net/dns/publicdns: permit more NextDNS profile bits in its IPv6 suffix

I brain-o'ed the math earlier. The NextDNS prefix is /32 (actually
/33, but will guarantee last bit is 0), so we have 128-32 = 96 bits
(12 bytes) of config/profile ID that we can extract. NextDNS doesn't
currently use all those, but might.

Updates #2452

Change-Id: I249bd28500c781e45425fd00fd3f46893ae226a2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/publicdns/publicdns.go

@@ -37,15 +37,15 @@ func DoHEndpointFromIP(ip netip.Addr) (dohBase string, dohOnly bool, ok bool) {
 	}
 
 	// NextDNS DoH URLs are of the form "https://dns.nextdns.io/c3a884"
-	// where the path component is the lower 8 bytes of the IPv6 address
+	// where the path component is the lower 12 bytes of the IPv6 address
 	// in lowercase hex without any zero padding.
 	if nextDNSv6RangeA.Contains(ip) || nextDNSv6RangeB.Contains(ip) {
 		a := ip.As16()
 		var sb strings.Builder
 		const base = "https://dns.nextdns.io/"
-		sb.Grow(len(base) + 8)
+		sb.Grow(len(base) + 12)
 		sb.WriteString(base)
-		for _, b := range bytes.TrimLeft(a[8:], "\x00") {
+		for _, b := range bytes.TrimLeft(a[4:], "\x00") {
 			fmt.Fprintf(&sb, "%02x", b)
 		}
 		return sb.String(), true, true
@@ -100,7 +100,7 @@ func DoHIPsOfBase(dohBase string) []netip.Addr {
 		// conventional for them and not required (it'll already be in the DoH path).
 		// (Really we shouldn't use either IPv4 or IPv6 anycast for DoH once we
 		// resolve "dns.nextdns.io".)
-		if b, err := hex.DecodeString(hexStr); err == nil && len(b) <= 8 && len(b) > 0 {
+		if b, err := hex.DecodeString(hexStr); err == nil && len(b) <= 12 && len(b) > 0 {
 			return []netip.Addr{
 				nextDNSv4One,
 				nextDNSv4Two,
@@ -215,7 +215,7 @@ var (
 // nextDNSv6Gen generates a NextDNS IPv6 address from the upper 8 bytes in the
 // provided ip and using id as the lowest 0-8 bytes.
 func nextDNSv6Gen(ip netip.Addr, id []byte) netip.Addr {
-	if len(id) > 8 {
+	if len(id) > 12 {
 		return netip.Addr{}
 	}
 	a := ip.As16()

net/dns/publicdns/publicdns_test.go

@@ -86,6 +86,19 @@ func TestDoHIPsOfBase(t *testing.T) {
 				"2a07:a8c1::c3:a884",
 			),
 		},
+		{
+			base: "https://dns.nextdns.io/112233445566778899aabbcc",
+			want: ips(
+				"45.90.28.0",
+				"45.90.30.0",
+				"2a07:a8c0:1122:3344:5566:7788:99aa:bbcc",
+				"2a07:a8c1:1122:3344:5566:7788:99aa:bbcc",
+			),
+		},
+		{
+			base: "https://dns.nextdns.io/112233445566778899aabbccdd",
+			want: ips(), // nothing; profile length is over 12 bytes
+		},
 		{
 			base: "https://dns.nextdns.io/c3a884/with/more/stuff",
 			want: ips(