Fix: DNS cache max-age (#2850)

Regarding DNS cache, it's advisable to not rely on the TTL values
mentioned in the Ns and Extra sections. Otherwise, any DNS queries
that do not yield any results (such as for non-existent.example.com)
will be accidentally cached.

The need for the ACME challenge hack has been eliminated and as
such, it has been removed.

dns/util.go

@@ -6,18 +6,19 @@ import (
 	"errors"
 	"fmt"
 	"net"
-	"strings"
 	"time"
 
 	"github.com/Dreamacro/clash/common/cache"
 	"github.com/Dreamacro/clash/common/picker"
-	"github.com/Dreamacro/clash/log"
 
 	D "github.com/miekg/dns"
 	"github.com/samber/lo"
 )
 
 func minimalTTL(records []D.RR) uint32 {
+	if len(records) == 0 {
+		return 0
+	}
 	return lo.MinBy(records, func(r1 D.RR, r2 D.RR) bool {
 		return r1.Header().Ttl < r2.Header().Ttl
 	}).Header().Ttl
@@ -34,26 +35,11 @@ func updateTTL(records []D.RR, ttl uint32) {
 }
 
 func putMsgToCache(c *cache.LruCache, key string, q D.Question, msg *D.Msg) {
-	// skip dns cache for acme challenge
-	if q.Qtype == D.TypeTXT && strings.HasPrefix(q.Name, "_acme-challenge.") {
-		log.Debugln("[DNS] dns cache ignored because of acme challenge for: %s", q.Name)
-		return
-	}
-
-	var ttl uint32
-	switch {
-	case len(msg.Answer) != 0:
-		ttl = minimalTTL(msg.Answer)
-	case len(msg.Ns) != 0:
-		ttl = minimalTTL(msg.Ns)
-	case len(msg.Extra) != 0:
-		ttl = minimalTTL(msg.Extra)
-	default:
-		log.Debugln("[DNS] response msg empty: %#v", msg)
+	ttl := minimalTTL(msg.Answer)
+	if ttl == 0 {
 		return
 	}
-
-	c.SetWithExpire(key, msg.Copy(), time.Now().Add(time.Second*time.Duration(ttl)))
+	c.SetWithExpire(key, msg.Copy(), time.Now().Add(time.Duration(ttl)*time.Second))
 }
 
 func setMsgTTL(msg *D.Msg, ttl uint32) {