I am researching IPS devices and their signatures both stateful and stateless. The test network I am looking to implement the IPS in has asymmetric traffic so stateful inspection would be nearly impossible. What percentage of threats can only be mitigated from stateful inspection?

If I were to turn off stateful inspection on the IPS what threats am I leaving the test network open to?

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    SomethingSmithe, welcome to Information Security. Dont forget to check the FAQ when you get a chance... Btw, Stateful Inspection, and IDS in general, do not mitigate threats, they only detect them. Though I'm sure that's what you meant :)
    – AviD
    Mar 25, 2012 at 17:28
  • @AviD IPS is preventative isn't it? IDS is detective.
    – lew
    Mar 27, 2012 at 0:48
  • @lew -ish. And that also depends on what threats you're talking about... But yeah, depending on product/implementation, there is some level of protection. Sorry, I got caught on the IDS part, and skipped the IPS... :$
    – AviD
    Mar 27, 2012 at 6:23
  • which IPS are you using?
    – YSY
    Mar 28, 2012 at 8:44
  • I was thinking of a Snort install on a FreeBSD box but not really decided of build versus buy. This stateful versus stateless question will probably need a decision on my part before I invest some time on some experimentation. Mar 28, 2012 at 10:43

1 Answer 1


Stateful network intrusion detection, and even more so prevention, is a tough task in face of multiple upstream links. When traffic can flow in at one link and exit at another, a single monitor cannot reconstruct end-host state effectively. You can either accept this situation with all its implications (e.g., mediocre protection at best) or resort to a distributed solution that propagates state amongst the nodes.

Ideally you would push the monitors to the network edge [1], but it sounds like you have no control over the edge. Without detailed understanding of the network topology, this is a hard question to answer. For the remainder of this answer, I assume that you have at least the ability to deploy several monitors in your network to approximate the ideal of inspecting all upstream traffic.

The NIDS Cluster

The Bro network security monitor has a cluster mode, where a node can propagate state to other nodes. For example, when node A sees a FTP control connection, it may propagate this information to node B, so that it knows how to interpret the following high-port data connection. Another scenario where this comes in handy is distributed scan detection. If you have an infected machine in your network that performs outbound scans through multiple links, each node may keep a local counter and exchange it regularly with other nodes. For a more detailed analysis of state exchange, have a look at the NIDS cluster paper.


This cluster approach scales well do to the following insights:

  • The granularity of analysis and clustering is an entire connection. As long as you can ensure that packets belonging to the same connection end up at the same cluster node, you get good scalability. In single-upstream environments, a frontend typically slices the input traffic to achieve this constraint. In medium-sized networks, a Click router that rewrites MAC address often suffices, but larger environments often have to use dedicated high-performance devices. Our research group has very good experiences with cPacket CVU frontends, which can load-balance multi-10-Gbps links at line rate without dropping packets.

  • The size of the exchanged state is typically orders of magnitude smaller than the traffic a node observes. For example, if one nodes observes an big ISO image download, it will not propagate the very ISO image to other nodes, but for example just the name of the ISO image. Moreover, intra-connection analysis are already stateful, e.g, Bro follows the TCP state machine and exposes the HTTP message pipeline to the user.

Applying this to your scenario is a little more complicated, because you do not have a single upstream link. In order to scale well, you need to ensure that traffic from the same connection is inspected by the same node. In other words, the notion of a frontend is a little bit blurry because you have to manually route the packets to the corresponding cluster node. The exact overhead of this approach depends on your concrete topology.

[1] This does not preclude defense in depth. Adding more monitors at the core is a good idea as well, for example when no "roadwarrior" subnets are established to isolate mobile devices.

  • Fantastic answer! Couple of quick follow-ups though. I have a feeling that the amount of throughput we are going to have across the cluster it will mean for some huge extra connections. Secondly, do you know if anyone has done any reports on purely stateless signatures versus statefull? Apr 2, 2012 at 12:44
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    Hopefully the added notes on scalability clarify this a bit. I do not use stateless signatures. Almost all important security incidents I come across are stateful. Also, signatures are essentially regular languages which are strictly inferior to the Turing-complete scripting language offered by NIDS like Bro.
    – mavam
    Apr 2, 2012 at 17:51

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