gistfile1.txt

foo:
    default_endpoint: endpoint_1     # this endpoint *must* be defined inside the endpoints array
    endpoints:
      - name: endpoint_1
        value: some_endpoint_1_data
      - name: endpoint_2
        value: some_endpoint_2_data
      - name: endpoint_3
        value: some_endpoint_3_data

You can use beforeNormalization on the foo node, to inspect values of both default_endpoint and endpoints, inside the closure throw an InvalidConfigurationException to indicate a problem with the keys.

Or you can just validate values in the container extension class itself, after everything is parsed (seems not very "strict" to me, but I don't think it really matters here because configuration and extension are always used together, never stand-alone.

By the way, DoctrineBundle has the same thing: default_connection and an array of connections, but it does not validate anything!