  We propose a metrological strategy reaching Heisenberg scaling precision in
the estimation of functions of any number $l$ of arbitrary parameters encoded
in a generic $M$-channel linear network. This scheme is experimentally feasible
since it only employs a single-mode squeezed vacuum and homodyne detection on a
single output channel. Two auxiliary linear network are required and their role
is twofold: to refocus the signal into a single channel after the interaction
with the interferometer, and to fix the function of the parameters to be
estimated according to the linear network analysed. Although the refocusing
requires some knowledge on the parameters, we show that the required precision
on the prior measurement is shot-noise, and thus achievable with a classic
measurement. We conclude by discussing two paradigmatic schemes in which the
choice of the auxiliary stages allows to change the function of the unknown
parameter to estimate.
