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

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CHAPTER 4 ■ SOCKET NAMES AND DNS Priority: 10 Hostname: pechora2.icann.org » Hostname pechora2.icann.org = A 192.0.43.8 ... Priority: 10 Hostname: pechora8.icann.org » Hostname pechora8.icann.org = A 192.0.43.8 By trying this script against many different domains, you will be able to see how both big and small organizations arrange for incoming e-mails to be routed to IP addresses. Zeroconf and Dynamic DNS There are two last technologies that you are perhaps not likely to implement yourself, but that deserve a quick mention because they allow machines to find each other when they lack permanent and stable IP addresses. The Zeroconf standard combines three techniques so that computers thrown on to a network segment with each other can discover each other's presence and names without a network administrator needing to install and configure a DHCP and DNS server. Apple computers use this technology extensively to find adjacent machines and printers, Linux machines often run an avahi service that implements the protocol, and there is an old pyzeroconf project that offers a complete Python implementation of the protocol suite. One of the technologies included in Zeroconf is “multicast DNS” (mDNS), which allows all of the machines on the local network to answer when another machine needs to look up a hostname. Dynamic DNS services are Internet sites built to serve users whose machines are regularly changing their IP address—perhaps because the address assigned by their ISP is not stable but is pulled from a pool of free addresses with every reconnect. By offering an API through which the user can offer her username, password, and new IP address, the DDNS service can update its database and point the user's domain name at the new IP. This technology was pioneered by the dyndns.com site, and it absolves the user of the need to rent and operate his or her own DNS server if he or she has only a few domain names to maintain. There appears to be a dyndnsc project on the Package Index that offers a client that can communicate with DDNS services. Summary Python programs often have to turn hostnames into socket addresses to which they can actually make connections. Most hostname lookup should occur through the getsockaddr() function in the socket module, since its intelligence is usually supplied by your operating system and it will know not only how to look up domain names, but also what flavor of address the local IP stack is configured to support. Old IPv4 addresses are still the most prevalent on the Internet, but IPv6 is becoming more and more common. By deferring all hostname and port name lookup to getsockaddr(), your Python program can treat addresses as opaque strings and not have to worry about parsing or interpreting them. Behind most name resolution is the DNS, a worldwide distributed database that forwards domain name queries directly to the servers of the organization that owns a domain. While not often used directly from Python, it can be very helpful in determining where to direct e-mail based on the e-mail domain named after the @ sign in an e-mail address. 70
C H A P T E R 5 ■ ■ ■ Network Data and Network Errors The first four chapters have given us a foundation: we have learned how hosts are named on an IP network, and we understand how to set up and tear down both TCP streams and UDP datagram connections between those hosts. But what data should we then send across those lengths? How should it be encoded and formatted? For what kinds of errors will our Python programs need to be prepared? These questions are all relevant regardless of whether we are using streams or datagrams. We will look at the basic answers in this chapter, and learn how to use sockets responsibly so that our data arrives intact. Text and Encodings If you were watching for it as you read the first few chapters, you may have caught me using two different terms for the same concept. Those terms were byte and octet, and by both words I always mean an 8-bit number—an ordered sequence of eight digits, that are each either a one or a zero. They are the fundamental units of data on modern computing systems, used both to represent raw binary numbers and to stand for characters or symbols. The binary number 1010000, for example, usually stands for either the number 80 or the letter P: >>> 0b1010000 80 >>> chr(0b1010000) 'P' The reason that the Internet RFCs are so inveterate in their use of the term “octet” instead of “byte” is that the earliest of RFCs date from a very ancient era in which bytes could be one of several different lengths—byte sizes from as little as 5 to as many as 16 bits were used on various systems. So the term “octet,” meaning a “group of eight things,” is always used in the standards so that their meaning is unambiguous. Four bits offer a mere sixteen values, which does not come close to even fitting our alphabet. But eight bits—the next-higher multiple of two—proved more than enough to fit both the upper and lower cases of our alphabet, all the digits, lots of punctuation, and 32 control codes, and it still left a whole half of the possible range of values empty. The problem is that many rival systems exist for the specific mapping used to turn characters into bytes, and the differences can cause problems unless both ends of your network connection use the same rules. The use of ASCII for the basic English letters and numbers is nearly universal among network protocols these days. But when you begin to use more interesting characters, you have to be careful. In Python you should always represent a meaningful string of text with a “Unicode string” that is denoted with a leading u, like this: >>> elvish = u'Namárië!' 71
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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.
Page 40 and 41: CHAPTER 2 ■ UDP them anywhere in
Page 42 and 43: CHAPTER 2 ■ UDP command-line argu
Page 44 and 45: CHAPTER 2 ■ UDP » » » » raise
Page 46 and 47: CHAPTER 2 ■ UDP world itself give
Page 48 and 49: CHAPTER 2 ■ UDP socket that is no
Page 50 and 51: CHAPTER 2 ■ UDP So binding to an
Page 52 and 53: CHAPTER 2 ■ UDP s.connect((hostna
Page 54 and 55: CHAPTER 2 ■ UDP else: » print >>
Page 56 and 57: C H A P T E R 3 ■ ■ ■ TCP The
Page 58 and 59: CHAPTER 3 ■ TCP situation), and t
Page 60 and 61: CHAPTER 3 ■ TCP » reply = recv_a
Page 62 and 63: CHAPTER 3 ■ TCP guess when the in
Page 64 and 65: CHAPTER 3 ■ TCP the system has no
Page 66 and 67: CHAPTER 3 ■ TCP » » » print '\
Page 68 and 69: CHAPTER 3 ■ TCP $ python tcp_dead
Page 70 and 71: CHAPTER 3 ■ TCP Using TCP Streams
Page 72 and 73: CHAPTER 4 ■ SOCKET NAMES AND DNS
Page 92 and 93: CHAPTER 5 ■ NETWORK DATA AND NETW
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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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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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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