diff --git a/agent/internal/proxy/proxy.go b/agent/internal/proxy/proxy.go
new file mode 100644
index 0000000..c87d3e8
--- /dev/null
+++ b/agent/internal/proxy/proxy.go
@@ -0,0 +1,114 @@
+// Package agentproxy relays a single TCP connection between a local service and
+// the control plane, so a control plane that cannot route to this host's network
+// can still open a console session.
+//
+// The dial host is hardcoded to loopback. The control plane supplies only a
+// port, and nothing in this package can be made to dial anywhere else.
+package agentproxy
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"strconv"
+	"time"
+
+	"gitea.hostxtra.co.uk/mrhid6/vantage/agent/internal/grpc/pb"
+)
+
+const (
+	loopbackHost = "127.0.0.1"
+	chunkSize    = 32 * 1024
+	dialTimeout  = 10 * time.Second
+)
+
+// Stream is the agent's half of a ProxyStream.
+type Stream interface {
+	Send(*pb.ProxyClientMsg) error
+	Recv() (*pb.ProxyServerMsg, error)
+	CloseSend() error
+}
+
+// Open dials the local port, announces itself on the stream, and relays until
+// either side ends. A refused dial is reported as an explicit close so the
+// operator sees a reason rather than a hang.
+func Open(stream Stream, serverID, agentToken, proxyID string, port uint32) error {
+	conn, dialErr := net.DialTimeout("tcp",
+		net.JoinHostPort(loopbackHost, strconv.Itoa(int(port))), dialTimeout)
+
+	if err := stream.Send(&pb.ProxyClientMsg{Open: &pb.ProxyOpen{
+		ServerId:   serverID,
+		AgentToken: agentToken,
+		ProxyId:    proxyID,
+	}}); err != nil {
+		if conn != nil {
+			_ = conn.Close()
+		}
+		return fmt.Errorf("send open: %w", err)
+	}
+
+	if dialErr != nil {
+		_ = stream.Send(&pb.ProxyClientMsg{Close: &pb.ProxyClose{
+			Reason: "dial_refused: " + dialErr.Error(),
+		}})
+		_ = stream.CloseSend()
+		return fmt.Errorf("dial 127.0.0.1:%d: %w", port, dialErr)
+	}
+	defer conn.Close()
+
+	return relay(conn, stream)
+}
+
+func relay(conn net.Conn, stream Stream) error {
+	errCh := make(chan error, 2)
+
+	// local service -> control plane
+	go func() {
+		buf := make([]byte, chunkSize)
+		for {
+			n, err := conn.Read(buf)
+			if n > 0 {
+				chunk := make([]byte, n)
+				copy(chunk, buf[:n])
+				if sendErr := stream.Send(&pb.ProxyClientMsg{Data: chunk}); sendErr != nil {
+					errCh <- sendErr
+					return
+				}
+			}
+			if err != nil {
+				errCh <- err
+				return
+			}
+		}
+	}()
+
+	// control plane -> local service
+	go func() {
+		for {
+			msg, err := stream.Recv()
+			if err != nil {
+				errCh <- err
+				return
+			}
+			if msg.Close != nil {
+				errCh <- fmt.Errorf("server closed relay: %s", msg.Close.Reason)
+				return
+			}
+			if len(msg.Data) > 0 {
+				if _, err := conn.Write(msg.Data); err != nil {
+					errCh <- err
+					return
+				}
+			}
+		}
+	}()
+
+	err := <-errCh
+	_ = conn.Close()
+	_ = stream.CloseSend()
+	if errors.Is(err, io.EOF) {
+		return nil
+	}
+	return err
+}