// externalName is the external reference that discovery mechanisms will
  // return as an alias for this service (e.g. a DNS CNAME record). No
  // proxying will be involved.  Must be a lowercase RFC-1123 hostname
  // (https://tools.ietf.org/html/rfc1123) and requires `type` to be "ExternalName".
  // +optional
  optional string externalName = 10;