We have a use case where we would like to log response headers when utilizing the pre-function plugin. Is this possible?
It is possible to log the response headers. Depending how you’d like the logs to be visualized, we have 2 different options:
config.log:
kong.log.inspect(kong.response.get_headers())Note: require("inspect") is blocked by Kong Gateway’s Lua sandbox (untrusted_lua) at both the default strict tier and the more permissive lax tier — it only works if untrusted_lua is set fully open (on), which is not recommended. kong.log.inspect() is the PDK-native equivalent: it performs the same pretty-printed table dump and is not gated by the sandbox at any tier, so no untrusted_lua configuration change is needed.
Sample output:
[notice] 2914#0: [kong] <source>:func:1 [pre-function]------------------------------------------------------------------------------------+
|{ |
| ["access-control-allow-credentials"] = "true", |
| ["access-control-allow-origin"] = "*", |
| connection = "close", |
| ["content-length"] = "387", |
| ["content-type"] = "application/json", |
| date = "Thu, 27 Aug 2026 07:26:10 GMT", |
| server = "gunicorn/19.9.0", |
| via = "1.1 kong/3.14.0.0-enterprise-edition", |
| ["x-kong-proxy-latency"] = "7", |
| ["x-kong-request-id"] = "d23fb44f401660746e13e1522c0c3e5e", |
| ["x-kong-upstream-latency"] = "2", |
| <metatable> = { |
| __index = <function 1> |
| } |
|} nil |
+------------------------------------------------------------------------------------------------------------------------+or if you wanted 1 header per log line:
config.log
for name, value in pairs(kong.response.get_headers()) do
if type(value) =="table" then
for i, v in ipairs(value) do
kong.log.info(name .. ":" .. v)
end
else
kong.log.info(name .. ":" .. value)
end
endSample output:
2026/08/27 07:27:15 [info] 2914#0: *3221 [kong] <source>:7 [pre-function] date:Thu, 27 Aug 2026 07:27:15 GMT while logging request, client: 172.66.147.243, server: kong, request: "GET /test/get HTTP/1.1", upstream: "http://172.18.0.6:80/get", host: "localhost:19000", request_id: "2ca915648910030237ff8045c9214d16"
2026/08/27 07:27:15 [info] 2914#0: *3221 [kong] <source>:7 [pre-function] content-length:387 while logging request, client: 172.66.147.243, server: kong, request: "GET /test/get HTTP/1.1", upstream: "http://172.18.0.6:80/get", host: "localhost:19000", request_id: "2ca915648910030237ff8045c9214d16"
2026/08/27 07:27:15 [info] 2914#0: *3221 [kong] <source>:7 [pre-function] x-kong-request-id:2ca915648910030237ff8045c9214d16 while logging request, client: 172.66.147.243, server: kong, request: "GET /test/get HTTP/1.1", upstream: "http://172.18.0.6:80/get", host: "localhost:19000", request_id: "2ca915648910030237ff8045c9214d16"
2026/08/27 07:27:15 [info] 2914#0: *3221 [kong] <source>:7 [pre-function] server:gunicorn/19.9.0 while logging request, client: 172.66.147.243, server: kong, request: "GET /test/get HTTP/1.1", upstream: "http://172.18.0.6:80/get", host: "localhost:19000", request_id: "2ca915648910030237ff8045c9214d16"
2026/08/27 07:27:15 [info] 2914#0: *3221 [kong] <source>:7 [pre-function] access-control-allow-origin:* while logging request, client: 172.66.147.243, server: kong, request: "GET /test/get HTTP/1.1", upstream: "http://172.18.0.6:80/get", host: "localhost:19000", request_id: "2ca915648910030237ff8045c9214d16"
2026/08/27 07:27:15 [info] 2914#0: *3221 [kong] <source>:7 [pre-function] access-control-allow-credentials:true while logging request, client: 172.66.147.243, server: kong, request: "GET /test/get HTTP/1.1", upstream: "http://172.18.0.6:80/get", host: "localhost:19000", request_id: "2ca915648910030237ff8045c9214d16"
2026/08/27 07:27:15 [info] 2914#0: *3221 [kong] <source>:7 [pre-function] via:1.1 kong/3.14.0.0-enterprise-edition while logging request, client: 172.66.147.243, server: kong, request: "GET /test/get HTTP/1.1", upstream: "http://172.18.0.6:80/get", host: "localhost:19000", request_id: "2ca915648910030237ff8045c9214d16"