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Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 1 Introduction to the Cisco ASA 5500 Series
Firewall Functional Overview
Applying Connection Limits and TCP Normalization
You can limit TCP and UDP connections and embryonic connections. Limiting the number of
connections and embryonic connections protects you from a DoS attack. The ASA uses the embryonic
limit to trigger TCP Intercept, which protects inside systems from a DoS attack perpetrated by flooding
an interface with TCP SYN packets. An embryonic connection is a connection request that has not
finished the necessary handshake between source and destination.
TCP normalization is a feature consisting of advanced TCP connection settings designed to drop packets
that do not appear normal.
Enabling Threat Detection
You can configure scanning threat detection and basic threat detection, and also how to use statistics to
analyze threats.
Basic threat detection detects activity that might be related to an attack, such as a DoS attack, and
automatically sends a system log message.
A typical scanning attack consists of a host that tests the accessibility of every IP address in a subnet (by
scanning through many hosts in the subnet or sweeping through many ports in a host or subnet). The
scanning threat detection feature determines when a host is performing a scan. Unlike IPS scan detection
that is based on traffic signatures, the ASA scanning threat detection feature maintains an extensive
database that contains host statistics that can be analyzed for scanning activity.
The host database tracks suspicious activity such as connections with no return activity, access of closed
service ports, vulnerable TCP behaviors such as non-random IPID, and many more behaviors.
You can configure the ASA to send system log messages about an attacker or you can automatically shun
the host.
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