---
title: "Stream Tutorial: Build a Private Echo Server"
description: "Step-by-step Rust tutorial to build an echo server and client communicating through the Nym mixnet using AsyncRead and AsyncWrite streams."
schemaType: "HowTo"
section: "Developers"
lastUpdated: "2026-03-26"
---

# Tutorial: Build a Private Echo Server

import { Callout } from 'nextra/components'
import { CodeVerified } from '../../../../components/code-verified'

In this tutorial you'll build two programs: a server that listens for incoming streams and echoes back whatever it receives, and a client that opens a stream, sends data, and reads the echo. Both communicate through the Nym Mixnet using `AsyncRead` and `AsyncWrite`, just like TCP sockets.

## What you'll learn

- Setting up a `MixnetListener` to accept incoming streams
- Opening an outbound stream with `open_stream()`
- Reading and writing with standard tokio I/O traits
- How streams are multiplexed over a single `MixnetClient`
- Clean shutdown and stream lifecycle

<CodeVerified />

## Prerequisites

- Rust toolchain (1.70+)
- A working internet connection (clients connect to the live Nym Mixnet)

## Step 1: Set up the project

```sh
cargo init nym-echo
cd nym-echo
rm src/main.rs
```

Add dependencies to `Cargo.toml`:

```toml
[dependencies]
nym-sdk = { git = "https://github.com/nymtech/nym", rev = "97068b2" }
nym-bin-common = { git = "https://github.com/nymtech/nym", rev = "97068b2", features = ["basic_tracing"] }
tokio = { version = "1", features = ["full"] }
rand = "0.8"
blake3 = "=1.7.0"  # required pin — see https://nymtech.net/docs/developers/rust/importing
```

## Step 2: Build the echo server

The server connects a `MixnetClient`, creates a listener, and accepts streams in a loop. Each stream gets its own task that reads data and writes it back.

Create `src/bin/server.rs`:

```rust
use nym_sdk::mixnet;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

#[tokio::main]
async fn main() {
    nym_bin_common::logging::setup_tracing_logger();

    // Connect to the Mixnet
    let mut client = mixnet::MixnetClient::connect_new().await.unwrap();
    println!("Echo server listening at: {}", client.nym_address());

    // Create a listener — this activates stream mode.
    // From this point, message-based methods are disabled.
    let mut listener = client.listener().unwrap();

    // Accept streams in a loop
    loop {
        let mut stream = match listener.accept().await {
            Some(s) => s,
            None => {
                println!("Listener closed");
                break;
            }
        };

        let stream_id = stream.id();
        println!("Accepted stream {stream_id}");

        // Spawn a task to handle each stream concurrently
        tokio::spawn(async move {
            let mut buf = vec![0u8; 32_000];

            loop {
                let n = match stream.read(&mut buf).await {
                    Ok(0) => break,     // EOF — stream closed
                    Ok(n) => n,
                    Err(e) => {
                        eprintln!("Stream {stream_id} read error: {e}");
                        break;
                    }
                };

                let data = &buf[..n];
                println!("Stream {stream_id} received {n} bytes");

                // Echo it back
                if let Err(e) = stream.write_all(data).await {
                    eprintln!("Stream {stream_id} write error: {e}");
                    break;
                }
                stream.flush().await.unwrap();
            }

            println!("Stream {stream_id} closed");
        });
    }
}
```

<Callout type="info">
**`listener()` can only be called once per client.** It takes exclusive ownership of the inbound message channel. A second call returns `Error::ListenerAlreadyTaken`.
</Callout>

## Step 3: Build the client

The client connects, opens a stream to the server, sends a few messages, reads back the echoes, and disconnects.

Create `src/bin/client.rs`:

```rust
use nym_sdk::mixnet::{self, Recipient};
use std::time::Duration;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

const TIMEOUT: Duration = Duration::from_secs(60);

#[tokio::main]
async fn main() {
    nym_bin_common::logging::setup_tracing_logger();

    // Read the server's Nym address from the command line
    let server_addr: Recipient = std::env::args()
        .nth(1)
        .expect("Usage: client <SERVER_NYM_ADDRESS>")
        .parse()
        .expect("Invalid Nym address");

    // Connect to the Mixnet
    let mut client = mixnet::MixnetClient::connect_new().await.unwrap();
    println!("Client address: {}", client.nym_address());

    // Open a stream to the server.
    // The second argument (None) uses the default number of reply SURBs.
    let mut stream = client.open_stream(server_addr, None).await.unwrap();
    println!("Stream opened: {}", stream.id());

    // Give the Open message time to traverse the mixnet and reach the server.
    // open_stream() returns immediately after sending — it doesn't wait for
    // the server to accept. Writing too soon risks the data arriving before
    // the Open, which the server would drop.
    tokio::time::sleep(Duration::from_secs(5)).await;

    // Send three payloads of different sizes and verify the echo.
    // Random bytes show that streams are binary-safe — not just text.
    let sizes = [320, 25_000, 1280];

    for (i, &size) in sizes.iter().enumerate() {
        let payload: Vec<u8> = (0..size).map(|_| rand::random::<u8>()).collect();
        println!("Sending message {} ({size} bytes)", i + 1);

        stream.write_all(&payload).await.unwrap();
        stream.flush().await.unwrap();

        // Read the echo
        let mut buf = vec![0u8; 32_000];
        let n = tokio::time::timeout(TIMEOUT, stream.read(&mut buf))
            .await
            .expect("timed out waiting for echo")
            .expect("read failed");

        assert_eq!(&buf[..n], &payload[..], "echo mismatch on message {}", i + 1);
        println!("Received echo: {n} bytes ok");
    }

    // Drop the stream to deregister it from the router
    drop(stream);

    // Disconnect the client
    client.disconnect().await;
    println!("Done!");
}
```

## Step 4: Run it

In one terminal, start the server:

```sh
RUST_LOG=info cargo run --bin server
```

It prints its Nym address:

```
Echo server listening at: 8gk4Y...@2xU4d...
```

In a second terminal, start the client with the server's address:

```sh
RUST_LOG=info cargo run --bin client -- 8gk4Y...@2xU4d...
```

You'll see the messages traverse the Mixnet and echo back:

```
Client address: F3qR7...@9nK2m...
Stream opened: 12345678
Sending message 1 (320 bytes)
Received echo: 320 bytes ok
Sending message 2 (25000 bytes)
Received echo: 25000 bytes ok
Sending message 3 (1280 bytes)
Received echo: 1280 bytes ok
Done!
```

On the server side:

```
Accepted stream 12345678
Stream 12345678 received 320 bytes
Stream 12345678 received 25000 bytes
Stream 12345678 received 1280 bytes
Stream 12345678 closed
```

## How it works internally

1. The server's `listener()` activates **stream mode**, which spawns a **router task** that decodes incoming Mixnet messages and dispatches them by stream ID.

2. The client's `open_stream()` generates a random 8-byte `StreamId`, sends an `Open` message through the Mixnet, and registers the stream in a local routing table.

3. When the server's router receives the `Open` message, it delivers it to `listener.accept()`, which creates the inbound `MixnetStream`.

4. Each `write_all()` prepends a 16-byte LP frame header (`[LpFrameKind: 2B][StreamId: 8B][MsgType: 1B][SequenceNum: 4B][Reserved: 1B]`) and sends the data through the Mixnet as a Sphinx packet.

5. On arrival, the router reads the `LpFrameKind` to identify it as stream traffic, decodes the header, finds the matching stream by ID, and delivers the raw payload to `read()`.

6. The inbound stream replies via **reply SURBs**, the same anonymous reply mechanism as the message API, applied transparently. The server never learns the client's Nym address.

7. When a stream is dropped, it deregisters from the local router. No close message is sent over the wire, since a close could race ahead of in-flight data.

See the [Architecture](./architecture) page for the full technical details.

## What you've learned

- `client.listener()` activates stream mode and returns a `MixnetListener`
- `listener.accept()` blocks until a remote peer opens a stream
- `client.open_stream(recipient, surbs)` opens an outbound stream to a Nym address
- `MixnetStream` implements `AsyncRead + AsyncWrite`, so standard tokio I/O works unchanged
- Multiple streams are multiplexed over a single client
- Streams deregister on `drop`; no close handshake is needed
- The server replies via SURBs and never learns the client's address

## Complete code

### Server (`src/bin/server.rs`)

```rust
use nym_sdk::mixnet;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

#[tokio::main]
async fn main() {
    nym_bin_common::logging::setup_tracing_logger();

    let mut client = mixnet::MixnetClient::connect_new().await.unwrap();
    println!("Echo server listening at: {}", client.nym_address());

    let mut listener = client.listener().unwrap();

    loop {
        let mut stream = match listener.accept().await {
            Some(s) => s,
            None => break,
        };

        let stream_id = stream.id();
        println!("Accepted stream {stream_id}");

        tokio::spawn(async move {
            let mut buf = vec![0u8; 32_000];
            loop {
                let n = match stream.read(&mut buf).await {
                    Ok(0) | Err(_) => break,
                    Ok(n) => n,
                };
                if let Err(_) = stream.write_all(&buf[..n]).await {
                    break;
                }
                stream.flush().await.unwrap();
            }
            println!("Stream {stream_id} closed");
        });
    }
}
```

### Client (`src/bin/client.rs`)

```rust
use nym_sdk::mixnet::{self, Recipient};
use std::time::Duration;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

const TIMEOUT: Duration = Duration::from_secs(60);

#[tokio::main]
async fn main() {
    nym_bin_common::logging::setup_tracing_logger();

    let server_addr: Recipient = std::env::args()
        .nth(1)
        .expect("Usage: client <SERVER_NYM_ADDRESS>")
        .parse()
        .expect("Invalid Nym address");

    let mut client = mixnet::MixnetClient::connect_new().await.unwrap();
    println!("Client address: {}", client.nym_address());

    let mut stream = client.open_stream(server_addr, None).await.unwrap();
    println!("Stream opened: {}", stream.id());

    // Wait for the Open message to reach the server through the mixnet
    tokio::time::sleep(Duration::from_secs(5)).await;

    let sizes = [320, 25_000, 1280];

    for (i, &size) in sizes.iter().enumerate() {
        let payload: Vec<u8> = (0..size).map(|_| rand::random::<u8>()).collect();
        println!("Sending message {} ({size} bytes)", i + 1);

        stream.write_all(&payload).await.unwrap();
        stream.flush().await.unwrap();

        let mut buf = vec![0u8; 32_000];
        let n = tokio::time::timeout(TIMEOUT, stream.read(&mut buf))
            .await
            .expect("timed out waiting for echo")
            .expect("read failed");

        assert_eq!(&buf[..n], &payload[..], "echo mismatch on message {}", i + 1);
        println!("Received echo: {n} bytes ok");
    }

    drop(stream);
    client.disconnect().await;
    println!("Done!");
}
```
