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Creating a Basic Gemini Server

I learned about the Gemini protocol last week. I don't know why, but the idea seemed very appealing to me. I like the idea of a simpler network where you don't feel like you're being tracked and monitored all of the time.

I did some looking around at the various servers that were available. Some of them seemed more full featured and configurable. Others were bare bones. Since my installation was going to run inside my own home network there were a few considerations:

I wanted to be able to verify the code myself
Generally, I want it to seem secure
I didn't want any CGI or scripting

Those might seem like odd considerations, but I really just wanted an extremely simple server that can run Gemini.

Implementation Notes

https://github.com/praetoriansentry/melchior

Based on what I wanted, I figured it was just easier to make my own server. Part of what's so awesome about the Gemini protocol is how simple it is. I feel like the simplicity of the protocol will encourage innovation and development.

In the case of a simple static Gemini server, there is very little that needs to be implemented. Basically comes down to:

Security
URL Parsing
Error Handing
Mime Types
Responses

The nice part about using Go is a lot can be borrowed from the standard libraries.

In my own deployment, there isn't a huge risk really. I guess if an attacker was able to totally own the box that's running Gemini, it could be a problem, but I have other controls to manage that. Even still, I'd like to minimize the risk of buffer over flows, directory traversal, remote code exec, remote/local file inclusion etc. By default Go helps with memory safety and had some decent control and libraries to help with some of the other risks. There are two main things that I used to help: ~net/http.Dir~ and ~path/filepath.Clean~.

https://golang.org/pkg/net/http/#Dir

https://golang.org/pkg/path/filepath/#Clean

The ~Dir~ type within the ~http~ package is almost like a chroot. It's able to open files at a certain directory. I had never used this type before, but I did some poking around within the go library to see how they implement the static file server, and they used this approach. It's a little odd that it's part of the HTTP library to be honest, but as far as I can tell, this particular type can be used for many purposes.

The other function I used was the ~Clean~ function in ~filepath~. I had never used this before either. I believe the combination of this method along with ~Dir~ should help with any risk of directory traversal. But, it's hard to say for sure. My own basic tests seem to look okay.

Here is an example from the Go HTTP library where they use ~path.Clean~:

func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
	upath := r.URL.Path
	if !strings.HasPrefix(upath, "/") {
		upath = "/" + upath
		r.URL.Path = upath
	}
	serveFile(w, r, f.root, path.Clean(upath), true)
}

URL parsing for my server is handled entirely by Golang. I didn't really have to do anything special:

https://golang.org/pkg/net/url/

In terms of detecting the content type and setting a response type header in Gemini, I ended up using another Go HTTP function.

DetectContentType

This approach isn't perfect, but it will work well enough for me since I don't plan on doing anything fancy.

I think with these few basic functions, we can hack together a pretty simple but secure (finger's crossed) server in very few lines of code.