DNS Request without Response

Network / Transport Level

Packet dropped in transit

Firewall or ACL blocks UDP/53 (or TCP/53 if used).

Middlebox/security appliance drops DNS queries or responses.

MTU issue causing packet fragmentation and loss.

Server not reachable

DNS server IP is wrong or unreachable (routing problem).

No listening service on port 53 (DNS server not running).

DNS server overloaded → silently drops queries.

Client / Server Side

Timeout on server

DNS server received the request but didn't send a reply (bug, misconfiguration).

Zone misconfiguration prevents response (e.g., no SOA/NS record).

UDP-specific behavior

If the response would exceed 512 bytes (without EDNS0), some servers may drop it instead of truncating.

Client may retry with TCP, but if TCP/53 is blocked → appears as "no response."