From 81bbbe4eece2ca88a33d813f3935c2ad2cbe8a55 Mon Sep 17 00:00:00 2001
From: Jiahao Lu <lujjjh@gmail.com>
Date: Mon, 16 Oct 2023 09:21:06 +0800
Subject: [PATCH] fix: DNS NCACHE TTL and OPT RRs (#2900)

* Fix: DNS NCACHE TTL and OPT RRs

1. DNS NCACHE was not correctly implemented.
2. OPT RRs must not be cached or forwarded.

Closes #2889.
---
 dns/resolver.go |  6 +++++-
 dns/util.go     | 47 +++++++++++++++++++++++------------------------
 2 files changed, 28 insertions(+), 25 deletions(-)

diff --git a/dns/resolver.go b/dns/resolver.go
index 2c429358..4696515f 100644
--- a/dns/resolver.go
+++ b/dns/resolver.go
@@ -195,7 +195,11 @@ func (r *Resolver) exchangeWithoutCache(ctx context.Context, m *D.Msg) (msg *D.M
 			msg := result.(*D.Msg)
 
 			if cache {
-				putMsgToCache(r.lruCache, q.String(), msg)
+				// OPT RRs MUST NOT be cached, forwarded, or stored in or loaded from master files.
+				msg.Extra = lo.Filter(msg.Extra, func(rr D.RR, index int) bool {
+					return rr.Header().Rrtype != D.TypeOPT
+				})
+				putMsgToCache(r.lruCache, q.String(), q, msg)
 			}
 		}()
 
diff --git a/dns/util.go b/dns/util.go
index 29de4e2a..433ea9e2 100644
--- a/dns/util.go
+++ b/dns/util.go
@@ -29,14 +29,16 @@ const (
 	MaxMsgSize = 65535
 )
 
+const serverFailureCacheTTL uint32 = 5
+
 func minimalTTL(records []D.RR) uint32 {
-	minObj := lo.MinBy(records, func(r1 D.RR, r2 D.RR) bool {
+	rr := lo.MinBy(records, func(r1 D.RR, r2 D.RR) bool {
 		return r1.Header().Ttl < r2.Header().Ttl
 	})
-	if minObj != nil {
-		return minObj.Header().Ttl
+	if rr == nil {
+		return 0
 	}
-	return 0
+	return rr.Header().Ttl
 }
 
 func updateTTL(records []D.RR, ttl uint32) {
@@ -49,28 +51,25 @@ func updateTTL(records []D.RR, ttl uint32) {
 	}
 }
 
-func putMsgToCache(c *cache.LruCache[string, *D.Msg], key string, msg *D.Msg) {
-	putMsgToCacheWithExpire(c, key, msg, 0)
-}
-
-func putMsgToCacheWithExpire(c *cache.LruCache[string, *D.Msg], key string, msg *D.Msg, sec uint32) {
-	if sec == 0 {
-		if sec = minimalTTL(msg.Answer); sec == 0 {
-			if sec = minimalTTL(msg.Ns); sec == 0 {
-				sec = minimalTTL(msg.Extra)
-			}
-		}
-		if sec == 0 {
-			return
-		}
-
-		if sec > 120 {
-			sec = 120 // at least 2 minutes to cache
-		}
-
+func putMsgToCache(c *cache.LruCache[string, *D.Msg], 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
 	}
 
-	c.SetWithExpire(key, msg.Copy(), time.Now().Add(time.Duration(sec)*time.Second))
+	var ttl uint32
+	if msg.Rcode == D.RcodeServerFailure {
+		// [...] a resolver MAY cache a server failure response.
+		// If it does so it MUST NOT cache it for longer than five (5) minutes [...]
+		ttl = serverFailureCacheTTL
+	} else {
+		ttl = minimalTTL(append(append(msg.Answer, msg.Ns...), msg.Extra...))
+	}
+	if ttl == 0 {
+		return
+	}
+	c.SetWithExpire(key, msg.Copy(), time.Now().Add(time.Duration(ttl)*time.Second))
 }
 
 func setMsgTTL(msg *D.Msg, ttl uint32) {