9-12 Firewalls317517-A Rev 009.5.5 Upper Layer ProtocolsSome higher layer protocols (such as FTP and RealAudio) utilize multiple network connectionssimultaneously. In general terms, they usually have a "control connection" which is used forsending commands between endpoints, and then "data connections" which are used for transmittingbulk information.Consider the FTP protocol. A user on the LAN opens a control connection to a server on theInternet and requests a file. At this point, the remote server will open a data connection from theInternet. For FTP to work properly, this connection must be allowed to pass through even though aconnection from the Internet would normally be rejected.In order to achieve this, the Contivity 221 inspects the application-level FTP data. Specifically, itsearches for outgoing "PORT" commands, and when it sees these; it adds a cache entry for theanticipated data connection. This can be done safely, since the PORT command contains addressand port information, which can be used to uniquely identify the connection.Any protocol that operates in this way must be supported on a case-by-case basis. You can use theWebGUI’s Custom Ports feature to do this.9.6 Guidelines For Enhancing Security With YourFirewall1. Change the default password via SMT or WebGUI.2. Think about access control before you connect a console port to the network in any way,including attaching a modem to the port. Be aware that a break on the console port might giveunauthorized individuals total control of the firewall, even with access control configured.3. Limit who can telnet into your router.4. Don't enable any local service (such as SNMP or NTP) that you don't use. Any enabled servicecould present a potential security risk. A determined hacker might be able to find creative waysto misuse the enabled services to access the firewall or the network.5. For local services that are enabled, protect against misuse. Protect by configuring the servicesto communicate only with specific peers, and protect by configuring rules to block packets forthe services at specific interfaces.6. Protect against IP spoofing by making sure the firewall is active.7. Keep the firewall in a secured (locked) room.9.7 Packet Filtering Vs FirewallBelow are some comparisons between the Contivity 221’s filtering and firewall functions.
