  We report the first experimental demonstration of frequency-locking of an
extended-cavity quantum-cascade-laser (EC-QCL) to a near-infrared frequency
comb. The locking scheme is applied to carry out absolute spectroscopy of N2O
lines near 7.87 {\mu}m with an accuracy of ~60 kHz. Thanks to a single mode
operation over more than 100 cm^{-1}, the comb-locked EC-QCL shows great
potential for the accurate retrieval of line center frequencies in a spectral
region that is currently outside the reach of broadly tunable cw sources,
either based on difference frequency generation or optical parametric
oscillation. The approach described here can be straightforwardly extended up
to 12 {\mu}m, which is the current wavelength limit for commercial cw EC-QCLs.
