isc-dhcp-server "No subnet declaration for enP4p65s0"

TL;DR: I'm aiming at configuring my own Ubuntu 22.04LTS-based router for my home network, using isc-dhcp-server, nftables & NetworkManager. I'm experiencing weird DHCP behaviors.

I'm using a Nano Pi R6S which has 3 ethernet interfaces :

• enP3p49s0 : WAN (connected to my ISP router - duh) - router is attributed an IP 192.168.0.22 via DHCP MAC address reservation configured on the ISP router
• enP4p65s0 : LAN1 (will be connected to a managed switch down the road) - 10.42.0.1/24
• eth0 : LAN2 (reserved for administrating the router via SSH and DHCP MAC address reservation) - 10.42.42.1/30

As a consequence, I'll have two local networks, defined as such:

# /etc/netplan/01-netcfg.yaml
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    enP3p49s0:
      dhcp4: true
    enP4p65s0:
      addresses:
      - 10.42.0.1/24
      dhcp4: false
    eth0:
      addresses:
      - 10.42.42.1/30
      dhcp4: false

DHCP is configured as followed:

# /etc/dhcp/dhcpd.conf
default-lease-time 86400;
max-lease-time 604800;
authoritative;

# Configuration for LAN1 (Home network)
subnet 10.42.0.0 netmask 255.255.255.0 {
  option routers          10.42.0.1;
  option subnet-mask      255.255.255.0;
  option domain-name-servers 8.8.8.8, 8.8.4.4;
  range 10.42.0.100 10.42.0.200;
}

# Configuration for LAN2 (Router Management)
subnet 10.42.42.0 netmask 255.255.255.252 {
  host novoo_adapter {
      hardware ethernet XX:XX:XX:XX:XX:XX;
      fixed-address 10.42.42.2;
  }
  option routers 10.42.42.1;
  option domain-name-servers 8.8.8.8, 8.8.4.4;
}

I'm experiencing a strange behavior: it seems the DHCP will behave as expected only if I plug something on the associated ethernet port and I restart the isc-dhcp-server.

For example, at the moment, if I plug to LAN2, I won't get the 10.42.42.2 address as expected despite using the right ethernet card with the right MAC address. I have to:

1. Plug the cable to LAN2
2. Set up the IP as fixed on the client (my laptop)
3. Restart DHCP via systemctl restart isc-dhcp-server on the server
4. Set back up IP to DHCP on the client
5. And then I finally have a DHCP-attributed IP

I've noticed the following before 1.:

$ sudo journalctl -u isc-dhcp-server
...
Jul 17 19:06:15 nanopi-r6s dhcpd[18158]: No subnet declaration for eth0 (no IPv4 addresses).
Jul 17 19:06:15 nanopi-r6s dhcpd[18158]: ** Ignoring requests on eth0.  If this is not what
Jul 17 19:06:15 nanopi-r6s dhcpd[18158]:    you want, please write a subnet declaration
Jul 17 19:06:15 nanopi-r6s dhcpd[18158]:    in your dhcpd.conf file for the network segment
Jul 17 19:06:15 nanopi-r6s dhcpd[18158]:    to which interface eth0 is attached. **
...

This mention disappears after step 3. (and I then have the same warning regarding eth0):

$ sudo journalctl -u isc-dhcp-server
...
Jul 17 19:15:28 nanopi-r6s dhcpd[18602]: No subnet declaration for enP4p65s0 (no IPv4 add>
Jul 17 19:15:28 nanopi-r6s dhcpd[18602]: ** Ignoring requests on enP4p65s0.  If this is n>
Jul 17 19:15:28 nanopi-r6s dhcpd[18602]:    you want, please write a subnet declaration
Jul 17 19:15:28 nanopi-r6s dhcpd[18602]:    in your dhcpd.conf file for the network segme>
Jul 17 19:15:28 nanopi-r6s dhcpd[18602]:    to which interface enP4p65s0 is attached. **

Jul 17 19:06:15 nanopi-r6s dhcpd[18158]:
Jul 17 19:06:15 nanopi-r6s dhcpd[18158]: Sending on   Socket/fallback/fallback-net
Jul 17 19:06:15 nanopi-r6s dhcpd[18158]: Server starting service.
Jul 17 18:45:59 nanopi-r6s dhcpd[13237]: DHCPDISCOVER from XX:XX:XX:XX:XX:XX via eth0
Jul 17 18:45:59 nanopi-r6s dhcpd[13237]: DHCPOFFER on 10.42.42.2 to XX:XX:XX:XX:XX:XX via eth0
Jul 17 18:46:00 nanopi-r6s dhcpd[13237]: DHCPREQUEST for 10.42.42.2 (10.42.42.1) from 94:05:bb:11:ae:49 via eth0
Jul 17 18:46:00 nanopi-r6s dhcpd[13237]: DHCPACK on 10.42.42.2 to XX:XX:XX:XX:XX:XX via eth0
...

And moreover, it seems that NetworkManager only assigns an IP to the interface IF a cable is plugged in (which is he expected behavior?):

# here with a cable plugged in eth0
$ ip addr
...
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether XX:XX:XX:XX:XX:XX brd ff:ff:ff:ff:ff:ff
    inet 10.42.42.1/30 brd 10.42.42.3 scope global noprefixroute eth0
       valid_lft forever preferred_lft forever
3: enP4p65s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN group default qlen 1000
    link/ether XX:XX:XX:XX:XX:XX brd ff:ff:ff:ff:ff:ff permaddr XX:XX:XX:XX:XX:XX
...

It seems like:

• isc-dhcp-server needs an interface with an assigned IP to work with
• the IP is only assign when a cable is plugged in the associated port
• that plugging a cable in will indeed lead to IP assignation
• but that the DHCP service won't recognized that an IP (and therefore a subnet declaration) is assigned
• this is corrected only with a DHCP service restart

I'm expecting to be able to hot-plug my laptop to LAN2 for administration without having to fixed-assign an IP (that defeats the whole purpose of DHCP MAC address reservation), and likewise for LAN1 for my future home network.

Any ideas please ?

Note: MAC Addresses have been scrambled

1. ISC DHCPD need interface to have an address because its the only way to associate declared subnet with the interface. And it ignore interface if it made to listen to all interfaces. Putting interface name in dhcpd startup option can help it to track its state (not obviously).

2. NetworkManager is known to put down logical interface when no link is up, but you can override this behavior. Try to create file like /usr/lib/NetworkManager/conf.d/00-server.conf :

   [main]

   no-auto-default=enP4p65s0

   ignore-carrier=enP4p65s0