rs: support ipv6 port-forwarded services

ipv6-only services are getting more common, especially with a Node 18
change that causes it to listen on ipv6 by default on most enabled
networks. In advance of releasing local forwarding for vscode, I think
we should support this.

This PR supports local ipv6 forwarding by reaching out to both the ipv6
and ipv4 interfaces by default, and using whichever one (if any)
accepts the TCP connection first.

rs/src/connections/relay_tunnel_host.rs

@@ -4,7 +4,7 @@
 use std::{
     collections::HashMap,
     env, io,
-    net::{Ipv4Addr, SocketAddr, SocketAddrV4},
+    net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr},
     pin::Pin,
     sync::Arc,
     task::Poll,
@@ -19,7 +19,7 @@ use crate::{
     },
 };
 use async_trait::async_trait;
-use futures::TryFutureExt;
+use futures::{stream::FuturesUnordered, StreamExt, TryFutureExt};
 use russh::{server::Server as ServerTrait, CryptoVec};
 use tokio::{
     io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt},
@@ -298,12 +298,7 @@ impl RelayTunnelHost {
     /// with the same port will result in an error.
     pub async fn add_port(&self, port_to_add: &TunnelPort) -> Result<(), TunnelError> {
         let rx = self.add_port_raw(port_to_add).await?;
-
-        tokio::spawn(forward_port_to_tcp(
-            SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), port_to_add.port_number).into(),
-            rx,
-        ));
-
+        tokio::spawn(forward_port_to_tcp(port_to_add.port_number, rx));
         Ok(())
     }
 
@@ -690,21 +685,41 @@ impl Server {
 
 /// Connects connections that are sent to the receiver to TCP services locally.
 /// Runs until the receiver is closed (usually via `delete_port()`).
-async fn forward_port_to_tcp(
-    addr: SocketAddr,
-    mut rx: mpsc::UnboundedReceiver<ForwardedPortConnection>,
-) {
+async fn forward_port_to_tcp(port: u16, mut rx: mpsc::UnboundedReceiver<ForwardedPortConnection>) {
+    let ipv4_addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), port);
+    let ipv6_addr = SocketAddr::new(IpAddr::V6(Ipv6Addr::LOCALHOST), port);
     while let Some(mut conn) = rx.recv().await {
-        let mut stream = match TcpStream::connect(&addr).await {
+        // Try connecting to ipv4 and ipv6 in parallel, using the first successful
+        // connection in the stream. A downside is that it means if different sevices
+        // are listening on the ports, the forwarded application is non-deterministic.
+        //
+        // But that's rare, and in other cases, one interface will time out and
+        // one interface will work, so this lets us respond as quickly as possible.
+        let mut futs = FuturesUnordered::new();
+        futs.push(TcpStream::connect(&ipv4_addr));
+        futs.push(TcpStream::connect(&ipv6_addr));
+
+        let mut last_result = None;
+        while let Some(r) = futs.next().await {
+            let ok = r.is_ok();
+            last_result = Some(r);
+            // stop on first successful:
+            if ok {
+                break;
+            }
+        }
+
+        // unwrap is safe since we know there will be at least one result:
+        let mut stream = match last_result.unwrap() {
             Ok(s) => s,
             Err(e) => {
-                log::info!("Error connecting forwarding to {}, {}", addr, e);
+                log::info!("Error connecting forwarding to port {}, {}", port, e);
                 conn.close().await;
                 continue;
             }
         };
 
-        log::debug!("Forwarded port to {}", addr);
+        log::debug!("Forwarded connection to port {}", port);
 
         tokio::spawn(async move {
             let mut read_buf = vec![0u8; 1024 * 64].into_boxed_slice();