Avaya Business Secure Router 252 Configuration - Basics Manual do Utilizador descarregar pdf (Página 164)

164 Chapter 10 Firewalls

NN47923-500

These custom rules work by evaluating the network traffic source IP address,

destination IP address, IP protocol type, and comparing these to rules set by the

administrator.

Below is a brief technical description of how these connections are tracked.

Connections can either be defined by the upper protocols (for instance, TCP), or

by the Business Secure Router itself (as with the virtual connections created for

UDP and ICMP).

TCP security

The Business Secure Router uses state information embedded in TCP packets.

The first packet of any new connection has its SYN flag set and its ACK flag

cleared; these are initiation packets. All packets that do not have this flag structure

are called subsequent packets, since they represent data that occurs later in the

TCP stream.

If an initiation packet originates on the WAN, someone is trying to make a

connection from the Internet into the LAN. Except in a few special cases, (see

“Upper layer protocols” on page 165), these packets are dropped and logged.

If an initiation packet originates on the LAN, someone is trying to make a

connection from the LAN to the Internet. Assuming that this is an acceptable part

of the security policy (as is the case with the default policy), the connection is

allowed. A cache entry is added, which includes connection information such as

IP addresses, TCP ports, and sequence numbers.

After the Business Secure Router receives any subsequent packet (from the

Internet or from the LAN), its connection information is extracted and checked

against the cache. A packet is only allowed to pass through if it corresponds to a

valid connection (that is, if it is a response to a connection that originated on the

LAN).

Note: The ability to define firewall rules is a very powerful tool. Using

custom rules, it is possible to disable all firewall protection or block all

access to the Internet. Use extreme caution when creating or deleting

firewall rules. Test changes after creating them to make sure they work

correctly.

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Avaya Business Secure Router 252 Configuration - Basics Manual do Utilizador Página 164