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FAQ

Why is Traefik Answering XXX HTTP Response Status Code?

Traefik is a dynamic reverse proxy, and while the documentation often demonstrates configuration options through file examples, the core feature of Traefik is its dynamic configurability, directly reacting to changes from providers over time.

Notably, a part of the configuration is static, and can be provided by a file on startup, whereas various providers, such as the file provider, contribute dynamically all along the traefik instance lifetime to its dynamic configuration changes.

In addition, the configuration englobes concepts such as the EntryPoint which can be seen as a listener on the Transport Layer (TCP), as apposed to the Router which is more about the Presentation (TLS) and Application layers (HTTP). And there can be as many routers as one wishes for a given EntryPoint.

In other words, for a given Entrypoint, at any given time the traffic seen is not bound to be just about one protocol. It could be HTTP, or otherwise. Over TLS, or not. Not to mention that dynamic configuration changes potentially make that kind of traffic vary over time.

Therefore, in this dynamic context, the static configuration of an entryPoint does not give any hint whatsoever about how the traffic going through that entryPoint is going to be routed. Or whether it's even going to be routed at all, i.e. whether there is a Router matching the kind of traffic going through it.

404 Not found

Traefik returns a 404 response code in the following situations:

  • A request reaching an EntryPoint that has no Routers
  • An HTTP request reaching an EntryPoint that has no HTTP Router
  • An HTTPS request reaching an EntryPoint that has no HTTPS Router
  • A request reaching an EntryPoint that has HTTP/HTTPS Routers that cannot be matched

From Traefik's point of view, every time a request cannot be matched with a router the correct response code is a 404 Not found.

In this situation, the response code is not a 503 Service Unavailable because Traefik is not able to confirm that the lack of a matching router for a request is only temporary. Traefik's routing configuration is dynamic and aggregated from different providers, hence it's not possible to assume at any moment that a specific route should be handled or not.

This behavior is consistent with rfc7231
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated after
some delay. If known, the length of the delay MAY be indicated in a
Retry-After header. If no Retry-After is given, the client SHOULD
handle the response as it would for a 500 response.

    Note: The existence of the 503 status code does not imply that a
    server must use it when becoming overloaded. Some servers may wish
    to simply refuse the connection.

Extract from rfc7231#section-6.6.4.

502 Bad Gateway

Traefik returns a 502 response code when an error happens while contacting the upstream service.

503 Service Unavailable

Traefik returns a 503 response code when a Router has been matched but there are no servers ready to handle the request.

This situation is encountered when a service has been explicitly configured without servers, or when a service has healthcheck enabled and all servers are unhealthy.

XXX Instead of 404

Sometimes, the 404 response code doesn't play well with other parties or services (such as CDNs).

In these situations, you may want Traefik to always reply with a 503 response code, instead of a 404 response code.

To achieve this behavior, a simple catchall router, with the lowest possible priority and routing to a service without servers, can handle all the requests when no other router has been matched.

The example below is a file provider only version (yaml) of what this configuration could look like:

# traefik.yaml

entrypoints:
  web:
    address: :80

providers:
  file:
    filename: dynamic.yaml
# dynamic.yaml

http:
  routers:
    catchall:
      # attached only to web entryPoint
      entryPoints:
        - "web"
      # catchall rule
      rule: "PathPrefix(`/`)"
      service: unavailable
      # lowest possible priority
      # evaluated when no other router is matched
      priority: 1

  services:
    # Service that will always answer a 503 Service Unavailable response
    unavailable:
      loadBalancer:
        servers: {}

Dedicated service

If there is a need for a response code other than a 503 and/or a custom message, the principle of the above example above (a catchall router) still stands, but the unavailable service should be adapted to fit such a need.