Forget about the intricacies of the DNS spec.

[timb_machine@infosec.exchange]

Forget about the intricacies of the DNS spec. This should have seemed like a bad life choice in any event. Turning user input into a command line when all you really wanted to do was write a 0 byte file:

sprintf(acfcommand, "/bin/touch %s/control/notlshosts/'%s'",
info->pw_dir, partner_fqdn);
fp = popen(acfcommand, "r");