Containers assigned an SDN won't start

despens

New Member
Oct 9, 2025
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Dear Proxmox people,

My apologies if the topic was already discussed… I'm not so vell-versed in networking questions and might not know what to search for.

I have configured an SDN, exactly as described here: https://pve.proxmox.com/wiki/Setup_Simple_Zone_With_SNAT_and_DHCP — the setup describes my desired use-case exactly.

These instructions were used to set up two PVEs in different networks, identical fresh install of version 9.1.6 on both. One is located in a basic lab network behind a consumer router. Everything works wonderfully as expected: containers assigned the self-made VNet as a bridge get internal IP addresses in the range 10.0.0.2-10.0.0.253, assigned via DHCP. The other PVE host is run in a DMZ. When I assign the VNet to a container as a bridge there, the container won't start at all. The error message is not very enlightening:

Code:
run_buffer: 571 Script exited with status 11
lxc_init: 845 Failed to run lxc.hook.pre-start for container "102"
__lxc_start: 2046 Failed to initialize container "102"
TASK ERROR: startup for container '102' failed

When assigned the standard vmbr0, the containers start just fine.

As said, the exact same config in the lab network works just fine. I'm a bit lost about what to do or where to search for more information.

The host's /etc/network/interfaces.d/sdn looks like this on both PVE hosts (public IP address replaced with 999.999.999.999):

Code:
#version:2

auto internal
iface internal
    address 10.0.0.1/24
    post-up iptables -t nat -A POSTROUTING -s '10.0.0.0/24' -o vmbr0 -j SNAT --to-source 999.999.999.999
    post-down iptables -t nat -D POSTROUTING -s '10.0.0.0/24' -o vmbr0 -j SNAT --to-source 999.999.999.999
    post-up iptables -t raw -I PREROUTING -i fwbr+ -j CT --zone 1
    post-down iptables -t raw -D PREROUTING -i fwbr+ -j CT --zone 1
    bridge_ports none
    bridge_stp off
    bridge_fd 0
    ip-forward on

Both hosts run tailscale, and logging in and accessing the web interface via the tailscale VPN service are no issue at all.

I'd be grateful for any help…