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DHCPv6 prefix delegation with systemd-networkd

··841 words·4 mins·
Fedora Linux Networking
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My home internet comes from Spectrum (formerly Time Warner Cable) and they offer IPv6 addresses for cable modem subscribers. One of the handy features they provide is DHCPv6 prefix delegation. If you’re not familiar with that topic, here’s a primer on how you get IPv6 addresses:

  • SLAAC: Your machine selects an IPv6 address based on router advertisements
  • DHCPv6: Your machine makes a DHCPv6 request (a lot like DHCP requests) and gets an address back to use
  • DHCPv6 with prefix delegation: Your machine makes a special DHCPv6 request where you provide a hint about the size of the IPv6 network prefix you want.

Are you new to IPv6 subnets and how they’re different from IPv4? If so, you might want to read up on IPv6 subnets first.

In a previous post, I wrote about using wide-dhcpv6 to get IPv6 addresses and that guide still works, but using systemd-networkd makes the process much easier.

Who needs this many IP addresses? #

Yes, I know that a /64 IPv6 network contains 18,446,744,073,709,551,616 addresses and that should be enough for most home networks. Using one /64 network block per interface has plenty of benefits for simplifying your network, though.

A /56 from your provider contains 256 /64 networks and this makes it easy to configure up to 256 internal networks with a /64 on each. Breaking up a /64 subnet into pieces becomes frustrating very quickly.

Laying out the basic network #

My Linux router is a Dell Optiplex running Fedora 34 with a dual-port Intel I350 network card. These little machines are assembled well and last a long time. My network interfaces are set up like this:

  • enp2s0f0: connected to my cable modem for internet access
  • enp2s0f1: connected to a network bridge (br0) for my LAN network

My LAN (192.168.10.0/24) gateway sits on br0 and masquerades traffic out through enp2s0f0, the external network interface.

All of the systemd-networkd configuration lives in /etc/systemd/network and we will add some files there. First off, we need to set up the external network:

# /etc/systemd/network/wan.network
[Match]
Name=enp2s0f0

[Network]
DHCP=yes

Now we need to set up the internal bridge br0:

# /etc/systemd/network/lanbridge.network
[NetDev]
Name=br0
Kind=bridge

Then we can configure the br0 network interface and IP address:

# /etc/systemd/network/lanbridge.network
[Match]
Name=br0

[Network]
Address=192.168.10.1/24
ConfigureWithoutCarrier=yes

🤔 Special note: I like to add the ConfigureWithoutCarrier option here because systemd-network sometimes takes a while to bring the bridge online after a reboot and that makes certain daemons, like dnsmasq, fail to start.

Now let’s connect the bridge to the physical network interface with a bind:

# /etc/systemd/network/lanbridge-bind.network
[Match]
Name=enp2s0f1

[Network]
Bridge=br0
ConfigureWithoutCarrier=true

Just run systemctl restart systemd-networkd and ensure all of your networks are alive:

$ networkctl
IDX LINK            TYPE     OPERATIONAL SETUP
  1 lo              loopback carrier     unmanaged
  2 enp2s0f0        ether    routable    configured
  4 enp2s0f1        ether    enslaved    configured
  5 br0             bridge   routable    configured

IPv6 time #

Every ISP is a bit different with how they assign IPv6 addresses and what size blocks they will allocate to you. I’ve seen where some will only give a /64, others give a /56, and others give something in between. As for Spectrum, they provide up to a /56 with a prefix delegation request. You may need to experiment with a /56 first and slowly back down towards /64 to see what your ISP might do.

Let’s go back to the configuration for the external interface and add our prefix delegation hint:

# /etc/systemd/networkd/wan.network
[Match]
Name=enp2s0f0

[Network]
DHCP=yes

[DHCPv6]
PrefixDelegationHint=::/56

When we apply this configuration, systemd-networkd will send a DHCPv6 request with a prefix hint included.

That’s half the battle. We also need a way to take a /64 block from the big /56 block and assign it to various network interfaces on our router. You can do this manually by looking at the /56, choosing how to subnet your network, and then manually assigning /64 blocks to each interface.

Manually assigning subnets is not a fun task. It gets worse when your ISP suddenly changes the network blocks assigned to you on a whim. 😱

Luckily, systemd-networkd has built in functionality to do this for you automatically! Let’s go back to the configuration for br0 and add a few lines:

# /etc/systemd/networkd/lanbridge.network
[Match]
Name=br0

[Network]
Address=192.168.10.1/24
ConfigureWithoutCarrier=yes

[Network]
IPv6SendRA=yes
DHCPv6PrefixDelegation=yes

Run systemctl restart systemd-networkd to apply the changes. You should see an IPv6 network assigned to the interface! 🎉

The IPv6SendRA option tells systemd-networkd to automatically announce the network block on the interface so that computers on the network will automatically assign their own addresses via SLAAC. (You can retire radvd if you used this in the past.)

Setting DHCPv6PrefixDelegation to yes will automatically pull a subnet from the prefix we asked for on the external network interface and add it to this interface (br0 in this case). There’s no need to calculate subnets, manage configurations, or deal with changes. It all happens automatically.

If you have other interfaces, such as VLANs, simply add the IPv6SendRA and DHCPv6PrefixDelegation options to their network configurations (the .network files, not the .netdev files), and apply the configuration.

Photo credit: tian kuan on Unsplash

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