  We discuss first results for the computation of short distance contributions
to semileptonic form factors for the rare $B$ decays $B \to K^{*} \ell^+\ell^-$
and $B_s \to \phi \ell^+ \ell^-$. Our simulations are based on RBC/UKQCD's
$N_f=2+1$ ensembles with domain wall light quarks and the Iwasaki gauge action.
For the valence $b$-quark we chose the relativistic heavy quark action.
