Bug ID 510612: syncookie and loose init do not work together

Last Modified: Sep 13, 2023

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

Known Affected Versions:
11.5.1, 11.5.2, 11.5.3, 11.5.4, 11.5.5, 11.5.6, 11.5.7, 11.5.8, 11.5.9, 11.5.10, 11.6.0, 11.6.1, 11.6.2, 11.6.3,,,,, 11.6.4, 11.6.5,,,, 12.0.0, 12.0.0 HF1, 12.1.0 HF1, 12.0.0 HF2, 12.1.0 HF2, 12.0.0 HF3, 12.0.0 HF4, 12.1.1 HF1, 12.1.1 HF2, 12.1.2 HF1, 12.1.2 HF2, 12.1.0, 12.1.1, 12.1.2, 12.1.3,,,,,,,, 12.1.4,, 12.1.5,,,, 12.1.6, 13.0.0, 13.0.0 HF1, 13.0.0 HF2, 13.0.0 HF3, 13.0.1, 13.1.0,,,,,,,,, 13.1.1,,,,, 13.1.3,,,,,,, 13.1.4,, 13.1.5,, 14.0.0,,,,,, 14.0.1,, 15.0.0, 15.0.1,,,,, 16.0.0,, 16.0.1,,, 17.0.0,,

Opened: Mar 04, 2015

Severity: 3-Major

Related Article: K99545311


If a TCP FastL4 virtual server is configured as loose init and also syncookie is enabled: - syncookie protection will be bypassed if non-SYN packets are received that do not match a flow. - connections may fail after a long idle period where the flow data has been deleted from TMM but the client can still try to resume via loose init. If it's software syn cookie protection, BIG-IP will proxy the TCP handshake and then initiate a TCP handshake to the back end server and attempt to send the data. If it's hardware syn cookie protection, BIG-IP will simply pass the first piece of data after the TCP handshake to the back end server without sending a syn. In both scenarios this is considered an improper configuration.


Failed ACK will be sent to virtual server and cause numerous RESETs. Normal traffic continues without error.


TCP virtual server with loose init and with syncookie enabled and triggered.


Avoid configuring syncookie and loose init together. This combination, using loose init together with syncookie, is not recommended. Essentially, loose init means to disable the 3-way handshake check at the BIG-IP system, while syncookie means to enforce the 3-way handshake check at the BIG-IP system (possibly by hardware). Configuring these two in combination will produce unexpected side effects.

Fix Information


Behavior Change

Guides & references

K10134038: F5 Bug Tracker Filter Names and Tips