Two logical interfaces on one physical, with Netplan

In my home network, I have a server which I want to appear under two hostnames, mainly so I can later move the functionality associated with one of them around to other hosts. I'm just using my ISP-supplied broadband router/modem to manage the network, but it doesn't exactly bristle with configuration options to make this directly possible with, say, a DNS alias. Nevertheless, I want to stick with it, as other solutions might involve duplicating a lot of its functionality, or splitting it across multiple hosts, both of which introduce their own risks.

The router provides local DNS resolution (in the .home domain), and it honours the hostnames specified by DHCP requests. By presenting two interfaces to it, a single host can get two IP addresses and so two distinct names. Yes, it's ugly and hacky, but it's a solution within the constraints.

Approach

In this specific example, enp3s0 is the physical interface, and the second hostname is media-centre. The approach is to create two virtual interface pairs (faux0-faux0br and faux1-faux1br), connect one end of each (faux0br and faux1br) to a virtual bridge (br0), and connect this to the physical interface enp3s0. The other two ends of the pairs (faux0 and faux1) are now on the same Ethernet network, and running DHCP on them causes them to acquire distinct IP addresses, and registers them under distinct DNS names.

IPv4 ARP

For IPv4, it's essential to prevent the two interfaces stepping on each other's toes regarding ARPs, and a Server Fault answer shows how. Put this in your /etc/sysctl.d/local.conf (or create a numbered file for it, say 99-dualiface.conf):

net.ipv4.conf.all.arp_ignore=1
net.ipv4.conf.all.arp_announce=2
net.ipv4.conf.all.rp_filter=2

That will apply on boot, but you can apply it immediately with sudo sysctl -p/etc/sysctl.d/local.conf.

Creating virtual interface pairs

At the time of writing, and as far as I can tell, Netplan can set up bridges, but not the veth pairs used in the previous solution. This Ask Ubuntu answer explains how to do it another way. For our case specifically, create /etc/systemd/network/25-faux0.netdev:

[NetDev]
Name=faux0
Kind=veth
[Peer]
Name=faux0br

Create /etc/systemd/network/25-faux1.netdev similarly:

[NetDev]
Name=faux1
Kind=veth
[Peer]
Name=faux1br

Connecting with a bridge

We create and define the bridge in the /network/bridges section of a YAML file in /etc/netplan/. I've called this one 99-bridgehack.yaml:

network:
  ethernets:
    enp3s0:
      dhcp4: false
    faux0:
      dhcp4: true
    faux0br: {}
    faux1:
      dhcp4: true
      dhcp4-overrides:
        hostname: media-centre
    faux1br: {}
  bridges:
    br0:
      interfaces:
        - faux0br
        - faux1br
        - enp3s0

We enable DHCP on faux0 and faux1. The former announces itself using the server's own name by default, but we set the name explicitly for the latter. Note that we also disable DHCP on our original interface enp3s0, overriding the setting in /etc/netplan/00-installer-config.yaml:

# This is the network config written by 'subiquity'
network:
  ethernets:
    enp3s0:
      dhcp4: true
  version: 2

The section /network/bridges/br0/interfaces binds the backends of the veth pairs together with the physical interface. faux0br and faux1br must have some presence in /network/ethernets in order to reference them here, so they are set empty.

Deployment

With /etc/netplan/99-bridgehack.yaml in place, you just need to tell Netplan about it. Any remote network reconfiguration risks you losing the very connection you're using to do it over, so this is best done on the server's console:

sudo netplan generate
sudo netplan apply

Maybe I did something wrong, but I would often find that Netplan would create new entities as requested, but not tear down old ones. A reboot ensures you're starting from a clean slate. If you make a mistake, you can always rename 99-bridgehack.yaml to disable it.

Déjà vu

I did this before, but without Netplan. I turned it off, and enabled legacy ifupdown functionality still available in Ubuntu 18.04. However, it's less clear how to do that on 22.04, so I had to find a way with Netplan. There was no need to mess with /etc/dhcp/dhclient.conf this time, which is good, as it didn't seem to make any difference. (Is dhclient being used any more?) The IPv4/ARP advice remains largely the same.