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This is a quick guide/tutorial to learning socket programming in python. Socket programming python is very similar to C.
Socket functions are documented at
To summarise the basics, sockets are the fundamental "things" behind any kind of network communications done by your computer. For example when you type www.google.com in your web browser, it opens a socket and connects to google.com to fetch the page and show it to you. Same with any chat client like gtalk or skype. Any network communication goes through a socket.
Before you begin
This tutorial assumes that you already have a basic knowledge of python.
So lets begin with sockets.Creating a socket
This first thing to do is create a socket. The socket.socket function does this. Quick Example :
#Socket client example in python import socket #for sockets #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)print 'Socket Created'
Function socket.socket creates a socket and returns a socket descriptor which can be used in other socket related functions
The above code will create a socket with the following properties ...
Address Family : AF_INET (this is IP version 4 or IPv4) Type : SOCK_STREAM (this means connection oriented TCP protocol)
Error handling
If any of the socket functions fail then python throws an exception called socket.error which must be caught.
#handling errors in python socket programsimport socket #for sockets import sys #for exit try: #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) except socket.error, msg: print 'Failed to create socket. Error code: ' + str(msg[0]) + ' , Error message : ' + msg[1] sys.exit(); print 'Socket Created'
Ok , so you have created a socket successfully. But what next ? Next we shall try to connect to some server using this socket. We can connect to www.google.com
Note
Apart from SOCK_STREAM type of sockets there is another type called SOCK_DGRAM which indicates the UDP protocol. This type of socket is non-connection socket. In this tutorial we shall stick to SOCK_STREAM or TCP sockets.
Connect to a Server
We connect to a remote server on a certain port number. So we need 2 things , IP address and port number to connect to. So you need to know the IP address of the remote server you are connecting to. Here we used the ip address of google.com as a sample.
First get the IP address of the remote host/url
Before connecting to a remote host, its ip address is needed. In python the getting the ip address is quite simple.
import socket #for socketsimport sys #for exittry: #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)except socket.error, msg: print 'Failed to create socket. Error code: ' + str(msg[0]) + ' , Error message : ' + msg[1] sys.exit();print 'Socket Created'host = 'www.google.com'try: remote_ip = socket.gethostbyname( host )except socket.gaierror: #could not resolve print 'Hostname could not be resolved. Exiting' sys.exit() print 'Ip address of ' + host + ' is ' + remote_ip
Now that we have the ip address of the remote host/system, we can connect to ip on a certain 'port' using the connect function.
Quick example
import socket #for socketsimport sys #for exittry: #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)except socket.error, msg: print 'Failed to create socket. Error code: ' + str(msg[0]) + ' , Error message : ' + msg[1] sys.exit();print 'Socket Created'host = 'www.google.com'port = 80try: remote_ip = socket.gethostbyname( host )except socket.gaierror: #could not resolve print 'Hostname could not be resolved. Exiting' sys.exit() print 'Ip address of ' + host + ' is ' + remote_ip#Connect to remote servers.connect((remote_ip , port))print 'Socket Connected to ' + host + ' on ip ' + remote_ip
Run the program
$ python client.py Socket Created Ip address of www.google.com is 74.125.236.83 Socket Connected to www.google.com on ip 74.125.236.83
It creates a socket and then connects. Try connecting to a port different from port 80 and you should not be able to connect which indicates that the port is not open for connection. This logic can be used to build a port scanner.
OK, so we are now connected. Lets do the next thing , sending some data to the remote server.
Free Tip
The concept of "connections" apply to SOCK_STREAM/TCP type of sockets. Connection means a reliable "stream" of data such that there can be multiple such streams each having communication of its own. Think of this as a pipe which is not interfered by data from other pipes. Another important property of stream connections is that packets have an "order" or "sequence".
Other sockets like UDP , ICMP , ARP dont have a concept of "connection". These are non-connection based communication. Which means you keep sending or receiving packets from anybody and everybody.
Sending Data
Function sendall will simply send data. Lets send some data to google.com
import socket #for socketsimport sys #for exittry: #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)except socket.error, msg: print 'Failed to create socket. Error code: ' + str(msg[0]) + ' , Error message : ' + msg[1] sys.exit();print 'Socket Created'host = 'www.google.com'port = 80try: remote_ip = socket.gethostbyname( host )except socket.gaierror: #could not resolve print 'Hostname could not be resolved. Exiting' sys.exit() print 'Ip address of ' + host + ' is ' + remote_ip#Connect to remote servers.connect((remote_ip , port))print 'Socket Connected to ' + host + ' on ip ' + remote_ip#Send some data to remote servermessage = "GET / HTTP/1.1\r\n\r\n"try : #Set the whole string s.sendall(message)except socket.error: #Send failed print 'Send failed' sys.exit()print 'Message send successfully'
In the above example , we first connect to an ip address and then send the string message "GET / HTTP/1.1\r\n\r\n" to it. The message is actually an "http command" to fetch the mainpage of a website.
Now that we have send some data , its time to receive a reply from the server. So lets do it.
Receiving Data
Function recv is used to receive data on a socket. In the following example we shall send the same message as the last example and receive a reply from the server.
#Socket client example in pythonimport socket #for socketsimport sys #for exit#create an INET, STREAMing sockettry: s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)except socket.error: print 'Failed to create socket' sys.exit() print 'Socket Created'host = 'www.google.com';port = 80;try: remote_ip = socket.gethostbyname( host )except socket.gaierror: #could not resolve print 'Hostname could not be resolved. Exiting' sys.exit()#Connect to remote servers.connect((remote_ip , port))print 'Socket Connected to ' + host + ' on ip ' + remote_ip#Send some data to remote servermessage = "GET / HTTP/1.1\r\n\r\n"try : #Set the whole string s.sendall(message)except socket.error: #Send failed print 'Send failed' sys.exit()print 'Message send successfully'#Now receive datareply = s.recv(4096)print reply
Here is the output of the above code :
$ python client.pySocket CreatedIp address of www.google.com is 74.125.236.81Socket Connected to www.google.com on ip 74.125.236.81Message send successfullyHTTP/1.1 302 FoundLocation: http://www.google.co.in/Cache-Control: privateContent-Type: text/html; charset=UTF-8Set-Cookie: expires=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=www.google.comSet-Cookie: path=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=www.google.comSet-Cookie: domain=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=www.google.comSet-Cookie: expires=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.www.google.comSet-Cookie: path=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.www.google.comSet-Cookie: domain=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.www.google.comSet-Cookie: expires=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=google.comSet-Cookie: path=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=google.comSet-Cookie: domain=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=google.comSet-Cookie: expires=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.google.comSet-Cookie: path=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.google.comSet-Cookie: domain=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.google.comSet-Cookie: PREF=ID=51f26964398d27b0:FF=0:TM=1343026094:LM=1343026094:S=pa0PqX9FCPvyhBHJ; expires=Wed, 23-Jul-2014 06:48:14 GMT; path=/; domain=.google.com
Google.com replied with the content of the page we requested. Quite simple! Now that we have received our reply, its time to close the socket.
Close socket
Function close is used to close the socket.
s.close()
Thats it.
Lets Revise
So in the above example we learned how to :
- Create a socket
- Connect to remote server
- Send some data
- Receive a reply
Its useful to know that your web browser also does the same thing when you open www.google.com This kind of socket activity represents a CLIENT. A client is a system that connects to a remote system to fetch data.
The other kind of socket activity is called a SERVER. A server is a system that uses sockets to receive incoming connections and provide them with data. It is just the opposite of Client. So www.google.com is a server and your web browser is a client. Or more technically www.google.com is a HTTP Server and your web browser is an HTTP client.
Now its time to do some server tasks using sockets.
Server Programming
OK now onto server things. Servers basically do the following :
- Open a socket
- Bind to a address(and port).
- Listen for incoming connections.
- Accept connections
- Read/Send
We have already learnt how to open a socket. So the next thing would be to bind it.
Bind a socket
Function bind can be used to bind a socket to a particular address and port. It needs a sockaddr_in structure similar to connect function.
Quick example
import socketimport sysHOST = '' # Symbolic name meaning all available interfacesPORT = 8888 # Arbitrary non-privileged ports = socket.socket(socket.AF_INET, socket.SOCK_STREAM)print 'Socket created'try: s.bind((HOST, PORT))except socket.error , msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete'
Now that bind is done, its time to make the socket listen to connections. We bind a socket to a particular IP address and a certain port number. By doing this we ensure that all incoming data which is directed towards this port number is received by this application.
This makes it obvious that you cannot have 2 sockets bound to the same port. There are exceptions to this rule but we shall look into that in some other article.
Listen for connections
After binding a socket to a port the next thing we need to do is listen for connections. For this we need to put the socket in listening mode. Function socket_listen is used to put the socket in listening mode. Just add the following line after bind.
s.listen(10)print 'Socket now listening'
The parameter of the function listen is called backlog. It controls the number of incoming connections that are kept "waiting" if the program is already busy. So by specifying 10, it means that if 10 connections are already waiting to be processed, then the 11th connection request shall be rejected. This will be more clear after checking socket_accept.
Now comes the main part of accepting new connections.
Accept connection
Function socket_accept is used for this.
import socketimport sysHOST = '' # Symbolic name meaning all available interfacesPORT = 8888 # Arbitrary non-privileged ports = socket.socket(socket.AF_INET, socket.SOCK_STREAM)print 'Socket created'try: s.bind((HOST, PORT))except socket.error , msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete's.listen(10)print 'Socket now listening'#wait to accept a connection - blocking callconn, addr = s.accept()#display client informationprint 'Connected with ' + addr[0] + ':' + str(addr[1])
Output
Run the program. It should show
$ python server.pySocket createdSocket bind completeSocket now listening
So now this program is waiting for incoming connections on port 8888. Dont close this program , keep it running. Now a client can connect to it on this port. We shall use the telnet client for testing this. Open a terminal and type
$ telnet localhost 8888
It will immediately show
$ telnet localhost 8888Trying 127.0.0.1...Connected to localhost.Escape character is '^]'.Connection closed by foreign host.
And the server output will show
$ python server.pySocket createdSocket bind completeSocket now listeningConnected with 127.0.0.1:59954
So we can see that the client connected to the server. Try the above steps till you get it working perfect.
We accepted an incoming connection but closed it immediately. This was not very productive. There are lots of things that can be done after an incoming connection is established. Afterall the connection was established for the purpose of communication. So lets reply to the client.
Function sendall can be used to send something to the socket of the incoming connection and the client should see it. Here is an example :
import socketimport sysHOST = '' # Symbolic name meaning all available interfacesPORT = 8888 # Arbitrary non-privileged ports = socket.socket(socket.AF_INET, socket.SOCK_STREAM)print 'Socket created'try: s.bind((HOST, PORT))except socket.error , msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete's.listen(10)print 'Socket now listening'#wait to accept a connection - blocking callconn, addr = s.accept()print 'Connected with ' + addr[0] + ':' + str(addr[1])#now keep talking with the clientdata = conn.recv(1024)conn.sendall(data)conn.close()s.close()
Run the above code in 1 terminal. And connect to this server using telnet from another terminal and you should see this :
$ telnet localhost 8888Trying 127.0.0.1...Connected to localhost.Escape character is '^]'.happyhappyConnection closed by foreign host.
So the client(telnet) received a reply from server.
We can see that the connection is closed immediately after that simply because the server program ends after accepting and sending reply. A server like www.google.com is always up to accept incoming connections.
It means that a server is supposed to be running all the time. Afterall its a server meant to serve. So we need to keep our server RUNNING non-stop. The simplest way to do this is to put the accept in a loop so that it can receive incoming connections all the time.
Live Server
So a live server will be alive always. Lets code this up
import socketimport sysHOST = '' # Symbolic name meaning all available interfacesPORT = 8888 # Arbitrary non-privileged ports = socket.socket(socket.AF_INET, socket.SOCK_STREAM)print 'Socket created'try: s.bind((HOST, PORT))except socket.error , msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete's.listen(10)print 'Socket now listening'#now keep talking with the clientwhile 1: #wait to accept a connection - blocking call conn, addr = s.accept() print 'Connected with ' + addr[0] + ':' + str(addr[1]) data = conn.recv(1024) reply = 'OK...' + data if not data: break conn.sendall(reply)conn.close()s.close()
We havent done a lot there. Just put the socket_accept in a loop.
Now run the server program in 1 terminal , and open 3 other terminals. From each of the 3 terminal do a telnet to the server port.
Each of the telnet terminal would show :
$ telnet localhost 5000Trying 127.0.0.1...Connected to localhost.Escape character is '^]'.happyOK .. happyConnection closed by foreign host.
And the server terminal would show
$ python server.pySocket createdSocket bind completeSocket now listeningConnected with 127.0.0.1:60225Connected with 127.0.0.1:60237Connected with 127.0.0.1:60239
So now the server is running nonstop and the telnet terminals are also connected nonstop. Now close the server program. All telnet terminals would show "Connection closed by foreign host."
Good so far. But still there is not effective communication between the server and the client. The server program accepts connections in a loop and just send them a reply, after that it does nothing with them. Also it is not able to handle more than 1 connection at a time. So now its time to handle the connections , and handle multiple connections together.
Handling Connections
To handle every connection we need a separate handling code to run along with the main server accepting connections. One way to achieve this is using threads. The main server program accepts a connection and creates a new thread to handle communication for the connection, and then the server goes back to accept more connections.
We shall now use threads to create handlers for each connection the server accepts.
import socketimport sysfrom thread import *HOST = '' # Symbolic name meaning all available interfacesPORT = 8888 # Arbitrary non-privileged ports = socket.socket(socket.AF_INET, socket.SOCK_STREAM)print 'Socket created'#Bind socket to local host and porttry: s.bind((HOST, PORT))except socket.error , msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete'#Start listening on sockets.listen(10)print 'Socket now listening'#Function for handling connections. This will be used to create threadsdef clientthread(conn): #Sending message to connected client conn.send('Welcome to the server. Type something and hit enter\n') #send only takes string #infinite loop so that function do not terminate and thread do not end. while True: #Receiving from client data = conn.recv(1024) reply = 'OK...' + data if not data: break conn.sendall(reply) #came out of loop conn.close()#now keep talking with the clientwhile 1: #wait to accept a connection - blocking call conn, addr = s.accept() print 'Connected with ' + addr[0] + ':' + str(addr[1]) #start new thread takes 1st argument as a function name to be run, second is the tuple of arguments to the function. start_new_thread(clientthread ,(conn,))s.close() Run the above server and open 3 terminals like before. Now the server will create a thread for each client connecting to it.
The telnet terminals would show :
$ telnet localhost 8888Trying 127.0.0.1...Connected to localhost.Escape character is '^]'.Welcome to the server. Type something and hit enterhiOK...hiasdOK...asdcvOK...cv
The server terminal might look like this
$ python server.pySocket createdSocket bind completeSocket now listeningConnected with 127.0.0.1:60730Connected with 127.0.0.1:60731 The above connection handler takes some input from the client and replies back with the same.
So now we have a server thats communicative. Thats useful now.
Conclusion
By now you must have learned the basics of socket programming in python. You can try out some experiments like writing a chat client or something similar.
When testing the code you might face this error
Bind failed. Error Code : 98 Message Address already in use
When it comes up, simply change the port number and the server would run fine.
If you think that the tutorial needs some addons or improvements or any of the code snippets above dont work then feel free to make a comment below so that it gets fixed.