Emulate DNAT in userland for loopback-to-loopback connections. This makes container ports available from localhost.

network.go

@@ -4,6 +4,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"io"
 	"log"
 	"net"
 	"os/exec"
@@ -221,10 +222,55 @@ func (mapper *PortMapper) Map(port int, dest net.TCPAddr) error {
 	if err := mapper.iptablesForward("-A", port, dest); err != nil {
 		return err
 	}
+
 	mapper.mapping[port] = dest
+	listener, err := net.Listen("tcp", fmt.Sprintf("127.0.0.1:%d", port))
+	if err != nil {
+		mapper.Unmap(port)
+		return err
+	}
+	// FIXME: store the listener so we can close it at Unmap
+	go proxy(listener, "tcp", dest.String())
 	return nil
 }
 
+// proxy listens for socket connections on `listener`, and forwards them unmodified
+// to `proto:address`
+func proxy(listener net.Listener, proto, address string) error {
+	Debugf("proxying to %s:%s", proto, address)
+	defer Debugf("Done proxying to %s:%s", proto, address)
+	for {
+		Debugf("Listening on %s", listener)
+		src, err := listener.Accept()
+		if err != nil {
+			return err
+		}
+		Debugf("Connecting to %s:%s", proto, address)
+		dst, err := net.Dial(proto, address)
+		if err != nil {
+			log.Printf("Error connecting to %s:%s: %s", proto, address, err)
+			src.Close()
+			continue
+		}
+		Debugf("Connected to backend, splicing")
+		splice(src, dst)
+	}
+	return nil
+}
+
+func halfSplice(dst, src net.Conn) error {
+	_, err := io.Copy(dst, src)
+	// FIXME: on EOF from a tcp connection, pass WriteClose()
+	dst.Close()
+	src.Close()
+	return err
+}
+
+func splice(a, b net.Conn) {
+	go halfSplice(a, b)
+	go halfSplice(b, a)
+}
+
 func (mapper *PortMapper) Unmap(port int) error {
 	dest, ok := mapper.mapping[port]
 	if !ok {