Handle malformed DNS replies better.

If we detect that that reply from usptream is malformed,
transform it into a SERVFAIL reply before sending to the
original requestor.

CHANGELOG

@@ -59,6 +59,9 @@ version 2.88
 	needed is O(n^2). Handle this case more intelligently.
 	Thanks to Ye Zhou for spotting the problem and an initial patch.
 	
+	If we detect that a DNS reply from upstream is malformed don't
+	return it to the requestor; send a SEVFAIL rcode instead.
+
 	
 version 2.87
         Allow arbitrary prefix lengths in --rev-server and

src/forward.c

@@ -821,12 +821,22 @@ static size_t process_reply(struct dns_header *header, time_t now, struct server
 	    n = rrfilter(header, n, RRFILTER_AAAA);
 	}
 
-      if (extract_addresses(header, n, daemon->namebuff, now, ipsets, nftsets, is_sign, check_rebind, no_cache, cache_secure, &doctored))
+      switch (extract_addresses(header, n, daemon->namebuff, now, ipsets, nftsets, is_sign, check_rebind, no_cache, cache_secure, &doctored))
 	{
+	case 1:
 	  my_syslog(LOG_WARNING, _("possible DNS-rebind attack detected: %s"), daemon->namebuff);
 	  munged = 1;
 	  cache_secure = 0;
 	  ede = EDE_BLOCKED;
+	  break;
+	  
+	  /* extract_addresses() found a malformed answer. */
+	case 2:
+	  munged = 1;
+	  SET_RCODE(header, SERVFAIL);
+	  cache_secure = 0;
+	  ede = EDE_OTHER;
+	  break;
 	}
 
       if (doctored)

src/rfc1035.c

@@ -538,7 +538,9 @@ static int print_txt(struct dns_header *header, const size_t qlen, char *name,
 /* Note that the following code can create CNAME chains that don't point to a real record,
    either because of lack of memory, or lack of SOA records.  These are treated by the cache code as 
    expired and cleaned out that way. 
-   Return 1 if we reject an address because it look like part of dns-rebinding attack. */
+   Return 1 if we reject an address because it look like part of dns-rebinding attack. 
+   Return 2 if the packet is malformed.
+*/
 int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t now, 
 		      struct ipsets *ipsets, struct ipsets *nftsets, int is_sign, int check_rebind,
 		      int no_cache_dnssec, int secure, int *doctored)
@@ -589,7 +591,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
   namep = p = (unsigned char *)(header+1);
   
   if (ntohs(header->qdcount) != 1 || !extract_name(header, qlen, &p, name, 1, 4))
-    return 0; /* bad packet */
+    return 2; /* bad packet */
   
   GETSHORT(qtype, p); 
   GETSHORT(qclass, p);
@@ -607,13 +609,13 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	{
 	cname_loop:
 	  if (!(p1 = skip_questions(header, qlen)))
-	    return 0;
+	    return 2;
 	  
 	  for (j = 0; j < ntohs(header->ancount); j++) 
 	    {
 	      int secflag = 0;
 	      if (!(res = extract_name(header, qlen, &p1, name, 0, 10)))
-		return 0; /* bad packet */
+		return 2; /* bad packet */
 	      
 	      GETSHORT(aqtype, p1); 
 	      GETSHORT(aqclass, p1);
@@ -651,7 +653,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 		    log_query(secflag | F_CNAME | F_FORWARD | F_UPSTREAM, name, NULL, NULL, 0);
 		  
 		  if (!extract_name(header, qlen, &p1, name, 1, 0))
-		    return 0;
+		    return 2;
 		  
 		  if (aqtype == T_CNAME)
 		    {
@@ -677,7 +679,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 
 	      p1 = endrr;
 	      if (!CHECK_LEN(header, p1, qlen, 0))
-		return 0; /* bad packet */
+		return 2; /* bad packet */
 	    }
 	}
       
@@ -722,14 +724,14 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
       
     cname_loop1:
       if (!(p1 = skip_questions(header, qlen)))
-	return 0;
+	return 2;
       
       for (j = 0; j < ntohs(header->ancount); j++) 
 	{
 	  int secflag = 0;
 	  
 	  if (!(res = extract_name(header, qlen, &p1, name, 0, 10)))
-	    return 0; /* bad packet */
+	    return 2; /* bad packet */
 	  
 	  GETSHORT(aqtype, p1); 
 	  GETSHORT(aqclass, p1);
@@ -747,7 +749,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	    {
 	      p1 = endrr;
 	      if (!CHECK_LEN(header, p1, qlen, 0))
-		return 0; /* bad packet */
+		return 2; /* bad packet */
 	      continue;
 	    }
 	  
@@ -790,7 +792,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	      
 	      namep = p1;
 	      if (!extract_name(header, qlen, &p1, name, 1, 0))
-		return 0;
+		return 2;
 	      
 	      if (qtype != T_CNAME)
 		goto cname_loop1;
@@ -813,25 +815,25 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 		  unsigned char *tmp = namep;
 		  
 		  if (!CHECK_LEN(header, p1, qlen, 6))
-		    return 0; /* bad packet */
+		    return 2; /* bad packet */
 		  GETSHORT(addr.srv.priority, p1);
 		  GETSHORT(addr.srv.weight, p1);
 		  GETSHORT(addr.srv.srvport, p1);
 		  if (!extract_name(header, qlen, &p1, name, 1, 0))
-		    return 0;
+		    return 2;
 		  addr.srv.targetlen = strlen(name) + 1; /* include terminating zero */
 		  if (!(addr.srv.target = blockdata_alloc(name, addr.srv.targetlen)))
 		    return 0;
 		  
 		  /* we overwrote the original name, so get it back here. */
 		  if (!extract_name(header, qlen, &tmp, name, 1, 0))
-		    return 0;
+		    return 2;
 		}
 	      else if (flags & (F_IPV4 | F_IPV6))
 		{
 		  /* copy address into aligned storage */
 		  if (!CHECK_LEN(header, p1, qlen, addrlen))
-		    return 0; /* bad packet */
+		    return 2; /* bad packet */
 		  memcpy(&addr, p1, addrlen);
 		  
 		  /* check for returned address in private space */
@@ -875,7 +877,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	      if (aqtype == T_TXT)
 		{
 		  if (!print_txt(header, qlen, name, p1, ardlen, secflag))
-		    return 0;
+		    return 2;
 		}
 	      else
 		log_query(flags | F_FORWARD | secflag | F_UPSTREAM, name, &addr, NULL, aqtype);
@@ -883,7 +885,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	  
 	  p1 = endrr;
 	  if (!CHECK_LEN(header, p1, qlen, 0))
-	    return 0; /* bad packet */
+	    return 2; /* bad packet */
 	}
       
       if (!found && (qtype != T_ANY || (flags & F_NXDOMAIN)))