Rate limiting

Rate limiting is a crucial defense method against service misuse. We have implemented a global rate controlling service called rated. It handles rate limiting for all processes, based on an "items per time interval" algorithm. Our implementation features a sliding rate limit (in contrast to leaking bucket; which has a fixed output rate regardless of exactly when the rate was exceeded).

Implementation

Rate limits may be "hit" using the HSL rate() function, and sometimes using the graphical blocks in the flows. It returns true as long as the rate() call interval is less than its rate limit, but false when exceeded. If the count parameter is zero, the current rate is returned instead. When a "hit" is counted, its expiration time is the fourth parameter (interval).

// get the rate limit number, without increasing
if (rate("failed-login:saslusername", $saslusername, 0, 60) >= 3)
    Reject("Too many failed logins");

// increase, and reject if exceeded
if ($is_spam)
    if (rate("spammers", $senderip, 100, 86400) == false)
        Reject("You may only send 100 suspicious messages per day"); 

Rate limits may be read and cleared using the SOAP API.

Clustering

Rate limits are synchronized in the cluster if a HMAC-SHA1 key is specified on the system settings page. The protocol is based on UDP and uses port 13131. In terms of security it may be wise to only activate this feature on local networks, as the HMAC-SHA1 only serves as a packet-authenticity firewall. It does not provide encryption nor protect against reply attacks. It should probably be protected by other means such as a VPN tunnel or psychical security (DMZ).

Performance

The rate limits performance is roughly O(log N + log M) and lookups are currently implemented as a C++ std::multimap.

Rate limit page in Web UI

The rate limit page in the Webui may display the rate limit database both in clustered mode (/cluster/ratelimit/) and standalone per unit (/cluster/ratelimit/), in both layouts different information is presented. Values may be alphabetically paged and filtered by name and minimal count (threshold).

Result	Description
Namespace	The namespace or "bucket"
Entry	The name of the rate limit item
Count	The maximum value a single node has, this should be the same on all units.
See table below
Interval	The time between first and last count
Most recent	When the rate limit was last hit

Cluster

If viewed in a cluster (/cluster/ratelimit/).

Synchronized

Result	Description
Count	The maximum value a single node has, this should be the same on all units.
Min	If shown, it is the lowest value a single node has that is not count.

Non-synchronized

Result	Description
Same as for synchronized
Sum	The total sum of all counts on all units.

Single unit

If viewed on a single unit (/ratelimit/).

Result	Description
Count	The count this node has.
Local count	The count created on this node (may be same as Count).