Different behaviour of netcat from telnet when connecting to public route server

Question

I have observed a different behaviour between netcat and telnet when connecting to the public route server bgp-view.tvnetwork.hu and issuing the command show ip bgp.

Using Telnet the output (which is normally some tens of thousands lines long) is truncated and in order to view it all you have to press space or enter to continue (like the man pages). When I connect with netcat it just starts dumping all the output and after the first few hundred lines it hangs. Even if I state explicitly that I want to pause after the first 100 lines using the command terminal length 100 netcat doesn't change behaviour.

Do you have any idea why this happens and how it can be resolved? My OS is ubuntu 10.4 and the route server runs Quagga (version 0.99.5). With other type of routers (cisco or juniper) that problem doesn't appear.

Thank you.

ps. I wanted to tag the question as route-server but I cannot create new tags :(

EDIT:

The problem is that netcat doesn't negotiate window size (see my answer bellow). Netcat's -t parameter is used to negotiate telnet options but it replies with negations (DON'T or WON'T) so the question is how to make netcat to negotiate telnet options. Maybe I'll post a new question for this matter.

Answer 1

Hello,

I found a similar discrepancy with SSH 1.5, on juniper routers. When I was implementing a network topology tool, I had to use two different SSH libraries in Java to get things to work. I never completely diagnosed the problem, but it looked like there was an issue with how one of my libraries was handshaking, as opposed to how the ssh server on the router was expecting things to be done. For this case, my connections were just being dropped. I had to use four libraries to support three protocols : telnet, ssh 1.5 and ssh 2.0.

I would not be surprised at all if this is a router specific issue. Unfortunately, I don't have any useful suggestions for you other than to try a different library or program to accomplish your goals. If you feel like troubleshooting the actual issue, you could watch the packets go through.

Thanks,

-Brian-

Answer 2

OK, mystery solved with wireshark.

The problem was the negotiation of telnet options. The server requests from the telnet client "Do Negotiate About Window Size" but the client wasn't negotiating and even with the -t option netcat replies "Won't Negotiate About Window Size".

I created a java telnet client with the apache commons library to negotiate about the window size using the WindowSizeOptionHandler() constructor and it works fine. Now I just have to find how to do it with netcat.