Package org.apache.catalina.filters

## Class RateLimitFilter

- java.lang.Object
- - jakarta.servlet.GenericFilter
  - - org.apache.catalina.filters.RateLimitFilter

- All Implemented Interfaces:
    `Filter`, `FilterConfig`, `Serializable`

      public class RateLimitFilter
      extends GenericFilter

  Servlet filter that can help mitigate Denial of Service (DoS) and Brute Force attacks by limiting the number of a requests that are allowed from a single IP address within a time window (also referred to as a time bucket), e.g. 300 Requests per 60 seconds.

  The filter works by incrementing a counter in a time bucket for each IP address, and if the counter exceeds the allowed limit then further requests from that IP are dropped with a "429 Too many requests" response until the bucket time ends and a new bucket starts.

  The filter is optimized for efficiency and low overhead, so it converts some configured values to more efficient values. For example, a configuration of a 60 seconds time bucket is converted to 65.536 seconds. That allows for very fast bucket calculation using bit shift arithmetic. In order to remain true to the user intent, the configured number of requests is then multiplied by the same ratio, so a configuration of 100 Requests per 60 seconds, has the real values of 109 Requests per 65 seconds.

  It is common to set up different restrictions for different URIs. For example, a login page or authentication script is typically expected to get far less requests than the rest of the application, so you can add a filter definition that would allow only 5 requests per 15 seconds and map those URIs to it.

  You can set `enforce` to `false` to disable the termination of requests that exceed the allowed limit. Then your application code can inspect the Request Attribute `org.apache.catalina.filters.RateLimitFilter.Count` and decide how to handle the request based on other information that it has, e.g. allow more requests to certain users based on roles, etc.

  WARNING: if Tomcat is behind a reverse proxy then you must make sure that the Rate Limit Filter sees the client IP address, so if for example you are using the Remote IP Filter, then the filter mapping for the Rate Limit Filter must come after the mapping of the Remote IP Filter to ensure that each request has its IP address resolved before the Rate Limit Filter is applied. Failure to do so will count requests from different IPs in the same bucket and will result in a self inflicted DoS attack.

  See Also:
    Serialized Form

- - ### Field Summary

    Fields
    Modifier and Type | Field | Description
    --- | --- | ---
    `static int` | `DEFAULT_BUCKET_DURATION` | default duration in seconds
    `static int` | `DEFAULT_BUCKET_REQUESTS` | default number of requests per duration
    `static boolean` | `DEFAULT_ENFORCE` | default value for enforce
    `static int` | `DEFAULT_STATUS_CODE` | default status code to return if requests per duration exceeded
    `static String` | `DEFAULT_STATUS_MESSAGE` | default status message to return if requests per duration exceeded
    `static String` | `PARAM_BUCKET_DURATION` | init-param to set the bucket duration in seconds
    `static String` | `PARAM_BUCKET_REQUESTS` | init-param to set the bucket number of requests
    `static String` | `PARAM_ENFORCE` | init-param to set the enforce flag
    `static String` | `PARAM_STATUS_CODE` | init-param to set a custom status code if requests per duration exceeded
    `static String` | `PARAM_STATUS_MESSAGE` | init-param to set a custom status message if requests per duration exceeded
    `static String` | `RATE_LIMIT_ATTRIBUTE_COUNT` | request attribute that will contain the number of requests per duration

  - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `RateLimitFilter()` |

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `void` | `destroy()` | Called by the web container to indicate to a filter that it is being taken out of service.
    `void` | `doFilter​(ServletRequest request, ServletResponse response, FilterChain chain)` | The `doFilter` method of the Filter is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain.
    `int` | `getActualDurationInSeconds()` |
    `int` | `getActualRequests()` |
    `void` | `init()` | Convenience method for sub-classes to save them having to call `super.init(config)`.

    - ### Methods inherited from class jakarta.servlet.GenericFilter

      `getFilterConfig, getFilterName, getInitParameter, getInitParameterNames, getServletContext, init`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### DEFAULT_BUCKET_DURATION

          public static final int DEFAULT_BUCKET_DURATION

      default duration in seconds
      See Also:
        Constant Field Values

    - #### DEFAULT_BUCKET_REQUESTS

          public static final int DEFAULT_BUCKET_REQUESTS

      default number of requests per duration
      See Also:
        Constant Field Values

    - #### DEFAULT_ENFORCE

          public static final boolean DEFAULT_ENFORCE

      default value for enforce
      See Also:
        Constant Field Values

    - #### DEFAULT_STATUS_CODE

          public static final int DEFAULT_STATUS_CODE

      default status code to return if requests per duration exceeded
      See Also:
        Constant Field Values

    - #### DEFAULT_STATUS_MESSAGE

          public static final String DEFAULT_STATUS_MESSAGE

      default status message to return if requests per duration exceeded
      See Also:
        Constant Field Values

    - #### RATE_LIMIT_ATTRIBUTE_COUNT

          public static final String RATE_LIMIT_ATTRIBUTE_COUNT

      request attribute that will contain the number of requests per duration
      See Also:
        Constant Field Values

    - #### PARAM_BUCKET_DURATION

          public static final String PARAM_BUCKET_DURATION

      init-param to set the bucket duration in seconds
      See Also:
        Constant Field Values

    - #### PARAM_BUCKET_REQUESTS

          public static final String PARAM_BUCKET_REQUESTS

      init-param to set the bucket number of requests
      See Also:
        Constant Field Values

    - #### PARAM_ENFORCE

          public static final String PARAM_ENFORCE

      init-param to set the enforce flag
      See Also:
        Constant Field Values

    - #### PARAM_STATUS_CODE

          public static final String PARAM_STATUS_CODE

      init-param to set a custom status code if requests per duration exceeded
      See Also:
        Constant Field Values

    - #### PARAM_STATUS_MESSAGE

          public static final String PARAM_STATUS_MESSAGE

      init-param to set a custom status message if requests per duration exceeded
      See Also:
        Constant Field Values

  - ### Constructor Detail

    - #### RateLimitFilter

          public RateLimitFilter()

  - ### Method Detail

    - #### getActualRequests

          public int getActualRequests()

      Returns:
        the actual maximum allowed requests per time bucket

    - #### getActualDurationInSeconds

          public int getActualDurationInSeconds()

      Returns:
        the actual duration of a time bucket in milliseconds

    - #### init

          public void init()
                    throws ServletException

      Description copied from class: `jakarta.servlet.GenericFilter`
      Convenience method for sub-classes to save them having to call `super.init(config)`. This is a NO-OP by default.
      Overrides:
        `init` in class `GenericFilter`
      Throws:
        `ServletException` - If an exception occurs that interrupts the Filter's normal operation

    - #### doFilter

          public void doFilter​(ServletRequest request,
                               ServletResponse response,
                               FilterChain chain)
                        throws IOException,
                               ServletException

      Description copied from interface: `jakarta.servlet.Filter`
      The `doFilter` method of the Filter is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain. The FilterChain passed in to this method allows the Filter to pass on the request and response to the next entity in the chain.

      A typical implementation of this method would follow the following pattern:-
      1. Examine the request
      2. Optionally wrap the request object with a custom implementation to filter content or headers for input filtering
      3. Optionally wrap the response object with a custom implementation to filter content or headers for output filtering
      4. a) Either invoke the next entity in the chain using the FilterChain object (`chain.doFilter()`),
      4. b) or not pass on the request/response pair to the next entity in the filter chain to block the request processing
      5. Directly set headers on the response after invocation of the next entity in the filter chain.

      Parameters:
        `request` - The request to process
        `response` - The response associated with the request
        `chain` - Provides access to the next filter in the chain for this filter to pass the request and response to for further processing
      Throws:
        `IOException` - if an I/O error occurs during this filter's processing of the request
        `ServletException` - if the processing fails for any other reason

    - #### destroy

          public void destroy()

      Description copied from interface: `jakarta.servlet.Filter`
      Called by the web container to indicate to a filter that it is being taken out of service. This method is only called once all threads within the filter's doFilter method have exited or after a timeout period has passed. After the web container calls this method, it will not call the doFilter method again on this instance of the filter.
      This method gives the filter an opportunity to clean up any resources that are being held (for example, memory, file handles, threads) and make sure that any persistent state is synchronized with the filter's current state in memory. The default implementation is a NO-OP.