辅导CS3214留学生程序、讲解c/c++程序语言、c++编程调试

日期：2021-04-27 08:13

CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
Due Date: Check course website for due date.
This project should be done in groups of 2 students.
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 today’s
Internet.
In addition, the assignment will introduce you to emerging standards for securely representing
claims between parties, specifically 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 standards documents (RFC
7230-7237), though the earlier RFC 2616 [1] provides a shorter read.
You may 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 directory
(the ’server root’) in the server’s file system. These files should appear under the /
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 appropriate content
type headers, based on the served file’s suffix. Support at least .html, .js, and .css
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.
Created by G. Back (gback@cs.vt.edu) 1 April 14, 2021
CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
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 /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 claim in the response, however, is not sufficient. The client must also
obtain a signature from the server that certifies that the server issued the token (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). You may use the
jansson and libjwt libraries which are installed as part of the provided code. Check
out the files jwt demo hs256.c and jwt demo rs256.c for examples.
See MDN for documentation on the Set-Cookie header. 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 Forbidden.
? When used in a GET request, /api/login should return the claims the client presented
in its request 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.
Created by G. Back (gback@cs.vt.edu) 2 April 14, 2021
CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
You should implement this without storing state 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 scalable
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
provides the 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 access
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 functionality
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.
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 appear
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 React.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.)
2.4 Streaming MP4
To support MP4 streaming, your server should advertise that it can handle Range requests
to transfer only part (a byte range) of a file. You should send an appropriate
Created by G. Back (gback@cs.vt.edu) 3 April 14, 2021
CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
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. Note that browsers will typically sever a connection
(and create a new one) if the user forwards or rewinds to a different point in the
stream.
To 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:
[
{
"size": 1659601458,
"name": "LectureVirtualMemory.mp4"
},
{
"size": 961734828,
"name": "Boggle.mp4"
},
{
"size": 1312962263,
"name": "OptimizingLocking.mp4"
},
{
"size": 423958714,
"name": "DemoFork.mp4"
}
]
Use the opendir(3) and readdir(3) calls to list all files in the server’s root directory,
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 eventbased
approach.1
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
1For the purposes of this project, a multi-process approach is not acceptable.
Created by G. Back (gback@cs.vt.edu) 4 April 14, 2021
CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
thread pool implementation from project 2, if this is useful.2
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 incurred
while handling one client’s request should impede your server’s ability to accept and handle
future clients.
This semester we will be using the american fuzzy lop (AFL) fuzzer 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 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 applications,
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 to accept connections both via IPv6 as well as via IPv4, depending on which versions
are enabled on a particular system. For a TCP/UDP client, this requires to identify
2 Please note, however, that the fork-join thread pool was implemented with a different goal in mind
and that some aspects here do not apply to this project, notably the fork-join aspect. We recommend trying
a thread-based approach first.
Created by G. Back (gback@cs.vt.edu) 5 April 14, 2021
CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
the addresses at which a particular server can be reached, and try them in order. Typically,
if a server is reachable via both IPv4 and IPv6, the IPv6 address is tried first, then a
fallback onto the IPv4 address is performed.
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 passed
using the type struct sockaddr * which is an opaque type that could refer to 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).
A tutorial on how to write protocol independent network code is given in this resource
and in the code for the textbook’s 3rd edition. However, both tutorials are fully correct
and will required 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 dualbind
is turned on. 3
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 number
in that case. Otherwise, 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 while testing. Even
3
I should point out, however, that this will make your code Linux-specific; truly portable socket code
will need to resort to handling accepts on multiple sockets.
Created by G. Back (gback@cs.vt.edu) 6 April 14, 2021
CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
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 connections using the same port number. To prevent 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). Don’t forget -f to allow strace to follow spawned
threads. A trick to easily verify that your Content-Length computation is correct is to
issue the body of each HTTP response in a separate system call.
4 Grading
4.1 Coding Style
Your service must be implemented in the C language. You should follow proper coding
conventions with respect to documentation, naming, and scoping. You must check the
return values of all system calls and library functions.
Your code should compile under -Wall without warnings, the use of the -Werror flag
as part of CFLAGS should have become a habit by now, as is the use of git for revision
control.
4.2 Submission
You should submit a .tar.gz file of your project, which must contain a Makefile. Your
project should build with ‘make clean all’ This command must build an executable ’server’
that must accept the following command line arguments:
Created by G. Back (gback@cs.vt.edu) 7 April 14, 2021
CS3214 Spring 2021 Project 4 - “Personal Web and Video Server”
? -p port When given, your web service must start accepting HTTP clients and
serving HTTP requests on port ’port.’ Multiple connection must be supported.
? -R path When given, ‘path’ specifies the root directory of your server.
? -s Silent mode (for benchmarking). When given, your server should suppress any
output to standard output.
? -e sec Specify the expiration time for the issued JWT in seconds. Your server must
enforce this expiration time.
? -a HTML5 Fallback mode. When given, requests for non-existing resources should
be responded to as if the request had been for /index.html.
Please test that ‘make clean’ removes all executables and object files. Issue ‘make clean’
before submitting to keep the size of the tar ball small. Please use the submit.py script or
web page and submit as ’p4’. Only one group member need submit.
Further submission instructions are posted on the course website.
This project will count for 120 points.
Good Luck!
References
[1] Roy Fielding, Jim Gettys, Jeff Mogul, H. Frystyk, L. Masinter, P. Leach, and Tim
Berners-Lee. Rfc 2616: Hypertext transfer protocol – http/1.1. http://www.w3.org/-
Protocols/rfc2616/rfc2616.html.
[2] M. Jones, J. Bradley, and N. Sakimura. Json web token (jwt), 2015. RFC7519.
Created by G. Back (gback@cs.vt.edu) 8 April 14, 2021