  We perform multi-wavelength light curve modeling of the recently discovered
low-luminosity gamma-ray burst (GRB) 171205A. The emission model is based on
the relativistic ejecta-circumstellar medium (CSM) interaction scenario. The
collision of freely expanding spherical ejecta traveling at mildly relativistic
velocities with the CSM produces the reverse and forward shocks, which
dissipate a part of the kinetic energy of the mildly relativistic ejecta. We
show that the early gamma-ray emission followed by an X-ray tail can be well
explained by the radiation diffusing out from the shocked gas. Mildly
relativistic ejecta with a kinetic energy of $5\times10^{50}$ erg and a
wind-like CSM with a mass-loss rate of a few $10^{-4}\ M_\odot$ yr$^{-1}$ for a
wind velocity of $10^3$ km s$^{-1}$, which extends up to $\sim 3\times 10^{13}$
cm, are required to account for the gamma-ray luminosity and duration of GRB
171205A. We also calculate the photospheric and non-thermal emission after the
optically thick stage, which can fit the late-time X-ray, optical, and radio
light curves. Our results suggest that the relativistic ejecta-CSM interaction
can be a potential power source for low-luminosity GRBs and other X-ray bright
transients.
