sub cmd_logfile
{
    my $self = shift;
    my $inner = $self->dl_inner_dir; defined $inner or die "Error: DL_INNER_DIR not set.";

    my $b4_ext = "$inner/transfer"; my $ext = "log";
    my $filename = "$b4_ext.$ext";
    my $i = 2;

    while (-s $filename)
    {
        $filename = "$b4_ext-$i.$ext";
        $i++;
    }

    return $filename;
}