Use preferred protocol first when resolving hostname

Prior to this change, LookupIPAddr() is used to do the DNS query,
which sends two DNS queries for A/AAAA records, even if the config has
preferred protocol set to ip4 and does not allow fallback protocol.

This change will change to use LookupIP() with the preferred protocol,
and only try fallback protocol if it's set to true. In most cases doing
this will save one RTT of DNS query.

prober/utils.go

@@ -15,6 +15,7 @@ package prober
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"hash/fnv"
 	"net"
@@ -26,6 +27,28 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
+var protocolToGauge = map[string]float64{
+	"ip4": 4,
+	"ip6": 6,
+}
+
+type resolver struct {
+	net.Resolver
+}
+
+// A simple wrapper around resolver.LookupIP.
+func (r *resolver) resolve(ctx context.Context, target string, protocol string) (*net.IPAddr, error) {
+	ips, err := r.LookupIP(ctx, protocol, target)
+	if err != nil {
+		return nil, err
+	}
+	for _, ip := range ips {
+		return &net.IPAddr{IP: ip}, nil
+	}
+	// Go doc did not specify when this could happen, better be defensive.
+	return nil, errors.New("calling LookupIP returned empty list of addresses")
+}
+
 // Returns the IP for the IPProtocol and lookup time.
 func chooseProtocol(ctx context.Context, IPProtocol string, fallbackIPProtocol bool, target string, registry *prometheus.Registry, logger log.Logger) (ip *net.IPAddr, lookupTime float64, err error) {
 	var fallbackProtocol string
@@ -55,63 +78,38 @@ func chooseProtocol(ctx context.Context, IPProtocol string, fallbackIPProtocol b
 		fallbackProtocol = "ip6"
 	}
 
-	level.Info(logger).Log("msg", "Resolving target address", "ip_protocol", IPProtocol)
 	resolveStart := time.Now()
 
 	defer func() {
 		lookupTime = time.Since(resolveStart).Seconds()
 		probeDNSLookupTimeSeconds.Add(lookupTime)
 	}()
 
-	resolver := &net.Resolver{}
-	ips, err := resolver.LookupIPAddr(ctx, target)
-	if err != nil {
-		level.Error(logger).Log("msg", "Resolution with IP protocol failed", "err", err)
-		return nil, 0.0, err
-	}
-
-	// Return the IP in the requested protocol.
-	var fallback *net.IPAddr
-	for _, ip := range ips {
-		switch IPProtocol {
-		case "ip4":
-			if ip.IP.To4() != nil {
-				level.Info(logger).Log("msg", "Resolved target address", "ip", ip.String())
-				probeIPProtocolGauge.Set(4)
-				probeIPAddrHash.Set(ipHash(ip.IP))
-				return &ip, lookupTime, nil
-			}
-
-			// ip4 as fallback
-			fallback = &ip
-
-		case "ip6":
-			if ip.IP.To4() == nil {
-				level.Info(logger).Log("msg", "Resolved target address", "ip", ip.String())
-				probeIPProtocolGauge.Set(6)
-				probeIPAddrHash.Set(ipHash(ip.IP))
-				return &ip, lookupTime, nil
-			}
-
-			// ip6 as fallback
-			fallback = &ip
-		}
+	r := &resolver{
+		Resolver: net.Resolver{},
 	}
 
-	// Unable to find ip and no fallback set.
-	if fallback == nil || !fallbackIPProtocol {
-		return nil, 0.0, fmt.Errorf("unable to find ip; no fallback")
+	level.Info(logger).Log("msg", "Resolving target address", "ip_protocol", IPProtocol)
+	if ip, err := r.resolve(ctx, target, IPProtocol); err == nil {
+		level.Info(logger).Log("msg", "Resolved target address", "ip", ip.String())
+		probeIPProtocolGauge.Set(protocolToGauge[IPProtocol])
+		probeIPAddrHash.Set(ipHash(ip.IP))
+		return ip, lookupTime, nil
+	} else if !fallbackIPProtocol {
+		level.Error(logger).Log("msg", "Resolution with IP protocol failed", "err", err)
+		return nil, 0.0, fmt.Errorf("unable to find ip; no fallback: %s", err)
 	}
 
-	// Use fallback ip protocol.
-	if fallbackProtocol == "ip4" {
-		probeIPProtocolGauge.Set(4)
-	} else {
-		probeIPProtocolGauge.Set(6)
+	level.Info(logger).Log("msg", "Resolving target address", "ip_protocol", fallbackProtocol)
+	ip, err = r.resolve(ctx, target, fallbackProtocol)
+	if err != nil {
+		// This could happen when the domain don't have A and AAAA record (e.g.
+		// only have MX record).
+		level.Error(logger).Log("msg", "Resolution with IP protocol failed", "err", err)
+		return nil, 0.0, fmt.Errorf("unable to find ip; exhausted fallback: %s", err)
 	}
-	probeIPAddrHash.Set(ipHash(fallback.IP))
-	level.Info(logger).Log("msg", "Resolved target address", "ip", fallback.String())
-	return fallback, lookupTime, nil
+	level.Info(logger).Log("msg", "Resolved target address", "ip", ip.String())
+	return ip, lookupTime, nil
 }
 
 func ipHash(ip net.IP) float64 {

prober/utils_test.go

@@ -24,6 +24,7 @@ import (
 	"math/big"
 	"net"
 	"os"
+	"strings"
 	"testing"
 	"time"
 
@@ -162,14 +163,25 @@ func TestChooseProtocol(t *testing.T) {
 	registry = prometheus.NewPedanticRegistry()
 
 	ip, _, err = chooseProtocol(ctx, "ip4", false, "ipv6.google.com", registry, logger)
-	if err != nil && err.Error() != "unable to find ip; no fallback" {
+	if err != nil && !strings.HasPrefix(err.Error(), "unable to find ip; no fallback") {
 		t.Error(err)
 	} else if err == nil {
 		t.Error("should set error")
 	}
 	if ip != nil {
 		t.Error("without fallback it should not answer")
 	}
+
+	registry = prometheus.NewPedanticRegistry()
+	ip, _, err = chooseProtocol(ctx, "ip4", true, "does-not-exist.google.com", registry, logger)
+	if err != nil && !strings.HasPrefix(err.Error(), "unable to find ip; exhausted fallback") {
+		t.Error(err)
+	} else if err == nil {
+		t.Error("should set error")
+	}
+	if ip != nil {
+		t.Error("with exhausted fallback it should not answer")
+	}
 }
 
 func checkMetrics(expected map[string]map[string]map[string]struct{}, mfs []*dto.MetricFamily, t *testing.T) {