Will Russia defend Iran against an attack?

It depends.

Prior to 1979 Iran was a wholly supported ally of the United States. It was specifically designated a U.S. special interest state and the government of Shah Reza Pahlavi was fully backed by the US with money, food and arms. After the fundamentalist revolution it was feared that Iran would become a Soviet bloc ally, or puppet state; this did not happen. Iran is almost as wary of Russia as it is the US. They do have a shared border and sea, and do have some trade agreements, yet they actually have had better relations with some European nations, including Germany and France.

Being as Iran does not want any part of the US or Russia dictating terms to them, it is unlikely that Russia would defend them against anything but a US invasion of Iran. That is not a highly likely proposition.

During the Iran - Iraq wars Russia actually provided some support to Iraq, while the US had made some backroom deals on both sides of the conflict (Iran-Contra scandal).