autoip: Choose next address after rate limit

[david-cermak]

Fix: AutoIP selects a new address after rate limiting

Issue

After hitting MAX_CONFLICTS (10), AutoIP enters rate limiting mode for 60 seconds. After this timeout, it tries to acquire an IP address again, but it incorrectly reuses the same address that previously had conflicts.

This causes devices to fail Bonjour Conformance Tests for rate limiting, as they should try different addresses after timeouts.

Root Cause

In lwIP pre-2.2.0, address conflict detection was handled directly in autoip.c, where tried_llipaddr was incremented in autoip_restart():

lwip/src/core/ipv4/autoip.c

Lines 125 to 130 in 6ca936f

	autoip_restart(struct netif *netif)
	{
	struct autoip *autoip = netif_autoip_data(netif);
	autoip->tried_llipaddr++;
	autoip_start(netif);
	}

When ACD was extracted into a separate module in 2.2.0, this increment was missing for the rate-limiting path in the callback handler. The ACD_DECLINE case cleared the address but didn't increment the counter.

Fix

The patch adds one line to increment the tried_llipaddr counter in the ACD_DECLINE case, ensuring that after rate limiting, a new address is tried rather than the same one.

Testing

With this patch, devices pass the Bonjour Conformance Test for rate limiting.

[yarrick]

This comment says _start will select a new address. Can we make it do that instead? If we increment it every time in autoip_start it should also pass the tests right?

[yarrick]

Or alternatively, edit the comment to match the new way of handling it.

[david-cermak]

Thanks for the suggestions.
I would prefer to update the comment, if it's okay.

If we increment it every time in autoip_start it should also pass the tests right?

Yes, it would probably pass the BCT, but might have some side effects, e.g. if we want to try the same address (like link-up / down), or at the startup. So we would need some way to pass the information about from ACD to AUTOIP.
(also this is the old behavior -- prior to v2.2.0)