In the current k8 ingress there's no way to control "Host" header sent
to upstream server; it's always the configured vhost.
It's desirable to support custom Host header when proxying for a number
of cases. One such case is forwarding to an Service that points to an
external host that expect a host paramater (like AWS S3 buckets).
There's a number of others.
By default nginx set Host to the upstreams domain name (and not the one
passed in) like K8 always forces. This provided an escape hatch to this
behavior.
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