I'm probably missing something because I've never used PPP with IPv6.

What does this solve? It's not like you are going to run out of space in fe80::/10 and if the PPP server is attacking your ndp table you have bigger problems...
-- 
Sent from a small device. Please excuse interesting auto-correct.

16 Nov 2024 19:50:32 Denis Fondras <denis@openbsd.org>:

> While working on IPv6 for pppd(8), I tried to use the EUI64 address as the PPP
> local address and failed because the kernel does not allow to change the netmask
> from /64 to /128.
> I propose we change the netmask for PPP interfaces to /128. Having a /64 is
> useless anyway on a PPP link.
> 
> (the alternative would be to generate another local address in pppd(8) but it
> seems overkill to me and we end up we 2 link-local addresses).
> 
> What do you think ?
> 
> Denis
> 
> Index: share/man/man4/pppoe.4
> ===================================================================
> RCS file: /cvs/src/share/man/man4/pppoe.4,v
> diff -u -p -r1.36 pppoe.4
> --- share/man/man4/pppoe.4  27 May 2022 15:45:02 -0000  1.36
> +++ share/man/man4/pppoe.4  16 Nov 2024 17:51:53 -0000
> @@ -102,7 +102,7 @@ inet 0.0.0.0 255.255.255.255 NONE \e
> dest 0.0.0.1
> inet6 eui64
> !/sbin/route add default -ifp pppoe0 0.0.0.1
> -!/sbin/route add -inet6 default -ifp pppoe0 fe80::%pppoe0
> +!/sbin/route add -inet6 default -ifp pppoe0 -iface fe80::%pppoe0
> .Ed
> .Pp
> The physical interface must also be marked
> Index: sys/netinet6/in6_ifattach.c
> ===================================================================
> RCS file: /cvs/src/sys/netinet6/in6_ifattach.c,v
> diff -u -p -r1.122 in6_ifattach.c
> --- sys/netinet6/in6_ifattach.c 9 Jul 2023 19:06:48 -0000   1.122
> +++ sys/netinet6/in6_ifattach.c 16 Nov 2024 17:51:53 -0000
> @@ -266,7 +266,11 @@ in6_ifattach_linklocal(struct ifnet *ifp
> 
>     ifra.ifra_prefixmask.sin6_len = sizeof(struct sockaddr_in6);
>     ifra.ifra_prefixmask.sin6_family = AF_INET6;
> -   ifra.ifra_prefixmask.sin6_addr = in6mask64;
> +   if (ifp->if_type == IFT_PPP)
> +       /* There is no need to get a /64 for P2P links */
> +       ifra.ifra_prefixmask.sin6_addr = in6mask128;
> +   else
> +       ifra.ifra_prefixmask.sin6_addr = in6mask64;
>     /* link-local addresses should NEVER expire. */
>     ifra.ifra_lifetime.ia6t_vltime = ND6_INFINITE_LIFETIME;
>     ifra.ifra_lifetime.ia6t_pltime = ND6_INFINITE_LIFETIME;