Computer Networks Project 1

Files

• project is folder to develop codes for future projects.
• docker-compose.yaml and Dockerfile are files configuring the containers.

Environment

• OS: ubuntu 22.04
• IP: 192.168.10.225. NOT accessible from the host machine.
• Port forwarding: container 8080 <-> host 8080
  • Use http://localhost:8080 to access the HTTP server in the container.
• Files in this repo are in the /project folder. That means, server.c is /project/project/server.c in the container.

Project Report

Name: Justin Hu

High level Design:

The main function of my server has 2 simple parts, setup and loop. In the setup area, I create and bind the socket to be used (port 8080), create a list of all available files under current directory, and setup some premade strings to be used in generating HTTP responses. In the loop area, I accept a single new connection, extract desired filename, search directory, and construct my response.

Major Problems:

Everything up to receiving HTTP request was pretty straightforward, and using scandir to enumerate files wasn't as painful as I thought it would be. Case insensitivity wasn't very difficult either. At some point I was getting segfaults, so I used valgrind to clean up my code which took about an hour.

First major issue I faced was constructing the HTTP response. Somehow I got the idea that the entire HTTP response had to be within one buffer, which was a nightmare to allocate correctly and resulted in a lot of content length incorrect errors. I also didn't read that I needed an extra \r\n at the end of the HTTP header, which made my image files not work. Basically, I didn't understand HTTP as well as I should have.

After that, there was this weird issue of larger image file transmission somehow being cut short. I realized that removing close(socket) at the end of the loop completely solves the issue, and to now I still have no idea why that is the case. I tried removing the REUSEPORT option but that didn't change anything.

My last obstacle was dealing with escape and space characters in filename. This took me much more effort than I had initially imagined because I suppose I am not a proficient C programmer and dealing with string pointers was a nightmare. Thankfully I found a great reference on stack overflow that really helped me out, in particular the idea of using multiple string pointers just to help keep track of where I am in the original and new strings.

I never had to debug anything for case insensitivity, filetype, or large binary data. Everything just worked first try, which I suppose was nice. I only ever tested on html, txt, and jpg.

Code References:

https://beej.us/guide/bgnet/ Beej's guide got me started, learning all about the socket API and how to do the project basically.

https://stackoverflow.com/questions/779875/what-function-is-to-replace-a-substring-from-a-string-in-c Forum post about substring search and replacement

Also looked up quite a few linux man pages.