Thanks for bisecting and fixing!

The broadcast_pmtu.sh selftest provided with the original patch still
passes with this patch applied.

Hm, yes, AFACT we're losing PMTU discovery but perhaps original commit
wasn't concerned with that. Hopefully Oscar can comment.

On Fri, 22 Aug 2025 16:50:51 +0000 Brett A C Sheffield wrote:

+		if (type == RTN_BROADCAST) {
+			/* ensure MTU value for broadcast routes is retained */
+			ip_dst_init_metrics(&rth->dst, res->fi->fib_metrics);

You need to check if res->fi is actually set before using it

Could you add a selftest / test case for the scenario we broke?
selftests can be in C / bash / Python. If bash hopefully socat
can be used to repro, cause it looks like wakeonlan is not very
widely packaged.