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Definitions I'm using in this question:

  • Main apiserver: the core kube-apiserver
  • Extension apiserver: an addon like metrics-server

I am reading through the configure aggregation layer guide and I don't understand the main apiserver's use of --requestheader-allowed-names. In section Kubernetes Apiserver Client Authentication it says:

The connection must be made using a client certificate whose CN is one of those listed in --requestheader-allowed-names. Note: You can set this option to blank as --requestheader-allowed-names="". This will indicate to an extension apiserver that any CN is acceptable.

It makes it sound like the main apiserver is responsible for setting this. Surely the extension apiserver would be in control of this and determine what is acceptable? Why configure this on the main apiserver at all? I.e. The client certificate common names are what they are and it's up to the extension apiserver to accept/reject these?

Or is that doc section mixing options that are passed to both the main and extension apiservers?

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I think I understand now why the main kube-apiserver needs this allowed names list: The kube-apiserver is a frontend for all the extension apiservers. So the requests always go through kube-apiserver first; it aggregates the requests and then passes them onto the correct backend extension apiserver. Hence it'll need to know which CN's to accept. I.e. a request to an extension server would go:

client request -> kube-apiserver -> extension-apiserver

The extension apiserver would then also check the CN once it received the request after it was routed there by main kube-apiserver.

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