LXC and VM in same subnet as Proxmox host cannot ping Proxmox host

[Aura]

Currently at my wits' end here and the forum is probably my last hope.

The issue boils down to the tile: I have 1 LXC and 1 VM that are in the same subnet as the Proxmox host, but they cannot ping the Proxmox host.

LXC - n8n - 192.168.50.30
VM - elk - 192.168.50.20
Proxmox - 192.168.50.10

Here's the network configuration of the Proxmox host. It has 2 NIC. Both are plugged in a USW Pro Max 24. The reason I have this setup is because I want the traffic from the Proxmox management interface to come out of its own port, and the rest of the VMs are going through vmrb1, associated with enp5s0 (NIC) which is in another port.



Here's the n8n network information:



Here's the elk network information:



IF from the Proxmox shell, I try to ping n8n, it doesn't work, because it looks like it's trying to use vmbr1.



However, when I force it to use the vmbr0.50 interface for the ping, it works.



n8n and elk can ping each other, but they cannot ping Proxmox.
Proxmox cannot ping n8n nor elk through vmbr1, the default interface used by ping, but can through vmbr0.50.

I have honestly no idea of what's going on ...

If it's of interest, I have 2 other LXC (adguard and pihole) that are in another subnet (192.168.1.0/24). They both can ping the Proxmox, n8n and elk.

I discovered this issue while trying to configure a n8n workflow leveraging the Proxmox API and realized that it couldn't reach it.

Let me know if you need any other information.

 

[abobakr]

It’s not advised to have two NICs on the same subnet, as in your case: 192.168.50.0/24.
When a machine has multiple interfaces configured with the same subnet, it can access the same network through different interfaces. This may lead to routing issues, where the system might choose the "wrong" interface based on interface metrics.

Could you please try changing one of the interfaces to a different subnet and then check the status?
Afterward, In case the problem still persist, please provide the output of:

ip --details route show

from proxmox

 

[Aura]

Alright so before trying to change the vmbr1 IP to put it in another subnet (e.g.: 192.168.51.0/24), I tried the ip command above just to see the output:

unicast default via 192.168.50.1 dev vmbr0.50 proto kernel scope global onlink
unicast 192.168.50.0/24 dev vmbr0.50 proto kernel scope link src 192.168.50.10
unicast 192.168.50.0/24 dev vmbr1 proto kernel scope link src 192.168.50.100

I understood that vmbr0.50 was then the default interface for routing to 192.168.50.1 and when trying to ping the LXCs in 192.168.50.0/24 well ... now it works. Same for the LXCs, I'm able to ping the Proxmox (192.168.50.10) from them.

I have no idea of what happened, nor what changed. But now it works. The ONLY thing that I did, and I don't know if that's what "fixed" it, because afterwards, it didn't work anyway, is changing the order of the interfaces in /etc/network/interfaces. Previously, vmbr1 was above vmbr0 and vmbr0.50.

# network interface settings; autogenerated
# Please do NOT modify this file directly, unless you know what
# you're doing.
#
# If you want to manage parts of the network configuration manually,
# please utilize the 'source' or 'source-directory' directives to do
# so.
# PVE will preserve these directives, but will NOT read its network
# configuration from sourced files, so do not attempt to move any of
# the PVE managed interfaces into external files!

auto lo
iface lo inet loopback

iface eno1 inet manual

iface enp5s0 inet manual

auto vmbr0
iface vmbr0 inet static
        bridge-ports eno1
        bridge-stp off
        bridge-fd 0
        bridge-vlan-aware yes
        bridge-vids 2-4094

auto vmbr0.50
iface vmbr0.50 inet static
       address 192.168.50.10/24
       gateway 192.168.50.1


auto vmbr1
iface vmbr1 inet static
        address 192.168.50.100/24
        bridge-ports enp5s0
        bridge-stp off
        bridge-fd 0
        bridge-vlan-aware yes
        bridge-vids 2-4094

source /etc/network/interfaces.d/*

 

[Maximiliano]

Hello Aura,

As abobakr said, assigning overlapping network ranges to two different network interfaces is a sure way to run into these issues. We generally advice against it. The problem might come back after restarting the host or network.

 

[Aura]

Understood! In that case, I'll perform the IP address change on vmbr1 to put it in another subnet (e.g.: 192.168.51.0/24). My goal is mainly just to have all the VM-related traffic come out of 1 NIC, and have the other "main" NIC being "reserved" for Proxmox management.

Thank you!