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Catchall (Fallback backend)

Document updated on Feb 7, 2024

Catchall (Fallback backend)

The catchall endpoint is a particular type of endpoint that, when added, receives all the traffic from routes and methods that do not resolve to any of the other existing endpoints in the configuration.

Its primary use case is for initial stages of a migration, when you want to test a small subset of endpoints on KrakenD and have all the rest of the traffic forwarded to a common backend.

The wildcard endpoints are similar in functionality, but they only listen to a starting path, not everything, and they are a better option.

Catchall means forwarding the good and the bad
Although this endpoint might be super-useful, keep in mind that, generally speaking, you should avoid having a catchall route. All types of unknown requests will hit your fallback backend, opening the door for malicious users to test and scan it. From a security perspective, you are safer without this because you cut any traffic that you don’t explicitly declare.

Catchall configuration

To enable the catchall endpoint, all it takes is to create a regular endpoint with the reserved endpoint name /__catchall.

As with any other endpoint, you must define the url_pattern and the host in its backend section, but in this case, it is acting as the fallback backend. Like this:

{
    "endpoint": "/__catchall",
    "output_encoding": "no-op",
    "input_headers": ["*"],
    "input_query_strings": ["*"],
    "backend": [{
        "encoding": "no-op",
        "url_pattern": "/",
        "host":["http://mybackend:1234"]
    }]
}

The catchall endpoint acts as a reverse proxy and internally works using a no-op endpoint. The route name /__catchall is not registered during startup, it is our convention to mark the fallback route. But effectively it makes little difference for the end-user as all unregistered routes (including /__callback) will fall here anyway.

In connection with working as a no-op endpoint, a few remarks worth noticing:

  • You cannot change this endpoint’s output_encoding or encoding. If you do, it will be automatically reverted during startup to no-op and an error log will be shown in the console.
  • All query strings and headers sent by the consumer of your API are forwarded to the fallback backend. Regardless of your input_query_strings and input_headers settings, it will revert your settings to behave with a wildcard ["*"], passing everything to the backend.
  • As the response is a no-op, the client will get all the headers, encodings, content and status codes of the fallback backend as originally received.
  • You can still use components loaded under the extra_config of your backend that are compatible with no-op, or endpoint functionality that acts at the proxy level.

Catchall limitations

All functionality that is declared at the extra_config of the service level (config’s root) and all functionality inside a backend works normally.

Components loaded in the endpoint’s extra_config are not loaded, except for the following list that still work:

  • Security Policies (except for HTTP)
  • JMESpath
  • Body generators
  • Observability (New Relic, OpenCensus, OpenTelemetry)
  • Shadow proxy
  • JSON schema validation
  • CEL
  • LUA (except for HTTP)
  • Request and Response modifiers
  • Flatmap
  • Static responses

Any other HTTP-related components loaded in the extra_config are ignored. For instance, you cannot apply JWT validation to all connections intercepted by a catchall, so you should move this logic to a wildcard instead.

Scarf

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