sub _recurse {
	my ( $self, $query, $apex ) = @_;
	$self->_diag("using cached nameservers for $apex");
	my $cache  = $self->{persistent}->{$apex};
	my @nslist = keys %$cache;
	my @glue   = grep { $$cache{$_} } @nslist;
	my @noglue = grep { !$$cache{$_} } @nslist;
	my $reply;
	foreach my $ns ( @glue, @noglue ) {
		if ( my $iplist = $$cache{$ns} ) {
			$self->nameservers(@$iplist);
		} else {
			$self->_diag("recover missing glue for $ns");
			next if substr( lc($ns), -length($apex) ) eq $apex;
			my @ip = $self->nameservers($ns);
			$$cache{$ns} = \@ip;
		}
		$query->header->id(undef);
		last if $reply = $self->SUPER::send($query);
		$$cache{$ns} = undef;				# park non-responder
	}
	$self->_callback($reply);
	return unless $reply;
	my $zone = $self->_referral($reply) || return $reply;
	die '_recurse exceeded depth limit' if $self->{recurse_depth}++ > 50;
	my $qname  = lc( ( $query->question )[0]->qname );
	my $suffix = substr( $qname, -length($zone) );
	return $zone eq $suffix ? $self->_recurse( $query, $zone ) : undef;
}