arcbox_proxy/

inbound_relay.rs

//! Inbound port forwarding via L2 frame injection.
//!
//! Instead of using utun + kernel routing, we inject crafted L2 Ethernet frames
//! directly into the guest FD (socketpair) so that host-side TCP/UDP listeners
//! can reach services inside the guest VM.
//!
//! # Architecture
//!
//! ```text
//! External client (host:8080)
//!     │
//!     ▼
//! InboundListenerManager (TcpListener / UdpSocket per rule)
//!     │ accept / recv
//!     ▼
//! InboundCommand channel  ──►  NetworkDatapath select! arm
//!     │
//!     ▼
//! InboundRelay
//!     └─ UDP: inject datagram → guest reply → forward to client
//!     │
//!     ▼
//! reply_tx ──► datapath ──► guest_fd (socketpair) ──► Guest VM
//! ```

use std::collections::HashMap;
use std::net::{Ipv4Addr, SocketAddr, SocketAddrV4};
use std::time::Instant;

use std::sync::Arc;

use socket2::SockRef;
use tokio::net::{TcpListener, UdpSocket};
use tokio::sync::mpsc;
use tokio::task::JoinHandle;
use tokio_util::sync::CancellationToken;

use arcbox_packet::ethernet::{ETH_HEADER_LEN, build_udp_ip_ethernet};

/// Socket buffer size applied to accepted inbound TCP streams.
///
/// The OS default on macOS is ~128 KiB which forces TCP to shrink the window
/// under high-throughput bulk transfers (e.g. iperf3). Raising to 4 MiB lets
/// the window grow to match the BDP of localhost / high-speed paths.
///
/// Requires `kern.ipc.maxsockbuf` to allow at least this value (default 8 MiB
/// on macOS; confirm with `sysctl kern.ipc.maxsockbuf`). setsockopt silently
/// clamps to the maxsockbuf ceiling, so oversizing is harmless.
const INBOUND_TCP_BUF_SIZE: usize = 4 * 1024 * 1024;

// ---------------------------------------------------------------------------
// Ephemeral port allocator
// ---------------------------------------------------------------------------

/// Start of the inbound ephemeral port range (guest kernel uses 32768-60999).
const EPHEMERAL_START: u16 = 61000;
/// End of the inbound ephemeral port range (inclusive).
const EPHEMERAL_END: u16 = 65535;

/// Wrapping ephemeral port allocator for inbound connections.
pub(crate) struct EphemeralPorts {
    next: u16,
}

impl EphemeralPorts {
    pub(crate) fn new() -> Self {
        Self {
            next: EPHEMERAL_START,
        }
    }

    /// Allocates the next ephemeral port, wrapping at the end of the range.
    pub(crate) fn allocate(&mut self) -> u16 {
        let port = self.next;
        self.next = if self.next == EPHEMERAL_END {
            EPHEMERAL_START
        } else {
            self.next + 1
        };
        port
    }

    /// Returns whether `port` falls within the inbound ephemeral range.
    #[inline]
    pub(crate) fn in_range(port: u16) -> bool {
        (EPHEMERAL_START..=EPHEMERAL_END).contains(&port)
    }
}

// ---------------------------------------------------------------------------
// Inbound command (sent from listener tasks to the datapath)
// ---------------------------------------------------------------------------

/// Command sent from `InboundListenerManager` listener tasks to the datapath.
pub enum InboundCommand {
    /// A new TCP connection was accepted on a host listener.
    TcpAccepted {
        host_port: u16,
        container_port: u16,
        stream: tokio::net::TcpStream,
    },
    /// A UDP datagram was received on a host listener.
    UdpReceived {
        host_port: u16,
        container_port: u16,
        data: Vec<u8>,
        /// Channel to send reply datagrams back to the host-side client.
        reply_tx: mpsc::Sender<Vec<u8>>,
        client_addr: SocketAddr,
    },
}

// ---------------------------------------------------------------------------
// UDP flow state
// ---------------------------------------------------------------------------

/// Per-flow inbound UDP state.
struct InboundUdpFlow {
    /// Channel to send reply datagrams back to the host-side client.
    client_tx: mpsc::Sender<Vec<u8>>,
    /// Last time traffic was seen on this flow.
    last_active: Instant,
}

// ---------------------------------------------------------------------------
// InboundRelay
// ---------------------------------------------------------------------------

/// Handles inbound (host → guest) connections by injecting L2 Ethernet frames
/// directly into the guest FD through the `reply_tx` channel.
pub(crate) struct InboundRelay {
    /// Active UDP flows keyed by (gateway_ip, ephemeral_port, guest_ip, container_port).
    udp_flows: HashMap<(Ipv4Addr, u16, Ipv4Addr, u16), InboundUdpFlow>,
    /// Channel to inject L2 frames towards the guest.
    reply_tx: mpsc::Sender<Vec<u8>>,
    gateway_mac: [u8; 6],
    gateway_ip: Ipv4Addr,
    guest_ip: Ipv4Addr,
    /// Guest link MTU; injected datagrams above it are IPv4-fragmented.
    mtu: usize,
    ephemeral_ports: EphemeralPorts,
}

impl InboundRelay {
    pub(crate) fn new(
        reply_tx: mpsc::Sender<Vec<u8>>,
        gateway_mac: [u8; 6],
        gateway_ip: Ipv4Addr,
        guest_ip: Ipv4Addr,
        mtu: usize,
    ) -> Self {
        Self {
            udp_flows: HashMap::new(),
            reply_tx,
            gateway_mac,
            gateway_ip,
            guest_ip,
            mtu,
            ephemeral_ports: EphemeralPorts::new(),
        }
    }

    // -----------------------------------------------------------------------
    // Frame matching — called on every outbound guest frame
    // -----------------------------------------------------------------------

    /// Attempts to match an outbound guest frame as a reply to an inbound
    /// connection. Returns `true` if the frame was consumed.
    ///
    /// Fast-path: `EphemeralPorts::in_range(dst_port)` rejects 99%+ of
    /// outbound frames before any `HashMap` lookup.
    pub(crate) fn try_handle_reply(&mut self, frame: &[u8], _guest_mac: [u8; 6]) -> bool {
        if frame.len() < ETH_HEADER_LEN + 20 {
            return false;
        }

        let ip_start = ETH_HEADER_LEN;
        let protocol = frame[ip_start + 9];

        let ihl = ((frame[ip_start] & 0x0F) as usize) * 4;
        let l4_start = ip_start + ihl;

        match protocol {
            6 => false, // TCP is handled by TcpBridge, not the inbound relay
            17 => self.try_handle_udp_reply(frame, ip_start, l4_start),
            _ => false,
        }
    }

    /// Checks if a UDP frame is a reply to an inbound flow.
    fn try_handle_udp_reply(&mut self, frame: &[u8], ip_start: usize, udp_start: usize) -> bool {
        if frame.len() < udp_start + 8 {
            return false;
        }

        let dst_port = u16::from_be_bytes([frame[udp_start + 2], frame[udp_start + 3]]);
        if !EphemeralPorts::in_range(dst_port) {
            return false;
        }

        let src_ip = Ipv4Addr::new(
            frame[ip_start + 12],
            frame[ip_start + 13],
            frame[ip_start + 14],
            frame[ip_start + 15],
        );
        let dst_ip = Ipv4Addr::new(
            frame[ip_start + 16],
            frame[ip_start + 17],
            frame[ip_start + 18],
            frame[ip_start + 19],
        );
        let src_port = u16::from_be_bytes([frame[udp_start], frame[udp_start + 1]]);

        let key = (dst_ip, dst_port, src_ip, src_port);

        if let Some(flow) = self.udp_flows.get_mut(&key) {
            let udp_len = u16::from_be_bytes([frame[udp_start + 4], frame[udp_start + 5]]) as usize;
            if udp_len >= 8 && udp_start + udp_len <= frame.len() {
                let payload = frame[udp_start + 8..udp_start + udp_len].to_vec();
                flow.last_active = Instant::now();
                let _ = flow.client_tx.try_send(payload);
            }
            return true;
        }

        false
    }

    // -----------------------------------------------------------------------
    // UDP: inject datagram to guest
    // -----------------------------------------------------------------------

    /// Injects a UDP datagram to the guest and sets up a flow for replies.
    pub(crate) fn inject_udp(
        &mut self,
        container_port: u16,
        data: &[u8],
        client_tx: mpsc::Sender<Vec<u8>>,
        guest_mac: [u8; 6],
    ) {
        let ephemeral_port = self.ephemeral_ports.allocate();
        let key = (
            self.gateway_ip,
            ephemeral_port,
            self.guest_ip,
            container_port,
        );

        self.udp_flows.insert(
            key,
            InboundUdpFlow {
                client_tx,
                last_active: Instant::now(),
            },
        );

        let frames = build_udp_ip_ethernet(
            self.gateway_ip,
            self.guest_ip,
            ephemeral_port,
            container_port,
            data,
            self.gateway_mac,
            guest_mac,
            self.mtu,
        );

        for frame in frames {
            if self.reply_tx.try_send(frame).is_err() {
                // Dropping a fragment kills the whole datagram; stop early.
                break;
            }
        }

        tracing::debug!(
            "Inbound UDP: injected {} bytes  gw:{} → guest:{}",
            data.len(),
            ephemeral_port,
            container_port,
        );
    }

    // -----------------------------------------------------------------------
    // Maintenance
    // -----------------------------------------------------------------------

    /// Removes expired UDP flows.
    pub(crate) fn cleanup(&mut self) {
        let now = Instant::now();
        self.udp_flows
            .retain(|_, flow| now.duration_since(flow.last_active).as_secs() < 60);
    }
}

// ---------------------------------------------------------------------------
// InboundListenerManager
// ---------------------------------------------------------------------------

/// Protocol for port forwarding rules.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum InboundProtocol {
    Tcp,
    Udp,
}

/// Manages host-side listeners that accept incoming connections / datagrams
/// and send `InboundCommand` messages to the datapath.
pub struct InboundListenerManager {
    cmd_tx: mpsc::Sender<InboundCommand>,
    listeners: HashMap<(Ipv4Addr, u16, InboundProtocol), (JoinHandle<()>, CancellationToken)>,
}

impl InboundListenerManager {
    /// Creates a new listener manager.
    #[must_use]
    pub fn new(cmd_tx: mpsc::Sender<InboundCommand>) -> Self {
        Self {
            cmd_tx,
            listeners: HashMap::new(),
        }
    }

    /// Adds a forwarding rule and spawns a listener task.
    ///
    /// # Errors
    ///
    /// Returns an error if the listener cannot bind.
    pub async fn add_rule(
        &mut self,
        host_ip: Ipv4Addr,
        host_port: u16,
        container_port: u16,
        protocol: InboundProtocol,
    ) -> std::io::Result<()> {
        let key = (host_ip, host_port, protocol);
        if self.listeners.contains_key(&key) {
            return Ok(()); // Already listening.
        }

        let cancel = CancellationToken::new();
        let cmd_tx = self.cmd_tx.clone();

        let handle = match protocol {
            InboundProtocol::Tcp => {
                let listener =
                    TcpListener::bind(SocketAddr::V4(SocketAddrV4::new(host_ip, host_port)))
                        .await?;
                tracing::info!(
                    "Inbound listener: TCP {}:{} → container :{}",
                    host_ip,
                    host_port,
                    container_port,
                );
                let cancel_clone = cancel.clone();
                tokio::spawn(async move {
                    tcp_listener_task(listener, container_port, cmd_tx, cancel_clone).await;
                })
            }
            InboundProtocol::Udp => {
                let socket =
                    UdpSocket::bind(SocketAddr::V4(SocketAddrV4::new(host_ip, host_port))).await?;
                tracing::info!(
                    "Inbound listener: UDP {}:{} → container :{}",
                    host_ip,
                    host_port,
                    container_port,
                );
                let cancel_clone = cancel.clone();
                tokio::spawn(async move {
                    udp_listener_task(socket, container_port, cmd_tx, cancel_clone).await;
                })
            }
        };

        self.listeners.insert(key, (handle, cancel));
        Ok(())
    }

    /// Removes a forwarding rule and stops its listener.
    pub fn remove_rule(&mut self, host_ip: Ipv4Addr, host_port: u16, protocol: InboundProtocol) {
        let key = (host_ip, host_port, protocol);
        if let Some((handle, cancel)) = self.listeners.remove(&key) {
            cancel.cancel();
            handle.abort();
            tracing::debug!(
                "Inbound listener removed: {:?} {}:{}",
                protocol,
                host_ip,
                host_port
            );
        }
    }

    /// Stops all listeners.
    pub fn stop_all(&mut self) {
        for ((ip, port, proto), (handle, cancel)) in self.listeners.drain() {
            cancel.cancel();
            handle.abort();
            tracing::debug!("Inbound listener stopped: {:?} {}:{}", proto, ip, port);
        }
    }
}

// ---------------------------------------------------------------------------
// Listener tasks
// ---------------------------------------------------------------------------

/// TCP listener task: accepts connections and sends `InboundCommand::TcpAccepted`.
async fn tcp_listener_task(
    listener: TcpListener,
    container_port: u16,
    cmd_tx: mpsc::Sender<InboundCommand>,
    cancel: CancellationToken,
) {
    let host_port = listener.local_addr().map_or(0, |a| a.port());
    loop {
        tokio::select! {
            biased;
            () = cancel.cancelled() => break,
            result = listener.accept() => {
                match result {
                    Ok((stream, peer)) => {
                        tracing::debug!(
                            "Inbound TCP accept: {} → host:{} → container:{}",
                            peer, host_port, container_port,
                        );
                        // Raise send/recv buffers so the TCP window can grow to
                        // localhost BDP. Failures here are non-fatal — the OS
                        // default still works, just throttles throughput.
                        let sock = SockRef::from(&stream);
                        if let Err(e) = sock.set_recv_buffer_size(INBOUND_TCP_BUF_SIZE) {
                            tracing::warn!("Failed to set SO_RCVBUF on inbound stream: {e}");
                        }
                        if let Err(e) = sock.set_send_buffer_size(INBOUND_TCP_BUF_SIZE) {
                            tracing::warn!("Failed to set SO_SNDBUF on inbound stream: {e}");
                        }
                        let cmd = InboundCommand::TcpAccepted {
                            host_port,
                            container_port,
                            stream,
                        };
                        if cmd_tx.send(cmd).await.is_err() {
                            break;
                        }
                    }
                    Err(e) => {
                        tracing::warn!("Inbound TCP accept error on :{}: {}", host_port, e);
                    }
                }
            }
        }
    }
}

/// UDP listener task: receives datagrams and sends `InboundCommand::UdpReceived`.
async fn udp_listener_task(
    socket: UdpSocket,
    container_port: u16,
    cmd_tx: mpsc::Sender<InboundCommand>,
    cancel: CancellationToken,
) {
    let host_port = socket.local_addr().map_or(0, |a| a.port());
    let socket = Arc::new(socket);
    let mut reply_flows: HashMap<SocketAddr, mpsc::Sender<Vec<u8>>> = HashMap::new();
    let mut buf = vec![0u8; 65535];

    loop {
        tokio::select! {
            biased;
            () = cancel.cancelled() => break,
            result = socket.recv_from(&mut buf) => {
                match result {
                    Ok((n, client_addr)) => {
                        let reply_tx = if let Some(tx) = reply_flows.get(&client_addr) {
                            if tx.is_closed() {
                                reply_flows.remove(&client_addr);
                                create_udp_reply_flow(client_addr, &socket, &cancel, &mut reply_flows)
                            } else {
                                tx.clone()
                            }
                        } else {
                            create_udp_reply_flow(client_addr, &socket, &cancel, &mut reply_flows)
                        };

                        let cmd = InboundCommand::UdpReceived {
                            host_port,
                            container_port,
                            data: buf[..n].to_vec(),
                            reply_tx,
                            client_addr,
                        };
                        if cmd_tx.send(cmd).await.is_err() {
                            break;
                        }
                    }
                    Err(e) => {
                        tracing::warn!("Inbound UDP recv error on :{}: {}", host_port, e);
                    }
                }
            }
        }
    }
}

fn create_udp_reply_flow(
    client_addr: SocketAddr,
    socket: &Arc<UdpSocket>,
    cancel: &CancellationToken,
    reply_flows: &mut HashMap<SocketAddr, mpsc::Sender<Vec<u8>>>,
) -> mpsc::Sender<Vec<u8>> {
    let (reply_tx, mut reply_rx) = mpsc::channel::<Vec<u8>>(16);
    let reply_sock = Arc::clone(socket);
    let flow_cancel = cancel.clone();
    tokio::spawn(async move {
        loop {
            tokio::select! {
                biased;
                () = flow_cancel.cancelled() => break,
                maybe_data = reply_rx.recv() => {
                    let Some(data) = maybe_data else {
                        break;
                    };
                    let _ = reply_sock.send_to(&data, client_addr).await;
                }
            }
        }
    });
    reply_flows.insert(client_addr, reply_tx.clone());
    reply_tx
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;

    const GW_IP: Ipv4Addr = Ipv4Addr::new(192, 168, 64, 1);
    const GUEST_IP: Ipv4Addr = Ipv4Addr::new(192, 168, 64, 2);
    const GW_MAC: [u8; 6] = [0x02, 0xAB, 0xCD, 0x00, 0x00, 0x01];
    const GUEST_MAC: [u8; 6] = [0x02, 0x00, 0x00, 0x00, 0x00, 0x99];

    #[test]
    fn ephemeral_ports_allocation() {
        let mut ep = EphemeralPorts::new();
        assert_eq!(ep.allocate(), 61000);
        assert_eq!(ep.allocate(), 61001);
    }

    #[test]
    fn ephemeral_ports_wrap_around() {
        let mut ep = EphemeralPorts::new();
        ep.next = EPHEMERAL_END;
        assert_eq!(ep.allocate(), EPHEMERAL_END);
        assert_eq!(ep.allocate(), EPHEMERAL_START);
    }

    #[test]
    fn ephemeral_ports_in_range() {
        assert!(EphemeralPorts::in_range(61000));
        assert!(EphemeralPorts::in_range(65535));
        assert!(EphemeralPorts::in_range(63000));
        assert!(!EphemeralPorts::in_range(60999));
        assert!(!EphemeralPorts::in_range(32768));
        assert!(!EphemeralPorts::in_range(80));
    }

    #[test]
    fn inbound_relay_rejects_non_ephemeral() {
        let (tx, _rx) = mpsc::channel(16);
        let mut relay = InboundRelay::new(tx, GW_MAC, GW_IP, GUEST_IP, 1500);

        // Build a minimal TCP frame with dst_port=80 (not in ephemeral range).
        let mut frame = vec![0u8; ETH_HEADER_LEN + 40];
        frame[12..14].copy_from_slice(&0x0800u16.to_be_bytes());
        let ip = &mut frame[ETH_HEADER_LEN..];
        ip[0] = 0x45;
        ip[9] = 6; // TCP
        ip[12..16].copy_from_slice(&GUEST_IP.octets());
        ip[16..20].copy_from_slice(&GW_IP.octets());
        // TCP header: src_port=8080, dst_port=80
        let tcp = &mut frame[ETH_HEADER_LEN + 20..];
        tcp[0..2].copy_from_slice(&8080u16.to_be_bytes());
        tcp[2..4].copy_from_slice(&80u16.to_be_bytes());
        tcp[12] = 0x50; // data offset = 5

        assert!(!relay.try_handle_reply(&frame, GUEST_MAC));
    }

    #[tokio::test]
    async fn inject_udp_sends_frame_and_tracks_flow() {
        let (tx, mut rx) = mpsc::channel(16);
        let mut relay = InboundRelay::new(tx, GW_MAC, GW_IP, GUEST_IP, 1500);

        let (client_tx, _client_rx) = mpsc::channel(16);
        relay.inject_udp(53, b"dns query", client_tx, GUEST_MAC);

        // Flow should be tracked.
        let key = (GW_IP, EPHEMERAL_START, GUEST_IP, 53);
        assert!(relay.udp_flows.contains_key(&key));

        // A UDP frame should have been sent.
        let frame = rx.recv().await.expect("should receive UDP frame");
        assert!(frame.len() >= ETH_HEADER_LEN + 28, "UDP frame too short");

        // Verify IP protocol = UDP (17).
        assert_eq!(frame[ETH_HEADER_LEN + 9], 17);

        // Verify ports.
        let udp_start = ETH_HEADER_LEN + 20;
        let src_port = u16::from_be_bytes([frame[udp_start], frame[udp_start + 1]]);
        let dst_port = u16::from_be_bytes([frame[udp_start + 2], frame[udp_start + 3]]);
        assert_eq!(src_port, EPHEMERAL_START);
        assert_eq!(dst_port, 53);
    }

    #[test]
    fn cleanup_removes_expired_udp_flows() {
        let (tx, _rx) = mpsc::channel(16);
        let mut relay = InboundRelay::new(tx, GW_MAC, GW_IP, GUEST_IP, 1500);

        let (client_tx, _client_rx) = mpsc::channel(16);
        let key = (GW_IP, 61000, GUEST_IP, 53);
        relay.udp_flows.insert(
            key,
            InboundUdpFlow {
                client_tx,
                last_active: Instant::now()
                    .checked_sub(std::time::Duration::from_secs(120))
                    .unwrap(),
            },
        );
        assert_eq!(relay.udp_flows.len(), 1);

        relay.cleanup();
        assert_eq!(
            relay.udp_flows.len(),
            0,
            "expired UDP flow should be removed"
        );
    }

    #[tokio::test]
    async fn listener_manager_add_and_remove_rule() {
        let (cmd_tx, mut cmd_rx) = mpsc::channel(16);
        let mut manager = InboundListenerManager::new(cmd_tx);

        // Add a TCP rule on an ephemeral port.
        manager
            .add_rule(Ipv4Addr::LOCALHOST, 0, 80, InboundProtocol::Tcp)
            .await
            .expect("should bind to port 0 (OS-assigned)");

        // Remove it.
        manager.remove_rule(Ipv4Addr::LOCALHOST, 0, InboundProtocol::Tcp);

        // The cmd_rx channel should still be valid (no panic).
        assert!(cmd_rx.try_recv().is_err(), "no commands expected yet");
    }

    #[tokio::test]
    async fn listener_manager_stop_all() {
        let (cmd_tx, _cmd_rx) = mpsc::channel(16);
        let mut manager = InboundListenerManager::new(cmd_tx);

        manager
            .add_rule(Ipv4Addr::LOCALHOST, 0, 80, InboundProtocol::Tcp)
            .await
            .unwrap();
        manager
            .add_rule(Ipv4Addr::LOCALHOST, 0, 53, InboundProtocol::Udp)
            .await
            .unwrap();

        manager.stop_all();
        // After stop_all, the internal map should be empty. Since we can't
        // inspect it directly, adding the same rule again should succeed (no
        // duplicate key).
        manager
            .add_rule(Ipv4Addr::LOCALHOST, 0, 80, InboundProtocol::Tcp)
            .await
            .unwrap();
    }

    #[tokio::test]
    async fn same_port_different_ip_coexist() {
        let (cmd_tx, _cmd_rx) = mpsc::channel(16);
        let mut manager = InboundListenerManager::new(cmd_tx);

        // Bind the same container port on two different host IPs (port 0 = OS-assigned).
        manager
            .add_rule(Ipv4Addr::LOCALHOST, 0, 80, InboundProtocol::Tcp)
            .await
            .unwrap();
        manager
            .add_rule(Ipv4Addr::UNSPECIFIED, 0, 80, InboundProtocol::Tcp)
            .await
            .unwrap();

        // Remove only the localhost rule; re-adding it should succeed (not a dup).
        manager.remove_rule(Ipv4Addr::LOCALHOST, 0, InboundProtocol::Tcp);
        manager
            .add_rule(Ipv4Addr::LOCALHOST, 0, 80, InboundProtocol::Tcp)
            .await
            .unwrap();
    }

    #[test]
    fn invalid_host_ip_is_rejected() {
        // Verify that HostIp parsing used by runtime rejects non-IPv4 strings.
        // The runtime calls `host_ip_str.parse::<Ipv4Addr>()` and skips on Err.
        assert!(
            "::1".parse::<Ipv4Addr>().is_err(),
            "IPv6 should fail Ipv4Addr parse"
        );
        assert!("not-an-ip".parse::<Ipv4Addr>().is_err());
        assert!("".parse::<Ipv4Addr>().is_err());
        // Valid cases the runtime accepts:
        assert!("127.0.0.1".parse::<Ipv4Addr>().is_ok());
        assert!("0.0.0.0".parse::<Ipv4Addr>().is_ok());
    }
}