If it's confirmed that ISIS planted a bomb on the Russian plane, what will Russia's response be?

Typically, ISIS is quick to trumpet how and who carried out any attacks for purposes of praise and propaganda. The fact they haven't in this case raises doubts about their claim. However maybe ISIS is getting wise not to show one's hand and look to use similar methods. However I wouldn't be surprised if the CIA had something to do with it. Some sort of effort to exert some pressure on Russia to back off. In either case I don't see Putin becoming sheepish with Syria. If any thing this will put boots on the ground.

SPC Andrew Griffin The response will be brutal and ugly.

Putin is defending his last port and interest in the Middle East, if they did down the plane then there would escalation of actions in the Middle East but I believe it would happen regardless.