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Build a network router and firewall with Fedora 22 and systemd-networkd

··926 words·5 mins·
Fedora Firewall Networking Security Systemd
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This post originally appeared on the Fedora Magazine blog.

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
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