replaceresponseonce

Caddy replace_response_once handler module

A fork of the replace_response module, with modifications to only allow single-instance replacements (i.e., replacing only the first occurrence of a match).

Note: Does not support regex-based replacements.

Module name: http.handlers.replace_response

JSON examples

Substring substitution:

{
	"handler": "replace_response_once",
	"replacements": [
		{
			"search": "Foo",
			"replace": "Bar"
		}
	]
}

Same, but with streaming mode (we just set "stream": true in the handler):

{
	"handler": "replace_response_once",
	"replacements": [
		{
			"search": "Foo",
			"replace": "Bar"
		}
	],
	"stream": true

With a response matcher:

{
	"handler": "replace_response_once",
	"replacements": [
		{
			"search": "Foo",
			"replace": "Bar"
		}
	],
	"match": {
		"headers": {
			"Content-Type": ["application/json*"]
		}
	}
}

Caddyfile

This module has Caddyfile support. It registers the replace directive, by default to be after the standard encode directive. Make sure to change it with the order global option in case that is not suitable for your needs.

Syntax:

replace [<matcher>] [stream | <search> <replace>] {
	stream
	match {
		header Content-Type application/json*
	}
	[re] <search> <replace>
}

• stream enables streaming mode.
• match defines a response matcher. If defined, replacements in this directive will only be performed on responses that match the matcher.
• Note that you can use a matcher token to filter which requests have replacements performed.

Simple substring substitution:

replace Foo Bar

Streaming mode:

replace stream {
	Foo Bar
}

Multiple replacements:

replace {
	Foo Bar
	A B
}

Limitations:

• Compressed responses (e.g. from an upstream proxy which gzipped the response body) will not be decoded before attempting to replace. To work around this, you may send the Accept-Encoding: identity request header to the upstream to tell it not to compress the response. For example:

  reverse_proxy localhost:8080 {
      header_up Accept-Encoding identity
  }