Thanks for your explanation, I need more clarification: who is Dara? Is he Darius king of Ancient Persian? I found another information in the Internet that Dhul-Qarnain is probably Cyrus the Great founder of Persian Empire whose empire was from Black Sea in the West to Indus Valley in the East, is it correct?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. You should know, dear brother, that the story in the Noble Qur'an is not narrated in the same way or with the same technique used by story-tellers in their books. The story-tellers usually shed more light on some characters to make main actors out of them giving these actors names that keep on recurring. Qur'an, however, most of the time, does not mention the name of the actor. It rather mentions the main characteristics of the actor without naming him. The story of Dhul Qurnain (the pious king) is not an exception to this rule. So, the main characteristics of his personality are mentioned. He was a king to whom Allah gave the rule of the earth. So, he went as far as the place where the sun goes down. He also went as far as the place where the sun rises. He dominated some very strong and fierce people. But Allah gave him the choice between punishing them or pardoning them. He helped the needy and weak people in building the great dam, hence lifting harm that was befalling them. He was then a pious slave, who defended the right and fought for establishing it. This is what imports from the story of Dhul Qurnain. As for his name, race and colour, they are not important. Indeed Qur'an is deemed far above following such details. Now, since neither the Noble Qur'an nor the Sunnah had mentioned Dhul Qurnain's real name, then the researcher could learn about pious good kings throughout history and find out the name of the one of them who was characterized by the characteristics of Dhul Qurnain. This was the way followed by the researcher Abd Al-Halim Abd Al-Rahman Geography teacher in Imam Muhammad Ibn Saoud University. He wrote a book entitled: Some geographic notions in the Qur'an stories: The story of Dhu Qurnain. After a thorough research, he came to the conclusion that Dhul Qurnain was most probably, the Persian king who appeared in the middle of the sixth century AD (529-559 AD). His name was "Cyrus the Persian". His empire included Today Iran as well as the countries between the White Sea, the Sind River and the Aral Lake. He also dominated Syria and Palestine. His empire extended to include the Mediterranean Sea, …etc. However, this remains a very probable option, only. Allah knows best.