Varnish-generated transaction ID for a request or response.

Fastly writes this header into requests and responses. It is proprietary to Fastly.

Fastly cache servers run software derived from the open source Varnish 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 and 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:

unset bereq.http.X-Varnish;