The goal of a router is forward packet streams from incoming network interfaces to outgoing network interfaces. Firewall rules add another layer of granularity to what is allowed to be forwarded - and additionally what is allowed to be inputted to, and outputted from,l the router itself. This section discusses the relationships between the firewall code and the network interfaces.
At the heart of all routers is a hardware switch with a number interface ports. When a packet enters one of the switch ports, the hardware switch matches a fixed field in the packet and forwards the packet to an output port which transmits it. The switch generally uses the the layer-2 destination MAC address in the packet to switch on. Each port has a MAC cache, which gradually ages-out. If the destination MAC is not bound to a port, other protocols (e.g. ARP) are used to discover which switch port can deliver a packet to the destination MAC.
OpenWrt routers have two types of LAN interface: wired ethernet (IEEE802.3 or RFC894 Ethernet II, Ethernet II being the most common) and wireless ethernet (IEEE802.11.)
The wired LAN ports each map directly to a single switch port. Generally there is one 802.11 wifi port attached to a Wifi radio chip (2.4Ghz, 5Ghz). Each handles one or more IEEE802.11 standard protocols (e.g. 802.11a, 802.11n) and ancillary support for wireless networks (e.g. 802.11s mesh networking). The wifi chips convert the 802.11 signal into a canonical ethernet frame injected into the switch port for routing. All wifi stations connected to the 802.11 Access Point use the same radio(s) and the same switch port.
Layer-2 frames with a known destination MAC are switched to the desired LAN port. If the MAC is not present in the switch cache, a broadcast packet (e.g. ARP) is flooded to all LAN ports to discover which has access to the destination MAC. After that the destination MAC is bound to that port for a period of time.
The LAN bridge combines the WLAN interface(s) with the wired LAN ports to
create a single logical network. In the
interface configuration set
option type bridge or in LuCI Network→Interfaces→LAN
Bridge interfaces box and select the physical interfaces to bridge
together. All switch ports in the bridge will act as a single network.
The new psuedo-interface has a
br- prepended to the interface name,
Use bridging when combining WLAN and wired ethernet ports. Otherwise partition the ports into VLANs.
The firewall of an OpenWrt router is able to collect interfaces into
to more logically filter traffic. A zone can be configured to any set of
interfaces but generally there are at least two zones:
lan for the
collection of LAN interfaces and
wan for the WAN interfaces.
This simplifies the firewall rule logic somewhat by conceptually grouping the interfaces:
recognize the zone concept does not significantly simplify a simple
SOHO router with a single
br-lan interface and a single
Each interface has a one-to-one mapping with a zone.
VLAN provisioning and use is documented in:
A switch partitioned into multiple VLANs futher helps to organize the switch ports. It is recommended that each VLAN map one-to-one with a zone. The advantage to using a VLAN architecture is the packets are tagged with the VLAN id to disambiguate routing/firewall decisions.