27 June, 2021

The QUICK quick guide to setting up a WireGurd VPN tunnel

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 Background story (skippable)

So I've been maintaining a set of OpenVPN tunnels into various NAT-routed networks for several years now and every now and then a software update will declare some of the keys too old and insecure to be used. This means I have to dust-off the TLS PKI files and remember how to use it so I can press-out a new set of keys and distribute these to all the places they need to be. PKI is generally a drag and I decided it was finally time to learn how to set-up the new and hip WireGuard tunnelling solution.
Unfortunately it took me a couple of hours to set it up just the way I wanted as there are a few large differences between the two systems:
  • Wireguard does not use TLS/PKI, it just uses pre-shared public/private key pairs
  • Wireguard uses a peer-to-peer setup, not client-server like OpenVPN
  • Wireguard ignores NAT by default and is fairly "quiet" so you may need keep-alive messages to keep UDP ports open trhough NAT routers

The situation

We want to replace an OpenVPN client/server pair with WireGuard peer equivalents. The OpenVPN server uses a static DNS address with a port-forward through a NAT router. The OpenVPN client is behind another NAT router, but with a dynamic port and address.

Solution

Do this on both nodes:
  • Install wireguard-tools
  • Go to /etc/wireguard and execute (as root) wg genkey | tee privatekey | wg pubkey > publickey.
  • Edit the config file wg0.conf.
    • Only on the "server" peer, enter:
      •  [Interface]
        Address = {{ SERVER_VPN_INTERNAL_ADDRESS }}
        PrivateKey = {{ SERVER_PRIVATE_KEY }}

        [Peer]
        PublicKey = {{ CLIENT_PUBLIC_KEY }}
        AllowedIPs = {{ CLIENT_VPN_INTERNAL_ADDRESS }}/32


    •   Only on the "client" peer, enter
      • [Interface]
        Address = {{ CLIENT_VPN_INTERNAL_ADDRESS }}
        PrivateKey = {{ CLIENT_PRIVATE_KEY }}

        [Peer]
        PublicKey = {{ SERVER_PUBLIC_KEY }}
        Endpoint = {{ SERVER_DNS_ADDRESS }}:{{ FORWARDED_PORT }}
        AllowedIPs = {{ SERVER_VPN_INTERNAL_ADDRESS }}/32
        PersistentKeepalive = 25

  • Issue (as root) nmcli con import type wireguard file /etc/wireguard/wg0.conf
  •  Start pinging each end

Details

The VPN_INTERNAL_ADDRESS fields are just any RFC-1918 private address pair (e.g. 192.168.10.1 and 192.168.10.2). So long as you're just connecting two hosts (not forwarding/bounching traffic), use the single host "/32" netmask for the "AllowedIPs" fields.

The PersistentKeepalive field ensures the dynamic-port "client" keeps the connection alive every 25 seconds and thus the NAT router will keep the port open. Without this, If the connection goes "quiet" for a while, the router might drop the port-forward and the "server" won't be able to send any traffic in until the "client" sends something out again.

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