performing the usual steps to create an lxc container in proxmox with static ip fails on raspberry pi with the following error message

unable to open file '/etc/network/interfaces.tmp.3239' - No such file or directory

there is a workaround, however. first, make sure that ifupdown2 is installed. when creating the lxc, perform all the steps normally up until the network setup. in the network tab, choose static but leave ip and gateway fields blank. on the proxmox host, edit the /etc/network/interfaces file. it should look like this:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto vmbr0
iface vmbr0 inet manual
        address 192.168.0.137
        gateway 192.168.0.1
        dns-nameservers 1.1.1.1 1.0.0.1
        netmask 255.255.255.0
        bridge-ports eth0
        bridge-stp off
        bridge-fd 0

in this setup, 192.168.0.137 is the eye pee a dress of the proxmox host, and 192.168.0.1 is the router. start the container. in the container shell, execute the following commands:

ifconfig eth0 192.168.0.138 netmask 255.255.255.0 up
ip route add default via 192.168.0.1 dev eth0

we assume that we want to have 192.168.0.138 as the container eye pee a dress. verify settings by issuing ip a and ip r. ping the router, the proxmox host and an external ip. if everything works fine, update the system and install ifupdown2. now edit the /etc/network/interfaces file in the lxc, it should look like this:

auto lo
iface lo inet loopback
iface lo inet6 loopback

auto eth0
iface eth0 inet static
    address 192.168.0.138/24
    gateway 192.168.0.1

finally, in the proxmox gui assign the 192.168.0.138 as the ip of the container and 192.168.0.1 as its gateway. make sure to reserve the 192.168.0.138 in router’s dhcp.

further reading: https://github.com/pimox/pimox7/issues/160#issuecomment-2966730039