// 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 lets users build TCP proxies, optionally making
// routing decisions based on HTTP/1 Host headers and the SNI hostname
// in TLS connections.
//
// Typical usage:
//
//	var p tcpproxy.Proxy
//	p.AddHTTPHostRoute(":80", "foo.com", tcpproxy.To("10.0.0.1:8081"))
//	p.AddHTTPHostRoute(":80", "bar.com", tcpproxy.To("10.0.0.2:8082"))
//	p.AddRoute(":80", tcpproxy.To("10.0.0.1:8081")) // fallback
//	p.AddSNIRoute(":443", "foo.com", tcpproxy.To("10.0.0.1:4431"))
//	p.AddSNIRoute(":443", "bar.com", tcpproxy.To("10.0.0.2:4432"))
//	p.AddRoute(":443", tcpproxy.To("10.0.0.1:4431")) // fallback
//	log.Fatal(p.Run())
//
// Calling Run (or Start) on a proxy also starts all the necessary
// listeners.
//
// For each accepted connection, the rules for that ipPort are
// matched, in order. If one matches (currently HTTP Host, SNI, or
// always), then the connection is handed to the target.
//
// The two predefined Target implementations are:
//
// 1) DialProxy, proxying to another address (use the To func to return a
// DialProxy value),
//
// 2) TargetListener, making the matched connection available via a
// net.Listener.Accept call.
//
// But Target is an interface, so you can also write your own.
//
// Note that tcpproxy does not do any TLS encryption or decryption. It
// only (via DialProxy) copies bytes around. The SNI hostname in the TLS
// header is unencrypted, for better or worse.
//
// This package makes no API stability promises. If you depend on it,
// vendor it.
package tcpproxy

import (
	"bufio"
	"context"
	"errors"
	"fmt"
	"io"
	"log"
	"net"
	"time"
)

// Proxy is a proxy. Its zero value is a valid proxy that does
// nothing. Call methods to add routes before calling Start or Run.
//
// The order that routes are added in matters; each is matched in the order
// registered.
type Proxy struct {
	configs map[string]*config // ip:port => config

	lns   []net.Listener
	donec chan struct{} // closed before err
	err   error         // any error from listening

	// ListenFunc optionally specifies an alternate listen
	// function. If nil, net.Dial is used.
	// The provided net is always "tcp".
	ListenFunc func(net, laddr string) (net.Listener, error)
}

// Matcher reports whether hostname matches the Matcher's criteria.
type Matcher func(ctx context.Context, hostname string) bool

// equals is a trivial Matcher that implements string equality.
func equals(want string) Matcher {
	return func(_ context.Context, got string) bool {
		return want == got
	}
}

// config contains the proxying state for one listener.
type config struct {
	routes []route
}

// A route matches a connection to a target.
type route interface {
	// match examines the initial bytes of a connection, looking for a
	// match. If a match is found, match returns a non-nil Target to
	// which the stream should be proxied. match returns nil if the
	// connection doesn't match.
	//
	// match must not consume bytes from the given bufio.Reader, it
	// can only Peek.
	//
	// If an sni or host header was parsed successfully, that will be
	// returned as the second parameter.
	match(*bufio.Reader) (Target, string)
}

func (p *Proxy) netListen() func(net, laddr string) (net.Listener, error) {
	if p.ListenFunc != nil {
		return p.ListenFunc
	}
	return net.Listen
}

func (p *Proxy) configFor(ipPort string) *config {
	if p.configs == nil {
		p.configs = make(map[string]*config)
	}
	if p.configs[ipPort] == nil {
		p.configs[ipPort] = &config{}
	}
	return p.configs[ipPort]
}

func (p *Proxy) addRoute(ipPort string, r route) {
	cfg := p.configFor(ipPort)
	cfg.routes = append(cfg.routes, r)
}

// AddRoute appends an always-matching route to the ipPort listener,
// directing any connection to dest.
//
// This is generally used as either the only rule (for simple TCP
// proxies), or as the final fallback rule for an ipPort.
//
// The ipPort is any valid net.Listen TCP address.
func (p *Proxy) AddRoute(ipPort string, dest Target) {
	p.addRoute(ipPort, fixedTarget{dest})
}

type fixedTarget struct {
	t Target
}

func (m fixedTarget) match(*bufio.Reader) (Target, string) { return m.t, "" }

// Run is calls Start, and then Wait.
//
// It blocks until there's an error. The return value is always
// non-nil.
func (p *Proxy) Run() error {
	if err := p.Start(); err != nil {
		return err
	}
	return p.Wait()
}

// Wait waits for the Proxy to finish running. Currently this can only
// happen if a Listener is closed, or Close is called on the proxy.
//
// It is only valid to call Wait after a successful call to Start.
func (p *Proxy) Wait() error {
	<-p.donec
	return p.err
}

// Close closes all the proxy's self-opened listeners.
func (p *Proxy) Close() error {
	for _, c := range p.lns {
		c.Close()
	}
	return nil
}

// Start creates a TCP listener for each unique ipPort from the
// previously created routes and starts the proxy. It returns any
// error from starting listeners.
//
// If it returns a non-nil error, any successfully opened listeners
// are closed.
func (p *Proxy) Start() error {
	if p.donec != nil {
		return errors.New("already started")
	}
	p.donec = make(chan struct{})
	errc := make(chan error, len(p.configs))
	p.lns = make([]net.Listener, 0, len(p.configs))
	for ipPort, config := range p.configs {
		ln, err := p.netListen()("tcp", ipPort)
		if err != nil {
			p.Close()
			return err
		}
		p.lns = append(p.lns, ln)
		go p.serveListener(errc, ln, config.routes)
	}
	go p.awaitFirstError(errc)
	return nil
}

func (p *Proxy) awaitFirstError(errc <-chan error) {
	p.err = <-errc
	close(p.donec)
}

func (p *Proxy) serveListener(ret chan<- error, ln net.Listener, routes []route) {
	for {
		c, err := ln.Accept()
		if err != nil {
			ret <- err
			return
		}
		go p.serveConn(c, routes)
	}
}

// serveConn runs in its own goroutine and matches c against routes.
// It returns whether it matched purely for testing.
func (p *Proxy) serveConn(c net.Conn, routes []route) bool {
	br := bufio.NewReader(c)
	for _, route := range routes {
		if target, hostName := route.match(br); target != nil {
			if n := br.Buffered(); n > 0 {
				peeked, _ := br.Peek(br.Buffered())
				c = &Conn{
					HostName: hostName,
					Peeked:   peeked,
					Conn:     c,
				}
			}
			target.HandleConn(c)
			return true
		}
	}
	// TODO: hook for this?
	log.Printf("tcpproxy: no routes matched conn %v/%v; closing", c.RemoteAddr().String(), c.LocalAddr().String())
	c.Close()
	return false
}

// Conn is an incoming connection that has had some bytes read from it
// to determine how to route the connection. The Read method stitches
// the peeked bytes and unread bytes back together.
type Conn struct {
	// HostName is the hostname field that was sent to the request router.
	// In the case of TLS, this is the SNI header, in the case of HTTPHost
	// route, it will be the host header.  In the case of a fixed
	// route, i.e. those created with AddRoute(), this will always be
	// empty. This can be useful in the case where further routing decisions
	// need to be made in the Target impementation.
	HostName string

	// Peeked are the bytes that have been read from Conn for the
	// purposes of route matching, but have not yet been consumed
	// by Read calls. It set to nil by Read when fully consumed.
	Peeked []byte

	// Conn is the underlying connection.
	// It can be type asserted against *net.TCPConn or other types
	// as needed. It should not be read from directly unless
	// Peeked is nil.
	net.Conn
}

func (c *Conn) Read(p []byte) (n int, err error) {
	if len(c.Peeked) > 0 {
		n = copy(p, c.Peeked)
		c.Peeked = c.Peeked[n:]
		if len(c.Peeked) == 0 {
			c.Peeked = nil
		}
		return n, nil
	}
	return c.Conn.Read(p)
}

// Target is what an incoming matched connection is sent to.
type Target interface {
	// HandleConn is called when an incoming connection is
	// matched. After the call to HandleConn, the tcpproxy
	// package never touches the conn again. Implementations are
	// responsible for closing the connection when needed.
	//
	// The concrete type of conn will be of type *Conn if any
	// bytes have been consumed for the purposes of route
	// matching.
	HandleConn(net.Conn)
}

// To is shorthand way of writing &tcpproxy.DialProxy{Addr: addr}.
func To(addr string) *DialProxy {
	return &DialProxy{Addr: addr}
}

// DialProxy implements Target by dialing a new connection to Addr
// and then proxying data back and forth.
//
// The To func is a shorthand way of creating a DialProxy.
type DialProxy struct {
	// Addr is the TCP address to proxy to.
	Addr string

	// KeepAlivePeriod sets the period between TCP keep alives.
	// If zero, a default is used. To disable, use a negative number.
	// The keep-alive is used for both the client connection and
	KeepAlivePeriod time.Duration

	// DialTimeout optionally specifies a dial timeout.
	// If zero, a default is used.
	// If negative, the timeout is disabled.
	DialTimeout time.Duration

	// DialContext optionally specifies an alternate dial function
	// for TCP targets. If nil, the standard
	// net.Dialer.DialContext method is used.
	DialContext func(ctx context.Context, network, address string) (net.Conn, error)

	// OnDialError optionally specifies an alternate way to handle errors dialing Addr.
	// If nil, the error is logged and src is closed.
	// If non-nil, src is not closed automatically.
	OnDialError func(src net.Conn, dstDialErr error)

	// ProxyProtocolVersion optionally specifies the version of
	// HAProxy's PROXY protocol to use. The PROXY protocol provides
	// connection metadata to the DialProxy target, via a header
	// inserted ahead of the client's traffic. The DialProxy target
	// must explicitly support and expect the PROXY header; there is
	// no graceful downgrade.
	// If zero, no PROXY header is sent. Currently, version 1 is supported.
	ProxyProtocolVersion int
}

// UnderlyingConn returns c.Conn if c of type *Conn,
// otherwise it returns c.
func UnderlyingConn(c net.Conn) net.Conn {
	if wrap, ok := c.(*Conn); ok {
		return wrap.Conn
	}
	return c
}

func tcpConn(c net.Conn) (t *net.TCPConn, ok bool) {
	if c, ok := UnderlyingConn(c).(*net.TCPConn); ok {
		return c, ok
	}
	if c, ok := c.(*net.TCPConn); ok {
		return c, ok
	}
	return nil, false
}

type closeReader interface{ CloseRead() error }
type closeWriter interface{ CloseWrite() error }

func goCloseConn(c net.Conn) { go c.Close() }

func closeRead(c net.Conn) {
	// prefer the interfaces, for compatibility with e.g. gvisor/netstack.
	if c, ok := UnderlyingConn(c).(closeReader); ok {
		c.CloseRead()
	}
}

func closeWrite(c net.Conn) {
	// prefer the interfaces, for compatibility with e.g. gvisor/netstack.
	if c, ok := UnderlyingConn(c).(closeWriter); ok {
		c.CloseWrite()
	}
}

// HandleConn implements the Target interface.
func (dp *DialProxy) HandleConn(src net.Conn) {
	ctx := context.Background()
	var cancel context.CancelFunc
	if dp.DialTimeout >= 0 {
		ctx, cancel = context.WithTimeout(ctx, dp.dialTimeout())
	}
	dst, err := dp.dialContext()(ctx, "tcp", dp.Addr)
	if cancel != nil {
		cancel()
	}
	if err != nil {
		dp.onDialError()(src, err)
		return
	}
	defer goCloseConn(dst)

	if err = dp.sendProxyHeader(dst, src); err != nil {
		dp.onDialError()(src, err)
		return
	}
	defer goCloseConn(src)

	if ka := dp.keepAlivePeriod(); ka > 0 {
		for _, c := range []net.Conn{src, dst} {
			if c, ok := tcpConn(c); ok {
				c.SetKeepAlive(true)
				c.SetKeepAlivePeriod(ka)
			}
		}
	}

	errc := make(chan error, 2)
	go proxyCopy(errc, src, dst)
	go proxyCopy(errc, dst, src)
	<-errc
	<-errc
}

func (dp *DialProxy) sendProxyHeader(w io.Writer, src net.Conn) error {
	switch dp.ProxyProtocolVersion {
	case 0:
		return nil
	case 1:
		var srcAddr, dstAddr *net.TCPAddr
		if a, ok := src.RemoteAddr().(*net.TCPAddr); ok {
			srcAddr = a
		}
		if a, ok := src.LocalAddr().(*net.TCPAddr); ok {
			dstAddr = a
		}

		if srcAddr == nil || dstAddr == nil {
			_, err := io.WriteString(w, "PROXY UNKNOWN\r\n")
			return err
		}

		family := "TCP4"
		if srcAddr.IP.To4() == nil {
			family = "TCP6"
		}
		_, err := fmt.Fprintf(w, "PROXY %s %s %s %d %d\r\n", family, srcAddr.IP, dstAddr.IP, srcAddr.Port, dstAddr.Port)
		return err
	default:
		return fmt.Errorf("PROXY protocol version %d not supported", dp.ProxyProtocolVersion)
	}
}

// proxyCopy is the function that copies bytes around.
// It's a named function instead of a func literal so users get
// named goroutines in debug goroutine stack dumps.
func proxyCopy(errc chan<- error, dst, src net.Conn) {
	defer closeRead(src)
	defer closeWrite(dst)

	// Before we unwrap src and/or dst, copy any buffered data.
	if wc, ok := src.(*Conn); ok && len(wc.Peeked) > 0 {
		if _, err := dst.Write(wc.Peeked); err != nil {
			errc <- err
			return
		}
		wc.Peeked = nil
	}

	// Unwrap the src and dst from *Conn to *net.TCPConn so Go
	// 1.11's splice optimization kicks in.
	src = UnderlyingConn(src)
	dst = UnderlyingConn(dst)

	_, err := io.Copy(dst, src)
	errc <- err
}

func (dp *DialProxy) keepAlivePeriod() time.Duration {
	if dp.KeepAlivePeriod != 0 {
		return dp.KeepAlivePeriod
	}
	return time.Minute
}

func (dp *DialProxy) dialTimeout() time.Duration {
	if dp.DialTimeout > 0 {
		return dp.DialTimeout
	}
	return 10 * time.Second
}

var defaultDialer = new(net.Dialer)

func (dp *DialProxy) dialContext() func(ctx context.Context, network, address string) (net.Conn, error) {
	if dp.DialContext != nil {
		return dp.DialContext
	}
	return defaultDialer.DialContext
}

func (dp *DialProxy) onDialError() func(src net.Conn, dstDialErr error) {
	if dp.OnDialError != nil {
		return dp.OnDialError
	}
	return func(src net.Conn, dstDialErr error) {
		var remoteAddr string
		if ra := src.RemoteAddr(); ra != nil {
			remoteAddr = ra.String()
		} else {
			remoteAddr = fmt.Sprintf("[%T with nil RemoteAddr]", src)
		}
		log.Printf("tcpproxy: for incoming conn %v, error dialing %q: %v", remoteAddr, dp.Addr, dstDialErr)
		src.Close()
	}
}
