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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package revdial implements a Dialer and Listener which work together
// to turn an accepted connection (for instance, a Hijacked HTTP request) into
// a Dialer which can then create net.Conns connecting back to the original
// dialer, which then gets a net.Listener accepting those conns.
//
// This is basically a very minimal SOCKS5 client & server.
//
// The motivation is that sometimes you want to run a server on a
// machine deep inside a NAT. Rather than connecting to the machine
// directly (which you can't, because of the NAT), you have the
// sequestered machine connect out to a public machine. Both sides
// then use revdial and the public machine can become a client for the
// NATed machine.
package revdial
/*
Protocol:
7-byte frame header:
uint8: frame type
0 new conn (server to peer only)
1 close conn (either way)
2 write (either way)
uint32: conn id (coordinator chooses, no ack from peer)
uint16: length of rest of data (for all frame types)
TODO(bradfitz): health checking PING packet type? since we can't use
TCP keep-alives at this layer. I guess we can just assume our caller
set up TCP keep-alives or similar. But it's actually tedious/hard to
do.
*/
import (
"bufio"
"bytes"
"encoding/binary"
"errors"
"fmt"
"io"
"log"
"net"
"sync"
"time"
)
// The Dialer can create new connections.
type Dialer struct {
rw *bufio.ReadWriter
closer io.Closer
mu sync.Mutex // guards following, and writes to rw
err error // non-nil when closed or peer dies
closed bool
conns map[uint32]*conn
nextID uint32
donec chan struct{}
}
// NewDialer returns the side of the connection which will initiate
// new connections. This will typically be the side which did the
// HTTP Hijack. The io.Closer is what gets closed by the Close
// or by any errors. It will typically be the hijacked Conn.
func NewDialer(rw *bufio.ReadWriter, c io.Closer) *Dialer {
d := &Dialer{
rw: rw,
closer: c,
conns: map[uint32]*conn{},
nextID: 1, // just for debugging, not seeing zeros
donec: make(chan struct{}),
}
go func() {
err := readFrames(rw.Reader, d)
if err == nil {
err = errors.New("revdial: Dialer.readFrames terminated with success")
}
d.closeWithError(err)
}()
return d
}
// Done returns a channel which is closed when d is either closed or closed
// by the peer.
func (d *Dialer) Done() <-chan struct{} { return d.donec }
var errDialerClosed = errors.New("revdial: Dialer closed")
// Close closes the Dialer and all still-open connections from it.
func (d *Dialer) Close() error {
return d.closeWithError(errDialerClosed)
}
func (d *Dialer) closeWithError(err error) error {
d.mu.Lock()
defer d.mu.Unlock()
if d.closed {
return nil
}
d.closed = true
d.err = err
for _, c := range d.conns {
// TODO(bradfitz): propagate err to peers. For now they'll just fail with
// EOF, which works but isn't as nice as it could be.
c.peerClose()
}
closeErr := d.closer.Close()
close(d.donec)
if err == errDialerClosed || err == nil {
return closeErr
}
return closeErr
}
func (d *Dialer) conn(id uint32) (*conn, error) {
d.mu.Lock()
defer d.mu.Unlock()
c, ok := d.conns[id]
if !ok {
return nil, fmt.Errorf("revdial.Dialer saw reference to unknown conn %v", id)
}
return c, nil
}
var (
errRole = errors.New("revdial: invalid frame type received for role")
errConnID = errors.New("revdial: invalid connection ID")
)
func (d *Dialer) onFrame(f frame) error {
switch f.command {
case frameNewConn:
return errRole
case frameCloseConn:
c, err := d.conn(f.connID)
if err != nil {
// Oh well.
return nil
}
c.peerClose()
return nil
case frameWrite:
c, err := d.conn(f.connID)
if err != nil {
// Ignore writes on bogus conn IDs; assume it
// just recently closed.
return nil
}
if _, err := c.peerWrite(f.payload); err != nil {
c.mu.Lock()
closed := c.closed
c.mu.Unlock()
if closed {
// Conn is now closed. Assume error
// was "io: read/write on closed pipe"
// and it was just data in-flight
// while this side closed. So, don't abort
// the frame-reading loop.
return nil
}
return err
}
return nil
default:
// Ignore unknown frame types.
}
return nil
}
// Dial creates a new connection back to the Listener.
func (d *Dialer) Dial() (net.Conn, error) {
d.mu.Lock()
defer d.mu.Unlock()
if d.closed {
return nil, errors.New("revdial: Dial on closed client")
}
var id uint32
for {
id = d.nextID
d.nextID++ // wrapping is okay; we check for free ones, assuming sparse
if _, inUse := d.conns[id]; inUse {
continue
}
break
}
c := &conn{
id: id,
wmu: &d.mu,
w: d.rw.Writer,
unregConn: d.unregConn,
}
c.cond = sync.NewCond(&c.mu)
d.conns[id] = c
err := writeFrame(c, frame{
command: frameNewConn,
connID: id,
})
return c, err
}
// c.wmu must be held.
func writeFrame(c *conn, f frame) error {
if len(f.payload) > 0xffff {
return errors.New("revdial: frame too long")
}
w := c.w
hdr := [7]byte{
byte(f.command),
byte(f.connID >> 24),
byte(f.connID >> 16),
byte(f.connID >> 8),
byte(f.connID),
byte(len(f.payload) >> 8),
byte(len(f.payload)),
}
if _, err := w.Write(hdr[:]); err != nil {
return err
}
if _, err := w.Write(f.payload); err != nil {
return err
}
return w.Flush()
}
type conn struct {
id uint32
wmu *sync.Mutex // held while writing & calling unreg
w *bufio.Writer
unregConn func(id uint32) // called with wmu held
mu sync.Mutex
cond *sync.Cond
buf []byte // unread data
eof bool // remote side closed
closed bool // our side closed (with Close)
}
var errUnsupported = errors.New("revdial: unsupported Conn operation")
func (c *conn) SetDeadline(t time.Time) error { return errUnsupported }
func (c *conn) SetReadDeadline(t time.Time) error { return errUnsupported }
func (c *conn) SetWriteDeadline(t time.Time) error { return errUnsupported }
func (c *conn) LocalAddr() net.Addr { return fakeAddr{} }
func (c *conn) RemoteAddr() net.Addr { return fakeAddr{} }
func (c *conn) Close() error {
c.mu.Lock()
if c.closed {
c.mu.Unlock()
return nil
}
c.closed = true
c.cond.Signal()
c.mu.Unlock()
c.wmu.Lock()
c.unregConn(c.id)
defer c.wmu.Unlock()
return writeFrame(c, frame{
command: frameCloseConn,
connID: c.id,
})
}
func (d *Dialer) unregConn(id uint32) {
delete(d.conns, id)
}
func (c *conn) peerWrite(p []byte) (n int, err error) {
c.mu.Lock()
defer c.mu.Unlock()
defer c.cond.Signal()
// TODO(bradfitz): bound this, like http2's buffer/pipe code
c.buf = append(c.buf, p...)
return len(p), nil
}
func (c *conn) peerClose() {
c.mu.Lock()
defer c.mu.Unlock()
defer c.cond.Broadcast()
c.eof = true
}
func (c *conn) Read(p []byte) (n int, err error) {
c.mu.Lock()
defer c.mu.Unlock()
defer c.cond.Signal() // for when writers block
for len(c.buf) == 0 && !c.eof && !c.closed {
c.cond.Wait()
}
if c.closed {
return 0, errors.New("revdial: Read on closed connection")
}
if len(c.buf) == 0 && c.eof {
return 0, io.EOF
}
n = copy(p, c.buf)
c.buf = c.buf[:copy(c.buf, c.buf[n:])] // slide down
return n, nil
}
func (c *conn) Write(p []byte) (n int, err error) {
c.mu.Lock()
if c.closed {
c.mu.Unlock()
return 0, errors.New("revdial: Write on Closed conn")
}
c.mu.Unlock()
const max = 0xffff // max chunk size
for len(p) > 0 {
chunk := p
if len(chunk) > max {
chunk = chunk[:max]
}
c.wmu.Lock()
err = writeFrame(c, frame{
command: frameWrite,
connID: c.id,
payload: chunk,
})
c.wmu.Unlock()
if err != nil {
return n, err
}
n += len(chunk)
p = p[len(chunk):]
}
return n, nil
}
type frameType uint8
const (
frameNewConn frameType = 'N'
frameCloseConn frameType = 'C'
frameWrite frameType = 'W'
)
type frame struct {
command frameType
connID uint32
payload []byte // not owned
}
func (f frame) String() string {
p := f.payload
if len(p) > 64 {
p = p[:64]
}
return fmt.Sprintf("[frame %q conn %v, %q]", f.command, f.connID, p)
}
// onFramer is the interface for something that can get callbacks on
// new frames being received.
type onFramer interface {
onFrame(f frame) error
}
const debug = false
func readFrames(br *bufio.Reader, of onFramer) error {
var hdr [7]byte
var payload bytes.Buffer
for {
_, err := io.ReadFull(br, hdr[:])
if err != nil {
return err
}
f := frame{
command: frameType(hdr[0]),
connID: binary.BigEndian.Uint32(hdr[1:5]),
}
paySize := binary.BigEndian.Uint16(hdr[5:7])
if debug {
log.Printf("Read frame header: %+v (len %v)", f, paySize)
}
payload.Reset()
if paySize > 0 {
if _, err := io.CopyN(&payload, br, int64(paySize)); err != nil {
return err
}
if payload.Len() != int(paySize) {
panic("invariant")
}
}
f.payload = payload.Bytes()
if debug {
log.Printf("Read full frame: %+v (len %v)", f, paySize)
}
err = of.onFrame(f)
if debug {
log.Printf("onFrame = %v", err)
}
if err != nil {
return err
}
}
}
// NewListener returns a new Listener, accepting connections which
// arrive from rw.
func NewListener(rw *bufio.ReadWriter) *Listener {
ln := &Listener{
connc: make(chan net.Conn, 8), // arbitrary
conns: map[uint32]*conn{},
rw: rw,
}
go func() {
err := readFrames(rw.Reader, ln)
ln.mu.Lock()
defer ln.mu.Unlock()
if ln.closed {
return
}
if err == nil {
err = errors.New("revdial: Listener.readFrames terminated with success")
}
ln.readErr = err
for _, c := range ln.conns {
c.peerClose()
}
go ln.Close()
}()
return ln
}
var _ net.Listener = (*Listener)(nil)
// Listener is a net.Listener, returning new connections which arrive
// from a corresponding Dialer.
type Listener struct {
rw *bufio.ReadWriter
connc chan net.Conn
mu sync.Mutex // guards below, closing connc, and writing to rw
readErr error
conns map[uint32]*conn
closed bool
}
// Closed reports whether the listener has been closed.
func (ln *Listener) Closed() bool {
ln.mu.Lock()
defer ln.mu.Unlock()
return ln.closed
}
// Accept blocks and returns a new connections, or an error.
func (ln *Listener) Accept() (net.Conn, error) {
c, ok := <-ln.connc
if !ok {
ln.mu.Lock()
err := ln.readErr
ln.mu.Unlock()
if err != nil {
return nil, fmt.Errorf("revdial: Listener closed; %v", err)
}
return nil, ErrListenerClosed
}
return c, nil
}
// ErrListenerClosed is returned by Accept after Close has been called.
var ErrListenerClosed = errors.New("revdial: Listener closed")
// Close closes the Listener, making future Accept calls return an
// error.
func (ln *Listener) Close() error {
ln.mu.Lock()
defer ln.mu.Unlock()
if ln.closed {
return nil
}
ln.closed = true
close(ln.connc)
return nil
}
// Addr returns a dummy address. This exists only to conform to the
// net.Listener interface.
func (ln *Listener) Addr() net.Addr { return fakeAddr{} }
func (ln *Listener) closeConn(id uint32) error {
ln.mu.Lock()
c, ok := ln.conns[id]
if ok {
delete(ln.conns, id)
}
ln.mu.Unlock()
if ok {
c.peerClose()
}
return nil
}
func (ln *Listener) newConn(id uint32) error {
ln.mu.Lock()
if _, dup := ln.conns[id]; dup {
ln.mu.Unlock()
return errors.New("revdial: peer newConn with already-open connID")
}
c := &conn{
id: id,
wmu: &ln.mu,
w: ln.rw.Writer,
unregConn: ln.unregConn,
}
c.cond = sync.NewCond(&c.mu)
ln.conns[id] = c
ln.mu.Unlock()
ln.connc <- c
return nil
}
func (ln *Listener) unregConn(id uint32) {
// Do nothing, unlike the outbound side.
}
func (ln *Listener) conn(id uint32) (*conn, error) {
ln.mu.Lock()
defer ln.mu.Unlock()
c, ok := ln.conns[id]
if !ok {
return nil, fmt.Errorf("revdial.Listener saw reference to unknown conn %v", id)
}
return c, nil
}
func (ln *Listener) onFrame(f frame) error {
switch f.command {
case frameNewConn:
return ln.newConn(f.connID)
case frameCloseConn:
return ln.closeConn(f.connID)
case frameWrite:
c, err := ln.conn(f.connID)
if err != nil {
// Ignore writes on bogus conn IDs; assume it
// just recently closed.
return nil
}
if _, err := c.peerWrite(f.payload); err != nil {
c.mu.Lock()
closed := c.closed
c.mu.Unlock()
if closed {
// Conn is now closed. Assume error
// was "io: read/write on closed pipe"
// and it was just data in-flight
// while this side closed. So, don't abort
// the frame-reading loop.
return nil
}
return err
}
default:
// Ignore unknown frame types.
}
return nil
}
type fakeAddr struct{}
func (fakeAddr) Network() string { return "revdial" }
func (fakeAddr) String() string { return "revdialconn" }