Cisco ASA 5500 Series Configuration Guide using the CLI, 8.2
Introduction to the Security Appliance
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Introduction to the ASA

Table Of Contents

Introduction to the ASA

Supported Software, Models, and Modules

VPN Specifications

New Features

New Features in Version 8.2(5)

New Features in Version 8.2(4.4)

New Features in Version 8.2(4.1)

New Features in Version 8.2(4)

New Features in Version 8.2(3.9)

New Features in Version 8.2(3)

New Features in Version 8.2(2)

New Features in Version 8.2(1)

Firewall Functional Overview

Security Policy Overview

Permitting or Denying Traffic with Access Lists

Applying NAT

Protecting from IP Fragments

Using AAA for Through Traffic

Applying HTTP, HTTPS, or FTP Filtering

Applying Application Inspection

Sending Traffic to the Advanced Inspection and Prevention Security Services Module

Sending Traffic to the Content Security and Control Security Services Module

Applying QoS Policies

Applying Connection Limits and TCP Normalization

Enabling Threat Detection

Firewall Mode Overview

Stateful Inspection Overview

VPN Functional Overview

Security Context Overview


Introduction to the ASA


The ASA combines advanced stateful firewall and VPN concentrator functionality in one device, and for some models, an integrated intrusion prevention module called the AIP SSM/SSC or an integrated content security and control module called the CSC SSM. The ASA includes many advanced features, such as multiple security contexts (similar to virtualized firewalls), transparent (Layer 2) firewall or routed (Layer6 3) firewall operation, advanced inspection engines, IPSec VPN, SSL VPN, and clientless SSL VPN support, and many more features.

This chapter includes the following sections:

Supported Software, Models, and Modules

VPN Specifications

New Features

Firewall Functional Overview

VPN Functional Overview

Security Context Overview

Supported Software, Models, and Modules

For a complete list of supported ASA software, models, and modules, see Cisco ASA 5500 Series Hardware and Software Compatibility:

http://www.cisco.com/en/US/docs/security/asa/compatibility/asamatrx.html

VPN Specifications

See the Supported VPN Platforms, Cisco ASA 5500 Series at http://www.cisco.com/en/US/docs/security/asa/compatibility/asa-vpn-compatibility.html

New Features

This section includes the following topics:

New Features in Version 8.2(5)

New Features in Version 8.2(4.4)

New Features in Version 8.2(4.1)

New Features in Version 8.2(4)

New Features in Version 8.2(3.9)

New Features in Version 8.2(3)

New Features in Version 8.2(2)

New Features in Version 8.2(1)


Note New, changed, and deprecated syslog messages are listed in Cisco ASA 5500 Series System Log Messages.


New Features in Version 8.2(5)

New Features in Version 8.2(4.4)

New Features in Version 8.2(4.1)

New Features in Version 8.2(4)

New Features in Version 8.2(3.9)

New Features in Version 8.2(3)

New Features in Version 8.2(2)

Released: January 11, 2010

Table 1-1 lists the new features forASA Version 8.2(2).

Table 1-1 New Features for ASA Version 8.2(2) 

Feature
Description
Remote Access Features

Scalable Solutions for Waiting-to-Resume VPN Sessions

An administrator can now keep track of the number of users in the active state and can look at the statistics. The sessions that have been inactive for the longest time are marked as idle (and are automatically logged off) so that license capacity is not reached and new users can log in.

Also available in Version 8.0(5).

Application Inspection Features

Inspection for IP Options

You can now control which IP packets with specific IP options should be allowed through the ASA. You can also clear IP options from an IP packet, and then allow it through the ASA. Previously, all IP options were denied by default, except for some special cases.

Note This inspection is enabled by default. The following command is added to the default global service policy: inspect ip-options. Therefore, the ASA allows RSVP traffic that contains packets with the Router Alert option (option 20) when the ASA is in routed mode.

The following commands were introduced: policy-map type inspect ip-options, inspect ip-options, eool, nop.

 

Enabling Call Set up Between H.323 Endpoints

You can enable call setup between H.323 endpoints when the Gatekeeper is inside the network. The ASA includes options to open pinholes for calls based on the RegistrationRequest/RegistrationConfirm (RRQ/RCF) messages.

Because these RRQ/RCF messages are sent to and from the Gatekeeper, the calling endpoint IP address is unknown and the ASA opens a pinhole through source IP address/port 0/0. By default, this option is disabled.

The following command was introduced: ras-rcf-pinholes enable (under the policy-map type inspect h323 > parameters commands).

Also available in Version 8.0(5).

Unified Communication Features

Mobility Proxy application no longer requires Unified Communications Proxy license

The Mobility Proxy no longer requires the UC Proxy license.

Interface Features

In multiple context mode, auto-generated MAC addresses now use a user-configurable prefix, and other enhancements

The MAC address format was changed to allow use of a prefix, to use a fixed starting value (A2), and to use a different scheme for the primary and secondary unit MAC addresses in a failover pair.

The MAC addresess are also now persistent accross reloads.

The command parser now checks if auto-generation is enabled; if you want to also manually assign a MAC address, you cannot start the manual MAC address with A2.

The following command was modified: mac-address auto prefix prefix.

Also available in Version 8.0(5).

Support for Pause Frames for Flow Control on the ASA 5580 10 Gigabit Ethernet Interfaces

You can now enable pause (XOFF) frames for flow control.

The following command was introduced: flowcontrol.

 

Firewall Features

Botnet Traffic Filter Enhancements

The Botnet Traffic Filter now supports automatic blocking of blacklisted traffic based on the threat level. You can also view the category and threat level of malware sites in statistics and reports. Reporting was enhanced to show infected hosts. The 1 hour timeout for reports for top hosts was removed; there is now no timeout.

The following commands were introduced or modified: dynamic-filter ambiguous-is-black, dynamic-filter drop blacklist, show dynamic-filter statistics, show dynamic-filter reports infected-hosts, and show dynamic-filter reports top.

 

Connection timeouts for all protocols

The idle timeout was changed to apply to all protocols, not just TCP.

The following command was modified: set connection timeout.

 

Routing Features

DHCP RFC compatibility (rfc3011, rfc3527) to resolve routing issues

This enhancement introduces ASA support for DHCP RFCs 3011 (The IPv4 Subnet Selection Option) and 3527 (Link Selection Sub-option for the Relay Agent Information Option). For each DHCP server configured for VPN clients, you can now configure the ASA to send the Subnet Selection option or the Link Selection option.

The following command was modified: dhcp-server [subnet-selection | link-selection].

Also available in Version 8.0(5).

High Availablility Features

IPv6 Support in Failover Configurations

IPv6 is now supported in failover configurations. You can assign active and standby IPv6 addresses to interfaces and use IPv6 addresses for the failover and Stateful Failover interfaces.

The following commands were modified: failover interface ip, ipv6 address.

 

No notifications when interfaces are brought up or brought down during a switchover event

To distinguish between link up/down transitions during normal operation from link up/down transitions during failover, no link up/link down traps are sent during a failover. Also, no syslog messages about link up/down transitions during failover are sent.

Also available in Version 8.0(5).

AAA Features

100 AAA Server Groups

You can now configure up to 100 AAA server groups; the previous limit was 15 server groups.

The following command was modified: aaa-server.

 

Monitoring Features

Smart Call Home

Smart Call Home offers proactive diagnostics and real-time alerts on the ASA and provides higher network availability and increased operational efficiency. Customers and TAC engineers get what they need to resolve problems quickly when an issue is detected.

Note Smart Call Home server Version 3.0(1) has limited support for the ASA. See the "Important Notes" for more information.

The following commands were introduced: call-home, call-home send alert-group, call-home test, call-home send, service call-home, show call-home, show call-home registered-module status.

 


New Features in Version 8.2(1)

Released: May 6, 2009

Hi

Table 1-2 lists the new features for ASA Version 8.2(1).

Table 1-2 New Features for ASA Version 8.2(1) 

Feature
Description
Remote Access Features

One Time Password Support for ASDM Authentication

ASDM now supports administrator authentication using one time passwords (OTPs) supported by RSA SecurID (SDI). This feature addresses security concerns about administrators authenticating with static passwords.

New session controls for ASDM users include the ability to limit the session time and the idle time. When the password used by the ASDM administrator times out, ASDM prompts the administrator to re-authenticate.

The following commands were introduced: http server idle-timeout and http server session-timeout. The http server idle-timeout default is 20 minutes, and can be increased up to a maximum of 1440 minutes.

 

Pre-fill Username from Certificate

The pre-fill username feature enables the use of a username extracted from a certificate for username/password authentication. With this feature enabled, the username is "pre-filled" on the login screen, with the user being prompted only for the password. To use this feature, you must configure both the pre-fill username and the username-from-certificate commands in tunnel-group configuration mode.

The double-authentication feature is compatible with the pre-fill username feature, as the pre-fill username feature can support extracting a primary username and a secondary username from the certificate to serve as the usernames for double authentication when two usernames are required. When configuring the pre-fill username feature for double authentication, the administrator uses the following new tunnel-group general-attributes configuration mode commands:

secondary-pre-fill-username—Enables username extraction for Clientless or AnyConnect client connection.

secondary-username-from-certificate—Allows for extraction of a few standard DN fields from a certificate for use as a username.

 

Double Authentication

The double authentication feature implements two-factor authentication for remote access to the network, in accordance with the Payment Card Industry Standards Council Data Security Standard. This feature requires that the user enter two separate sets of login credentials at the login page. For example, the primary authentication might be a one-time password, and the secondary authentication might be a domain (Active Directory) credential. If either authentication fails, the connection is denied.

Both the AnyConnect VPN client and Clientless SSL VPN support double authentication. The AnyConnect client supports double authentication on Windows computers (including supported Windows Mobile devices and Start Before Logon), Mac computers, and Linux computers. The IPsec VPN client, SVC client, cut-through-proxy authentication, hardware client authentication, and management authentication do not support double authentication.

Double authentication requires the following new tunnel-group general-attributes configuration mode commands:

secondary-authentication-server-group—Specifies the secondary AAA server group, which cannot be an SDI server group.

secondary-username-from-certificate—Allows for extraction of a few standard DN fields from a certificate for use as a username.

secondary-pre-fill-username—Enables username extraction for Clientless or AnyConnect client connection.

authentication-attr-from-server—Specifies which authentication server authorization attributes are applied to the connection.

authenticated-session-username—Specifies which authentication username is associated with the session.

Note The RSA/SDI authentication server type cannot be used as the secondary username/password credential. It can only be used for primary authentication.

 

AnyConnect Essentials

AnyConnect Essentials is a separately licensed SSL VPN client, entirely configured on the ASA, that provides the full AnyConnect capability, with the following exceptions:

No CSD (including HostScan/Vault/Cache Cleaner)

No clientless SSL VPN

Optional Windows Mobile Support

The AnyConnect Essentials client provides remote end users running Microsoft Windows Vista, Windows Mobile, Windows XP or Windows 2000, Linux, or Macintosh OS X, with the benefits of a Cisco SSL VPN client.

To configure AnyConnect Essentials, the administrator uses the following command:

anyconnect-essentials—Enables the AnyConnect Essentials feature. If this feature is disabled (using the no form of this command), the SSL Premium license is used. This feature is enabled by default.

Note This license cannot be used at the same time as the shared SSL VPN premium license.

 

Disabling Cisco Secure Desktop per Connection Profile

When enabled, Cisco Secure Desktop automatically runs on all computers that make SSL VPN connections to the ASA. This new feature lets you exempt certain users from running Cisco Secure Desktop on a per connection profile basis. It prevents the detection of endpoint attributes for these sessions, so you might need to adjust the Dynamic Access Policy (DAP) configuration.

CLI: [no] without-csd command

Note "Connect Profile" in ASDM is also known as "Tunnel Group" in the CLI. Additionally, the group-url command is required for this feature. If the SSL VPN session uses connection-alias, this feature will not take effect.

 

Certificate Authentication Per Connection Profile

Previous versions supported certificate authentication for each ASA interface, so users received certificate prompts even if they did not need a certificate. With this new feature, users receive a certificate prompt only if the connection profile configuration requires a certificate. This feature is automatic; the ssl certificate authentication command is no longer needed, but the ASA retains it for backward compatibility.

 

EKU Extensions for Certificate Mapping

This feature adds the ability to create certificate maps that look at the Extended Key Usage extension of a client certificate and use these values in determining what connection profile the client should use. If the client does not match that profile, it uses the default group. The outcome of the connection then depends on whether or not the certificate is valid and the authentication settings of the connection profile.

The following command was introduced: extended-key-usage.

 

SSL VPN SharePoint Support for Win 2007 Server

Clientless SSL VPN sessions now support Microsoft Office SharePoint Server 2007.

Shared license for SSL VPN sessions

You can purchase a shared license with a large number of SSL VPN sessions and share the sessions as needed among a group of ASAs by configuring one of the ASAs as a shared license server, and the rest as clients. The following commands were introduced: license-server commands (various), show shared license.

Note This license cannot be used at the same time as the AnyConnect Essentials license.

 

Firewall Features

TCP state bypass

If you have asymmetric routing configured on upstream routers, and traffic alternates between two ASAs, then you can configure TCP state bypass for specific traffic. The following command was introduced: set connection advanced tcp-state-bypass.

 

Per-Interface IP Addresses for the Media-Termination Instance Used by the Phone Proxy

In Version 8.0(4), you configured a global media-termination address (MTA) on the ASA. In Version 8.2, you can now configure MTAs for individual interfaces (with a minimum of two MTAs). As a result of this enhancement, the old CLI has been deprecated. You can continue to use the old configuration if desired. However, if you need to change the configuration at all, only the new configuration method is accepted; you cannot later restore the old configuration.

 

Displaying the CTL File for the Phone Proxy

The Cisco Phone Proxy feature includes the show ctl-file command, which shows the contents of the CTL file used by the phone proxy. Using the show ctl-file command is useful for debugging when configuring the phone proxy instance.

This command is not supported in ASDM.

Clearing Secure-phone Entries from the Phone Proxy Database

The Cisco Phone Proxy feature includes the clear phone-proxy secure-phones command, which clears the secure-phone entries in the phone proxy database. Because secure IP phones always request a CTL file upon bootup, the phone proxy creates a database that marks the IP phones as secure. The entries in the secure phone database are removed after a specified configured timeout (via the timeout secure-phones command). Alternatively, you can use the clear phone-proxy secure-phones command to clear the phone proxy database without waiting for the configured timeout.

This command is not supported in ASDM.

H.239 Message Support in H.323 Application Inspection

In this release, the ASA supports the H.239 standard as part of H.323 application inspection. H.239 is a standard that provides the ability for H.300 series endpoints to open an additional video channel in a single call. In a call, an endpoint (such as a video phone), sends a channel for video and a channel for data presentation. The H.239 negotiation occurs on the H.245 channel. The ASA opens a pinhole for the additional media channel. The endpoints use open logical channel message (OLC) to signal a new channel creation. The message extension is part of H.245 version 13. The decoding and encoding of the telepresentation session is enabled by default. H.239 encoding and decoding is preformed by ASN.1 coder.

 

Processing H.323 Endpoints When the Endpoints Do Not Send OLCAck

H.323 application inspection has been enhanced to process common H.323 endpoints. The enhancement affects endpoints using the extendedVideoCapability OLC with the H.239 protocol identifier. Even when an H.323 endpoint does not send OLCAck after receiving an OLC message from a peer, the ASA propagates OLC media proposal information into the media array and opens a pinhole for the media channel (extendedVideoCapability).

 

IPv6 in transparent firewall mode

Transparent firewall mode now participates in IPv6 routing. Prior to this release, the ASA could not pass IPv6 traffic in transparent mode. You can now configure an IPv6 management address in transparent mode, create IPv6 access lists, and configure other IPv6 features; the ASA recognizes and passes IPv6 packets.

All IPv6 functionality is supported unless specifically noted.

 

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, and then logs any suspicious activity. You can also supplement the dynamic database with a static database by entering IP addresses or domain names in a local "blacklist" or "whitelist."

Note This feature requires the Botnet Traffic Filter license. See the following licensing document for more information:

http://www.cisco.com/en/US/docs/security/asa/asa82/license/license82.html

The following commands were introduced: dynamic-filter commands (various), and the inspect dns dynamic-filter-snoop keyword.

 

AIP SSC card for the ASA 5505

The AIP SSC offers IPS for the ASA 5505 ASA. Note that the AIP SSM does not support virtual sensors. The following commands were introduced: allow-ssc-mgmt, hw-module module ip, and hw-module module allow-ip.

 

IPv6 support for IPS

You can now send IPv6 traffic to the AIP SSM or SSC when your traffic class uses the match any command, and the policy map specifies the ips command.

 

Management Features

SNMP version 3 and encryption

This release provides DES, 3DES, or AES encryption and support for SNMP Version 3, the most secure form of the supported security models. This version allows you to configure authentication characteristics by using the User-based Security Model (USM).

The following commands were introduced:

show snmp engineid

show snmp group

show snmp-server group

show snmp-server user

snmp-server group

snmp-server user

The following command was modified:

snmp-server host

 

NetFlow

This feature was introduced in Version 8.1(1) for the ASA 5580; this version introduces the feature to the other platforms. The new NetFlow feature enhances the ASA logging capabilities by logging flow-based events through the NetFlow protocol.

 

Routing Features

Multicast NAT

The ASA now offers Multicast NAT support for group addresses.

Troubleshooting Features

Coredump functionality

A coredump is a snapshot of the running program when the program has terminated abnormally. Coredumps are used to diagnose or debug errors and save a crash for later or off-site analysis. Cisco TAC may request that users enable the coredump feature to troubleshoot application or system crashes on the ASA.

To enable coredump, use the coredump enable command.


Firewall Functional Overview

Firewalls protect inside networks from unauthorized access by users on an outside network. A firewall can also protect inside networks from each other, for example, by keeping a human resources network separate from a user network. If you have network resources that need to be available to an outside user, such as a web or FTP server, you can place these resources on a separate network behind the firewall, called a demilitarized zone (DMZ). The firewall allows limited access to the DMZ, but because the DMZ only includes the public servers, an attack there only affects the servers and does not affect the other inside networks. You can also control when inside users access outside networks (for example, access to the Internet), by allowing only certain addresses out, by requiring authentication or authorization, or by coordinating with an external URL filtering server.

When discussing networks connected to a firewall, the outside network is in front of the firewall, the inside network is protected and behind the firewall, and a DMZ, while behind the firewall, allows limited access to outside users. Because the ASA lets you configure many interfaces with varied security policies, including many inside interfaces, many DMZs, and even many outside interfaces if desired, these terms are used in a general sense only.

This section includes the following topics:

Security Policy Overview

Firewall Mode Overview

Stateful Inspection Overview

Security Policy Overview

A security policy determines which traffic is allowed to pass through the firewall to access another network. By default, the ASA allows traffic to flow freely from an inside network (higher security level) to an outside network (lower security level). You can apply actions to traffic to customize the security policy. This section includes the following topics:

Permitting or Denying Traffic with Access Lists

Applying NAT

Protecting from IP Fragments

Using AAA for Through Traffic

Applying HTTP, HTTPS, or FTP Filtering

Applying Application Inspection

Sending Traffic to the Advanced Inspection and Prevention Security Services Module

Sending Traffic to the Content Security and Control Security Services Module

Applying QoS Policies

Applying Connection Limits and TCP Normalization

Permitting or Denying Traffic with Access Lists

You can apply an access list to limit traffic from inside to outside, or allow traffic from outside to inside. For transparent firewall mode, you can also apply an EtherType access list to allow non-IP traffic.

Applying NAT

Some of the benefits of NAT include the following:

You can use private addresses on your inside networks. Private addresses are not routable on the Internet.

NAT hides the local addresses from other networks, so attackers cannot learn the real address of a host.

NAT can resolve IP routing problems by supporting overlapping IP addresses.

Protecting from IP Fragments

The ASA provides IP fragment protection. This feature performs full reassembly of all ICMP error messages and virtual reassembly of the remaining IP fragments that are routed through the ASA. Fragments that fail the security check are dropped and logged. Virtual reassembly cannot be disabled.

Using AAA for Through Traffic

You can require authentication and/or authorization for certain types of traffic, for example, for HTTP. The ASA also sends accounting information to a RADIUS or TACACS+ server.

Applying HTTP, HTTPS, or FTP Filtering

Although you can use access lists to prevent outbound access to specific websites or FTP servers, configuring and managing web usage this way is not practical because of the size and dynamic nature of the Internet. We recommend that you use the ASA in conjunction with a separate server running one of the following Internet filtering products:

Websense Enterprise

Secure Computing SmartFilter

Applying Application Inspection

Inspection engines are required for services that embed IP addressing information in the user data packet or that open secondary channels on dynamically assigned ports. These protocols require the ASA to do a deep packet inspection.

Sending Traffic to the Advanced Inspection and Prevention Security Services Module

If your model supports the AIP SSM for intrusion prevention, then you can send traffic to the AIP SSM for inspection. The AIP SSM is an intrusion prevention services module that monitors and performs real-time analysis of network traffic by looking for anomalies and misuse based on an extensive, embedded signature library. When the system detects unauthorized activity, it can terminate the specific connection, permanently block the attacking host, log the incident, and send an alert to the device manager. Other legitimate connections continue to operate independently without interruption. For more information, see Configuring the Cisco Intrusion Prevention System Sensor Using the Command Line Interface.

Sending Traffic to the Content Security and Control Security Services Module

If your model supports it, the CSC SSM provides protection against viruses, spyware, spam, and other unwanted traffic. It accomplishes this by scanning the FTP, HTTP, POP3, and SMTP traffic that you configure the adaptive ASA to send to it.

Applying QoS Policies

Some network traffic, such as voice and streaming video, cannot tolerate long latency times. QoS is a network feature that lets you give priority to these types of traffic. QoS refers to the capability of a network to provide better service to selected network traffic.

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.

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.

A stateful firewall like the ASA, however, takes into consideration the state of a packet:

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

Performing route lookups

Allocating NAT translations (xlates)

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?

If the connection is already established, the ASA does not need to re-check packets; most matching packets can go through the "fast" path in both directions. The fast path is responsible for the following tasks:

IP checksum verification

Session lookup

TCP sequence number check

NAT translations based on existing sessions

Layer 3 and Layer 4 header adjustments

Data packets for protocols that require Layer 7 inspection can also go through the fast path.

Some established session packets must continue to go through the session management path or the control plane path. Packets that go through the session management path include HTTP packets that require inspection or content filtering. Packets that go through the control plane path include the control packets for protocols that require Layer 7 inspection.

VPN Functional Overview

A VPN is a secure connection across a TCP/IP network (such as the Internet) that appears as a private connection. This secure connection is called a tunnel. The ASA uses tunneling protocols to negotiate security parameters, create and manage tunnels, encapsulate packets, transmit or receive them through the tunnel, and unencapsulate them. The ASA functions as a bidirectional tunnel endpoint: it can receive plain packets, encapsulate them, and send them to the other end of the tunnel where they are unencapsulated and sent to their final destination. It can also receive encapsulated packets, unencapsulate them, and send them to their final destination. The ASA invokes various standard protocols to accomplish these functions.

The ASA performs the following functions:

Establishes tunnels

Negotiates tunnel parameters

Authenticates users

Assigns user addresses

Encrypts and decrypts data

Manages security keys

Manages data transfer across the tunnel

Manages data transfer inbound and outbound as a tunnel endpoint or router

The ASA invokes various standard protocols to accomplish these functions.

Security Context Overview

You can partition a single ASA into multiple virtual devices, known as security contexts. Each context is an independent device, with its own security policy, interfaces, and administrators. Multiple contexts are similar to having multiple standalone devices. Many features are supported in multiple context mode, including routing tables, firewall features, IPS, and management. Some features are not supported, including VPN and dynamic routing protocols.

In multiple context mode, the ASA includes a configuration for each context that identifies the security policy, interfaces, and almost all the options you can configure on a standalone device. The system administrator adds and manages contexts by configuring them in the system configuration, which, like a single mode configuration, is the startup configuration. The system configuration identifies basic settings for the ASA. The system configuration does not include any network interfaces or network settings for itself; rather, when the system needs to access network resources (such as downloading the contexts from the server), it uses one of the contexts that is designated as the admin context.

The admin context is just like any other context, except that when a user logs into the admin context, then that user has system administrator rights and can access the system and all other contexts.


Note You can run all your contexts in routed mode or transparent mode; you cannot run some contexts in one mode and others in another.

Multiple context mode supports static routing only.