How to get ip address automatically from different dhcp server

[dave jones]

On Sat, Aug 13, 2011 at 2:32 AM, Christopher J. Ruwe  wrote:
> On Fri, 12 Aug 2011 11:46:34 +0800
> dave jones <s.dave.jones at gmail.com> wrote:
>
> I rearrange your mail and post bottom to enable others to have a look.
>
>>On Thu, Jul 28, 2011 at 4:30 AM, Christopher J. Ruwe  wrote:
>>> On Wed, 27 Jul 2011 10:49:42 +0800
>>> dave jones <s.dave.jones at gmail.com> wrote:
>>>
>>>> Hi,
>>>>
>>>> I cat get an IP address from dhcp server by adding the line
>>>> in /etc/rc.conf:
>>>>
>>>> ifconfig_em0="DHCP"
>>>>
>>>> If I move my laptop to another place, I have to manually run
>>>> "dhclient em0" to get an IP. Otherwise, it won't get an IP
>>>> automatically.
>>>>
>>>> My question is it's possible to get ip address automatically from
>>>> different dhcp server? thanks.
>>>>
>>>> Regards,
>>>> Dave.
>>>> _______________________________________________
>>>> freebsd-questions at freebsd.org mailing list
>>>> http://lists.freebsd.org/mailman/listinfo/freebsd-questions
>>>> To unsubscribe, send any mail to
>>>> "freebsd-questions-unsubscribe at freebsd.org"
>>>
>>> Have a look at /etc/devd.conf. Mine include a portion
>>>
>>> #
>>> # Try to start dhclient on Ethernet like interfaces when the link comes
>>> # up.  Only devices that are configured to support DHCP will actually
>>> # run it.  No link down rule exists because dhclient automaticly exits
>>> # when the link goes down.
>>> #
>>> notify 0 {
>>>        match "system"          "IFNET";
>>>        match "type"            "LINK_UP";
>>>        media-type              "ethernet";
>>>        action "/etc/rc.d/dhclient quietstart $subsystem";
>>> };
>>> #
>>> notify 0 {
>>>        match "system"          "IFNET";
>>>        match "type"            "LINK_DOWN";
>>>        media-type              "ethernet";
>>>        action "/etc/rc.d/dhclient quietstop $subsystem ; ifconfig $subsystem inet 0.0.0.0";
>>> };
>>>
>>> I am under the impression that this rule does what you want to do.
>>>
>>> Cheers,
>>> --
>>> Christopher J. Ruwe
>>> TZ GMT + 2
>>>
>
>> Hi Christopher,
>>
>> Thanks for your solution. It does help a lot, but there's one problem.
>> For example, DHCP server is not started for some reason and my computer's
>> ethernet cable is plugged. Once dhcp server started, I can't get the IP unless
>> I unplug and then plug the ethernet cable. Do you know how to solve
>> this issue? Thank you.
>>
>> Regards,
>> Dave.
>>
>
> Ok. To check whether I understand what you are saying: Your computer is running, but an external DHCP-server is not. Your computer tries to get an IP from the external DHCP server, which is down, so dhclient is unsuccessful. You then kick the DHCP-server back to live and then you have to plug in and out to get an IP?

Right. I'm sorry for the confusion.

> Assuming I understand correctly, that is exactly what should happen. You see, normally DHCP-servers don't flood the network with "Hello all dhclients, I am dhcp-server, please tell me if you need an IP", usually the opposite direction is in order as in "hello dhcp-server, I am dhclient, I need an IP, please give me one".
> You now have two options: 1) You coerce a manual request be running dhclient. 2) You plug in and out, which runs dhclient as you have configured to do so in your devd.conf.
> Of course you can set the retry-time for dhclient (see `man dhclient`) to an absurldly low threshold, so you are saved doing the dhcp-discover-procedure manually. It is, however, dubious, whether you want to do so. It might be a smarter way to fix that DHCP-server of yours.

Thank you for the clear explanation. I will add "retry 10" in
dhclient.conf and give it a try.
I'm wondering if net/ifstated will work in this case. I'll give it a
shot as well.
Thanks again!

> Hope to have been of some help here,
> cheers
> --
> Christopher J. Ruwe
> TZ GMT + 2
>

Regards,
Dave.