Changing the DHCP address of a machine on my Local LAN
Bill Davidsen
davidsen at tmr.com
Mon Mar 8 02:15:36 UTC 2010
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 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.
>
--
Bill Davidsen <davidsen at tmr.com>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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