Interception & Rewriting Capabilities

This page lists every built-in traffic-rewriting capability in RelayCore, with a minimal JSON example for each (ready to feed to PUT /api/v1/rules or relay-core-cli rules validate). For the full filter schema see Rule API.

Mock Response

Replace the upstream response with a local fake. Use for frontend development, contract testing, error simulation.

{
  "id": "mock-health",
  "name": "Mock /v1/health",
  "filter": { "path": "/v1/health" },
  "action": {
    "type": "MockResponse",
    "status": 200,
    "headers": { "content-type": "application/json" },
    "body": { "type": "Text", "value": "{\"ok\":true}" }
  },
  "enabled": true
}

Map Local

Serve a local file in place of an HTTP request. Use for frontend offline dev.

{
  "id": "map-local-static",
  "name": "Serve /static from disk",
  "filter": { "path_starts_with": "/static/" },
  "action": {
    "type": "MapLocal",
    "path": "/Users/me/projects/app/static",
    "content_type": null
  },
  "enabled": true
}

Map Remote

Rewrite the URL to point at another address. Use to swap prod traffic to a staging environment.

{
  "id": "map-remote-prod-to-staging",
  "name": "api.example.com -> staging",
  "filter": { "host": "api.example.com" },
  "action": {
    "type": "MapRemote",
    "url": "https://api-staging.example.com",
    "preserve_host": false
  },
  "enabled": true
}

Redirect

Return a 3xx redirect.

{
  "id": "redirect-legacy",
  "name": "/old -> /new",
  "filter": { "path": "/old" },
  "action": {
    "type": "Redirect",
    "location": "https://example.com/new",
    "status": 301
  },
  "enabled": true
}

Header modification

Add, update, or delete request/response headers. All header actions support the {{previous}} variable to refer to the prior value.

{
  "id": "strip-cookie",
  "name": "Strip Set-Cookie on /api/*",
  "filter": { "path_starts_with": "/api/" },
  "action": { "type": "DeleteResponseHeader", "name": "set-cookie" },
  "enabled": true
}

{
  "id": "add-debug",
  "name": "Add X-Debug header",
  "filter": { "method": "POST" },
  "action": {
    "type": "AddRequestHeader",
    "name": "x-debug",
    "value": "relay-core"
  },
  "enabled": true
}

Body modification

Full replace, regex replace, and JSONPath set/delete.

{
  "id": "jsonpath-set",
  "name": "Force user.role = admin",
  "filter": { "host": "api.example.com" },
  "action": {
    "type": "TransformRequestBody",
    "transform": {
      "type": "JsonPathSet",
      "path": "$.user.role",
      "value": "\"admin\""
    }
  },
  "enabled": true
}

{
  "id": "regex-replace",
  "name": "Replace token in body",
  "filter": { "path": "/login" },
  "action": {
    "type": "TransformResponseBody",
    "transform": {
      "type": "RegexReplace",
      "pattern": "secret=([^&]+)",
      "replacement": "secret=REDACTED"
    }
  },
  "enabled": true
}

Intercept breakpoints

Pause live traffic for human decision. Rules create the breakpoint; resume_flow releases or drops it.

# Create a request-phase breakpoint
curl -X POST http://127.0.0.1:8082/api/v1/intercepts \
  -H 'Content-Type: application/json' \
  -d '{
    "url_pattern": "api.example.com",
    "phase": "request"
  }'

Hit breakpoints show up in the TUI's Intercepts panel, and can be listed via GET /api/v1/intercepts. When resuming, you can attach modifications:

curl -X POST http://127.0.0.1:8082/api/v1/intercepts/<key>/resume \
  -H 'Content-Type: application/json' \
  -d '{
    "action": "continue",
    "request_headers": { "x-test": "value" }
  }'

Modifiable fields: method, url, request_headers, request_body, status_code, response_headers, response_body, message_content (WebSocket).

Flow replay

Resend a captured request/response, with optional modifications.

curl -X POST http://127.0.0.1:8082/api/v1/flows/fld_01HX.../replay \
  -H 'Content-Type: application/json' \
  -d '{
    "modifications": {
      "headers": { "x-test": "value" },
      "body": "raw body content"
    }
  }'

# Skip TLS verification (debug only!)
curl -X POST 'http://127.0.0.1:8082/api/v1/flows/fld_01HX.../replay?accept_invalid_certs=true'

WebSocket message rewriting

WebSocket traffic has two dedicated actions:

{
  "id": "ws-mock",
  "name": "Mock chat messages",
  "filter": { "host": "chat.example.com" },
  "action": {
    "type": "MockWebSocketMessage",
    "direction": "Incoming",
    "message": "{\"type\":\"welcome\",\"user\":\"bot\"}"
  },
  "enabled": true
}

Or unconditionally drop matching messages:

{
  "id": "ws-drop-pings",
  "name": "Drop ping frames",
  "action": { "type": "DropWebSocketMessage" },
  "enabled": true
}

L3/L4 actions (forward-looking)

The following action types are declared in the rule schema but not all are enabled in every build. See Rule API for current status:

• RedirectIp — redirect to a different IP
• SetTtl — set the IP TTL
• ForwardPort — forward to a different port