Response rate limiting definition
Response rate limiting (RRL) is a server-side traffic control mechanism that restricts the number of similar or identical responses a service — most often a DNS server — sends to the same IP address or subnet within a short time window. When the limit is reached, additional packets are either dropped, delayed, or returned as minimal “truncated” replies. Unlike traditional rate limiting, which slows incoming requests, RRL throttles outgoing responses. This makes it significantly more difficult for attackers to exploit your DNS server as an amplifier in a distributed denial-of-service (DDoS) attack.
See also: DDoS mitigation
How response rate limiting works
- Counting: The server tracks identical or similar replies sent to each destination during a one-second interval.
- Ceiling: After, for instance, five identical replies, subsequent packets in that interval are suppressed or replaced with minimal-size responses that force the requester to retry over TCP, which is more effortful and rate-limited by design.
- Reset: At the start of the next second, the counter resets, resulting in little to no perceptible delay for normal users.
Why response rate limiting matters
- Prevents DNS reflection and amplification attacks: Attackers can’t use your DNS server to flood victims with spoofed traffic.
- Slows credential-stuffing bots: Automated login attempts receive fewer error messages, reducing the feedback attackers rely on.
- Reduces congestion and cost: Dropping unnecessary packets decreases outbound data usage and helps maintain low latency for legitimate users.
