WIP

peer/forwarder.go

@@ -0,0 +1,84 @@
+package peer
+
+import (
+	"log"
+	"net"
+	"net/netip"
+	"sync/atomic"
+)
+
+const ForwarderBasePort = 5000
+
+// Forwarder is a relay-side UDP forwarding service. It listens on one UDP port
+// per peer on the relay's VPN interface and forwards received bytes to each
+// peer's physical WireGuard port. Port assignment: ForwarderBasePort +
+// peer_vpn_ip_byte (peer .11 → port 5011).
+//
+// Incoming packets are authenticated for free by the relay's outer WireGuard
+// session before they reach the forwarder.
+type Forwarder struct {
+	vpnIP netip.Addr
+	peers [256]atomic.Pointer[fwdPeer]
+}
+
+// fwdPeer is immutable once created. Endpoint changes produce a new fwdPeer.
+type fwdPeer struct {
+	listenConn *net.UDPConn
+	endpoint   netip.AddrPort
+}
+
+// NewForwarder creates a Forwarder that will bind listeners to vpnIP.
+func NewForwarder(vpnIP netip.Addr) *Forwarder {
+	return &Forwarder{vpnIP: vpnIP}
+}
+
+// SetEndpoint registers or updates the physical WireGuard endpoint for the
+// peer identified by peerIPByte (last octet of its VPN IP). The old fwdPeer,
+// if any, is closed before the new one is opened on the same port.
+func (f *Forwarder) SetEndpoint(peerIPByte byte, ep netip.AddrPort) {
+	if old := f.peers[peerIPByte].Swap(nil); old != nil {
+		old.listenConn.Close()
+	}
+
+	listenAddr := netip.AddrPortFrom(f.vpnIP, ForwarderBasePort+uint16(peerIPByte))
+	listenConn, err := net.ListenUDP("udp4", net.UDPAddrFromAddrPort(listenAddr))
+	if err != nil {
+		log.Printf("[Forwarder] listen %v: %v", listenAddr, err)
+		return
+	}
+
+	p := &fwdPeer{listenConn: listenConn, endpoint: ep}
+	f.peers[peerIPByte].Store(p)
+	go p.run()
+}
+
+// RemoveEndpoint stops forwarding for the given peer and closes its listener.
+func (f *Forwarder) RemoveEndpoint(peerIPByte byte) {
+	if old := f.peers[peerIPByte].Swap(nil); old != nil {
+		old.listenConn.Close()
+	}
+}
+
+func (p *fwdPeer) run() {
+	network := "udp4"
+	if p.endpoint.Addr().Is6() {
+		network = "udp6"
+	}
+	sendConn, err := net.DialUDP(network, nil, net.UDPAddrFromAddrPort(p.endpoint))
+	if err != nil {
+		log.Printf("[Forwarder] dial %v: %v", p.endpoint, err)
+		return
+	}
+	defer sendConn.Close()
+
+	buf := make([]byte, 1<<16)
+	for {
+		n, err := p.listenConn.Read(buf)
+		if err != nil {
+			return
+		}
+		if _, err := sendConn.Write(buf[:n]); err != nil {
+			log.Printf("[Forwarder] write to %v: %v", p.endpoint, err)
+		}
+	}
+}