Writing a telnet client

Question

HI,

I have a device that exposes a telnet interface which you can log into using a username and password and then manipulate the working of the device.

I have to write a C program that hides the telnet aspect from the client and instead provides an interface for the user to control the device.

What would be a good way to proceed. I tried writing a simple socket program but it stops at the login prompt. My guess is that i am not following the TCP protocol.

Has anyone attempted this, is there an opensource library out there to do this?

Thanks

Addition: Eventually i wish to expose it through a web api/webservice. The platform is linux.

Answer 1

If Python is an option you could use telnetlib.

Code example:

#!/usr/bin/env python
import getpass
import sys
import telnetlib

HOST = "localhost"
user = raw_input("Enter your remote account: ")
password = getpass.getpass()

tn = telnetlib.Telnet(HOST)

tn.read_until("login: ")
tn.write(user + "\n")
if password:
    tn.read_until("Password: ")
    tn.write(password + "\n")

tn.write("ls\n")
tn.write("exit\n")

print tn.read_all()

Answer 2

This might be useful reading for you.

Answer 3

Download Putty source code and look for TELNET.C file, it's the telnet backend for Putty.

Answer 4

telnet's protocol is pretty straightforward... you just create a TCP connection, and send and receive ASCII data. That's pretty much it.

So all you really need to do is create a program that connects via TCP, then reads characters from the TCP socket and parses it to update the GUI, and/or writes characters to the socket in response to the user manipulating controls in the GUI.

How you would implement that depends a lot on what software you are using to construct your interface. On the TCP side, a simple event loop around select() would be sufficient.

Answer 5

Take a look at expect. You might not even need to write a C program:

• http://en.wikipedia.org/wiki/Expect
• http://www.linuxjournal.com/article/3065

These use the Telnet protocol:

• http://www.davidjarvis.ca/dave/telnet.zip
• http://www.davidjarvis.ca/dave/jexpect.zip

I will post a simple C-based Telnet application shortly.

Answer 6

I really find Beej's Guide to Network Programming a good introduction to network programming in C.

Answer 7

Unless the application is trivial, a better starting point would be to figure out how you're going to create the GUI. This is a bigger question and will have more impact on your project than how exactly you telnet into the device. You mention C at first, but then start talking about Python, which makes me believe you are relatively flexible in the matter.

Once you are set on a language/platform, then look for a telnet library -- you should find something reasonable already implemented.

Answer 8

While telnet is almost just a socket tied to a terminal it's not quite. I believe that there can be some control characters that get passed shortly after the connection is made. If your device is sending some unexpected control data then it may be confusing your program. If you haven't already, go download wireshark (or tshark or tcpdump) and monitor your connection. Wireshark (formerly ethereal) is cross platform and pretty easy to use for simple stuff. Filter with tcp.port == 23

Answer 9

Check out the source code here. It has helped me out a lot in understanding Telnet protocol.