Getting started with the Post-function plugin

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The following guide shows you how to set up the Post-function plugin to adjust request header names. In this example, we’ll edit two types of headers: headers set via a plugin (in this case, Rate Limiting), and latency headers from Kong Gateway).

• The Rate Limiting plugin returns headers such as X-RateLimit-Remaining-<time> and X-RateLimit-Limit-<time>, where <time> is the configured time span for the limit.
• Kong Gateway adds latency headers to responses, such as X-Kong-Upstream-Latency and X-Kong-Proxy-Latency. While you can turn these headers on or off in kong.conf, they have fixed names that can’t be configured.

To change any header names, set up a Post-function plugin instance that runs in the header_filter phase.

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. Add a rate limiting plugin so that we can change its headers:

    curl -X POST http://localhost:8001/services/example-service/plugins \
      --data "name=rate-limiting"  \
      --data "config.second=5"  \
      --data "config.minute=30"  \
      --data "config.policy=local"
   

4. Create a Lua file for the plugin named serverless.lua (any name is fine). For example:

    return function()
   
    -- Rename rate-limit plugin headers
    -- X-RateLimit-Remaining-second: 1
    -- X-RateLimit-Limit-second: 2
    -- X-RateLimit-Limit-second changed to X-Rlls
    -- X-RateLimit-Remaining-second changed to X-Rlrs
   
      local kong_rl_headers = {}
      kong_rl_headers["x-ratelimit-limit-second"]="X-Rlls"
      kong_rl_headers["x-ratelimit-remaining-second"]="X-Rlrs"
      kong_rl_headers["x-ratelimit-limit-minute"]="X-Rllm"
      kong_rl_headers["x-ratelimit-remaining-minute"]="X-Rlrm"
      kong_rl_headers["x-ratelimit-limit-hour"]="X-Rllh"
      kong_rl_headers["x-ratelimit-remaining-hour"]="X-Rlrh"
      kong_rl_headers["x-ratelimit-limit-day"]="X-Rlld"
      kong_rl_headers["x-ratelimit-remaining-day"]="X-Rlrd"
      kong_rl_headers["x-ratelimit-limit-month"]="X-Rlln"
      kong_rl_headers["x-ratelimit-remaining-month"]="X-Rlrn"
      kong_rl_headers["x-ratelimit-limit-year"]="X-Rlly"
      kong_rl_headers["x-ratelimit-remaining-year"]="X-Rlry"
   
      local headers = kong.response.get_headers()
      for k, v in pairs(headers) do
        if kong_rl_headers[k] ~= nil then
          kong.response.set_header(kong_rl_headers[k], v)
          kong.response.clear_header(k)
        end
      end
   
      -- Add custom headers for latency
      kong.response.set_header("My-Custom-Proxy-Latency", ngx.ctx.KONG_PROXY_LATENCY)
      kong.response.set_header("My-Custom-Upstream-Latency", ngx.ctx.KONG_WAITING_TIME)
   
    end
   

5. Add the serverless function to the appropriate Kong entity by enabling the post-function plugin. For example, let’s apply it to the test route:

    curl -i -X POST https://localhost:8001/routes/test/plugins \
      --form "name=post-function" \
      --form "config.header_filter=@/tmp/serverless.lua"
   

6. Verify the changes. Make a call to the Admin API and check that the response header names have been changed:

    curl -v http://localhost:8000/test
   

   The response should show the new header names:

    *   Trying 127.0.0.1)...
    * TCP_NODELAY set
    * Connected to kong (127.0.0.1)) port 8000 (#0)
    > GET /test HTTP/1.1
    > Host: localhost:8000
    > User-Agent: curl/7.86.0
    > Accept: */*
    >
    < HTTP/1.1 200 OK
    < Content-Type: application/json
    < Content-Length: 374
    < Connection: keep-alive
    < Server: gunicorn/19.9.0
    < Date: Mon, 27 Apr 2023 09:27:20 GMT
    < Access-Control-Allow-Origin: *
    < Access-Control-Allow-Credentials: true
    < X-Rlls: 2
    < X-Rllm: 5
    < X-Rlrm: 3
    < X-Rlrs: 1
    < My-Custom-Proxy-Latency: 1
    < My-Custom-Upstream-Latency: 4
   

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