[Bug 194672] New: [carp] Changing advskew to 0 from another value doesn't work

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194672

            Bug ID: 194672
           Summary: [carp] Changing advskew to 0 from another value
                    doesn't work
           Product: Base System
           Version: 10.1-RC2
          Hardware: Any
                OS: Any
            Status: Needs Triage
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: cmb at pfsense.org

When you configure advskew in CARP to be something other than 0, then try to
change it back to 0, FreeBSD refuses to do so. It can be changed to any value
other than 0, but never back to 0. This is a semi-common scenario, as it's how
people would generally demote a system from master to backup for maintenance or
other purposes, and would generally want to set it back to 0 afterwards. 

Initially setting it to 0 works fine. 

Tested on 10.0 release and 10.1-RC3, both behave the same. The following shows
the issue and how to replicate. 

# ifconfig em0 vhid 55 advskew 0 pass PASSWORD alias 192.168.124.222/24
# ifconfig em0 
em0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu
1500
        options=9b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM>
        ether 00:0c:29:11:5e:be
        inet 192.168.124.132 netmask 0xffffff00 broadcast 192.168.124.255
        inet 192.168.124.222 netmask 0xffffff00 broadcast 192.168.124.255 vhid
55
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
        media: Ethernet autoselect (1000baseT <full-duplex>)
        status: active
        carp: MASTER vhid 55 advbase 1 advskew 0

Change it, it'll work. 
# ifconfig em0 vhid 55 advskew 100
# ifconfig em0 | grep carp:
        carp: MASTER vhid 55 advbase 1 advskew 100

Try changing it back to 0, and it won't. 
# ifconfig em0 vhid 55 advskew 0
# ifconfig em0 | grep carp:
        carp: MASTER vhid 55 advbase 1 advskew 100

But can change it to 1 or other values. 
# ifconfig em0 vhid 55 advskew 1
# ifconfig em0 | grep carp:
        carp: MASTER vhid 55 advbase 1 advskew 1

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