Debian 13 as a dual WAN router

Debian 13 can be configured as a dual WAN router, using a single N100 mini PC with six gigabit ports and an NVMe disk. This setup replaces a Cisco router that provided Internet access through a Ziggo cable line and a KPN fiber line.

The N100 mini PC has six ports, three of which are currently in use: enp1s0 for the Ziggo modem, enp2s0 for the KPN fiber connection via a media converter, and enp5s0 for a 16-port PoE LAN switch. The remaining ports remain spare.

The network configuration is managed using ifupdown, with separate configuration files for each port in /etc/network/interfaces.d/. The Ziggo modem uses DHCP for its interface, while the KPN line requires PPPoE with VLAN 6 and a larger MTU. This configuration is handled through /etc/network/interfaces.d/1_kpn, which sets up the physical port, VLAN, and PPPoE session.

To forward packets, the sysctl.conf file is modified to enable IP forwarding. Additionally, dnsmasq is installed to serve DHCP and DNS services on the LAN, forwarding requests to Cloudflare and Google DNS servers. This ensures DNS functionality remains active regardless of which Internet line is active.

Firewall and NAT rules are managed by nftables, which create tables and chains for input, forward, and output. These rules allow traffic to flow between the two WAN lines while limiting access to unnecessary services. Additionally, the MSS clamping rules ensure proper TCP connections by setting the maximum segment size based on the route MTU.