For DCF the petitioner MUST provide I-864. I-864 requires 2 things (IRS return for past year, or a letter explaining how you were EXEMPT from filing, AND US Citizen needs to prove that they maintained US Domicile while living abroad)
US Citizens may be exempt from paying tax on foreign income, but they are NOT exempt from filing returns unless income is below a set figure determined by IRS.
Some good reading about I-864 requirements.
http://travel.state.gov/pdf/I-864GenInfo-FAQ-Final.pdfMany DCF filers use a joint sponsor.