I have setup ssh agent forwarding, and I know it works. I also have many keys setup in the agent. The problem is there are so many keys that I get:

Received disconnect from XXX.XXX.XXX.XXX: 2: Too many authentication failures for bob

The way around this is to use IdentitiesOnly=yes so that ssh will only send the identity you want it to for the specified host. I've also gotten this implemented and I know it works, without agent forwarding.

Now, I'm trying to combine the two features. That is, I want to use agent forwarding, but also be able to specify which identity to use when connecting. Problem is, I can't figure out how to do this.

So, I want to connect from box A through box B to box C. Box A has all of the identity files and the ssh agent running. I want to edit box A or B's ssh config file(s) to use a specific identity that exists in box A's agent (which is being forwarded).