[Bug 214746] Carp ipv6 duplicate address detection

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214746

            Bug ID: 214746
           Summary: Carp ipv6 duplicate address detection
           Product: Base System
           Version: 10.3-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: aduitsis at gmail.com
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

Somewhat difficult to ascertain whether this has been previously reported,
apologies if this is a duplicate bug.

Trying to use carp and IPv6 on a 10.3-RELEASE. Two machines, each one with its
own IPv6 address and one common IPv6. Fairly simple.

When a machine boots (the other one is obviously MASTER and has acquired the
common IPv6 address), complaints about duplicate address detection of the
common IPv6 are logged by the booting machine's kernel.

kernel: vmx0: DAD detected duplicate IPv6 address <commonipv6address>: NS
in/out/loopback=0/1/0, NA in=1
kernel: vmx0: DAD complete for <commonipv6address> - duplicate found

As a result, the machine that has just booted has the duplicated flag for that
address in ifconfig and services that want to bind to that common address fail
to start. In other words, the BACKUP cannot bind to the common address.

Trying to setup the common address and carp by hand using ifconfig initially
fails with:

#ifconfig vmx0 inet6 <commonipv6address>/64 vhid 34 pass <my_pass>
ifconfig: ioctl (SIOCAIFADDR): No such file or directory

What's curious, second time that command is issued, no errors are printed and
the address is assigned to the interface.

Using the no_dad ifconfig flag seems to solve the problem, but I suspect that
this workaround is not extremely good as it disables DAD.

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