smartvpn/rust/src/bridge.rs

//! L2 Bridge forwarding engine.
//!
//! Provides server-side bridging: receives L3 IP packets from VPN clients,
//! wraps them in Ethernet frames, and injects them into a Linux bridge
//! connected to the host's physical network interface.
//!
//! Return traffic from the bridge is stripped of its Ethernet header and
//! routed back to VPN clients via `tun_routes`.

use anyhow::Result;
use std::collections::HashMap;
use std::net::Ipv4Addr;
use std::sync::Arc;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::sync::mpsc;
use tracing::{debug, error, info, warn};

use crate::server::ServerState;

/// Configuration for the bridge forwarding engine.
pub struct BridgeConfig {
    /// TAP device name (e.g., "svpn_tap0")
    pub tap_name: String,
    /// Linux bridge name (e.g., "svpn_br0")
    pub bridge_name: String,
    /// Physical interface to bridge (e.g., "eth0")
    pub physical_interface: String,
    /// Gateway IP on the bridge (host's LAN IP)
    pub gateway_ip: Ipv4Addr,
    /// Subnet prefix length (e.g., 24)
    pub prefix_len: u8,
    /// MTU for the TAP device
    pub mtu: u16,
}

/// Ethernet frame constants
const ETH_HEADER_LEN: usize = 14;
const ETH_TYPE_IPV4: [u8; 2] = [0x08, 0x00];
const ETH_TYPE_ARP: [u8; 2] = [0x08, 0x06];
const BROADCAST_MAC: [u8; 6] = [0xff; 6];

/// Generate a deterministic locally-administered MAC from an IPv4 address.
/// Uses prefix 02:53:56 (locally administered, "SVP" in hex-ish).
fn mac_from_ip(ip: Ipv4Addr) -> [u8; 6] {
    let octets = ip.octets();
    [0x02, 0x53, 0x56, octets[1], octets[2], octets[3]]
}

/// Wrap an IP packet in an Ethernet frame.
fn wrap_in_ethernet(ip_packet: &[u8], src_mac: [u8; 6], dst_mac: [u8; 6]) -> Vec<u8> {
    let mut frame = Vec::with_capacity(ETH_HEADER_LEN + ip_packet.len());
    frame.extend_from_slice(&dst_mac);
    frame.extend_from_slice(&src_mac);
    frame.extend_from_slice(&ETH_TYPE_IPV4);
    frame.extend_from_slice(ip_packet);
    frame
}

/// Extract the EtherType and payload from an Ethernet frame.
fn unwrap_ethernet(frame: &[u8]) -> Option<([u8; 2], &[u8])> {
    if frame.len() < ETH_HEADER_LEN {
        return None;
    }
    let ether_type = [frame[12], frame[13]];
    Some((ether_type, &frame[ETH_HEADER_LEN..]))
}

/// Extract destination IPv4 from a raw IP packet header.
fn dst_ip_from_packet(packet: &[u8]) -> Option<Ipv4Addr> {
    if packet.len() < 20 {
        return None;
    }
    // Version must be 4
    if (packet[0] >> 4) != 4 {
        return None;
    }
    Some(Ipv4Addr::new(packet[16], packet[17], packet[18], packet[19]))
}

/// Extract source IPv4 from a raw IP packet header.
fn src_ip_from_packet(packet: &[u8]) -> Option<Ipv4Addr> {
    if packet.len() < 20 {
        return None;
    }
    if (packet[0] >> 4) != 4 {
        return None;
    }
    Some(Ipv4Addr::new(packet[12], packet[13], packet[14], packet[15]))
}

/// Build a gratuitous ARP announcement frame.
fn build_garp(ip: Ipv4Addr, mac: [u8; 6]) -> Vec<u8> {
    let ip_bytes = ip.octets();
    let mut frame = Vec::with_capacity(42); // 14 eth + 28 ARP
    // Ethernet header
    frame.extend_from_slice(&BROADCAST_MAC); // dst: broadcast
    frame.extend_from_slice(&mac);           // src: our MAC
    frame.extend_from_slice(&ETH_TYPE_ARP);  // EtherType: ARP
    // ARP payload
    frame.extend_from_slice(&[0x00, 0x01]); // Hardware type: Ethernet
    frame.extend_from_slice(&[0x08, 0x00]); // Protocol type: IPv4
    frame.push(6);                          // Hardware addr len
    frame.push(4);                          // Protocol addr len
    frame.extend_from_slice(&[0x00, 0x01]); // Operation: ARP Request (GARP uses request)
    frame.extend_from_slice(&mac);          // Sender hardware addr
    frame.extend_from_slice(&ip_bytes);     // Sender protocol addr
    frame.extend_from_slice(&[0x00; 6]);    // Target hardware addr (ignored in GARP)
    frame.extend_from_slice(&ip_bytes);     // Target protocol addr (same as sender for GARP)
    frame
}

// ============================================================================
// Linux bridge management (ip commands)
// ============================================================================

async fn run_ip_cmd(args: &[&str]) -> Result<String> {
    let output = tokio::process::Command::new("ip")
        .args(args)
        .output()
        .await?;
    if !output.status.success() {
        let stderr = String::from_utf8_lossy(&output.stderr);
        anyhow::bail!("ip {} failed: {}", args.join(" "), stderr.trim());
    }
    Ok(String::from_utf8_lossy(&output.stdout).to_string())
}

/// Create a Linux bridge interface.
pub async fn create_bridge(name: &str) -> Result<()> {
    run_ip_cmd(&["link", "add", name, "type", "bridge"]).await?;
    info!("Created bridge {}", name);
    Ok(())
}

/// Add an interface to a bridge.
pub async fn bridge_add_interface(bridge: &str, iface: &str) -> Result<()> {
    run_ip_cmd(&["link", "set", iface, "master", bridge]).await?;
    info!("Added {} to bridge {}", iface, bridge);
    Ok(())
}

/// Bring an interface up.
pub async fn set_interface_up(iface: &str) -> Result<()> {
    run_ip_cmd(&["link", "set", iface, "up"]).await?;
    Ok(())
}

/// Remove a bridge interface.
pub async fn remove_bridge(name: &str) -> Result<()> {
    // First bring it down, ignore errors
    let _ = run_ip_cmd(&["link", "set", name, "down"]).await;
    run_ip_cmd(&["link", "del", name]).await?;
    info!("Removed bridge {}", name);
    Ok(())
}

/// Detect the default network interface from the routing table.
pub async fn detect_default_interface() -> Result<String> {
    let output = run_ip_cmd(&["route", "show", "default"]).await?;
    // Format: "default via X.X.X.X dev IFACE ..."
    let parts: Vec<&str> = output.split_whitespace().collect();
    if let Some(idx) = parts.iter().position(|&s| s == "dev") {
        if let Some(iface) = parts.get(idx + 1) {
            return Ok(iface.to_string());
        }
    }
    anyhow::bail!("Could not detect default network interface from route table");
}

/// Get the IP address and prefix length of a network interface.
pub async fn get_interface_ip(iface: &str) -> Result<(Ipv4Addr, u8)> {
    let output = run_ip_cmd(&["-4", "addr", "show", "dev", iface]).await?;
    // Parse "inet X.X.X.X/NN" from output
    for line in output.lines() {
        let trimmed = line.trim();
        if let Some(rest) = trimmed.strip_prefix("inet ") {
            let addr_cidr = rest.split_whitespace().next().unwrap_or("");
            let parts: Vec<&str> = addr_cidr.split('/').collect();
            if parts.len() == 2 {
                let ip: Ipv4Addr = parts[0].parse()?;
                let prefix: u8 = parts[1].parse()?;
                return Ok((ip, prefix));
            }
        }
    }
    anyhow::bail!("Could not find IPv4 address on interface {}", iface);
}

/// Migrate the host's IP from a physical interface to a bridge.
/// This is the most delicate operation — briefly interrupts connectivity.
pub async fn migrate_host_ip_to_bridge(
    physical_iface: &str,
    bridge: &str,
    ip: Ipv4Addr,
    prefix: u8,
) -> Result<()> {
    let cidr = format!("{}/{}", ip, prefix);
    // Remove IP from physical interface
    let _ = run_ip_cmd(&["addr", "del", &cidr, "dev", physical_iface]).await;
    // Add IP to bridge
    run_ip_cmd(&["addr", "add", &cidr, "dev", bridge]).await?;
    info!("Migrated IP {} from {} to {}", cidr, physical_iface, bridge);
    Ok(())
}

/// Restore the host's IP from bridge back to the physical interface.
pub async fn restore_host_ip(
    physical_iface: &str,
    bridge: &str,
    ip: Ipv4Addr,
    prefix: u8,
) -> Result<()> {
    let cidr = format!("{}/{}", ip, prefix);
    let _ = run_ip_cmd(&["addr", "del", &cidr, "dev", bridge]).await;
    run_ip_cmd(&["addr", "add", &cidr, "dev", physical_iface]).await?;
    info!("Restored IP {} to {}", cidr, physical_iface);
    Ok(())
}

/// Enable proxy ARP on an interface via sysctl.
pub async fn enable_proxy_arp(iface: &str) -> Result<()> {
    let path = format!("/proc/sys/net/ipv4/conf/{}/proxy_arp", iface);
    tokio::fs::write(&path, "1").await?;
    info!("Enabled proxy_arp on {}", iface);
    Ok(())
}

/// Create a TAP device (L2) using the tun crate.
pub fn create_tap(name: &str, mtu: u16) -> Result<tun::AsyncDevice> {
    let mut config = tun::Configuration::default();
    config
        .tun_name(name)
        .layer(tun::Layer::L2)
        .mtu(mtu)
        .up();

    #[cfg(target_os = "linux")]
    config.platform_config(|p| {
        p.ensure_root_privileges(true);
    });

    let device = tun::create_as_async(&config)?;
    info!("TAP device {} created (L2, mtu={})", name, mtu);
    Ok(device)
}

// ============================================================================
// BridgeEngine — main event loop
// ============================================================================

/// The BridgeEngine wraps/unwraps Ethernet frames and bridges VPN traffic
/// to the host's physical LAN via a Linux bridge + TAP device.
pub struct BridgeEngine {
    state: Arc<ServerState>,
    /// Learned MAC addresses for LAN peers (dst IP → MAC).
    /// Populated from ARP replies and Ethernet frame src MACs.
    arp_cache: HashMap<Ipv4Addr, [u8; 6]>,
}

impl BridgeEngine {
    pub fn new(state: Arc<ServerState>) -> Self {
        Self {
            state,
            arp_cache: HashMap::new(),
        }
    }

    /// Run the bridge engine event loop.
    /// Receives L3 IP packets from VPN clients, wraps in Ethernet, writes to TAP.
    /// Reads Ethernet frames from TAP, strips header, routes back to VPN clients.
    pub async fn run(
        mut self,
        mut tap_device: tun::AsyncDevice,
        mut packet_rx: mpsc::Receiver<Vec<u8>>,
        mut shutdown_rx: mpsc::Receiver<()>,
    ) -> Result<()> {
        let mut buf = vec![0u8; 2048];

        info!("BridgeEngine started");

        loop {
            tokio::select! {
                // Packet from VPN client → wrap in Ethernet → write to TAP
                Some(ip_packet) = packet_rx.recv() => {
                    if let Some(dst_ip) = dst_ip_from_packet(&ip_packet) {
                        let src_ip = src_ip_from_packet(&ip_packet).unwrap_or(Ipv4Addr::UNSPECIFIED);
                        let src_mac = mac_from_ip(src_ip);
                        let dst_mac = self.arp_cache.get(&dst_ip)
                            .copied()
                            .unwrap_or(BROADCAST_MAC);
                        let frame = wrap_in_ethernet(&ip_packet, src_mac, dst_mac);
                        if let Err(e) = tap_device.write_all(&frame).await {
                            warn!("TAP write error: {}", e);
                        }
                    }
                }

                // Frame from TAP (LAN) → strip Ethernet → route to VPN client
                result = tap_device.read(&mut buf) => {
                    match result {
                        Ok(len) if len >= ETH_HEADER_LEN => {
                            let frame = &buf[..len];

                            // Learn src MAC from incoming frames
                            if let Some((ether_type, payload)) = unwrap_ethernet(frame) {
                                // Learn ARP cache from src MAC + src IP
                                let src_mac: [u8; 6] = frame[6..12].try_into().unwrap_or([0; 6]);
                                if ether_type == ETH_TYPE_IPV4 {
                                    if let Some(src_ip) = src_ip_from_packet(payload) {
                                        self.arp_cache.insert(src_ip, src_mac);
                                    }
                                }

                                // Only forward IPv4 packets to VPN clients
                                if ether_type == ETH_TYPE_IPV4 {
                                    if let Some(dst_ip) = dst_ip_from_packet(payload) {
                                        // Look up VPN client by dst IP in tun_routes
                                        let routes = self.state.tun_routes.read().await;
                                        if let Some(sender) = routes.get(&dst_ip) {
                                            let _ = sender.try_send(payload.to_vec());
                                        }
                                    }
                                }
                            }
                        }
                        Ok(_) => {} // Frame too short, ignore
                        Err(e) => {
                            warn!("TAP read error: {}", e);
                        }
                    }
                }

                _ = shutdown_rx.recv() => {
                    info!("BridgeEngine shutting down");
                    break;
                }
            }
        }

        Ok(())
    }

    /// Send a gratuitous ARP for a VPN client IP.
    pub async fn announce_client(tap: &mut tun::AsyncDevice, ip: Ipv4Addr) -> Result<()> {
        let mac = mac_from_ip(ip);
        let garp = build_garp(ip, mac);
        tap.write_all(&garp).await?;
        debug!("Sent GARP for {} (MAC {:02x}:{:02x}:{:02x}:{:02x}:{:02x}:{:02x})",
            ip, mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
        Ok(())
    }
}