add transparent proxy support

add transparent proxy support for dynamic port forward on linux

on linux, can use iptables to redirect all tcp packet to obfssh
 client dynamic listen port(-D), and forward through ssh channel

client.go

@@ -255,10 +255,37 @@ func (cc *Client) handleRemoteForward(conn net.Conn, local string) {
 }
 
 func (cc *Client) handleDynamicForward(conn net.Conn) {
+	addr, err := getOriginDst(conn)
+	if err == nil {
+		if addr.String() != conn.LocalAddr().String() {
+			// transparent proxy
+			// iptables redirect the packet to this port
+			cc.handleTransparentProxy(conn, addr)
+			return
+		}
+	} else {
+		// SO_ORIGNAL_DST failed
+		// ipv6 not support this syscall
+		// so ignore it
+		Log(DEBUG, "get original destination on %s failed: %s, ignore",
+			conn.LocalAddr(), err)
+	}
+
+	// socks5 to this port
 	s := socks.Conn{Conn: conn, Dial: cc.client.Dial}
 	s.Serve()
 }
 
+func (cc *Client) handleTransparentProxy(c net.Conn, addr net.Addr) {
+	c2, err := cc.client.Dial("tcp", addr.String())
+	if err != nil {
+		Log(ERROR, "%s", err)
+		c.Close()
+		return
+	}
+	PipeAndClose(c2, c)
+}
+
 func (cc *Client) keepAlive(interval time.Duration, maxCount int) {
 	count := 0
 	c := time.NewTicker(interval)

redir.go

@@ -0,0 +1,11 @@
+// +build !linux
+
+package obfssh
+
+import (
+	"net"
+)
+
+func getOriginDst(c net.Conn) (net.Addr, error) {
+	return c.LocalAddr(), nil
+}

redir_iptables.go

@@ -0,0 +1,76 @@
+// +build linux
+
+package obfssh
+
+import (
+	"encoding/binary"
+	"fmt"
+	"net"
+	"syscall"
+	"unsafe"
+)
+
+const (
+	// SO_ORIGINAL_DST in linux/netfilter_ipv4.h
+	soOriginalDst = 80
+)
+
+func getOriginDst(c net.Conn) (net.Addr, error) {
+	var sockaddr syscall.RawSockaddrAny
+	var len = unsafe.Sizeof(sockaddr)
+
+	cc, ok := c.(*net.TCPConn)
+	if !ok {
+		return nil, fmt.Errorf("only tcp socket supported")
+	}
+
+	f, err := cc.File()
+	if err != nil {
+		return nil, err
+	}
+
+	defer f.Close()
+
+	// get original ip destination, in C like this
+	//
+	// struct sockaddr addr;
+	// memset(&addr, 0, sizeof(addr);
+	// int len = sizeof(addr);
+	// getsocketopt(fd, SOL_IP, SO_ORIGINAL_DST, &addr, &len);
+	//
+	_, _, errno := syscall.Syscall6(sysGetSockOpt, f.Fd(),
+		uintptr(syscall.SOL_IP), uintptr(soOriginalDst),
+		uintptr(unsafe.Pointer(&sockaddr)),
+		uintptr(unsafe.Pointer(&len)), 0)
+
+	if errno != 0 {
+		return nil, fmt.Errorf("syscall error %d", errno)
+	}
+
+	var port uint16
+	var ip net.IP
+
+	switch sockaddr.Addr.Family {
+	case syscall.AF_INET:
+		a := (*syscall.RawSockaddrInet4)(unsafe.Pointer(&sockaddr))
+		ip = net.IP(a.Addr[0:])
+		port = ntohl(a.Port)
+	case syscall.AF_INET6:
+		a := (*syscall.RawSockaddrInet6)(unsafe.Pointer(&sockaddr))
+		ip = net.IP(a.Addr[0:])
+		port = ntohl(a.Port)
+	default:
+		return nil, fmt.Errorf("unknown socket family: %d",
+			sockaddr.Addr.Family)
+	}
+
+	addr := &net.TCPAddr{IP: ip, Port: int(port)}
+	return addr, nil
+}
+
+func ntohl(a uint16) uint16 {
+	b := make([]byte, 2)
+	binary.BigEndian.PutUint16(b, a)
+	c := binary.LittleEndian.Uint16(b)
+	return c
+}

redir_linux_386.go

@@ -0,0 +1,5 @@
+package obfssh
+
+// getsockopt syscall number
+// refer to $GOROOT/src/syscall/syscall_linux_386.go
+const sysGetSockOpt = 15

redir_vars.go

@@ -0,0 +1,11 @@
+// +build linux, !386
+// +build linux
+
+package obfssh
+
+import (
+	"syscall"
+)
+
+// getsockopt syscall number
+const sysGetSockOpt = syscall.SYS_GETSOCKOPT