  With the newest version of our Monte Carlo code for ultra-high-energy cosmic
ray (UHECR) propagation, CRPropa 3, the flux of neutrinos and photons due to
interactions of UHECRs with extragalactic background light can be predicted.
Together with the recently updated data for the isotropic diffuse gamma-ray
background (IGRB) by Fermi LAT, it is now possible to severely constrain UHECR
source models. The evolution of the UHECR sources especially plays an important
role in the determination of the expected secondary photon spectrum. Pure
proton UHECR models are already strongly constrained, primarily by the highest
energy bins of Fermi LAT's IGRB, as long as their number density is not
strongly peaked at recent times.
