Python Notes 12 : Network clients

After we have explored the basics of network programming in brief in the previous post, we will discuss network clients in more details in this post.

Understanding Sockets
Sockets are an extension to the operating system's I/O system that enable communication between processes and machines. It can be treated the same as standard files, with the same interface methods so in many cases, a program need not know whether it's writing data to a file, the terminal, or a TCP connection. While many files are opened with the open () call, sockets are created with the socket () call and additional calls are needed to connect and activate them.

Creating Sockets

For a client program, creating a socket is generally a two-step process.

1. Create the actual socket object.
2. Connect the socket to the remote server.

When you create a socket object, you need to tell the system two things:

• The communication type: the underlying protocol used to transmit data. Examples of protocols include IPv4 (current Internet standard), IPv6 (future Internet standard), IPX/ SPX (NetWare), and AFP (Apple file sharing). By far the most common is IPv4.
• The protocol family: defines how data is transmitted.
  For Internet communications, which make up the bulk of this book, the communication type is almost always AF_INET (corresponding to IPv4). The protocol family is typically either:
  • SOCK_STREAM for TCP communications or
  • SOCK_DGRAM for UDP communications

For a TCP connection, creating a socket generally uses
code like this:

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) To connect the socket, you'll generally need to provide a tuple containing the remote hostname or IP address and the remote port. Connecting a socket typically looks like this:
s.connect(("www.example.com", 80))

Finding the port number

Most operating systems ship with a list of well-known server port numbers which you can query. On windows systems, you can find this file at C:\Windows\System32\drivers\etc\services. To query this list, you need two parameters:

• A protocol name
• A port name.

This query is like:

>>>print socket.getservbyname(‘ftp’,’tcp’)

21

You didn't have to know in advance that FTP uses port 80.

Getting Information from a Socket
Once you've established a socket connection, you can find out some useful information from it.

s.getsockname() #Get your IP address and port number

s.getpeername() #Get the remote machine IP address and port number

Socket Exceptions

Different network calls can raise different exceptions when network errors occur. Python's socket module actually defines four possible exceptions:

• socket.error for general I/O and communication problems.
• socket.gaierror for errors looking up address information
• socket.herror for other addressing errors.
• socket.timeout for handling timeouts that occur after settimeout() has been called on a socket.

Complete Example

The example program takes three command-line arguments: a host to which it will connect, a port number or name on the server, and a file to request from the server. The program will connect to the server, send a simple HTTP
request for the given filename, and display the result. Along the way, it exercises care to handle various types of potential errors.

import socket, sys
host = sys.argv[l]
textport = sys.argv[2]
filename = sys.argv[3]
try:
    s = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
except socket.error, e: 
    print "Strange error creating socket: %s" % e 
    sys.exit(l)
    # Try parsing it as a numeric port number.
try:
    port = int(textport)
except ValueError:
    # That didn't work, so it's probably a protocol name.
    # Look it up instead,
try:
    port = socket.getservbyname(textport, 'tcp')
    except socket.error, e:
    print "Couldn't find your port: %s" % e
    sys.exit(i)

try:
    s.connect((host, port))
except socket.gaierror, e:
    print "Address-related error connecting to server: %s" % e
    sys.exit(i)
except socket.error, e:
    print "Connection error: %s" % e
    sys.exit(l)
try:
    s.sendall("CET %s HTTP/1.0\r\n\r\n" % filename)
except socket.error, e:
    print "Error sending data: %s" % e
    sys.exit(i)
while 1:
    try:
        buf = s.recvB048)
    except socket.error, e:
        print "Error receiving data: %s" % e
        sys.exit(l)
    if not len(buf):
        break
    sys.stdout.write(buf)

Using User Datagram Protocol

In UDP there is no sufficient control over how data is sent and received. Working with UDP clients differs than TCP clients in the following:

• When create the socket ask for SOCKDGRAM
  instead of SOCKSTREAM; this indicates to the operating system that the socket will
  be used for UDP instead of TCP communications.
• When call socket.getservbyname(), pass ‘udp’ instead of ‘tcp’.

In this post we discussed network clients in a little bit depth. In the next post we will discuss network servers.