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net/dns/resolver: return an empty successful response instead of NXDomain when resolving A records for 4via6 domains

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As quad-100 is an authoritative server for 4via6 domains, it should always return responses
with a response code of 0 (indicating no error) when resolving records for these domains.
If there's no resource record of the specified type (e.g. A), it should return a response
with an empty answer section rather than NXDomain. Such a response indicates that there
is at least one RR of a different type (e.g., AAAA), suggesting the Windows stub resolver
to look for it.

Fixes tailscale/corp#20767

Signed-off-by: Nick Khyl <nickk@tailscale.com>
pull/12367/head
Nick Khyl 2 weeks ago committed by Nick Khyl
parent 4b6a0c42c8
commit 4cdc4ed7db
2 changed files with 28 additions and 4 deletions
Show Stats Download Patch File Download Diff File

26
net/dns/resolver/tsdns.go
Unescape Escape View File

@ -610,7 +610,7 @@ func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netip.Addr,
}
}
// Special-case: 4via6 DNS names.
if ip, ok := r.parseViaDomain(domain, typ); ok {
if ip, ok := r.resolveViaDomain(domain, typ); ok {
return ip, dns.RCodeSuccess
}
@ -689,7 +689,7 @@ func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netip.Addr,
}
}
// parseViaDomain synthesizes an IP address for quad-A DNS requests of the form
// resolveViaDomain synthesizes an IP address for quad-A DNS requests of the form
// `<IPv4-address-with-hypens-instead-of-dots>-via-<siteid>[.*]`. Two prior formats that
// didn't pan out (due to a Chrome issue and DNS search ndots issues) were
// `<IPv4-address>.via-<X>` and the older `via-<X>.<IPv4-address>`,
@ -697,13 +697,27 @@ func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netip.Addr,
//
// This exists as a convenient mapping into Tailscales 'Via Range'.
//
// It returns a zero netip.Addr and true to indicate a successful response with
// an empty answers section if the specified domain is a valid Tailscale 4via6
// domain, but the request type is neither quad-A nor ALL.
//
// TODO(maisem/bradfitz/tom): `<IPv4-address>.via-<X>` was introduced
// (2022-06-02) to work around an issue in Chrome where it would treat
// "http://via-1.1.2.3.4" as a search string instead of a URL. We should rip out
// the old format in early 2023.
func (r *Resolver) parseViaDomain(domain dnsname.FQDN, typ dns.Type) (netip.Addr, bool) {
func (r *Resolver) resolveViaDomain(domain dnsname.FQDN, typ dns.Type) (netip.Addr, bool) {
fqdn := string(domain.WithoutTrailingDot())
if typ != dns.TypeAAAA {
switch typ {
case dns.TypeA, dns.TypeAAAA, dns.TypeALL:
// For Type A requests, we should return a successful response
// with an empty answer section rather than an NXDomain
// if the specified domain is a valid Tailscale 4via6 domain.
//
// Therefore, we should continue and parse the domain name first
// before deciding whether to return an IPv6 address,
// a zero (invalid) netip.Addr and true to indicate a successful empty response,
// or a zero netip.Addr and false to indicate that it is not a Tailscale 4via6 domain.
default:
return netip.Addr{}, false
}
if len(fqdn) < len("via-X.0.0.0.0") {
@ -756,6 +770,10 @@ func (r *Resolver) parseViaDomain(domain dnsname.FQDN, typ dns.Type) (netip.Addr
return netip.Addr{}, false // badly formed, don't respond
}
if typ == dns.TypeA {
return netip.Addr{}, true // the name exists, but cannot be resolved to an IPv4 address
}
// MapVia will never error when given an IPv4 netip.Prefix.
out, _ := tsaddr.MapVia(uint32(prefix), netip.PrefixFrom(ip4, ip4.BitLen()))
return out.Addr(), true

6
net/dns/resolver/tsdns_test.go
Unescape Escape View File

@ -401,6 +401,12 @@ func TestResolveLocal(t *testing.T) {
// suffixes are currently hard-coded and not plumbed via the netmap)
{"via_form3_dec_example.com", dnsname.FQDN("1-2-3-4-via-1.example.com."), dns.TypeAAAA, netip.Addr{}, dns.RCodeRefused},
{"via_form3_dec_examplets.net", dnsname.FQDN("1-2-3-4-via-1.examplets.net."), dns.TypeAAAA, netip.Addr{}, dns.RCodeRefused},
// Resolve A and ALL types of resource records.
{"via_type_a", dnsname.FQDN("1-2-3-4-via-1."), dns.TypeA, netip.Addr{}, dns.RCodeSuccess},
{"via_invalid_type_a", dnsname.FQDN("1-2-3-4-via-."), dns.TypeA, netip.Addr{}, dns.RCodeRefused},
{"via_type_all", dnsname.FQDN("1-2-3-4-via-1."), dns.TypeALL, netip.MustParseAddr("fd7a:115c:a1e0:b1a:0:1:1.2.3.4"), dns.RCodeSuccess},
{"via_invalid_type_all", dnsname.FQDN("1-2-3-4-via-."), dns.TypeALL, netip.Addr{}, dns.RCodeRefused},
}
for _, tt := range tests {

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