ip-up.local or ip-up is called by pppd, rc.local (which run once at system start) doesnt come in picture.pppd calls ip-up script everytime it connects to internet.$4 indicates which ppp interface (ppp0, ppp1 etc - mostly ppp0)man pppd for more info--interface option of curl tells it to use that interface for outgoing connection, dnsomatic sees originating IP and automatically updates IPs in opendns, dyndns etc.PS: I am using this method from 2 yrs now, never faced any problem