---
title: X-Varnish
summary: null
url: https://www.fastly.com/documentation/reference/http/http-headers/X-Varnish
---

Fastly cache servers run software derived from the open source [Varnish](https://varnish-cache.org/) caching daemon.  Each time a request is forwarded through a Varnish process, it will append a transaction ID in the form of an `X-Varnish` header.  Due to the effects of [clustering](https://www.fastly.com/documentation/guides/vcl/clustering/) and [shielding](https://www.fastly.com/documentation/guides/concepts/shielding/), there can often be up to four of these attached to a request when it arrives at a backend server.

It is not possible to correlate the identifier with any other customer-visible data, but these IDs can occasionally be used as part of joint debugging activity with Fastly engineers.  To remove them, unset the header in `vcl_miss` and `vcl_pass`:

```vcl
unset bereq.http.X-Varnish;
```