package main

import (
	"bufio"
	"bytes"
	"context"
	"flag"
	"fmt"
	"log"
	"net"
	"net/http"
	"net/url"
	"regexp"

	"github.com/elazarl/goproxy"
	"github.com/inconshreveable/go-vhost"
)

func orPanic(err error) {
	if err != nil {
		panic(err)
	}
}

func main() {
	verbose := flag.Bool("v", true, "should every proxy request be logged to stdout")
	http_addr := flag.String("httpaddr", ":3129", "proxy http listen address")
	https_addr := flag.String("httpsaddr", ":3128", "proxy https listen address")
	flag.Parse()

	proxy := goproxy.NewProxyHttpServer()
	proxy.Verbose = *verbose
	if proxy.Verbose {
		log.Printf("Server starting up! - configured to listen on http interface %s and https interface %s", *http_addr, *https_addr)
	}

	proxy.NonproxyHandler = http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
		if req.Host == "" {
			fmt.Fprintln(w, "Cannot handle requests without Host header, e.g., HTTP 1.0")
			return
		}
		req.URL.Scheme = "http"
		req.URL.Host = req.Host
		proxy.ServeHTTP(w, req)
	})
	proxy.OnRequest(goproxy.ReqHostMatches(regexp.MustCompile("^.*$"))).
		HandleConnect(goproxy.AlwaysMitm)
	proxy.OnRequest(goproxy.ReqHostMatches(regexp.MustCompile("^.*:80$"))).
		HijackConnect(func(req *http.Request, client net.Conn, ctx *goproxy.ProxyCtx) {
			defer func() {
				if e := recover(); e != nil {
					ctx.Logf("error connecting to remote: %v", e)
					client.Write([]byte("HTTP/1.1 500 Cannot reach destination\r\n\r\n"))
				}
				client.Close()
			}()
			clientBuf := bufio.NewReadWriter(bufio.NewReader(client), bufio.NewWriter(client))

			remote, err := connectDial(req.Context(), proxy, "tcp", req.URL.Host)
			orPanic(err)
			remoteBuf := bufio.NewReadWriter(bufio.NewReader(remote), bufio.NewWriter(remote))
			for {
				req, err := http.ReadRequest(clientBuf.Reader)
				orPanic(err)
				orPanic(req.Write(remoteBuf))
				orPanic(remoteBuf.Flush())
				resp, err := http.ReadResponse(remoteBuf.Reader, req)
				orPanic(err)
				orPanic(resp.Write(clientBuf.Writer))
				orPanic(clientBuf.Flush())
			}
		})

	go func() {
		log.Fatalln(http.ListenAndServe(*http_addr, proxy))
	}()

	// listen to the TLS ClientHello but make it a CONNECT request instead
	ln, err := net.Listen("tcp", *https_addr)
	if err != nil {
		log.Fatalf("Error listening for https connections - %v", err)
	}
	for {
		c, err := ln.Accept()
		if err != nil {
			log.Printf("Error accepting new connection - %v", err)
			continue
		}
		go func(c net.Conn) {
			tlsConn, err := vhost.TLS(c)
			if err != nil {
				log.Printf("Error accepting new connection - %v", err)
			}
			if tlsConn.Host() == "" {
				log.Printf("Cannot support non-SNI enabled clients")
				return
			}
			connectReq := &http.Request{
				Method: http.MethodConnect,
				URL: &url.URL{
					Opaque: tlsConn.Host(),
					Host:   net.JoinHostPort(tlsConn.Host(), "443"),
				},
				Host:       tlsConn.Host(),
				Header:     make(http.Header),
				RemoteAddr: c.RemoteAddr().String(),
			}
			resp := dumbResponseWriter{tlsConn}
			proxy.ServeHTTP(resp, connectReq)
		}(c)
	}
}

// copied/converted from https.go
func dial(ctx context.Context, proxy *goproxy.ProxyHttpServer, network, addr string) (c net.Conn, err error) {
	if proxy.Tr.DialContext != nil {
		return proxy.Tr.DialContext(ctx, network, addr)
	}
	var d net.Dialer
	return d.DialContext(ctx, network, addr)
}

// copied/converted from https.go
func connectDial(ctx context.Context, proxy *goproxy.ProxyHttpServer, network, addr string) (c net.Conn, err error) {
	if proxy.ConnectDial == nil {
		return dial(ctx, proxy, network, addr)
	}
	return proxy.ConnectDial(network, addr)
}

type dumbResponseWriter struct {
	net.Conn
}

func (dumb dumbResponseWriter) Header() http.Header {
	panic("Header() should not be called on this ResponseWriter")
}

func (dumb dumbResponseWriter) Write(buf []byte) (int, error) {
	if bytes.Equal(buf, []byte("HTTP/1.0 200 OK\r\n\r\n")) {
		return len(buf), nil // throw away the HTTP OK response from the faux CONNECT request
	}
	return dumb.Conn.Write(buf)
}

func (dumb dumbResponseWriter) WriteHeader(code int) {
	panic("WriteHeader() should not be called on this ResponseWriter")
}

func (dumb dumbResponseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	return dumb, bufio.NewReadWriter(bufio.NewReader(dumb), bufio.NewWriter(dumb)), nil
}
