Patch: fix for ndp -s saying "writing to routing socket: Network is unreachable"
Here's a patch that fixes it for me. Quite similar to the OpenBSD and FreeBSD ones.
I discovered something else is broken: expiry times don't seem to be set even with "temp". Specifically, after the command in my previous comment, running "ndp -a" shows the word "permanent" in the "Expire" column. On OpenBSD, running a similar command results in a time slightly less than 20min shown in the "Expire" column, which makes more sense.