go to this site and it says everything
http://www.usajobs.opm.gov/ will tell you everything.
http://www.usajobs.opm.gov/EI9.htm answer what kind non-citizen can aply for the federal job and they are as follows. Check if you qualify
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Appropriations Act Restrictions
Congress prohibits the use of appropriated funds to employ noncitizens within the United States. Certain groups of noncitizens are not included in this ban.
They are:
Persons who owe permanent allegiance to the United States (for example, natives of American Samoa and Swains Island).
Aliens from Cuba, Poland, South Vietnam, countries of the former Soviet Union, or the Baltic countries (Estonia, Latvia, and Lithuania) lawfully admitted to the United States for permanent residence.
South Vietnamese, Cambodian or Laotian refugees paroled into the United States after January 1, 1975.
Nationals of the People's Republic of China who qualify for adjustment of status pursuant to the Chinese Student Protection Act of 1992.
Citizens of Ireland, Israel, or the Republic of the Philippines.
Nationals of countries currently allied with the United States in a defense effort, (as determined by the Office of the Assistant Legal Adviser for Treaty Affairs, Department of State).
International broadcasters employed by the U.S. Information Agency.
Translators employed temporarily.
People employed up to 60 days on an emergency basis in the field service.