How to connect 2 virtual bridges so traffic goes from one to the other?

Question

Debian system, current situation, configured via etc/network/interfaces

bridge "br0" has 2 physical ethernet interfaces eth0 (WAN side, connected to ISP modem/router) eth1 (LAN side going to a physical switch)

Traffic between eth0 and eth1 is passing through the IPtables firewall and Suricata as an IPS. This setup is currently working ok.

Now I want to add "pi-hole" to this setup via a Linux container. The linux container needs a virtual bridge to add its virtual ethernet interface.

So I want to achieve the following:

Virtual Bridge "br0" - with eth0 (WAN)

<-->

connected to virtual bridge "lanbr0" - with eth1 (LAN) - with veth0 (virtual ethernet interface for LXC)

This way I can add more interfaces to the "lanbr0" bridge if needed in the future.

edit I was able to do this via the following commands ip link add veth0 type veth peer name veth1

then adding veth0 to br0 and veth1 to lanbr0 + moving eth1 from br0 to lanbr0 after bringing all interfaces up

However, how do I do this via /etc/network/interfaces (so it loads during startup like this?

Answer 1

There is no provision for specifically handling virtual dual interfaces type veth. So a bit of tinkering is in order mainly by using the pre-up option. I kept veth0 and veth1 here, but you should really consider choosing any other names (eg veth-br0 and veth-lanbr0), because those are the default names chosen if not supplied (eg: ip link add type veth), and they might clash.

For the veth interfaces:

iface veth0 inet manual
        pre-up ip link add veth0 type veth peer name veth1 || :
        hwaddress 02:00:00:01:00:00

iface veth1 inet manual
        pre-up ip link add veth1 type veth peer name veth0 || :
        hwaddress 02:00:00:01:00:01

This enables both interface when bringing up any of them. The || : is here to not fail because the peer interface was already set up by the "other" interface. Please note that for ifup's internal state's sake, both should be brought up in the end even if bringing up one will make the other appear. For the intended usage, the hwaddress is probably optional and can anyway be set to any other correct value. The bridge takes the MAC address of the first enslaved interface.

Now use them in the bridges. I won't put all your setup (because you didn't really give it), just the veth related parts. You will have to adapt what I couldn't guess and where I left some XXXX or ... (eg, is inet XXXX inet manual inet static or inet dhcp?)

auto br0
iface br0 inet XXXX
    pre-up ifup veth0
    bridge_ports eth0 veth0        
    ...

auto lanbr0
iface lanbr0 inet XXXX
    pre-up ifup veth1
    bridge_ports veth1 eth1
    address xx.xx.xx.xx
    netmask 255.255.255.0
    ...

Note I prepended an ifup vethX command inside, because else it will complain the interface didn't exist, will bring up the bridge without this interface and consider it done. I didn't have to use || : here because bringing up an already logically (meaning ifupdown's state) up interface isn't an error.

After completing the missing parts, put these in separate (or not) configuration files in /etc/network/interfaces.d (or even /etc/network/interfaces) and that should be working.

If it didn't work at first, try bringing logically down all interfaces (by logically I mean using ifdown) and try again: that would be because of a wrong logical state preventing some interfaces to be (re)configured correctly.

Also note that there might be interactions with other network managers, like... NetworkManager. There are probably options to have them ignore some interfaces, but that's out of the scope of this answer.

Answer 2

For reference, here's the full interfaces script (adding xx for some addresses):

# The loopback network interface
auto lo
iface lo inet loopback

# IPv4 address
auto enp8s0
iface enp8s0 inet manual
        offload-gro off
        offload-lro off

auto enp0s31f6
iface enp0s31f6 inet manual

iface veth0 inet manual
        pre-up ip link add veth0 type veth peer name veth1 || :
        hwaddress 02:00:00:01:00:00

iface veth1 inet manual
        pre-up ip link add veth1 type veth peer name veth0 || :
        hwaddress 02:00:00:01:00:01

# Bridge setup
auto br0
iface br0 inet static
    pre-up ifup veth0
    bridge_ports enp8s0 veth0
    bridge_fd 0
    bridge_maxwait 0
        address 192.168.xx.xx
        broadcast 192.168.xx.xx
        network 192.168.xx.xx
        netmask 255.255.255.0
        gateway 192.168.xx.xx
        dns-nameservers xx.xx.xx.xx xx.xx.xx.xx

auto lanbr0
iface lanbr0 inet manual
    pre-up ifup veth1
    bridge_ports veth1 enp0s31f6

Thanks again A.B