Changing the DHCP address of a machine on my Local LAN

[Sam Varshavchik]

Aaron Konstam writes:

> On Sat, 2010-03-06 at 15:22 -0600, Mikkel wrote: 
>> On 03/06/2010 03:06 PM, Aaron Konstam wrote:
>> > 
>> > I am not using my neighbors wireless. The machine is hard-wired to the
>> > router but does not use the router as a DHCP server. It did until this
>> > morning but now it does not. How does one change that behavior?
>> > 
>> Check the configuration of the router. You can also try to reset the
>> router. Outside of giving the machine a static IP address, there is
>> not much you can do on the machine. This is NOT a problem with the
>> Linux machine - it is a router problem.
>> 
>> If you want to know why this is a router problem, find some good
>> documentation on how DHCP works. The machine making the request
>> responds to the first DHCP server that replies...
>> 
>> Mikkel
> 
> I agree with your analysis of the problem. But that leaves us with the
> following mystery.. There are 4 machines on the LAN. Why does only the
> one get a response from the providers DHCP server first? Also why this
> only occurred this morning after using the routers DHCP server for 6
> months?

DHCP requests are, by definition, sent to the local link broadcast address, 
and can only be received by DHCP servers on the same network segment. DHCP 
requests cannot cross a router or hop to a different network segment. A DHCP 
server is always required to be on the same network segment as its clients, 
in order to receive DHCP requests and manage the clients.

In other words, I'm firmly convinced that the laws of physics of our shared 
universe prohibit a machine on your local LAN from being able to obtain an 
IP address from some DHCP server outside of your LAN. Although I understand 
that this is what you claim has happened, it is simply not possible, 
according to everything I know about DHCP. Although DHCP is not actually one 
of my areas of deep expertise, I'm fairly certain that that's not how DHCP 
works.

So far, looking over this thread, you've merely paraphrased what you think 
is happening. You've stated what you think has happened, but in your own 
words only. I don't recall you posting the actual raw, hard data. Maybe I 
missed it, but if so, instead of interpreting what you think has happened, 
you should actually post what's actually happening. Without actually 
looking at your actual machine configuration, any advice you receive is 
indistinguishable from a random guess. By configuration I mean:

>From your LAN machines which are configured and are working correctly:

* The contents of your /etc/sysconfig/ifcfg-<interface> configuration files

* The output of the "ifconfig <interface>" command

* The output of the "route" command, with and without the -n option.

* The contents of /etc/resolv.conf

* The output of "grep dhclient /var/log/messages", presuming that your most 
recent DHCP configuration is still logged there, and the log file has not 
been rotated, since then.

* A traceroute to some well-known site, such as www.google.com.

Then, from your questionable host, which you believe has accomplished the 
impossible feat of contacting a DHCP server on a different network segment, 
the same exact stuff. The above may not necessarily be an exclusive list, 
somewhat else may also suggest some other useful tidbit to look into, but 
that's the bare minimum required for anyone, other than yourself, to have 
any idea how your machine is configured, network-wise. That does, also, mean 
that you should not mask or hide the actual data, like replacing IP 
addresses or hostnames with dummy labels, thinking that they're secret, in 
some way.

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