Cyllenian

A minimalist HTTPS server that I built to better understand HTTP, SSL, and the POSIX socket API. It proved to be a highly educational way to begin exploring low level networking concepts.

Tech Stack

Category	Technology Used
Language	C
Compiler	GCC
Build System	GNU Make
Libraries	OpenSSL
Operating System	Linux
License	GNU General Public License V2

Features

Supported HTTP Methods:

HEAD
GET

Supported HTTP Response Codes: 200 OK, 403 Forbidden, 404 Not Found, and 405 Method Not Allowed
Concurrency via fork() for incoming connections
Graceful shutdown on SIGINT
Static File Serving

Challenges & Solutions

Handling binary files: I initially stored binary file buffers as signed char arrays, which resulted in overflow due to the data exceeding the signed char range. Switching to unsigned fixed this.
Address already in use: I encountered this error when terminating and re-launching the server in quick succession. The solution was to implement signal handling to ensure that the socket is never left open on shutdown.

Testing

The project was tested manually on each build by ensuring the following:

CSS and images were rendering properly
JavaScript executes as expected
Requests for nonexistent pages return a 404 response
Requests containing traversal patterns return a 403 response
PUT requests return a 405 response
Rapid restarts don't result in "Address already in use" error

Lessons Learned

HTTP is a text-based protocol, so parsing and constructing headers is simple string manipulation.

The SSL structure and context must be freed in addition to closing the socket file descriptor. Since the user may choose to terminate the program at any point, it was essential to learn how to implement signal handling for SIGINT.

In C, the range of a signed char is -128 to 127 while the range of an unsigned char is 0 to 255 because the former uses the most significant bit for the sign. Binary data can use any value within the 8-bit range, so it is essential to use an unsigned char array when storing binary data in a buffer.

It is easy to run into corruption when trying to send binary data and text together for a variety of reasons, the potential for the data to contain a null byte being chief among them, so sending the HTTP header and body separately is ideal.

Another protocol family supported by the POSIX socket API, PF_UNIX, is widely used on POSIX-like operating systems for inter-process communication.

Future Plans

While I consider the project to be mostly feature complete, I plan to add integration testing and epoll support in the future.