system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] How do Doctor Stange's sling ring portals work?

Can he open a portal to anywhere he wants in the universe, or does he need to have been there before?
gpt
Not anywhere in the universe, but certainly anywhere on Earth. In Infinity War when he, Stark and Parker are stuck on Ebony Maw's ship and are arguing about turning it around or not, if Strange could simply sling-ring his way home he would have done.