Fix duplicate address detection

src/firejail/arp.c

@@ -230,15 +230,26 @@ int arp_check(const char *dev, uint32_t destaddr) {
  if (framerx[12] != (ETH_P_ARP / 256) || framerx[13] != (ETH_P_ARP % 256))
  continue;
  memcpy(&hdr, framerx + 14, sizeof(ArpHdr));
- if (hdr.opcode == htons(1))
- continue;
- if (hdr.opcode == htons(2)) {
- // check my mac and my address
- if (memcmp(ifr.ifr_hwaddr.sa_data, hdr.target_mac, 6) != 0)
- continue;
+ if (hdr.opcode == htons(1)) {
+ // request, check if someone else is probing the same IP
+ if (memcmp(ifr.ifr_hwaddr.sa_data, hdr.sender_mac, 6) == 0)
+ continue; // it was our own probe, ignore it
+
  uint32_t ip;
  memcpy(&ip, hdr.target_ip, 4);
- if (ip != srcaddr) {
+ if (ip != destaddr) {
+ continue;
+ }
+ close(sock);
+ return -1;
+ }
+ if (hdr.opcode == htons(2)) {
+ // reply, check if someone else has the address we are probing for
+ /*if (memcmp(ifr.ifr_hwaddr.sa_data, hdr.target_mac, 6) != 0)
+ continue;*/
+ uint32_t ip;
+ memcpy(&ip, hdr.sender_ip, 4);
+ if (ip != destaddr) {
  continue;
  }
  close(sock);