LXC cannot connect to network in default config

J

JacobHM

New Member
Feb 12, 2026
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Hi, every new unmodified LXC on the debian-12-standard_12.12-1 template will not connect to my network, local or otherwise

With static ip
Code: 
root@CT100:~# ping 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
From 192.168.0.101 icmp_seq=1 Destination Host Unreachable
From 192.168.0.101 icmp_seq=2 Destination Host Unreachable
From 192.168.0.101 icmp_seq=3 Destination Host Unreachable
^C
--- 192.168.0.1 ping statistics ---
5 packets transmitted, 0 received, +3 errors, 100% packet loss, time 4094ms

With DHCP
Code: 
root@CT100:~# ping 192.168.0.1
ping: connect: Network is unreachable

The server is connected as shown
Router ---> Switch1 ---> Switch2 ---> Server

The host has a full connection and can ping locally with no issues

Any help appreciated,
Thanks!
 
cat /etc/network/interfaces returns

Code: 
auto lo
iface lo inet loopback

auto enp1s0
iface enp1s0 inet manual

auto vmbr0
iface vmbr0 inet static
        address 192.168.0.100/24
        gateway 192.168.0.1
        bridge-ports enp1s0
        bridge-stp off
        bridge-fd 0

source /etc/network/interfaces.d/*

cat /etc/pve/nodes/localhost124/lxc/100.conf returns

Code: 
arch: amd64
cores: 1
hostname: CT100
memory: 512
net0: name=eth0,bridge=vmbr0,firewall=1,hwaddr=BC:24:11:3E:CA:5C,ip=dhcp,type=veth
ostype: debian
rootfs: local-lvm:vm-100-disk-0,size=16G
swap: 512

Thanks!
 
Last edited:
Hi Jacob,
you posted the LXC configuration with the dhcp setup, can you add the one with the static ip address assignment?

And maybe also add the routes the container is aware of and the state of the interfaces (host and LXC), with the static setup?
Code: 
## ip routes
ip route

## interfaces configurationen
ip address

Just to get sure, your pve host can connect to the router? And the router should actually work as dhcp-server?

BR, Lucas
 
Hello, yes my router does work as a dhcp-server, most the devices in my network are dhcp and yes the host can ping external and internal ip's.

Output of cat /etc/network/interfaces (static ip setup)

Code: 
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 192.168.0.101/24
        gateway 192.168.0.1

192.168.0.101 is free on my router

ip route returns

Code: 
default via 192.168.0.1 dev eth0 onlink
192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.101

ip address returns

Code: 
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute
       valid_lft forever preferred_lft forever
2: eth0@if85: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether bc:24:11:3e:ca:5c brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 192.168.0.101/24 brd 192.168.0.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::be24:11ff:fe3e:ca5c/64 scope link
       valid_lft forever preferred_lft forever

The ip of my router is 192.168.0.1
My subnet mask is 255.255.255.0
My router cannot see the lxc but can see the host

Thanks!
 
Hi Jacob,

that basically looks all right, so the troubleshooting might be a little bit more complex.

mhm, one think I noticed in the lxc config is the activated firewall for the container.
This should basically not influence anything, as long as the firewall for host and cluster are not active (the default).

On the host can you check if the package forwarding is active?
Code: 
sysctl -a | grep '.forwarding'

As next step you could try to connect directly to the host,
assign a static ip address within the subnet of 192.168.0.0/24
and try to ping the host, as well as the lxc.
That might help sorting, where the blocking occurs.

For my understanding, what way of installation did you use?
The one via the official iso-installer?

BR, Lucas
 
Hi, yes i did use the official debain 12 standard lxc template from the ct template installer, also I made another lxc with ubuntu 22.04 (official template) and got the same result, cannot ping internally or externally.


sysctl -a | grep '.forwarding' returns

Code: 
net.ipv4.conf.all.bc_forwarding = 0
net.ipv4.conf.all.forwarding = 0
net.ipv4.conf.all.mc_forwarding = 0
net.ipv4.conf.default.bc_forwarding = 0
net.ipv4.conf.default.forwarding = 0
net.ipv4.conf.default.mc_forwarding = 0
net.ipv4.conf.enp1s0.bc_forwarding = 0
net.ipv4.conf.enp1s0.forwarding = 0
net.ipv4.conf.enp1s0.mc_forwarding = 0
net.ipv4.conf.fwbr100i0.bc_forwarding = 0
net.ipv4.conf.fwbr100i0.forwarding = 0
net.ipv4.conf.fwbr100i0.mc_forwarding = 0
net.ipv4.conf.fwln100i0.bc_forwarding = 0
net.ipv4.conf.fwln100i0.forwarding = 0
net.ipv4.conf.fwln100i0.mc_forwarding = 0
net.ipv4.conf.fwpr100p0.bc_forwarding = 0
net.ipv4.conf.fwpr100p0.forwarding = 0
net.ipv4.conf.fwpr100p0.mc_forwarding = 0
net.ipv4.conf.lo.bc_forwarding = 0
net.ipv4.conf.lo.forwarding = 0
net.ipv4.conf.lo.mc_forwarding = 0
net.ipv4.conf.veth100i0.bc_forwarding = 0
net.ipv4.conf.veth100i0.forwarding = 0
net.ipv4.conf.veth100i0.mc_forwarding = 0
net.ipv4.conf.vmbr0.bc_forwarding = 0
net.ipv4.conf.vmbr0.forwarding = 1
net.ipv4.conf.vmbr0.mc_forwarding = 0
net.ipv6.conf.all.forwarding = 0
net.ipv6.conf.all.mc_forwarding = 0
net.ipv6.conf.default.forwarding = 0
net.ipv6.conf.default.mc_forwarding = 0
net.ipv6.conf.enp1s0.forwarding = 0
net.ipv6.conf.enp1s0.mc_forwarding = 0
net.ipv6.conf.fwbr100i0.forwarding = 0
net.ipv6.conf.fwbr100i0.mc_forwarding = 0
net.ipv6.conf.fwln100i0.forwarding = 0
net.ipv6.conf.fwln100i0.mc_forwarding = 0
net.ipv6.conf.fwpr100p0.forwarding = 0
net.ipv6.conf.fwpr100p0.mc_forwarding = 0
net.ipv6.conf.lo.forwarding = 0
net.ipv6.conf.lo.mc_forwarding = 0
net.ipv6.conf.veth100i0.forwarding = 0
net.ipv6.conf.veth100i0.mc_forwarding = 0
net.ipv6.conf.vmbr0.forwarding = 1
net.ipv6.conf.vmbr0.mc_forwarding = 0

Is it worth reinstalling proxmox entirely?
Thanks!
 
Hi Jacob,

the forwarding parameters look very usual, too.

I would expect, that reinstalling the pve server will not solve the issue.
If you used the offical iso and copied it to an USB-Stick, the installation should be the same afterwards.

Like mentioned earlier, there are more devices involved in the connection.
By this reason my approach would be to sort out potential issue reasons,
e.g. the network devices, or the router configuration.

Basically I would verify, where the communication is dropped.
The remote solution for this would be to use tcpdump, to verify if the host sends out and forwards the packages of the LXC. Then the PVE- + LXC-configuration is fine and the reason of the issue might be outsite of the server. (You can try tcpdump -veni <interface e.g. vmbr0 or enp1s0> host 192.168.0.101)

The more manual way would be to directly connected a device and check if the communication to the LXC works. If this is the case, I would possibly connect the device and the PVE host to the switches and check the communication.

Best regards,
Lucas
 
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