Feature: support fd device

Experimental support for file descriptor based device, may be used like `fd://3`

core/device/fd/fd.go

@@ -1,6 +1,7 @@
 package fd
 
 import (
+	"fmt"
 	"strconv"
 
 	"github.com/xjasonlyu/tun2socks/core/device"
@@ -17,6 +18,14 @@ type FD struct {
 	mtu uint32
 }
 
+func Open(name string, mtu uint32) (device.Device, error) {
+	fd, err := strconv.Atoi(name)
+	if err != nil {
+		return nil, fmt.Errorf("cannot open fd: %s", name)
+	}
+	return open(fd, mtu)
+}
+
 func (f *FD) Type() string {
 	return Driver
 }

core/device/fd/open_linux.go

@@ -7,7 +7,7 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/link/fdbased"
 )
 
-func Open(fd int, mtu uint32) (device.Device, error) {
+func open(fd int, mtu uint32) (device.Device, error) {
 	f := &FD{fd: fd, mtu: mtu}
 
 	ep, err := fdbased.New(&fdbased.Options{

core/device/fd/open_others.go

@@ -10,7 +10,7 @@ import (
 	"github.com/xjasonlyu/tun2socks/core/device/rwbased"
 )
 
-func Open(fd int, mtu uint32) (device.Device, error) {
+func open(fd int, mtu uint32) (device.Device, error) {
 	f := &FD{fd: fd, mtu: mtu}
 
 	ep, err := rwbased.New(os.NewFile(uintptr(fd), f.Name()), mtu)

engine/parse.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/xjasonlyu/tun2socks/core/device"
+	"github.com/xjasonlyu/tun2socks/core/device/fd"
 	"github.com/xjasonlyu/tun2socks/core/device/tun"
 	"github.com/xjasonlyu/tun2socks/proxy"
 	"github.com/xjasonlyu/tun2socks/proxy/proto"
@@ -28,6 +29,8 @@ func parseDevice(s string, mtu uint32) (device.Device, error) {
 	switch driver {
 	case tun.Driver:
 		return tun.Open(name, mtu)
+	case fd.Driver:
+		return fd.Open(name, mtu)
 	default:
 		return nil, fmt.Errorf("unsupported driver: %s", driver)
 	}