If I work for the federal government can I work part-time as a consultant in an unrelated field?

I’m considering accepting a job offer w/ the federal government. For some reason I have it in my head that federal employees aren’t allowed to have side jobs? Is that true? I’d like to do part-time consulting work on the weekends in a field COMPLETELY unrelated to the work I’d be doing for the gov. Is that allowed or do I have to live entirely off of a federal salary? :P