From 5a957fd750b205f38cb3440522a279421f8b7bdc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Berkay=20=C3=96zdemirci?= <berkayozdemirci@gmail.com>
Date: Fri, 10 Apr 2026 09:05:07 +0300
Subject: [PATCH] Fix EDNS OPT record corruption in DNS cache

The TTL computation and assignment loops treat OPT record's Hdr.Ttl
as a regular TTL, but per RFC 6891 it encodes EDNS0 metadata
(ExtRCode|Version|Flags). This corrupts cached responses causing
systemd-resolved to reject them with EDNS version 255.

Also fix pointer aliasing: storeCache() stored raw *dns.Msg pointer
so subsequent mutations by Exchange() corrupted cached data.

- Skip OPT records in all TTL loops (Exchange + loadResponse)
- Use message.Copy() in storeCache() to isolate cache from mutations
---
 dns/client.go | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/dns/client.go b/dns/client.go
index 70b53c951..1a2ee8f8c 100644
--- a/dns/client.go
+++ b/dns/client.go
@@ -283,6 +283,9 @@ func (c *Client) Exchange(ctx context.Context, transport adapter.DNSTransport, m
 	if timeToLive == 0 {
 		for _, recordList := range [][]dns.RR{response.Answer, response.Ns, response.Extra} {
 			for _, record := range recordList {
+				if record.Header().Rrtype == dns.TypeOPT {
+					continue
+				}
 				if timeToLive == 0 || record.Header().Ttl > 0 && record.Header().Ttl < timeToLive {
 					timeToLive = record.Header().Ttl
 				}
@@ -294,6 +297,9 @@ func (c *Client) Exchange(ctx context.Context, transport adapter.DNSTransport, m
 	}
 	for _, recordList := range [][]dns.RR{response.Answer, response.Ns, response.Extra} {
 		for _, record := range recordList {
+			if record.Header().Rrtype == dns.TypeOPT {
+				continue
+			}
 			record.Header().Ttl = timeToLive
 		}
 	}
@@ -381,21 +387,21 @@ func (c *Client) storeCache(transport adapter.DNSTransport, question dns.Questio
 	}
 	if c.disableExpire {
 		if !c.independentCache {
-			c.cache.Add(question, message)
+			c.cache.Add(question, message.Copy())
 		} else {
 			c.transportCache.Add(transportCacheKey{
 				Question:     question,
 				transportTag: transport.Tag(),
-			}, message)
+			}, message.Copy())
 		}
 	} else {
 		if !c.independentCache {
-			c.cache.AddWithLifetime(question, message, time.Second*time.Duration(timeToLive))
+			c.cache.AddWithLifetime(question, message.Copy(), time.Second*time.Duration(timeToLive))
 		} else {
 			c.transportCache.AddWithLifetime(transportCacheKey{
 				Question:     question,
 				transportTag: transport.Tag(),
-			}, message, time.Second*time.Duration(timeToLive))
+			}, message.Copy(), time.Second*time.Duration(timeToLive))
 		}
 	}
 }
@@ -486,6 +492,9 @@ func (c *Client) loadResponse(question dns.Question, transport adapter.DNSTransp
 		var originTTL int
 		for _, recordList := range [][]dns.RR{response.Answer, response.Ns, response.Extra} {
 			for _, record := range recordList {
+				if record.Header().Rrtype == dns.TypeOPT {
+					continue
+				}
 				if originTTL == 0 || record.Header().Ttl > 0 && int(record.Header().Ttl) < originTTL {
 					originTTL = int(record.Header().Ttl)
 				}
@@ -500,12 +509,18 @@ func (c *Client) loadResponse(question dns.Question, transport adapter.DNSTransp
 			duration := uint32(originTTL - nowTTL)
 			for _, recordList := range [][]dns.RR{response.Answer, response.Ns, response.Extra} {
 				for _, record := range recordList {
+					if record.Header().Rrtype == dns.TypeOPT {
+						continue
+					}
 					record.Header().Ttl = record.Header().Ttl - duration
 				}
 			}
 		} else {
 			for _, recordList := range [][]dns.RR{response.Answer, response.Ns, response.Extra} {
 				for _, record := range recordList {
+					if record.Header().Rrtype == dns.TypeOPT {
+						continue
+					}
 					record.Header().Ttl = uint32(nowTTL)
 				}
 			}