HTTP header for indicating originating server behind load balancer
AFAIK there isn't a formal standard for this. What I've seen is:
A custom HTTP header. It's really your choice as long as you stay in the X- namespace. For me personally
X-Backend-Serveris descriptive, and it seems popular.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.