[2022] CS3214 Computer Systems - Project 4: Personal Web and Video Server
CS3214 Spring 2022 Project 4 - “Personal Web and Video Server”
This project must be done in groups of 2 students. Please read the syllabus for instruc- tions, deadlines, penalties, and accommodations regarding group formation and management.
1 Introduction
This assignment introduces you to the principles of internetwork communication using the HTTP and TCP protocols, which form two of the most widely used protocols in to- day’s Internet.
In addition, the assignment will introduce you to existing methods for securely representing claims between parties, using the example of JSON Web Tokens as described in RFC 7519 [2].
Last but not least, it will provide an example of how to implement a concurrent server that can handle multiple clients simultaneously.
2 Functionality
The goal of the project is to build a small personal web server that can serve files, stream MP4 video, and provides a simple token-based authentication API.
The web server should implement persistent connections as per the HTTP/1.1 protocol. HTTP/1.1 is specified in a series of “request for comments” (RFC) standards documents (RFC 7230-7237), though the earlier RFC 2616 [1] provides a shorter read.
You should use code we provide as a base from which to start. To that end, fork the repository at https://git.cs.vt.edu/cs3214-staff/pserv. Be sure to set your fork to be private!
2.1 Serving Files
Your web server should, like a traditional web server, support serving files from a direc- tory (the ’server root’) in the server’s file system. These files should appear under the root (/) URL. For instance, if the URL /private/secure.html is visited and the root directory is set to a directory $DIR that contains the directory private, the content of the file $DIR/private/secure.html should be served. You should return appropri- ate content type headers, based on the served file’s suffix. Support at least .html, .js, .css, and .mp4 files; see /etc/mime.types for a complete list.
Make sure that you do not accidentally expose other files by ensuring that the request url’s path does not contain .. (two adjacent periods), such as /public/../../../../../etc/passwd. You should return appropriate error codes for requests to URLs you do not support.
2.2 Authentication
You must, at a minimum, support a single user that will authenticate with a username and password. If the user is authenticated, they should have access to the secure portion of your server, which are all files located under /private. Otherwise, such access should be denied.
Your server should implement the entry point /api/login as follows:
• When used as the target of a POST request, the body of the request must contain
{"username":"user0","password":"thepassword"}
where ‘user0‘ is the name of the user and ‘thepassword‘ is their password. If the password is correct, your server should respond with a JSON object that describes claims that the client can later use to prove it has successfully authenticated.
Send (at least) the following claims: (a) sub - to describe the subject (the principal as which the server will recognize the bearer of the claim), (b) iat - the time at which the claim was issued, in seconds since Jan 1, 1970, and (c) exp - the time at which the claim will expire.
For example, a claim may look like this:
{"exp":1523737086,"iat":1523650686,"sub":"user0"}
Returning the claims in the response, however, is not sufficient. The client must also obtain a signature from the server that certifies that the server issued the to- ken (i.e., that the user’s password was correct and thus the user has successfully authenticated).
This signature is obtained in the form of a JSON Web Token, which the server should return as a cookie to the client. You may choose an appropriate signing mechanism (either HMAC or using a private/public key pair using RSA, suggested is HMAC). You may use the jansson and libjwt libraries which are installed as part of the pro- vided code. Check out the files jwt demo hs256.c and jwt demo rs256.c for examples.
See MDN for documentation on the format of the Set-Cookie header which you must follow. Make sure to set the cookie’s path to / so that the cookie is sent along for all URIs. You may choose a suitable cookie name such as auth token.
• If the username/password does not match, your server should return 403 Forbid- den.
When used in a GET request, /api/login should return the claims the client pre- sented in its request as a JSON object if the user is authenticated, or an empty object {} if not.
Be sure to validate tokens before deciding whether the client is authenticated or not; do not accept tokens that have expired or whose signature does not validate.
You should implement this without storing information about which cookies your server has issued server-side, but rather simply by validating the token the client presents.
The type of “stateless authentication” can be used to provide a simple, yet scal- able form of authentication. Unlike in traditional schemes in which the server must maintain a session store to remember past actions by a client, the presented token contains proof of past authentication, and thus the server can directly proceed in handling the request if it can validate the token. Moreover, this way of securely presenting claims allows authentication servers that are separate from the servers that provide a resource or service: for instance, if you log onto a website via Google or Facebook, their authentication server will present a signed token to you which you can later use to prove to a third server that Google or Facebook successfully authenticated you.
However, such stateless authentication also has drawbacks: revoking a user’s ac- cess can be more difficult since a token, once issued, cannot be taken away. Thus, the server either has to keep revocation lists (in which case a session-like function- ality must be implemented), or keep token expiration times short (requiring more frequent reauthentication or a token refresh scheme), or by changing the server’s key (which invalidates all tokens for all users). For this assignment, you do not need to implement revocation.
We recommend you read the Introduction to JSON Web Tokens tutorial by Auth0. Note that JSON Web Tokens are not the only technology that make uses of crypto- graphically signed tokens. Others include PASETO and IRON Session.
2.3 Supporting HTML5 Fallback
Modern web applications exploit the History API, which is a feature by which JavaScript code in the client can change the URL that’s displayed in the address bar, making it ap- pear to the user that they have navigated to a new URL when in fact all changes to the page were driven by JavaScript code that was originally loaded. This is also known as “client-side routing,” see React Router for how this is accomplished in the popular Re- act.js framework.
When a URL that was modified in this way is bookmarked and later retrieved, or if the user refreshes the page while the modified URL is displayed, a request with this URL will be sent to the server, but it does not correspond to an existing server resource. In this case, the server should be programmed to return a “fallback” resource rather than 404. When your server is run with the -a flag it should return this fallback resource, specifically the file index.html in its root directory. (As a sidenote, this ability is provided by the nginx server using the try files directive.)
When HTML5 fallback is enabled, your server should return index.html in its root di- rectory also for requests to /. Otherwise, you may select a suitable response for requests to other, existing directories. (Some servers disallow such accesses, others can be config- ured to return an HTML page with a listing of the files in a directory.)
2.4 Streaming MP4
To support MP4 streaming, your server should advertise that it can handle Range re- quests to transfer only part (a byte range) of a file. You should send an appropriate Accept-Ranges header and your server should interpret Range headers sent by a client.
To support a basic streaming server, it is sufficient to support only single-range requests such as Range: bytes=203232- or Range: bytes=500-700. Be sure to return an appropriate Content-Range header. Browsers will typically close a connection (and create a new one) if the user forwards or rewinds to a different point in the stream.
To let clients learn about which videos are available for streaming, your server should support an entry point /api/video. GET requests to this entry point should return a JSON object that is a list of videos that can be served, in the following format:
1 [
2 {
3 "size": 1659601458,
4 "name": "LectureVirtualMemory.mp4"
5 }, {
6 "size": 961734828,
7 "name": "Boggle.mp4"
8 }, {
9 "size": 1312962263,
10 "name": "OptimizingLocking.mp4"
11 }, {
12 "size": 423958714,
13 "name": "DemoFork.mp4"
14 } ]
Use the opendir(3) and readdir(3) calls to list all files in the server’s root directory (or a subdirectory, at your choosing), selecting those that carry the suffix .mp4. Use the stat(2) system call to find the size of each file.
2.5 Multiple Client Support
For all of the above services, your implementation should support multiple clients simultaneously. This means that it must be able to accept new clients and process HTTP requests even while HTTP transactions with already accepted clients are still in progress. You must use a single-process approach, either using multiple threads, or using an event-
If using a thread-based approach, it is up to you whether you spawn new threads for every client, or use a thread pool. You may modify or reuse parts of your thread pool
To test that your implementation supports multiple clients correctly, we will connect to your server, then delay the sending of the HTTP request. While your server has accepted one client and is waiting for the first HTTP request by that client, it must be ready to accept and serve additional clients. Your server may impose a reasonable limit on the number of clients it simultaneously serves in this way.
2.6 Robustness
Network servers are designed for long running use. As such, they must be programmed in a manner that is robust, even when individual clients send ill-formed requests, crash, delay responses, or violate the HTTP protocol specification in other ways. No error in- curred while handling one client’s request should impede your server’s ability to accept and handle future clients.
This semester we will be using a research prototype of a new fuzzing software to test your server software. Instructions for how to do this will be separately provided.
2.7 Performance and Scalability
We will benchmark your service to figure out the maximum number of clients and rate of requests it can support. Note that for your server to be benchmarked, it must obtain a full score in the robustness category first. We will publish a script to benchmark your server. A scoreboard will be posted to compare your results with the rest of the class.
2.8 Protocol Independence
The Internet has been undergoing a transition from IPv4 to IPv6 over the last 2.5 decades. To see a current data point, Google publishes current statistics on the number of users that use IPv6 to access Google’s services. This transition is spurred by the exhaustion of the IPv4 address space as well as by political mandates.
Since IPv4 addresses can be used to communicate only between IPv4-enabled applica- tions, and since IPv6 addresses can be used to communicate only between IPv6-enabled applications, applications need to be designed to support both protocols and addresses, using whichever is appropriate for a particular connection. For a TCP/UDP server, this requires accepting connections both via IPv6 as well as via IPv4, depending on which ver- sions are available on a particular system. For a TCP/UDP client, this requires to identify the addresses at which a particular server can be reached, and try them in order. Typi- cally, if a server is reachable via both IPv4 and IPv6, the IPv6 address is tried first, falling back to the IPv4 address if that fails.
Ensuring protocol independence requires avoiding any dependence on a specific protocol in your code. Fortunately, the socket API was designed to support multiple protocols from the beginning as its designers foresaw that protocols and addressing mechanisms would evolve. For instance, the bind() and connect() calls refer to the addresses passedusingthetypestruct sockaddr *whichisanopaquetypethatcouldreferto either a IPv4 or IPv6 address.
To implement protocol independence, you need to avoid any dependence on a particular address family. Accordingly, you should use the getaddrinfo(3) or getnameinfo(3) functions to translate from symbolic names to addresses and vice versa and you should avoid the outdated functions gethostbyname(3), getaddrbyname(3), or inet ntoa(3) or inet ntop(3).
Tutorials on how to write protocol independent network code are given in this resource and in the code for the textbook’s 3rd edition. However, neither tutorial is fully correct and will require (minor) adaptations.
Ensuring that your server can accept both IPv4 and IPv6 clients can be implemented using two separate sockets, one bound to either family. Two separate threads can then be devoted to these sockets to accept clients that connect using either of the two protocol families.
However, the Linux kernel provides a convenience feature that provides a simpler facility for accepting both IPv6 and IPv4 clients. This so-called dual-bind feature allows a socket bound to an IPv6 socket to accept IPv4 clients. Linux activates this feature if /proc/sys/net/ipv6/bindv6only contains 0. You may assume in your code that dual bind is turned on.
Our starter code uses protocol independent functions, but it is tested with IPv4 only. Augmenting it to implement protocol independence is part of your assignment.
2.9 Choice of Port Numbers
Port numbers are shared among all processes on a machine. To reduce the potential for conflicts, use a port number that is 10, 000 + last four digits of the student id of a team member.
If a port number is already in use, bind() will fail with EADDRINUSE. If you weren’t using that port number before, someone else might have. Choose a different port num- ber in that case. Otherwise, and more frequently, it may be that the port number is still in use because of your testing. Check that you have killed all processes you may have started on the machine you are working on while testing. Even after you have killed your processes, binding to a port number may fail for an additional 2 min period if that port number recently accepted clients. This timeout is built into the TCP protocol to avoid mistaking delayed packets sent on old connections for packets that belong to new con- nections using the same port number. To override that, you may use setsockopt() with the SO REUSEADDR flag to allow address reuse.
3 Strategy
Make sure you understand the roles of DNS host names, IP addresses, and port numbers in the context of TCP communication. Study the roles of the necessary socket API calls.
Since you may be using a multi-threaded design, use thread-safe versions of all functions.
Familiarize yourselves with the commands wget(1) and curl(1) and the specific flags that show you headers and protocol versions. These programs can be extremely helpful in debugging web servers.
Refresh your knowledge of strace(1), which is an essential tool to debug your server’s interactions with the outside world. Use -s 1024 to avoid cutting off the contents of reads and writes (or recv and send calls). Don’t forget -f to allow strace to follow spawned threads.
