  We consider massless Dirac operators on the real line with compactly
supported potentials. We solve two inverse problems (including
characterization): in terms of zeros of reflection coefficient and in terms of
poles of reflection coefficients (i.e. resonances). We prove that a potential
is uniquely determined by zeros of reflection coefficients and there exist
distinct potentials with the same resonances. We describe the set of
"isoresonance potentials". Moreover, we prove the following:
  1) a zero of the reflection coefficient can be arbitrarily shifted, such that
we obtain the sequence of zeros of the reflection coefficient for an other
compactly supported potential,
  2) the forbidden domain for resonances is estimated,
  3) asymptotics of resonances counting function is determined,
  4) these results are applied to canonical systems.
