The following excerpt from the Mahabaratha clarifies that Narakasura was the son of Earth and Krishna (in another form) gave him a boon on Earth's request:
Vasudeva replied unto him, saying, 'Listen, O Partha, to this secret and ancient history as it is, O sinless one! I have four forms, eternally engaged as I am in protecting the worlds. Dividing my own Self, I ordain the good of the worlds. One form of mine, staying on the earth, is engaged in the practice of ascetic austerities. Another beholdeth the good and the evil deeds in the world. My third form, coming into the world of men, is engaged in action. My fourth form lieth down in sleep for a thousand years. The form of mine which awaketh from sleep at the end of a thousand years, granteth, upon awakening, excellent boons to persons deserving of them. The earth, knowing (on one occasion) that that time had come, asked of me a boon for (her son) Naraka. Hear, O Partha, what that boon was. Possessed of the Vaishnava weapon, let my son become incapable of being slain by the gods and the Asuras. It behoveth thee to grant me that weapon. Hearing this prayer, I then gave, in days of old, the supreme and infallible Vaishnava weapon to the Earth's son. I said also at that time these words, 'O Earth, let this weapon be infallible for the protection of Naraka. None will be able to slay him. Protected by this weapon, thy son will always, in all the worlds, be invincible and crush all hostile hosts.' Saying, So be it! the intelligent goddess went away, her wishes fulfilled.
In Baghavatham, there are many slokhas stating that Naraka was the son of Earth, but in which slokha of which Purana was the father of Narakasura was stated as Varaha?