Anyone understand dhcpd and dynamic dns?

[Phil Meyer]

On 01/20/2010 10:26 AM, Tom Horsley wrote:
> Is this wacko time keeping in the virtual guest making dhcpclient expire the
> lease sooner than the dhcp server can renew it?
>    


I can speak somewhat to the time issues in VMs.

In both Xen and KVM based VMs which I have personally installed MANY of, 
the VM clock can be a real issue.

It is somewhat complicated, but simply put, when the host time-slices 
the VMs, they lose clock ticks.  On decent systems, ntpd can keep up, 
but not if the clock is way off when the system boots.  If the host 
time-slices at just the wrong moment in the VM startup, the clock 
defaults to a consistent offset from GMT.  For example, our VMs here are 
almost always GMT -1 when they should be GMT -6.  Anyways, always 5 
hours off.  Without ntpd running, that clock can vary an hour or two in 
a week or so.

Fedora, Centos, and RHE all have the initial ntpdate turned off by 
default when ntpd starts.  For VMs, this is critical.  Turn it back on 
simply by editing /etc/sysconfig/ntpd (add -x to OPTIONS).

The other thing that we do with VMs is turn off prelinking.  Having 
100VMs per server doing prelink every day is just short of ridiculous.

Here is a snippit from our kickstart %post section.

Good Luck!

...
touch /var/lock/makewhatis.lock
chattr +i /var/lock/makewhatis.lock
egrep '^OPTIONS.*-x' /etc/sysconfig/ntpd > /dev/null 2>&1 || \
sed -ir 's/^OPTIONS=\"/OPTIONS=\"-x /' /etc/sysconfig/ntpd
sed -ir 's/^PRELINKING=yes/PRELINKING=no/' /etc/sysconfig/prelink
...