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package cache
import (
"encoding/binary"
"fmt"
"hash/fnv"
"net"
"sync"
"time"
"github.com/miekg/dns"
)
// Cache represents a cache of DNS entries. Use New to initialize a new cache.
type Cache struct {
capacity int
now func() time.Time
maintainer *maintainer
mu sync.RWMutex
wg sync.WaitGroup
entries map[uint32]*Value
keys []uint32
}
type maintainer struct {
interval time.Duration
done chan bool
}
func maintain(cache *Cache, interval time.Duration) {
m := &maintainer{
interval: interval,
done: make(chan bool),
}
cache.maintainer = m
cache.wg.Add(1)
go m.run(cache)
}
func (m *maintainer) run(cache *Cache) {
defer cache.wg.Done()
ticker := time.NewTicker(m.interval)
for {
select {
case <-ticker.C:
cache.deleteExpired()
case <-m.done:
ticker.Stop()
return
}
}
}
// Value represents a value stored in the cache.
type Value struct {
Question string
Qtype uint16
Answer net.IP
CreatedAt time.Time
msg *dns.Msg
}
// TTL returns the TTL of this cache value.
func (v *Value) TTL() time.Duration { return minTTL(v.msg) }
// New creates a new cache of given capacity. Stale cache entries are removed at expiryInterval.
func New(capacity int, expiryInterval time.Duration) (*Cache, error) {
if capacity < 0 {
return nil, fmt.Errorf("invalid capacity: %d", capacity)
}
if expiryInterval == 0 {
expiryInterval = 10 * time.Minute
}
cache := &Cache{
now: time.Now,
capacity: capacity,
entries: make(map[uint32]*Value, capacity),
}
maintain(cache, expiryInterval)
return cache, nil
}
// NewKey creates a new cache key for the DNS name, qtype and qclass
func NewKey(name string, qtype, qclass uint16) uint32 {
h := fnv.New32a()
h.Write([]byte(name))
binary.Write(h, binary.LittleEndian, qtype)
binary.Write(h, binary.LittleEndian, qclass)
return h.Sum32()
}
// Close closes the cache.
func (c *Cache) Close() error {
c.maintainer.done <- true
c.wg.Wait()
return nil
}
// Get returns the DNS message associated with key k. Get will return nil if any TTL in the answer section of the
// message is exceeded according to time t.
func (c *Cache) Get(k uint32) (*dns.Msg, bool) {
v, ok := c.getValue(k)
if !ok {
return nil, false
}
return v.msg, true
}
func (c *Cache) getValue(k uint32) (*Value, bool) {
c.mu.RLock()
v, ok := c.entries[k]
c.mu.RUnlock()
if !ok || c.isExpired(v) {
return nil, false
}
return v, true
}
// List returns the n most recent cache values.
func (c *Cache) List(n int) []*Value {
values := make([]*Value, 0, n)
c.mu.RLock()
for i := len(c.keys) - 1; i >= 0; i-- {
if len(values) == n {
break
}
v, _ := c.getValue(c.keys[i])
values = append(values, v)
}
c.mu.RUnlock()
return values
}
// Set associated key k with the DNS message v. Message msg will expire from the cache according to its TTL. Setting a
// new key in a cache that has reached its capacity will remove the first key.
func (c *Cache) Set(k uint32, msg *dns.Msg) {
if c.capacity == 0 {
return
}
if !isCacheable(msg) {
return
}
now := c.now()
c.mu.Lock()
if len(c.entries) == c.capacity && c.capacity > 0 {
delete(c.entries, c.keys[0])
c.keys = c.keys[1:]
}
c.entries[k] = &Value{
Question: question(msg),
Answer: answer(msg),
Qtype: qtype(msg),
CreatedAt: now,
msg: msg,
}
c.keys = append(c.keys, k)
c.mu.Unlock()
}
func qtype(m *dns.Msg) uint16 { return m.Question[0].Qtype }
func question(m *dns.Msg) string { return m.Question[0].Name }
func answer(m *dns.Msg) net.IP {
rr := m.Answer[0]
switch v := rr.(type) {
case *dns.A:
return v.A
case *dns.AAAA:
return v.AAAA
}
return net.IPv4zero
}
func (c *Cache) deleteExpired() {
c.mu.Lock()
for k, v := range c.entries {
if c.isExpired(v) {
delete(c.entries, k)
}
}
c.mu.Unlock()
}
func (c *Cache) isExpired(v *Value) bool {
now := c.now()
for _, answer := range v.msg.Answer {
if now.After(v.CreatedAt.Add(ttl(answer))) {
return true
}
}
return false
}
func min(x, y uint32) uint32 {
if x < y {
return x
}
return y
}
func minTTL(m *dns.Msg) time.Duration {
var ttl uint32 = 1<<32 - 1 // avoids importing math.MaxUint32
for _, answer := range m.Answer {
ttl = min(answer.Header().Ttl, ttl)
}
for _, ns := range m.Ns {
ttl = min(ns.Header().Ttl, ttl)
}
return time.Duration(ttl) * time.Second
}
func isCacheable(m *dns.Msg) bool {
if minTTL(m) == 0 {
return false
}
return m.Rcode == dns.RcodeSuccess || m.Rcode == dns.RcodeNameError
}
func ttl(rr dns.RR) time.Duration {
ttlSecs := rr.Header().Ttl
return time.Duration(ttlSecs) * time.Second
}
|
