122 lines
2.9 KiB
Markdown
122 lines
2.9 KiB
Markdown
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# Chapter I: Using a custom UDP resolver
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In this chapter we will write together a `main.go` file that
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uses a custom UDP DNS resolver to lookup domain names.
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This program is very similar to the one in the previous chapter
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except that we'll be configuring a custom resolver.
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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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There's not much to say about the beginning of the program
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since it is equal to the one in the previous chapter.
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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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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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serverAddr := flag.String("server-addr", "1.1.1.1:53", "DNS server address")
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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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Here's where we start to diverge. We create a dialer without a resolver,
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which is going to be used by the UDP resolver.
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```Go
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dialer := netxlite.NewDialerWithoutResolver(log.Log)
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```
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Then, we create an UDP resolver. The arguments are the same as for
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creating a system resolver, except that we also need to specify the
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UDP endpoint address at which the server is listening.
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```Go
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reso := netxlite.NewResolverUDP(log.Log, dialer, *serverAddr)
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```
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The API we invoke is the same as in the previous chapter, though,
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and the rest of the program is equal to the one in the previous chapter.
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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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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/chapter06
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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
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```bash
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go run -race ./internal/tutorial/netxlite/chapter06 -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/chapter06 -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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```bash
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go run -race ./internal/tutorial/netxlite/chapter06 -server-addr 1.1.1.1:1
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```
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should also cause a timeout, because 1.1.1.1:1 is not an endpoint
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where a DNS-over-UDP resolver is listening.
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## Conclusions
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We have seen how to use a custom DNS-over-UDP resolver.
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