Too soon to be sure of any of the facts, other than what we know from residents in Ashdod.
The incoming rocket warning system (Tzeva Adom, or Color Red) began sounding around 10:50 pm tonight (Tuesday), followed very shortly afterwards by the sound of explosions. Early indications are that at least one in-bound rocket, clearly from the nearby Gaza Strip, was intercepted in midair by the Iron Dome anti-rocket defence system. And here's a posted video showing the intercept. There may have been a second rocket and a second intercept.
Ashdod, home to Israel's main seaport, has a population of about 240,000.
There are reports from residents there hearing considerable aircraft activity over the city and in the direction of the Gaza Strip.
Most news reporting - at least outside Israel - will likely kick in only after there has been a counter-attack by Israel against the rocket-rich forces arrayed in the Hamas enclave. That's the point at which the predictable narrative of allegedly-unprovoked Israeli attacks on the teeming masses of Gaza get started. So predictable, so counter-factual, so unhelpful.
UPDATE 11:45 pm Tuesday 29-Sep-15: From Haaretz -
"At least one rocket was fired at southern Israel Tuesday night, but failed to cause damage or injuries, the Israel Police said. The Iron Dome defense system intercepted the rocket - most likely fired from Gaza Strip - over the skies of the coastal city of Ashdod. It was preceded by two sirens alert of an incoming attack."UPDATE 3:00 pm October 1, 2015: See "Missile attack on Ashdod gets fifteen words of BBC coverage" via BBCwatch