[Bug 241529] ifconfig ... alias behaves in non-POLA manner.

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241529

            Bug ID: 241529
           Summary: ifconfig ... alias behaves in non-POLA manner.
           Product: Base System
           Version: 11.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: dgilbert at eicat.ca

This may be related to PR 228929, but it also may not.

consider this sequence of events:

ifconfig lo0 alias 192.168.1.1/32
ifconfig lo0 192.168.1.2/32
ifconfig lo0 192.168.1.3/32

... now if lo0 is configured as per normal, the 2nd command will delete the
address 127.0.0.1 (sorta expected) and the 3rd command will delete 192.168.1.1.
 In general, I would characterize the actual operation of the "alias" word to
ifconfig as follows.

ifconfig always adds the address to the bottom of the list of addresses. 
Without ALIAS, ifconfig also subtracts the top address.

That is, to be clear.  Using alias means there is +1 ip addresses, not using
alias means that there is the same number of addresses.  The top address is
always the one to go away and the new address is added to the bottom.  AFAICT,
this behavior is not described anywhere.

Additionally, as a doc bug, what ifconfig's man page says about alias --- about
a non-conflicting netmask --- is no longer true.

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