CS 471G: Networking and Distributed Operating Systems Spring 2009
Programming Assignment 1
Assigned: Feb. 11, 2009 Due: Feb. 27, 2009
In this programming assignment, you will implement a HTTP server and a HTTP client in C or
C++. The goal is to use basic socket programming interfaces to implement a very small (yet core)
part of the HTTP protocol and practice TCP socket programming using multiple processes. The ba-
sic requirement is that your client can request web pages from your server and other web servers such
as www.cs.uky.edu, news and search servers, and you can also use Firefox/Internet Explorer/Netscape
to get web pages (possibly with embedded images) from your server. You can start with the sample
codes from the “TCP/IP Sockets in C” book, but you have to implement the major functions of the as-
signment. The sample codes are TCPEchoClient.c and TCPEchoServer-Fork.c. They can be found at
http://cs.ecs.baylor.edu/~donahoo/practical/CSockets/textcode.html.
Server
The server can host any number of web pages and its interface should be:
myserver port_no http_root_dir_name
where port_no is the port number that the server will listen to, and http_root_dir_name is the directory
under which you will put all the html and image files of the web pages. The server should be able to process
GET requests from clients. It will ignore request messages using other methods. If the requested document
exists at the server, it will send the client a reply formatted according to the HTTP protocol with status
code 200 and the content of the file. The server should be able to serve four types of files: txt, html, jpg,
and gif. File types are determined by the extension part of file names. If the document does not exist, it will
reply with the status code 404 and other relevant information based on the HTTP protocol for the browser
to display.
When the server is processing a request from one client, requests from other clients may come. In order
to process these requests immediately, the server should use multiple processes. Basically, the main process
of the server keeps listening to new connection requests from clients. Once a connection request is accepted,
the main process will create a child process to respond the request from the client on that connection while
the main process is watching out for other connection requests.
Client
The client can request a web page and store it to a file in the current directory. Its interface should be:
myclient URL local_filename
where URL should be in the form of http://host_name:port_no/file_name and port_no and file_name
are optional. Here is an example:
myclient http://squill.cs.uky.edu:9872/mywebpage.html storepage.html.
When we use the client to access a web page from other web servers, we may run the client as
myclient http://www.yahoo.com yahoopage.html.
HTTP Protocol
You will get a better understanding of the HTTP protocol through implementing its core part. Here we
outline the major steps taken by the client and the server, and messages exchanged between them.
When a client is running with a URL http://host_name:port_no/file_name, it will use DNS (i.e.,
gethostbyname()) to get the IP address of the host represented by host_name. It opens a TCP connection
to the host at port indicated by port_no (use the default port number 80 if no port_no is given). Then the
client will form a HTTP request and send it through the TCP connection to the server. If the URL is in
the form of http://host_name:port_no, i.e., without file_name, the client should use “/” as the file name
in the “GET” request. The last step is to read from the TCP connection and store the reply (header+file)
obtained to the local file named local_filename.
