lib: fix IP selection in DNS caching

Fixes #677

lib/attack.go

@@ -375,6 +375,33 @@ func DNSCaching(ttl time.Duration) func(*Attacker) {
 	}
 }
 
+// firstOfEachIPFamily returns the first IP of each IP family in the input slice.
+func firstOfEachIPFamily(ips []string) []string {
+	if len(ips) == 0 {
+		return ips
+	}
+
+	var (
+		lastV4 bool
+		each   = ips[:0]
+	)
+
+	for i := 0; i < len(ips) && len(each) < 2; i++ {
+		ip := net.ParseIP(ips[i])
+		if ip == nil {
+			continue
+		}
+
+		isV4 := ip.To4() != nil
+		if len(each) == 0 || isV4 != lastV4 {
+			each = append(each, ips[i])
+			lastV4 = isV4
+		}
+	}
+
+	return each
+}
+
 type attack struct {
 	name  string
 	began time.Time

lib/attack_test.go

@@ -17,6 +17,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/google/go-cmp/cmp"
 )
 
 func TestAttackRate(t *testing.T) {
@@ -420,3 +422,79 @@ func TestDNSCaching_Issue649(t *testing.T) {
 	atk := NewAttacker(DNSCaching(0))
 	_ = atk.hit(tr, &attack{name: "TEST", began: time.Now()})
 }
+
+func TestFirstOfEachIPFamily(t *testing.T) {
+	tests := []struct {
+		name  string
+		input []string
+		want  []string
+	}{
+		{
+			name:  "empty list",
+			input: []string{},
+			want:  []string{},
+		},
+		{
+			name:  "single IPv4",
+			input: []string{"192.168.1.1"},
+			want:  []string{"192.168.1.1"},
+		},
+		{
+			name:  "single IPv6",
+			input: []string{"fe80::1"},
+			want:  []string{"fe80::1"},
+		},
+		{
+			name:  "multiple IPv6",
+			input: []string{"fe80::1", "fe80::2"},
+			want:  []string{"fe80::1"},
+		},
+		{
+			name:  "one IPv4 and one IPv6",
+			input: []string{"192.168.1.1", "fe80::1"},
+			want:  []string{"192.168.1.1", "fe80::1"},
+		},
+		{
+			name:  "one IPv6 and one IPv4",
+			input: []string{"fe80::1", "192.168.1.1"},
+			want:  []string{"fe80::1", "192.168.1.1"},
+		},
+		{
+			name:  "multiple IPs with alternating versions",
+			input: []string{"192.168.1.1", "fe80::1", "192.168.1.2", "fe80::2"},
+			want:  []string{"192.168.1.1", "fe80::1"},
+		},
+		{
+			name:  "multiple IPs with same versions",
+			input: []string{"192.168.1.1", "192.168.1.2", "192.168.1.3"},
+			want:  []string{"192.168.1.1"},
+		},
+		{
+			name:  "multiple IPs with non-alternating versions",
+			input: []string{"192.168.1.1", "fe80::1", "192.168.1.2", "192.168.1.3", "fe80::2"},
+			want:  []string{"192.168.1.1", "fe80::1"},
+		},
+		{
+			name:  "invalid IP addresses",
+			input: []string{"invalid", "192.168.1.1", "fe80::1"},
+			want:  []string{"192.168.1.1", "fe80::1"},
+		},
+		{
+			name:  "IPv4 with embedded IPv6",
+			input: []string{"192.168.1.1", "::ffff:c000:280", "fe80::1"},
+			want:  []string{"192.168.1.1", "fe80::1"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := firstOfEachIPFamily(tt.input)
+			if len(result) != len(tt.want) {
+				t.Fatalf("want %v, got %v", tt.want, result)
+			}
+			if diff := cmp.Diff(tt.want, result); diff != "" {
+				t.Errorf("unexpected result (-want +got):\n%s", diff)
+			}
+		})
+	}
+}