The dns.query.xfr() takes two inputs similar to the dig utility (dig axfr @nsztm1.digi.ninja zonetransfer.me). It gets the first input parameter (in the case of zonetransfer.me, it is nsztm1.digi.ninja), by processing master_answer[0].address, which is essentially retrieved from the single command-line argument (and also the second input parameter), address. We print the sorted names after the zone transfer.