all: add local caching

This commit adds config params to enable local DNS response caching and
control its behavior, allow tweaking the cache size, ttl override and
serving stale response.

internal/dnscache/cache.go

@@ -0,0 +1,76 @@
+package dnscache
+
+import (
+	"strings"
+	"time"
+
+	lru "github.com/hashicorp/golang-lru/v2"
+	"github.com/miekg/dns"
+)
+
+// Cacher is the interface for caching DNS response.
+type Cacher interface {
+	Get(Key) *Value
+	Add(Key, *Value)
+}
+
+// Key is the caching key for DNS message.
+type Key struct {
+	Qtype  uint16
+	Qclass uint16
+	Name   string
+}
+
+type Value struct {
+	Expire time.Time
+	Msg    *dns.Msg
+}
+
+var _ Cacher = (*LRUCache)(nil)
+
+// LRUCache implements Cacher interface.
+type LRUCache struct {
+	cacher *lru.ARCCache[Key, *Value]
+}
+
+func (l *LRUCache) Get(key Key) *Value {
+	v, _ := l.cacher.Get(key)
+	return v
+}
+
+func (l *LRUCache) Add(key Key, value *Value) {
+	l.cacher.Add(key, value)
+}
+
+// NewLRUCache creates a new LRUCache instance with given size.
+func NewLRUCache(size int) (*LRUCache, error) {
+	cacher, err := lru.NewARC[Key, *Value](size)
+	return &LRUCache{cacher: cacher}, err
+}
+
+// NewKey creates a new cache key for given DNS message.
+func NewKey(msg *dns.Msg) Key {
+	q := msg.Question[0]
+	return Key{Qtype: q.Qtype, Qclass: q.Qclass, Name: normalizeQname(q.Name)}
+}
+
+// NewValue creates a new cache value for given DNS message.
+func NewValue(msg *dns.Msg, expire time.Time) *Value {
+	return &Value{
+		Expire: expire,
+		Msg:    msg,
+	}
+}
+
+func normalizeQname(name string) string {
+	var b strings.Builder
+	b.Grow(len(name))
+	for i := 0; i < len(name); i++ {
+		c := name[i]
+		if 'A' <= c && c <= 'Z' {
+			c += 'a' - 'A'
+		}
+		b.WriteByte(c)
+	}
+	return b.String()
+}