README
¶
Chapter I: TLS handshakes
In this chapter we will write together a main.go file that
uses netxlite to establish a new TCP connection and then performs
a TLS handshake using the established connection.
(This file is auto-generated from the corresponding source file, so make sure you don't edit it manually.)
The main.go file
We define main.go file using package main.
package main
import (
"context"
"crypto/tls"
"errors"
"flag"
"net"
"os"
"time"
"github.com/apex/log"
"github.com/ooni/probe-cli/v3/internal/netxlite"
)
Main function
func main() {
The beginning of main is just like in the previous chapter
except that here we also have a -sni flag.
log.SetLevel(log.DebugLevel)
address := flag.String("address", "8.8.4.4:443", "Remote endpoint address")
sni := flag.String("sni", "dns.google", "SNI to use")
timeout := flag.Duration("timeout", 60*time.Second, "Timeout")
flag.Parse()
ctx, cancel := context.WithTimeout(context.Background(), *timeout)
defer cancel()
We create a TLS config. In general you always want to specify these three fields when you're performing handshakes:
-
ServerName, which controls the SNI -
NextProtos, which controls the ALPN -
RootCAs, which we are forcing here to be the CA pool bundled with OONI (so we don't have to trust the system-wide certificate store)
tlsConfig := &tls.Config{
ServerName: *sni,
NextProtos: []string{"h2", "http/1.1"},
RootCAs: netxlite.NewDefaultCertPool(),
}
The logic to dial and handshake have been factored
into a function called dialTLS.
conn, state, err := dialTLS(ctx, *address, tlsConfig)
If there is an error, we bail, like before. Otherwise we
print information about the established TLS connection, which
is returned by dialTLS and assigned to state. Finally,
like in the previous chapter, we close the connection.
if err != nil {
fatal(err)
}
log.Infof("Conn type : %T", conn)
log.Infof("Cipher suite : %s", netxlite.TLSCipherSuiteString(state.CipherSuite))
log.Infof("Negotiated protocol: %s", state.NegotiatedProtocol)
log.Infof("TLS version : %s", netxlite.TLSVersionString(state.Version))
conn.Close()
}
Dialing and handshaking
The dialTCP function is exactly as in the previous chapter.
func dialTCP(ctx context.Context, address string) (net.Conn, error) {
d := netxlite.NewDialerWithoutResolver(log.Log)
return d.DialContext(ctx, "tcp", address)
}
The handshakeTLS function performs the handshake given a TCP
connection and a TLS config. This function creates a new handshaker
using the stdlib to manage TLS conns (we will see how to use
alternative TLS libraries in the next chapter). Then, once it
has constructed an handshaker, it invokes its Handshake method
to obtain a TLS conn (nil on failure), a TLS connection state
(empty on failure), and an error (nil on success).
While the returned connection is a net.Conn, the Handshake
function guarantees that the returned connection is always
compatible with the netxlite.TLSConn interface. Basically
this interface is an extension of net.Conn that also
allows to perform TLS specific operations, such as handshaking
and obtaining the connection state. (We will see in a later
chapter why this guarantee helps when writing more complex code.)
func handshakeTLS(ctx context.Context, tcpConn net.Conn,
config *tls.Config) (net.Conn, tls.ConnectionState, error) {
th := netxlite.NewTLSHandshakerStdlib(log.Log)
return th.Handshake(ctx, tcpConn, config)
}
Lastly, dialTLS combines dialTCP and handshakeTLS
together. The code you see here is a stripped down version
of the code in the measurex library that helps to
perform this dial+handshake operation in a single function call.
func dialTLS(ctx context.Context, address string,
config *tls.Config) (net.Conn, tls.ConnectionState, error) {
tcpConn, err := dialTCP(ctx, address)
if err != nil {
return nil, tls.ConnectionState{}, err
}
tlsConn, state, err := handshakeTLS(ctx, tcpConn, config)
if err != nil {
tcpConn.Close()
return nil, tls.ConnectionState{}, err
}
return tlsConn, state, nil
}
Printing the error
This code did not change since the previous chapter.
func fatal(err error) {
var ew *netxlite.ErrWrapper
if !errors.As(err, &ew) {
log.Fatal("cannot get ErrWrapper")
}
log.Warnf("error string : %s", err.Error())
log.Warnf("OONI failure : %s", ew.Failure)
log.Warnf("failed operation: %s", ew.Operation)
log.Warnf("underlying error: %+v", ew.WrappedErr)
os.Exit(1)
}
Running the code
Vanilla run
You can now run this code as follows:
go run -race ./internal/tutorial/netxlite/chapter02
You will see debug logs describing what is happening along with timing info.
Connect timeout
go run -race ./internal/tutorial/netxlite/chapter02 -address 8.8.4.4:1
should cause a connect timeout error. Try lowering the timout adding, e.g.,
the -timeout 5s flag to the command line.
Connection refused
go run -race ./internal/tutorial/netxlite/chapter02 -address '[::1]:1'
should give you a connection refused error in most cases. (We are quoting
the ::1 IPv6 address using [ and ] here.)
SNI mismatch
go run -race ./internal/tutorial/netxlite/chapter02 -sni example.com
should give you a TLS invalid hostname error (for historical reasons
named ssl_invalid_hostname).
TLS handshake reset
If you're on Linux, build Jafar (go build -v ./internal/cmd/jafar)
and then run:
sudo ./jafar -iptables-reset-keyword dns.google
Then run in another terminal
go run ./internal/tutorial/netxlite/chapter02
Then you can interrupt Jafar using ^C.
TLS handshake timeout
If you're on Linux, build Jafar (go build -v ./internal/cmd/jafar)
and then run:
sudo ./jafar -iptables-drop-keyword dns.google
Then run in another terminal
go run ./internal/tutorial/netxlite/chapter02
Then you can interrupt Jafar using ^C.
Conclusions
We have seen how to use netxlite to establish a TCP connection and perform a TLS handshake using such a connection.
Documentation
¶
Overview ¶
-=-=- StartHere -=-=-
Chapter I: TLS handshakes ¶
In this chapter we will write together a `main.go` file that uses netxlite to establish a new TCP connection and then performs a TLS handshake using the established connection.
(This file is auto-generated from the corresponding source file, so make sure you don't edit it manually.)
## The main.go file
We define `main.go` file using `package main`.
```Go
Source Files