ooni-probe-cli/internal/tutorial/netxlite/chapter05

Chapter I: Using the "system" DNS resolver

In this chapter we will write together a main.go file that uses the "system" DNS resolver to lookup domain names.

The "system" resolver is the one used by getaddrinfo on Unix.

(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"
	"errors"
	"flag"
	"os"
	"time"

	"github.com/apex/log"
	"github.com/ooni/probe-cli/v3/internal/netxlite"
)

func main() {

The beginning of the program is equal to the previous chapters, so there is not much to say about it.

	log.SetLevel(log.DebugLevel)
	hostname := flag.String("hostname", "dns.google", "Hostname to resolve")
	timeout := flag.Duration("timeout", 60*time.Second, "Timeout")
	flag.Parse()
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()

We create a new resolver using the standard library to perform domain name resolutions. Unless you're cross compiling, this resolver will call the system resolver using a C API. On Unix the called C API is getaddrinfo.

The returned resolver implements an interface that is very close to the API of the net.Resolver struct.

	reso := netxlite.NewStdlibResolver(log.Log)

We call LookupHost to map the hostname to IP addrs. The returned value is either a list of addrs or an error.

	addrs, err := reso.LookupHost(ctx, *hostname)
	if err != nil {
		fatal(err)
	}
	log.Infof("resolver addrs: %+v", addrs)
}

This function is exactly like it was in previous chapters.

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/chapter05

You will see debug logs describing what is happening along with timing info.

NXDOMAIN error

go run -race ./internal/tutorial/netxlite/chapter05 -hostname antani.ooni.io

should cause a dns_nxdomain_error, because the domain does not exist.

Timeout

go run -race ./internal/tutorial/netxlite/chapter05 -timeout 10us

should cause a timeout error, because the timeout is ridicolously small.

Conclusions

We have seen how to use the "system" DNS resolver.