http://www.usajobs.gov/ is the official federal jobs web site. Because the right wingers have for the past generation attempted to make it look like the federal government was not expanding, many things the federal government used to do are now done by contractors even though it usually costs more. (I used to work for the contractors!) Therefore, many jobs require “status” which usually means you are already a federal government employee or have been laid off.
Almost all jobs with the federal government require US Citizenship. Jobs that involve national security information require a security clearance which is almost impossible to get if you have any family members living in countries deemed hostile to the US.