From ff30fa4b916a1b93f563f5728e231f67759d41c1 Mon Sep 17 00:00:00 2001
From: Simon Kelley <simon@thekelleys.org.uk>
Date: Sun, 10 Aug 2025 23:32:59 +0100
Subject: [PATCH] Fix caching bug with negative records.

To provoke this bug, at least the following must be true.

A reply must be a CNAME.
The target of the CNAME must not exist for the queried RR (a No Data reply).
The No Data reply must include an SOA record in the NS section.
The query must take place over TCP.

The result is the caching of a CNAME whose target is the
name of the SOA record, instead of the No data record.

If there is a RR at this target, then a subsequent query for that
RRtype will get a qrong reply.

Thanks to the testers extraordinaire at Pi-Hole for spotting this
and providing enough information to chase it down.
---
 src/cache.c   |  7 +++----
 src/rfc1035.c | 28 +++++++++++++++++++++-------
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/src/cache.c b/src/cache.c
index a93bfc6..7d1b0f4 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -938,15 +938,14 @@ int cache_recv_insert(time_t now, int fd)
 		if (newc)
 		  {
 		    newc->addr.cname.is_name_ptr = 0;
+		    newc->addr.cname.target.cache = crecp;
 		    
-		    if (!crecp)
-		      newc->addr.cname.target.cache = NULL;
-		    else
+		    if (crecp)
 		      {
 			next_uid(crecp);
-			newc->addr.cname.target.cache = crecp;
 			newc->addr.cname.uid = crecp->uid;
 		      }
+		    crecp = newc;
 		  }
 	      }
 	    else
diff --git a/src/rfc1035.c b/src/rfc1035.c
index 7eb3b0d..f0e1082 100644
--- a/src/rfc1035.c
+++ b/src/rfc1035.c
@@ -516,7 +516,7 @@ int do_doctor(struct dns_header *header, size_t qlen, char *namebuff)
    Cache said SOA and return the difference in length between name and the name of the 
    SOA RR so we can look it up again.
 */
-static int find_soa(struct dns_header *header, size_t qlen, char *name, int *substring, unsigned long *ttlp, int no_cache, time_t now)
+static int find_soa(struct dns_header *header, size_t qlen, char *name, int *substring, unsigned long *ttlp, int cache, time_t now)
 {
   unsigned char *p, *psave;
   int qtype, qclass, rdlen;
@@ -556,7 +556,7 @@ static int find_soa(struct dns_header *header, size_t qlen, char *name, int *sub
 	    {
 	      int prefix = name_len - soa_len;
 	      
-	      if (!no_cache)
+	      if (cache)
 		{
 		  if (!(addr.rrblock.rrdata = blockdata_alloc(NULL, 0)))
 		    return 0;
@@ -568,12 +568,12 @@ static int find_soa(struct dns_header *header, size_t qlen, char *name, int *sub
 		{
 		  if (!extract_name(header, qlen, &p, daemon->workspacename, EXTR_NAME_EXTRACT, 0))
 		    {
-		      if (!no_cache)
+		      if (cache)
 			blockdata_free(addr.rrblock.rrdata);
 		      return 0;
 		    }
 		  
-		  if (!no_cache)
+		  if (cache)
 		    {
 		      len = to_wire(daemon->workspacename);
 		      if (!blockdata_expand(addr.rrblock.rrdata, addr.rrblock.datalen, daemon->workspacename, len))
@@ -588,13 +588,13 @@ static int find_soa(struct dns_header *header, size_t qlen, char *name, int *sub
 
 	      if (!CHECK_LEN(header, p, qlen, 20))
 		{
-		  if (!no_cache)
+		  if (cache)
 		    blockdata_free(addr.rrblock.rrdata);
 		  return 0;
 		}
 	      
 	      /* rest of RR */
-	      if (!no_cache)
+	      if (cache)
 		{
 		  int secflag = 0;
 
@@ -1079,7 +1079,17 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	  
 	  if (insert && !option_bool(OPT_NO_NEG))
 	    {
-	      int substring, have_soa = find_soa(header, qlen, name, &substring, &ttl, no_cache_dnssec, now);
+	      /* The order of records going into the cache matters (see  cache_recv_insert()).
+		 The target of a CNAME must immediately follow the CNAME.
+		 (CNAME has already gone into the cache at this point)
+		 Here we call find_soa to get the ttl and substring, but
+		 we DON'T LET IT INSERT the SOA into the cache if our negative record is a CNAME target
+		 so that the SOA doesn't come before the CNAME target.
+
+		 We call find_soa() again after inserting the CNAME target to insert the SOA
+		 if necessary. */
+
+	      int substring, have_soa = find_soa(header, qlen, name, &substring, &ttl, cpp == NULL, now);
 	      
 	      if (have_soa || daemon->neg_ttl)
 		{		  
@@ -1101,6 +1111,10 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 		      next_uid(newc);
 		      cpp->addr.cname.target.cache = newc;
 		      cpp->addr.cname.uid = newc->uid;
+		    
+		      /* we didn't insert the SOA before a CNAME target above, do it now. */
+		      if (have_soa)
+			find_soa(header, qlen, name, NULL, NULL, 1, now);
 		    }
 		}
 	    }