The firewall takes care to only allow necessary network traffic to and from Gluetun. It also effectively acts as a kill switch in case the VPN connection goes down, blocking all traffic.
The firewall is rather strict by default:
- it drops any incoming traffic except from your Docker network on the (non VPN) default route found.
- it drops any outgoing traffic except to the combination VPN network interface + VPN server IP address + VPN server port + VPN server protocol.
- it drops any forwarding traffic
The firewall can be adjusted, see the Firewall options.
Its setup happens at container start and takes about 15 milliseconds from start. This cannot be reduced further, since setting the firewall rules already takes 10 milliseconds. It is also unlikely Docker connected containers would connect to Gluetun before it sets up its firewall, and the built-in proxies only get started after the firewall is enabled.
The firewall is never de-activated.
For now the firewall uses iptables and ip6tables (or iptables-nft and ip6tables-nft) which are called using custom Go code using a subshell. Most of this code resides in the internal/firewall package.