From: Stefan Sperling <stsp@stsp.name>
Subject: Re: Fwd: Re: PPPoE assigns link local fe80::200:0:0:1 instead of fe80::1
To: Bjorn Ketelaars <bket@openbsd.org>
Cc: tech@openbsd.org
Date: Mon, 20 Jul 2026 22:07:18 +0200

On Mon, Jul 20, 2026 at 09:26:01PM +0200, Bjorn Ketelaars wrote:
> Bringing this from misc@ to tech@ as I'm looking for comments/OK.
> 
> 
> netinet6: keep negotiated IPv6CP IFID unchanged for link-local address
> 
> When sppp/PPPoE installs a peer-suggested IPv6CP interface identifier,
> in6_ifattach_linklocal() forced EUI64 G/U bit adjustments on the
> provided IFID. This incorrectly rewrote a valid negotiated IFID such as
> ::1 into ::200:0:0:1, yielding fe80::200:0:0:1 instead of fe80::1.
> 
> Do not apply EUI64 bit mangling when an explicit IFID is supplied; use
> the negotiated value as-is (RFC 5072). Other callers are unaffected
> since they pass NULL and continue using in6_get_ifid().
> 
> Reported and tested by Martin Crossley <admin AT crossleys DOT biz>.
> Tested by me on a connection that allows an explicit IFID.

I am fine with this change in principle. I am not sure if we can safely
assume that any potential future callers passing an ifid won't need EUI64
bit mangling.

Would it make sense to guard EUI64 bit mangling with a check for
point-to-point interfaces? Something like this?

	if ((ifp->if_flags & IFF_POINTOPOINT) == 0) {
		ifra.ifra_addr.sin6_addr.s6_addr[8] &= ~EUI64_GBIT;
		ifra.ifra_addr.sin6_addr.s6_addr[8] |= EUI64_UBIT;
	}
 
> diff --git sys/netinet6/in6_ifattach.c sys/netinet6/in6_ifattach.c
> index 975ccbf5056..c8c7033c058 100644
> --- sys/netinet6/in6_ifattach.c
> +++ sys/netinet6/in6_ifattach.c
> @@ -244,8 +244,6 @@ in6_ifattach_linklocal(struct ifnet *ifp, struct in6_addr *ifid)
>  		ifra.ifra_addr.sin6_addr.s6_addr16[0] = htons(0xfe80);
>  		ifra.ifra_addr.sin6_addr.s6_addr16[1] = htons(ifp->if_index);
>  		ifra.ifra_addr.sin6_addr.s6_addr32[1] = 0;
> -		ifra.ifra_addr.sin6_addr.s6_addr[8] &= ~EUI64_GBIT;
> -		ifra.ifra_addr.sin6_addr.s6_addr[8] |= EUI64_UBIT;
>  	} else
>  		in6_get_ifid(ifp, &ifra.ifra_addr.sin6_addr);
>  
>