Use random ports when port number is 0 (#79)

* Use random ports when port number is 0

* Add support for --max-port CLI option

* Fix typo

* Fix another typo

* Update README

Cargo.toml

@@ -19,6 +19,7 @@ path = "src/main.rs"
 anyhow = { version = "1.0.56", features = ["backtrace"] }
 clap = { version = "4.0.22", features = ["derive", "env"] }
 dashmap = "5.2.0"
+fastrand = "1.9.0"
 futures-util = { version = "0.3.21", features = ["sink"] }
 hex = "0.4.3"
 hmac = "0.12.1"

README.md

@@ -19,7 +19,7 @@ This will expose your local port at `localhost:8000` to the public internet at `
 
 Similar to [localtunnel](https://github.com/localtunnel/localtunnel) and [ngrok](https://ngrok.io/), except `bore` is intended to be a highly efficient, unopinionated tool for forwarding TCP traffic that is simple to install and easy to self-host, with no frills attached.
 
-(`bore` totals less than 400 lines of safe, async Rust code and is trivial to set up — just run a single binary for the client and server.)
+(`bore` totals about 400 lines of safe, async Rust code and is trivial to set up — just run a single binary for the client and server.)
 
 ## Installation
 
@@ -93,7 +93,8 @@ Runs the remote proxy server
 Usage: bore server [OPTIONS]
 
 Options:
-      --min-port <MIN_PORT>  Minimum TCP port number to accept [default: 1024]
+      --min-port <MIN_PORT>  Minimum accepted TCP port number [default: 1024]
+      --max-port <MAX_PORT>  Maximum accepted TCP port number [default: 65535]
   -s, --secret <SECRET>      Optional secret for authentication [env: BORE_SECRET]
   -h, --help                 Print help information
 ```

src/client.rs

@@ -3,9 +3,7 @@
 use std::sync::Arc;
 
 use anyhow::{bail, Context, Result};
-
-use tokio::io::AsyncWriteExt;
-use tokio::{net::TcpStream, time::timeout};
+use tokio::{io::AsyncWriteExt, net::TcpStream, time::timeout};
 use tracing::{error, info, info_span, warn, Instrument};
 use uuid::Uuid;
 

src/main.rs

@@ -1,6 +1,6 @@
 use anyhow::Result;
 use bore_cli::{client::Client, server::Server};
-use clap::{Parser, Subcommand};
+use clap::{error::ErrorKind, CommandFactory, Parser, Subcommand};
 
 #[derive(Parser, Debug)]
 #[clap(author, version, about)]
@@ -35,10 +35,14 @@ enum Command {
 
     /// Runs the remote proxy server.
     Server {
-        /// Minimum TCP port number to accept.
+        /// Minimum accepted TCP port number.
         #[clap(long, default_value_t = 1024)]
         min_port: u16,
 
+        /// Maximum accepted TCP port number.
+        #[clap(long, default_value_t = 65535)]
+        max_port: u16,
+
         /// Optional secret for authentication.
         #[clap(short, long, env = "BORE_SECRET", hide_env_values = true)]
         secret: Option<String>,
@@ -58,8 +62,18 @@ async fn run(command: Command) -> Result<()> {
             let client = Client::new(&local_host, local_port, &to, port, secret.as_deref()).await?;
             client.listen().await?;
         }
-        Command::Server { min_port, secret } => {
-            Server::new(min_port, secret.as_deref()).listen().await?;
+        Command::Server {
+            min_port,
+            max_port,
+            secret,
+        } => {
+            let port_range = min_port..=max_port;
+            if port_range.is_empty() {
+                Args::command()
+                    .error(ErrorKind::InvalidValue, "port range is empty")
+                    .exit();
+            }
+            Server::new(port_range, secret.as_deref()).listen().await?;
         }
     }
 

src/server.rs

@@ -1,8 +1,6 @@
 //! Server implementation for the `bore` service.
 
-use std::net::SocketAddr;
-use std::sync::Arc;
-use std::time::Duration;
+use std::{io, net::SocketAddr, ops::RangeInclusive, sync::Arc, time::Duration};
 
 use anyhow::Result;
 use dashmap::DashMap;
@@ -17,8 +15,8 @@ use crate::shared::{proxy, ClientMessage, Delimited, ServerMessage, CONTROL_PORT
 
 /// State structure for the server.
 pub struct Server {
-    /// The minimum TCP port that can be forwarded.
-    min_port: u16,
+    /// Range of TCP ports that can be forwarded.
+    port_range: RangeInclusive<u16>,
 
     /// Optional secret used to authenticate clients.
     auth: Option<Authenticator>,
@@ -29,9 +27,10 @@ pub struct Server {
 
 impl Server {
     /// Create a new server with a specified minimum port number.
-    pub fn new(min_port: u16, secret: Option<&str>) -> Self {
+    pub fn new(port_range: RangeInclusive<u16>, secret: Option<&str>) -> Self {
+        assert!(!port_range.is_empty(), "must provide at least one port");
         Server {
-            min_port,
+            port_range,
             conns: Arc::new(DashMap::new()),
             auth: secret.map(Authenticator::new),
         }
@@ -61,6 +60,43 @@ impl Server {
         }
     }
 
+    async fn create_listener(&self, port: u16) -> Result<TcpListener, &'static str> {
+        let try_bind = |port: u16| async move {
+            TcpListener::bind(("0.0.0.0", port))
+                .await
+                .map_err(|err| match err.kind() {
+                    io::ErrorKind::AddrInUse => "port already in use",
+                    io::ErrorKind::PermissionDenied => "permission denied",
+                    _ => "failed to bind to port",
+                })
+        };
+        if port > 0 {
+            // Client requests a specific port number.
+            if !self.port_range.contains(&port) {
+                return Err("client port number not in allowed range");
+            }
+            try_bind(port).await
+        } else {
+            // Client requests any available port in range.
+            //
+            // In this case, we bind to 150 random port numbers. We choose this value because in
+            // order to find a free port with probability at least 1-δ, when ε proportion of the
+            // ports are currently available, it suffices to check approximately -2 ln(δ) / ε
+            // independently and uniformly chosen ports (up to a second-order term in ε).
+            //
+            // Checking 150 times gives us 99.999% success at utilizing 85% of ports under these
+            // conditions, when ε=0.15 and δ=0.00001.
+            for _ in 0..150 {
+                let port = fastrand::u16(self.port_range.clone());
+                match try_bind(port).await {
+                    Ok(listener) => return Ok(listener),
+                    Err(_) => continue,
+                }
+            }
+            Err("failed to find an available port")
+        }
+    }
+
     async fn handle_connection(&self, stream: TcpStream) -> Result<()> {
         let mut stream = Delimited::new(stream);
         if let Some(auth) = &self.auth {
@@ -77,22 +113,15 @@ impl Server {
                 Ok(())
             }
             Some(ClientMessage::Hello(port)) => {
-                if port != 0 && port < self.min_port {
-                    warn!(?port, "client port number too low");
-                    return Ok(());
-                }
-                info!(?port, "new client");
-                let listener = match TcpListener::bind(("0.0.0.0", port)).await {
+                let listener = match self.create_listener(port).await {
                     Ok(listener) => listener,
-                    Err(_) => {
-                        warn!(?port, "could not bind to local port");
-                        stream
-                            .send(ServerMessage::Error("port already in use".into()))
-                            .await?;
+                    Err(err) => {
+                        stream.send(ServerMessage::Error(err.into())).await?;
                         return Ok(());
                     }
                 };
                 let port = listener.local_addr()?.port();
+                info!(?port, "new client");
                 stream.send(ServerMessage::Hello(port)).await?;
 
                 loop {
@@ -133,16 +162,7 @@ impl Server {
                 }
                 Ok(())
             }
-            None => {
-                warn!("unexpected EOF");
-                Ok(())
-            }
+            None => Ok(()),
         }
     }
 }
-
-impl Default for Server {
-    fn default() -> Self {
-        Server::new(1024, None)
-    }
-}

src/shared.rs

@@ -7,7 +7,6 @@ use futures_util::{SinkExt, StreamExt};
 use serde::de::DeserializeOwned;
 use serde::{Deserialize, Serialize};
 use tokio::io::{self, AsyncRead, AsyncWrite};
-
 use tokio::time::timeout;
 use tokio_util::codec::{AnyDelimiterCodec, Framed, FramedParts};
 use tracing::trace;

tests/e2e_test.rs

@@ -17,7 +17,7 @@ lazy_static! {
 
 /// Spawn the server, giving some time for the control port TcpListener to start.
 async fn spawn_server(secret: Option<&str>) {
-    tokio::spawn(Server::new(1024, secret).listen());
+    tokio::spawn(Server::new(1024..=65535, secret).listen());
     time::sleep(Duration::from_millis(50)).await;
 }
 
@@ -117,3 +117,11 @@ async fn very_long_frame() -> Result<()> {
     }
     panic!("did not exit after a 1 MB frame");
 }
+
+#[test]
+#[should_panic]
+fn empty_port_range() {
+    let min_port = 5000;
+    let max_port = 3000;
+    let _ = Server::new(min_port..=max_port, None);
+}