Which statement about backlog limitations in TCP servers is accurate?

• A. Backlog limitations cap the queue of established connections.
• B. Backlog limitations cap the number of half-open connections waiting to complete the handshake.
• C. Backlog limitations control DNS query rates.
• D. Backlog limitations cap the queue of pending connections waiting to complete the TCP handshake.

Answer: (D)

Backlog defines how many incoming connection attempts can be queued by a listening socket before the server accepts them. When a client starts a TCP connection, the handshake progresses and the connection sits in this backlog while the server processes it and calls accept. If the backlog is full, new connection attempts may be delayed or rejected, which is the behavior you want to understand for capacity planning and tuning.

This best description matches the fact that the backlog is about pending connections in the process of being established, not about connections that are already established, and it has nothing to do with DNS query rates.