attack: support IPv6 only in DNSCaching

The previous code had a bug that assumed we could always find an IPv4
first.

Fixes #649

lib/attack.go

@@ -334,13 +334,14 @@ func DNSCaching(ttl time.Duration) func(*Attacker) {
 				for i := 0; i < len(ips) && j < 2; i++ {
 					ip := net.ParseIP(ips[i])
 					switch {
-					case len(ip.To4()) == net.IPv4len && j == 0:
+					case len(ip) == net.IPv4len && (j == 0 || len(ips[j-1]) == net.IPv6len):
 						fallthrough
-					case len(ip) == net.IPv6len && j == 1:
+					case len(ip) == net.IPv6len && (j == 0 || len(ips[j-1]) == net.IPv4len):
 						ips[j] = ips[i]
 						j++
 					}
 				}
+
 				ips = ips[:j]
 
 				type result struct {

lib/attack_test.go

@@ -407,3 +407,16 @@ func TestVegetaHeaders(t *testing.T) {
 		}
 	}
 }
+
+// https://github.com/tsenart/vegeta/issues/649
+func TestDNSCaching_Issue649(t *testing.T) {
+	defer func() {
+		if err := recover(); err != nil {
+			t.Fatalf("panic: %v", err)
+		}
+	}()
+
+	tr := NewStaticTargeter(Target{Method: "GET", URL: "https://[2a00:1450:4005:802::200e]"})
+	atk := NewAttacker(DNSCaching(0))
+	_ = atk.hit(tr, &attack{name: "TEST", began: time.Now()})
+}