Re: [PATCH] net: ipv6: don't generate link-local address in any addr_gen_mode

This is all going in the wrong direction. Link-local addresses are not
optional on an interface, all IPv6 enabled interfaces are required to
have one:

RFC4291, "IP Version 6 Addressing Architecture"

"2.1.  Addressing Model

All interfaces are required to have at least one Link-Local unicast
   address (see Section 2.8 for additional required addresses)."

Regards,
Mark.



On Fri, 10 Sept 2021 at 05:13, David Ahern [off-list ref] wrote:

On 9/9/21 12:20 AM, Lorenzo Colitti wrote:

quoted

quoted

I think another addr_gen_mode is better than a separate sysctl. It looks
like IN6_ADDR_GEN_MODE_STABLE_PRIVACY and IN6_ADDR_GEN_MODE_RANDOM are
the ones used for RAs, so add something like:

IN6_ADDR_GEN_MODE_STABLE_PRIVACY_NO_LLA,
IN6_ADDR_GEN_MODE_RANDOM_NO_LLA,

I think the real requirement here (which wasn't clear in this thread)
is that the network needs to control the interface ID (i.e., the
bottom 64 bits) of the link-local address, but the device is free to
use whatever interface IDs to form global addresses. See:
https://www.etsi.org/deliver/etsi_ts/129000_129099/129061/15.03.00_60/ts_129061v150300p.pdf

How do you think that would best be implemented?

There is an established paradigm for configuring how an IPv6 address is
created or whether it is created at all - the IFLA_INET6_ADDR_GEN_MODE
attribute.

quoted

1. The actual interface ID could be passed in using IFLA_INET6_TOKEN,
but there is only one token, so that would cause all future addresses
to use the token, disabling things like privacy addresses (bad).
2. We could add new IN6_ADDR_GEN_MODE_STABLE_PRIVACY_LL_TOKEN,
IN6_ADDR_GEN_MODE_RANDOM_LL_TOKEN, etc., but we'd need to add one such
mode for every new mode we add.
3. We could add a separate sysctl for the link-local address, but you
said that per-device sysctls aren't free.

per-device sysctl's are one of primary causes of per netdev memory usage.

Besides that there is no reason to add complexity by having a link
attribute and a sysctl for this feature.

quoted

4. We could change the behaviour so that if the user configures a
token and then sets IN6_ADDR_GEN_MODE_*, then we use the token only
for the link-local address. But that would impact backwards
compatibility.

Thoughts?

We can have up to 255 ADDR_GEN_MODEs (GEN_MODE is a u8). There is
established code for handling the attribute and changes to it. Let's
reuse it.