Customize a 404 error message

Uses: Kong Gateway decK

Deployment Platform:

Prerequisites

Kong Konnect

This is a Konnect tutorial and requires a Konnect personal access token.

Create a new personal access token by opening the Konnect PAT page and selecting Generate Token.
Export your token to an environment variable:
```
 export KONNECT_TOKEN='YOUR_KONNECT_PAT'
```
Run the quickstart script to automatically provision a Control Plane and Data Plane, and configure your environment:
```
 curl -Ls https://get.konghq.com/quickstart | bash -s -- -k $KONNECT_TOKEN --deck-output
```
This sets up a Konnect Control Plane named quickstart, provisions a local Data Plane, and prints out the following environment variable exports:
```
 export DECK_KONNECT_TOKEN=$KONNECT_TOKEN
 export DECK_KONNECT_CONTROL_PLANE_NAME=quickstart
 export KONNECT_CONTROL_PLANE_URL=https://us.api.konghq.com
 export KONNECT_PROXY_URL='http://localhost:8000'
```
Copy and paste these into your terminal to configure your session.

Kong Gateway running

This tutorial requires Kong Gateway Enterprise. If you don’t have Kong Gateway set up yet, you can use the quickstart script with an enterprise license to get an instance of Kong Gateway running almost instantly.

Export your license to an environment variable:

 export KONG_LICENSE_DATA='LICENSE-CONTENTS-GO-HERE'

Run the quickstart script:

 curl -Ls https://get.konghq.com/quickstart | bash -s -- -e KONG_LICENSE_DATA 

Once Kong Gateway is ready, you will see the following message:

 Kong Gateway Ready

decK

decK is a CLI tool for managing Kong Gateway declaratively with state files. To complete this tutorial you will first need to install decK.

Required entities

For this tutorial, you’ll need Kong Gateway entities, like Gateway Services and Routes, pre-configured. These entities are essential for Kong Gateway to function but installing them isn’t the focus of this guide. Follow these steps to pre-configure them:

Run the following command:

echo '
_format_version: "3.0"
services:
  - name: example-service
    url: http://httpbin.konghq.com/anything
routes:
  - name: example-route
    paths:
    - "/anything"
    service:
      name: example-service
' | deck gateway apply -

To learn more about entities, you can read our entities documentation.

Enable the Exit Transformer plugin

In this example, we want to customize the message that the client will receive when the requested Route isn’t found. To do this, we’ll use the Exit Transformer plugin. To transform 404 responses, we need to set the handle_unknown parameter to true, and then specify the transformations to perform using one or multiple Lua functions.

In this case, we’ll create a function that:

Replaces the existing error message with a custom one
Changes the response body structure to include error: true and the status code

echo '
_format_version: "3.0"
plugins:
  - name: exit-transformer
    config:
      handle_unknown: true
      functions:
      - return function(status, body, headers) if status == 404 then local new_body
        = { error = true, status = status, message = "This is not the Route you are
        looking for", } return status, new_body, headers else return status, body, headers
        end end
' | deck gateway apply -

Validate

To check that the response transformation is working, send a request to a Route that doesn’t exist. For example:

curl "$KONNECT_PROXY_URL/something" \
     -H "Accept: application/json"

curl "http://localhost:8000/something" \
     -H "Accept: application/json"

The response should contain the customized message.

Cleanup

Clean up Konnect environment

If you created a new control plane and want to conserve your free trial credits or avoid unnecessary charges, delete the new control plane used in this tutorial.

Destroy the Kong Gateway container

curl -Ls https://get.konghq.com/quickstart | bash -s -- -d