CIS 5930-04 – Spring 2001 Network Programming

http://aspen.csit.fsu.edu/it1spring01 Instructors: Geoffrey Fox , Bryan Carpenter Computational Science and Information Technology Florida State University Acknowledgements: Nancy McCracken Syracuse University dbc@npac.syr.edu

1

Sockets

 Sockets first appeared in BSD UNIX (designed by Joy —also one of the originators of Java) Bill circa 1982.

 They provided a cross-protocol API for networking. The original implementation, for example, provided access to protocols including: – – – TCP/IP Xerox NS Local UNIX inter-process communication.

 Today available in Windows, through the WinSock API.

 Sockets directly support a client/server architecture.

 They support connection-oriented well as connectionless protocols like protocols like UDP .

TCP , as  We will only discuss the connection-oriented case.

dbc@npac.syr.edu

2

BSD Socket Calls

Client

socket()

: create socket

connect()

:

write()

: send request

Network

Server

socket()

: create socket

bind()

: name socket

listen()

:

accept()

: accept connection

read()

: get request . . . process request . . .

write()

: send reply

read()

: get reply dbc@npac.syr.edu

3

Port Numbers

 The

bind()

call on the server side establishes a

well known address

for the listening socket.

 In the case of an TCP/IP socket (the only case we are interested in) the important part of this is the port number .

 A port number is an integer between 0 and 64K.

 On any given host, only one server socket can be listening on a particular port at a particular time.

 In UNIX, port numbers below 1024 can only be used by a privileged user (the super-user ). Any user can create a server socket listening on higher ports.

 Low port numbers are used by standard services, e.g.: – – 23 is the default port number for telnet 80 is the default port number for HTTP servers dbc@npac.syr.edu

4

Making a Connection

 The client makes a

connect()

call, specifying the remote host IP address , and the port number on that host for the server socket it wants to connect to.

 Meanwhile the server is waiting on an

accept()

the server socket.

call on  When the connection is established, the

accept()

call completes, returning a reference to a

new socket

.

 Data is exchanged through the

socket pair

consisting of the client socket, and the new socket returned by the

accept()

call on the server .

 The new socket on the server typically lasts for the duration of a single transaction with the client, although the connection transactions.

may

be kept open over multiple dbc@npac.syr.edu

5

Sockets in Java

 Using sockets from C is traditionally quite hard. The arguments of the BSD socket functions are complex, presumably in part because the historical need to support multiple protocols.

 Luckily the API has been greatly simplified in the Java binding for sockets.

 The associated classes are in the package

java.net

.

dbc@npac.syr.edu

6

Java Sockets from the Client Side

 A Java program can open a socket connection in one step using a constructor of the

Socket

class:

Socket t = new Socket(hostName, port) ;

Here

hostName

is a string, such as

“sirah.csit.fsu.edu”

, and

port

argument is an integer port number, such as 80 .

 This

Socket connect()

constructor subsumes the calls in the BSD API.

socket()

and  The

Socket

class has methods

getInputStream()

and

getOutputStream()

. These return normal Java stream objects that can be used to exchange data over a connected socket pair.

 The connection is bi-directional : both client an server can read and write.

dbc@npac.syr.edu

7

Java Sockets from the Server Side

 The BSD operations

socket()

,

bind()

and

listen()

for a server-side socket are subsumed in a constructor for the

ServerSocket

class:

ServerSocket s = new ServerSocket(port) ;

Here

port

is the integer port number, such as 80 (if you are writing a Web server), on which the server will listen.

 Next the Java server will call the

accept()

method and wait for clients to connect to it.

accept()

returns an ordinary socket, completing the socket-pair for the connection:

Socket connection = s.accept() ;

 After processing the request, the client goes back to waiting on

accept()

, for new client requests.

– Real servers fork a thread or process to deal with the request, and return immediately to waiting for the next client connection.

dbc@npac.syr.edu

8

A Simple Client

import java.io.* ; import java.net.* ; public class TrivialBrowser { public static void main(String [] args) { Socket sock = new Socket(“aspen.csit.fsu.edu”, 80) ; PrintWriter out = new PrintWriter( new OutputStreamWriter( sock.getOutputStream() ) ; out.println(“GET /it1spring01/index.html HTTP/1.0”) ; out.println(“”) ; out.flush() ; BufferedReader in = new BufferedReader( new InputStreamReader( sock.getInputStream() ) ; while(true) String line = in.readLine ; if(line == null) break ; System.out.println(line) ; } } }

dbc@npac.syr.edu

9

Remarks

 This implements a (drastically restricted) Web browser.

  If you run this program it will print out the

HTML source

for the IT1 course Home Page.

It connects to port 80 on aspen (the Web server’s port).

  It gets an output stream to write to the socket using

getOuputStream()

.

It sends an HTTP “

GET

” request on the stream, specifying the file

it1spring01/index.html

server’s document root.

relative to the  It gets an input stream to read from the socket using

getInputStream()

.

 It copies lines from the socket connection to the console.

dbc@npac.syr.edu

10

A Simple Server

public static void main(String [] args) throws Exception { ServerSocket server = new ServerSocket(8080) ; while(true) { Socket sock = server.accept() ; BufferedReader in = new BufferedReader( new InputStreamReader(sock.getInputStream()) ; String header = in.readLine() ;

. . . skip over any other lines in request packet . . .

String fileName = getFileName(header) ;

// Second field of

header byte [] bytes = readFile(“.” + fileName) ;

// Contents of local file

DataOutputStream out = new DateOutputStream(sock.getOutputStream()) ; if( . . .

) {

// File exists and is a

.html

file

out.writeBytes(“HTTP/1.0 200 OK\r\n”) ; out.writeBytes(“Content-Length: ” + bytes.length + “\r\n”) ; out.writeBytes(“Content-Type: text/html\r\n\r\n”) ; out.write(bytes) ; }

dbc@npac.syr.edu

11

} }

Remarks

 This implements a (somewhat restricted) Web server.

 It creates a server socket listening to port 8080 on the local host.

  It gets a socket connection from a client using the

accept()

method, and then gets the input stream from the socket using

getInputStream()

.

We handle only “

GET

” requests; the second field will be the file name.

 It reads the file and writes it to the output stream of the socket, in HTTP.

 A real server would spawn a new thread to deal with each transaction. The main loop would return immediately to waiting on

accept()

.

dbc@npac.syr.edu

12

URL Objects

 Instead of explicitly opening a socket connection to a Web server, a client can read information using the higher level

URL

class.

 A constructor takes a URL string and creates a

URL

object:

URL url = new URL(“http://aspen.csit.fsu.edu/it1spring01/”) ;

 This constructor may throw a

MalformedURLException

.

dbc@npac.syr.edu

13

Reading a File Using a URL Object

 Now if

url

is a

URL

object, the resource can be read by opening a stream on the URL :

BufferedReader in = new BufferedReader( new InputStreamReader( url.openStream() )) ;

 This example creates a character stream that can be read like any other.

dbc@npac.syr.edu

14

URL Connection Objects

 A class

java.net.URLConnection

provides additional functionality on URLs. A URLConnection is created by the

openConnection()

method:

URL url = . . . ; URLConnection connection = url.openConnection() ;

 Methods on

connection

HTTP header: allow to return fields from the

String getContentType() int getContentLength() . . .

 You can also open an

InputStream

or

OutputStream

on a URL connection. The latter is used for HTTP “

POST

” requests.

dbc@npac.syr.edu

15

Firewalls

Local Area Network

The Internet

Firewall Server dbc@npac.syr.edu

Server Client Client Server 16

Operation of Firewalls

 The firewall server forwards or blocks IP packets it receives.

 Typically it will allow hosts arbitrary

connect()

calls to inside the LAN to make servers outside the firewall . So it does not prevent users inside from visiting external Web servers, etc.

 However it will block external requests to

connect()

to hosts inside the firewall.

 The firewall configuration can be set up to allow connection requests to specific ports on specific hosts to get through —for example to port 80 on the company Web server.

dbc@npac.syr.edu

17