re-add $ipv6_enable support for backward compatibility. From
UPDATING:
1. To use IPv6, simply define $ifconfig_IF_ipv6 like $ifconfig_IF
for IPv4. For aliases, $ifconfig_IF_aliasN should be used.
Note that both variables need the "inet6" keyword at the head.
Do not set $ipv6_network_interfaces manually if you do not
understand what you are doing. It is not needed in most cases.
$ipv6_ifconfig_IF and $ipv6_ifconfig_IF_aliasN still work, but
they are obsolete.
2. $ipv6_enable is obsolete. Use $ipv6_prefer and/or
"inet6 accept_rtadv" keyword in ifconfig(8) instead.
If you define $ipv6_enable=YES, it means $ipv6_prefer=YES and
all configured interfaces have "inet6 accept_rtadv" in the
$ifconfig_IF_ipv6. These are for backward compatibility.
3. A new variable $ipv6_prefer has been added. If NO, IPv6
functionality of interfaces with no corresponding
$ifconfig_IF_ipv6 is disabled by using "inet6 ifdisabled" flag,
and the default address selection policy of ip6addrctl(8)
is the IPv4-preferred one (see rc.d/ip6addrctl for more details).
Note that if you want to configure IPv6 functionality on the
disabled interfaces after boot, first you need to clear the flag by
using ifconfig(8) like:
ifconfig em0 inet6 -ifdisabled
If YES, the default address selection policy is set as
IPv6-preferred.
The default value of $ipv6_prefer is NO.
4. If your system need to receive Router Advertisement messages,
define "inet6 accept_rtadv" in $ifconfig_IF_ipv6. The rc(8)
scripts automatically invoke rtsol(8) when the interface becomes
UP. The Router Advertisement messages are used for SLAAC
(State-Less Address AutoConfiguration).
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