Zadok was a priest during David’s reign. He anointed Solomon king at David’s behest. In the restored temple of Ezekiel only the sons of Zadok were permitted to serve as priests as only they had remained faithful to Yahweh when the Levites and the rest of Israel were unfaithful. See Ezk 40:46, 43:19,44:15, and 48:11.
reference: see John McKenzie, S.J., Dictionary of the Bible, Mqacmillan, 1965.page 946 for more.