Subject: Re: [RFC PATCH] net: ipv6: Make address flushing on ifdown optional

From: Stephen Hemminger <[email-masked]>
Date: Thu, 22 Jan 2015 22:40:33 -0800

On Wed, 14 Jan 2015 12:17:19 -0700
David Ahern <[email-masked]> wrote:

> Currently, ipv6 addresses are flushed when the interface is configured down:
> 
> [root@f20 ~]# ip -6 addr add dev eth1 2000:11:1:1::1/64
> [root@f20 ~]# ip addr show dev eth1
> 3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
>     link/ether 02:04:11:22:33:01 brd ff:ff:ff:ff:ff:ff
>     inet6 2000:11:1:1::1/64 scope global tentative
>        valid_lft forever preferred_lft forever
> [root@f20 ~]# ip link set dev eth1 up
> [root@f20 ~]# ip link set dev eth1 down
> [root@f20 ~]# ip addr show dev eth1
> 3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
>     link/ether 02:04:11:22:33:01 brd ff:ff:ff:ff:ff:ff
> 
> Add a new sysctl to make this behavior optional. Setting defaults to flush
> addresses to maintain backwards compatibility. When reset flushing is bypassed:
> 
> [root@f20 ~]# echo 0 > /proc/sys/net/ipv6/conf/eth1/flush_addr_on_down
> [root@f20 ~]# ip -6 addr add dev eth1 2000:11:1:1::1/64
> [root@f20 ~]# ip addr show dev eth1
> 3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
>     link/ether 02:04:11:22:33:01 brd ff:ff:ff:ff:ff:ff
>     inet6 2000:11:1:1::1/64 scope global tentative
>        valid_lft forever preferred_lft forever
> [root@f20 ~]#  ip link set dev eth1 up
> [root@f20 ~]#  ip link set dev eth1 down
> [root@f20 ~]# ip addr show dev eth1
> 3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
>     link/ether 02:04:11:22:33:01 brd ff:ff:ff:ff:ff:ff
>     inet6 2000:11:1:1::1/64 scope global
>        valid_lft forever preferred_lft forever
>     inet6 fe80::4:11ff:fe22:3301/64 scope link
>        valid_lft forever preferred_lft forever
> 
> Suggested-by: Hannes Frederic Sowa <[email-masked]>
> Signed-off-by: David Ahern <[email-masked]>
> Cc: Hannes Frederic Sowa <[email-masked]>

Would this break existing application expecting a particular semantic
by listening to netlink?  What happens to packets received with the static
address when interface is down? With IPv4 Linux is mostly a weak host
model, and IPv6 somewhere in between.

For vendors that control the application stack or have limited number
of services this would work fine, but what about RHEL?