Lightweight Internet communications with the simple Gemini Protocol


© Lead Image © Nelli Valova,

© Lead Image © Nelli Valova,

Article from Issue 261/2022

Create Gemini pages to show sensor data or control a Raspberry Pi rover.

The Gemini Protocol [1] is a relatively new Internet communication protocol for accessing remote documents. Unlike an HTML web page that contain layers of style sheets, JavaScript, and HTML tags, a Gemini document is a simple readable document.

Gemini's Gemtext format is easy to learn, requiring about five codes. Children or new coders could easily put together a small documentation server or custom application server without many programming skills.

In this article, I introduce the Gemini protocol with three simple projects. The first project creates a Gemini server and client with just one line of Bash code. In the second project, a Bash script creates a common gateway interface (CGI) page that connects to a sensor to show temperature and humidity data. The third project uses Gemini document links to control Raspberry Pi general purpose input/output (GPIO) pins to drive a rover.

Getting Started

Any text editor can be used to create Gemini documents. The Gemtext format is a little bit like a slimmed down version of the Markdown syntax. The default file extension for a Gemini document is .gmi.

The example file in Listing 1 (page1.gmi) shows all of the Gemtext format codes, including headings (three levels), document links, block quotes, lists, and preformatted text. I'll look a little later into the use of preformatted text for ASCII art and output from Bash commands.

Listing 1

Gemini Document

# Heading level 1 (H1)
## Heading level 2 (H2)
### Heading level 3 (H3)
=> testpage.gmi A link to another page.
> This line will show as a block-quote.
A list of items
* This is the first list item.
* This is another list item.
Code or ASCII Block
   _   ___  ___ ___ ___   _____       _
  /_\ / __|/ __|_ _|_ _| |_   _|__ __| |_
 / _ \\__ \ (__ | | | |    | |/ -_|_-<  _|
/_/ \_\___/\___|___|___|   |_|\___/__/\__|

A good selection of Gemini client applications are available, such as the Lagrange [2] desktop GUI client that works with both local files and Gemini network links. Figure 1 shows the example file (file://page1.gmi) accessed locally. Note that differences between the source file and the Gemini presentation are minimal.

Figure 1: Example document in a Gemini client.

Content Type

The content type is used by browsers and applications to determine how to manage the requested file. Typically, the content type is managed by the application server (e.g., a web server will send the "HTTP/1.0 200 OK" status code before sending an HTML file).

For the Gemini protocol, the content type is: "20 text/gemini". Depending on the Gemini server and the file extension of the requested file, the user might have to add the content type manually. (More about this when I look at Bash and CGI servers.)

Simple Bash Gemini Servers and Clients

For basic testing, a one-line Bash statement can be used for either a Gemini server or a client. The Gemini protocol typically uses the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) encryption, so the Bash ncat utility is needed. (Note: the simpler nc command does not support SSL.)

The Bash code to serve up the earlier single Gemini page would be:

while true; do { echo -ne "20 text/gemini\r\n"; cat page1.gmi; } | ncat -l -p 1965 --ssl; done

This Bash statement echos the Gemini content type ("20 text/gemini") and lists (cat) the example file when a request is received. The ncat utility listens (-l option) on port 1965 (the default Gemini port) for incoming requests.

The Bash script to connect to a Gemini server is:

echo "gemini://" | ncat --ssl 1965

Gemini clients start by sending "gemini://<requested_ip_address>". This string is echoed to ncat with SSL enabled (--ssl) and the Gemini server's IP and port defined. This Bash client statement will output the Gemini page to the screen.

The at scheduling utility could be used with the Bash client statement to save a Gemini page at a specific time, for example, at 11:20am:

at 1120 $(echo "gemini://" | ncat --ssl 1965 > page1120.gmi) &

One benefit of one-line Bash utilities is that you can easily configure your own custom network applications without the need to load back-end FTP or web servers. The downside of the ncat utility is that it could be a security issue if it is used improperly.

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