Quote of the Day: The easiest protocol

I recently found myself browsing the List of TCP and UDP Port Numbers on Wikipedia. Knowing what a few ports are reserved or commonly used for (such as 80 for HTTP, 443 for HTTPS, and 22 for SSH), I was wondering what the other ~1000 well-known ports are used for. There are a lot of interesting protocols listed on that page, but one that quickly jumped out to me was the Quote of the Day (QOTD) protocol, which uses port 17.

The Wikipedia page for QOTD is short and sweet and it made me wonder how quickly I could implement my own QOTD server program based on RFC 865, which standardized the protocol, in Rust. The RFC is delightfully short and easy to read, so I’ll quote it here in its entirety:

Quote of the Day Protocol

This RFC specifies a standard for the ARPA Internet community. Hosts on the ARPA Internet that choose to implement a Quote of the Day Protocol are expected to adopt and implement this standard.

A useful debugging and measurement tool is a quote of the day service. A quote of the day service simply sends a short message without regard to the input.

TCP Based Character Generator Service

One quote of the day service is defined as a connection based application on TCP. A server listens for TCP connections on TCP port 17. Once a connection is established a short message is sent out the connection (and any data received is thrown away). The service closes the connection after sending the quote.

UDP Based Character Generator Service

Another quote of the day service is defined as a datagram based application on UDP. A server listens for UDP datagrams on UDP port 17. When a datagram is received, an answering datagram is sent containing a quote (the data in the received datagram is ignored).

Quote Syntax

There is no specific syntax for the quote. It is recommended that it be limited to the ASCII printing characters, space, carriage return, and line feed. The quote may be just one or up to several lines, but it should be less than 512 characters.

So I used cargo new to start a new Rust project, opened up main.rs in my editor and the Rust standard library documentation in my browser, and started looking at the std::net::TcpListener struct documentation to learn how to do basic TCP networking in Rust. After copying the example program on that page and adding a writeln! call to the handle_client function, I realized I had created a fully compliant QOTD server:

use std::{
    io::Write,
    net::{TcpListener, TcpStream},
};

fn handle_client(mut stream: TcpStream) -> std::io::Result<()> {
    writeln!(&mut stream, "Heyo!")
}

fn main() -> std::io::Result<()> {
    let listener = TcpListener::bind("0.0.0.0:17")?;
    for stream in listener.incoming() {
        handle_client(stream?)?;
    }
    Ok(())
}

Let’s review the RFC and make a checklist of what a QOTD service needs:

• A TCP or UDP service listening on port 17
• For TCP, once a connection is established, a short message is sent out and the service closes the connection
• The quote is less than 512 characters

That’s it! It never says the quote actually has to change daily, or be a quote of anything at all; just that it’s a “short message” sent on port 17. “Heyo!” doesn’t communicate much, but I’d call it a short message.

I can run this server with cargo run and communicate with it using nc and get this deeply impactful quote whenever I desire:

$ cargo run &
$ nc localhost 17
Heyo!