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Cisco ASA Series CLI Configuration Guide
Chapter 1 Introduction to the Cisco ASA
Firewall Functional Overview
•
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
.
A stateful firewall like the ASA, however, takes into consideration the state of a packet:
•
Is this a new connection?
If it is a new connection, the ASA has to check the packet against access lists and perform other
tasks to determine if the packet is allowed or denied. To perform this check, the first packet of the
session goes through the “session management path,” and depending on the type of traffic, it might
also pass through the “control plane path.”
The session management path is responsible for the following tasks:
–
Performing the access list checks
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Performing route lookups
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Allocating NAT translations (xlates)
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Establishing sessions in the “fast path”
The ASA creates forward and reverse flows in the fast path for TCP traffic; the ASA also creates
connection state information for connectionless protocols like UDP, ICMP (when you enable ICMP
inspection), so that they can also use the fast path.
Note
For other IP protocols, like SCTP, the ASA does not create reverse path flows. As a result,
ICMP error packets that refer to these connections are dropped.
Some packets that require Layer 7 inspection (the packet payload must be inspected or altered) are
passed on to the control plane path. Layer 7 inspection engines are required for protocols that have
two or more channels: a data channel, which uses well-known port numbers, and a control channel,
which uses different port numbers for each session. These protocols include FTP, H.323, and SNMP.
•
Is this an established connection?
Summary of Contents for 5505 - ASA Firewall Edition Bundle
Page 28: ...Glossary GL 24 Cisco ASA Series CLI Configuration Guide ...
Page 61: ...P A R T 1 Getting Started with the ASA ...
Page 62: ......
Page 219: ...P A R T 2 Configuring High Availability and Scalability ...
Page 220: ......
Page 403: ...P A R T 2 Configuring Interfaces ...
Page 404: ......
Page 499: ...P A R T 2 Configuring Basic Settings ...
Page 500: ......
Page 533: ...P A R T 2 Configuring Objects and Access Lists ...
Page 534: ......
Page 601: ...P A R T 2 Configuring IP Routing ...
Page 602: ......
Page 745: ...P A R T 2 Configuring Network Address Translation ...
Page 746: ......
Page 845: ...P A R T 2 Configuring AAA Servers and the Local Database ...
Page 846: ......
Page 981: ...P A R T 2 Configuring Access Control ...
Page 982: ......
Page 1061: ...P A R T 2 Configuring Service Policies Using the Modular Policy Framework ...
Page 1062: ......
Page 1093: ...P A R T 2 Configuring Application Inspection ...
Page 1094: ......
Page 1191: ...P A R T 2 Configuring Unified Communications ...
Page 1192: ......
Page 1333: ...P A R T 2 Configuring Connection Settings and QoS ...
Page 1334: ......
Page 1379: ...P A R T 2 Configuring Advanced Network Protection ...
Page 1380: ......
Page 1475: ...P A R T 2 Configuring Modules ...
Page 1476: ......
Page 1549: ...P A R T 2 Configuring VPN ...
Page 1550: ......
Page 1965: ...P A R T 2 Configuring Logging SNMP and Smart Call Home ...
Page 1966: ......
Page 2059: ...P A R T 2 System Administration ...
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Page 2098: ...1 8 Cisco ASA Series CLI Configuration Guide Chapter 1 Troubleshooting Viewing the Coredump ...
Page 2099: ...P A R T 2 Reference ...
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