ARP: improve policies, guards and neighbors cache. Fix related Fenrir claims.

[danielinux]

Featuring the following addressed claims:

d146f2a F-4045: self-review fixes for the DAD state machine
d7d2199 F-4045: run RFC 4331 DAD after DHCPACK before the lease binds
64288da F-4057: drop queued frame when the ARP pending queue is full
9ba8611 F-10261: retry the pure ACK when the immediate send misses ARP
99fb19b F-6035: remove SYN-driven ARP neighbor learning
3c194d9 F-6212: evict the least recently used neighbor when the table is full
88c8feb F-9805: require a pending request for ARP reply installs
b5fd480 F-10274: drop ARP frames with a group sender MAC

[Copilot]

Pull request overview

This PR strengthens ARP/DHCP behavior in wolfIP by tightening neighbor-cache update policy, improving cache/queue eviction behavior, and adding RFC 4331/5227-style DHCP address conflict detection (DAD) after DHCPACK to address a set of Fenrir claims/regressions.

Changes:

• Add DHCP post-ACK DAD state machine (ARP probes, conflict handling, and DECLINE) before binding the lease.
• Harden ARP neighbor learning: require a pending request for ARP reply installs, reject group (bcast/mcast) sender MACs, remove SYN-driven neighbor learning, and implement LRU eviction + use-based aging.
• Adjust TCP pure-ACK handling on ARP miss to return -WOLFIP_EAGAIN so callers can schedule a retry; add/extend unit/regression coverage for the new behaviors.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/wolfip.c	Implements DHCP DAD + DECLINE path, tightens ARP reply install policy, adds neighbor LRU/aging refresh, and updates TCP pure-ACK retry behavior.
src/test/unit/unit.c	Registers new/renamed unit tests for ARP regressions, DHCP DAD, and pure-ACK ARP-miss retry.
src/test/unit/unit_tests_tcp_ack.c	Updates ARP tests to reflect pending-only install gate and queue-full drop behavior; adds neighbor aging/eviction coverage.
src/test/unit/unit_tests_proto.c	Updates ARP reply tests to record pending requests and adjusts MACs to satisfy new unicast-sender requirement.
src/test/unit/unit_tests_ip_arp_recv.c	Adds coverage ensuring group sender MAC ARP frames are dropped (request and reply).
src/test/unit/unit_tests_dns_dhcp.c	Updates DHCP ACK tests to complete DAD before asserting bound behavior.
src/test/unit/unit_tests_dhcp_edges.c	Adds DHCP DAD unit tests and updates existing ACK tests to transition through DAD.
src/test/unit/unit_tests_branches.c	Updates branch tests for ARP queue-full drop behavior and LRU eviction semantics.
src/test/unit/unit_tests_arp_regression.c	New regression suite asserting pending-only neighbor installs and “no SYN-driven learning” behavior.
src/test/unit/unit_shared.c	Adds dhcp_test_complete_dad() helper to drive DAD completion in tests.
Makefile	Adds new ARP regression test source to the unit-test build list.

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[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

src/wolfip.c:9084

• arp_neighbor_index() now refreshes the neighbor timestamp unconditionally on any successful lookup. This means even callers doing a conditional refresh (e.g. arp_recv()'s ARP REQUEST path, which intends to refresh only when the sender MAC matches the cached MAC) will still extend the lifetime of an entry on MAC mismatch, making it easier to keep a stale/poisoned mapping from aging out.

        if (s->arp.neighbors[i].ip == ip && s->arp.neighbors[i].if_idx == if_idx) {
            if (s->last_tick >= s->arp.neighbors[i].ts &&
                    (s->last_tick - s->arp.neighbors[i].ts) > (uint64_t)ARP_AGING_TIMEOUT_MS) {
                s->arp.neighbors[i].ip = IPADDR_ANY;
                s->arp.neighbors[i].if_idx = 0;
                s->arp.neighbors[i].ts = 0;
                memset(s->arp.neighbors[i].mac, 0, 6);
                return -1;
            }
            s->arp.neighbors[i].ts = s->last_tick;
            return i;