What are the requirements for a post-graduate work permit (Doktorand)?

The requirements to obtain a post graduate work permit are the following:

• Applicant to have studied at a recognised foreign university (Anabin for the university required);
• Applicant to be offered a post graduate job (Doktorandenstelle) by a German based company;
• Salary to be in the standard range for post-graduate activity (can be lower than Blue Card level).