Connecting for co-op: LAN or VPN

Co-op runs directly between the players' machines. Each machine listens on UDP port 7777, and nothing gets through a home router from the internet, so every machine must be able to reach the others directly:

• Same home network (LAN)? You are done; check the firewall.
• Friends elsewhere? Put everyone on one free virtual network with ZeroTier or Tailscale. It takes a few minutes, once.

When you are connected, the co-op guide takes over.

ZeroTier

One player creates the network (once):

1. Sign in at my.zerotier.com (a free account).
2. Press Create A Network. Copy its Network ID, 16 characters like 8056c2e21c000001, and send it to your friends.

Every player joins it, the creator too:

1. Install ZeroTier:

   sudo pacman -S zerotier-one                       # Arch
   curl -s https://install.zerotier.com | sudo bash  # other Linux
   sudo systemctl enable --now zerotier-one          # Linux: start it

   On macOS, run the installer from zerotier.com/download.

2. Join the network:

   sudo zerotier-cli join <network-id>

   On macOS you can also use the ZeroTier menu-bar icon: Join New Network.

The creator lets everyone in. On my.zerotier.com, open the network and scroll to Members. Each machine that joined is listed; tick its Auth box. A new network is private, so nobody gets an address until this is done.

Every player checks it works:

sudo zerotier-cli listnetworks   # your network should say OK, with an address
ping <a friend's ZeroTier address>

Which address is the ZeroTier one? ermod-engine coop id lists every address this machine has, each with its interface in brackets, so it lists your home network's address as well. Pick the ZeroTier one in either of two ways:

• Match it. The last column of sudo zerotier-cli listnetworks is your ZeroTier address, for example 10.147.20.5/24. Drop the /24: the coop id line with 10.147.20.5 is the one to send. The network page on my.zerotier.com shows the same address under Members, as Managed IPs.
• Read the interface name. On Linux, ZeroTier's interface starts with zt (for example ztabcd1234). On macOS it starts with feth (for example feth956).

Do not go by the number. A ZeroTier network may hand out 10.…, 172.… or 192.168.… addresses, so its address can look just like a home network's. Trust the interface name, or the match with listnetworks.

Linux:
Address    192.168.1.10      (enp5s0)        ← your home network: not this one
           10.147.20.5       (ztabcd1234)    ← ZeroTier: send this join line

macOS:
Address    192.168.1.23      (en0)           ← your home network: not this one
           192.168.194.177   (feth956)       ← ZeroTier: send this join line

With three or more players, every address a joiner names (the host's and the earlier joiners') is a ZeroTier address: a friend elsewhere cannot reach your home network's address.

Tailscale

1. Every player installs Tailscale and runs sudo tailscale up.
2. Everyone signs in to the same tailnet, or the owner shares their machine with friends.
3. tailscale ip -4 prints your Tailscale address (100.…). In coop id it is the one on the tailscale0 interface (on macOS, a utun interface); send the join line with that address. Ping a friend's to check.

The firewall

If a machine runs a firewall, let in UDP on port 7777 (on the VPN interface too, if you use one):

sudo ufw allow 7777/udp                                   # Linux, ufw
sudo firewall-cmd --add-port=7777/udp --permanent && sudo firewall-cmd --reload   # Linux, firewalld

On macOS, if the system asks whether Wine may accept incoming connections, allow it.

A friend still not getting through? See The friend never shows up.