Bug : -f option does not provide output

[proabiral]

--fast , -f option does not provide any output when the number of input domains are lower.

However, there is not present when the number of input domain is increased.

[ghost]

If I understand the pictures correctly /tmp/a has only one root domain. I think the problem just lies in that the -f option only permutes on observed subdomain words, which one root domain does not provide.
When you then run it without -f, the default dnsgen wordlist is used to generate subdomains.
So this is actually correct behavior. For -f you need to supply (valid) subdomains so dnsgen can observe words occurring and create a list with permutations.