Robert Foster only states that it is somewhere in Middle-Earth. I havn't come across it in my reading so let me just comment on the mention that it may have been south of the Sea of Rhun.
For one thing, the sea of rhun is very close to mordor as we all notice. Who would be daring enough to live there and maintain a garden for the making of wine, I can not imagine.
However, we know that the area around the Sea of Rhun was a nice at one point because we are told that the Elves spent some time there on their way from Cuiviénen(where the elves first awoke) to Beleriand and on to Valinor for some. Whether it was still nice some 12,000 years later, I can not say.
One last point. The Forest River passes into the River Running which ofcourse flows all they way to the Sea of Rhun. However, it flows south. So the heavy barrels full of wine would either have to be carried over land from Dorwinion or paddled up the River Running, which is only feasible if the River flows very slow.
If someone can direct me to the text that would suggest that Dorwinion is near the Sea of Rhun, i'd surely appreciate it.
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