CS 43 Lab 2: A Concurrent Web Server

Handy References:

• RFC 1945: HTTP 1.0 Specification. Sections 4, 5, and 6 are probably the most helpful.
• Manual pages for bind, listen, and accept.
• Manual pages for pthread_create, and pthread_detach.

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Lab Audio, day 1 Lab Audio, day 2

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Lab 2 Goals:

• Implement the server side of a HTTP / TCP connection.
• Use threading to serve multiple concurrent clients.
• More practice with sockets, send(), and receive().

Overview

Having built a Web client, for this lab we'll look at the other end of the HTTP protocol -- the Web server. As real web clients (e.g., browsers like Firefox) send requests to your server, you'll be finding the requested files and serving them back to the clients.

Your server program will receive two arguments: 1) the port number it should listen on for incoming connections, and 2) the directory out of which it will serve files (typically called the document root). For example:

./lab2 8080 test_documents

This command will tell your Web server to listen for connections on port 8080 and serve files out of the test_documents directory. That is, the test_documents directory is considered '/' when responding to requests. If you're asked for /index.html, you should respond with the file that resides in test_documents/index.html. If you're asked for /dir1/dir2/file.ext, you should respond with the file test_documents/dir1/dir2/file.ext.

High-level checklist

Roughly, your server should follow this sequence:

1. Read the arguments, bind to the specified port, and find your document root (you might find the chdir() system call helpful).
2. Accept a connection, and hand it off to a new thread for concurrent processing. (Initially, you can just call a function rather than spawning a thread.)
3. Receive and parse a request from the client.
4. Look for the path that was requested, starting from your document root (the second argument to your program). One of four things should happen:
   1. If the path exists and it's a file, formulate a response (with the Content-Type header set) and send it back to the client.
   2. If the path exists and it's a directory that contains an index.html file, respond with that file.
   3. If the path exists and it's a directory that does NOT contain an index.html file, respond with a directory listing.
   4. If the path does not exist, respond with a 404 code with a basic HTML error page.
   You might want to make each of these cases a separate function!
5. Close the connection, and continue serving other clients.

Requirements

In addition to serving requested files, your server should handle at least the following cases:

1. A variety of file formats should all display properly, including both text and binary formats. You'll need to return the proper HTTP Content-Type header in your response. You don't need to handle everything on that list, but you should at least be able to handle files with html, txt, jpeg, gif, png, pdf, and ico extensions. You may assume that the file extension is correct (e.g., I'm not going to name a PDF file with a ".txt" suffix). For .ico, use: image/x-icon


2. If asked for a file that does not exist, you should respond with a 404 error code with a readable error page, just like a Web server would. It doesn't need to be fancy, but it should contain some basic HTML so that the browser renders something and makes the error clear.


3. Some clients may be slow to complete a connection or send a request. Your server should be able to serve multiple clients concurrently, not just back-to-back. For this lab, use multithreading with pthreads to handle concurrent connections. (We'll try an alternative to threads, event-based concurrency, in a future lab assignment.)


4. If the path requested by the client is a directory, you should handle the request as if it was for the file "index.html" inside that directory if such a file exists. Hint: use the stat() system call to determine if a path is a directory or a file. The st_mode field in the stat struct has what you need.


5. The Web server should respond with a list of files when the user requests a directory that does not contain an index.html file. You can read the contents of a directory using the opendir() and readdir() calls. Together they behave like an iterator. That is, you can open a (DIR *) with opendir and then continue calling readdir(), which returns info for one file, on that (DIR *) until it returns NULL. Note that there should be no additional files created on the server's disk to respond to the request. The response should mimic result of running:

python -m SimpleHTTPServer

When testing, you should be able to retrieve byte-for-byte copies of files from your server. Use wget or curl to fetch files and md5sum or diff to compare the fetched file with the original. I will do this when grading. For full credit, the files need to be exact replicas of the original.

Grading Rubric

This assignment is worth five points, which I'm planning to assign as follows:

• 1 point for serving both text and binary files (that can be rendered correctly -- set Content-Type) to a standard Web browser (e.g., Firefox).
• 1 point for serving exact copies of text and binary files to command line clients like wget or curl. The MD5 sums should match!
• ½ point for correctly returning a 404 error code and HTML message when a request asks for a file that does not exist.
• ½ point for serving index.html, if it exists, when asked for a file that is a directory.
• 1 point for handling multiple clients concurrently. Your server should be able to have one or more open telnet connections, doing nothing, while still being able to handle browser/wget/curl requests.
• 1 point for returning a file listing when asked for a directory that does not contain an index.html file. The listing should use simple HTML to provide clickable links.

Hints / Tips / Assumptions

• You may assume that file suffixes correctly correspond to their type (e.g., if a file ends in ".pdf" that it is a PDF file).


• You may assume that requests sent to your server are at most 4 KB in length.


• You may assume that if the user requests a path that is a directory, the path will end in a trailing '/'. When generating the list of files in a directory, make sure your server also sends back URLs that end in '/' for directories. This is for the benefit of your browser, which keeps track of its current location based on the absence or presence of slashes.


• Unlike many of your prior experiences with threading (e.g., parallel GOL in CS 31), the threads in this assignment don't need to coordinate their actions. This makes the threading relatively easy, and it's something that can be added on once the main serving functionality is implemented. When starting out, organize your code such that it calls a function on any newly-accepted client sockets, and let that function do all the work for that connection. This'll make adding pthread support quite simple!


• Take compiler warnings seriously. Unless it's an unused variable, you should address the warning as soon as you see it. Dealing with a pile of warnings just makes things more difficult later.


• Test your code in small increments. It's much easier to localize a bug when you've only changed a few lines.


• If you need to copy a specific number of bytes from one buffer to another, and you're not 100% sure that the data will be entirely text, use memcpy() rather than strncpy(). The latter still terminates early if it finds a null terminator ('\0').


• If you're trying to do some sort of specific string or memory manipulation, feel free to ask if there's a better/recommended way to do it rather than brute force. Often there may be a standard library function that will make things easier.

Reminder

Always, always, always check the return value of any system calls you make! This is especially important for send, recv, read, and write calls that tell you how many bytes were read or written.

If you have any questions about the lab requirements or specification, please post on Piazza.

Submitting

Please remove any debugging output prior to submitting.

To submit your code, simply commit your changes locally using git add and git commit. Then run git push while in your lab directory. Only one partner needs to run the final push, but make sure both partners have pulled and merged each others changes. See the section on Using a shared repo on the git help page.