Re: How to force DHCP renew?

On Wed, 2017-10-04 at 11:07 -0700, Stephen Hemminger wrote:
On Azure (and probably other VM environments), there is a desire to
cause guest VM to renew DHCP
lease in response to host infrastructure changes. These include
things like spinning up
lots of VM's then suspending them and later unfreezing them as
needed. The other scenario
is doing renew following migration.

The current kernel code has a hack to try and simulate link bounce,
but it doesn't
work very well and the old ISC DHCP client does nothing. Network
manager might be
able to do something better??


link bounce might not work with NetworkManager, in the current form.

NM has a 5 seconds grace period, during which it ignores carrier going
away. It does so because the link can go down unexpectedly for unknown
Actually, after NM changes the MTU, the 5 seconds are extended to 10
seconds, because [1].
So, kernel would have to bounce the link for at least 10 seconds.

Then there is a setting ignore-carrier (`man NetworkManager.conf`).
That causes NM to totally ignore when the link goes down. That setting
is quite common, for example on the package "NetworkManager-config-
server" enables it by default on RHEL.

Appart from the above obstacles, carrier going away causes the active
connection to disconnect. Later, another (or the same) connection may
autoconnect again. So, that sounds like what you requested, but I don't
think it works well.



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