> ## Documentation Index
> Fetch the complete documentation index at: https://docs.elding.app/llms.txt
> Use this file to discover all available pages before exploring further.

# The proxy

> How Elding injects HTTP API keys without exposing them to your application.

<Note>
  The proxy is designed exclusively for **HTTP API keys** such as OpenAI, Stripe, Resend, and
  Anthropic: keys sent through an `Authorization` or `x-api-key` header to an HTTPS API.
  Non-HTTP variables (`DATABASE_URL`, `JWT_SECRET`, `REDIS_URL`, and similar values) remain in
  your host's environment variables. Elding does not store or inject them.
</Note>

## How it works

In proxy mode, your application uses a `{{KEY_NAME}}` **placeholder** for an HTTP API key.
The local proxy intercepts each outgoing request, replaces the placeholder with the real
value, and forwards the request upstream.

```
App → http://127.0.0.1:PORT → [proxy replaces {{OPENAI_API_KEY}}] → https://api.openai.com
```

The real key lives inside the separate proxy process and **never** enters your application's
memory.

## Request processing

<Steps>
  <Step title="Authentication">
    The proxy verifies `x-elding-token` using a constant-time comparison. Only the child
    process started by `elding proxy` receives this token.
  </Step>

  <Step title="Anti-SSRF">
    The target hostname is resolved. Private, loopback, link-local, and cloud metadata IP
    addresses such as `169.254.169.254` are blocked. DNS pinning prevents rebinding attacks.
  </Step>

  <Step title="Host locking">
    For every placeholder used, the proxy verifies that the key's authorized domain matches
    the target. A mismatch returns HTTP 403 and prevents exfiltration.
  </Step>

  <Step title="Injection and forwarding">
    Placeholders are replaced in request headers, then the HTTPS request is streamed to the
    upstream service.
  </Step>
</Steps>

## Proxy security

* Binds to `127.0.0.1` only and is never exposed to the network.
* Uses a random session token with constant-time comparison.
* Allows HTTPS only and strips hop-by-hop headers.
* Logs placeholder names only, never API key values.

<Note>
  A compromised dependency or AI agent inside your application only sees
  `{{OPENAI_API_KEY}}`, never the real key.
</Note>