Access internal devices through the WireGuard tunnel

Enable IP forwarding on the server

Enable IP forwarding on your server by removing the comments in front of

net.ipv4.ip_forward = 1
net.ipv6.conf.all.forwarding = 1

in the file /etc/sysctl.d/99-sysctl.conf

Then apply the new option with the command below.

sudo sysctl -p

If you see the options repeated like

net.ipv4.ip_forward = 1
net.ipv6.conf.all.forwarding = 1

they were enabled successfully.

A properly configured firewall is highly recommended for any Internet-facing device. Configuring a firewall (iptables, ufw, etc.) is not part of this guide.

Enable NAT on the server

nftables (most distributions)

On your server, add the following to the [INTERFACE] section of your /etc/wireguard/wg0.conf:

PostUp = nft add table ip wireguard; nft add chain ip wireguard wireguard_chain {type nat hook postrouting priority srcnat\; policy accept\;}; nft add rule ip wireguard wireguard_chain counter packets 0 bytes 0 masquerade; nft add table ip6 wireguard; nft add chain ip6 wireguard wireguard_chain {type nat hook postrouting priority srcnat\; policy accept\;}; nft add rule ip6 wireguard wireguard_chain counter packets 0 bytes 0 masquerade
PostDown = nft delete table ip wireguard; nft delete table ip6 wireguard

You may need to install nftables using sudo apt-get install nftables.

iptables (older distributions)

If you are using a different distribution, you may need to use iptables instead of nftables. In this case, add the following to the [INTERFACE] section of your /etc/wireguard/wg0.conf:

PostUp = iptables -w -t nat -A POSTROUTING -o eth0 -j MASQUERADE; ip6tables -w -t nat -A POSTROUTING -o eth0 -j MASQUERADE
PostDown = iptables -w -t nat -D POSTROUTING -o eth0 -j MASQUERADE; ip6tables -w -t nat -D POSTROUTING -o eth0 -j MASQUERADE

Important: Substitute interface

Without the correct interface name, this will not work!

Substitute eth0 in the preceding lines to match the Internet-facing interface. This may be enp2s0 or similar on more recent Ubuntu versions. If you are unsure, you can use ip a to find the correct interface name. The interface name is the one that is connected to the Internet.

If you are using the nftables method, you do not need to specify the interface name in the PostUp and PostDown lines.

PostUp and PostDown defines steps to be run after the interface is turned on or off, respectively. In this case, iptables is used to set Linux IP masquerade rules to allow all the clients to share the server’s IPv4 and IPv6 address. The rules will then be cleared once the tunnel is down.

Exemplary server config file with this change

[Interface]
PrivateKey = [your server's private key]
Address = [Wireguard-internal IPs of the server, e.g. 10.100.0.1/24, fd08:4711::1/64]
ListenPort = 47111

PostUp = nft add table ip wireguard; nft add chain ip wireguard wireguard_chain {type nat hook postrouting priority srcnat\; policy accept\;}; nft add rule ip wireguard wireguard_chain counter packets 0 bytes 0 masquerade; nft add table ip6 wireguard; nft add chain ip6 wireguard wireguard_chain {type nat hook postrouting priority srcnat\; policy accept\;}; nft add rule ip6 wireguard wireguard_chain counter packets 0 bytes 0 masquerade
PostDown = nft delete table ip wireguard; nft delete table ip6 wireguard

# Android phone
[Peer]
PublicKey = [public key of this client]
PresharedKey = [pre-shared key of this client]
AllowedIPs = [Wireguard-internal IP of this client, e.g., 10.100.0.2/32, fd08:4711::2/128]

# maybe more [Peer] sections for more clients

The important change is the extra PostUp and PostDown in the [Interface] section.

Allow clients to access other devices

In our standard configuration, we have configured the clients in such a way that they can only speak to the server. Add the network range of your local network in CIDR notation (e.g., 192.168.2.1 - 192.168.2.254 -> 192.168.2.0/24) in the [Peers] section of all clients you want to have this feature:

[Peer]
AllowedIPs = 10.0.0.0/24, fd08:4711::/64, 192.168.2.0/24

It is possible to add this only for a few clients, leaving the others isolated to only the Pi-hole server itself.

Exemplary client config file with this change

[Interface]
PrivateKey = [your client's private key]
Address = [Wireguard-internal IPs of your client, e.g. 10.100.0.2/32, fd08:4711::2/128]
DNS = 10.100.0.1

[Peer]
AllowedIPs = 10.100.0.0/24, fd08:4711::/64, 192.168.2.0/24
Endpoint = [your server's public IP or domain]:47111
PublicKey = [public key of the server]
PresharedKey = [pre-shared key of this client]
PersistentKeepalive = 25

The important change is the extra 192.168.2.0/24 at the end of the [Peer] -> AllowedIPs entry.