Re: PPPoE assigns link local fe80::200:0:0:1 instead of fe80::1

> On 18 Jul 2026, at 15:21, Bjorn Ketelaars <[email protected]> wrote:
> 
> On Sat 18/07/2026 11:44, Martin Crossley wrote:
>> Clean build of OpenBSD 7.9 generic.mp#449 amd64.
>> 
>> During PPPoE negotiation the peer gives us fe80::1 and we accept it.
>> So why does pppoe set our link local address to fe80::200:0:0:1 ?
>> 
>> fe80::200:0:0:1 looks like the EUI64 conversion for a MAC address of
>> 00:00:00:00:00:01, so could it be that the offered IFID is being
>> Interpreted as a MAC?
>> 
>> (1) hostname.pppoe0 (NB: same result if we explicitly set inet6 fe80::1)
>> 
>> inet 0.0.0.0 255.255.255.255 NONE \
>>    pppoedev bge0 authproto chap \
>>    authname ‘xxxxx' authkey ‘xxxxx’ up
>> 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
>> 
>> (2) Ifconfig pppoe0 after negotiation
>> 
>> pppoe0: flags=8855<UP,DEBUG,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> mtu 1492
>> index 5 priority 0 llprio 3
>> dev: bge0 state: session
>> sid: 0xca5 PADI retries: 0 PADR retries: 0 time: 00:04:27
>> sppp: phase network authproto chap authname “xxxxx" 
>> dns: 212.23.3.100 212.23.6.100
>> groups: pppoe egress
>> status: active
>> inet6 fe80::200:0:0:1%pppoe0 -->  prefixlen 64 scopeid 0x5
>> inet 217.155.184.174 --> 51.148.77.132 netmask 0xffffffff
>> 
>> 
>> (3) Packet capture on the ethernet (our mac address is 00:9c:02:ab:d9:44)
>> 
>> 21:17:19.111881 00:9c:02:ab:d9:44 80:7f:f8:74:01:4b 8864 36: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 16
>>        IPV6CP Configure-Request Id=0x03: IPv6-Interface-Id=029c:02ff:feab:d944
>> 21:17:19.117305 80:7f:f8:74:01:4b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 12
>>        IPCP Configure-Request Id=0x30: IP-Address=51.148.77.133
>> 21:17:19.117307 80:7f:f8:74:01:4b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 24
>>        IPCP Configure-Nak Id=0x02: IP-Address=217.155.184.174 Primary-DNS-Server=212.23.3.100 Secondary-DNS-Server=212.23.6.100
>> 21:17:19.117308 80:7f:f8:74:01:4b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 16
>>        IPV6CP Configure-Request Id=0x62: IPv6-Interface-Id=827f:f8ff:fe74:014b
>> 21:17:19.117779 00:9c:02:ab:d9:44 80:7f:f8:74:01:4b 8864 32: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 12
>>        IPCP Configure-Ack Id=0x30: IP-Address=51.148.77.133
>> 21:17:19.117780 00:9c:02:ab:d9:44 80:7f:f8:74:01:4b 8864 44: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 24
>>        IPCP Configure-Request Id=0x04: IP-Address=217.155.184.174 Primary-DNS-Server=212.23.3.100 Secondary-DNS-Server=212.23.6.100
>> 21:17:19.117781 00:9c:02:ab:d9:44 80:7f:f8:74:01:4b 8864 36: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 16
>>        IPV6CP Configure-Ack Id=0x62: IPv6-Interface-Id=827f:f8ff:fe74:014b
>> 21:17:19.117895 80:7f:f8:74:01:4b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 16
>>        IPV6CP Configure-Nak Id=0x03: IPv6-Interface-Id=0000:0000:0000:0001
>> 21:17:19.118149 00:9c:02:ab:d9:44 80:7f:f8:74:01:4b 8864 36: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 16
>>        IPV6CP Configure-Request Id=0x05: IPv6-Interface-Id=0000:0000:0000:0001
>> 21:17:19.118595 00:9c:02:ab:d9:44 80:7f:f8:74:01:4b 8864 94: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 74
>>        IPv6 fe80::29c:2ff:feab:d944 > ff02::2: HBH icmp6: multicast listener done max resp delay: 0 addr: ff02::2 [hlim 1]
>> 21:17:19.118597 00:9c:02:ab:d9:44 80:7f:f8:74:01:4b 8864 94: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 74
>>        IPv6 fe80::200:0:0:1 > ff02::1:ff00:1: HBH icmp6: multicast listener report max resp delay: 0 addr: ff02::1:ff00:1 [hlim 1]
>> 21:17:19.226019 80:7f:f8:74:01:4b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 24
>>        IPCP Configure-Ack Id=0x04: IP-Address=217.155.184.174 Primary-DNS-Server=212.23.3.100 Secondary-DNS-Server=212.23.6.100
>> 21:17:19.226621 80:7f:f8:74:01:4b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 16
>>        IPV6CP Configure-Ack Id=0x05: IPv6-Interface-Id=0000:0000:0000:0001
>> 21:17:19.402414 80:7f:f8:74:01:4b 00:9c:02:ab:d9:44 8864 110: PPPoE-Session
>>        code Session, version 1, type 1, id 0x07e4, length 90
>>        IPv6 fe80::827f:f8ff:fe74:14b > ff02::1: icmp6: router advertisement [class 0xc0]
>> 
> 
> I think your diagnosis is correct. The peer NAKs your initial interface
> ID and suggests 0000:0000:0000:0001. The sppp state machine accepts it
> and schedules installation of fe80::1. The problem is in
> in6_ifattach_linklocal(), which then unconditionally applies an EUI-64
> U/L bit transformation to the provided interface ID:
> 
> ifra.ifra_addr.sin6_addr.s6_addr[8] &= ~EUI64_GBIT;
> ifra.ifra_addr.sin6_addr.s6_addr[8] |=  EUI64_UBIT; /* 0x00 → 0x02 */
> 
> That turns fe80::1 into fe80::200:0:0:1. This transformation is
> appropriate when deriving an interface ID from a MAC address, but RFC
> 5072 requires PPP-negotiated interface IDs to be used verbatim — they
> carry no EUI-64 semantics.
> 
> The fix is to remove those two lines, see diff below.
> 
> There are three call sites for in6_ifattach_linklocal(). Only
> sys/net/if_spppsubr.c reaches this branch. The other two — in
> sys/netinet/ip_carp.c and sys/netinet6/in6_ifattach.c itself — both pass
> NULL as the ifid argument and never reach this branch, so there is no
> impact on any other interface type.
> 
> The lines were introduced by r1.66 of sys/netinet6/in6_ifattach.c
> (stsp@, January 2014), which added the ifid parameter to allow sppp to
> install peer-suggested interface IDs — a correct change overall. The
> EUI-64 bit manipulation was apparently carried over by analogy with the
> MAC-to-EUI-64 code nearby, but does not belong in the PPP context.
> 
> Could you test this against your setup?
> 
> 
> 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);
> 

Happy to confirm that works for me - many thanks for the very quick response

(1) hostname.pppoe0 (same as before)

inet 0.0.0.0 255.255.255.255 NONE \
   pppoedev bge0 authproto chap \
   authname ‘xxxxx' authkey ‘xxxxx’ up
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

(2) ifconfig pppoe0 after negotiation

pppoe0: flags=8855<UP,DEBUG,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> mtu 1492
	index 5 priority 0 llprio 3
	dev: bge0 state: session
	sid: 0x6bd PADI retries: 0 PADR retries: 0 time: 00:01:04
	sppp: phase network authproto chap authname "zen103022@zen" 
	dns: 212.23.3.100 212.23.6.100
	groups: pppoe egress
	status: active
	inet6 fe80::1%pppoe0 -->  prefixlen 64 scopeid 0x5
	inet 217.155.184.174 --> 51.148.77.133 netmask 0xffffffff

(3) packet capture on the ethernet (our mac address is 00:9c:02:ab:d9:44)

18:01:09.142927 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 44: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 24
        IPCP Configure-Request Id=0x02: IP-Address=0.0.0.0 Primary-DNS-Server=0.0.0.0 Secondary-DNS-Server=0.0.0.0
18:01:09.142928 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 36: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 16
        IPV6CP Configure-Request Id=0x03: IPv6-Interface-Id=029c:02ff:feab:d944
18:01:09.148350 80:7f:f8:74:01:3b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 12
        IPCP Configure-Request Id=0xe9: IP-Address=51.148.77.133
18:01:09.148352 80:7f:f8:74:01:3b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 24
        IPCP Configure-Nak Id=0x02: IP-Address=217.155.184.174 Primary-DNS-Server=212.23.3.100 Secondary-DNS-Server=212.23.6.100
18:01:09.148353 80:7f:f8:74:01:3b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 16
        IPV6CP Configure-Request Id=0x2a: IPv6-Interface-Id=827f:f8ff:fe74:013b
18:01:09.158496 80:7f:f8:74:01:3b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 16
        IPV6CP Configure-Nak Id=0x03: IPv6-Interface-Id=0000:0000:0000:0001
18:01:09.177925 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 32: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 12
        IPCP Configure-Ack Id=0xe9: IP-Address=51.148.77.133
18:01:09.177927 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 44: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 24
        IPCP Configure-Request Id=0x04: IP-Address=217.155.184.174 Primary-DNS-Server=212.23.3.100 Secondary-DNS-Server=212.23.6.100
18:01:09.177928 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 36: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 16
        IPV6CP Configure-Ack Id=0x2a: IPv6-Interface-Id=827f:f8ff:fe74:013b
18:01:09.187336 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 36: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 16
        IPV6CP Configure-Request Id=0x05: IPv6-Interface-Id=0000:0000:0000:0001
18:01:09.205733 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 94: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 74
        IPv6 fe80::29c:2ff:feab:d944 > ff02::2: HBH icmp6: multicast listener done max resp delay: 0 addr: ff02::2 [hlim 1]
18:01:09.205735 00:9c:02:ab:d9:44 80:7f:f8:74:01:3b 8864 94: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 74
        IPv6 fe80::1 > ff02::1:ff00:1: HBH icmp6: multicast listener report max resp delay: 0 addr: ff02::1:ff00:1 [hlim 1]
18:01:09.239512 80:7f:f8:74:01:3b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 24
        IPCP Configure-Ack Id=0x04: IP-Address=217.155.184.174 Primary-DNS-Server=212.23.3.100 Secondary-DNS-Server=212.23.6.100
18:01:09.239515 80:7f:f8:74:01:3b 00:9c:02:ab:d9:44 8864 60: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 16
        IPV6CP Configure-Ack Id=0x05: IPv6-Interface-Id=0000:0000:0000:0001
18:01:09.426405 80:7f:f8:74:01:3b 00:9c:02:ab:d9:44 8864 110: PPPoE-Session
        code Session, version 1, type 1, id 0x06bd, length 90
        IPv6 fe80::827f:f8ff:fe74:13b > ff02::1: icmp6: router advertisement [class 0xc0]