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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package main
import (
"context"
"encoding/binary"
"encoding/json"
"expvar"
"log"
"math/rand/v2"
"net"
"net/http"
"net/netip"
"strconv"
"strings"
"sync/atomic"
"time"
"tailscale.com/syncs"
"tailscale.com/util/mak"
"tailscale.com/util/slicesx"
)
const refreshTimeout = time.Minute
type dnsEntryMap struct {
IPs map[string][]net.IP
Percent map[string]float64 // "foo.com" => 0.5 for 50%
}
var (
dnsCache atomic.Pointer[dnsEntryMap]
dnsCacheBytes syncs.AtomicValue[[]byte] // of JSON
unpublishedDNSCache atomic.Pointer[dnsEntryMap]
bootstrapLookupMap syncs.Map[string, bool]
)
var (
bootstrapDNSRequests = expvar.NewInt("counter_bootstrap_dns_requests")
publishedDNSHits = expvar.NewInt("counter_bootstrap_dns_published_hits")
publishedDNSMisses = expvar.NewInt("counter_bootstrap_dns_published_misses")
unpublishedDNSHits = expvar.NewInt("counter_bootstrap_dns_unpublished_hits")
unpublishedDNSMisses = expvar.NewInt("counter_bootstrap_dns_unpublished_misses")
unpublishedDNSPercentMisses = expvar.NewInt("counter_bootstrap_dns_unpublished_percent_misses")
)
func init() {
expvar.Publish("counter_bootstrap_dns_queried_domains", expvar.Func(func() any {
return bootstrapLookupMap.Len()
}))
}
func refreshBootstrapDNSLoop() {
if *bootstrapDNS == "" && *unpublishedDNS == "" {
return
}
for {
refreshBootstrapDNS()
refreshUnpublishedDNS()
time.Sleep(10 * time.Minute)
}
}
func refreshBootstrapDNS() {
if *bootstrapDNS == "" {
return
}
ctx, cancel := context.WithTimeout(context.Background(), refreshTimeout)
defer cancel()
dnsEntries := resolveList(ctx, *bootstrapDNS)
// Randomize the order of the IPs for each name to avoid the client biasing
// to IPv6
for _, vv := range dnsEntries.IPs {
slicesx.Shuffle(vv)
}
j, err := json.MarshalIndent(dnsEntries.IPs, "", "\t")
if err != nil {
// leave the old values in place
return
}
dnsCache.Store(dnsEntries)
dnsCacheBytes.Store(j)
}
func refreshUnpublishedDNS() {
if *unpublishedDNS == "" {
return
}
ctx, cancel := context.WithTimeout(context.Background(), refreshTimeout)
defer cancel()
dnsEntries := resolveList(ctx, *unpublishedDNS)
unpublishedDNSCache.Store(dnsEntries)
}
// resolveList takes a comma-separated list of DNS names to resolve.
//
// If an entry contains a slash, it's two DNS names: the first is the one to
// resolve and the second is that of a TXT recording containing the rollout
// percentage in range "0".."100". If the TXT record doesn't exist or is
// malformed, the percentage is 0. If the TXT record is not provided (there's no
// slash), then the percentage is 100.
func resolveList(ctx context.Context, list string) *dnsEntryMap {
ents := strings.Split(list, ",")
ret := &dnsEntryMap{}
var r net.Resolver
for _, ent := range ents {
name, txtName, _ := strings.Cut(ent, "/")
addrs, err := r.LookupIP(ctx, "ip", name)
if err != nil {
log.Printf("bootstrap DNS lookup %q: %v", name, err)
continue
}
mak.Set(&ret.IPs, name, addrs)
if txtName == "" {
mak.Set(&ret.Percent, name, 1.0)
continue
}
vals, err := r.LookupTXT(ctx, txtName)
if err != nil {
log.Printf("bootstrap DNS lookup %q: %v", txtName, err)
continue
}
for _, v := range vals {
if v, err := strconv.Atoi(v); err == nil && v >= 0 && v <= 100 {
mak.Set(&ret.Percent, name, float64(v)/100)
}
}
}
return ret
}
func handleBootstrapDNS(w http.ResponseWriter, r *http.Request) {
bootstrapDNSRequests.Add(1)
w.Header().Set("Content-Type", "application/json")
// Bootstrap DNS requests occur cross-regions, and are randomized per
// request, so keeping a connection open is pointlessly expensive.
w.Header().Set("Connection", "close")
// Try answering a query from our hidden map first
if q := r.URL.Query().Get("q"); q != "" {
bootstrapLookupMap.Store(q, true)
if bootstrapLookupMap.Len() > 500 { // defensive
bootstrapLookupMap.Clear()
}
if m := unpublishedDNSCache.Load(); m != nil && len(m.IPs[q]) > 0 {
unpublishedDNSHits.Add(1)
percent := m.Percent[q]
if remoteAddrMatchesPercent(r.RemoteAddr, percent) {
// Only return the specific query, not everything.
m := map[string][]net.IP{q: m.IPs[q]}
j, err := json.MarshalIndent(m, "", "\t")
if err == nil {
w.Write(j)
return
}
} else {
unpublishedDNSPercentMisses.Add(1)
}
}
// If we have a "q" query for a name in the published cache
// list, then track whether that's a hit/miss.
m := dnsCache.Load()
var inPub bool
var ips []net.IP
if m != nil {
ips, inPub = m.IPs[q]
}
if inPub {
if len(ips) > 0 {
publishedDNSHits.Add(1)
} else {
publishedDNSMisses.Add(1)
}
} else {
// If it wasn't in either cache, treat this as a query
// for the unpublished cache, and thus a cache miss.
unpublishedDNSMisses.Add(1)
}
}
// Fall back to returning the public set of cached DNS names
j := dnsCacheBytes.Load()
w.Write(j)
}
// percent is [0.0, 1.0].
func remoteAddrMatchesPercent(remoteAddr string, percent float64) bool {
if percent == 0 {
return false
}
if percent == 1 {
return true
}
reqIPStr, _, err := net.SplitHostPort(remoteAddr)
if err != nil {
return false
}
reqIP, err := netip.ParseAddr(reqIPStr)
if err != nil {
return false
}
if reqIP.IsLoopback() {
// For local testing.
return rand.Float64() < 0.5
}
reqIP16 := reqIP.As16()
rndSrc := rand.NewPCG(binary.LittleEndian.Uint64(reqIP16[:8]), binary.LittleEndian.Uint64(reqIP16[8:]))
rnd := rand.New(rndSrc)
return percent > rnd.Float64()
}
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