  We assume that QCD can be effectively described with string-like variables.
The hadronic string is built over the chirally non-invariant QCD vacuum by
means of the boundary interaction with background chiral fields associated with
pions. By making this interaction compatible with the conformal symmetry of the
string and with the unitarity constraint on chiral fields we reconstruct the
equations of motion for the latter ones and furthermore recover the Lagrangian
of non-linear sigma model of pion interactions. The estimated chiral structural
constants of Gasser and Leutwyler fit well the phenomenological values.
