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This patch improves ctnetlink event reliability if one broadcast listener has set the NETLINK_BROADCAST_ERROR socket option. The logic is the following: if an event delivery fails, we keep the undelivered events in the missed event cache. Once the next packet arrives, we add the new events (if any) to the missed events in the cache and we try a new delivery, and so on. Thus, if ctnetlink fails to deliver an event, we try to deliver them once we see a new packet. Therefore, we may lose state transitions but the userspace process gets in sync at some point. At worst case, if no events were delivered to userspace, we make sure that destroy events are successfully delivered. Basically, if ctnetlink fails to deliver the destroy event, we remove the conntrack entry from the hashes and we insert them in the dying list, which contains inactive entries. Then, the conntrack timer is added with an extra grace timeout of random32() % 15 seconds to trigger the event again (this grace timeout is tunable via /proc). The use of a limited random timeout value allows distributing the "destroy" resends, thus, avoiding accumulating lots "destroy" events at the same time. Event delivery may re-order but we can identify them by means of the tuple plus the conntrack ID. The maximum number of conntrack entries (active or inactive) is still handled by nf_conntrack_max. Thus, we may start dropping packets at some point if we accumulate a lot of inactive conntrack entries that did not successfully report the destroy event to userspace. During my stress tests consisting of setting a very small buffer of 2048 bytes for conntrackd and the NETLINK_BROADCAST_ERROR socket flag, and generating lots of very small connections, I noticed very few destroy entries on the fly waiting to be resend. A simple way to test this patch consist of creating a lot of entries, set a very small Netlink buffer in conntrackd (+ a patch which is not in the git tree to set the BROADCAST_ERROR flag) and invoke `conntrack -F'. For expectations, no changes are introduced in this patch. Currently, event delivery is only done for new expectations (no events from expectation expiration, removal and confirmation). In that case, they need a per-expectation event cache to implement the same idea that is exposed in this patch. This patch can be useful to provide reliable flow-accouting. We still have to add a new conntrack extension to store the creation and destroy time. Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org> Signed-off-by: Patrick McHardy <kaber@trash.net> |
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