net/dnscache: parse passed CIDR

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

net/dnscache/dnscache.go

@@ -133,7 +133,7 @@ func (r *Resolver) addIPCache(host string, ip net.IP, d time.Duration) net.IP {
 }
 
 func mustCIDR(s string) *net.IPNet {
-	_, ipNet, err := net.ParseCIDR("100.64.0.0/10")
+	_, ipNet, err := net.ParseCIDR(s)
 	if err != nil {
 		panic(err)
 	}

net/dnscache/dnscache_test.go

@@ -0,0 +1,24 @@
+package dnscache
+
+import (
+	"net"
+	"testing"
+)
+
+func TestIsPrivateIP(t *testing.T) {
+	tests := []struct {
+		ip   string
+		want bool
+	}{
+		{"10.1.2.3", true},
+		{"172.16.1.100", true},
+		{"192.168.1.1", true},
+		{"1.2.3.4", false},
+	}
+
+	for _, test := range tests {
+		if got := isPrivateIP(net.ParseIP(test.ip)); got != test.want {
+			t.Errorf("isPrivateIP(%q)=%v, want %v", test.ip, got, test.want)
+		}
+	}
+}