In an interview if you've ever asked a question that you don't know the answer to, the appropriate response is "I don't know". Then, follow that with "but I had a similar experience", and think of some experience that you had that relates to the question, because you don't want to stop with "I don't know" and then just leave them hanging.
You want to give such responses as follows:
-- "I don't know, but I will find out"
-- "I don't know but I know someone who does" (e.g., I could ask a senior engineer or search on google, etc.)
-- "I don't know and here is an example that may relates to that"
Then that says about you that you are resourceful, you may not know the answer but you probably know someone who does or you are willing to go and do some research to find out the answer and get back to them.