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> I don't think I've ever seen two computers communicate over a 169.254 address. I have. Used to do it often in the dorms when the uni's DHCP server was on the

> I don't think I've ever seen two computers communicate over a 169.254 address. I have. Used to do it often in the dorms when the uni's DHCP server was on the fritz. Zeroconf/Avahi wasn't really a thing, so we'd use Network Neighborhood to get people's IP addresses to browse files or play on servers they were hosting or whatever. I have also (as the RFC envisions) done it when connected to ad-hoc WiFi networks, and also while directly connecting two Windows PCs. In those scenarios, I asked my peer what their IP address was, rather than bothering with going to Network Neighborhood. > Also, OS network stacks normally don't even try to get a link-local IPv4 address if they can get to a DHCP server or have a static address assigned. Correct. From the second paragraph of the Abstract of RFC 3927: [0] > IPv4 Link-Local addresses are not suitable for communication with devices not directly connected to the same physical (or logical) link, and are only used where stable, routable addresses are not available (such as on ad hoc or isolated networks). > The RFC even recommends not using the link-local address if any other address is available. It's quite different from the way IPv6 works (as usual). No, it's also recommended when using IPv6 to not use link-local addresses if any other address is available. (Both because those addresses are only link-local, and because one usually needs to put an outbound interface specifier on the address when you go to contact another host, which is pretty obnoxious.) It just happens that IPv6's address autoconfiguration has been upgraded to be useful for globally-routable addressing, too. So, yeah, a usual mode of both IPv4 and IPv6 networks is to have autoconfigured addresses, as well as DHCP-assigned addresses. It just so happens that most of the time IPv4 addresses are not autoconfigured, and most of the time IPv6 addresses are not assigned by DHCP. That's equivalent complexity. [0] <https://www.rfc-editor.org/rfc/rfc3927 https://www.rfc-editor.org/rfc/rfc3927>

tsimionescu 2y ago
> No, it's also recommended when using IPv6 to not use link-local addresses if any other address is available. (Both because those addresses are only link-local, and because one usually needs to put an outbound interface specifier on the address when you go to contact another host, which is pretty obnoxious.) It just happens that IPv6's address autoconfiguration has been upgraded to be useful for globally-routable addressing, too. With what seems to be the recommended IPv6 setup at least for client machines, the Link-local address is the only stable identifier of that machine, as the actual reputable address changes all the time. So, the machine has both a link-local and a routable IPv6 at the same time, and both are going to be used. This is still different from IPv4, where link-local addresses are, again, an obscure feature that almost no one uses.
- deleted 2y ago
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- simoncion 2y ago
  > With what seems to be the recommended IPv6 setup at least for client machines, the Link-local address is the only stable identifier of that machine... What? ULA space is the IPv6 equivalent of RFC 1918 space. If you want a prefix you know will not change, then you go generate you a /48 and use that on your internal network(s). Why on earth would people expect ISPs that have a "We won't give you a guaranteed-static IPv4 address." policy to have a "We will give you a guaranteed-static IPv6 prefix" policy? Thinking that way is madness. Perhaps you're confused because you're looking at guidance for client machines, when you should be looking at guidance for customer edge routers? Check out RFC 7084, and in particular section 3 and its subsections, and section 4.3 and its subsections. [0] > ...as the actual reputable address changes all the time Are you talking about IPv6 address randomization? (AKA "privacy" addresses?) IF you are, then know that it's a) Controlled by the operator of the computer, rather than the operator of the network it's connected to. b) Optional, and SHOULD be able to be turned off by the computer operator. c) RECOMMENDED that a stable address be generated for the machine, in addition to the random ones. Thing c) is in part to deal with "But what if I want to put a AAAA into the local DNS?" problem, and in part to give expected-to-be-long-lived connections an address that the system knows will hang around for at least as long as the interface is up. After all, unless you want to tear down the connections associated with it, you can't remove an address from an interface until those connections have closed. More addresses allocated to a host means more multicast groups joined for that host, which eats up resources on the local networking infrastructure... so (if you're using "privacy" addresses) you really, really wanna steer ephemeral connections to addresses that are temporary, and long-lived ones to ones that are not. [0] <https://datatracker.ietf.org/doc/html/rfc7084 https://datatracker.ietf.org/doc/html/rfc7084>

tsimionescu 2y ago

> No, it's also recommended when using IPv6 to not use link-local addresses if any other address is available. (Both because those addresses are only link-local, and because one usually needs to put an outbound interface specifier on the address when you go to contact another host, which is pretty obnoxious.) It just happens that IPv6's address autoconfiguration has been upgraded to be useful for globally-routable addressing, too. With what seems to be the recommended IPv6 setup at least for client machines, the Link-local address is the only stable identifier of that machine, as the actual reputable address changes all the time. So, the machine has both a link-local and a routable IPv6 at the same time, and both are going to be used. This is still different from IPv4, where link-local addresses are, again, an obscure feature that almost no one uses.

deleted 2y ago
[deleted]
simoncion 2y ago
> With what seems to be the recommended IPv6 setup at least for client machines, the Link-local address is the only stable identifier of that machine... What? ULA space is the IPv6 equivalent of RFC 1918 space. If you want a prefix you know will not change, then you go generate you a /48 and use that on your internal network(s). Why on earth would people expect ISPs that have a "We won't give you a guaranteed-static IPv4 address." policy to have a "We will give you a guaranteed-static IPv6 prefix" policy? Thinking that way is madness. Perhaps you're confused because you're looking at guidance for client machines, when you should be looking at guidance for customer edge routers? Check out RFC 7084, and in particular section 3 and its subsections, and section 4.3 and its subsections. [0] > ...as the actual reputable address changes all the time Are you talking about IPv6 address randomization? (AKA "privacy" addresses?) IF you are, then know that it's a) Controlled by the operator of the computer, rather than the operator of the network it's connected to. b) Optional, and SHOULD be able to be turned off by the computer operator. c) RECOMMENDED that a stable address be generated for the machine, in addition to the random ones. Thing c) is in part to deal with "But what if I want to put a AAAA into the local DNS?" problem, and in part to give expected-to-be-long-lived connections an address that the system knows will hang around for at least as long as the interface is up. After all, unless you want to tear down the connections associated with it, you can't remove an address from an interface until those connections have closed. More addresses allocated to a host means more multicast groups joined for that host, which eats up resources on the local networking infrastructure... so (if you're using "privacy" addresses) you really, really wanna steer ephemeral connections to addresses that are temporary, and long-lived ones to ones that are not. [0] <https://datatracker.ietf.org/doc/html/rfc7084 https://datatracker.ietf.org/doc/html/rfc7084>