Ytdyklly

Problem Summary

I have two network interfaces in the machine, and need them to behave in the following way:

• enp3s0: Main network interface for the machine. Receive over DHCP an IP address (will be in 172.16.0.0/24), a gateway, and a DNS resolver for the office network and the world at large. Default route is to this received gateway.




• enp2s0: Local subnet consisting of nothing more than 2 meters away, and nothing that should have access to the main network. Serve DHCP on this network (192.168.0.0/24, domain is .localnet), and provide the local machine with DNS resolution of .localnet clients.

I had a working solution for all this under 16.04 using /etc/network/ and dnsmasq (no NetworkManager), but I'm just lost in the new era of netplan and systemd-resolved. Everything just keeps fighting everything else.

So, what's my right, modern solution to make this work right? Ideally in such a way that I won't have to change it for 20.04.

Additional information

For reference, here is the script that did what I needed under 16.04.

#!/bin/bash

#

# Configure the PC to use dual network adapters.

#

# usage: dualnetwork.sh <hinetif> <localnetif>

#

# This configuration is based around the use of the classic ifupdown

# configuration files rather than NetworkManager.  This is substantially

# more stable and easier to work with.  It is probably prudent to also

# uninstall NetworkManager once this is done:

#   sudo apt-get remove network-manager

#

# Must run as root.

#

# Rob Gaddi, 29 Jan 2019



GENERATED="Generated by $0 at $(date)"

HINET=$1

LOCALNET=$2



abort () {

    echo $*

    exit 1

}



[[ ! -z "$LOCALNET" ]] || abort Must provide hinet and localnet interfaces.

ip -br addr | grep $HINET    || abort Interface $HINET not found

ip -br addr | grep $LOCALNET || abort Interface $LOCALNET not found



####################################################################

echo Configuring network interfaces

####################################################################

mkdir -p /etc/network/interfaces.d

echo source-directory /etc/network/interfaces.d > /etc/network/interfaces

cat <<-CONF > /etc/network/interfaces.d/hinet

    # $GENERATED

    auto $HINET

    iface $HINET inet dhcp

CONF

cat <<-CONF > /etc/network/interfaces.d/localnet 

    # $GENERATED

    allow-hotplug $LOCALNET

    iface $LOCALNET inet static

        address 192.168.0.20/24



    iface $LOCALNET inet static

        address 192.168.254.20/24

CONF



####################################################################

echo Installing and configuring dnsmasq

####################################################################

apt-get install --reinstall -y dnsmasq

mkdir -p /etc/dnsmasq.d

cat <<-CONF > /etc/dnsmasq.d/dualnetwork.conf

    # $GENERATED

    no-negcache

    dhcp-range=192.168.0.100,192.168.0.150,4h

    interface=$LOCALNET

    domain=localnet

    dhcp-fqdn

    log-dhcp

CONF

systemctl enable --now dnsmasq.service

Edit

This differs from Convert /etc/network/interfaces to netplan for floating IP in that the problem is not netplanning the machine IP addresses; that part is pretty easy. The problem is making the system serve DHCP on one network. When I tried it the obvious way, dnsmasq and systemd-resolved kept stepping on one another's toes. When I tried just disabling systemd-resolvd and letting dnsmasq do all the work it never got the upstream DNS server when it accepted the main network DHCP.

Per systemd-resolved.service(8), there are two possible ways to configure systemd-resolved to know about your dnsmasq server:

• add it to /etc/systemd/resolved.conf as a "global" DNS server


• add it to a *.network file in one of the directories read by systemd-networkd.

For the latter, it may be sufficient to set a nameservers option in your netplan config for enp2s0, pointing to the local IP that dnsmasq will run on.