Re: we should enable RFC7217 by default

On 1/27/26 20:17, Shawn Webb wrote:
> On Tue, Jan 27, 2026 at 07:27:28PM +0100, Guido Falsi wrote:
>> On 1/27/26 19:17, Shawn Webb wrote:
>>> On Tue, Jan 27, 2026 at 03:35:16AM +0330, Pouria Mousavizadeh Tehrani wrote:
>>>> Hi everyone,
>>>>
>>>> With `net.inet6.ip6.use_stableaddr` now available, I believe we should
>>>> enable it by default in CURRENT at least.
>>>> As you may already know, we currently use the EUI64 method for generating
>>>> stable IPv6 addresses, which has serious privacy issues.
>>>>
>>>> IMHO, trying to maintain backward compatibility defeats the purpose of a
>>>> privacy RFC.
>>>>
>>>> To be clear, we don't want to change the ip addresses of existing servers.
>>>> However, it's reasonable for users to expect changes during a major upgrade
>>>> (15 -> 16), a fresh install of a new major release, or living on CURRENT.
>>>> So, for obvious reasons, changing the default value would not be MFCed.
>>>>
>>>> What do you think?
>>>
>>> I think this would be a good step for FreeBSD. In HardenedBSD, we set
>>> net.inet6.ip6.{prefer,use}_tempaddr to 1, which creates completely
>>> random IPv6 addresses (scoped to the prefix, of course).
>>>
>>> The one thing I would hope is that support for completely random IPv6
>>> addresses via SLAAC does not go the way of the dodo.
>>>
>>> (If net.inet6.ip6.use_stableaddr becomes the default, we will likely
>>> keep it at 0 in favor of the other aforementioned sysctl nodes.)
>>
>> Those are two orthogonal things.
>>
>> stableaddress enabled replaces the current algorithm for deriving the main
>> interface address, that stays attached to the interface indefinitely.
>>
>> tempaddr creates additional addresses for the interface that are used (and
>> preferred if the prefer flag is enabled) for outgoing connections, and are
>> generated again periodically, with old ones remaining attached to the
>> interface, since old connections could still use them, till reboot.
>>
>> The two can live together, there is no reason to disable one of them.
>>
>>
>> BTW while developing my patch, in one of the first iterations, I did break
>> the tempaddr mechanism, so I can assure you I took special care for them to
>> not interfere with each other.
> 
> Seems I was indeed a bit confused. Thank you for the explanation.
> 
> So looking at one of my current SLAAC systems, I see:
> 
> ==== BEGIN LOG ====
> bridge0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
>          options=10<VLAN_HWTAGGING>
>          ether 58:9c:fc:10:d7:7e
>          inet 192.168.1.251 netmask 0xfffff000 broadcast 192.168.15.255
>          inet6 fe80::5a9c:fcff:fe10:d77e%bridge0 prefixlen 64 scopeid 0x3
>          inet6 2001:470:4001:1:5a9c:fcff:fe10:d77e prefixlen 64 autoconf pltime 14400 vltime 86400
>          inet6 2001:470:4001:1:c001:f868:c587:cdd7 prefixlen 64 deprecated autoconf temporary pltime 0 vltime 44033
>          inet6 2001:470:4001:1:c139:85be:79b3:e3ec prefixlen 64 autoconf temporary pltime 12610 vltime 86400
>          id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
>          maxage 20 holdcnt 6 proto rstp maxaddr 2000 timeout 1200
>          root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0
>          bridge flags=0<>
> ==== END LOG ====
> 
>  From what I understand now, the only thing that would change is the
> 2001:470:4001:1:5a9c:fcff:fe10:d77e address. Instead of incorporating
> the MAC address in that IP address, it would be the stableaddr
> address.
> 
> Amy I understanding that correctly?

You are correct.

AFAIK the relevant RFCs implemented here  were studied to be compatible 
with one another.



To give some details:

The net.inet6.ip6.use_stableaddr sysctl changes the algorithm that 
incorporates the MAC address in the IPv6 address with one deriving the 
IPv6 address with an hash (sha256 HMAC) of the concatenation of various 
sources, as described in RFC 7217, specifically:

- the network prefix

- MAC address, interface name or interface id (configurable via 
net.inet6.ip6.stableaddr_netifsource, default uses MAC address)

- hostid (this is a UUID, constant on the machine)

- a counter, usually 0, incremented if there are DAD conflicts (another 
host with same address is detected on the network, counter incremented 
and a new address is checked, by default for 3 times, configurable via 
net.inet6.ip6.use_stableaddr)

- we use an additional counter to cater for the very rare case the 
algorithm should generate an invalid address, in such a case the counter 
is incremented and another address generated and verified.


-- 
Guido Falsi <madpilot@FreeBSD.org>