[mananaysiempre]

It’s not exclusively an IPv6 feature: RFC 3927 defines link-local IPv4 addresses, to be assigned randomly from 169.254.0.0/16 after a bit of ceremony to detect collisions.

Ideally, you’d be able to connect a PC and a printer with an Ethernet cable, they would both (having failed to find a better alternative) allocate a link-local address for themselves, and then the PC would use DNS-SD over mDNS to discover the printer and show it to you. Similar story with PCs exporting their media files over the network, a—say—set-top box, and a switch they’re all plugged into.

And for some combinations of parts this actually works. It’s just that the functionality is not always well-exposed by the OS, that a switch + DHCP server in a box (in practice, a consumer router) can work just as well with no configuration as an unmanaged switch can, and that people are not that interested in local-only wired networks anymore.

There’s also the “having failed to find a better alternative” part: unlike with IPv6, the RFC does not endorse always assigning a link-local address as the second one next to a static or DHCP-provided one, I’m guessing for software compatibility. Thus you really only see 169.254.* in your interface configuration when DHCP is borked, and it’s kind of useless in that case.

[dcrazy]

The IPv6 feature isn’t link-local addresses, it’s being able to specify the interface to bind to as part of the address specification. This lets you demand that your IPv6-based tool use your wired Ethernet connection, for example.

[AshamedCaptain]

You cannot use zones for global addresses, so zones are indeed mostly a feature of link-local addresses only.

[dcow]

That is not a design goal of IPv6. It’s a terrible leak in the abstraction.

[dcrazy]

How is it not a design goal? Why else would this syntax have been invented?

[dcow]

You said:

> The IPv6 feature isn’t link-local addresses, it’s being able to specify the interface to bind to as part of the address specification. This lets you demand that your IPv6-based tool use your wired Ethernet connection, for example.

I don’t think a design goal of IPv6 was to enable users to demand which link the kernel uses or open up some rich new world of link local ip services. I think it’s more like “because IPv6 hosts assign everything an address all at once, there’s a new problem when two interfaces use the same link local addresses since we can’t guarantee uniqueness so we have to invent this stupid zone_id convention to work around poor protocol design and implementation”. Design goals are different than constraints.

[sedatk]

It looks like a practical solution for a rare but unsolved problem to me. What do you think a better solution would look like?

[dcow]

Have hosts put a guid in its LL address and print that on the sticker next to the mac address… idk. Actually it’d be nice if we only did link-exclusive things at the link layer, eh?

[themafia]

> to be assigned randomly from 169.254.0.0/16

Yes, but the question is, "what if an address in this range is assigned to _two_ interfaces at the same time?" Now your local routing information base cannot distinguish which interface to use when trying to reach other hosts in that same network. So, it's fair to say, it's not a feature even available in IPv4.

The second difference is IPv6 is almost always going to have link local addresses assigned and machines with multiple network interfaces are the norm rather than the exception.

[xp84]

I literally had to interrogate an LLM to explain what this was about, because to me, indeed, when I see 169.254 I think "Ah, someone unplugged something critical and the network is now completely down." I didn't even know that in ipv6 land there are any reasons to use link-local addresses for anything. I mean, there still basically isn't a reason for 99.99% of people, I think. But it's interesting.

I also didn't realize that part of the idea behind these LL things was one of the rounds of wishful networking ideas of the 90s or 2000s, kind of a cousin of UPnP and mDNS in that way (in increasing order of eventual usefulness).

Considered completely in a vacuum, especially ignoring the WAN, I can see how it seemed silly that if you plugged three computers and a printer into a switch, rolling random IP addresses like this could have allowed things to be discoverable and to function locally (I thought mDNS or "Bonjour"/"Rendezvous" as Apple called it came much later, but I know my PCs could "see" each other with NetBIOS or whatever long before mDNS was invented).

[RiverCrochet]

Link-local addresses (LLAs) are needed in IPv6 because IPv6 doesn't have broadcast. IPv6 uses multicast instead.

Broadcasts go to all IPv4 addresses in the subnet, multicasts only go to those who subscribed to a multicast group. To subscribe to a IPv6 multicast group you need an IPv6 address. So all IPv6 interfaces will have at least one LLA self-generated.

One thing that IPv6 uses multicast heavily for is NDP, which is the IPv6 version of ARP. This is how IP addresses on your LAN/WLAN are converted to MAC addresses which is required info for the NIC in your node to talk to another node on your Ethernet LAN/WLAN.

End users don't typically have to use LLAs directly but you can use them if you want to 100% ensure things won't leave your LAN as routers don't forward LLAs.

[Dagger2]

mDNS on link-locals is what makes the "plug computers and printers into switch" case work. It would have been NetBIOS originally but mDNS is how it's done today.