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From: The Natural Philosopher <tnp@invalid.invalid>
Newsgroups: comp.os.linux.misc
Subject: Re: I never thought of this scenario
Date: Mon, 8 Apr 2024 08:34:40 +0100
Organization: A little, after lunch
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On 07/04/2024 14:29, Robert Heller wrote:
> At Sun, 7 Apr 2024 13:24:28 +0100 The Natural Philosopher <tnp@invalid.invalid> wrote:
> 
>>
>> On 07/04/2024 12:37, Robert Heller wrote:
>>>> My question is, if that had already been occupied by another piece of
>>>> kit, would I have ended up with an IP address clash?
>>> This would not happen.  I believe that the DHCP server process does something
>>> like an "arp" before it assigns leaases.
>>
>> I think it may in fact do a *ping*. In fact this led me to the
>> realisation that none of my Raspberry PI PICO W widgets were responding
>> to pings...another 2 hours of my life working out why...:-)
>>
>> "To prevent a newly allocated IP address conflicting with existing IP
>> addresses, the DHCP server sends an ICMP Echo Request packet before
>> sending a DHCP Offer message. This ICMP packet contains the IP address
>> to be allocated in both the source and destination IP address fields.
>> The server can allocate the IP address if it receives no ICMP Echo Reply
>> packet within the detection period (no client is using this IP address).
>> If the server receives an ICMP Echo Reply packet within the detection
>> period, the DHCP server lists this IP address as a conflicting IP
>> address (as it is in use by another client), and then waits for the next
>> DHCP Discover message to start the IP address selection process again."
>>
>> https://support.huawei.com/enterprise/en/doc/EDOC1100126920/5cef90ad/how-a-dhcp-server-allocates-network-parameters-to-new-dhcp-clients
>>
>>    >Also, anything with an "old"
>>> (pre-reboot) lease would eventually ask for a lease renewal sooner or later
>>> (eg when the lease runs out). It would ask for its existing IP address and the
>>> router would likely grant that and create an entry int its lease table.
>>>
>> yes.
>>
>>> So, even if the router does not have any persistant lease data (eg saved
>>> across reboots), the data would eventually be re-created is pretty shourt
>>> order. I suspect the lease time for most od these little routersis fairly
>>> should (an hour?),
>>
>> 24 hours...
>>
>>> so the problem you fear is a non-problem.
>>
>> It was a  potential  problem when my Pi Picos didnt respond to pings
> 
> Which is not the *router's* problem, but a mis-configuration problem with the
> Pi Picos...

No argument there!

>>
> 

-- 
"In our post-modern world, climate science is not powerful because it is 
true: it is true because it is powerful."

Lucas Bergkamp