Bug ID 1086517: TMM may not properly exit hardware SYN cookie mode

Last Modified: May 29, 2024

Affected Product(s):
BIG-IP TMOS(all modules)

Known Affected Versions:
15.0.0, 15.0.1,,,,, 15.1.0,,,,,, 15.1.1, 15.1.2,, 15.1.3,, 15.1.4,, 15.1.5,, 15.1.6, 16.0.0,, 16.0.1,,, 16.1.0, 16.1.1, 16.1.2,,, 16.1.3,,,,,, 17.0.0,,

Fixed In:
17.1.0, 16.1.4,

Opened: Mar 11, 2022

Severity: 3-Major


Due to a race condition, when one TMM exits SYN cookie mode, another may immediately re-enter hardware SYN cookie mode, keeping the virtual server in SYN cookie mode and the mitigation offloaded to hardware. The SYN cookie status of the virtual server is not properly updated and will show 'not-activated'.


A virtual server that once entered hardware SYN cookie mode may remain in that state indefinitely. The reduced MSS size may affect performance of that virtual server.


Hardware SYN cookie protection is enabled and SYN cookie mode is triggered.


Disable hardware SYN cookie either locally via the TCP or FastL4 profile, or globally by the PvaSynCookies.Enabled BigDB variable. Software SYN cookie mode is unaffected.

Fix Information

The race condition is eliminated, virtual servers properly exit hardware SYN cookie mode.

Behavior Change

Guides & references

K10134038: F5 Bug Tracker Filter Names and Tips