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// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause
//go:build !ts_omit_appconnectors
package appc
import (
"net/netip"
"strings"
"golang.org/x/net/dns/dnsmessage"
"tailscale.com/util/mak"
)
// ObserveDNSResponse is a callback invoked by the DNS resolver when a DNS
// response is being returned over the PeerAPI. The response is parsed and
// matched against the configured domains, if matched the routeAdvertiser is
// advised to advertise the discovered route.
func (e *AppConnector) ObserveDNSResponse(res []byte) error {
var p dnsmessage.Parser
if _, err := p.Start(res); err != nil {
return err
}
if err := p.SkipAllQuestions(); err != nil {
return err
}
// cnameChain tracks a chain of CNAMEs for a given query in order to reverse
// a CNAME chain back to the original query for flattening. The keys are
// CNAME record targets, and the value is the name the record answers, so
// for www.example.com CNAME example.com, the map would contain
// ["example.com"] = "www.example.com".
var cnameChain map[string]string
// addressRecords is a list of address records found in the response.
var addressRecords map[string][]netip.Addr
for {
h, err := p.AnswerHeader()
if err == dnsmessage.ErrSectionDone {
break
}
if err != nil {
return err
}
if h.Class != dnsmessage.ClassINET {
if err := p.SkipAnswer(); err != nil {
return err
}
continue
}
switch h.Type {
case dnsmessage.TypeCNAME, dnsmessage.TypeA, dnsmessage.TypeAAAA:
default:
if err := p.SkipAnswer(); err != nil {
return err
}
continue
}
domain := strings.TrimSuffix(strings.ToLower(h.Name.String()), ".")
if len(domain) == 0 {
continue
}
if h.Type == dnsmessage.TypeCNAME {
res, err := p.CNAMEResource()
if err != nil {
return err
}
cname := strings.TrimSuffix(strings.ToLower(res.CNAME.String()), ".")
if len(cname) == 0 {
continue
}
mak.Set(&cnameChain, cname, domain)
continue
}
switch h.Type {
case dnsmessage.TypeA:
r, err := p.AResource()
if err != nil {
return err
}
addr := netip.AddrFrom4(r.A)
mak.Set(&addressRecords, domain, append(addressRecords[domain], addr))
case dnsmessage.TypeAAAA:
r, err := p.AAAAResource()
if err != nil {
return err
}
addr := netip.AddrFrom16(r.AAAA)
mak.Set(&addressRecords, domain, append(addressRecords[domain], addr))
default:
if err := p.SkipAnswer(); err != nil {
return err
}
continue
}
}
e.mu.Lock()
defer e.mu.Unlock()
for domain, addrs := range addressRecords {
domain, isRouted := e.findRoutedDomainLocked(domain, cnameChain)
// domain and none of the CNAMEs in the chain are routed
if !isRouted {
continue
}
// advertise each address we have learned for the routed domain, that
// was not already known.
var toAdvertise []netip.Prefix
for _, addr := range addrs {
if !e.isAddrKnownLocked(domain, addr) {
toAdvertise = append(toAdvertise, netip.PrefixFrom(addr, addr.BitLen()))
}
}
if len(toAdvertise) > 0 {
e.logf("[v2] observed new routes for %s: %s", domain, toAdvertise)
e.scheduleAdvertisement(domain, toAdvertise...)
}
}
return nil
}
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