proxy.go

// toy http "proxy" (not adhereing the proxy protocol,
// only a pre-configured static host), first go programm.
// replaces some words with l33t alternative and replaces
// all <img> tags.
// call with './proxy www.mmix.cs.hm.edu'; only works for
// http sites, no https.

// known issues:
// - not a real proxy
// - not valid http
// - no error checking \wrt sockets
// - inefficient and DoS'able by downloading large files

package main

import "net"
import "strings"
import "fmt"
import "bufio"
import "io"
import "bytes"
import "os"
import "net/textproto"
import "regexp"

func getHeaders(con net.Conn) []string {
	reader := bufio.NewReader(con)
	tp := textproto.NewReader(reader)

	var headers []string

	for {
		line, _ := tp.ReadLine()
		if len(line) == 0 {
			return headers
		} else {
			headers = append(headers, line)
		}
	}
}

func prepareRequestHeader(headers []string, host string) []string {
	var output []string
	for _, element := range headers {
		if (strings.HasPrefix(element, "Accept-Encoding:") || strings.HasPrefix(element, "Host:") || strings.HasPrefix(element, "Connection:")) {
			fmt.Printf("DROPPING CLIENT HEADER: %v\n", element)
		} else {
			output = append(output, element)
		}
	}
	output = append(output, "Host: " + host + "\r")
	output = append(output, "Connection: close\r")
	return   append(output, "Accept-Encoding: identity\r")
}

func leetify(b string) string {
	img_url := "https://w3-mediapool.hm.edu/mediapool/media/_technik/img/_technik_1/hm_logo_header.de.jpg"
	re := regexp.MustCompile("<img (.*)>")

	ret := strings.Replace(strings.Replace(strings.Replace(strings.Replace(
		    strings.Replace(strings.Replace(strings.Replace(strings.Replace(
		    strings.Replace(strings.Replace(
		    strings.Replace(strings.Replace(b,
			"MMIX", "MM1X", -1),
			"Java", "J4v4", -1),
			"Computer", "C0mpu73r", -1),
			"RISC", "R15C", -1),
			"CISC", "C15C", -1),
			"Debugger", "D3bu663r", -1),
			"Informatik", "1nfr0m471k", -1),
			"Studentin", "57ud3n71n", -1),
			"Student", "57ud3n7", -1),
			"Studierende", "57ud13r3nd3", -1),
			"Windows", "w1nd0w5", -1),
			"Linux", "l1nux", -1)
	return re.ReplaceAllString(ret, "<img src='" + img_url + "'>")
}

func main() {
	ln, _ := net.Listen("tcp", "localhost:8080")
	dest := os.Args[1]
	for {
		browser, _ := ln.Accept()
		remote, _ := net.Dial("tcp", dest + ":80")

		// rewrite request headers, send them to the actual remote server
		// line by line
		for _, element := range prepareRequestHeader(getHeaders(browser), dest) {
			fmt.Printf("> %v\n", element)
			fmt.Fprintf(remote, "%v\n", element)
		}
		fmt.Fprintf(remote, "\r\n\r\n")
		fmt.Printf("\r\n\r\n")

		var b bytes.Buffer

		// buffer the entire response (memory intensive...), leetify it
		// and rewrite the <img> tags, then send back to the browser and
		// close the connection (OK since we changed to
		// "connection: close" above)
		io.Copy(&b, remote)

		re_content_length := regexp.MustCompile("(Content-Length: [0-9]*\r\n)")
		c := re_content_length.ReplaceAllString(b.String(), "")

		fmt.Fprintf(browser, "%v", leetify(c))
		browser.Close()
	}
}