It’s of no comfort or even any use to you but I had a need to control some PCs via WoL and your suspicion about the ARP cache is correct. I ended up writing a C++ class that scanned the ARP table and if it couldn’t find the entry, it added it, after doing this I could then send the WoL packet and the PC would wake from its slumber.
const int WOLSender::setARP(const char * pAddr, const char * pMAC) {
int ok(0), arpSocket(socket(AF_INET, SOCK_DGRAM, IPPROTO_IP)) ;
if (arpSocket < 0) {
syslog(LOG_CRIT, "%s", "Unable to get Socket DataGram for APR set!") ;
}
else {
struct in_addr ipaddr ;
if (inet_aton(pAddr, &ipaddr) == 0) {
syslog(LOG_INFO, "Invalid IP Address %s", pAddr) ;
}
else {
struct arpreq arp ;
std::memset(&arp, 0, sizeof(arpreq)) ;
arp.arp_ha.sa_family = ARPHRD_ETHER ;
std::memcpy(&arp.arp_ha.sa_data, pMAC, std::strlen(pMAC)) ;
arp.arp_flags = ATF_PERM | ATF_COM ;
struct sockaddr_in * sin = (struct sockaddr_in *) &arp.arp_pa ;
sin->sin_addr = ipaddr ;
sin->sin_family = AF_INET ;
strncpy(arp.arp_dev, ETH_IF, std::strlen(ETH_IF)) ;
if (ioctl(arpSocket, SIOCSARP, (caddr_t) &arp) < 0) {
syslog(LOG_INFO, "arp ioctl set request failed : %d", errno) ;
}
else {
syslog(LOG_INFO, "Added %s's MAC %s to ARP Cache!", pAddr, pMAC) ;
ok = 1 ;
}
}
}
close(arpSocket) ;
return ok ;
}
