net/{dnscache,interfaces}: use netaddr.IP.IsPrivate, delete copied code

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dnscache/dnscache.go

@@ -249,7 +249,7 @@ func (r *Resolver) lookupIP(host string) (ip, ip6 net.IP, allIPs []net.IPAddr, e
 }
 
 func (r *Resolver) addIPCache(host string, ip, ip6 net.IP, allIPs []net.IPAddr, d time.Duration) {
-	if isPrivateIP(ip) {
+	if naIP, _ := netaddr.FromStdIP(ip); naIP.IsPrivate() {
 		// Don't cache obviously wrong entries from captive portals.
 		// TODO: use DoH or DoT for the forwarding resolver?
 		if debug {
@@ -275,24 +275,6 @@ func (r *Resolver) addIPCache(host string, ip, ip6 net.IP, allIPs []net.IPAddr,
 	}
 }
 
-func mustCIDR(s string) *net.IPNet {
-	_, ipNet, err := net.ParseCIDR(s)
-	if err != nil {
-		panic(err)
-	}
-	return ipNet
-}
-
-func isPrivateIP(ip net.IP) bool {
-	return private1.Contains(ip) || private2.Contains(ip) || private3.Contains(ip)
-}
-
-var (
-	private1 = mustCIDR("10.0.0.0/8")
-	private2 = mustCIDR("172.16.0.0/12")
-	private3 = mustCIDR("192.168.0.0/16")
-)
-
 type DialContextFunc func(ctx context.Context, network, address string) (net.Conn, error)
 
 // Dialer returns a wrapped DialContext func that uses the provided dnsCache.

net/dnscache/dnscache_test.go

@@ -14,24 +14,6 @@ import (
 
 var dialTest = flag.String("dial-test", "", "if non-empty, addr:port to test dial")
 
-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)
-		}
-	}
-}
-
 func TestDialer(t *testing.T) {
 	if *dialTest == "" {
 		t.Skip("skipping; --dial-test is blank")