I'm trying to find a way to track users by impressions. Is it possible to either read the doubleclick cookie ID or Google user ID when serving an ad? Alternatively is it possible to send a custom value to DFA on the impression level? I know it's possible to do this by passing custom variables to a floodlight tag but I really need to do it for impressions too. Thanks.
Not possible in impressions. Only possible to pass this on click using %m macro. I imagine it can only read the cookie once the impression has been delivered even if a previous cookie exists.
That said you could do a hack whereby in your creative you setup a fake click in a hidden iframe that sends the cookie ID up / makes it available.
This has a pretty good breakdown of the macro's available and where they can be used/what level in DFP - I imagine DFA would be the same.
https://support.google.com/dfp/answer/158101?hl=en#Tracking2