For debugging purposes, we would like to configure our pool of load balanced web servers to output an HTTP header identifying the "true" server which served the request. However, we don't know what header name this should be! Is there any pre-existing header for this? Do you have a favorite?
AFAIK there isn't a formal standard for this. What I've seen is:
Writing the backend server ID into an HTML comment at the end of the content body.
Update: I found this survey of HTTP headers in the wild from 2011. My suggestion
X-Backend-Server is number 119, while
X-Server-Name is number 56 on the list. So I'd say
X-Server-Name seems the most common header for this.