Custom error responses

Custom error responses set overrides for HTTP response statuses generated either by Emissary or upstream services.

They can be configured either on the Emissary Module or on an Mapping, the schema is identical. See below for more information on rule precedence.

  • on_status_code: HTTP status code to match for this rewrite rule. Only 4xx and 5xx classes are supported.
  • body: Describes the response body contents and format.
    • content_type: A string that sets the content type of the response.
    • text_format: A string whose value will be used as the new response body. Content-Type will default to text/plain if unspecified.
    • json_format: A config object whose keys and values will be serialized as JSON and used as the new response body.
    • text_format_source: Describes a file to be used as the response. If used, filename must be set and the file must exist on the Emissary pod.
      • filename: A file path on the Emissary pod that will be used as the new response body.

Only one of text_format, json_format, or text_format_source may be provided.

Custom response bodies are subject to Envoy’s AccessLog substitution syntax and variables, see Envoy’s documentation for more information.

Note that the AccessLog substitutions use % as a delimiter (for example, %RESPONSE_CODE%). To include a literal % in a custom response body, use %%. For example,

%%RESPONSE_CODE%% %RESPONSE_CODE%

would render as

%RESPONSE_CODE% 401

for a request that resulted in a response code of 401.

If the % symbol is not escaped as above (%%), it may only be as part of an AccessLog substitution, for example %RESPONSE_CODE% or %PROTOCOL%. If a % is neither part of a valid substitution nor an escape, Emissary will ignore the custom error response.

Simple response bodies

Simple responses can be be added quickly for convenience. They are inserted into the manifest as either text or JSON:

apiVersion: getambassador.io/v3alpha1
kind: Module
metadata:
  name: ambassador
  namespace: ambassador
spec:
  config:
    error_response_overrides:
      - on_status_code: 404
        body:
          text_format: "File not found"
      - on_status_code: 500
        body:
          json_format:
            error: "Application error"
            status: "%RESPONSE_CODE%"
            cluster: "%UPSTREAM_CLUSTER%"

File response bodies

For more complex response bodies a file can be returned as the response. This could be used for a customer friendly HTML document for example. Use text_format_source with a filename set as a path on the Emissary pod. content_type should be used set the specific file type, such as text/html.

First configure the Emissary module:

apiVersion: getambassador.io/v3alpha1
kind: Module
metadata:
  name: ambassador
  namespace: ambassador
spec:
  config:
    error_response_overrides:
      - on_status_code: 404
        body:
          content_type: "text/html"
          text_format_source:
            filename: '/ambassador/ambassador-errorpages/404.html'

Then create the config map containing the HTML file:

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: ambassador-errorpages
  namespace: ambassador
data:
  404.html: |
    <html>
      <h1>File not found</h1>
      <p>Uh oh, looks like you found a bad link.</p>
      <p>Click <a href="index.html">here</a> to go back home.</p>
    </html>

Finally, mount the configmap to the Emissary pod:

NOTE: The following YAML is in patch format and does not represent the entire deployment spec.

spec:
  template:
    spec:
      containers:
        - name: aes
          volumeMounts:
            - name: ambassador-errorpages
              mountPath: /ambassador/ambassador-errorpages
      volumes:
        - name: ambassador-errorpages
          configMap:
            name: ambassador-errorpages

Known limitations

  • text_formatand text_format_source perform no string escaping on expanded variables. This may break the structural integrity of your response body if, for example, the variable contains HTML data and the response content type is text/html. Be careful when using variables in this way, and consider whether the value may be coming from an untrusted source like request or response headers.
  • The json_format field does not support sourcing from a file. Instead consider using text_format_source with a JSON file and content_type set to application/json.

Rule precedence

If rules are set on both the Module and on a Mapping, the rule set on the Mapping will take precedence, ignoring any Module rules. This is true even if the rules are for different status codes. For example, consider this configuration:

apiVersion: getambassador.io/v3alpha1
kind: Module
metadata:
  name: ambassador
  namespace: ambassador
spec:
  config:
    error_response_overrides:
      - on_status_code: 404
        body:
          text_format: "Global 404"
---
apiVersion: getambassador.io/v3alpha1
kind: Mapping
metadata:
  name: ambassador
  namespace: ambassador
spec:
  hostname: "*"
  prefix: /api/
  service: quote
  error_response_overrides:
    - on_status_code: 429
      body:
         text_format: "Per-mapping 429"

The Mapping rule will prevent an override on the 404 rule defined on the Module for this Mapping. The rule on the Mapping will cause all rules on the Module to be ignored, regardless of the status codes specified. A seperate Mapping with no override rules defined will follow the 404 rule on the Module.

Disabling response overrides

If error response overrides are set on the Module, they can be disabled on individual mappings by setting bypass_error_response_overrides: true on those mappings:

---
apiVersion: getambassador.io/v3alpha1
kind: Mapping
metadata:
  name: quote-backend
  namespace: ambassador
spec:
  hostname: "*"
  prefix: /api/
  service: quote
  bypass_error_response_overrides: true

This is useful if a portion of the domain serves an API whose errors should not be rewritten, but all other APIs should contain custom errors.


Last modified July 4, 2024: Fix all metadata for 3.8 for navigation (7c97b54)