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syn cache overflows. Whether this is due to an attack or due to the system having more legitimate connections than the syn cache can hold, this situation can quickly impact performance. To make the system perform better during these periods, the code will now switch to exclusively using cookies until the syn cache stops overflowing. In order for this to occur, the system must be configured to use the syn cache with syn cookie fallback. If syn cookies are completely disabled, this change should have no functional impact. When the system is exclusively using syn cookies (either due to configuration or the overflow detection enabled by this change), the code will now skip acquiring a lock on the syn cache bucket. Additionally, the code will now skip lookups in several places (such as when the system receives a RST in response to a SYN|ACK frame). Reviewed by: rrs, gallatin (previous version) Discussed with: tuexen Sponsored by: Netflix, Inc. Differential Revision: https://reviews.freebsd.org/D21644
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