Re: Bridge stays down until a port is added

On Mon, 26 Sep 2011 22:02:21 +0200
Nicolas de Pesloüan [off-list ref] wrote:

Le 26/09/2011 17:47, Stephen Hemminger a écrit :

quoted

The root of this whole problem is really that IPv6 reports addresses
in a tentative state to applications that can not be passed to the bind() system call.
For most cases, this problem never happens because the tentative addresses are
resolved by Duplicate Address Detection before the application starts. But
I have seen (and fixed) this happen before this whole discussion started.

1. The problem is not unique to bridges. It happens with bridge, macvtap,
    even on wireless networks where the device is available but carrier is
    not asserted.

2. Any change to what the kernel does (like not reporting tentative addresses)
    would break applications even worse.

3. When the bridge was always reporting carrier, it was in effect breaking
    IPv6 Duplicate Address Detection. And that is bad.

Stephen,

What do you think about a generic per-interface option that would cause bind() to accept tentative 
address hold by a particular interface? This of course violate IPv6 principle, but we are talking 
about interfaces that are unable to do DAD, either permanently or until something happens on the 
underlying device.

echo 1 > /sys/class/net/br0/allow_bind_on_tentative_address
echo 1 > /sys/class/net/dummy0/allow_bind_on_tentative_address
echo 1 > /sys/class/net/wlan0/allow_bind_on_tentative_address
and so on...

And we may possibly automatically reset this option to 0 if DAD eventually causes the address to be 
considered duplicate.

The issue is that if DAD rejects a duplicate, the socket is dead and application is
out of luck.

Has anyone looked at this issue in relation to systemd which does early
opportunistic binding of services?