Re: multiple «hardware ethernet» for the same IP address

[glenn.satchell at uniq.com.au]

On 2023-02-19 04:03, Simon wrote:
> Vincent LE NEZET <vincent.le.nezet at ens.psl.eu> wrote:
> 
>> I have a question about the support of multiple mac address for the 
>> same IP in isc-dhcp-server.
>> 
>> The user support team register client's mac address in the dhcp.
>> 
>> So that clients always have the same IP address, they create two 
>> «host» blocks in the conf with the same IP, but two differents mac : 
>> one for the ethernet card of the PC and one for the dock on the user's 
>> desk.
>> 
>> Given that it could result in an address conflict, it surprises me 
>> that isc-dhcp accept this. Is it because of a RFC ?
> 
> Strictly speaking, it violates the RFCs (I think), and if you tried to
> do it via any other mechanism (classes & subclasses, reservations) it
> wouldn’t work. But when you use a fixed-address statement, this causes
> the server to bypass most of the lease handling processes - and in
> particular there is no check to see if the lease is free (the admin
> said to do it, so do it on the basis that they (should) know their
> network).
> 
> Simon

I just tried this, because I do it at home with the ethernet and wifi 
connections on my laptops.

Turns out both interfaces get the same IP. In your case Vincent, the 
user would need to attach the laptop to the dock *and* plug an ethernet 
cable into laptop.

I'm using dhcpd 4.4.1 on Ubuntu with an Ubuntu client. Other clients 
might behave differently.

regards,
Glenn