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Write Your Own Caching Web Proxy Using Basic Concurrency Solution
Introduction
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.
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.
In this lab, you will write a simple HTTP proxy that caches web objects. For the first part of the lab, 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. This first part will involve learning about basic HTTP operation and how to use sockets to write programs that communicate over network connections. In the second part, you will upgrade your proxy to deal with multiple concurrent connections using a simple, thread-based model. This will introduce you to dealing with concurrency, a crucial systems concept. In the next lab, you will modify your concurrency approach to use a threadpool and add caching to your proxy using a simple main memory cache of recently accessed web content. In a future lab, you will modify your proxy to use I/O multiplexing.
Logistics
This is an individual project.
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Handout instructions
The files for the lab will made available from a link in the “assignment” page for the lab on the course site on Learning Suite, as a single archive file, proxylab-handout.tar.
Start by copying proxylab-handout.tar to a protected Linux directory in which you plan to do your work. Then give the command
linux tar xvf proxylab-handout.tar
This will generate a handout directory called proxylab-handout. The README file describes the various files.
Part I: Implementing a sequential web proxy
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.
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.
4.1 HTTP/1.0 GET requests
When an end user enters a URL such as http://www.cmu.edu/hub/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:
GET http://www.cmu.edu/hub/index.html HTTP/1.1
In that case, the proxy should parse the request into at least the following fields: the hostname, www.cmu.edu; and the path or query and everything following it, /hub/index.html. That way, the proxy can deter-mine that it should open a connection to www.cmu.edu and send an HTTP request of its own starting with a line of the following form:
GET /hub/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 important is that every HTTP request is terminated by an empty line: "\r\n".
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.
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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.
4.2 Request headers
The important request headers for this lab are the Host, User-Agent, Connection, and Proxy-Connection headers:
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.
The Host header describes the hostname of the end server. For example, to access http://www. cmu.edu/hub/index.html, your proxy would send the following header:
Host: www.cmu.edu
It is possible that web browsers will attach their own Host headers to their HTTP requests. If that is the case, your proxy should use the same Host header as the browser.
You may choose to always send the following User-Agent header:
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.
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.
Always send the following Connection header: Connection: close
Always send the following Proxy-Connection header: Proxy-Connection: close
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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. While it is not the most efficient use of resources, it is perfectly acceptable to have your proxy open a new connection for each request—and it will simplify your proxy implementation. Specifying close as the value of these headers alerts web servers that your proxy intends to close connections after the first request/response exchange.
For your convenience, the values of the described User-Agent header is provided to you as a string constant in proxy.c.
Finally, if a browser sends any additional request headers as part of an HTTP request, your proxy should forward them unchanged.
4.3 Port numbers
There are two significant classes of port numbers for this lab: HTTP request ports and your proxy’s listening port.
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.cmu.edu:8080/hub/index.html, in which case your proxy should connect to the host www.cmu.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.
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:
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:
linux ./port-for-user.pl droh
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.
Please don’t pick your own random port. If you do, you run the risk of interfering with another user.
Part II: Dealing with multiple concurrent requests
Once you have a working sequential proxy, you should alter it to simultaneously handle multiple requests. The simplest way to implement a concurrent server is to spawn a new thread to handle each new connection
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request.
Note that with this particular thread paradigm, you should run your threads in detached mode to avoid memory leaks. When a new thread is spawned, you can put it in detached mode by calling within the thread routine itself:
pthread_detach(pthread_self());
5.1 Robustness
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.
Robustness implies other requirements as well, including invulnerability to error cases like segmentation faults and a lack of memory leaks and file descriptor leaks.
Testing and debugging
Besides the simple autograder, you will not have any sample inputs or a test program to test your imple-mentation. 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.
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.
6.1 telnet
As described in your textbook (11.5.3), you can use telnet to open a connection to your proxy and send it HTTP requests.
6.2 curl
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:
linux curl -v --proxy http://localhost:15214 http://localhost:15213/home.html
About to connect() to proxy localhost port 15214 (#0)
Trying 127.0.0.1... connected
Connected to localhost (127.0.0.1) port 15214 (#0)
GET http://localhost:15213/home.html HTTP/1.1
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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
<
<html
<head<titletest</title</head
<body
<img align="middle" src="godzilla.gif" Dave O’Hallaron
</body
</html
Closing connection #0
6.3 netcat
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 catshark using port 12345 you can do something like the following to manually test your proxy:
sh nc catshark.ics.cs.cmu.edu 12345
GET http://www.cmu.edu/hub/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 12345:
sh nc -l 12345
Once you have set up a netcat server, you can generate a request to a phony object on it through your proxy, and you will be able to inspect the exact request that your proxy sent to netcat.
6.4 Web browsers
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.
To configure Firefox to work with a proxy, visit
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PreferencesAdvancedNetworkSettings
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.
An important caveat is that you must be very careful when testing caching using a Web browser. All modern Web browsers have caches of their own, which you should disable before attempting to test your proxy’s cache.
6.5 Autograding
Your handout materials include an autograder, called driver.sh, which will evaluate your proxy server.
Use the following command line to run the driver for this lab:
linux ./driver.sh multithread 70 25 0 2 0
This will provide you a grade and feedback for your assignment. Note that maximum possible points that the script will output is 97. The remaining three points are for no warnings with compilation.
Resources, Hints, and Example Code
7.1 Resources
Chapters 10-12 of the textbook contains useful information on system-level I/O, network program-ming, HTTP protocols, and concurrent programming.
RFC 1945 (http://www.ietf.org/rfc/rfc1945.txt) is the complete specification for the HTTP/1.0 protocol.
7.2 Tiny web server / CS:APP code
Your handout directory the source code for the CS:APP Tiny web server. While not as powerful as thttpd, the CS:APP Tiny web server will be easy for you to modify as you see fit. It’s also a reasonable starting point for much of your proxy code. And it’s the server that the driver code uses to fetch pages.
That being said, do not use CS:APP’s code for preparing sockets (e.g., Open listenfd) or its code robust I/O code (i.e., functions starting with rio ). See more information and guidance below and setting up sockets and reading and writing robustly from a socket. You will find much useful code that you can adapt for your needs, including the code for parsing the request line and the headers (which needs to be modified to not use the rio functions). You have been warned.
Using the error-handling functions provide in csapp.c is allowed but not encouraged, as I would rather have you know what you are doing in terms of error handling. Anyway, some of them would need to be modified to behave appropriately for your proxy because once a server begins accepting connections, it is not supposed to terminate (some of the actions in those functions is to simply exit).
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7.3 Socket setup code
The essential functions used for setting up a socket for listening for incoming HTTP connections are socket(), bind(), and listen(). To help you with this part, we include some code below, which will set up a TCP socket to listen on any IPv4 address.
ip4addr.sin_family = AF_INET; ip4addr.sin_port = htons(atoi(argv[1])); ip4addr.sin_addr.s_addr = INADDR_ANY;
if ((sfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("socket error");
exit(EXIT_FAILURE);
}
if (bind(sfd, (struct sockaddr *)&ip4addr, sizeof(struct sockaddr_in)) < 0) { close(sfd);
perror("bind error");
exit(EXIT_FAILURE);
}
if (listen(sfd, 100) < 0) {
close(sfd);
perror("listen error");
exit(EXIT_FAILURE);
}
Note that this code is similar to that already provided to you in your sockets homework assignment.
The essential functions used for setting up a socket and connecting to a remote address and port are getaddrinfo(), socket(), and connect(). To help you with this part, we include some code below, which will set up a socket to and connect to a remote host and port over whichever protocol.
/* Obtain address(es) matching host/port */
memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */ hints.ai_socktype = SOCK_STREAM; /* TCP socket */ hints.ai_flags = 0;
hints.ai_protocol = 0; /* Any protocol */
s = getaddrinfo(host, port, &hints, &result); if (s != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s)); exit(EXIT_FAILURE);
}
/* getaddrinfo() returns a list of address structures.
Try each address until we successfully connect(2).
If socket(2) (or connect(2)) fails, we (close the socket and) try the next address. */
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for (rp = result; rp != NULL; rp = rp-ai_next) { sfd = socket(rp-ai_family, rp-ai_socktype, rp-ai_protocol);
if (sfd == -1)
continue;
if (connect(sfd, rp-ai_addr, rp-ai_addrlen) != -1)
break; /* Success */
close(sfd);
}
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not connect\n");
}
freeaddrinfo(result); /* No longer needed */
7.4 Other Hints
As discussed in Section 10.11 of your textbook, using standard I/O functions for socket input and output is a problem. Use your own robust read() (or recv()) loop (i.e., similar to that you created in your sockets homework assignment). The general idea is to keep reading from the socket into a buffer until you have some assurance that you have everything you need in the buffer to move on.
For example, when receiving an HTTP request from a client, you can call read() within a loop to keep reading bytes from the socket until you know you have the entire HTTP request (i.e., as indicated with "\r\n\r\n" which signal the end of headers). Similarly, when receiving an HTTP response from a server, you can call read() within a loop to keep reading bytes from the socket until you know you have received the entire response. Because we are only using HTTP/1.0 in this lab, your signal here that you have received the entire response is when the remote server disconnects, and your read() returns 0 (With HTTP/1.1, the server would, by default, keep the TCP connection open for future requests, so you would need to read the Content-length: header to determine how many bytes to read).
You are free to modify the files in the handout directory any way you like. For example, for the sake of good modularity, you might implement your cache functions as a library in files called cache.c and cache.h. Of course, adding new files will require you to update the provided Makefile, so we can compile it properly.
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. For example, you will want to depend heavily on the return value from read(), rather than calling (for example) strlen() to determine how many bytes have been received.
Finally, forward all requests as HTTP/1.0 even if the original request was HTTP/1.1.
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7.5 Error handling
As discussed in the Aside on page 964 of the CS:APP3e text, your proxy must ignore SIGPIPE signals and should deal gracefully with write operations that return EPIPE errors.
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 either.
Good luck!
Grading
The following is the point breakdown:
- 70 points for basic proxy operation
- 25 points for handling concurrent requests - 3 compiles without any warnings
- 2 no memory leaks
The maxmimum number of points is 100.
Handin instructions
The provided Makefile includes functionality to build your final handin file. Issue the following command from your working directory:
linux make handin
The output is the file ../proxylab-handin.tar, which you can then handin.
To submit your lab, please upload your proxylab-handin.tar file to the appropriate assignment page on Learning Suite.
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