Characteristics of hadronic interactions in the forward direction

A selection of results obtained with very forward detectors of the CMS experiment at LHC is presented. Studies of the energy flow, $\mathrm{d}E/\mathrm{d}\eta$, are extended into very forward region with CASTOR calorimeter ($-6.6 < \eta < 5.2$) for pp collisions at 3 different center-of-mass energies ($0.9$, $2.76$ and $7\,\mathrm{TeV}$) and PbPb collisions at $2.76\,\mathrm{A TeV}$. Results are compared to hadronic interaction models as implemented in standard collider physics generators and generators used in cosmic-ray physics.