One of my favorite features of Fedora 22 is systemd-networkd and all of the new features that came with it in recent systemd versions. The configuration files are easy to read, bridging is simple, and tunnels are resilient.

I’ve recently started using a small Linux server at home again as a network router and firewall. However, I used systemd-networkd this time and had some great results. Let’s get started!

Overview

Our example router in this example has two network interfaces:

eth0: public internet connectivity

eth1: private LAN (192.168.3.1/24)

We want machines on the private LAN to route their traffic through the router to the public internet via NAT. Also, we want clients on the LAN to get their IP addresses assigned automatically.

Network configuration

All of the systemd-networkd configuration files live within

/etc/systemd/network

and we need to create that directory:

mkdir /etc/systemd/network

We need to write a network configuration file for our public interface that systemd-networkd can read. Open up

/etc/systemd/network/eth0.network

and write these lines:

[Match] Name=eth0 [Network] Address=PUBLIC_IP_ADDRESS/CIDR Gateway=GATEWAY DNS=8.8.8.8 DNS=8.8.4.4 IPForward=yes

If we break this configuration file down, we’re telling systemd-networkd to apply this configuration to any devices that are called

eth0

. Also, we’re specifying a public IP address and CIDR mask (like /24 or /22) so that the interface can be configured. The gateway address will be added to the routing table. We’ve also provided DNS servers to use with systemd-resolved (more on that later).

I added

IPForward=yes

so that systemd-networkd will automatically enable forwarding for the interface via sysctl. (That always seems to be the step I forget when I build a Linux router.)

Let’s do the same for our LAN interface. Create this configuration file and store it as

/etc/systemd/network/eth1.network

:

[Match] Name=eth1 [Network] Address=192.168.3.1/24 IPForward=yes

We don’t need to specify a gateway address here because this interface will be the gateway for the LAN.

Prepare the services

If we’re planning to use systemd-networkd, we need to ensure that it runs instead of traditional network scripts or NetworkManager:

systemctl disable network systemctl disable NetworkManager systemctl enable systemd-networkd

Also, let’s be sure to use systemd-resolved to handle our

/etc/resolv.conf

:

systemctl enable systemd-resolved systemctl start systemd-resolved rm -f /etc/resolv.conf ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf

Reboot

We’re now set to reboot! It’s possible to bring up systemd-networkd without rebooting but I’d rather verify with a reboot now than get goosed with a broken network after a reboot later.

Once your router is back up, run

networkctl

and verify that you have routable in the output for both interfaces:

[root@router ~]# networkctl IDX LINK TYPE OPERATIONAL SETUP 1 lo loopback carrier unmanaged 2 eth0 ether routable configured 3 eth1 ether routable configured

DHCP

Now that both network interfaces are online, we need something to tell our clients about the IP configuration they should be using. There are plenty of good options here, but I prefer dnsmasq. It has served me well over the years and it provides some handy features along with DHCP, such as DNS caching, TFTP and IPv6 router announcements.

Let’s install dnsmasq and enable it at boot:

dnf -y install dnsmasq systemctl enable dnsmasq

Open

/etc/dnsmasq.conf

in your favorite text editor and edit a few lines:

Uncomment dhcp-authoritative

This tells dnsmasq that it’s the exclusive DHCP server on the network and that it should answer all requests

Uncomment interface= and add eth1 on the end (should look like interface=eth1 when you’re done)

Most ISP’s filter DHCP replies on their public networks, but we don’t want to take chances here. We need to restrict DHCP to our public interface only.

Look for the dhcp-range line and change it to dhcp-range=192.168.3.50,192.168.3.150,12h

We’re giving clients 12 hour leases on 192.168.3.0/24

Save the file and start dnsmasq:

systemctl start dnsmasq

Firewall

We’re almost done! Now it’s time to tell iptables to masquerade any packets from our LAN to the internet. But wait, it’s 2015 and we have tools like

firewall-cmd

to do that for us in Fedora.

Let’s enable masquerading, allow DNS, and allow DHCP traffic. We can then make the state permanent:

firewall-cmd --add-masquerade firewall-cmd --add-service=dns --add-service=dhcp firewall-cmd --runtime-to-permanent

Testing

Put a client machine on your LAN network and you should be able to ping some public sites from the client:

[root@client ~]# ping -c 4 icanhazip.com PING icanhazip.com (104.238.141.75) 56(84) bytes of data. 64 bytes from lax.icanhazip.com (104.238.141.75): icmp_seq=1 ttl=52 time=69.8 ms 64 bytes from lax.icanhazip.com (104.238.141.75): icmp_seq=2 ttl=52 time=69.7 ms 64 bytes from lax.icanhazip.com (104.238.141.75): icmp_seq=3 ttl=52 time=69.6 ms 64 bytes from lax.icanhazip.com (104.238.141.75): icmp_seq=4 ttl=52 time=69.7 ms --- icanhazip.com ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3005ms rtt min/avg/max/mdev = 69.659/69.758/69.874/0.203 ms

Extras

If you need to adjust your network configuration, just run

systemctl restart systemd-networkd

afterwards. I’ve found that it’s quite intelligent about the network devices and it won’t reconfigure anything that hasn’t changed.

The

networkctl

command is very powerful. Check out the

status

and

lldp

functions to get more information about your network devices and the networks they’re connected to.

When something goes wrong, look in your systemd journal:

[root@router ~]# journalctl -u systemd-networkd -- Logs begin at Fri 2015-07-31 01:22:38 UTC, end at Fri 2015-07-31 02:11:24 UTC. -- Jul 31 01:46:14 router systemd[1]: Starting Network Service... Jul 31 01:46:14 router systemd-networkd[286]: Enumeration completed Jul 31 01:46:14 router systemd[1]: Started Network Service. Jul 31 01:46:15 router systemd-networkd[286]: eth1 : link configured Jul 31 01:46:15 router systemd-networkd[286]: eth0 : gained carrier Jul 31 01:46:15 router systemd-networkd[286]: eth0 : link configured Jul 31 01:46:16 router systemd-networkd[286]: eth1 : gained carrier