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hellocap.go
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hellocap.go
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// Package hellocap is used to capture sample TLS ClientHellos from web browsers on this machine.
package hellocap
import (
"bytes"
"context"
"crypto/tls"
"errors"
"fmt"
"io"
"net"
"net/http"
"os/exec"
"sync"
utls "github.com/refraction-networking/utls"
"github.com/getlantern/golog"
"github.com/getlantern/tlsutil"
)
var log = golog.LoggerFor("hellocap")
type browser interface {
// Uses the browser to make an HTTP GET request of the input address.
//
// This function must return an error if and only if the GET was not sent. Any errors after the
// request has been sent must be ignored. In cases where it is unclear whether the GET was sent
// successfully, it is better to assume that it was. The caller can always time out if they
// never see the request.
get(ctx context.Context, addr string) error
// The browser's name, e.g. Google Chrome.
name() string
// Free up resources used by this browser instance.
close() error
}
// GetDefaultBrowserHello returns a sample ClientHello from the system's default web browser. This
// function may take a couple of seconds.
//
// Note that this may generate a log from the http package along the lines of "http: TLS handshake
// error... use of closed network connection".
func GetDefaultBrowserHello(ctx context.Context) ([]byte, error) {
b, err := defaultBrowser(ctx)
if err != nil {
return nil, err
}
defer b.close()
return getBrowserHello(ctx, b)
}
func getBrowserHello(ctx context.Context, browser browser) ([]byte, error) {
log.Debug("Getting browser hello")
type helloResult struct {
hello []byte
err error
}
var (
helloChan = make(chan helloResult, 1)
serverErrChan = make(chan error, 1)
browserErrChan = make(chan error, 1)
sendHelloOnce = sync.Once{}
)
s, err := newCapturingServer(func(hello []byte, err error) {
sendHelloOnce.Do(func() { helloChan <- helloResult{hello, err} })
})
if err != nil {
return nil, fmt.Errorf("failed to start hello-capture server: %w", err)
}
defer s.Close()
_, port, err := net.SplitHostPort(s.address())
if err != nil {
return nil, fmt.Errorf("failed to split hello-capture server address: %w", err)
}
go func() {
if err := s.listenAndServeTLS(); err != nil {
serverErrChan <- err
}
}()
go func() {
if err := browser.get(ctx, fmt.Sprintf("https://localhost:%s", port)); err != nil {
browserErrChan <- err
}
}()
select {
case result := <-helloChan:
return result.hello, result.err
case err := <-serverErrChan:
return nil, fmt.Errorf("error serving browser connection: %w", err)
case err := <-browserErrChan:
return nil, fmt.Errorf("failed to make test request with %s: %w", browser.name(), err)
case <-ctx.Done():
return nil, ctx.Err()
}
}
// onHello is a callback invoked when a ClientHello is captured. Only one of hello or err will be
// non-nil. This callback is not invoked for incomplete ClientHellos. In other words, err is non-nil
// only if what has been read off the connection could not possibly constitute a ClientHello,
// regardless of further data from the connection.
type onHello func(hello []byte, err error)
type capturingConn struct {
// Wraps a TCP connection.
net.Conn
helloLock sync.Mutex
helloRead bool
helloBuf bytes.Buffer
onHello onHello
}
func (c *capturingConn) Read(b []byte) (n int, err error) {
n, err = c.Conn.Read(b)
c.checkHello(b[:n])
return
}
func (c *capturingConn) Write(b []byte) (n int, err error) {
return c.Conn.Write(b)
}
func (c *capturingConn) checkHello(newBytes []byte) {
c.helloLock.Lock()
if !c.helloRead {
c.helloBuf.Write(newBytes)
nHello, parseErr := tlsutil.ValidateClientHello(c.helloBuf.Bytes())
if parseErr == nil {
f := &utls.Fingerprinter{
AllowBluntMimicry: false,
AlwaysAddPadding: false,
}
_, err := f.FingerprintClientHello(c.helloBuf.Bytes()[:nHello])
if err != nil {
c.onHello(nil, fmt.Errorf("fingerprint failure: %w", err))
} else {
c.onHello(c.helloBuf.Bytes()[:nHello], nil)
}
c.helloRead = true
} else if !errors.Is(parseErr, io.ErrUnexpectedEOF) {
c.onHello(nil, fmt.Errorf("could not parse captured bytes as a ClientHello: %w", parseErr))
c.helloRead = true
}
}
c.helloLock.Unlock()
}
type capturingListener struct {
net.Listener
onHello onHello
}
func (l capturingListener) Accept() (net.Conn, error) {
tcpConn, err := l.Listener.Accept()
if err != nil {
return nil, err
}
return &capturingConn{Conn: tcpConn, onHello: l.onHello}, nil
}
func listenAndCaptureTCP(onHello onHello) (*capturingListener, error) {
l, err := net.Listen("tcp", "localhost:0")
if err != nil {
return nil, err
}
return &capturingListener{l, onHello}, nil
}
type capturingServer struct {
s *http.Server
l capturingListener
}
func newCapturingServer(onHello onHello) (*capturingServer, error) {
l, err := listenAndCaptureTCP(onHello)
if err != nil {
return nil, fmt.Errorf("failed to start TLS server for capturing hellos: %w", err)
}
s := http.Server{
TLSConfig: &tls.Config{Certificates: []tls.Certificate{cert}},
Handler: http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
w.WriteHeader(http.StatusNoContent)
}),
}
return &capturingServer{&s, *l}, nil
}
func (cs *capturingServer) listenAndServeTLS() error {
return cs.s.ServeTLS(cs.l, "", "")
}
func (cs *capturingServer) address() string {
return cs.l.Addr().String()
}
func (cs *capturingServer) Close() error {
return cs.s.Close()
}
func wrapExecError(msg string, err error) error {
var execErr *exec.ExitError
if errors.As(err, &execErr) {
return fmt.Errorf("%w: %s", err, string(execErr.Stderr))
}
return err
}
var (
certPem = []byte(`-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----`)
keyPem = []byte(`-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIIrYSSNQFaA2Hwf1duRSxKtLYX5CB04fSeQ6tF1aY/PuoAoGCCqGSM49
AwEHoUQDQgAEPR3tU2Fta9ktY+6P9G0cWO+0kETA6SFs38GecTyudlHz6xvCdz8q
EKTcWGekdmdDPsHloRNtsiCa697B2O9IFA==
-----END EC PRIVATE KEY-----`)
cert tls.Certificate
)
func init() {
var err error
cert, err = tls.X509KeyPair(certPem, keyPem)
if err != nil {
panic(err)
}
}