npppd_get_ppp_by_ip
npppd_ppp *npppd_get_ppp_by_ip(npppd *, struct in_addr);
ppp = npppd_get_ppp_by_ip(_this->npppd, _this->ip4addr);
ppp = npppd_get_ppp_by_ip(_npppd, iphdr->ip_dst);
ppp = npppd_get_ppp_by_ip(_this, ina);