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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 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 údp) 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.

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 an outside packet arrive at the NAT device it can not deduce which client to send it to. Here are examples of protocols that has problem:

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

Difficult protocols

Some protocols can port jump and 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 proxies.

Some examples of protocols that can port jump and create additional connections are:

  • FTP
  • Media streams (Media Player, iTunes...):
    • RTSP
    • Realmedia
    • Conferencing
    • VoIP, IP telephony:
      • H323
      • SIP
  • Some gaming protocols


Your network device typically has more than two interfaces. DD-WRT has a statefull firewall, NAT, proxy functionality via netfilter with among others iptables.

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 and ip configurations are leased out by a DHCP server.
  • vlan1 - WAN with some ip address 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 also

External links