Re: question: Any way to trigger DHCP renewal from kernel code?

[Dan Williams <dcbw@redhat.com>]

On Thu, 2014-05-22 at 22:24 +0000, Haiyang Zhang wrote:
> Hi,
> 
> Our synthetic network driver is notified when the Hyper-V host 
> resume from sleep/hibernation. We need to renew DHCP in this case.
> I'm looking for a way to trigger DHCP renewal from kernel mode 
> code.

Another approach could be to change the 'operstate' of your synthetic
netdevice to IF_OPER_DORMANT and then back to IF_OPER_UP.  Possibly like
drivers/net/hsr/hsr_device.c does in __hsr_set_operstate().

The netdev_state_change() call will send out a netlink message that the
device has changed, which includes the device flags.  Before changing
the operstate, the flags will include IFF_RUNNING, which indicates the
netdevice is operating and passing traffic.  When you change the
operstate to IF_OPER_DORMANT, a netlink message will be sent which does
*not* include IFF_RUNNING.  Then changing back to IF_OPER_UP will emit a
netlink message that *does* include IFF_RUNNING again.

The DHCP client can listen for device flag changes and trigger a DHCP
renew when the IFF_RUNNING flag re-appears on the device.  There are
some notes about this in Documentation/networking/operstates.txt too.

(Random note: long ago when WiMAX was relevant there was also a need to
trigger a DHCP renew in userspace based on events from the driver; a
renew should be performed when the device comes out of fast sleep, which
can happen quite often.  I don't remember how Inaky solved this, but I
think it was out-of-band driver messages to the userspace control
daemon.  A more standardized kernel facility somewhat less convoluted
than manually munging operstates would have been nice...)

Dan