feat(server): add bridge forwarding mode and per-client destination policy overrides

rust/src/bridge.rs

@@ -0,0 +1,352 @@
+//! 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(())
+    }
+}