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

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C H A P T E R 9 ■ ■ ■ HTTP The protocols of yore tended to be dense, binary, and decipherable only by Boolean machine logic. But the workhorse protocol of the World Wide Web, named the Hypertext Transfer Protocol (HTTP), is instead based on friendly, mostly-human-readable text. There is probably no better way to start this chapter than to show you what an actual request and response looks like; that way, you will already know the layout of a whole request as we start digging into each of its features. Consider what happens when you ask the urllib2 Python Standard Library to open this URL, which is the RFC that defines the HTTP protocol itself: www.ietf.org/rfc/rfc2616.txt The library will connect to the IETF web site, and send it an HTTP request that looks like this: GET /rfc/rfc2616.txt HTTP/1.1 Accept-Encoding: identity Host: www.ietf.org Connection: close User-Agent: Python-urllib/2.6 As you can see, the format of this request is very much like that of the headers of an e-mail message—in fact, both HTTP and e-mail messages define their header layout using the same standard: RFC 822. The HTTP response that comes back over the socket also starts with a set of headers, but then also includes a body that contains the document itself that has been requested (which I have truncated): HTTP/1.1 200 OK Date: Wed, 27 Oct 2010 17:12:01 GMT Server: Apache/2.2.4 (Linux/SUSE) mod_ssl/2.2.4 OpenSSL/0.9.8e PHP/5.2.6 with Suhosin- Patch mod_python/3.3.1 Python/2.5.1 mod_perl/2.0.3 Perl/v5.8.8 Last-Modified: Fri, 11 Jun 1999 18:46:53 GMT ETag: "1cad180-67187-31a3e140" Accept-Ranges: bytes Content-Length: 422279 Vary: Accept-Encoding Connection: close Content-Type: text/plain Network Working Group Request for Comments: 2616 Obsoletes: 2068 Category: Standards Track ... R. Fielding UC Irvine J. Gettys Compaq/W3C Note that those last four lines are the beginning of RFC 2616 itself, not part of the HTTP protocol. Two of the most important features of this format are not actually visible here, because they pertain to whitespace. First, every header line is concluded by a two-byte carriage-return linefeed sequence, or '\r\n' in Python. Second, both sets of headers are terminated—in HTTP, headers are always 137
CHAPTER 9 ■ HTTP terminated—by a blank line. You can see the blank line between the HTTP response and the document that follows, of course; but in this book, the blank line that follows the HTTP request headers is probably invisible. When viewed as raw characters, the headers end where two end-of-line sequences follow one another with nothing in between them: …Penultimate-Header: value\r\nLast-Header: value\r\n\r\n Everything after that final \n is data that belongs to the document being returned, and not to the headers. It is very important to get this boundary strictly correct when writing an HTTP implementation because, although text documents might still be legible if some extra whitespace works its way in, images and other binary data would be rendered unusable. As this chapter proceeds to explore the features of HTTP, we are going to illustrate the protocol using several modules that come built-in to the Python Standard Library, most notably its urllib2 module. Some people advocate the use of HTTP libraries that require less fiddling to behave like a normal browser, like mechanize or even PycURL, which you can find at these locations: http://wwwsearch.sourceforge.net/mechanize/ http://pycurl.sourceforge.net/ But urllib2 is powerful and, when understood, convenient enough to use that I am going to support the Python “batteries included” philosophy and feature it here. Plus, it supports a pluggable system of request handlers that we will find very useful as we progress from simple to complex HTTP exchanges in the course of the chapter. If you examine the source code of mechanize, you will find that it actually builds on top of urllib2; thus, it can be an excellent source of hints and patterns for adding features to the classes already in the Standard Library. It even supports cookies out of the box, which urllib2 makes you enable manually. Note that some features, like gzip compression, are not available by default in either framework, although mechanize makes compression much easier to turn on. I must acknowledge that I have myself learned urllib2, not only from its documentation, but from the web site of Michael Foord and from the Dive Into Python book by Mark Pilgrim. Here are links to each of those resources: http://www.voidspace.org.uk/python/articles/urllib2.shtml http://diveintopython.org/toc/index.html And, of course, RFC 2616 (the link was given a few paragraphs ago) is the best place to start if you are in doubt about some technical aspect of the protocol itself. URL Anatomy Before tackling the inner workings of HTTP, we should pause to settle a bit of terminology surrounding Uniform Resource Locators (URLs), the wonderful strings that tell your web browser how to fetch resources from the World Wide Web. They are a subclass of the full set of possible Uniform Resource Identifiers (URIs); specifically, they are URIs constructed so that they give instructions for fetching a document, instead of serving only as an identifier. For example, consider a very simple URL like the following: http://python.org If submitted to a web browser, this URL is interpreted as an order to resolve the host name python.org to an IP address (see Chapter 4), make a TCP connection to that IP address at the standard HTTP port 80 (see Chapter 3), and then ask for the root document / that lives at that site. Of course, many URLs are more complicated. Imagine, for example, that there existed a service offering pre-scaled thumbnail versions of various corporate logos for an international commerce site we were writing. And imagine that we wanted the logo for Nord/LB, a large German bank. The resulting URL might look something like this: http://example.com:8080/Nord%2FLB/logo?shape=square&dpi=96 138
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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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Page 106 and 107: C H A P T E R 6 ■ ■ ■ TLS and
Page 108 and 109: CHAPTER 6 ■ TLS AND SSL systems a
Page 110 and 111: CHAPTER 6 ■ TLS AND SSL • He wi
Page 112 and 113: CHAPTER 6 ■ TLS AND SSL discussio
Page 114 and 115: CHAPTER 6 ■ TLS AND SSL • The s
Page 116 and 117: CHAPTER 6 ■ TLS AND SSL The Links
Page 118 and 119: C H A P T E R 7 ■ ■ ■ Server
Page 120 and 121: CHAPTER 7 ■ SERVER ARCHITECTURE P
Page 122 and 123: CHAPTER 7 ■ SERVER ARCHITECTURE
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Page 126 and 127: CHAPTER 7 ■ SERVER ARCHITECTURE L
Page 128 and 129: CHAPTER 7 ■ SERVER ARCHITECTURE F
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Page 136 and 137: CHAPTER 7 ■ SERVER ARCHITECTURE L
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Page 144 and 145: C H A P T E R 8 ■ ■ ■ Caches,
Page 146 and 147: CHAPTER 8 ■ CACHES, MESSAGE QUEUE
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Page 160 and 161: CHAPTER 9 ■ HTTP Relative URLs Ve
Page 162 and 163: CHAPTER 9 ■ HTTP From now on, I a
Page 164 and 165: CHAPTER 9 ■ HTTP • 303 See Othe
Page 166 and 167: CHAPTER 9 ■ HTTP You cannot tell
Page 168 and 169: CHAPTER 9 ■ HTTP Instead of stuff
Page 170 and 171: CHAPTER 9 ■ HTTP POST And APIs Al
Page 172 and 173: CHAPTER 9 ■ HTTP Content Type Neg
Page 174 and 175: CHAPTER 9 ■ HTTP HTTP Caching Man
Page 176 and 177: CHAPTER 9 ■ HTTP If the connectio
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Page 182 and 183: C H A P T E R 10 ■ ■ ■ Screen
Page 184 and 185: CHAPTER 10 ■ SCREEN SCRAPING Figu
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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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CHAPTER 13 ■ SMTP This mechanism
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CHAPTER 13 ■ SMTP s = smtplib.SMT
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CHAPTER 13 ■ SMTP ETRN STARTTLS X
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CHAPTER 13 ■ SMTP » s = smtplib.
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CHAPTER 13 ■ SMTP exchange mail o
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CHAPTER 13 ■ SMTP username = sys.
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C H A P T E R 14 ■ ■ ■ POP PO
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CHAPTER 14 ■ POP ■ Caution! Whi
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CHAPTER 14 ■ POP finally: » p.qu
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CHAPTER 14 ■ POP Subject: Backup
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CHAPTER 15 ■ IMAP THE IMAP PROTOC
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CHAPTER 15 ■ IMAP '(\\HasNoChildr
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CHAPTER 15 ■ IMAP Examining Folde
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CHAPTER 15 ■ IMAP Listing 15-5. D
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CHAPTER 15 ■ IMAP key that IMAP h
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CHAPTER 15 ■ IMAP » » print def
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CHAPTER 15 ■ IMAP » From: Brando
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CHAPTER 15 ■ IMAP • \Flagged: T
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CHAPTER 15 ■ IMAP An IMAP message
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CHAPTER 15 ■ IMAP display or summ
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CHAPTER 16 ■ TELNET AND SSH cloud
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CHAPTER 16 ■ TELNET AND SSH Unix
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CHAPTER 16 ■ TELNET AND SSH Do yo
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CHAPTER 16 ■ TELNET AND SSH As we
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CHAPTER 16 ■ TELNET AND SSH tabif
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CHAPTER 16 ■ TELNET AND SSH repla
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CHAPTER 16 ■ TELNET AND SSH Listi
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CHAPTER 16 ■ TELNET AND SSH def p
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CHAPTER 16 ■ TELNET AND SSH We wi
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CHAPTER 16 ■ TELNET AND SSH • p
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CHAPTER 16 ■ TELNET AND SSH You w
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CHAPTER 16 ■ TELNET AND SSH » »
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CHAPTER 16 ■ TELNET AND SSH Listi
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CHAPTER 16 ■ TELNET AND SSH Summa
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CHAPTER 17 ■ FTP The biggest prob
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CHAPTER 17 ■ FTP f.login() print
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CHAPTER 17 ■ FTP if os.path.exist
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CHAPTER 17 ■ FTP f = FTP(host) f.
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CHAPTER 17 ■ FTP Windows servers
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CHAPTER 17 ■ FTP » try: » » f.
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C H A P T E R 18 ■ ■ ■ RPC Re
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CHAPTER 18 ■ RPC sort of proxy ex
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CHAPTER 18 ■ RPC The SimpleXMLRPC
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CHAPTER 18 ■ RPC Traceback (most
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CHAPTER 18 ■ RPC 8.0 If this
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CHAPTER 18 ■ RPC Note that the po
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CHAPTER 18 ■ RPC up being, simply
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CHAPTER 18 ■ RPC such as Python i
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CHAPTER 18 ■ RPC • Google Proto
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■ INDEX mod_python, 194 Qpid, 131
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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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