net: add IP.IsPrivate

Adds a new method IsPrivate to check if an IP is
private according to RFC 1918 & RFC 4193.

Fixes #29146

Change-Id: If77b9e1746d86029df66ae9f18437b1f65a18b59
GitHub-Last-Rev: 09f4ba7
GitHub-Pull-Request: #42793
Reviewed-on: https://go-review.googlesource.com/c/go/+/272668
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/net/ip.go

@@ -128,6 +128,27 @@ func (ip IP) IsLoopback() bool {
 	return ip.Equal(IPv6loopback)
 }
 
+// IsPrivate reports whether ip is a private address, according to
+// RFC 1918 (IPv4 addresses) and RFC 4193 (IPv6 addresses).
+func (ip IP) IsPrivate() bool {
+	if ip4 := ip.To4(); ip4 != nil {
+		// Following RFC 4193, Section 3. Local IPv6 Unicast Addresses which says:
+		//   The Internet Assigned Numbers Authority (IANA) has reserved the
+		//   following three blocks of the IPv4 address space for private internets:
+		//     10.0.0.0        -   10.255.255.255  (10/8 prefix)
+		//     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
+		//     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)
+		return ip4[0] == 10 ||
+			(ip4[0] == 172 && ip4[1]&0xf0 == 16) ||
+			(ip4[0] == 192 && ip4[1] == 168)
+	}
+	// Following RFC 4193, Section 3. Private Address Space which says:
+	//   The Internet Assigned Numbers Authority (IANA) has reserved the
+	//   following block of the IPv6 address space for local internets:
+	//     FC00::  -  FDFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF (FC00::/7 prefix)
+	return len(ip) == IPv6len && ip[0]&0xfe == 0xfc
+}
+
 // IsMulticast reports whether ip is a multicast address.
 func (ip IP) IsMulticast() bool {
 	if ip4 := ip.To4(); ip4 != nil {

src/net/ip_test.go

@@ -691,6 +691,28 @@ var ipAddrScopeTests = []struct {
 	{IP.IsGlobalUnicast, IP{0xfe, 0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, false},
 	{IP.IsGlobalUnicast, IP{0xff, 0x05, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, false},
 	{IP.IsGlobalUnicast, nil, false},
+	{IP.IsPrivate, nil, false},
+	{IP.IsPrivate, IPv4(1, 1, 1, 1), false},
+	{IP.IsPrivate, IPv4(9, 255, 255, 255), false},
+	{IP.IsPrivate, IPv4(10, 0, 0, 0), true},
+	{IP.IsPrivate, IPv4(10, 255, 255, 255), true},
+	{IP.IsPrivate, IPv4(11, 0, 0, 0), false},
+	{IP.IsPrivate, IPv4(172, 15, 255, 255), false},
+	{IP.IsPrivate, IPv4(172, 16, 0, 0), true},
+	{IP.IsPrivate, IPv4(172, 16, 255, 255), true},
+	{IP.IsPrivate, IPv4(172, 23, 18, 255), true},
+	{IP.IsPrivate, IPv4(172, 31, 255, 255), true},
+	{IP.IsPrivate, IPv4(172, 31, 0, 0), true},
+	{IP.IsPrivate, IPv4(172, 32, 0, 0), false},
+	{IP.IsPrivate, IPv4(192, 167, 255, 255), false},
+	{IP.IsPrivate, IPv4(192, 168, 0, 0), true},
+	{IP.IsPrivate, IPv4(192, 168, 255, 255), true},
+	{IP.IsPrivate, IPv4(192, 169, 0, 0), false},
+	{IP.IsPrivate, IP{0xfb, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, false},
+	{IP.IsPrivate, IP{0xfc, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, true},
+	{IP.IsPrivate, IP{0xfc, 0xff, 0x12, 0, 0, 0, 0, 0x44, 0, 0, 0, 0, 0, 0, 0, 0}, true},
+	{IP.IsPrivate, IP{0xfd, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, true},
+	{IP.IsPrivate, IP{0xfe, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, false},
 }
 
 func name(f interface{}) string {