Bug in ACD implementation RFC5227

[anhuba]

The ACD is not properly implemented as described in RFC5227 ch. 2.1.1
The implementation incorrectly detects an IP conflict, if an ARP request is received for the target IP.
The reason is that the current implementation checks for ARP requests instead of ARP probes.

To reproduce enable NET_IPV4_ACD and use DHCP to get an address. Immediately after address is bound send an arp request for the received IP. The DHCP server may already do that. Then a conflict is detected and a new IP requested and so on...

I fixed it with this change, which correctly checks for ARP probe instead of ARP request for the second condition:

--- zephyr/subsys/net/ip/ipv4_acd.c	2024-10-25 11:19:34.000000000 
+++ zephyr/subsys/net/ip/ipv4_acd.c	2024-10-24 16:00:44.000000000 
@@ -285,21 +285,25 @@
 			continue;
 		}
 
 		ll_addr = net_if_get_link_addr(addr_iface);
 
 		/* RFC 5227, ch. 2.1.1 Probe Details:
-		 * - Sender IP address match OR,
-		 * - Target IP address match with different sender HW address,
+		 * - ARP Request/Reply with Sender IP address match OR,
+		 * - ARP Probe where Target IP address match with different sender HW address,
 		 * indicate a conflict.
+		 * ARP Probe has an all-zero sender IP address
 		 */
+		uint32_t all_zero_ip = 0;
 		if (net_ipv4_addr_cmp_raw(arp_hdr->src_ipaddr,
 					  (uint8_t *)&ifaddr->address.in_addr) ||
 		    (net_ipv4_addr_cmp_raw(arp_hdr->dst_ipaddr,
 					  (uint8_t *)&ifaddr->address.in_addr) &&
-		     memcmp(&arp_hdr->src_hwaddr, ll_addr->addr, ll_addr->len) != 0)) {
+		     (memcmp(&arp_hdr->src_hwaddr, ll_addr->addr, ll_addr->len) != 0) &&
+		     (net_ipv4_addr_cmp_raw(arp_hdr->src_ipaddr,
+		                           (uint8_t *)&all_zero_ip)))) {
 			NET_DBG("Conflict detected from %s for %s",
 				net_sprint_ll_addr((uint8_t *)&arp_hdr->src_hwaddr,
 						   arp_hdr->hwlen),
 				net_sprint_ipv4_addr(&ifaddr->address.in_addr));
 
 			iface->config.ip.ipv4->conflict_cnt++;

[nordicjm]

Can you submit a PR? We do not accept pasted patched

[anhuba]

#80433