Re: MVP for a DHCP server (was Re: ISC's EoL dhcp suite, including dhcpd)

 ---- On Mon, 14 Jul 2025 17:28:11 +0100  Martin Husemann <[email protected]> wrote --- 
 > On Mon, Jul 14, 2025 at 04:53:01PM +0100, Roy Marples wrote:
 > > Why do you need to wire hosts to fixed IP addresses?
 > 
 > Several different reasons, the DNS entry is one of them - think about
 > printers that others need to reach over the network for example.
 > I am not sure if the old printer devices would work with mDNS.
 > 
 > Putting them outside the dynamic range should work (actually that is how
 > I run them currently, but I have way more then 10 non-dynamic IPs).
 > 
 > The other reason is the NFS server has hardwired permissions mapping root
 > file systems to IP addresses. So a diskless client mounting its / via NFS
 > needs a fixed IP and needs DHCP. I am not sure what you mean with the
 > INFORM variant.
 > 
 > And then there are "smart" devices where turning off DHCP does not
 > always work so you have to give them a fixed IP via DHCP and matching
 > firewall rules. The most popular variant of this is devices allowing
 > a fixed IPv4 configuration but then doing IPv6 auto configuration
 > w/o any option to disable IPv6 completely.

Would you consider an option to use ethers(5) for this instead?
This would allow IPv4 mapping, but not IPv6.
MVP I guess would be IP address only, no idea how hard it would be to make a DNS call to work out the IP address if it's a hostname.

 > 
 > > This is an important question because there is no equivalent for DHCPv6 as DHCPv6 does not carry a hardware address option.
 > > DHCPv6 and DHCPv4 Client ID MUST be treated as opaque values
 > 
 > Opaque value is fine, as long as I can note it in my config somehow.
 > No need to only allow this via MAC address.
 > 
 > > Even Infiniband with DHCP has this issue because the hardware address does not fit inside the chaddr field and needs to use a Client ID.
 > > https://gitlab.isc.org/isc-projects/kea/-/issues/3331
 > > 
 > > My view is that if you need a host to be a fixed IP address then
 > > please configure the fixed IP address on the host and use the INFORM
 > > function.
 > 
 > I don't know how you configure INFORM only (but I could look that up). This
 > is OK, but having central ways to update those devices to slightly changed
 > network settings is very convenient. This clearly is a corner case.

in dhcpcd.conf

interface bge0
   inform 192.168.1.12/24

The address is optional - if missed dhcpcd will use the first one on the interface.

Roy