Configuration
Middleware
String Rewrites

String Rewrites

The String Rewrite mechanism is used when configuring Itsi for

It allows you to create dynamic strings from a template by combining literal text with placeholders. Placeholders (denoted using curly braces: {}) are replaced at runtime with data derived from the HTTP request, response, or context.

Modifiers can be appended after a pipe | to transform the substituted value.

Modifiers

After a placeholder name, add |: (or for replace, |replace:,). Available modifiers:

strip_prefix:<text> If the substituted value starts with , remove that prefix.

strip_suffix:<text> If the substituted value ends with , remove that suffix.

replace:<from>,<to> Replace all occurrences of in the substituted value with .

Modifiers are applied in the order they appear. You can chain multiple modifiers by repeating the |: syntax (e.g. {path|strip_prefix:/rails|replace:old,new}).

Rewriting a Request

The following placeholders are supported:

  • request_id: A short unique identifier for the request.
  • request_id_full: The full request identifier.
  • method: The HTTP request method (e.g., GET, POST).
  • path: The URL path of the request.
  • addr: The client IP address.
  • host: The host portion of the URL (defaults to localhost if unspecified).
  • path_and_query: The combination of the URL path and query string.
  • query: The query string (prepended with a ? if non-empty).
  • port: The port number (defaulting to 80 if not available).
  • start_time: The formatted start time of the request.
  • <Header-Name>: Any existing response header. For example {Content-Type} or {Set-Cookie} will be replaced with its current value.

The mechanism also allows any available matching regex capture from routes defined in the location block. If no match is found, otherwise, the placeholder remains unchanged (i.e. it is rendered as {placeholder_name}).

Rewriting a Response

When you use String Rewrite in response_headers, you can refer to built‑in response fields and any header in the outgoing response:

  • status: The HTTP status code (e.g., 200, 404).
  • response_time: The computed response time, formatted (e.g., 12.345ms).
  • <Header-Name>: Any existing response header. For example {Content-Type} or {Set-Cookie} will be replaced with its current value.

If a header placeholder does not exist on the response, it will render as {Header-Name}.

Example Templates

Reverse Proxying

When acting as a reverse proxy, you might want to forward the request to a backend service. For example, if your backend service expects the complete path and query string appended to its URL, you could use:

"https://backend.example.com/api{path}{query}"

For an incoming request to /v1/resource?x=1, this template rewrites the target URL to: https://backend.example.com/api/v1/resource?x=1

Redirects

For redirect middleware, a common use case is to guide clients from an old URL to a new one. For instance:

"https://new.example.com{path}?source=redirect"

If a request comes in to /old-section?foo=bar, the rewrite produces: https://new.example.com/old-section?source=redirect

Static ResponseFAQs