net.Dial alike Dial for SOCKS4a, and SOCKS5, or how to use http.Client's Transport element to proxy HTTP calls

gistfile1.go

package main

import (
	"bytes"
	"encoding/binary"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"strconv"
)

func main() {
	tr := &http.Transport{
		Dial: DialSOCKS5, // or DialSOCKS4a
	}
	c := &http.Client{
		Transport: tr,
	}
	res, err := c.Get("http://echoip.com")
	if err != nil {
		log.Fatalf("%s", err)
	}
	body, err := ioutil.ReadAll(res.Body)
	if err != nil {
		log.Fatalf("%s", err)
	}
	log.Print(string(body))
}

func DialSOCKS4a(network, addr string) (c net.Conn, err error) {
	laddr, _ := net.ResolveTCPAddr("tcp", "127.0.0.1")
	raddr, _ := net.ResolveTCPAddr("tcp", "127.0.0.1:9999") // SOCKS4a server
	host, p, err := net.SplitHostPort(addr)
	if err != nil {
		return
	}
	port, err := strconv.Atoi(p)
	if err != nil {
		return
	}
	c, err = net.DialTCP("tcp", laddr, raddr)

	buffer := bytes.NewBuffer([]byte{})
	// SOCKS version number
	binary.Write(buffer, binary.BigEndian, []byte{0x04})
	// Command code. 0x01 to establish a TCP/IP stream connection
	binary.Write(buffer, binary.BigEndian, []byte{0x01})
	// Network byte order port number
	binary.Write(buffer, binary.BigEndian, uint16(port))
	// Deliberate invalid IP address
	binary.Write(buffer, binary.BigEndian, []byte{0x00, 0x00, 0x00, 0x01})
	// User ID
	binary.Write(buffer, binary.BigEndian, []byte{0x00})
	// Domain name of the host we want to contact
	binary.Write(buffer, binary.BigEndian, []byte(host))
	// Since domain name is variable lengthm terminate it with a null
	binary.Write(buffer, binary.BigEndian, []byte{0x00})
	binary.Write(c, binary.BigEndian, buffer.Bytes())
	data := make([]byte, 8)
	_, err = c.Read(data)
	if err != nil {
		return
	}

	return
}

func DialSOCKS5(network, addr string) (c net.Conn, err error) {
	laddr, _ := net.ResolveTCPAddr("tcp", "127.0.0.1")
	raddr, _ := net.ResolveTCPAddr("tcp", "127.0.0.1:9999") // SOCKS5 server
	host, p, err := net.SplitHostPort(addr)
	if err != nil {
		return
	}
	port, err := strconv.Atoi(p)
	if err != nil {
		return
	}
	c, err = net.DialTCP("tcp", laddr, raddr)

	version := []byte{0x05}
	buffer := bytes.NewBuffer([]byte{})
	// SOCKS version number
	binary.Write(buffer, binary.BigEndian, version)
	// In SOCKS5 initial handshake is different
	// A greeting which includes a list of authentication methods supported
	// Only one authentication method is supported
	binary.Write(buffer, binary.BigEndian, []byte{0x01})
	// Supported authentication method is "No authentication"
	binary.Write(buffer, binary.BigEndian, []byte{0x00})
	binary.Write(c, binary.BigEndian, buffer.Bytes())
	data := make([]byte, 8)
	_, err = c.Read(data)
	if err != nil {
		return
	}

	buffer = bytes.NewBuffer([]byte{})
	// SOCKS version number
	binary.Write(buffer, binary.BigEndian, version)
	// Command code. 0x01 to establish a TCP/IP stream connection
	binary.Write(buffer, binary.BigEndian, []byte{0x01})
	// Reserved, must be 0x00
	binary.Write(buffer, binary.BigEndian, []byte{0x00})
	// Address type. 0x03 is for domain name
	binary.Write(buffer, binary.BigEndian, []byte{0x03})
	// Length of domain name
	binary.Write(buffer, binary.BigEndian, []byte{byte(len(host))})
	// Domain name
	binary.Write(buffer, binary.BigEndian, []byte(host))
	// Network byte order port number
	binary.Write(buffer, binary.BigEndian, uint16(port))
	binary.Write(c, binary.BigEndian, buffer.Bytes())
	data = make([]byte, 16)
	_, err = c.Read(data)
	if err != nil {
		return
	}

	return
}