Expand random range of integers using probability distribution

I tried to solve the classic problem of generating a random integer between 1 and 7, given a function that generates a random integer between 1 and 5. My approach was to add the result of 2 calls to rand5(), effectively turning this into a "sum of dice rolls" problem. The probability of the sum dice rolls is fairly easy to calculate, so I used it here.

My question is: How can I calculate what the values of counter should be? The current values are incorrect, as verified by experiment. Do integer values exist that satisfy the probability? And is there a better way to solve this problem with this approach?

def rand5():
    return random.randint(1,5)

def rand7():
    counter = [1,2,3,4,5,4,3]
    while 0 not in counter:
        sum = rand5() + rand5() - 2
        if sum <= 6:
            counter[sum] -= 1
    return counter.index(0) + 1