A Simple Web Server in C

In this project, we'll finish the implementation of a web server in C.

What's already there:

• Skeleton code that handles all the network communication
• The main loop in main()
• Skeleton endpoint handler calls functions

What you need to write:

• Code that parses HTTP requests
• Code that builds HTTP responses
• Your code will interface with the existing code

What you don't need to write:

• Any system calls, including send() and recv()
• Any new functions from scratch--there's a skeleton for all functions you'll need

What is a Web Server?

A web server is a piece of software that accepts HTTP requests (commonly GET requests for HTML pages), and returns responses (commonly HTML pages). Other common uses are GET requests for images within web pages, and POST requests to upload data to the server (e.g. a form submission or file upload).

General Information about Networking

Before learning about the web server, let's take a look at some general information about how networking works. Some of these terms will be familiar to you, and we'll expand their definitions a bit.

Networking Protocols

This is background information. You will not need to use this directly in the web server.

A protocol is an agreement between two programs about how they will communicate. For the Internet, most protocols take the form of "If you send me x, I'll send you back y." Internet-related protocols are clearly written down in specifications, known as an RFC.

When you send some data out on the network, that data is wrapped up in several layers of additional data that provide information about data integrity, routing, and so on.

At the highest level, you have your data that you want to transmit. As it is prepared for transmission on the network, the data is encapsulated in other data to help it arrive at its destination. Any particular piece of data will be wrapped, partially unwrapped, and re-wrapped as it moves from wire to wire across the Internet to its destination.

The act of wrapping data puts a new header on the data. This header encapsulates the original data, and all the headers that have been added before it.

Here is an example of a fully-encapsulated HTTP data packet.

+-----------------+
| Ethernet Header |  Deals with routing on the LAN
+-----------------+
| IP Header       |  Deals with routing on the Internet
+-----------------+
| TCP Header      |  Deals with data integrity
+-----------------+
| HTTP Header     |  Deals with web data
+-----------------+
| <h1>Hello, worl |  Whatever you need to send
| d!</h1>         |
|                 |
+-----------------+

The details of what data exists in each header type is beyond the scope of what most people need to know. It is enough to know the short description of what each does.

As the data leaves your LAN and heads out in the world, the Ethernet header will be stripped off, the IP header will be examined to see how the data should be routed, and another header for potentially a different protocol will be put on to send the traffic over DSL, a cable modem, or fiber.

The Ethernet header is created and managed by the network drivers in the OS.

Sockets

This is background information. You will not need to use this directly in the web server. This code is written for you.

Under Unix-like operating systems, the sockets API is the one used to send Internet traffic. It supports both the TCP and UDP protocols, and IPv4 and IPv6.

The sockets API gives access to the IP and TCP layers in the diagram above.

A socket descriptor is a number used by the OS to keep track of open connections. It is used to send and receive data. In our web server, this variable is called fd.

You can create a new socket (socket descriptor) with the socket() system call.

Once created you still have to bind it to a particular IP address (which the OS associates with a particular network card). This is done with the bind() system call.

Once bound, you can read and write data to the socket using the recv() and send() system calls.

• See also Beej's Guide to Network Programming

HTTP

In the webserver, you will be writing code that parses down strings that hold HTTP requests, and builds strings that hold HTTP responses. Study what an HTTP request and response look like.

The final piece of information needed for web traffic is the HyperText Transport Protocol (HTTP). While TCP deals with general data integrity and IP deals with routine, HTTP is concerned with GET and POST requests of web data.

Like the other stages of networking, HTTP adds a header before the data it wants to send with the packet. Like IP and TCP, this header has a well-defined specification for exactly what needs to be sent.

Though the specification is complex, fortunately only a small amount of information is needed to implement a barebones version.

For each HTTP request from a client, the server sends back an HTTP response.

Here is an example HTTP GET request and response using version 1.1 of the HTTP protocol getting the page http://lambdaschool.com/example:

GET /example HTTP/1.1
Host: lambdaschool.com

And here is a sample HTTP response:

HTTP/1.1 200 OK
Date: Wed Dec 20 13:05:11 PST 2017
Connection: close
Content-Length: 41749
Content-Type: text/html

<!DOCTYPE html><html><head><title>Lambda School ...

The end of the header on both the request and response is marked by a blank line (i.e. two newlines in a row).

If the file is not found, a 404 response is generated and returned by the server:

HTTP/1.1 404 NOT FOUND
Date: Wed Dec 20 13:05:11 PST 2017
Connection: close
Content-Length: 13
Content-Type: text/plain

404 Not Found

If you've ever looked in the Network panel of your web browser's debugger, some of these headers might look familiar.

Important things to note:

• For HTTP/1.1, the request must include the Host header.
• The second word of the first line of the response gives you a success or failure indicator.
• Content-Length gives the length of the request or response body, not counting the blank line between the header and the body.
• Content-Type gives you the MIME type of the content in the body. This is how your web browser knows to display a page as plain text, as HTML, as a GIF image, or anything else. They all have their own MIME types.
• Even if your request has no body, a blank line still must appear after the header.
• Connection: close tells the web browser that the TCP connection will be closed after this response. This should be included.
• The Date should be the date right now, but this field is optional.

Assignment

We will write a simple web server that returns data on three GET endpoints:

• http://localhost:3490/ should contain some HTML, e.g. <h1>Hello, world!</h1>.
• http://localhost:3490/d20 should return a random number between 1 and 20 inclusive as text/plain data.
• http://localhost:3490/date should print the current date and time in GMT as text/plain data.

Examine the skeleton source code for which pieces you'll need to implement.

Spend some time inventorying the code to see what is where. Write down notes. Write an outline. Note which functions call which other functions. Time spent up front doing this will reduce overall time spent down the road.

For the portions that are already written, study the well-commented code to see how it works.

Don't worry: the networking code is already written.

There is a Makefile provided. On the command line, type make to build the server.

Type ./server to run the server.

Main Goals

Read through all the main and stretch goals before writing any code to get an overall view, then come back to goal #1 and dig in.

1. Examine handle_http_request() in the file server.c.

   You'll want to parse the first line of the HTTP request header to see if this is a GET or POST request, and to see what the path is. You'll use this information to decide which handler function to call.

   The variable request in handle_http_request() holds the entire HTTP request once the recv() call returns.

   Read the three components from the first line of the HTTP header. Hint: sscanf().

   Right after that, call the appropriate handler based on the request type (GET, POST) and the path (/, /d20, etc.) You can start by just checking for / and add the others later as you get to them.

   The handler for GET / is get_root() (search for the skeleton code). The handler for GET /d20 is get_d20(), etc.

   Hint: strcmp() for matching the request method and path. Another hint: strcmp() returns 0 if the strings are the same!

   Note: you can't switch() on strings in C since it will compare the string pointer values instead of the string contents. You have to use an if-else block with strcmp() to get the job done.

   If you can't find an appropriate handler, call resp_404() instead to give them a "404 Not Found" response.

2. Implement the get_root() handler. This will call send_response().

   See above at the beginning of the assignment for what get_root() should pass to send_response().

   If you need a hint as to what the send_response() call should look like, check out the usage of it in resp_404(), just above there.

   The fd variable that is passed widely around to all the functions holds a file descriptor. It's just a number use to represent an open communications path. Usually they point to regular files on disk, but in the case it points to an open socket network connection. All of the code to create and use fd has been written already, but we still need to pass it around to the points it is used.

3. Implement send_response().

   This needs to build a complete HTTP response with the given parameters. It should write the response to the string in the response variable.

   The total length of the header and body should be stored in the response_length variable so that the send() call knows how many bytes to send out over the wire.

   See the HTTP section above for an example of an HTTP response and use that to build your own.

   Hint: sprintf() for creating the HTTP response. strlen() for computing content length. sprintf() also returns the total number of bytes in the result string, which might be helpful.

   The HTTP Content-Length header only includes the length of the body, not the header. But the response_length variable used by send() is the total length of both header and body.

4. Implement the get_d20() handler. Hint: srand() with time(NULL), rand().

5. Implement the get_date() handler. Hint: time(NULL), gmtime().

Stretch Goals

Post a file:

1. Implement find_end_of_header() to locate the end of the HTTP request header (and start of the body).

2. Implement the post_save() handler. Modify the main loop to pass the body into it. Have this handler write the file to disk. Hint: open(), write(), close(). fopen(), fwrite(), and fclose() variants can also be used, but the former three functions will be slightly more straightforward to use in this case.

   The response from post_save() should be of type application/json and should be {"status":"ok"}.

Concurrency:

Convert the web server to be multiprocessed by using the fork() system call.

1. Examine and understand the signal handler on SIGCHLD that watches for when child processes exit. (This is already written for you.)

2. Modify the main while loop to fork() a new child process to handle each request.

   Be careful not to fork-bomb your system to its knees!

   Your child process must call exit() or you will risk having piles of extra processes at work!

3. Modify the post_save() function to get an exclusive lock on the file using flock(). The lock should be unlocked once the file has been written.

   What happens if multiple processes try to write to the POSTed file at the same time without locking the file?