---
id: proxying
title: Proxying
description: Documentation on Proxying
---

## Overview

The Backstage backend comes packaged with a basic HTTP proxy, that can aid in
reaching backend service APIs from frontend plugin code. See
[Call Existing API](call-existing-api.md) for a description of when the proxy
can be the best choice for communicating with an API.

## Getting Started

The plugin is already added to a default Backstage project.

In `packages/backend/src/index.ts`:

```ts
const proxyEnv = useHotMemoize(module, () => createEnv('proxy'));

const service = createServiceBuilder(module)
  .loadConfig(configReader)
  /** ... other routers ... */
  .addRouter('/proxy', await proxy(proxyEnv));
```

## Configuration

Configuration for the proxy plugin lives under a `proxy` root key of your
`app-config.yaml` file.

Example:

```yaml
# in app-config.yaml
proxy:
  '/simple-example': http://simple.example.com:8080
  '/larger-example/v1':
    target: http://larger.example.com:8080/svc.v1
    headers:
      Authorization:
        $env: EXAMPLE_AUTH_HEADER
```

Each key under the proxy configuration entry is a route to match, below the
prefix that the proxy plugin is mounted on. It must start with a slash. For
example, if the backend mounts the proxy plugin as `/proxy`, the above
configuration will lead to the proxy acting on backend requests to
`/api/proxy/simple-example/...` and `/api/proxy/larger-example/v1/...`.

The value inside each route is either a simple URL string, or an object on the
format accepted by
[http-proxy-middleware](https://www.npmjs.com/package/http-proxy-middleware).

If the value is a string, it is assumed to correspond to:

```yaml
target: <the string>
changeOrigin: true
pathRewrite:
  '^<url prefix><the string>/': '/'
```

When the target is an object, it is given verbatim to `http-proxy-middleware`
except with the following caveats for convenience:

- If `changeOrigin` is not specified, it is set to `true`. This is the most
  commonly useful value.
- If `pathRewrite` is not specified, it is set to a single rewrite that removes
  the entire prefix and route. In the above example, a rewrite of
  `'^/api/proxy/larger-example/v1/': '/'` is added. That means that a request to
  `/api/proxy/larger-example/v1/some/path` will be translated to a request to
  `http://larger.example.com:8080/svc.v1/some/path`.

There are also additional settings:

- `allowedMethods`: Limit the forwarded HTTP methods. For example
  `allowedMethods: ['GET']` enforces read-only access.
- `allowedHeaders`: A list of headers that should be forwarded to and received
  from the target.

By default, the proxy will only forward safe HTTP request headers to the target.
Those are based on the headers that are considered safe for CORS and includes
headers like `content-type` or `last-modified`, as well as all headers that are
set by the proxy. If the proxy should forward other headers like
`authorization`, this must be enabled by the `allowedHeaders` config, for
example `allowedHeaders: ['Authorization']`. This should help to not
accidentally forward confidential headers (`cookie`, `X-Auth-Request-User`) to
third-parties.

The same logic applies to headers that are sent from the target back to the
frontend.
