// Copyright 2017 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package tcpproxy

import (
	"bufio"
	"bytes"
	"crypto/tls"
	"io"
	"net"
)

// AddSNIRoute appends a route to the ipPort listener that says if the
// incoming TLS SNI server name is sni, the connection is given to
// dest. If it doesn't match, rule processing continues for any
// additional routes on ipPort.
//
// The ipPort is any valid net.Listen TCP address.
func (p *Proxy) AddSNIRoute(ipPort, sni string, dest Target) {
	p.addRoute(ipPort, sniMatch(sni), dest)
}

type sniMatch string

func (sni sniMatch) match(br *bufio.Reader) bool {
	return clientHelloServerName(br) == string(sni)
}

// clientHelloServerName returns the SNI server name inside the TLS ClientHello,
// without consuming any bytes from br.
// On any error, the empty string is returned.
func clientHelloServerName(br *bufio.Reader) (sni string) {
	const recordHeaderLen = 5
	hdr, err := br.Peek(recordHeaderLen)
	if err != nil {
		return ""
	}
	const recordTypeHandshake = 0x16
	if hdr[0] != recordTypeHandshake {
		return "" // Not TLS.
	}
	recLen := int(hdr[3])<<8 | int(hdr[4]) // ignoring version in hdr[1:3]
	helloBytes, err := br.Peek(recordHeaderLen + recLen)
	if err != nil {
		return ""
	}
	tls.Server(sniSniffConn{r: bytes.NewReader(helloBytes)}, &tls.Config{
		GetConfigForClient: func(hello *tls.ClientHelloInfo) (*tls.Config, error) {
			sni = hello.ServerName
			return nil, nil
		},
	}).Handshake()
	return
}

// sniSniffConn is a net.Conn that reads from r, fails on Writes,
// and crashes otherwise.
type sniSniffConn struct {
	r        io.Reader
	net.Conn // nil; crash on any unexpected use
}

func (c sniSniffConn) Read(p []byte) (int, error) { return c.r.Read(p) }
func (sniSniffConn) Write(p []byte) (int, error)  { return 0, io.EOF }