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.NewResolverStdlib(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.