Fix CVE-2017-13704, which resulted in a crash on a large DNS query.

A DNS query recieved by UDP which exceeds 512 bytes (or the EDNS0 packet size,
if different.) is enough to cause SIGSEGV.

CHANGELOG

@@ -17,6 +17,13 @@ version 2.78
 	--strict-order active. Thanks to Hans Dedecker
 	for the patch
 
+	Fix regression in 2.77, ironically added as a security
+	improvement, which resulted in a crash when a DNS
+	query exceeded 512 bytes (or the EDNS0 packet size,
+	if different.) Thanks to Christian Kujau, Arne Woerner
+	Juan Manuel Fernandez and Kevin Darbyshire-Bryant for
+	chasing this one down.  CVE-2017-13704 applies.
+
 	
 version 2.77
 	Generate an error when configured with a CNAME loop,

src/auth.c

@@ -119,11 +119,6 @@ size_t answer_auth(struct dns_header *header, char *limit, size_t qlen, time_t n
   struct cname *a, *candidate;
   unsigned int wclen;
   
-  /* Clear buffer beyond request to avoid risk of
-     information disclosure. */
-  memset(((char *)header) + qlen, 0, 
-	 (limit - ((char *)header)) - qlen);
-  
   if (ntohs(header->qdcount) == 0 || OPCODE(header) != QUERY )
     return 0;
 

src/forward.c

@@ -1189,6 +1189,10 @@ void receive_query(struct listener *listen, time_t now)
       (header->hb3 & HB3_QR))
     return;
 
+  /* Clear buffer beyond request to avoid risk of
+     information disclosure. */
+  memset(daemon->packet + n, 0, daemon->edns_pktsz - n);
+  
   source_addr.sa.sa_family = listen->family;
   
   if (listen->family == AF_INET)
@@ -1689,6 +1693,10 @@ unsigned char *tcp_request(int confd, time_t now,
       if (size < (int)sizeof(struct dns_header))
 	continue;
 
+      /* Clear buffer beyond request to avoid risk of
+	 information disclosure. */
+      memset(payload + size, 0, 65536 - size);
+      
       query_count++;
 
       /* log_query gets called indirectly all over the place, so 

src/rfc1035.c

@@ -1223,11 +1223,6 @@ size_t answer_request(struct dns_header *header, char *limit, size_t qlen,
   struct mx_srv_record *rec;
   size_t len;
 
-  /* Clear buffer beyond request to avoid risk of
-     information disclosure. */
-  memset(((char *)header) + qlen, 0, 
-	 (limit - ((char *)header)) - qlen);
-  
   if (ntohs(header->ancount) != 0 ||
       ntohs(header->nscount) != 0 ||
       ntohs(header->qdcount) == 0 ||