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Basic NAT and firewall policy

routerd applies IPv4 NAPT (NAT44) and a stateful firewall to a Linux router. This tutorial shows the minimum resources needed to put both in place on a freshly installed host.

Scenario​

The router has:

  • An upstream interface (wan) carrying IPv4 connectivity (any of: native dual-stack, PPPoE, or DS-Lite).
  • A LAN interface (lan) serving local clients with private addresses.
  • Optionally, a management interface (mgmt).

The goal is to:

  • Masquerade outbound IPv4 from the LAN.
  • Apply a sane default firewall posture (WAN cannot reach LAN, LAN can reach WAN, management can reach the router itself).

NAT44​

- apiVersion: net.routerd.net/v1alpha1
  kind: NAT44Rule
  metadata:
    name: lan-to-wan
  spec:
    outboundInterface: wan
    sourceCIDRs:
      - 192.0.2.0/24
    masquerade: true

routerd renders the rule into the routerd_nat nftables table. The same shape works for plain DHCP-acquired uplinks, PPPoE pseudo-interfaces, and DS-Lite tunnels — only outboundInterface changes.

Conntrack observation​

routerd consumes conntrack so the Web Console and routerctl connections can show live flows. Where /proc/net/nf_conntrack is missing, routerd falls back to a sysctl-derived summary; it does not stop on this condition, but the per-flow detail will be unavailable.

Firewall kinds​

FirewallZone, FirewallPolicy, and FirewallRule describe a stateful filter. routerd renders them into the inet routerd_filter nftables table.

The roles (untrust, trust, mgmt) provide the implicit accept/drop matrix. routerd injects internal openings for managed services (DHCP, DNS, DS-Lite control traffic, etc.) so they keep working when the firewall is on.

- apiVersion: firewall.routerd.net/v1alpha1
  kind: FirewallZone
  metadata: {name: wan}
  spec:
    role: untrust
    interfaces:
      - Interface/wan

- apiVersion: firewall.routerd.net/v1alpha1
  kind: FirewallZone
  metadata: {name: lan}
  spec:
    role: trust
    interfaces:
      - Interface/lan

- apiVersion: firewall.routerd.net/v1alpha1
  kind: FirewallPolicy
  metadata: {name: default}
  spec: {}

For exceptions to the implicit matrix, use the firewall rule guide.

Validation​

routerctl describe NAT44Rule/lan-to-wan
routerctl firewall test from=wan to=lan proto=tcp dport=22
nft list table inet routerd_filter
nft list table ip routerd_nat

See also​