I concluded the syntax from some Asus source code files available on googlecode.com: http://lamadanu-latest.googlecode.com/git-history/4d8ea9a0b3a055b77b18a0eed38aab6476a200e8/trunk/user/rc/services_ex.c
doSystem("ez-ipupdate -h %s -s ns1.asuscomm.com -S dyndns -i %s -A 1", ddns_hnm, wan_ifname);