/C-Web-Server

A simple webserver written in C

Primary LanguageC

A Simple Web Server in C

Networking Protocols

In networking, each layer adds a wrapper around the data with more information about data integrity or routing or another information.

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
+-----------------+
| Your Data       |  Whatever you need to send
|                 |
|                 |
+-----------------+

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 router, 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

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.

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.

HTTP

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, a fortunately 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

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.

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).

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.

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.

Main Goals

  1. Add parsing of the first line of the HTTP request header that arrives in the handle_http_request() function. It will be in the request array.

    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. Hint: strcmp(). Another hint: strcmp() returns `0 if the strings are the same!

    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(). 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(). Hint: sprintf(), 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: fopen(), fwrite(), fclose().

    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?