Re: [RFC] [PATCH 0/4] netfilter: "fail-open" feature support for NFQUEUE

[Florian Westphal <fw@strlen.de>]

Krishna Kumar <krkumar2@in.ibm.com> wrote:
> Many users of an IBM security product, which uses netfilter's NFQUEUE
> target to process packets in userspace, face a problem of dropped
> connections during heavy load. Incoming packets are queued and
> processed by the security module, which does deep packet analysis to
> decide whether to accept or reject them. However during heavy load,
> NFQUEUE queue (default 1024 entries) fills up and connections fail
> after large number of packets drop during enqueue. Increasing the
> queue size delays the problem and also worsens latency.
> 
> This patch set implements a "failopen" support to help keep connections
> open during such failures. This is achieved by allowing acceptance of
> packets temporarily when the queue is full, which enables existing
> connections to be kept alive. Customers prefer this option as similar
> feature is available on other systems.
> 
> This patch set implements failopen for NFQUEUE (though a similar patch
> for IPQUEUE is also implemented but not submitted at this time). I will
> submit the iptables changes which controls turning failopen mode on/off
> later. The original requirement for sysctl option is not implemented -
> please let me know whether that is acceptable/preferable.

I think that exposing this feature as userspace-changeable via netlink
(eg. by adding "NFQA_CFG_FAILOPEN" attribute) rather than via ruleset
would make most sense, as only the application can know wheter it
can cope with missing packets.