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Anonymous bridges in netplan

netplan is the default network configuration system for new installs of Ubuntu 18.04 (Bionic). Introduced as the default in Artful, it replaces /etc/network/interfaces.

One question that gets asked repeatedly is: "How do I set up an anonymous bridge in netplan?" (An anonymous bridge, I discovered, is one where the bridge doesn't have an IP address; it's more akin to a switch or hub.) It's been approached on Launchpad, and comes up on the IRC channel.

If you're trying to create a bridge without an IP address, the obvious first thing to try is this:

network:
    version: 2
    ethernets:
        ens8:
            match:
                macaddress: 52:54:00:f9:e9:dd
        ens9:
            match:
                macaddress: 52:54:00:56:0d:ce
    bridges:
        br0:
           interfaces: [ens8, ens9]

This is neat, plausible, and wrong - the bridge will be created but will stay 'down'. Per ip a:

5: br0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 0e:e3:1c:83:f8:e8 brd ff:ff:ff:ff:ff:ff

This is because systemd requires a 'network file' to bring up an interface, and netplan doesn't generate a systemd network file for the bridge. If you look at src/generate.c, in particular at write_network_file, you need at least one of a set of properties to trigger generation of a network file, and an anonymous bridge has none of them. This is clearly a bug - LP: #1736975.

There's no fix yet, but in the mean time, you can work around it by just manually telling systemd-networkd to bring up the interface. I created /etc/systemd/network/br0.network, containing the following:

[Match]
Name=br0

[Network]
LinkLocalAddressing=no
IPv6AcceptRA=no

Then upon restarting networking (netplan apply or just reboot), you will see that the bridge comes up, and - as desired - has no address:

5: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether 0e:e3:1c:83:f8:e8 brd ff:ff:ff:ff:ff:ff

Don't forget to include a comment in your netplan YAML to remind you that this extra file exists!

I will update this post when the bug is fixed to provide whatever canonical solution we land on.

Comments

  1. an alternate way, just add an systemd service?

    ========= /etc/systemd/system/up-bridge-100-interface.service ========
    [Unit]
    Description=Bring br100 interface up after network settings are done (bug: anonymous briges dont came up at boottime)
    Wants=network-online.target
    After=network-online.target

    [Service]
    Type=simple
    ExecStart=/bin/ip link set br100 up

    [Install]
    WantedBy=multi-user.target
    =========================================

    # sudo systemctl start up-bridge-100-interface
    # sudo systemctl status up-bridge-100-interface
    # sudo systemctl enable up-bridge-100-interface

    ReplyDelete
    Replies
    1. Yes, that should also work fine.

      Delete
    2. Do you find you get IPv6 link local addressing in that case though?

      Delete
  2. It would appear this has been fixed in bionic-updates:
    https://bugs.launchpad.net/ubuntu/+source/nplan/+bug/1736975/comments/31

    ReplyDelete
  3. I've got it working like this. The trick is to make it think it needs dhcp, but without IP. So this case interface will be up.

    network:
    version: 2
    ethernets:
    enp1s0:
    dhcp4: no
    enp2s0:
    dhcp4: no
    bridges:
    br0:
    interfaces: [enp1s0, enp2s0]

    ReplyDelete

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