dns: IPv6 roots generation

dns.go

@@ -14,6 +14,11 @@ const (
 	ByteSize = 8
 )
 
+const (
+	ipv4AddressLength = 32
+	ipv6AddressLength = 128
+)
+
 // generateMagicDNSRootDomains generates a list of DNS entries to be included in `Routes` in `MapResponse`.
 // This list of reverse DNS entries instructs the OS on what subnets and domains the Tailscale embedded DNS
 // server (listening in 100.100.100.100 udp/53) should be used for.
@@ -37,22 +42,25 @@ const (
 func generateMagicDNSRootDomains(ipPrefixes []netaddr.IPPrefix) []dnsname.FQDN {
 	fqdns := make([]dnsname.FQDN, 0, len(ipPrefixes))
 	for _, ipPrefix := range ipPrefixes {
-		var generateDnsRoot func(netaddr.IPPrefix) []dnsname.FQDN
+		var generateDNSRoot func(netaddr.IPPrefix) []dnsname.FQDN
 		switch ipPrefix.IP().BitLen() {
-		case 32:
+		case ipv4AddressLength:
-			generateDnsRoot = generateIPv4DNSRootDomain
+			generateDNSRoot = generateIPv4DNSRootDomain
+
+		case ipv6AddressLength:
+			generateDNSRoot = generateIPv6DNSRootDomain
 
 		default:
 			panic(fmt.Sprintf("unsupported IP version with address length %d", ipPrefix.IP().BitLen()))
 		}
 
-		fqdns = append(fqdns, generateDnsRoot(ipPrefix)...)
+		fqdns = append(fqdns, generateDNSRoot(ipPrefix)...)
 	}
 
 	return fqdns
 }
 
-func generateIPv4DNSRootDomain(ipPrefix netaddr.IPPrefix) (fqdns []dnsname.FQDN) {
+func generateIPv4DNSRootDomain(ipPrefix netaddr.IPPrefix) []dnsname.FQDN {
 	// Conversion to the std lib net.IPnet, a bit easier to operate
 	netRange := ipPrefix.IPNet()
 	maskBits, _ := netRange.Mask.Size()
@@ -76,6 +84,7 @@ func generateIPv4DNSRootDomain(ipPrefix netaddr.IPPrefix) (fqdns []dnsname.FQDN)
 	rdnsSlice = append(rdnsSlice, "in-addr.arpa.")
 	rdnsBase := strings.Join(rdnsSlice, ".")
 
+	fqdns := make([]dnsname.FQDN, 0, max-min+1)
 	for i := min; i <= max; i++ {
 		fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%d.%s", i, rdnsBase))
 		if err != nil {
@@ -84,7 +93,55 @@ func generateIPv4DNSRootDomain(ipPrefix netaddr.IPPrefix) (fqdns []dnsname.FQDN)
 		fqdns = append(fqdns, fqdn)
 	}
 
-	return
+	return fqdns
+}
+
+func generateIPv6DNSRootDomain(ipPrefix netaddr.IPPrefix) []dnsname.FQDN {
+	const nibbleLen = 4
+
+	maskBits, _ := ipPrefix.IPNet().Mask.Size()
+	expanded := ipPrefix.IP().StringExpanded()
+	nibbleStr := strings.Map(func(r rune) rune {
+		if r == ':' {
+			return -1
+		}
+
+		return r
+	}, expanded)
+
+	// TODO?: that does not look the most efficient implementation,
+	// but the inputs are not so long as to cause problems,
+	// and from what I can see, the generateMagicDNSRootDomains
+	// function is called only once over the lifetime of a server process.
+	prefixConstantParts := []string{}
+	for i := 0; i < maskBits/nibbleLen; i++ {
+		prefixConstantParts = append([]string{string(nibbleStr[i])}, prefixConstantParts...)
+	}
+
+	makeDomain := func(variablePrefix ...string) (dnsname.FQDN, error) {
+		prefix := strings.Join(append(variablePrefix, prefixConstantParts...), ".")
+
+		return dnsname.ToFQDN(fmt.Sprintf("%s.ip6.arpa", prefix))
+	}
+
+	var fqdns []dnsname.FQDN
+	if maskBits%4 == 0 {
+		dom, _ := makeDomain()
+		fqdns = append(fqdns, dom)
+	} else {
+		domCount := 1 << (maskBits % nibbleLen)
+		fqdns = make([]dnsname.FQDN, 0, domCount)
+		for i := 0; i < domCount; i++ {
+			varNibble := fmt.Sprintf("%x", i)
+			dom, err := makeDomain(varNibble)
+			if err != nil {
+				continue
+			}
+			fqdns = append(fqdns, dom)
+		}
+	}
+
+	return fqdns
 }
 
 func getMapResponseDNSConfig(

dns_test.go

@@ -10,8 +10,10 @@ import (
 )
 
 func (s *Suite) TestMagicDNSRootDomains100(c *check.C) {
-	prefix := netaddr.MustParseIPPrefix("100.64.0.0/10")
+	prefixes := []netaddr.IPPrefix{
-	domains := generateMagicDNSRootDomains(prefix)
+		netaddr.MustParseIPPrefix("100.64.0.0/10"),
+	}
+	domains := generateMagicDNSRootDomains(prefixes)
 
 	found := false
 	for _, domain := range domains {
@@ -45,8 +47,10 @@ func (s *Suite) TestMagicDNSRootDomains100(c *check.C) {
 }
 
 func (s *Suite) TestMagicDNSRootDomains172(c *check.C) {
-	prefix := netaddr.MustParseIPPrefix("172.16.0.0/16")
+	prefixes := []netaddr.IPPrefix{
-	domains := generateMagicDNSRootDomains(prefix)
+		netaddr.MustParseIPPrefix("172.16.0.0/16"),
+	}
+	domains := generateMagicDNSRootDomains(prefixes)
 
 	found := false
 	for _, domain := range domains {
@@ -69,6 +73,40 @@ func (s *Suite) TestMagicDNSRootDomains172(c *check.C) {
 	c.Assert(found, check.Equals, true)
 }
 
+// Happens when netmask is a multiple of 4 bits (sounds likely).
+func (s *Suite) TestMagicDNSRootDomainsIPv6Single(c *check.C) {
+	prefixes := []netaddr.IPPrefix{
+		netaddr.MustParseIPPrefix("fd7a:115c:a1e0::/48"),
+	}
+	domains := generateMagicDNSRootDomains(prefixes)
+
+	c.Assert(len(domains), check.Equals, 1)
+	c.Assert(domains[0].WithTrailingDot(), check.Equals, "0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa.")
+}
+
+func (s *Suite) TestMagicDNSRootDomainsIPv6SingleMultiple(c *check.C) {
+	prefixes := []netaddr.IPPrefix{
+		netaddr.MustParseIPPrefix("fd7a:115c:a1e0::/50"),
+	}
+	domains := generateMagicDNSRootDomains(prefixes)
+
+	yieldsRoot := func(dom string) bool {
+		for _, candidate := range domains {
+			if candidate.WithTrailingDot() == dom {
+				return true
+			}
+		}
+
+		return false
+	}
+
+	c.Assert(len(domains), check.Equals, 4)
+	c.Assert(yieldsRoot("0.0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa."), check.Equals, true)
+	c.Assert(yieldsRoot("1.0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa."), check.Equals, true)
+	c.Assert(yieldsRoot("2.0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa."), check.Equals, true)
+	c.Assert(yieldsRoot("3.0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa."), check.Equals, true)
+}
+
 func (s *Suite) TestDNSConfigMapResponseWithMagicDNS(c *check.C) {
 	namespaceShared1, err := app.CreateNamespace("shared1")
 	c.Assert(err, check.IsNil)