Cisco ASA 5505 - Manual (Page 97)

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Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 1 Introduction to the Cisco ASA 5500 Series
Firewall Functional Overview
Enabling the Botnet Traffic Filter
Malware is malicious software that is installed on an unknowing host. Malware that attempts network
activity such as sending private data (passwords, credit card numbers, key strokes, or proprietary data)
can be detected by the Botnet Traffic Filter when the malware starts a connection to a known bad IP
address. The Botnet Traffic Filter checks incoming and outgoing connections against a dynamic database
of known bad domain names and IP addresses (the blacklist), and then logs any suspicious activity. When
you see syslog messages about the malware activity, you can take steps to isolate and disinfect the host.
Configuring Cisco Unified Communications
The Cisco ASA 5500 series is a strategic platform to provide proxy functions for unified
communications deployments. The purpose of a proxy is to terminate and reoriginate connections
between a client and server. The proxy delivers a range of security functions such as traffic inspection,
protocol conformance, and policy control to ensure security for the internal network. An increasingly
popular function of a proxy is to terminate encrypted connections in order to apply security policies
while maintaining confidentiality of connections.
Firewall Mode Overview
The ASA runs in two different firewall modes:
• Routed
• Transparent
In routed mode, the ASA is considered to be a router hop in the network.
In transparent mode, the ASA acts like a “bump in the wire,” or a “stealth firewall,” and is not considered
a router hop. The ASA connects to the same network on its inside and outside interfaces.
You might use a transparent firewall to simplify your network configuration. Transparent mode is also
useful if you want the firewall to be invisible to attackers. You can also use a transparent firewall for
traffic that would otherwise be blocked in routed mode. For example, a transparent firewall can allow
multicast streams using an EtherType access list.
Stateful Inspection Overview
All traffic that goes through the ASA is inspected using the Adaptive Security Algorithm and either
allowed through or dropped. A simple packet filter can check for the correct source address, destination
address, and ports, but it does not check that the packet sequence or flags are correct. A filter also checks
every packet against the filter, which can be a slow process.
Note The TCP state bypass feature allows you to customize the packet flow. See the “TCP State Bypass”
section on page 53-3 .
A stateful firewall like the ASA, however, takes into consideration the state of a packet:
• Is this a new connection?