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.

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

@@ -1188,6 +1188,10 @@ void receive_query(struct listener *listen, time_t now)
       (msg.msg_flags & MSG_TRUNC) ||
       (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;
   
@@ -1688,6 +1692,10 @@ 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++;
 

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 ||