From 257fcef0b817c2cd461640dc6e9a808a3257f942 Mon Sep 17 00:00:00 2001
From: KaitoHH <hh.kaito@gmail.com>
Date: Sun, 30 Apr 2023 12:18:20 +0800
Subject: [PATCH] Fix: adjust DNS TTL values based on minimum value (#2706)

This commit adds an updated function that adjusts
the TTL values of DNS records are based on the minimum TTL
the value found in the records list so that all records share the
same TTL value. This ensures consistency in the cache
expiry time for all records to prevent caching issues.
---
 dns/resolver.go |  3 ++-
 dns/util.go     | 29 ++++++++++++++++++++++++++---
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/dns/resolver.go b/dns/resolver.go
index fa20764..50e0c3d 100644
--- a/dns/resolver.go
+++ b/dns/resolver.go
@@ -143,7 +143,8 @@ func (r *Resolver) ExchangeContext(ctx context.Context, m *D.Msg) (msg *D.Msg, e
 			setMsgTTL(msg, uint32(1)) // Continue fetch
 			go r.exchangeWithoutCache(ctx, m)
 		} else {
-			setMsgTTL(msg, uint32(time.Until(expireTime).Seconds()))
+			// updating TTL by subtracting common delta time from each DNS record
+			updateMsgTTL(msg, uint32(time.Until(expireTime).Seconds()))
 		}
 		return
 	}
diff --git a/dns/util.go b/dns/util.go
index 90d2ff9..0600360 100644
--- a/dns/util.go
+++ b/dns/util.go
@@ -14,8 +14,25 @@ import (
 	"github.com/Dreamacro/clash/log"
 
 	D "github.com/miekg/dns"
+	"github.com/samber/lo"
 )
 
+func minimalTTL(records []D.RR) uint32 {
+	return lo.MinBy(records, func(r1 D.RR, r2 D.RR) bool {
+		return r1.Header().Ttl < r2.Header().Ttl
+	}).Header().Ttl
+}
+
+func updateTTL(records []D.RR, ttl uint32) {
+	if len(records) == 0 {
+		return
+	}
+	delta := minimalTTL(records) - ttl
+	for i := range records {
+		records[i].Header().Ttl = lo.Clamp(records[i].Header().Ttl-delta, 1, records[i].Header().Ttl)
+	}
+}
+
 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.") {
@@ -26,11 +43,11 @@ func putMsgToCache(c *cache.LruCache, key string, q D.Question, msg *D.Msg) {
 	var ttl uint32
 	switch {
 	case len(msg.Answer) != 0:
-		ttl = msg.Answer[0].Header().Ttl
+		ttl = minimalTTL(msg.Answer)
 	case len(msg.Ns) != 0:
-		ttl = msg.Ns[0].Header().Ttl
+		ttl = minimalTTL(msg.Ns)
 	case len(msg.Extra) != 0:
-		ttl = msg.Extra[0].Header().Ttl
+		ttl = minimalTTL(msg.Extra)
 	default:
 		log.Debugln("[DNS] response msg empty: %#v", msg)
 		return
@@ -53,6 +70,12 @@ func setMsgTTL(msg *D.Msg, ttl uint32) {
 	}
 }
 
+func updateMsgTTL(msg *D.Msg, ttl uint32) {
+	updateTTL(msg.Answer, ttl)
+	updateTTL(msg.Ns, ttl)
+	updateTTL(msg.Extra, ttl)
+}
+
 func isIPRequest(q D.Question) bool {
 	return q.Qclass == D.ClassINET && (q.Qtype == D.TypeA || q.Qtype == D.TypeAAAA)
 }