Re: IP routing with remote DNS, but server & client on same subnet - how?

Hello,

Randy Brick MacKenna a écrit :

1. With the cable modem plugged in, I surfed to my website www.foodomain.net
(which is a server on my home network)
2. I surfed to a link to a large document (32Meg)
3. *Before* clicking on the link, I unplugged the cable modem
(disconnecting from the internet)
4. I clicked on the file link -- and it timed out, server
unreachable.

Next experiment:

1. With the cable modem plugged in, I surfed to my website www.foodomain.net
2. I surfed to a link to a large document (32Meg)
3. I *first* clicked on the link, then during the file transfer, I
unplugged the cable modem
4. The file transfer completed successfully!

So, from this I deduce that the internet connection is required only
for DNS -- but after that the actual routing of packets from the
server and the client (all within my home network) do not involve the
internet connect

IHMO this sounds a bit optimistic.

First, I believe there is more than just DNS. If you browsed the website before unplugging the cable, it is likely that the DNS resolution was already cached by the client. You said in your first post that you told your registrar to redirect www.foodomain.net to [your.public.ip.address]:32004. DNS does not handle ports (neither does the the "hosts" file), so I guess there is some form of web indirection (HTTP redirect, frame...) performed not by a DNS server but by a web server hosting www.foodomain.net operated by your registrar. So I guess the path is as follows :

1) User browses URL <http://www.foodomain.net/some/file>.
2) Browser sends a DNS query asking for the IP address of host "www.foodomain.net".
3) DNS replies with the IP address of a registrar's web server.
4) Browser connects to that IP address on port 80 and asks for "/some/file" on host "www.foodomain.net".
5) Web server replies that the document is actually at URL <http://[your.public.ip.address]:32004/some/file>.
6) Browser connects to your.public.ip.address on port 32004.
7) Router transparently forwards the connection to 192.168.1.10.
8) Browser asks for page "/some/file" on that connection.

So, even though you cache the DNS resolution for www.foodomain.net, you still need the internet connection in order to use the web redirection performed by your registrar.

Second, even though your second test was successul, as Stefan wrote, the port forwarding rule on the router may be bound to the internet link state. I'll take for example the way the port forwarding is done in Linux - I believe some Linksys routers run Linux. The port forwarding rules is used only for the first packet of the redirected connection ; then a NAT mapping is created for all other packets of that connection. So if the transfer has started before the port forwarding rule is removed, it does not stop. But any subsequent new HTTP connection will fail because the port forwarding rule does not exist any more.

A workaround to all this would be a local DNS server resolving www.foodomain.net into the private web server address and having the web server listening on port 80.
.

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