netplan config is ignored, who reconfigures my network?

Question

I have a machine with multiple NIC, for the sake of simplicity we call them

• ext0: uses DHCP, goes to the outside
• int0 and int1: have static IP adresses, go to some private network, where there is also a DHCP server running, which other machines should use.

I have only one netplan config file (/etc/netplan/01-netcfg.yaml), which reads:

network:
  renderer: networkd
  version: 2
  ethernets:
    ext0:
      dhcp4: true
    int0:
      dhcp4: false
      dhcp6: false
      addresses:
        - 192.168.X.X/24
    int1:
      dhcp4: false
      dhcp6: false
      addresses:
        - 192.168.X.X/24

I can run netplan apply and everything works fine. After some time, however int0 and int1 get additional IPs over DHCP, although this is explicitly prohibited in my netplan config. I would not really mind that, if not additionally at some point, the default route would be transferred to one of the internal network interfaces.

Why is this happening? Which service is interferring with my network config? This is on a clean install of ubuntu 22.04.

Answer 1

I am seeing the same on a new 24.04 install. My /etc/netplan/01-netcfg.yml looks like:

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s25:
      addresses: [172.16.0.1/17]
      dhcp4: false
      dhcp6: false
      mtu: 1500
    enp7s4:
      dhcp4: true
      mtu: 1500

I am running the dhcpd server on this machine itself. I thought that the dhcp4: false configuration means that that interface should not perform a dhcpdiscover to try and get an address, but my syslog shows:

2024-05-15T19:45:52.953502-05:00 tlcadmin dhcpd[14134]: DHCPDISCOVER from 00:23:7d:49:70:59 via enp0s25: network 172.16.0.0/17: no free leases