DD-WRT:Firewall

A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass.

Many personal computer operating systems include software-based firewalls to protect against threats from the public Internet. Many routers that pass data between networks contain firewall components and, conversely, many firewalls can perform basic routing functions.

The purpose of the firewall is to moderate traffic and/or log it. Most firewall are made for moderating ip traffic and are called ip firewalls.

The simplest ip firewall has two physical interfaces normally referred to as inside (LAN) and outside (WAN, the internet). It has two main access control lists (ACL) - e.g. named inside2outside and outside2inside.

Packet filter firewall
The simplest ip firewall - a packet filter firewall - can pass packet by packet or drop them based on:
 * source ip address
 * destination ip address
 * If tcp or udp:
 * source tcp/udp port
 * destination tcp/udp port

Statefull firewall
The better ip firewall - a statefull firewall - can pass packet by packet - and if possible (e.g. tcp and udp) track the connection. A statefull firewall can additionally moderate trackable traffic by:
 * number of connections per (src/dst) ip address
 * number of connections per interface
 * number of connections attempt - "SYN"-attacks, packet storms

NAT - Network address Translation
Due to IPv4 address shortage, the internet society began to use NAT, and therefore the firewall also need to be NAT aware.

NAT incompatible protocols
A real problem with NAT is when more than one inside clients (e.g. C1, C2) connect to the same outside server ip address (S) and the traffic is not tcp and udp. When a response outside packet later arrives at the NAT device (firewall), it can not deduce which client to send it to. Here are examples of protocols that has that problem:
 * IPsec (over IP protocol 51)
 * PPTP (over IP protocol 47)
 * L2TP (over IP protocol 50)

Even if the traffic is unencrypted it can not be deduced where to NAT a response outside packet, if more than one inside client uses the same protocol to the same outside ip address. UDP and TCP are special because they have 65536 possible src and dst ports that can help connection tracking.

Firewall difficult protocols
Some protocols can in-line signal a port jump and/or create connections one or both ways "at will". A firewall that can moderate that kind of traffic, need to inspect the traffic stream. To do that a firewall must have transparent proxies and are then called an application firewall.

Some examples of protocols that can port jump and/or create additional connections are:
 * FTP passive
 * FTP active - if you enable proxy support for active FTP, you firewall can be "punctured" from the internet and is therefore almost useless.
 * Media streams (Media Player, iTunes...):
 * RTSP
 * Realmedia
 * Conferencing
 * VoIP, IP telephony:
 * H323
 * SIP
 * Some gaming protocols

DD-WRT firewall - iptables
DD-WRT has a packet filtering firewall, statefull firewall, NAT and proxy functionality.

The default internal device network has two networks (non-802.11n example!):
 * vlan0(built-in hardware switch) software-bridged with eth1(wireless access point) - LAN private ip subnet 192.168.1.0/24 and ip configurations are leased to clients by a DHCP server.
 * vlan1 - WAN with some ip configuration normally acquired via a DHCP client.

There is a default ip firewall with NAT between vlan0 and vlan1 (on non-802.11n) network devices.

See Internal Device Network#Examples of Changed Internal Network for other firewall examples.

Netfilter iptables architecture

 * sns.ias.edu: Kernel space structure - simple packet journey through kernel
 * The left and right upper red arrows together, is the input and output of your network device logical network interfaces (bridges=switches, - and vlans). The five blue balls represent the default firewall chains hook points. The "local process" is your network device's service process(es) - e.g. remote management (WEB server, Telnet or SSH server), Samba server, PPPoE client, DHCP server(s) or client and so on.