The rules defining a valid session prescribe, for example, the destination and source addresses and ports, the time of day, the protocol being used, the user and the password. circuits) at the transport layer (typically TCP connections) against a table of allowed connections, before a session can be opened and data exchanged. It checks the validity of connections (i.e. However, because circuit level firewalls work at the transport layer, they require substantial modification to the programming that normally provides transport functions for example, Windows Socket Interface (WinSock).Ĥ-10 Securing Cisco Network Devices (SND) v2.0 © 2006 Cisco Systems, Inc.A circuit level gateway operates at the transport layer of the OSI or internet reference models and, as the name implies, implements circuit level filtering rather than packet level filtering. This makes IP spoofing much more difficult. Once the firewall terminates a connection, it removes the table entry for that connection, and the virtual circuit between the two peer transport layers is closed.Ĭircuit level filtering has an advantage over packet filtering because it can make up for the shortcomings of the UDP protocol in which the source address is never validated as a function of the protocol. This is useful for hiding information about protected networks.Ī circuit level firewall maintains a table (including complete session state and sequencing information) of valid connections and allows network packets containing data to pass through when network packet information matches an entry in the virtual circuit table.
Any information passed to a remote computer through a circuit level firewall appears to have originated from the gateway. In addition, the firewall will not forward data packets until the handshake is complete. To validate a session, a circuit level firewall examines each connection setup to ensure that the connection follows a legitimate TCP handshake.
In addition to allowing or disallowing packets, the circuit level firewall also determines whether the connection between both ends is valid according to configurable rules. Requires reprogramming of transport handling ^Ī circuit level firewall, also called a circuit level gateway, is second-generation firewall technology that validates that a packet is either a connection request or a data packet belonging to a connection or virtual circuit between two peer transport layers.
SCND This topic describes the operation of a circuit level firewall.
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