Using Network Layer Firewalls with Lync Server

Wikipedia defines a firewall as a part of a computer system or network that is designed to block unauthorized access while permitting authorized communications. It is a device or set of devices configured to permit or deny computer applications based on a set of rules and other criteria.

There are several types of firewall techniques including

• Packet filtering—Packet filtering inspects packets as they are passed through the network and rejects or accepts these packets based on defined rules. Typically, these rules are in the form of source address to destination address on port XYZ, allow. Packet-filtering firewalls are generally fast but can be difficult to configure for applications that dynamically choose ports for communications after an initial handshake.

• Application gateway—Application gateways apply security enforcement to specific applications. In other words, the gateway understands the applications and can recognize its packets. It makes its decisions based on which applications are allowed to pass through the firewall. Application gateways can be relatively easy to configure but are generally processor intensive and thus cannot handle as much throughput as a packet-filtering firewall.

• Proxy/reverse proxy server—A proxy server intercepts all messages entering and leaving the network. It inspects the packets and then continues the conversation on behalf of the protected system. In this way, packets never go directly from the source to the protected destination or from the protected source directly to the uncontrolled destination. Not unlike applications gateways, proxy servers are processor intensive.