packet header which
is different from the address in the IP header (apparently). This can be
caused by having two DNS servers with different IP addresses or, by a
machine doing 'proxy' DNS where it claims to be 'you' so it will get
your packets which it will then forward to you. This latter example has
been found in a situation where SLIP IP addresses were being 'spoken for'
by the slip server. Unfortunately, we don't have good information about
what can be done to really solve this problem. I would recommend having
your host and actual IP address in a HOSTS file locally so the DNS call
is either not made or not favored over the local hosts entry.