# Bypassing Reverse Proxy, Cache, and Content Delivery Networks (CDN)

When using a reverse proxy (Varnish, Fastly, HA Proxy, Nginx...), a load balancer or a content delivery network (CDN) sitting in front of your application, you need to bypass them when profiling with Blackfire.

In such context, your infrastructure **must**:

- Serve a **non-cached** response;
- **Not remove the specific HTTP headers** injected by the Blackfire browser extension, the Blackfire CLI client, or the Blackfire Player (read further below);
- **Target a specific server** running your application, behind the proxy.

## HTTP Headers

In order to successfully profile your application, several HTTP headers are processed by the client. These headers are required to pass through to and from your application (and the Blackfire probe).

### Request Headers

Any Blackfire client, whether it is the Blackfire browser extension, the Blackfire CLI client, or the Blackfire Player, injects a specific HTTP header into each Request to profile a resource: `X-Blackfire-Query`. This header contains mandatory information for the probe, including the signature (read [How does Blackfire work for further details](../php/training-resources/book/08-profiling-all-the-things#how-does-blackfire-work)).

### Response Headers

The following HTTP response headers need to be preserved:

- `X-Blackfire-Response`
- `X-Blackfire-Error`

Those headers are needed by the Blackfire clients.

## Load Balancer

If you are using multiple application servers behind a load-balancer:

> - Install the **Blackfire Probe** and the **Blackfire Agent** on every server;

## Reverse Proxies and CDNs

We provide various integration examples with different reverse proxies and CDNs:

- [Varnish](../integrations/proxies/varnish)
- [Fastly](../integrations/proxies/fastly)
- [Cloudflare](../integrations/proxies/cloudflare)
- [Nginx](../integrations/proxies/nginx)