[Bug 218508] Tunneling and aliases using the tun device, reusing a destination address works with IPv4, but not IPv6

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218508

            Bug ID: 218508
           Summary: Tunneling and aliases using the tun device, reusing a
                    destination address works with IPv4, but not IPv6
           Product: Base System
           Version: 11.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: reezer at reezer.org

I can create a tun device by using:

ifconfig tun0 create
ifconfig tun 10.1.2.0 196.2.3.4 
ifconfig tun alias 196.2.3.3 196.2.3.4
ifconfig up

This works great and allows me to use the 10.1.2.0 and have it tunneled over
the second (aliased) tunnel when I set up the correct route using /sbin/route
add -net 10.0.0.0/8 -iface tun0.

I have this setup, using IPv6 only working as it is supposed to. To explain
what I am am actually achieving with this: In the background I am using BGP
over a one-to-one VPN software using the tunnel interface.

Doing the same thing on IPv6, however doesn't work when I add an alias, using
the same destination address twice:

ifconfig tun0 inet6 fdaa:abc:abc:abc::1 fe80::111:0 prefixlen /128
ifconfig tun0 inet6 alias fe80::111:1 fe80::111:0 prefixlen /128
ifconfig: ioctl (SIOCAIFADDR): File exists

Using another destination address this works however. Since this works on IPv4
I'd expect it to work here as well.

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