Kong request transformer plugin

Synopsis

This plugin transforms the request sent by a client on the fly on Kong, before hitting the upstream server. It can match complete or portions of incoming requests using regular expressions, save those matched strings into variables, and substitute those strings into transformed requests via flexible templates.

Configuration

Enabling the plugin on a Service

Configure this plugin on a Service by making the following request:

$ curl -X POST http://kong:8001/services/{service}/plugins \
    --data "name=request-transformer"

service: the id or name of the Service that this plugin configuration will target.

Enabling the plugin on a Route

Configure this plugin on a Route with:

$ curl -X POST http://kong:8001/routes/{route_id}/plugins \
    --data "name=request-transformer"

route_id: the id of the Route that this plugin configuration will target.

Enabling the plugin on a Consumer

You can use the http://localhost:8001/plugins endpoint to enable this plugin on specific Consumers:

$ curl -X POST http://kong:8001/plugins \
    --data "name=request-transformer" \
    --data "consumer_id={consumer_id}"

Where consumer_id is the id of the Consumer we want to associate with this plugin.

You can combine consumer_id and service_id in the same request, to furthermore narrow the scope of the plugin.

form parameter	default	description
name		The name of the plugin to use, in this case request-transformer
service_id		The id of the Service which this plugin will target.
route_id		The id of the Route which this plugin will target.
enabled	true	Whether this plugin will be applied.
consumer_id		The id of the Consumer which this plugin will target.
config.http_method		Changes the HTTP method for the upstream request
config.remove.headers		List of header names. Unset the headers with the given name.
config.remove.querystring		List of querystring names. Remove the querystring if it is present.
config.remove.body		List of parameter names. Remove the parameter if and only if content-type is one the following [application/json,multipart/form-data, application/x-www-form-urlencoded] and parameter is present.
config.replace.headers		List of headername:value pairs. If and only if the header is already set, replace its old value with the new one. Ignored if the header is not already set.
config.replace.querystring		List of queryname:value pairs. If and only if the querystring name is already set, replace its old value with the new one. Ignored if the header is not already set.
config.replace.uri		Updates the upstream request URI with given value. This value can only be used to update the path part of the URI, not the scheme, nor the hostname.
config.replace.body		List of paramname:value pairs. If and only if content-type is one the following [application/json,multipart/form-data, application/x-www-form-urlencoded] and the parameter is already present, replace its old value with the new one. Ignored if the parameter is not already present.
config.rename.headers		List of headername:value pairs. If and only if the header is already set, rename the header. The value is unchanged. Ignored if the header is not already set.
config.rename.querystring		List of queryname:value pairs. If and only if the field name is already set, rename the field name. The value is unchanged. Ignored if the field name is not already set.
config.rename.body		List of parameter name:value pairs. Rename the parameter name if and only if content-type is one the following [application/json,multipart/form-data, application/x-www-form-urlencoded] and parameter is present.
config.add.headers		List of headername:value pairs. If and only if the header is not already set, set a new header with the given value. Ignored if the header is already set.
config.add.querystring		List of queryname:value pairs. If and only if the querystring name is not already set, set a new querystring with the given value. Ignored if the querystring name is already set.
config.add.body		List of paramname:value pairs. If and only if content-type is one the following [application/json,multipart/form-data, application/x-www-form-urlencoded] and the parameter is not present, add a new parameter with the given value to form-encoded body. Ignored if the parameter is already present.
config.append.headers		List of headername:value pairs. If the header is not set, set it with the given value. If it is already set, a new header with the same name and the new value will be set.
config.append.querystring		List of queryname:value pairs. If the querystring is not set, set it with the given value. If it is already set, a new querystring with the same name and the new value will be set.
config.append.body		List of paramname:value pairs. If the content-type is one the following [application/json, application/x-www-form-urlencoded], add a new parameter with the given value if the parameter is not present, otherwise if it is already present, the two values (old and new) will be aggregated in an array.

Notes:

• If the value contains a , then the comma-separated format for lists cannot be used. The array notation must be used instead.
• The X-Forwarded-* fields are non-standard header fields written by Nginx to inform the upstream about client details and can't be overwritten by this plugin. If you need to overwrite these header fields, see the post-function plugin in Serverless Functions.

Template as Value

You can use any of the current request headers, query params, and captured URI groups as a template to populate the above supported configuration fields.

Request Param	Template
header	$(headers.<header_name>), $(headers["<Header-Name>"]) or $(headers["<header-name>"]))
querystring	$(query_params.<query-param-name>) or $(query_params["<query-param-name>"]))
captured URIs	$(uri_captures.<group-name>) or $(uri_captures["<group-name>"]))
Nginx var	$(ngx_var.<var-name>) or $(ngx_var["<var-name>"]))

To escape a template, wrap it inside quotes and pass it inside another template.
$('$(some_escaped_template)')

Note: The plugin creates a non-mutable table of request headers, querystrings, and captured URIs before transformation. Therefore, any update or removal of params used in template does not affect the rendered value of a template.

Advanced templates

The content of the placeholder $(...) is evaluated as a Lua expression, so logical operators may be used. For example:

Header-Name:$(uri_captures["user-id"] or query_params["user"] or "unknown")

This will first look for the path parameter (uri_captures). If not found, it will return the query parameter. If that also doesn't exist, it returns the default value '"unknown"'.

Constant parts can be specified as part of the template outside the dynamic placeholders. For example, creating a basic-auth header from a query parameter called auth that only contains the base64-encoded part:

Authorization:Basic $(query_params["auth"])

Lambdas are also supported if wrapped as an expression like this:

$((function() ... implementation here ... end)())

A complete Lambda example for prefixing a header value with "Basic" if not already there:

Authorization:$((function()
    local value = headers.Authorization
    if not value then
      return
    end
    if value:sub(1, 6) == "Basic " then
      return value            -- was already properly formed
    end
    return "Basic " .. value  -- added proper prefix
  end)())

NOTE: Especially in multi-line templates like the example above, make sure not to add any trailing white-space or new-lines. Because these would be outside the placeholders, they would be considered part of the template, and hence would be appended to the generated value.

The environment is sandboxed, meaning that Lambdas will not have access to any library functions, except for the string methods (like sub() in the example above).

Examples Using Template as Value

Add an API test with uris configured with a named capture group user_id

$ curl -X POST http://localhost:8001/apis \
    --data 'name=test' \
    --data 'upstream_url=http://mockbin.com' \
    --data-urlencode 'uris=/requests/user/(?<user_id>\w+)' \
    --data "strip_uri=false"

Enable the ‘request-transformer’ plugin to add a new header x-consumer-id whose value is being set with the value sent with header x-user-id or with the default value alice is header is missing.

$ curl -X POST http://localhost:8001/apis/test/plugins \
    --data "name=request-transformer" \
    --data-urlencode "config.add.headers=x-consumer-id:\$(headers['x-user-id'] or 'alice')" \
    --data "config.remove.headers=x-user-id"

Now send a request without setting header x-user-id

$ curl -i -X GET localhost:8000/requests/user/foo

Plugin will add a new header x-consumer-id with value alice before proxying request upstream. Now try sending request with header x-user-id set

$ curl -i -X GET localhost:8000/requests/user/foo \
  -H "X-User-Id:bob"

This time the plugin will add a new header x-consumer-id with the value sent along with the header x-user-id, i.e.bob

Order of execution

Plugin performs the response transformation in the following order:

• remove → rename → replace → add → append

Examples

Kubernetes users: version v1beta1 of the Ingress specification does not allow the use of named regex capture groups in paths. If you use the ingress controller, you should use unnamed groups, e.g. (\w+)/ instead of (?<user_id>\w+). You can access these based on their order in the URL path, e.g. $(uri_captures[1]) will obtain the value of the first capture group.

In these examples we have the plugin enabled on a Service. This would work similarly for Routes.

• Add multiple headers by passing each header:value pair separately:

With a database

$ curl -X POST http://localhost:8001/services/example-service/plugins \
  --data "name=request-transformer" \
  --data "config.add.headers[1]=h1:v1" \
  --data "config.add.headers[2]=h2:v1"

** Without a database **

plugins:
- name: request-transformer
  config:
    add:
      headers: ["h1:v1", "h2:v1"]

incoming request headers	upstream proxied headers:
h1: v1	h1: v1 h2: v1

• Add multiple headers by passing comma separated header:value pair (only possible with a database):

$ curl -X POST http://localhost:8001/services/example-service/plugins \
  --data "name=request-transformer" \
  --data "config.add.headers=h1:v1,h2:v2"

incoming request headers	upstream proxied headers:
h1: v1	h1: v1 h2: v1

• Add multiple headers passing config as JSON body (only possible with a database):

$ curl -X POST http://localhost:8001/services/example-service/plugins \
  --header 'content-type: application/json' \
  --data '{"name": "request-transformer", "config": {"add": {"headers": ["h1:v2", "h2:v1"]}}}'

incoming request headers	upstream proxied headers:
h1: v1	h1: v1 h2: v1

• Add a querystring and a header:

** With a database **

$ curl -X POST http://localhost:8001/services/example-service/plugins \
  --data "name=request-transformer" \
  --data "config.add.querystring=q1:v2,q2:v1" \
  --data "config.add.headers=h1:v1"

** Without a database **

plugins:
- name: request-transformer
  config:
    add:
      headers: ["h1:v1"],
      querystring: ["q1:v1", "q2:v2"]

incoming request headers	upstream proxied headers:
h1: v2	h1: v2 h2: v1
h3: v1	h1: v1 h2: v1 h3: v1

incoming request querystring	upstream proxied querystring
?q1=v1	?q1=v1&q2=v1
	?q1=v2&q2=v1

• Append multiple headers and remove a body parameter:

** With a database **

$ curl -X POST http://localhost:8001/services/example-service/plugins \
  --header 'content-type: application/json' \
  --data '{"name": "request-transformer", "config": {"append": {"headers": ["h1:v2", "h2:v1"]}, "remove": {"body": ["p1"]}}}'

** Without a database **

plugins:
- name: request-transformer
  config:
    add:
      headers: ["h1:v1", "h2:v1"]
    remove:
      body: [ "p1" ]

incoming request headers	upstream proxied headers:
h1: v1	h1: v1 h1: v2 h2: v1

incoming url encoded body	upstream proxied url encoded body
p1=v1&p2=v1	p2=v1
p2=v1	p2=v1