Not sure about JTR but you could always use crunch.
As i'm living outside the europe, exactly at Tunisia, the available wordlists on he net didn't gave any good result to find a wpa psk2 key i'm trying to determine.
Then, i have seen that here in Tunisia, all Modem passwords are 5 to 12 alpha-numeric letter that can be upper/lower case, and sometimes it can contain a special caracter like "-" or "_".
So, i've decided to use John the ripper to build a new heuristic dictionnary containing all probabilities, and for that i user this command to have words with 8 characters:
"./john --incremental=alnum --stdout=8 > dico.txt"
then, i had a big problem.
JTR fucctionned for approximatly 2 hours, and then staurated my had disk and it created a 19Gb dico.txt file , telling me that he stopped on the "t" letter.
so, first of all, how can i precise the length of the words generated between a minimum (5) and a maximum (12) instead of giving only the max size.
Then, is it normal that my outcoming file is as big as this ? I think backtrack can't handle it.
thanks for all
even generating all possible combinations of an 8 characters gets ridiculously large really fast, if you really insist on a brute force style of attack id look into piping crunch through your cracking program instead of eating up all of your disk space, but be warned this can take an unrealistic amount of time.