  Mid infrared (MIR) photonics is a key region for molecular physics [1].
High-resolution spectroscopy in the 1--10 {\mu}m region, though, has never been
fully tackled for the lack of widely-tunable and practical light sources.
Indeed, all solutions proposed thus far suffer from at least one of three
issues: they are feasible only in a narrow spectral range; the power available
for spectroscopy is limited; the frequency accuracy is poor. Here, we present a
setup for high-resolution spectroscopy that can be applied in the whole 1--10
{\mu}m range by combining the power of quantum cascade lasers (QCLs) and the
accuracy achievable by difference frequency generation using an OP-GaP crystal.
The frequency is measured against a primary frequency standard using the
Italian metrological fibre link network. We demonstrate the performance of the
setup by measuring a vibrational transition in a highly-excited metastable
state of CO around 6 {\mu}m with 11 digits of precision, four orders of
magnitude better than the value available in the literature [2].
