9ef4d9df7d
The main tutorial will be the one at https://github.com/ooni/probe-cli/pull/506, but it's useful to also document the primitives used by measurex. So, here's the companion tutorial, which explains how to use the features in netxlite to perform measurements. This work is part of https://github.com/ooni/ooni.org/issues/361.
118 lines
2.6 KiB
Markdown
118 lines
2.6 KiB
Markdown
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# Chapter I: Using the "system" DNS resolver
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In this chapter we will write together a `main.go` file that
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uses the "system" DNS resolver to lookup domain names.
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The "system" resolver is the one used by `getaddrinfo` on Unix.
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(This file is auto-generated from the corresponding source file,
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so make sure you don't edit it manually.)
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## The main.go file
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We define `main.go` file using `package main`.
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```Go
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package main
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import (
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"context"
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"errors"
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"flag"
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"os"
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"time"
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"github.com/apex/log"
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"github.com/ooni/probe-cli/v3/internal/netxlite"
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)
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func main() {
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```
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The beginning of the program is equal to the previous chapters,
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so there is not much to say about it.
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```Go
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log.SetLevel(log.DebugLevel)
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hostname := flag.String("hostname", "dns.google", "Hostname to resolve")
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timeout := flag.Duration("timeout", 60*time.Second, "Timeout")
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flag.Parse()
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ctx, cancel := context.WithTimeout(context.Background(), *timeout)
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defer cancel()
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```
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We create a new resolver using the standard library to perform
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domain name resolutions. Unless you're cross compiling, this
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resolver will call the system resolver using a C API. On Unix
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the called C API is `getaddrinfo`.
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The returned resolver implements an interface that is very
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close to the API of the `net.Resolver` struct.
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```Go
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reso := netxlite.NewResolverStdlib(log.Log)
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```
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We call `LookupHost` to map the hostname to IP addrs. The returned
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value is either a list of addrs or an error.
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```Go
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addrs, err := reso.LookupHost(ctx, *hostname)
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if err != nil {
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fatal(err)
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}
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log.Infof("resolver addrs: %+v", addrs)
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}
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```
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This function is exactly like it was in previous chapters.
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```Go
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func fatal(err error) {
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var ew *netxlite.ErrWrapper
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if !errors.As(err, &ew) {
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log.Fatal("cannot get ErrWrapper")
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}
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log.Warnf("error string : %s", err.Error())
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log.Warnf("OONI failure : %s", ew.Failure)
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log.Warnf("failed operation: %s", ew.Operation)
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log.Warnf("underlying error: %+v", ew.WrappedErr)
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os.Exit(1)
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}
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```
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## Running the code
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### Vanilla run
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You can now run this code as follows:
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```bash
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go run -race ./internal/tutorial/netxlite/chapter05
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```
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You will see debug logs describing what is happening along with timing info.
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### NXDOMAIN error
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```bash
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go run -race ./internal/tutorial/netxlite/chapter05 -hostname antani.ooni.io
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```
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should cause a `dns_nxdomain_error`, because the domain does not exist.
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### Timeout
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```bash
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go run -race ./internal/tutorial/netxlite/chapter05 -timeout 10us
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```
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should cause a timeout error, because the timeout is ridicolously small.
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## Conclusions
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We have seen how to use the "system" DNS resolver.
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