CSE 333 18au Homework #4

out: Friday, November 16, 2018
due: Thursday, December 6, 2018 by 11:00 pm.

[ summary | part a | part b | part c | bonus | how to submit | grading ]


For homework #4, you will build on your homework #3 solution to implement a multithreaded Web server front-end to your query processor. In Part A, you will read through some of our code to learn about the infrastructure we have built for you. In Part B, you will complete some of our classes and routines to finish the implementation of a simple Web server. In Part C, you will fix some security problems in our Web server.

As before, please read through this entire document before beginning the assignment, and please start early!

In HW4, as with HWs 2 and 3, you don't need to worry about propagating errors back to callers in all situations. You will use Verify333()'s to spot some kinds of errors and cause your program to crash out. However, no matter what a client does, your web server must handle that; only internal issues (such as out of memory) should cause your web server to crash out.

To help you schedule your time, here's a suggested order for the parts of this assignment. We're not going to enforce a schedule; it's up to you to manage your time.

  • Read over the project specifications and understand which code is responsible for what.

  • Finish ServerSocket.cc. Make sure to cover all functionality, not just what is in the unit tests.

  • Implement FileReader.cc, which should be very easy, and GetNextRequest in HttpConnection.cc.

  • Complete ParseRequest in HttpConnection.cc. This can be tricky as it involves both Boost and string parsing.

  • Finish the code for http333d.cc. Implement HttpServer_ThrFn in HttpServer.cc.

  • Complete ProcessFileRequest and ProcessQueryRequest in HttpServer.cc. At this point, you should be able to search the "333gle" site and view the webpages available under /static/, e.g. http://localhost:5555/static/bikeapalooza_2011/index.html.

  • Fix the security issues with the website, if you have any.

  • Make sure everything works as it is supposed to.
As before, you may not modify any of the existing header files or class definitions distributed with the code. If you wish to add extra "helper" functions you can to do that by including additional static functions in the implementation (.cc) files. You also may not modify the Makefile distributed with the project. In particular, there are reasonable ways to do the necessary string handling without using the Boost Regex library.

Part A -- read through our code.


Our web server is a fairly straightforward multithreaded application. Every time a client connects to the server, the server dispatches a thread to handle all interactions with that client. Threads do not interact with each other at all, which greatly simplifies the design of the server.

The figure to the right shows the high-level architecture of the server. There is a main class called "HttpServer" that uses a "ServerSocket" class to create a listening socket, and then sits in a loop waiting to accept new connections from clients. For each new connection that the HttpServer receives, it dispatches a thread from a ThreadPool class to handle the connection. The dispatched thread springs to life in a function called "HttpServer_ThrFn" within the HttpServer.cc file.

The HttpServer_ThrFn function handles reading requests from one client. For each request that the client sends, the HttpServer_ThrFn invokes GetNextRequest on an HttpConnection object to read in the next request and parse it.

To read a request, the GetNextRequest method invokes WrappedRead() some number of times until it spots the end of the request. To parse a request, the method invokes the ParseRequest method (also within HttpConnection). At this point, the HttpServer_ThrFun has a fully parsed HttpRequest object (defined in HttpRequest.h).

The next job of HttpServer_ThrFn is to process the request. To do this, it invokes the ProcessRequest() function, which looks at the request URI to determine if this is a request for a static file, or if it is a request associated with the search functionality. Depending on what it discovers, it either invokes ProcessFileRequest() or ProcessSearchRequest().

Once those functions return an HttpResponse, the HttpServer_ThrFn invokes the WriteResponse method on the HttpConnection object to write the response back to the client.

Our web server isn't too complicated, but there is a fair amount of plumbing to get set up. In this part of the assignment, we want you to read through a bunch of lower-level code that we've provided for you. You need to understand how this code works to finish our web server implementation, but we won't have you modify this plumbing.

What to do.

  1. Change to the directory that has your hw1, hw2, hw3, and projdocs directories in it. Use git pull to retrieve the new hw4 folder with the starter code for this assignment. You will need the hw1, hw2, and hw3 directories in the same folder as your new hw4 folder since hw4 links to files in those previous directories. Also, as with previous parts of the project, you can use the solution binary versions of the previous parts of the project if you wish.

  2. Run "make" to compile the HW4 binaries. One of them is the usual unit test binary called "test_suite". Run it, and you'll see the unit tests fail, crash out, and you won't yet earn the automated grading points tallied by the test suite. The second binary is the web server itself (http333d). Try running it to see its command line arguments. When you're ready to run it for real, you can use a command like:
    ./http333d 5555 ../projdocs ../hw3/unit_test_indices/*
    (You might need to pick a different port than 5555 if someone else is using that port on the same machine as you.)

    Try using our solution_binaries server, and running it using a similar command line:

    ./solution_binaries/http333d 5555 ../projdocs ../hw3/unit_test_indices/*
    Next, launch Firefox or Chrome on that machine, visit http://localhost:5555/, and try issuing some searches. As well, visit http://localhost:5555/static/bikeapalooza_2011/Bikeapalooza.html and click around. This is what your finished web server will be capable of.

    To shut down a http333d server when you are done with it, open another terminal window on the same machine and run the command

    kill pid
    where pid is the server process number. Use the ps command to find that number.

  3. Read through ThreadPool.h and ThreadPool.cc. You don't need to implement anything in either, but several pieces of the project rely on this code. The header file is well-documented, so it ought to be clear how it's used. (There's also a unit test file that you can peek at.)

  4. Read through HttpUtils.h and HttpUtils.cc. This class defines a number of utility functions that the rest of HW4 uses. Make sure that you understand what each of them does, and why.

  5. Finally, read through HttpRequest.h and HttpResponse.h. These files define the HttpRequest and HttpResponse classes, which represent a parsed HTTP request and response, respectively.

It's time to start coding in Part B.

Part B -- get the basic web server working.


You are now going to finish a basic implementation of the http333d web server. We'll have you implement some of the event handling routines at different layers of abstraction in the web server, culminating with generating HTTP and HTML to send to the client.

What to do.

  1. Take a look at ServerSocket.h. This file contains a helpful class for creating a server-side listening socket, and accepting a new connection from a client. We've provided you with the class declaration in ServerSocket.h but no implementation in ServerSocket.cc; your next job is to build it. You'll need to make the code handle either IPv4 or IPv6 clients. Run the test_suite to see if you make it past the server socket unit tests.

  2. Read through FileReader.h and FileReader.cc. Note that the implementation of FileReader.cc is missing; go ahead and implement it. See if you make it past the filereader unit test code.

  3. Read through HttpConnection.h and HttpConnection.cc. The two major functions in HttpConnection.cc have their implementations missing, but have generous comments for you to follow. Implement the missing functions, and see if you make it past the httpconnection unit test code.

  4. Now comes the hardest part of the assignment. Read through HttpServer.cc, HttpServer.h, and http333d.cc. Note that some parts of HttpServer.cc and http333d.cc are missing. Go ahead and implement those missing functions. Once you have them working, test your http333d binary to see if it works. Make sure you exercise both the web search functionality as well as the static file serving functionality. You'll probably need to look at the source of pages that our solution binary serves and emulate that HTML to get the same "look and feel" to your server as ours.

At this point, your web server should run correctly, and everything should compile with no warnings. Try running your web server and connecting to it from a browser. Also try running the test_suite under valgrind to make sure there are no memory issues. Finally, launch the web server under valgrind to make sure there are no issues or leaks; after the web server has launched, exercise it by issuing a few queries, then kill the web server. (The supplied code does have some leaks, but your code should not make things significantly worse.)

Part C - fix security vulnerabilities.


Now that the basic web server works, you will discover that your web server (probably) has two security vulnerabilities. We are going to point these out to you, and you will repair them.

What to do.

We'll bet that your implementation has two security flaws.

  • The first is called a "cross-site scripting" flaw. See this for background if you're curious:
    Try typing the following query into our example web server, and into your web server, and compare the two. (Note: do this with Firefox or Safari; it turns out that Chrome will attempt to help out web servers by preventing this attack from the client-side!)
    hello <script>alert("Boo!");</script>
    To fix this flaw, you need "escape" untrusted input from the client before you relay it to output. We've provided you with an escape function in HttpUtils.

  • Try telnet'ing to your web server, and manually typing in a request for the following URL. (Browsers are smart enough to help defend against this attack, so you can't just type it into the URL bar, but nothing prevents attackers from directly connecting to your server with a program of their own!)
    This is called a directory traversal attack. Instead of trusting the file pathname provided by a client, you need to normalize the path and verify that it names a file within your document subdirectory tree (which would be projdocs/ if the example command shown in part A was used to start the server). If the file names something outside of that subdirectory, you should return an error message instead of the file contents. We've provided you with a function in HttpUtils to help you test to see if a path is safe or not.

Fix these two security flaws, assuming they do in fact exist in your server. As a point of reference, in solution_binaries/, we've provided a version of our web server that has both of these flaws in place (http333d_withflaws). Feel free to try it out, but DO NOT leave this server running, as it will potentially expose all of your files to anybody that connects to it.

Congrats, you're done with the HW4 project sequence!!


There are two bonus tasks for this assignment. As before, you can do them, or not; if you don't, there will be no negative impact on your grade. You should not attempt either bonus task unless and until the basic assignment is working properly. We will not award any bonus credit if the basic assignment is not substantially correct.

If you want to do any of the bonus parts, first create a hw4-final tag in your repository to mark the version of the assignment with the required parts of the project. That will allow us to more easily evaluate how well you did on the basic requirements of the assignment.

Then, when you are done adding additional bonus parts, create a new tag hw4-bonus after committing and pushing your additions, and push the new tag to your GitLab repository. If we find a hw4-bonus tag in your repository we'll grade the extra credit parts; otherwise we'll assume that you just did the required parts.

If you do any of the bonus parts, you should add a file named readme_bonus.txt in your top-level hw4 directory giving a brief summary of the additions in your project.

  • The first bonus task is to perform a performance analysis of your web server implementation, determining what throughput your server can handle (measured both in requests per second and bytes per second), what latency clients experience (measure in seconds per request), and what the performance bottleneck is. You might want to look at the "httperf" tool for Linux to generate synthetic load.

    You should conduct this performance analysis for a few different usage scenarios; e.g., you could vary the size of the web page you request, and see its impact on the number of pages per second your server can deliver. If you choose to do the bonus, please include a PDF file in your submission containing relevant performance graphs and analysis.

  • The second bonus task is to figure out some interesting feature to add to your web server, and implement it! As one idea, find the implementation of a "chat bot", such as Eliza, and add it to your web server. As another idea, implement logging functionality; every time your server serves content, write out some record with a timestamp to a log file; make the log file available through the web server itself. As a third idea, change the results page to show excerpts from matching documents, similar to how Google shows excerpts from matching pages; specifically, make it so that each result in the result list shows:
    x words + <bold>hit word</bold> + y words
    for one or more of the query words that hit. If you do this part of the assignment, please add a readme_bonus.txt file to your hw4 directory describing what you've added and how to use it.

    This part of the assignment is deliberately open-ended, with much less structure than earlier parts. The (small) amount of extra credit granted will depend on how interesting your extension is and how well it is implemented.

What to turn in

When you are ready to turn in your assignment, you should follow the same procedures you used for previous assignments, except this time tag the repository with hw4-final. Remember to clean up, commit, and push all necessary files to your repository before you add the tag and push it. After you have created and pushed the tag, remember to test everything by creating a new clone of the repository in a separate, empty directory, checkout the hw4-final tag, and verify that everything works as expected. Refer to the hw0 turnin instructions for details, and follow those steps carefully.

If you do any of the bonus parts, create an additional hw4-bonus tag and push that after adding and pushing the bonus code to the repository. Be sure to clone the repo, checkout that tag, and verify that everything works as expected. Also verify that the hw4-final tag is still present, that it includes (only) the required parts of the project, and that those parts still work as required.

As with previous parts of the project, when you clone your repository to check it, it will normally not include previous solution files like hw1/libhw1.a. You should either run make in the hw1 through hw3 directories to recreate those archives, or else copy the versions from the solution_binaries folders into the right places. These are needed in order to build hw4 and test it.


We will be basing your grade on several elements:

  • The degree to which your code passes the unit tests. If your code fails a test, we won't attempt to understand why: we're planning on just including the number of points that the test drivers print out.

  • We have some additional unit tests that test a few additional cases that aren't in the supplied test drivers. We'll be checking to see if your code passes these.

  • The quality of your code. We'll be judging this on several qualitative aspects, including whether you've sufficiently factored your code and whether there is any redundancy in your code that could be eliminated.

  • The readability of your code. For this assignment, we don't have formal coding style guidelines that you must follow; instead, attempt to mimic the style of code that we've provided you. Aspects you should mimic are conventions you see for capitalization and naming of variables, functions, and arguments, the use of comments to document aspects of the code, and how code is indented.