Add Headers

Overview

The Add Headers policy action enables the addition of headers to both inbound requests and outbound responses.

Example

Traffic Policy for adding headers to an outbound response.

YAML

# snippet
---
actions:
  - type: "add-headers"
    config:
      headers:
        x-custom-header: "custom-value"
        x-template-example: "started at ${conn.ts.start}"

JSON

// snippet
{
  "actions": [
    {
      "type": "add-headers",
      "config": {
        "headers": {
          "x-custom-header": "custom-value",
          "x-template-example": "started at ${conn.ts.start}"
        }
      }
    }
  ]
}

Request:

curl https://example.ngrok.app/api/v1

Result:

< HTTP/2 200
< x-custom-header: custom-value
< x-template-example: started at 2024-06-24T15:30:00Z

Behavior

When executed as an inbound policy, this action will add headers on an incoming http request before reaching the upstream server with the configured headers. When executed as an outbound policy, the configured headers are added to the response from the upstream server.

Reference

Supported Directions

• Inbound
• Outbound

Configuration

Type
add-headers

Parameter		Description
headers	Map<string, string>	Headers to be added to the either the request or response. Supports interpolating the results of cel expressions in header values.

Templating Syntax

The results of CEL expressions can be interpolated into your policy's config using ngrok's ${} templating syntax. For a complete list of available variables and functions or to see a more detailed explanation, checkout the docs.

More Examples

Simple cel expression

YAML

# snippet
---
actions:
  - type: "add-headers"
    config:
      headers:
        x-time: "${time.now}"

JSON

// snippet
{
  "actions": [
    {
      "type": "add-headers",
      "config": {
        "headers": {
          "x-time": "${time.now}"
        }
      }
    }
  ]
}

Request:

curl https://example.ngrok.app/api/v1

Result:

< HTTP/2 200
< x-time: 2024-06-24T15:30:00Z

---

Complex cel expression

YAML

# complex expression
---
actions:
  - type: "add-headers"
    config:
      headers:
        x-complex: "${size(getReqHeader('Content-Type')) > 0}"

JSON

// complex expression
{
  "actions": [
    {
      "type": "add-headers",
      "config": {
        "headers": {
          "x-complex": "${size(getReqHeader('Content-Type')) > 0}"
        }
      }
    }
  ]
}

Request:

curl https://example.ngrok.app/api/v1

Result:

< HTTP/2 200
< x-complex: true

---

Action Result Variables

The following variables are available following the invocation of this action.

Name	Type	Description
actions.ngrok.add_headers.headers_added	map[string]string	The headers and associated values that were added by the action.