From 36dc883c7c288a4cc5e5b40aa2c9825da239e44f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E4=B8=96=E7=95=8C?= <i@sekai.icu>
Date: Thu, 9 Oct 2025 22:56:18 +0800
Subject: [PATCH] Fix DNS negative caching to comply with RFC 2308

---
 dns/client.go | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/dns/client.go b/dns/client.go
index c1c98c796..8db45d412 100644
--- a/dns/client.go
+++ b/dns/client.go
@@ -95,6 +95,20 @@ func (c *Client) Start() {
 	}
 }
 
+func extractNegativeTTL(response *dns.Msg) (uint32, bool) {
+	for _, record := range response.Ns {
+		if soa, isSOA := record.(*dns.SOA); isSOA {
+			soaTTL := soa.Header().Ttl
+			soaMinimum := soa.Minttl
+			if soaTTL < soaMinimum {
+				return soaTTL, true
+			}
+			return soaMinimum, true
+		}
+	}
+	return 0, false
+}
+
 func (c *Client) Exchange(ctx context.Context, transport adapter.DNSTransport, message *dns.Msg, options adapter.DNSQueryOptions, responseChecker func(responseAddrs []netip.Addr) bool) (*dns.Msg, error) {
 	if len(message.Question) == 0 {
 		if c.logger != nil {
@@ -214,7 +228,7 @@ func (c *Client) Exchange(ctx context.Context, transport adapter.DNSTransport, m
 			response.Answer = append(response.Answer, validResponse.Answer...)
 		}
 	}*/
-	disableCache = disableCache || (response.Rcode != dns.RcodeSuccess && response.Rcode != dns.RcodeNameError) || len(response.Answer) == 0
+	disableCache = disableCache || (response.Rcode != dns.RcodeSuccess && response.Rcode != dns.RcodeNameError)
 	if responseChecker != nil {
 		var rejected bool
 		// TODO: add accept_any rule and support to check response instead of addresses
@@ -251,10 +265,17 @@ func (c *Client) Exchange(ctx context.Context, transport adapter.DNSTransport, m
 		}
 	}
 	var timeToLive uint32
-	for _, recordList := range [][]dns.RR{response.Answer, response.Ns, response.Extra} {
-		for _, record := range recordList {
-			if timeToLive == 0 || record.Header().Ttl > 0 && record.Header().Ttl < timeToLive {
-				timeToLive = record.Header().Ttl
+	if len(response.Answer) == 0 {
+		if soaTTL, hasSOA := extractNegativeTTL(response); hasSOA {
+			timeToLive = soaTTL
+		}
+	}
+	if timeToLive == 0 {
+		for _, recordList := range [][]dns.RR{response.Answer, response.Ns, response.Extra} {
+			for _, record := range recordList {
+				if timeToLive == 0 || record.Header().Ttl > 0 && record.Header().Ttl < timeToLive {
+					timeToLive = record.Header().Ttl
+				}
 			}
 		}
 	}