We demonstrate the generation of multimode entangled states of propagating microwaves. The entangled
states are generated by our parametrically pumping a multimode superconducting cavity. By
combining different pump frequencies, applied simultaneously to the device, we can produce different
entanglement structures in a programable fashion. The Gaussian output states are fully characterized by
our measuring the full covariance matrices of the modes. The covariance matrices are absolutely calibrated
by our using an in situ microwave calibration source, a shot-noise tunnel junction. Applying a variety of
entanglement measures, we demonstrate both full inseparability and genuine tripartite entanglement of the
states. Our method is easily extensible to more modes.