[Bug 236105] carp MASTER/MASTER status when adding or removing carp IP addresses

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236105

            Bug ID: 236105
           Summary: carp MASTER/MASTER status when adding or removing carp
                    IP addresses
           Product: Base System
           Version: 11.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: vegeta at tuxpowered.net

I'm not sure how much of a bug this is, rather a feature request for special
cases. 

carp HMAC is calculated using a hash of carp IP addresses for a given VHID.
This means that if two routes are configured with different carp IP addresses,
verifying signatures will fail and routers will ignore carp annoucements from
eachother resulting in MASTER/MASTER operation.

However I understand this is a feature for enforcing configuration check, there
are cases when this is a bad idea: on some routers I have carp addresses
reconfigured tens of times a day. As it is impossible to configure both routers
exactly at the same time, there is often a difference big enough to triger
MASTER/MASTER operation which is then soon resolved. I value network stability
more than configuration check enforced on me and if configuration was wrong,
things s would break anyway and I would be notified by other means.

Please see
https://github.com/innogames/freebsd/commit/a1b1ff410c0f532b67660c86593ef7bda2398be8
for my first attempt. It still lacks sysctl value set hook, so works fine only
after carp addresses are configured. If you like the idea and believe it could
be merged into kernel, I will add a set hook recalculating HMAC and post the
patch here.

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