Http 400 on BAN method

Istio Newbie here,

I’m doing my first tests with Istio (on version 1.3.0). Most things run nice without much effort.
What I’m having an issue is a service that talks with varnish to clean up the cache. This service makes a HTTP request to every pod behind a headless service and its failing with a HTTP 400 (Bad Request) error. This request uses the HTTP Method “BAN” which I belive is the source of the problem since other request method reach varnish without problems.

As a temporary workaround I changed the port name from http to varnish and everything start working again.

I’m reading the docs but not finding a way to tell the envoy-proxy that the request-method BAN is a valid one.