telnet

telnet has default open port 23, which will remote login. But it has much more functions and can almost

fullfill any network service.

it'll help you learn network protocols and debug new developed ones.

1. use telnet to access web

telnet baidu.com 80

and use http protocol to access :

GET / HTTP/1.1

Host: baidu,com

and then you will get response

2. use telnet can access any network service:

here we implement a simple echo server and use telnet to access it

#! /usr/bin/python                                                              

from socket import *
import sys

host = ''
port = 50000
bufsize = 1024
addr = (host, port)

sock = socket(AF_INET, SOCK_STREAM)
sock.bind(addr)
sock.listen(5)

try:
    while True:
        c_sock, addr = sock.accept()
        data = c_sock.recv(bufsize)
	if data:
            print "receive data  %s" %data
            c_sock.send(data)
        else:
            break
        c_sock.close()

    sock.close()
except KeyboardInterrupt:
        sock.close()
        sys.exit()

open a terminal and start the echo server:

bash-4.3> python echo_server.py

open another terminal and telnet it:

bash-4.3> telnet localhost 50000
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
hello
hello
Connection closed by foreign host.

final: to exit telnet session:

^] q

see man telnet