Rate Limit Plugin

The rate_limit plugin provides basic mechanism for how much traffic a particular service (remap rule) is allowed to do. Currently, the only implementation is a limit on how many active client transactions a service can have. However, it would be easy to refactor this plugin to allow for adding new limiter policies later on.

The limit counters and queues are per remap rule only, i.e. there is (currently) no way to group transaction limits from different remap rules into a single rate limiter.


This is still work in progress, in particularly the configuration and the IP reputation system needs some work. In particular:

  • The remap configuration needs YAML support.

  • We need reloadable configurations.

  • The IP reputation currently only works with the global plugin settings.

  • There is no support for adding allow listed IPs to the IP reputation.

Remap Plugin

All configuration is done via remap.config, and the following options are available:


The maximum number of active client transactions. This option can also be used in conjunction with --rate.


The acceptable rate, in transaction or connections per second, that we will allow. This option can also be used in conjunction with --limit.


When the limit (above) has been reached, all new transactions are placed on a FIFO queue. This option (optional) sets an upper bound on how many queued transactions we will allow. When this threshold is reached, all additional transactions are immediately served with an error message.

The queue is effectively disabled if this is set to 0, which implies that when the transaction limit is reached, we immediately start serving error responses.

The default queue size is UINT_MAX, which is essentially unlimited.


An optional HTTP status error code, to be used together with the --queue option above. The default is 429.


An optional retry-after value, which if set will cause rejected (e.g. 429) responses to also include a header Retry-After.


This is an optional HTTP header name, which will be added to the client request header IF the transaction was delayed (queued). The value of the header is the delay, in milliseconds. This can be useful to for example log the delays for later analysis.

It is recommended that an @ header is used here, e.g. @RateLimit-Delay, since this header will not leave the ATS server instance.


An optional max-age for how long a transaction can sit in the delay queue. The value (default 0) is the age in seconds.


An optional metric prefix to use instead of the default (plugin.rate_limiter).


An optional metric tag to use instead of the default. When a tag is not specified the plugin will use the scheme, FQDN, and port when it is non-standard. For example a default plugin tag might be “https.example.com” or “http.example.com:8080” noting that in the latter exampe, the non-standard scheme and port led to “:8080” being appended to the string.


This flag tells the limiter that rather than limiting the number of active transactions, it should limit the number of active connections. This allows an established connection to make any number of transactions, but limits the number of connections that can be active at any one time.

Note that it’s highly recommended that you keep a very low keep-alive timeout for the connections that are using this rate limiter.

Global Plugin

As a global plugin, the rate limiting currently applies only for TLS enabled connections, based on the SNI from the TLS handshake. As a global plugin we also have the support of an IP reputation system, see below for configurations.

In addition, the global plugin must be configured via a reloadable YAML configuration file. The basic use is as:

rate_limit.so some_config.yaml


As a global plugin, it’s highly recommended to also reduce the Keep-Alive inactive timeout for the service(s) controlled by this plugin. This avoids the risk of having idle connections consume too many of the available resources. This is easily done using e.g. the conf_remap plugin, proxy.config.http.keep_alive_no_activity_timeout_in.

The YAML configuration can have the following format, where the various sections and nodes are documented below.

   - sni: test1.example.com
      limit: 1000
      rate: 200
         size: 1000
         max-age: 30
         tag: example.com
         prefix: ddos
      ip-rep: main
      exclude: internal
   - sni: test2.example.com
      aliases: [test3.example.com, test4.example.com]
      limit: 100
   - name: main
      buckets: 10
      size: 15
      percentage: 90
      max-age: 300
         limit: 100
         threshold: 1
         max-age: 1800
   - name: internal

For the top level selector node, the following options are available:


The SNI to match for this rate limiter.


The maximum number of active client transactions. This can also be used in conjunction with the rate option.


This limits the number of new sessions per second. It can be used in conjunction with the limit option.


A list of aliases for the SNI, which will also be matched by this rate limiter.


The name of the IP reputation node to use for this rate limiter. If not specified, the IP reputation system is not used for this rate limiter.


A list of IP CIDR ranges to exclude from any rate limiting. Any IP matching this list will not be rate limited, even if the SNI matches.


If enabled, when the limit (above) has been reached, all new connections are placed on a FIFO queue. This option sets an upper bound on how many queued transactions we will allow. When this threshold is reached, all additional connections are immediately errored out in the TLS handshake.

The queue option can include a size and a max-age option. The size is default to UINT_MAX, which is essentially unlimited. The max-age is default to 0, which means no age limit.

No queue is enabled without this configuration directive, but it can also be disabled explicitly if the size is set to 0.


This is an optional node, which can be used to configure the metrics for this rate limiter. If not specified, no metrics will be added.

The metrics node can include a tag and a prefix option. The tag is default to the SNI, and the prefix is default to plugin.rate_limiter.

The lists node is used to configure IP lists, which can be used to exclude certain address ranges from the rate limiting. The following options are used:


The name of the IP reputation setup, used to refer to it from the rate limiters.


A list of CIDR ranges to add to this rule. The format is e.g.

The ip-rep` node is used to configure the IP reputation system, there can be zero, one or many IP reputation setups. Each setup is configured with a name, and the following options:


The name of the IP reputation setup, used to refer to it from the rate limiters.


The number of LRU buckets to use for the IP reputation. A good number here is 10, which is the default, but can be configured. The reason for the different buckets is to account for a pseudo-sorted list of IPs on the frequency seen. Too few buckets will not be enough to keep such sorting, rendering the algorithm useless. To function in our setup, the number of buckets must be less than 100.


This is the size of the largest LRU bucket (the entry bucket), 15 is a good value. This is a power of 2, so 15 means the largest LRU can hold 32768 entries. Note that this option must be bigger then the --iprep_buckets setting, for the bucket halfing to function.

The default here is 0, which means the IP reputation filter is not enabled!


This is the minimum percentage of the limit that the pressure must be at, before we start blocking IPs. The default is 0.9 which means 90% of the limit.


This is used for aging out entries out of the LRU, the default is 0 which means no aging happens. Even with no aging, entries will eventually fall out of buckets because of the LRU mechanism that kicks in. The aging is here to make sure a spike in traffic from an IP doesn’t keep the entry for too long in the LRUs.

In addition, there’s an optional configuration for the permanently blocking buckets, perma-block. This is a special bucket, which is only used for IPs which have been blocked for a long time. The configuration for this bucket is:


The minimum number of hits an IP must reach to get moved to the permanent bucket. In this bucket, entries will stay for 2x


This option specifies from which bucket an IP is allowed to move from into the perma block bucket. A good value here is likely 0 or 1, which is very conservative.


Like above, but only applies to the long term (perma-block) bucket. Default is 0, which means no aging to this bucket is applied.


Metric names are generated either using defaults or user-supplied values. In either case, the format of the metric names is as follows:


A user can specify their own prefixes and tags, but not types or metrics.


The default prefix for all metrics is plugin.rate_limiter.


There are two types of metrics: sni and remap. Each type corresponds with the type of configuration used to generate the metric. The global configuration is for rate limiting requests during TLS negotiation, hence, the type of sni. Similarly remap connotes a remap configuration.


By default the metric tag is derived from a description that is set conditionally. When configured in global mode, the SNI argument allows a comma separated list of FQDNs that require rate limiting. Each FQDN is associated with an instance of the rate limiter, and the description of each limiter is set to the FQDN.

When configured on a remap, the plugin will generate a description based on the configuration. When the scheme and port number are standard, the port is omitted from the generated description, however, when the scheme and port combination are non-standard, the port is appended. For example, a standard scheme and port would lead to a description of http.example.com or https.example.com but if a non-standard port was used, a description might be https.example.com:8443 or http.example.com:8080. This approach allows each limiter to increment metrics for the correct remaps.


There are four metrics that may be incremented, depending on which action the plugin takes:




Request queued due to being at the limit but under the queue limit.


Request rejected due to being over the defined limits.


Queued connection is too old to be resumed and is rejected.


Queued connection is resumed.

IP Reputation

The goal of the IP reputation system is to simply try to identify IPs which are more likely to be abusive than others. It’s not a perfect system, and it relies heavily on the notion of pressure. The Sieve LRUs are always filled, so you have to make sure that you only start using them when the system thinks it’s under pressure.

The Sieve LRU is a chained set of (configurable) LRUs, each with smaller and smaller capacity. This essentially adds a notion of partially sorted elements; All IPs in LRU <n> generally are more active than the IPs in LRU <n+1>. LRU is specially marked for longer term blocking, only the most abusive elements would end up here.



This example shows a simple rate limiting of 128 concurrently active client transactions, with a maximum queue size of 256. The default of HTTP status code 429 is used when queue is full:

map http://cdn.example.com/ http://some-server.example.com \
  @plugin=rate_limit.so @pparam=--limit=128 @pparam=--queue=256

This example would put a hard transaction (in) limit to 256, with no backoff queue, and add a header with the transaction delay if it was queued:

map http://cdn.example.com/ http://some-server.example.com \
  @plugin=rate_limit.so @pparam=--limit=256 @pparam=--queue=0 \

This final example will limit the active transaction, queue size, and also add a Retry-After header once the queue is full and we return a 429 error:

map http://cdn.example.com/ http://some-server.example.com \
  @plugin=rate_limit.so @pparam=--limit=256 @pparam=--queue=1024 \
  @pparam=--retry=3600 @pparam=--header=@RateLimit-Delay

In this case, the response would look like this when the queue is full:

HTTP/1.1 429 Too Many Requests
Date: Fri, 26 Mar 2021 22:42:38 GMT
Connection: keep-alive
Server: ATS/10.0.0
Cache-Control: no-store
Content-Type: text/html
Content-Language: en
Retry-After: 3600
Content-Length: 207

Metric Examples

The following examples show the metric names that result from various settings using a hypothetical domain of example.com with both global and remap configurations. Note that in this example the remap configuration contains both TLS and non-TLS remap rules.





Defaults with non-standard scheme+port combinations in the remap rules:



  • --prefix=limiter on the global configuration

  • --tag=tls.example.com on the global configuration

  • @pparam=--prefix=limiter on the remap configurations

  • @pparam=--tag=secure.example.com on the TLS-enabled remap configuration

  • @pparam=--tag=insecure.example.com on the non-TLS-enabled remap configuration