Rate limiting is a crucial defense method against service misuse. The Halon SMTP software implements this in a global rate controlling service called rated. It handles rate limiting for all processes (HSL contexts), 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).
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 (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.
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 replay attacks. It should probably be protected by other means such as a VPN tunnel or psychical security (DMZ).
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 Web UI may display different fields depending on how it is configured. Entries may be alphabetically paged and filtered by name and by a minimal count (threshold).
|Namespace||The namespace or "bucket".|
|Entry||The name of the rate limit entry.|
|See table below.|
|Rate||count divided by the interval.|
|Interval||The time between first and last count.|
|Most recent||When a count was last added to this entry.|
If viewed in a cluster (/cluster/ratelimit/). The rate limit may or may not be synchronized.
|Count||The maximum count a single unit has; this should preferable be the same on all units.|
|Min||If shown, it is the lowest count a single node has.|
|Same as for synchronized|
|Sum||The total sum of all counts on all units.|
If viewed on a single unit (/ratelimit/).
|Count||The count this unit has.|
|Local count||If shown, the count created on this unit (otherwise the same as Count).|