replacing large rear "dogbone" link on a 300e
 

I've never done this job before, and a search didn't help. When I went under my parts car I noticed the inside bolt was not a standard 6 sided one but that one with many indentations, like a spline. What kind of special socket do I need to hold this bolt while ratcheting the outside bolt?

You've somehow posted in the "video" forum. If a mod doesn't move this, try "Tech" at the top of the page.

it's an XZN head, aka triple square. I am struggling to remember if it is m12 or m10.

I'm not familiar with this particular part but I bet that a standard 12 pt. socket or box end wrench will fit.

I guess the real question is whether this control arm/thrust arm will come out just by removing the bolt, or whether the XZN head also has to be held so it doesn't turn.