Configure Kernel to Rate Limit Sending of Duplicate TCP Acknowledgments

Description

Make sure that the system is configured to limit the maximal rate for sending duplicate acknowledgments in response to incoming TCP packets that are for an existing connection but that are invalid due to any of these reasons: (a) out-of-window sequence number, (b) out-of-window acknowledgment number, or (c) PAWS (Protection Against Wrapped Sequence numbers) check failure This measure protects against or limits effects of DoS attacks against the system. Set the system to implement rate-limiting measures by adding the following line to /etc/sysctl.conf or a configuration file in the /etc/sysctl.d/ directory (or modify the line to have the required value):

net.ipv4.tcp_invalid_ratelimit = 

Issue the following command to make the changes take effect:

# sysctl --system

Rationale

Denial of Service (DoS) is a condition when a resource is not available for legitimate users. When this occurs, the organization either cannot accomplish its mission or must operate at degraded capacity.

This can help mitigate simple “ack loop” DoS attacks, wherein a buggy or malicious middlebox or man-in-the-middle can rewrite TCP header fields in manner that causes each endpoint to think that the other is sending invalid TCP segments, thus causing each side to send an unterminating stream of duplicate acknowledgments for invalid segments.