Foundations of Python Network Programming 978-1-4302-3004-5

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CHAPTER 16 ■ TELNET AND SSH replaced by more text. Often these were the characters Ctrl+S for “Stop” and Ctrl+Q for “Keep going,” and it was a source of great annoyance that if binary data worked its way into an automated Telnet connection that the first Ctrl+S that happened to pass across the channel would pause the terminal and probably ruin the session. Again, this setting can be turned off with stty: $ stty -ixon -ixoff Those are the two biggest problems you will run into with terminals doing buffering, but there are plenty of less famous settings that can also cause you grief. Because there are so many—and because they vary between Unix implementations—the stty command actually supports two modes, cooked and raw, that turn dozens of settings like icanon and ixon on and off together: $ stty raw $ stty cooked In case you make your terminal settings a hopeless mess after some experimentation, most Unix systems provide a command for resetting the terminal back to reasonable, sane settings (and note that if you have played with stty too severely, you might need to hit Ctrl+J to submit the reset command, since your Return key, whose equivalent is Ctrl+M, actually only functions to submit commands because of a terminal setting called icrnl!): $ reset If, instead of trying to get the terminal to behave across a Telnet or SSH session, you happen to be talking to a terminal from Python, check out the termios module that comes with the Standard Library. By puzzling through its example code and remembering how Boolean bitwise math works, you should be able to control all of the same settings that we just accessed through the stty command. This book lacks the space to look at terminals in any more detail (since one or two chapters of examples could easily be inserted right here to cover all of the interesting techniques and cases), but there are lots of great resources for learning more about them—a classic is Chapter 19, “Pseudo Terminals,” of W. Richard Stevens' Advanced Programming in the UNIX Environment. Telnet This brief section is all you will find in this book about the ancient Telnet protocol. Why? Because it is insecure: anyone watching your Telnet packets fly by will see your username, password, and everything you do on the remote system. It is clunky. And it has been completely abandoned for most systems administration. 274
CHAPTER 16 ■ TELNET AND SSH THE TELNET PROTOCOL Purpose: Remote shell access Standard: RFC 854 (1989) Runs atop: TCP/IP Default port: 23 Library: telnetlib Exceptions: socket.error, socket.gaierror The only time I ever find myself needing Telnet is when speaking to small embedded systems, like a Linksys router or DSL modem or network switch. In case you are having to write a Python program that has to speak Telnet to one of these devices, here are a few pointers on using the Python telnetlib. First, you have to realize that all Telnet does is to establish a channel—in fact, a fairly plain TCP socket (see Chapter 3)—and to send the things you type, and receive the things the remote system says, back and forth across that channel. This means that Telnet is ignorant of all sorts of things of which you might expect a remote-shell protocol to be aware. For example, it is conventional that when you Telnet to a Unix machine, you are presented with aa login: prompt at which you type your username, and a password: prompt where you enter your password. The small embedded devices that still use Telnet these days might follow a slightly simpler script, but they, too, often ask for some sort of password or authentication. But the point is that Telnet knows nothing about this! To your Telnet client, password: is just nine random characters that come flying across the TCP connection and that it must print to your screen. It has no idea that you are being prompted, that you are responding, or that in a moment the remote system will know who you are. The fact that Telnet is ignorant about authentication has an important consequence: you cannot type anything on the command line itself to get yourself pre-authenticated to the remote system, nor avoid the login and password prompts that will pop up when you first connect! If you are going to use plain Telnet, you are going to have to somehow watch the incoming text for those two prompts (or however many the remote system supplies) and issue the correct replies. Obviously, if systems vary in what username and password prompts they present, then you can hardly expect standardization in the error messages or responses that get sent back when your password fails. That is why Telnet is so hard to script and program from a language like Python and a library like telnetlib. Unless you know every single error message that the remote system could produce to your login and password—which might not just be its “bad password” message, but also things like “cannot spawn shell: out of memory,” “home directory not mounted,” and “quota exceeded: confining you to a restricted shell”—your script will sometimes run into situations where it is waiting to see either a command prompt or else an error message it recognizes, and will instead simply wait forever without seeing anything on the inbound character stream that it recognizes. So if you are using Telnet, then you are playing a text game: you watch for text to arrive, and then try to reply with something intelligible to the remote system. To help you with this, the Python telnetlib provides not only basic methods for sending and receiving data, but also a few routines that will watch and wait for a particular string to arrive from the remote system. In this respect, telnetlib is a little bit like the third-party Python pexpect library that we mentioned early in this chapter, and therefore a bit like the venerable Unix expect command that largely exists because Telnet makes us play a textual pattern-matching game. In fact, one of these telnetlib routines is, in honor of its predecessor, named expect()! 275
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Foundations of Python Network Progr
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To the Python community for creatin
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Contents ■Contents at a Glance ..
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■ CONTENTS Asking getaddrinfo() W
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■ CONTENTS Using Message Queues f
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■ CONTENTS Parsing Dates ........
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■ CONTENTS Telnet ...............
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About the Authors ■ Brandon Craig
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Acknowledgements This book owes its
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■ INTRODUCTION If you do know som
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C H A P T E R 1 ■ ■ ■ Introdu
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CHAPTER 1 ■ INTRODUCTION TO CLIEN
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C H A P T E R 2 ■ ■ ■ UDP The
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CHAPTER 2 ■ UDP server with SSH.
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CHAPTER 2 ■ UDP them anywhere in
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CHAPTER 2 ■ UDP command-line argu
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CHAPTER 2 ■ UDP » » » » raise
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CHAPTER 2 ■ UDP world itself give
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CHAPTER 2 ■ UDP socket that is no
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CHAPTER 2 ■ UDP So binding to an
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CHAPTER 2 ■ UDP s.connect((hostna
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CHAPTER 2 ■ UDP else: » print >>
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C H A P T E R 3 ■ ■ ■ TCP The
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CHAPTER 3 ■ TCP situation), and t
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CHAPTER 3 ■ TCP » reply = recv_a
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CHAPTER 3 ■ TCP guess when the in
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CHAPTER 3 ■ TCP the system has no
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CHAPTER 3 ■ TCP » » » print '\
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CHAPTER 3 ■ TCP $ python tcp_dead
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CHAPTER 3 ■ TCP Using TCP Streams
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CHAPTER 4 ■ SOCKET NAMES AND DNS
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CHAPTER 5 ■ NETWORK DATA AND NETW
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C H A P T E R 6 ■ ■ ■ TLS and
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CHAPTER 6 ■ TLS AND SSL systems a
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CHAPTER 6 ■ TLS AND SSL • He wi
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CHAPTER 6 ■ TLS AND SSL discussio
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CHAPTER 6 ■ TLS AND SSL • The s
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CHAPTER 6 ■ TLS AND SSL The Links
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C H A P T E R 7 ■ ■ ■ Server
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CHAPTER 7 ■ SERVER ARCHITECTURE P
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CHAPTER 7 ■ SERVER ARCHITECTURE
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CHAPTER 7 ■ SERVER ARCHITECTURE N
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CHAPTER 7 ■ SERVER ARCHITECTURE L
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CHAPTER 7 ■ SERVER ARCHITECTURE F
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CHAPTER 7 ■ SERVER ARCHITECTURE p
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CHAPTER 7 ■ SERVER ARCHITECTURE N
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CHAPTER 7 ■ SERVER ARCHITECTURE L
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CHAPTER 7 ■ SERVER ARCHITECTURE s
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CHAPTER 7 ■ SERVER ARCHITECTURE c
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C H A P T E R 8 ■ ■ ■ Caches,
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CHAPTER 8 ■ CACHES, MESSAGE QUEUE
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C H A P T E R 9 ■ ■ ■ HTTP Th
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CHAPTER 9 ■ HTTP Here, the URL sp
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CHAPTER 9 ■ HTTP Relative URLs Ve
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CHAPTER 9 ■ HTTP From now on, I a
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CHAPTER 9 ■ HTTP • 303 See Othe
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CHAPTER 9 ■ HTTP You cannot tell
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CHAPTER 9 ■ HTTP Instead of stuff
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CHAPTER 9 ■ HTTP POST And APIs Al
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CHAPTER 9 ■ HTTP Content Type Neg
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CHAPTER 9 ■ HTTP HTTP Caching Man
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CHAPTER 9 ■ HTTP If the connectio
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CHAPTER 9 ■ HTTP >>> import cooki
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CHAPTER 9 ■ HTTP So the technique
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C H A P T E R 10 ■ ■ ■ Screen
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CHAPTER 10 ■ SCREEN SCRAPING Figu
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CHAPTER 10 ■ SCREEN SCRAPING cont
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CHAPTER 10 ■ SCREEN SCRAPING Thir
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CHAPTER 10 ■ SCREEN SCRAPING Ther
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CHAPTER 10 ■ SCREEN SCRAPING Beau
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CHAPTER 10 ■ SCREEN SCRAPING If y
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CHAPTER 10 ■ SCREEN SCRAPING Cond
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C H A P T E R 11 ■ ■ ■ Web Ap
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CHAPTER 11 ■ WEB APPLICATIONS Thi
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CHAPTER 11 ■ WEB APPLICATIONS But
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CHAPTER 11 ■ WEB APPLICATIONS the
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CHAPTER 11 ■ WEB APPLICATIONS •
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CHAPTER 11 ■ WEB APPLICATIONS hig
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CHAPTER 11 ■ WEB APPLICATIONS The
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CHAPTER 11 ■ WEB APPLICATIONS the
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CHAPTER 11 ■ WEB APPLICATIONS The
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C H A P T E R 12 ■ ■ ■ E-mail
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CHAPTER 12 ■ E-MAIL COMPOSITION A
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C H A P T E R 13 ■ ■ ■ SMTP A
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CHAPTER 13 ■ SMTP anyway. Outgoin
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CHAPTER 13 ■ SMTP How SMTP Is Use
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Page 278 and 279: CHAPTER 15 ■ IMAP An IMAP message
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■ INDEX Common Gateway Interface.
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■ INDEX international characters
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■ INDEX front-end web servers, 17
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■ INDEX deleting folders, 260 del
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■ INDEX mechanize, 138, 163 Memca
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■ INDEX pausing terminal output,
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■ INDEX resources. See also RFCs
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■ INDEX shutdown(), 48 shutting d
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■ INDEX terminals, 270-74 bufferi
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■ INDEX ■ V validating cached r
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