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

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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>
This commit is contained in:
Brad Fitzpatrick
2022-09-28 17:19:30 -07:00
committed by Brad Fitzpatrick
parent c581ce7b00
commit bb7be74756
2 changed files with 18 additions and 5 deletions
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10
net/dns/publicdns/publicdns.go
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@ -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()