@ -7,26 +7,98 @@
package publicdns
import (
"bytes"
"encoding/hex"
"fmt"
"net/netip"
"sort"
"strings"
"sync"
"tailscale.com/util/strs"
)
var knownDoH = map [ netip . Addr ] string { } // 8.8.8.8 => "https://..."
// dohOfIP maps from public DNS IPs to their DoH base URL.
//
// This does not include NextDNS which is handled specially.
var dohOfIP = map [ netip . Addr ] string { } // 8.8.8.8 => "https://..."
var dohIPsOfBase = map [ string ] [ ] netip . Addr { }
var populateOnce sync . Once
// KnownDoH returns a map of well-known public DNS IPs to their DoH URL.
// The returned map should not be modified.
func KnownDoH ( ) map [ netip . Addr ] string {
// DoHEndpointFromIP returns the DNS-over-HTTPS base URL for a given IP
// and whether it's DoH-only (not speaking DNS on port 53).
//
// The ok result is whether the IP is a known DNS server.
func DoHEndpointFromIP ( ip netip . Addr ) ( dohBase string , dohOnly bool , ok bool ) {
populateOnce . Do ( populate )
if b , ok := dohOfIP [ ip ] ; ok {
return b , false , true
}
// 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
// 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 . WriteString ( base )
for _ , b := range bytes . TrimLeft ( a [ 8 : ] , "\x00" ) {
fmt . Fprintf ( & sb , "%02x" , b )
}
return sb . String ( ) , true , true
}
return "" , false , false
}
// KnownDoHPrefixes returns the list of DoH base URLs.
//
// It returns a new copy each time, sorted. It's meant for tests.
//
// It does not include providers that have customer-specific DoH URLs like
// NextDNS.
func KnownDoHPrefixes ( ) [ ] string {
populateOnce . Do ( populate )
return knownDoH
ret := make ( [ ] string , 0 , len ( dohIPsOfBase ) )
for b := range dohIPsOfBase {
ret = append ( ret , b )
}
sort . Strings ( ret )
return ret
}
// DoHIPsOfBase returns a map of DNS server IP addresses keyed
// by their DoH URL. It is the inverse of KnownDoH.
func DoHIPsOfBase ( ) map [ string ] [ ] netip . Addr {
// DoHIPsOfBase returns the IP addresses to use to dial the provided DoH base
// URL.
//
// It is basically the inverse of DoHEndpointFromIP with the exception that for
// NextDNS it returns IPv4 addresses that DoHEndpointFromIP doesn't map back.
func DoHIPsOfBase ( dohBase string ) [ ] netip . Addr {
populateOnce . Do ( populate )
return dohIPsOfBase
if s := dohIPsOfBase [ dohBase ] ; len ( s ) > 0 {
return s
}
if hexStr , ok := strs . CutPrefix ( dohBase , "https://dns.nextdns.io/" ) ; ok {
// TODO(bradfitz): using the NextDNS anycast addresses works but is not
// ideal. Some of their regions have better latency via a non-anycast IP
// which we could get by first resolving A/AAAA "dns.nextdns.io" over
// DoH using their anycast address. For now we only use the anycast
// addresses. The IPv4 IPs we use are just the first one in their ranges.
// For IPv6 we put the profile ID in the lower bytes, but that seems just
// 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 {
return [ ] netip . Addr {
nextDNSv4One ,
nextDNSv4Two ,
nextDNSv6Gen ( nextDNSv6RangeA . Addr ( ) , b ) ,
nextDNSv6Gen ( nextDNSv6RangeB . Addr ( ) , b ) ,
}
}
}
return nil
}
// DoHV6 returns the first IPv6 DNS address from a given public DNS provider
@ -45,7 +117,7 @@ func DoHV6(base string) (ip netip.Addr, ok bool) {
// adds it to both knownDoH and dohIPsOFBase maps.
func addDoH ( ipStr , base string ) {
ip := netip . MustParseAddr ( ipStr )
knownDoH [ ip ] = base
dohOfIP [ ip ] = base
dohIPsOfBase [ base ] = append ( dohIPsOfBase [ base ] , ip )
}
@ -106,3 +178,43 @@ func populate() {
addDoH ( "193.19.108.3" , "https://adblock.doh.mullvad.net/dns-query" )
addDoH ( "2a07:e340::3" , "https://adblock.doh.mullvad.net/dns-query" )
}
var (
// The NextDNS IPv6 ranges (primary and secondary). The customer ID is
// encoded in the lower bytes and is used (in hex form) as the DoH query
// path.
nextDNSv6RangeA = netip . MustParsePrefix ( "2a07:a8c0::/33" )
nextDNSv6RangeB = netip . MustParsePrefix ( "2a07:a8c1::/33" )
// The first two IPs in the /24 v4 ranges can be used for DoH to NextDNS.
//
// They're Anycast and usually okay, but NextDNS has some locations that
// don't do BGP and can get results for querying them over DoH to find the
// IPv4 address of "dns.mynextdns.io" and find an even better result.
//
// Note that the Tailscale DNS client does not do any of the "IP address
// linking" that NextDNS can do with its IPv4 addresses. These addresses
// are only used for DoH.
nextDNSv4RangeA = netip . MustParsePrefix ( "45.90.28.0/24" )
nextDNSv4RangeB = netip . MustParsePrefix ( "45.90.30.0/24" )
nextDNSv4One = nextDNSv4RangeA . Addr ( )
nextDNSv4Two = nextDNSv4RangeB . Addr ( )
)
// 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 {
return netip . Addr { }
}
a := ip . As16 ( )
copy ( a [ 16 - len ( id ) : ] , id )
return netip . AddrFrom16 ( a )
}
// IPIsDoHOnlyServer reports whether ip is a DNS server that should only use
// DNS-over-HTTPS (not regular port 53 DNS).
func IPIsDoHOnlyServer ( ip netip . Addr ) bool {
return nextDNSv6RangeA . Contains ( ip ) || nextDNSv6RangeB . Contains ( ip ) ||
nextDNSv4RangeA . Contains ( ip ) || nextDNSv4RangeB . Contains ( ip )
}
Brad Fitzpatrick
2 years ago
committed by