Re: Changing the DHCP address of a machine on my Local LAN

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Sam Varshavchik wrote:
> 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.
I'm firmly convinced that the machine in question is visible to the ISP DHCP 
server, although I can't say why that would be so.

> 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
> 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.

Bill Davidsen <[email protected]>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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