samples: net: add basic echo server sample using UDP

Add an echo server sample to test UDP networking functionality.

Author: mrodgers-witekio

dt-rust.yaml

@@ -45,6 +45,7 @@
       - "nordic,nrf52-flash-controller"
       - "nordic,nrf51-flash-controller"
       - "raspberrypi,pico-flash-controller"
+      - "st,stm32-flash-controller"
       level: 0
   actions:
   - type: instance

samples/net/echo_server/CMakeLists.txt

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(echo_server)
+
+rust_cargo_application()

samples/net/echo_server/Cargo.toml

@@ -0,0 +1,21 @@
+# Copyright (c) 2025 Witekio
+# SPDX-License-Identifier: Apache-2.0
+
+[package]
+# This must be rustapp for now.
+name = "rustapp"
+version = "0.1.0"
+edition = "2021"
+description = "Echo's back any data received from a UDP network socket to the sender"
+license = "Apache-2.0 OR MIT"
+
+[lib]
+crate-type = ["staticlib"]
+
+[dependencies]
+zephyr = "0.1.0"
+log = "0.4.22"
+static_cell = "2.1.0"
+
+[build-dependencies]
+zephyr-build = "0.1.0"

samples/net/echo_server/boards/qemu_cortex_m3.conf

@@ -0,0 +1 @@
+CONFIG_TEST_RANDOM_GENERATOR=y

samples/net/echo_server/build.rs

@@ -0,0 +1,9 @@
+fn main() {
+    // This call will make make config entries available in the code for every device tree node, to
+    // allow conditional compilation based on whether it is present in the device tree.
+    // For example, it will be possible to have:
+    // ```rust
+    // #[cfg(dt = "aliases::led0")]
+    // ```
+    zephyr_build::dt_cfgs();
+}

samples/net/echo_server/prj.conf

@@ -0,0 +1,21 @@
+CONFIG_RUST=y
+CONFIG_MAIN_STACK_SIZE=2048
+
+# Networking config
+CONFIG_NETWORKING=y
+CONFIG_NET_IPV4=y
+CONFIG_NET_IPV6=y
+CONFIG_NET_UDP=y
+CONFIG_NET_SOCKETS=y
+
+# Net config settings for sample
+CONFIG_NET_CONFIG_SETTINGS=y
+CONFIG_NET_CONFIG_NEED_IPV6=y
+CONFIG_NET_CONFIG_MY_IPV6_ADDR="2001:db8::1"
+CONFIG_NET_CONFIG_PEER_IPV6_ADDR="2001:db8::2"
+CONFIG_NET_CONFIG_NEED_IPV4=y
+CONFIG_NET_CONFIG_MY_IPV4_ADDR="192.0.2.1"
+CONFIG_NET_CONFIG_PEER_IPV4_ADDR="192.0.2.2"
+
+CONFIG_SHELL=y
+CONFIG_NET_SHELL=y

samples/net/echo_server/sample.yaml

@@ -0,0 +1,15 @@
+# See doc/develop/test/twister.rst for what is here.
+sample:
+  name: Echo Server Sample
+common:
+  filter: CONFIG_RUST_SUPPORTED
+  platform_allow:
+    - qemu_cortex_m3
+tests:
+  sample.rust.net.echo_server:
+    tags:
+      - net
+    depends_on: netif
+    harness: net
+    integration_platforms:
+      - qemu_cortex_m3

samples/net/echo_server/src/lib.rs

@@ -0,0 +1,49 @@
+// Copyright (c) 2025 Witekio
+// SPDX-License-Identifier: Apache-2.0
+
+#![no_std]
+#![allow(unexpected_cfgs)]
+
+use log::{error, info, warn};
+use static_cell::ConstStaticCell;
+use zephyr::error::Result;
+use zephyr::net::udp::UdpSocket;
+
+#[no_mangle]
+extern "C" fn rust_main() {
+    unsafe {
+        zephyr::set_logger().unwrap();
+    }
+
+    let res = run_echo();
+
+    if let Err(e) = res {
+        error!("Echo server terminated with error {}", e);
+    }
+}
+
+fn run_echo() -> Result<()> {
+    // Don't allocate the large RX buffer on the stack
+    static RX_BUF: ConstStaticCell<[u8; 2048]> = ConstStaticCell::new([0; 2048]);
+    let rx_buf = RX_BUF.take();
+
+    let sockaddr = "0.0.0.0:4242".parse().unwrap();
+    let sock = UdpSocket::bind(&sockaddr)?;
+
+    info!("Waiting for UDP packets on port 4242");
+
+    loop {
+        let (n, peer) = sock.recv_from(rx_buf)?;
+
+        // Note that being able to set the MSG_TRUNC sockopt is not implemented yet so it should
+        // not be possible to get n > rx_buf.len(), but it's probably still worth including the
+        // check for when this sockopt is implemented.
+        let n_trunc = n.min(rx_buf.len());
+        if n != n_trunc {
+            warn!("Data truncated, got {} / {} bytes", n_trunc, n);
+        }
+
+        info!("Echoing {} bytes back to peer address {:?}", n_trunc, peer);
+        let _ = sock.send_to(&rx_buf[0..n_trunc], &peer)?;
+    }
+}