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CPTS 360 Systems Programming: Lab Assignment 5 Building a Web Proxy

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1 Introduction CourseNana.COM

CPTS 360: Lab Assignment 5 Building a Web Proxy CourseNana.COM

Notes: CourseNana.COM

  • This is an individual lab. CourseNana.COM

  • You must complete the lab on a Linux environment (either a dedicated machine or VM). Using WSL (Windows Subsystem for Linux) on Windows is NOT recommended. CourseNana.COM

  • Your submission will be tested on a Linux environment. You will NOT receive any points if your submitted program does not work on a Linux system. CourseNana.COM

  • Start EARLY. The lab may take more time than you anticipated. CourseNana.COM

  • Read the entire document carefully before you start working on the lab. CourseNana.COM

  • The code and other answers you submit MUST be entirely your own work, and you are bound by the WSU Academic Integrity Policy (https://www.communitystandards.wsu. edu/policies-and-reporting/academic-integrity-policy/). CourseNana.COM

  • You MAY consult with other students about the conceptualization of the tasks and the meaning of the questions, but you MUST NOT look at any part of someone else’s solution or collaborate with anyone. CourseNana.COM

  • You may consult published references or search online archives, provided that you appropriately cite them in your reports/programs. CourseNana.COM

  • The use of artificial intelligence (AI)–generated texts/code-snippets must be CLEARLY DISCLOSED, including the prompt used to generate the results and the corresponding outputs the tool(s) provide. CourseNana.COM

    Good Luck! CourseNana.COM

A Web proxy is a program that acts as a middleman between a Web browser and an end server. Instead of contacting the end server directly to get a Web page, the browser contacts the proxy, which forwards the request on to the end server. When the end server replies to the proxy, the proxy sends the reply on to the browser. CourseNana.COM

Proxies are useful for many purposes. Sometimes proxies are used in firewalls, so that browsers behind a firewall can only contact a server beyond the firewall via the proxy. Proxies can also act as anonymizers: by stripping requests of all identifying information, a proxy can make the browser anonymous to Web servers. Proxies can even be used to cache web objects by storing local copies of objects from servers then responding to future requests by reading them out of its cache rather than by communicating again with remote servers. CourseNana.COM

In this lab, you will write a simple HTTP proxy that caches web objects. You will set up the proxy to accept incoming connections, read and parse requests, forward requests to web servers, read the servers’ responses, and forward those responses to the corresponding clients. The lab will involve learning about basic HTTP operation and how to use sockets to write programs that communicate over network connections. CourseNana.COM

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2 Lab Tasks: Implementing a Web Proxy CourseNana.COM

The first step is implementing a basic sequential proxy that handles HTTP/1.0 GET requests. Other requests type, such as POST, are strictly optional. CourseNana.COM

When started, your proxy should listen for incoming connections on a port whose number will be specified on the command line. Once a connection is established, your proxy should read the entirety of the request from the client and parse the request. It should determine whether the client has sent a valid HTTP request; if so, it can then establish its own connection to the appropriate web server then request the object the client specified. Finally, your proxy should read the server’s response and forward it to the client. CourseNana.COM

2.1 HTTP/1.0 GET requests CourseNana.COM

When an end user enters a URL such as http://www.wsu.edu/eecs/index.html into the address bar of a web browser, the browser will send an HTTP request to the proxy that begins with a line that might resemble the following: CourseNana.COM

    GET http://www.wsu.edu/eecs/index.html HTTP/1.1

In that case, the proxy should parse the request into at least the following fields: the hostname, www.wsu.edu; and the path or query and everything following it, /eecs/index.html. That way, the proxy can determine that it should open a connection to www.wsu.edu and send an HTTP request of its own starting with a line of the following form: CourseNana.COM

GET /eecs/index.html HTTP/1.0
Note that all lines in an HTTP request end with a carriage return, ‘\r’, followed by a newline, ‘\n’. Also CourseNana.COM

important is that every HTTP request is terminated by an empty line: "\r\n". CourseNana.COM

You should notice in the above example that the web browser’s request line ends with HTTP/1.1, while the proxy’s request line ends with HTTP/1.0. Modern web browsers will generate HTTP/1.1 requests, but your proxy should handle them and forward them as HTTP/1.0 requests. CourseNana.COM

It is important to consider that HTTP requests, even just the subset of HTTP/1.0 GET requests, can be incredibly complicated. The textbook describes certain details of HTTP transactions, but you should refer to RFC 1945 for the complete HTTP/1.0 specification. Ideally your HTTP request parser will be fully robust according to the relevant sections of RFC 1945, except for one detail: while the specification allows for multiline request fields, your proxy is not required to properly handle them. Of course, your proxy should never prematurely abort due to a malformed request. CourseNana.COM

2.2 Request Headers CourseNana.COM

The important request headers for this lab are the Host, User-Agent, Connection, and Proxy-Connection headers: CourseNana.COM

• Always send a Host header. While this behavior is technically not sanctioned by the HTTP/1.0 specification, it is necessary to coax sensible responses out of certain Web servers, especially those that use virtual hosting. CourseNana.COM

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The Host header describes the hostname of the end server. For example, to access http://www. wsu.edu/eecs/index.html, your proxy would send the following header: CourseNana.COM

Host: www.wsu.edu
It is possible that web browsers will attach their own Host headers to their HTTP requests. If that is CourseNana.COM

the case, your proxy should use the same Host header as the browser. CourseNana.COM

  • You may choose to always send the following User-Agent header: CourseNana.COM

             User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.3)
                 Gecko/20120305 Firefox/10.0.3

    The header is provided on two separate lines because it does not fit as a single line in the writeup, but your proxy should send the header as a single line. CourseNana.COM

    The User-Agent header identifies the client (in terms of parameters such as the operating system and browser), and web servers often use the identifying information to manipulate the content they serve. Sending this particular User-Agent: string may improve, in content and diversity, the material that you get back during simple telnet-style testing. CourseNana.COM

  • Always send the following Connection header: Connection: close CourseNana.COM

  • Always send the following Proxy-Connection header: Proxy-Connection: close CourseNana.COM

    The Connection and Proxy-Connection headers are used to specify whether a connection will be kept alive after the first request/response exchange is completed. It is perfectly acceptable (and suggested) to have your proxy open a new connection for each request. Specifying close as the value of these headers alerts web servers that your proxy intends to close connections after the first request/response exchange. CourseNana.COM

    For your convenience, the values of the described User-Agent header is provided to you as a string constant in proxy.c. CourseNana.COM

    Finally, if a browser sends any additional request headers as part of an HTTP request, your proxy should forward them unchanged. CourseNana.COM

2.3 Port Numbers CourseNana.COM

There are two significant classes of port numbers for this lab: HTTP request ports and your proxy’s listening port. CourseNana.COM

The HTTP request port is an optional field in the URL of an HTTP request. That is, the URL may be of the form, http://www.wsu.edu:8080/eecs/index.html, in which case your proxy should connect to CourseNana.COM

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the host www.wsu.edu on port 8080 instead of the default HTTP port, which is port 80. Your proxy must properly function whether or not the port number is included in the URL. CourseNana.COM

The listening port is the port on which your proxy should listen for incoming connections. Your proxy should accept a command line argument specifying the listening port number for your proxy. For example, with the following command, your proxy should listen for connections on port 15213: CourseNana.COM

    linux> ./proxy 15213

You may select any non-privileged listening port (greater than 1,024 and less than 65,536) as long as it is not used by other processes. Since each proxy must use a unique listening port and many people will simultaneously be working on each machine, the script port-for-user.pl is provided to help you pick your own personal port number. Use it to generate port number based on your user ID: CourseNana.COM

    linux> ./port-for-user.pl cougs
    droh: 45806

The port, p, returned by port-for-user.pl is always an even number. So if you need an additional port number, say for the Tiny server, you can safely use ports p and p + 1. CourseNana.COM

Please don’t pick your own random port. If you do, you run the risk of interfering with another user. CourseNana.COM

2.4 Robustness CourseNana.COM

As always, you must deliver a program that is robust to errors and even malformed or malicious input. Servers are typically long-running processes, and web proxies are no exception. Think carefully about how long-running processes should react to different types of errors. For many kinds of errors, it is certainly inappropriate for your proxy to immediately exit. CourseNana.COM

Robustness implies other requirements as well, including invulnerability to error cases like segmentation faults and a lack of memory leaks and file descriptor leaks. CourseNana.COM

3 Testing and Debugging CourseNana.COM

We do not provide any sample inputs or a test program to test your implementation. You will have to come up with your own tests and perhaps even your own testing harness to help you debug your code and decide when you have a correct implementation. This is a valuable skill in the real world, where exact operating conditions are rarely known and reference solutions are often unavailable. CourseNana.COM

Fortunately, there are many tools you can use to debug and test your proxy. Be sure to exercise all code paths and test a representative set of inputs, including base cases, typical cases, and edge cases. CourseNana.COM

3.1 Tiny Web Server CourseNana.COM

The GitHub repository contains source code for the Tiny web server (discussed in class). The Tiny web server will be easy for you to modify as you see fit. It is also a reasonable starting point for your proxy code. CourseNana.COM

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3.2 telnet
As in the class, you can use telnet to open a connection to your proxy and send it HTTP requests. CourseNana.COM

3.3 curl CourseNana.COM

You can use curl to generate HTTP requests to any server, including your own proxy. It is an extremely useful debugging tool. For example, if your proxy and Tiny are both running on the local machine (, Tiny is listening on port 15213, and proxy is listening on port 15214, then you can request a page from Tiny via your proxy using the following curl command: CourseNana.COM

linux> curl -v --proxy http://localhost:15214 http://localhost:15213/home.html
* About to connect() to proxy localhost port 15214 (#0)
*   Trying connected
* Connected to localhost ( port 15214 (#0)
> GET http://localhost:15213/home.html HTTP/1.1
> User-Agent: curl/7.19.7 (x86_64-redhat-linux-gnu)...
> Host: localhost:15213
> Accept: */*
> Proxy-Connection: Keep-Alive
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Server: Tiny Web Server
< Content-length: 120
< Content-type: text/html
<img align="middle" src="wsu.png">
* Closing connection #0

3.4 netcat CourseNana.COM

netcat, also known as nc, is a versatile network utility. You can use netcat just like telnet, to open connections to servers. Hence, imagining that your proxy were running on cougs using port 12345 you can do something like the following to manually test your proxy: CourseNana.COM

    sh> nc cougs.eecs.wsu.edu 12345
    GET http://www.wsu.edu/eecs/index.html HTTP/1.0
    HTTP/1.1 200 OK

In addition to being able to connect to Web servers, netcat can also operate as a server itself. With the following command, you can run netcat as a server listening on port 1235: CourseNana.COM

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sh> nc -l 1235
Once you have set up a netcat server, you can generate a request to a phony object on it through your CourseNana.COM

proxy, and you will be able to inspect the exact request that your proxy sent to netcat.
For example, if a
netcat server is running at port 1235 and the proxy is at port 1234, we can generate CourseNana.COM

request to the server through our proxy using the following command: CourseNana.COM

sh> pkill proxy
sh> ./proxy 1234
sh> curl -v --proxy$(date +"%T").dat

This generates request a (pseudo) .dat file indexed by the current time. You may expect the following output: CourseNana.COM

*   Trying
* Connected to ( port 1234 (#0)
> GET HTTP/1.1
> Host:
> User-Agent: curl/7.74.0
> Accept: */*
> Proxy-Connection: Keep-Alive

3.5 Web Browsers CourseNana.COM

Eventually you should test your proxy using the most recent version of Mozilla Firefox. Visiting About Firefox will automatically update your browser to the most recent version. CourseNana.COM

Figure 1: Configuring Firefox with a proxy running at localhost ( and port 23361. To configure Firefox to work with a proxy, visit (also see Figure 1): CourseNana.COM


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It will be very exciting to see your proxy working through a real Web browser. Although the functionality of your proxy will be limited, you will notice that you are able to browse the vast majority of websites through your proxy. CourseNana.COM

Visit few websites, for example www.google.com, www.bing.com, and www.wiki.com and report which ones can be accessible with your proxy. CourseNana.COM

4 Hints CourseNana.COM

  • Chapters 10-12 of the textbook (CSAPP) contains useful information on system-level I/O, network programming, HTTP protocols, and concurrent programming. CourseNana.COM

  • RFC 1945 (http://www.ietf.org/rfc/rfc1945.txt) is the complete specification for the HTTP/1.0 protocol. CourseNana.COM

  • As discussed in class, using standard I/O functions for socket input and output is a problem. Instead, we recommend that you use the Robust I/O (RIO) package, which is provided in the csapp.c file in the handout directory. CourseNana.COM

  • The error-handling functions provide in csapp.c are not appropriate for your proxy because once a server begins accepting connections, it is not supposed to terminate. You’ll need to modify them or write your own. CourseNana.COM

  • You are free to add new files or modify the provided files any way you like. Of course, adding new files will require you to update the provided Makefile. CourseNana.COM

  • Sometimes, calling read to receive bytes from a socket that has been prematurely closed will cause read to return -1 with errno set to ECONNRESET. Your proxy should not terminate due to this error. CourseNana.COM

  • Remember that not all content on the web is ASCII text. Much of the content on the web is binary data, such as images and video. Ensure that you account for binary data when selecting and using functions for network I/O. CourseNana.COM

  • Forward all requests as HTTP/1.0 even if the original request was HTTP/1.1. CourseNana.COM

    5 Deliverable CourseNana.COM

  • Complete the implementation of proxy. CourseNana.COM

    [40 Points] CourseNana.COM

    The point split for your implementation is as follows (see below for details): * 35 points for correctness and
    * 5 points for good code organization, indentation, and proper comments. CourseNana.COM

  • A demonstration (screencast) of your implementation (a key part of your submission, see below). [50 Points] CourseNana.COM

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A PDF document (no more than 2 pages, 11 points Times font, 1-inch margin) containing a summary of how you implemented your proxy. You should include your report in the GitHub repository. CourseNana.COM

[10 Points + 5 Bonus Points] CourseNana.COM

  • –  Your PDF filename should be lab5 lastname.pdf, where lastname is your surname. Include your full name and WSU ID inside the report. We will deduct 5 points if the filename/contents does not follow this convention. CourseNana.COM

  • –  Bonus points [5 Points]: While browsing using Firefox, you may find some of the URLs can not be accessible using your proxy. Why? Explain in your report with a clear heading. CourseNana.COM

    Requirements for the Demonstration CourseNana.COM

You need to record your screen and show how your code works. The demonstration contains a major part of your grades (50 Points). Record your VM/terminal using any screen recorder tools. We recommend using Zoom for this purpose. For instance, you can record any application window using Zoom by following the steps listed here: CourseNana.COM

https://blog.smu.edu/itconnect/2017/08/23/ using-zoom-create-quick-easy-screen-recordings-free/ CourseNana.COM

5.2 What to Record CourseNana.COM

  1. Show a communication between your proxy (running on and Tiny web server using the telnet utility as demonstrated in the class. For example, you can initiate a GET request using telnet. We expect you open two terminal windows side-by-side and record input/output operations (starting your proxy, tiny server, and issuing commands using telnet). CourseNana.COM

  2. A screencast of the curl command as mentioned in Section 3.3. CourseNana.COM

  3. A screencast of the netcat command as mentioned in Section 3.4. CourseNana.COM

  4. A screencast of browsing websites using Firefox configured with your proxy and port (see Section 3.5). Browse the following URLs: www.google.com, www.bing.com, and www.wiki.com and show which one is loading with your proxy. CourseNana.COM

Submission Guidelines CourseNana.COM

Commit and push your work (source and report) on GitHub Classroom Lab 5 repository. Make sure you can successfully push commits on GitHub Classroom — last minute excuses will not be considered). Get the Git commit id of your work by running the command: git log -1 --format=oneline. Paste the commit ID (the hexadecimal string) to the corresponding lab assignment section of Canvas. Check the course website for additional details. CourseNana.COM

Note: On Canvas, you can submit any commit ID (not necessarily the last one) from your GitHub Classroom Git history. You can also submit as many times as you want. However, we will grade the last commit ID submitted to Canvas before the deadline. CourseNana.COM

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Your work on GitHub Classroom will not be considered for grading unless you confirm your commit ID on Canvas. Hence, You will NOT receive any points if you fail to submit your commit ID on Canvas by the due date. CourseNana.COM

How to Submit Your Recording: CourseNana.COM

• Upload the video file to WSU OneDrive and enable link sharing. You must ensure that the video file is accessible by the instructor. A good rule of thumb is to enable “People in WSU can access” so that you do not require to provide explicit permissions. Note: If your video file is not accessible by the instructor, you will not receive any points. CourseNana.COM

• In your Canvas submission page, include your GitHub commit id and OneDrive link of the video file. CourseNana.COM

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