🐣 so that's how multicast groups work

.gitignore

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+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "anyhow"
+version = "1.0.48"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "62e1f47f7dc0422027a4e370dd4548d4d66b26782e513e98dca1e689e058a80e"
+
+[[package]]
+name = "lookaround"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+]

Cargo.toml

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+[package]
+name = "lookaround"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+anyhow = "1.0.48"

src/main.rs

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+use std::{
+	env,
+	net::{
+		Ipv4Addr,
+		UdpSocket,
+	},
+	str::FromStr,
+};
+
+use anyhow::{
+	self,
+	Result,
+	bail,
+};
+
+fn main () -> Result <()> {
+	let mut args = env::args ();
+	
+	let _exe_name = args.next ();
+	
+	match args.next ().as_ref ().map (|s| &s[..]) {
+		None => bail! ("First argument must be a subcommand"),
+		Some ("client") => client ()?,
+		Some ("server") => server ()?,
+		Some (x) => bail! ("Unknown subcommand {}", x),
+	}
+	
+	Ok (())
+}
+
+fn client () -> Result <()> {
+	let socket = UdpSocket::bind ("0.0.0.0:9041").unwrap ();
+	
+	socket.join_multicast_v4 (&(Ipv4Addr::from_str ("225.100.99.98").unwrap ()), &([0u8, 0, 0, 0].into ())).unwrap ();
+	
+	socket.send_to ("hi there".as_bytes (), ("225.100.99.98", 9040)).unwrap ();
+	
+	Ok (())
+}
+
+fn server () -> Result <()> {
+	let socket = UdpSocket::bind ("0.0.0.0:9040").unwrap ();
+	
+	socket.join_multicast_v4 (&(Ipv4Addr::from_str ("225.100.99.98").unwrap ()), &([0u8, 0, 0, 0].into ())).unwrap ();
+	
+	let mut buf = vec! [0u8; 4096];
+	let (bytes_recved, remote_addr) = socket.recv_from (&mut buf).unwrap ();
+	
+	dbg! (bytes_recved, remote_addr);
+	
+	Ok (())
+}