Add a way to allow for NAT64 mapped addresses

This introduces the IPv6NAT64Prefix constant that can be passed to
WithAllowedV6Prefixes if so desired.

cmd/ssrfgen/templates/ssrf.tmpl

@@ -42,6 +42,11 @@ var (
     // assignments, i.e "the internet"
     IPv6GlobalUnicast = netip.MustParsePrefix("{{ .V6GlobalUnicast }}")
 
+    // IPv6NAT64Prefix is the prefix set aside for NAT64. This allows a server
+    // to only have an IPv6 address but still be able to talk to an IPv4-only
+    // server through DNS64+NAT64
+    IPv6NAT64Prefix = netip.MustParsePrefix("64:ff9b::/96")
+
     // IPv6DeniedPrefixes contains IPv6 special purpose IP prefixes from IANA
     // within the IPv6 Global Unicast range that we wish to block by default
     // https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml

ssrf.go

@@ -48,6 +48,17 @@ func WithAllowedV4Prefixes(prefixes ...netip.Prefix) Option {
 // ranges outside of the global unicast range or connections to
 // otherwise denied prefixes within the global unicast range.
 //
+// This function should be called with [IPv6NAT64Prefix] as one of the
+// prefixes, if you run in an IPv6-only environment but provide IPv4
+// connectivity through a combination of DNS64+NAT64. The NAT64 prefix
+// is outside of the IPv6 global unicast range and as such blocked by
+// default. Allowing it is typically harmless in dual-stack setups as
+// your clients need an explicit route for 64:ff9b::/96 configured which
+// won't be the case by default. Beware that allowing this prefix may
+// allow for an address like 64:ff9b::7f00:1, i.e 127.0.0.1 mapped to
+// NAT64. A NAT64 gateway should drop this. Ideally a DNS64 server
+// would never generate an address for an RFC1918 IP in an A-record.
+//
 // This function overrides the allowed IPv6 prefixes, it does not accumulate.
 func WithAllowedV6Prefixes(prefixes ...netip.Prefix) Option {
 	return func(g *Guardian) {

ssrf_gen.go

@@ -57,6 +57,11 @@ var (
 	// assignments, i.e "the internet"
 	IPv6GlobalUnicast = netip.MustParsePrefix("2000::/3")
 
+	// IPv6NAT64Prefix is the prefix set aside for NAT64. This allows a server
+	// to only have an IPv6 address but still be able to talk to an IPv4-only
+	// server through DNS64+NAT64
+	IPv6NAT64Prefix = netip.MustParsePrefix("64:ff9b::/96")
+
 	// IPv6DeniedPrefixes contains IPv6 special purpose IP prefixes from IANA
 	// within the IPv6 Global Unicast range that we wish to block by default
 	// https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml

ssrf_test.go

@@ -76,6 +76,11 @@ func TestOptions(t *testing.T) {
 			Options: []Option{WithAllowedV6Prefixes(netip.MustParsePrefix("2002::/8"))},
 			Result:  &Guardian{networks: []string{"tcp4", "tcp6"}, ports: []uint16{80, 443}, allowedv6Prefixes: []netip.Prefix{netip.MustParsePrefix("2002::/8")}, deniedv4Prefixes: IPv4DeniedPrefixes, deniedv6Prefixes: IPv6DeniedPrefixes, strNetworks: "tcp4, tcp6", strPorts: "80, 443"},
 		},
+		{
+			Name:    "with allowed NAT64 prefix",
+			Options: []Option{WithAllowedV6Prefixes(IPv6NAT64Prefix)},
+			Result:  &Guardian{networks: []string{"tcp4", "tcp6"}, ports: []uint16{80, 443}, allowedv6Prefixes: []netip.Prefix{IPv6NAT64Prefix}, deniedv4Prefixes: IPv4DeniedPrefixes, deniedv6Prefixes: IPv6DeniedPrefixes, strNetworks: "tcp4, tcp6", strPorts: "80, 443"},
+		},
 		{
 			Name:    "with multiple allowed v6 prefix calls",
 			Options: []Option{WithAllowedV6Prefixes(netip.MustParsePrefix("2002::/23")), WithAllowedV6Prefixes(netip.MustParsePrefix("2002::/8"))},
@@ -155,6 +160,7 @@ func TestDefaultGuardian(t *testing.T) {
 		{Addr: "invalid network", Network: "udp6", Err: ErrProhibitedNetwork},
 		{Addr: "invalid address", Network: "tcp4", Err: ErrInvalidHostPort},
 		{Addr: "[::ffff:129.144.52.38]:80", Network: "tcp6", Err: ErrProhibitedIP},
+		{Addr: "[64:ff9b::7f00:1]:80", Network: "tcp6", Err: ErrProhibitedIP},
 	}
 
 	s := New()
@@ -224,6 +230,12 @@ func TestCustomGuardian(t *testing.T) {
 			Addr:     "[2001::1]:80",
 			Network:  "tcp6",
 		},
+		{
+			Name:     "allow IPv6 NAT64 prefix",
+			Guardian: New(WithAllowedV6Prefixes(IPv6NAT64Prefix)),
+			Addr:     "[64:ff9b::7f00:1]:80",
+			Network:  "tcp6",
+		},
 		{
 			Name:     "deny IPv4 prefix",
 			Guardian: New(WithDeniedV4Prefixes(netip.MustParsePrefix("8.8.8.0/24"))),