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package cache
import (
"encoding/binary"
"hash/fnv"
"sync"
"time"
"github.com/miekg/dns"
"github.com/mpolden/zdns/dnsutil"
)
// Cache is a cache of DNS messages.
type Cache struct {
capacity int
values map[uint64]*Value
keys []uint64
mu sync.RWMutex
done chan bool
now func() time.Time
}
// Value wraps a DNS message stored in the cache.
type Value struct {
CreatedAt time.Time
msg *dns.Msg
}
// Rcode returns the response code of the cached value v.
func (v *Value) Rcode() int { return v.msg.Rcode }
// Question returns the first question the cached value v.
func (v *Value) Question() string { return v.msg.Question[0].Name }
// Qtype returns the query type of the cached value v
func (v *Value) Qtype() uint16 { return v.msg.Question[0].Qtype }
// Answers returns the answers of the cached value v.
func (v *Value) Answers() []string { return dnsutil.Answers(v.msg) }
// TTL returns the time to live of the cached value v.
func (v *Value) TTL() time.Duration { return dnsutil.MinTTL(v.msg) }
// New creates a new cache of given capacity.
func New(capacity int) *Cache { return newCache(capacity, time.Minute, time.Now) }
func newCache(capacity int, interval time.Duration, now func() time.Time) *Cache {
if capacity < 0 {
capacity = 0
}
cache := &Cache{
now: now,
capacity: capacity,
values: make(map[uint64]*Value, capacity),
done: make(chan bool),
}
go maintain(cache, interval)
return cache
}
// NewKey creates a new cache key for the DNS name, qtype and qclass
func NewKey(name string, qtype, qclass uint16) uint64 {
h := fnv.New64a()
h.Write([]byte(name))
binary.Write(h, binary.BigEndian, qtype)
binary.Write(h, binary.BigEndian, qclass)
return h.Sum64()
}
func maintain(cache *Cache, interval time.Duration) {
ticker := time.NewTicker(interval)
for {
select {
case <-cache.done:
ticker.Stop()
return
case <-ticker.C:
cache.evictExpired()
}
}
}
// Close stops any outstanding maintenance tasks.
func (c *Cache) Close() error {
c.done <- true
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 uint64) (*dns.Msg, bool) {
v, ok := c.getValue(k)
if !ok {
return nil, false
}
return v.msg, true
}
func (c *Cache) getValue(k uint64) (*Value, bool) {
c.mu.RLock()
v, ok := c.values[k]
c.mu.RUnlock()
if !ok || c.isExpired(v) {
return nil, false
}
return v, true
}
// List returns the n most recent values in cache c.
func (c *Cache) List(n int) []Value {
values := make([]Value, 0, n)
c.mu.RLock()
defer c.mu.RUnlock()
for i := len(c.keys) - 1; i >= 0; i-- {
if len(values) == n {
break
}
v, ok := c.getValue(c.keys[i])
if !ok {
continue
}
values = append(values, *v)
}
return values
}
// Set associates key k with the DNS message msg. Message msg will expire from the cache according to its TTL. Setting a
// new key in a cache that has reached its capacity will evict values in a FIFO order.
func (c *Cache) Set(k uint64, msg *dns.Msg) {
if c.capacity == 0 {
return
}
if !canCache(msg) {
return
}
now := c.now()
c.mu.Lock()
defer c.mu.Unlock()
if len(c.values) == c.capacity && c.capacity > 0 {
delete(c.values, c.keys[0])
c.keys = c.keys[1:]
}
c.values[k] = &Value{CreatedAt: now, msg: msg}
c.keys = append(c.keys, k)
}
// Reset removes all values contained in cache c.
func (c *Cache) Reset() {
c.mu.Lock()
defer c.mu.Unlock()
c.values = make(map[uint64]*Value)
c.keys = nil
}
func (c *Cache) evictExpired() {
c.mu.Lock()
defer c.mu.Unlock()
evictedKeys := make(map[uint64]bool)
for k, v := range c.values {
if c.isExpired(v) {
delete(c.values, k)
evictedKeys[k] = true
}
}
if len(evictedKeys) > 0 {
// At least one entry was evicted. The ordered list of keys must be updated.
var keys []uint64
for _, k := range c.keys {
if _, ok := evictedKeys[k]; ok {
continue
}
keys = append(keys, k)
}
c.keys = keys
}
}
func (c *Cache) isExpired(v *Value) bool {
expiresAt := v.CreatedAt.Add(dnsutil.MinTTL(v.msg))
return c.now().After(expiresAt)
}
func canCache(m *dns.Msg) bool {
if dnsutil.MinTTL(m) == 0 {
return false
}
return m.Rcode == dns.RcodeSuccess || m.Rcode == dns.RcodeNameError
}
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