code: func (h *Handle) GetNetNsIdByFd(fd int) (int, error) {
  	return h.getNetNsId(NETNSA_FD, uint32(fd))
  }
docs: // GetNetNsIdByFd looks up the network namespace ID for a given fd.
  // fd must be an open file descriptor to a namespace file.
  // Returns -1 if the namespace does not have an ID set.