How to log response body and upstream by using pre-function plugin
How to log response body and upstream by using pre-function plugin
Uses:
Kong Gateway
TL;DR
How do I log the response body and upstream target using the pre-function plugin?
Enable a pre-function plugin on the service with a config.log phase function that calls kong.log(kong.service.response.get_raw_body()), and a config.access phase function that enables request buffering and logs ngx.ctx.balancer_data fields. The response body and resolved upstream then appear in Kong’s error log for each request. Logging the response body forces Nginx to buffer the full body, which affects proxy performance for large responses.
Steps
See the pre-function plugin docs to learn how to enable a pre-function plugin.
Use the command below to enable a pre-function plugin with a Lua function to log the response body on a service:
curl -X POST http://\<kong\>:8001/services/\<service\>/plugins \
--data "name=pre-function" \
--data "config.log[1]=kong.log(string.gsub(kong.service.response.get_raw_body(), '\n', ''))" \
--data "config.access[1]=kong.service.request.enable_buffering()" \
--data "config.access[2]=kong.log(ngx.ctx.balancer_data.scheme, '://', ngx.ctx.balancer_data.host, ':', ngx.ctx.balancer_data.port, ngx.var.upstream_uri)"If you send a request to the route associated with the service that has the pre-function plugin enabled, you should be able to see the response body inside kong error.log:
2026/05/07 07:03:01 [notice] 27#0: *207173 [kong] ',..."]:1 [pre-function] https://httpbin.org:443/anything, client: 172.21.0.1, server: kong, request: "GET /ttt HTTP/1.1", host: "localhost:8000"
...
2026/05/07 07:03:02 [notice] 27#0: *207173 [kong] [string "kong.log(string.gsub(kong.service.response.ge..."]:1 [pre-function] { "args": {}, "data": "", "files": {}, "form": {}, "headers": { "Accept": "*/*", "Host": "httpbin.org", "User-Agent": "curl/7.64.1", "X-Amzn-Trace-Id": "Root=1-6094e626-44a735441ee0fc2b51ef72a0", "X-Forwarded-Host": "localhost", "X-Forwarded-Path": "/ttt", "X-Forwarded-Prefix": "/ttt" }, "json": null, "method": "GET", "origin": "172.21.0.1, 175.177.45.138", "url": "https://localhost/anything"}19 while logging request, client: 172.21.0.1, server: kong, request: "GET /ttt HTTP/1.1", host: "localhost:8000"Be aware that logging the response body requires Nginx to buffer the whole body, which affects proxy performance when the body is large.