Overwrite link-local address with my own ULA

[Hoeze]

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Is your feature request related to a problem? Please describe.

On my server, there are multiple interfaces using fe80:: prefixes (e.g. podman).
I tried to solve that issue by setting a static IPv6 configuration with my own private ULA (fdb0:[...]).
However, I noticed that now I simply get an additional IPv6 address:

    inet6 fdb0:[...]/64 scope global dynamic noprefixroute 
       valid_lft 86355sec preferred_lft 14355sec
    inet6 fe80::[...]/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

Describe the solution you like

Is there a way to replace the fe80:: address with my own ULA?
Or would it be possible to disable the fe80:: address and set the fdb0:[...] as default gateway?

Describe alternatives you considered

Additional context

[Hoeze]

Actually the following issue sounds quite similar to mine:
#817

[AdSchellevis]

I wouldn't know why you would like to remove the default link-local (it's likely not a good idea), but my best guess is setting the tunable net.inet6.ip6.auto_linklocal to 0 stops assigning one automatically. See also https://man.freebsd.org/cgi/man.cgi?ifconfig(8)

[fichtner]

In practice this will likely lead to defunct IPv6 unless you add static valid link-locals to every interface that should speak IPv6.

[Hoeze]

What I want to reach is to get from the current state:

    inet6 fdb0:[...]/64 scope global dynamic noprefixroute 
       valid_lft 86355sec preferred_lft 14355sec
    inet6 fe80::[...]/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

to this state:

    inet6 fdb0:[...]/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

so I can avoid having overlapping link-local addresses.

[Monviech]

Link local addresses are local to their link.

Each interface can have fe80::1 and it does not matter since the full link local address is fe80::1%interface for the operating system.

Do not mess with link local if you do not want IPv6 to break.