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CHAPTER 16 TELNET AND SSH
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Listing 16 3 connects to localhost, which in this case is my Ubuntu laptop, where I have just run aptitude install telnetd so that a Telnet daemon is now listening on its standard port 23. Yes, I actually changed my password to mypass to test the scripts in this chapter; and, yes, I un-installed telnetd and changed my password again immediately after! Listing 16 3. Logging In to a Remote Host Using Telnet #!/usr/bin/env python # Foundations of Python Network Programming - 16 - telnet_login.py # Connect to localhost, watch for a login prompt, and try logging in import telnetlib t = telnetlib.Telnet('localhost') # t.set_debuglevel(1) # uncomment this for debugging messages t.read_until('login:') t.write('brandon\n') t.read_until('assword:') # let "P" be capitalized or not t.write('mypass\n') n, match, previous_text = t.expect([r'Login incorrect', r'\$'], 10) if n == 0: print "Username and password failed - giving up" else: t.write('exec uptime\n') print t.read_all() # keep reading until the connection closes If the script is successful, it shows you what the simple uptime command prints on the remote system: $ python telnet_login.py 10:24:43 up 5 days, 12:13, 14 users, load average: 1.44, 0.91, 0.73
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The listing shows you the general structure of a session powered by telnetlib. First, a connection is established, which is represented in Python by an instance of the Telnet object. Here only the hostname is specified, though you can also provide a port number to connect to some other service port than standard Telnet. You can call set_debuglevel(1) if you want your Telnet object to print out all of the strings that it sends and receives during the session. This actually turned out to be important for writing even the very simple script shown in the listing, because in two different cases it got hung up, and I had to re-run it with debugging messages turned on so that I could see the actual output and fix the script. (Once I was failing to match the exact text that was coming back, and once I forgot the '\r' at the end of the uptime command.) I generally turn off debugging only once a program is working perfectly, and turn it back on whenever I want to do more work on the script. Note that Telnet does not disguise the fact that its service is backed by a TCP socket, and will pass through to your program any socket.error and socket.gaierror exceptions that are raised. Once the Telnet session is established, interaction generally falls into a receive-and-send pattern, where you wait for a prompt or response from the remote end, then send your next piece of information. The listing illustrates two methods of waiting for text to arrive: The very simple read_until() method watches for a literal string to arrive, then returns a string providing all of the text that it received from the moment it started listing until the moment it finally saw the string you were waiting for.
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CHAPTER 16 TELNET AND SSH
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The more powerful and sophisticated expect() method takes a list of Python regular expressions. Once the text arriving from the remote end finally adds up to something that matches one of the regular expressions, expect() returns three items: the index in your list of the pattern that matched, the regular expression SRE_Match object itself, and the text that was received leading up to the matching text. For more information on what you can do with a SRE_Match, including finding the values of any sub-expressions in your pattern, read the Standard Library documentation for the re module.
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Regular expressions, as always, have to be written carefully. When I first wrote this script, I used '$' as the expect() pattern that watched for the shell prompt to appear which, of course, is a special character in a regular expression! So the corrected script shown in the listing escapes the $ so that expect() actually waits until it sees a dollar sign arrive from the remote end. If the script sees an error message because of an incorrect password and does not get stuck waiting forever for a login or password prompt that never arrives or that looks different than it was expecting then it exits: $ python telnet_login.py Username and password failed - giving up If you wind up writing a Python script that has to use Telnet, it will simply be a larger or more complicated version of the same simple pattern shown here. Both read_until() and expect() take an optional second argument named timeout that places a maximum limit on how long the call will watch for the text pattern before giving up and returning control to your Python script. If they quit and give up because of the timeout, they do not raise an error; instead awkwardly enough they just return the text they have seen so far, and leave it to you to figure out whether that text contains the pattern! There are a few odds and ends in the Telnet object that we need not cover here. You will find them in the telnetlib Standard Library documentation including an interact() method that lets the user talk directly over your Telnet connection using the terminal! This kind of call was very popular back in the old days, when you wanted to automate login but then take control and issue normal commands yourself. The Telnet protocol does have a convention for embedding control information, and telnetlib follows these protocol rules carefully to keep your data separate from any control codes that appear. So you can use a Telnet object to send and receive all of the binary data you want, and ignore the fact that control codes might be arriving as well. But if you are doing a sophisticated Telnet-based project, then you might need to process options. Normally, each time a Telnet server sends an option request, telnetlib flatly refuses to send or receive that option. But you can provide a Telnet object with your own callback function for processing options; a modest example is shown in Listing 16 4. For most options, it simply re-implements the default telnetlib behavior and refuses to handle any options (and always remember to respond to each option one way or another; failing to do so will often hang the Telnet session as the server waits forever for your reply). But if the server expresses interest in the terminal type option, then this client sends back a reply of mypython, which the shell command it runs after logging in then sees as its $TERM environment variable. Listing 16 4. How to Process Telnet Option Codes #!/usr/bin/env python # Foundations of Python Network Programming - 16 - telnet_codes.py # How your code might look if you intercept Telnet options yourself from telnetlib import Telnet, IAC, DO, DONT, WILL, WONT, SB, SE, TTYPE
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