Dhclient and Virtual Router IP changes

[Nick Wales <ni...@nickwales.co.uk>]

Does anyone have advice for how to deal with linux hosts when the virtual
router gets destroyed and comes back on a different IP address?

I'm no dhclient expert, having run everything in a traditional datacenter
up until now with static IP's so looking for some useful advice / best
practise ideas here!

My experience so far is that guests no longer know their fully qualified
hostname which can cause problems with agent software that has to check in
with an SSL cert like puppet or a known name like teamcity.

In theory something that caused dhclient to re-check for newly active dhcp
servers on a regular basis would do the job for me.

Nick

Re: Dhclient and Virtual Router IP changes

[Kirk Kosinski <ki...@gmail.com>]

Hi, for isolated networks this shouldn't be a problem.  A recreated
virtual router will keep the same IP on the guest network (10.1.1.1 by
default).  For shared networks or for the network in a basic zone, I
guess this might be a problem.

dhclient and other DHCP clients will automatically try to renew their
DHCP based on the lease time.  However, CloudStack assigns clients
infinite lease times.  This causes dhclient to use a lease of 4294967295
seconds, and not try to renew the lease until about half that time, as
seen in the log:

Jul 16 18:37:58 web11 dhclient[5276]: bound to 10.1.1.152 -- renewal in
2147483646 seconds.

Luckily, dhclient is very configurable.  You can force it to use a lease
or renewal time of your choice.  For example:

[root@web11 ~]# cat /etc/dhcp/dhclient.conf
interface "eth0" {
   supersede dhcp-renewal-time = encode-int(600, 32);
}

Result:

Jul 16 18:39:45 web11 dhclient[5914]: bound to 10.1.1.152 -- renewal in
544 seconds.

Best regards,
Kirk


On 07/16/2013 12:54 PM, Nick Wales wrote:
> Does anyone have advice for how to deal with linux hosts when the virtual
> router gets destroyed and comes back on a different IP address?
> 
> I'm no dhclient expert, having run everything in a traditional datacenter
> up until now with static IP's so looking for some useful advice / best
> practise ideas here!
> 
> My experience so far is that guests no longer know their fully qualified
> hostname which can cause problems with agent software that has to check in
> with an SSL cert like puppet or a known name like teamcity.
> 
> In theory something that caused dhclient to re-check for newly active dhcp
> servers on a regular basis would do the job for me.
> 
> Nick
>