[systemd-devel] "Correct" way to obtain DHCP lease info?

[Bruce A. Johnson]

I'm still trying to get an explanation of why having a valid DHCP 
address is not in itself good enough. The only reason I've been able to 
see is that after the lease is issued, and before the time comes to 
refresh the lease, there could be a communication failure somewhere 
between the switch the DHCP client is on and the home office where the 
DHCP server is. One would assume that application failures would be a 
reasonable clue.... Regardless, it seems to me that it's not 
unreasonable for an application outside of systemd-networkd to be able 
to obtain the DHCP lease information. Am I off base here?

Bruce A. Johnson | Firmware Engineer
Blue Ridge Networks, Inc.
14120 Parke Long Court Suite 103 | Chantilly, VA 20151
Main: 1.800.722.1168 | Direct: 703-633-7332
http://www.blueridgenetworks.com
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On 22/04/2021 12:00, Mantas Mikulėnas wrote:
> On Thu, Apr 22, 2021 at 6:17 PM Bruce A. Johnson 
> <bjohnson at blueridgenetworks.com 
> <mailto:bjohnson at blueridgenetworks.com>> wrote:
>
>     Silvio, thanks for the suggestion. I'm not concerned with keeping
>     the lease forever; the system actually experiences a topology
>     change as it's switched from one network to another, and I can
>     catch that from the DBus events that occur. The problem we're
>     trying to solve is to contact some address that we're sure exists
>     on the network, without knowing anything about that network. The
>     default gateway was an obvious choice, but someone wants to cover
>     the case of there being a private LAN with no gateway. The only
>     other choice I could see is the DHCP server that issues the lease.
>
> Hmm, don't you also have the case of there being a private LAN with no 
> gateway and no DHCP? Or possibly the case of a DHCP relay. And since 
> you don't know anything about the network, you also don't know whether 
> the address will respond to your communication attempts (other than 
> ARP) -- it might be pingable but it might be not.
>
> I'm curious about what brought this problem into existence in the 
> first place. Why *is* it necessary to contact a random address within 
> the network? (If it's to check that the physical interface is working, 
> then just the fact that you somehow acquired a lease would be enough. no?)
>
> -- 
> Mantas Mikulėnas
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