6. SYN Cookie: Hardware vs Software


Currently, you know the differences between LTM and AFM when talking about SYN Cookie capabilities and configuration. In this article I describe how SYN Cookie can perform its tasks using two different ways, in software or offloaded in hardware.

The most important difference between these functioning modes is clearly the resource consuming. In hardware based platforms SYN Cookie is a countermeasure that will not consume extra memory nor CPU, while in software this task must be handled by TMM, so some load could be added comparing to normal operation.

Hardware SYN Cookie

If you have a hardware offloading capable platform and you configure SYN Cookie to work in hardware, the creation and validation of SYN Cookie challenges will be managed by dedicated FPGA (Programmable Field Array) or NSP (Neuron Search Processor) In a few words this means that SYN Cookie running in hardware basically allows you to forget about TCP SYN flood attacks. You do not have to care about it, if it is correctly configured, since BIG-IP will not only mitigate attack without impacting legitimate users, who will still access to application during the attack, but also no extra resources will be needed.

The only handicap you have to take into account currently when hardware SYN Cookie is activated is that you cannot keep TCP options information. In order to keep this information you need to enable SYN Cookie in software and client connection need to activate TCP TS option (this is enabled by default in BIG-IP). See second article in this series for more details (SYN Cookie Operation).


Platforms shipped with Neuron chip and running a TMOS version greater than 14, improve performance because this chip, which is directly connected to HSBe2, provide with extra functionalities to SYN Cookie. Neuron improves performance and solves limitations that you could find in HSBe2 only platforms when SYN Cookie is activated for wildcard virtual servers. A brief description of this limitation can be found in K50955355.

Note that Neuron does not only offloads SYN Cookie tasks from software, if you have an AFM provisioned device you can take advantage of other features like extended whitelists and blacklists in hardware. However these article series are only focused on SYN Cookie. With this in mind the functioning of Neuron is simple, HSBe2 makes requests to Neuron when a TCP SYN reaches BIG-IP, and depending of the type of request Neuron then will create a SYN Cookie for the connection or it will validate SYN Cookie response presented by client. For example, as I showed in previous articles, when an ACK comes from a client then HSBe2 will ask Neuron if SYN Cookie is correct, then Neuron will response with the requested information, so TMM will know if it must drop or allow the connection through the datapath.

SYN Cookie Neuron does not offload AFM Global SYN Cookie, it works for:

  • LTM Global SYN Cookie
  • Per VLAN SYN Cookie
  • LTM Per Virtual SYN Cookie
  • AFM Per Virtual SYN Cookie

In order to enable SYN Cookie Neuron in your platform you need to activate the Turboflex profile for Security/Securityv1 (AFM) or ADC (LTM). There is many literature in AskF5 about Turboflex, but in a few words it allows you to group several related features to be accelerated in hardware.

I will not give technical details about Neuron since it is not the intention of these article series. For troubleshooting issues with Neuron please raise a case to F5 Networks, so expert engineer can investigate it. Making internal changes to Neuron could lead to a worst result.

Disabling SYN Cookie

SYN Cookie is enabled by default in software and hardware, if we disable hardware SYN Cookie then software SYN Cookie comes into play. There is a DB key for disabling Hardware SYN Cookie in all contexts:


tmsh list sys db pvasyncookies.enabled
sys db pvasyncookies.enabled {
   value "false"


Of course, if the specific platform cannot offload SYN Cookie into hardware then above DB key has no effect. In this case software SYN Cookie is enabled by default. There is not such DB key for enabling/disabling software SYN Cookie as we have for hardware, instead, if you want to disable software SYN Cookie then you have different options depending on context that you can consult in table below.

Remember that AFM SYN Cookie has precedence over LTM SYN Cookie, this means that if AFM SYN Cookie is configured and you want to disable completely SYN Cookie you need to disable both, AFM and LTM SYN Cookie. The simplest path to disable completely SYN Cookie assuming default config, that is, hardware SYN Cookie is enabled, it would be:


Fig14. Disabling SYN Cookie


In table below I summarize how to disable hardware and software SYN Cookie in each context for the different scopes.


For AFM disabling SYN Cookie is quite easier, you just need to change DoS Device and DoS profiles TCP Half Open vector to 0/Infinite.


It can be possible that SYN Cookie works in software in your device whilst you expected SYN Cookie working in hardware. Sometimes you end up in this situation due to a wrong configuration. Below I make a list of possible reasons that you can check:

  • You have disabled autolasthop. This option must be enabled if you want to offload SYN Cookie into hardware (K16887).
  • You have changed DB key connection.syncookies.algorithm manually. This key defines which algorithm is used for generating hash that is part of the SYN Cookie challenge (as I described in the first article of this series). When set a value of Hardware to this DB key the error detecting code used for validating the hash will be understood by both, software and hardware. If set to Software then it will only be understood by software. If your version also has the option ‘both’, this means that BIG-IP dynamically discovers which algorithm should be used rather than forcing to Software or Hardware. In summary, if virtual is configured for hardware SYN Cookie but algorithm is configured in software then hardware will inspect SYN Cookies, it will confirm it cannot validate them and then it will send it to TMM for validation.
  • Until v14.1 SYN cookie hardware it is not recommended for protecting networks (wildcard virtual servers), this is because SYN Cookie will only protect a single flood network destination when flooding towards multiple network destination at a time. The other networks will be protected by software SYN cookie. This could cause unexpected extra CPU utilization. This is greatly improved in Neuron capable platforms.
  • If there are collisions when SYN Cookie is configured in hardware you will still see non zero stats for software SYN Cookie. Also, these stats can increase due to validation of first challenges, when SYN Cookie is activated, and TMMs handles TCP SYN until it enables hardware to do it. This is expected and it does not mean that hardware SYN Cookie is not working. I will show examples in article dedicated to stats.

In order to end up this section with clear ideas, remember that hardware SYN Cookie and hardware flow acceleration are two different concepts.



In this article you learnt how behaves SYN Cookie when working in software and hardware and the advantages or disadvantages. In next two articles I will show you how to troubleshoot SYN Cookie issues.

Updated Dec 08, 2022
Version 2.0

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