feat: implement whoami.v4.powerdns.org (#47)

Part of ooni/probe#2669

dnsserver.go

@@ -91,6 +91,20 @@ func NewDNSConfig() *DNSConfig {
 	}
 }
 
+// Clone clones a [DNSConfig].
+func (dc *DNSConfig) Clone() *DNSConfig {
+	defer dc.mu.Unlock()
+	dc.mu.Lock()
+	out := NewDNSConfig()
+	for key, value := range dc.r {
+		out.r[key] = &DNSRecord{
+			A:     append([]net.IP{}, value.A...),
+			CNAME: value.CNAME,
+		}
+	}
+	return out
+}
+
 // ErrNotIPAddress indicates that a string is not a serialized IP address.
 var ErrNotIPAddress = errors.New("netem: not a valid IP address")
 
@@ -132,6 +146,21 @@ func (dc *DNSConfig) Lookup(name string) (*DNSRecord, bool) {
 	return record, found
 }
 
+func dnsConfigWithWhoami(config *DNSConfig, endpoint net.Addr) *DNSConfig {
+	// make sure we operate on a copy
+	config = config.Clone()
+
+	// extract the endpoint address
+	ipAddr, _, err := net.SplitHostPort(endpoint.String())
+	if err != nil {
+		return config
+	}
+
+	// add whoami.v4.powerdns.org record
+	_ = config.AddRecord("whoami.v4.powerdns.org", "", ipAddr)
+	return config
+}
+
 // dnsServerWorker is the [DNSServer] worker.
 func dnsServerWorker(
 	logger Logger,
@@ -156,7 +185,7 @@ func dnsServerWorker(
 		}
 		rawQuery := buffer[:count]
 
-		rawResponse, err := DNSServerRoundTrip(config, rawQuery)
+		rawResponse, err := DNSServerRoundTrip(dnsConfigWithWhoami(config, addr), rawQuery)
 		if err != nil {
 			logger.Warnf("netem: dnsServerRoundTrip: %s", err.Error())
 			continue

dnsserver_test.go

@@ -47,4 +47,25 @@ func TestDNSConfig(t *testing.T) {
 			})
 		})
 	})
+
+	t.Run("we can clone a DNSConfig", func(t *testing.T) {
+		config := NewDNSConfig()
+		config.AddRecord("www.example.com", "", "130.192.91.211")
+		other := config.Clone()
+		config.RemoveRecord("www.example.com")
+		if _, good := config.Lookup("www.example.com"); good {
+			t.Fatal("expected record to be missing")
+		}
+		record, good := other.Lookup("www.example.com")
+		if !good {
+			t.Fatal("expected to see record")
+		}
+		expect := &DNSRecord{
+			A:     []net.IP{net.IPv4(130, 192, 91, 211)},
+			CNAME: "",
+		}
+		if diff := cmp.Diff(expect, record); diff != "" {
+			t.Fatal(diff)
+		}
+	})
 }

example_star_test.go

@@ -1,6 +1,7 @@
 package netem_test
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"log"
@@ -114,3 +115,63 @@ func Example_starTopologyHTTPSAndDNS() {
 	// Bonsoir, Elliot!
 	//
 }
+
+// This example shows how DNS servers implement whoami.v4.powerdns.org
+func Example_starTopologyDNSWhoami() {
+	// Create a star topology for our hosts.
+	topology := netem.MustNewStarTopology(&netem.NullLogger{})
+	defer topology.Close()
+
+	// Add client stack to topology. Note that we don't need to
+	// close the clientStack: the topology will do that.
+	clientStack, err := topology.AddHost(
+		"130.192.91.211",    // host IP address
+		"8.8.8.8",           // host DNS resolver IP address
+		&netem.LinkConfig{}, // link with no PLR, RTT, DPI
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Add DNS server stack to topology.
+	dnsServerStack, err := topology.AddHost(
+		"8.8.8.8",
+		"8.8.8.8", // this host is its own DNS resolver
+		&netem.LinkConfig{},
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// spawn a DNS server with empty configuration.
+	dnsConfig := netem.NewDNSConfig()
+	dnsServer, err := netem.NewDNSServer(
+		&netem.NullLogger{},
+		dnsServerStack,
+		"8.8.8.8",
+		dnsConfig,
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer dnsServer.Close()
+
+	// create the DNS query to use
+	query := netem.NewDNSRequestA("whoami.v4.powerdns.org")
+
+	// issue a DNS request to the server
+	response, err := netem.DNSRoundTrip(context.Background(), clientStack, "8.8.8.8", query)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// parse the DNS response
+	addrs, _, err := netem.DNSParseResponse(query, response)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s\n", addrs)
+	// Output:
+	// [130.192.91.211]
+	//
+}