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Getting started with the Pre-function plugin

By Kong Inc.

The following guide shows you how set the plugin up in the access phase. In this example, the plugin will look for a request matching x-custom-auth. If the header exists in the request, it lets the request through. If the header doesn’t exist, it terminates the request early.

Let’s test out the Pre-function plugin by filtering requests based on header names.

Set up the plugin

  1. Create a Kong Gateway service:

     curl -i -X POST http://localhost:8001/services/ \
   --data "name=example-service" \
   --data "url=https://httpbin.konghq.com/headers"

  2. Add a route to the service:

     curl -i -X POST http://localhost:8001/services/example-service/routes \
   --data "name=test" \
   --data "paths[]=/test"

  3. Create a file named custom-auth.lua with the following content:

       -- Get list of request headers
   local custom_auth = kong.request.get_header("x-custom-auth")

   -- Terminate request early if our custom authentication header
   -- does not exist
   if not custom_auth then
     return kong.response.exit(401, "Invalid Credentials")
   end

   -- Remove custom authentication header from request
   kong.service.request.clear_header('x-custom-auth')

  4. Apply the Lua code using the pre-function plugin endpoint:

     curl -i -X POST http://localhost:8001/services/example-service/plugins \
   --form "name=pre-function" \
   --form "config.access[1]=@custom-auth.lua" \
   --form "config.access[2]=kong.log.err('Test Access')" \
   --form "config.header_filter[1]=kong.log.err('Test Header_Filter!')" \
   --form "config.body_filter[1]=kong.log.err('Test Body_Filter!')" \
   --form "config.log[1]=kong.log.err('Test Log!')"

    If successful, the API returns a 201 response code.

  1. Create the service, route, and associated plugin in the declarative config file:

     services:
 - name: example-service
   url: https://httpbin.konghq.com/headers

 routes:
 - service: example-service
   paths: [ "/test" ]

 plugins:
 - name: pre-function
   config:
     access:
     - |2
         -- Get list of request headers
         local custom_auth = kong.request.get_header("x-custom-auth")

         -- Terminate request early if the custom authentication header
         -- does not exist
         if not custom_auth then
           return kong.response.exit(401, "Invalid Credentials")
         end

         -- Remove custom authentication header from request
         kong.service.request.clear_header('x-custom-auth')
     - kong.log.err('Test Access!')
     header_filter:
     - kong.log.err('Test Header_Filter!')
     body_filter:
     - kong.log.err('Test Body_Filter!')
     log:
     - kong.log.err('Test Log!')

  2. Sync the file to your Kong Gateway instance:

     deck gateway sync kong.yaml

Validate the code

  1. Test that the code will terminate the request when no header is passed:

     curl -i -X GET http://localhost:8000/test

    You should get a 401 response:

     HTTP/1.1 401 Unauthorized
 ...
 "Invalid Credentials"

    The logs will show the following messages:

     [pre-function] Test Header_Filter!
 [pre-function] Test Body_Filter!
 [pre-function] Test Body_Filter!
 [pre-function] Test Log!

    The “Access” message is missing because the first function in that phase does an early exit, throwing the 401. Hence the subsequent functions are not executed.

  2. This time, test the code by making a valid request:

     curl -i -X GET http://localhost:8000/test \
   --header "x-custom-auth: demo"

    You should get a 200 response:

     HTTP/1.1 200 OK

    Now the logs will also have the “Access” message.

This is just a small demonstration of the power this plugin grants. You were able to dynamically inject Lua code into the plugin phases to dynamically terminate, or transform the request without creating a custom plugin or reloading and redeploying Kong Gateway.

In summary, serverless functions give you the full capabilities of a custom plugin without requiring redeploying or restarting Kong.