What are the visa & residency requirements in Sudan?

"In order to visit Sudan, travelers must obtain a visa prior to arrival. The process of applying for a visa involves submitting an application form, a valid passport, two passport-sized photographs, and a fee. Depending on the type of visa, additional documents may be required. In order to obtain residency in Sudan, applicants must submit an application form, a valid passport, two passport-sized photographs, a medical certificate, and a fee. The application must also include a letter of recommendation from a Sudanese citizen or a company in Sudan. Once the application is approved, the applicant will be issued a residency permit," remarked another expat in Sudan.