tailscale/net/dnscache/dnscache.go

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// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package dnscache contains a minimal DNS cache that makes a bunch of
// assumptions that are only valid for us. Not recommended for general use.
package dnscache
import (
"context"
"fmt"
"net"
"sync"
"time"
"golang.org/x/sync/singleflight"
)
var single = &Resolver{
Forward: &net.Resolver{PreferGo: true},
}
// Get returns a caching Resolver singleton.
func Get() *Resolver { return single }
const fixedTTL = 10 * time.Minute
// Resolver is a minimal DNS caching resolver.
//
// The TTL is always fixed for now. It's not intended for general use.
// Cache entries are never cleaned up so it's intended that this is
// only used with a fixed set of hostnames.
type Resolver struct {
// Forward is the resolver to use to populate the cache.
// If nil, net.DefaultResolver is used.
Forward *net.Resolver
sf singleflight.Group
mu sync.Mutex
ipCache map[string]ipCacheEntry
}
type ipCacheEntry struct {
ip net.IP
expires time.Time
}
func (r *Resolver) fwd() *net.Resolver {
if r.Forward != nil {
return r.Forward
}
return net.DefaultResolver
}
// LookupIP returns the first IPv4 address found, otherwise the first IPv6 address.
func (r *Resolver) LookupIP(ctx context.Context, host string) (net.IP, error) {
if ip := net.ParseIP(host); ip != nil {
if ip4 := ip.To4(); ip4 != nil {
return ip4, nil
}
return ip, nil
}
if ip, ok := r.lookupIPCache(host); ok {
return ip, nil
}
ch := r.sf.DoChan(host, func() (interface{}, error) {
ip, err := r.lookupIP(host)
if err != nil {
return nil, err
}
return ip, nil
})
select {
case res := <-ch:
if res.Err != nil {
return nil, res.Err
}
return res.Val.(net.IP), nil
case <-ctx.Done():
return nil, ctx.Err()
}
}
func (r *Resolver) lookupIPCache(host string) (ip net.IP, ok bool) {
r.mu.Lock()
defer r.mu.Unlock()
if ent, ok := r.ipCache[host]; ok && ent.expires.After(time.Now()) {
return ent.ip, true
}
return nil, false
}
func (r *Resolver) lookupIP(host string) (net.IP, error) {
if ip, ok := r.lookupIPCache(host); ok {
return ip, nil
}
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
ips, err := r.fwd().LookupIPAddr(ctx, host)
if err != nil {
return nil, err
}
if len(ips) == 0 {
return nil, fmt.Errorf("no IPs for %q found", host)
}
for _, ipa := range ips {
if ip4 := ipa.IP.To4(); ip4 != nil {
return r.addIPCache(host, ip4, fixedTTL), nil
}
}
return r.addIPCache(host, ips[0].IP, fixedTTL), nil
}
func (r *Resolver) addIPCache(host string, ip net.IP, d time.Duration) net.IP {
if isPrivateIP(ip) {
// Don't cache obviously wrong entries from captive portals.
// TODO: use DoH or DoT for the forwarding resolver?
return ip
}
r.mu.Lock()
defer r.mu.Unlock()
if r.ipCache == nil {
r.ipCache = make(map[string]ipCacheEntry)
}
r.ipCache[host] = ipCacheEntry{ip: ip, expires: time.Now().Add(d)}
return ip
}
func mustCIDR(s string) *net.IPNet {
_, ipNet, err := net.ParseCIDR("100.64.0.0/10")
if err != nil {
panic(err)
}
return ipNet
}
func isPrivateIP(ip net.IP) bool {
return private1.Contains(ip) || private2.Contains(ip) || private3.Contains(ip)
}
var (
private1 = mustCIDR("10.0.0.0/8")
private2 = mustCIDR("172.16.0.0/12")
private3 = mustCIDR("192.168.0.0/16")
)