Introduction to the Cisco ASA

The Cisco ASA provides advanced stateful firewall and VPN concentrator functionality in one device as well as integrated services with add-on modules. The ASA includes many advanced features, such as multiple security contexts (similar to virtualized firewalls), clustering (combining multiple firewalls into a single firewall), transparent (Layer 2) firewall or routed (Layer 3) firewall operation, advanced inspection engines, IPsec VPN, SSL VPN, and clientless SSL VPN support, and many more features.

Hardware and Software Compatibility

For a complete list of supported hardware and software, see Cisco ASA Compatibility.

New Features

This section lists new features for each release.


Note

New, changed, and deprecated syslog messages are listed in the syslog message guide.

New Features in ASA 9.8(4)

Released: April 24, 2019

Feature

Description

VPN Features

Add subdomains to webVPN HSTS

Allows domain owners to submit what domains should be included in the HSTS preload list for web browsers.

New/Modified commands: hostname(config-webvpn) includesubdomains

Also in 9.12(1).

Administrative Features

Allow non-browser-based HTTPS clients to access the ASA

You can allow non-browser-based HTTPS clients to access HTTPS services on the ASA. By default, ASDM, CSM, and REST API are allowed. Many specialty clients (for example, python libraries, curl, and wget) do not support Cross-site request forgery (CSRF) token-based authentication, so you need to specifically allow these clients to use the ASA basic authentication method. For security purposes, you should only allow required clients.

New/Modified commands: http server basic-auth-client

Also in 9.12(1).

show tech-support includes additional output

The output of the show tech-support is enhanced to display the output of the following:

  • show ipv6 interface

  • show aaa-server

  • show fragment

New/Modified commands: show tech-support

Also in 9.12(1).

Support to enable and disable the results for free memory and used memory statistics during SNMP walk operations

To avoid overutilization of CPU resources, you can enable and disable the query of free memory and used memory statistics collected through SNMP walk operations.

New/Modified command: snmp-server enable oid

Also in 9.10(1).

New Features in ASA 9.8(3)

Released: July 2, 2018

Feature

Description

Platform Features

Firepower 2100 Active LED now lights amber when in standby mode

Formerly, the Active LED was unlit in standby mode.

Firewall Features

Support for removing the logout button from the cut-through proxy login page.

If you configure the cut-through proxy to obtain user identity information (the AAA authentication listener), you can now remove the logout button from the page. This is useful in case where users connect from behind a NAT device and cannot be distinguished by IP address. When one user logs out, it logs out all users of the IP address.

New/Modified commands: aaa authentication listener no-logout-button .

Trustsec SXP connection configurable delete hold down timer

The default SXP connection hold down timer is 120 seconds. You can now configure this timer, between 120 to 64000 seconds.

New/Modified commands: cts sxp delete-hold-down period , show cts sxp connection brief , show cts sxp connections

VPN Features

Support for legacy SAML authentication

If you deploy an ASA with the fix for CSCvg65072, then the default SAML behavior is to use the embedded browser, which is not supported on AnyConnect 4.4 or 4.5. Therefore, to continue to use AnyConnect 4.4 or 4.5, you must enable the legacy external browser SAML authentication method. Because of security limitations, use this option only as part of a temporary plan to migrate to AnyConnect 4.6. This option will be deprecated in the near future.

New/Modified commands: saml external-browser

Interface Features

Unique MAC address generation for single context mode

You can now enable unique MAC address generation for VLAN subinterfaces in single context mode. Normally, subinterfaces share the same MAC address with the main interface. Because IPv6 link-local addresses are generated based on the MAC address, this feature allows for unique IPv6 link-local addresses.

New or modified command: mac-address auto

Also in 9.9(2) and later.

New Features in ASA 9.8(2)

Released: August 28, 2017

Feature

Description

Platform Features

ASA for the Firepower 2100 series

We introduced the ASA for the Firepower 2110, 2120, 2130, and 2140. Similar to the Firepower 4100 and 9300, the Firepower 2100 runs the base FXOS operating system and then the ASA operating system as an application. The Firepower 2100 implementation couples FXOS more closely with the ASA than the Firepower 4100 and 9300 do (pared down FXOS functions, single device image bundle, easy management access for both ASA and FXOS).

FXOS owns configuring hardware settings for interfaces, including creating EtherChannels, as well as NTP services, hardware monitoring, and other basic functions. You can use the Firepower Chassis Manager or the FXOS CLI for this configuration. The ASA owns all other functionality, including Smart Licensing (unlike the Firepower 4100 and 9300). The ASA and FXOS each have their own IP address on the Management 1/1 interface, and you can configure management of both the ASA and FXOS instances from any data interface.

We introduced the following commands: connect fxos, fxos https, fxos snmp, fxos ssh, ip-client

Department of Defense Unified Capabilities Approved Products List

The ASA was updated to comply with the Unified Capabilities Approved Products List (UC APL) requirements. In this release, when you enter the fips enable command, the ASA will reload. Both failover peers must be in the same FIPS mode before you enable failover.

We modified the following command: fips enable

ASAv for Amazon Web Services M4 instance support

You can now deploy the ASAv as an M4 instance.

We did not modify any commands.

ASAv5 1.5 GB RAM capability

Starting in Version 9.7(1), the ASAv5 may experience memory exhaustion where certain functions such as enabling AnyConnect or downloading files to the ASAv fail. You can now assign 1.5 GB (up from 1 GB) of RAM to the ASAv5.

We did not modify any commands.

VPN Features

HTTP Strict Transport Security (HSTS) header support

HSTS protects websites against protocol downgrade attacks and cookie hijacking on clientless SSL VPN. It lets web servers declare that web browsers (or other complying user agents) should only interact with it using secure HTTPS connections, and never via the insecure HTTP protocol. HSTS is an IETF standards track protocol and is specified in RFC 6797.

We introduced the following commands: hsts enable, hsts max-age age_in_seconds

Interface Features

VLAN support for the ASAv50

The ASAv50 now supports VLANs on the ixgbe-vf vNIC for SR-IOV interfaces.

We did not modify any commands.

New Features in ASA 9.8(1.200)

Released: July 30, 2017


Note

This release is only supported on the ASAv for Microsoft Azure. These features are not supported in Version 9.8(2).

Feature

Description

High Availability and Scalability Features

Active/Backup High Availability for ASAv on Microsoft Azure

A stateless Active/Backup solution that allows for a failure of the active ASAv to trigger an automatic failover of the system to the backup ASAv in the Microsoft Azure public cloud.

We introduced the following commands: failover cloud

No ASDM support.

New Features in ASA 9.8(1)

Released: May 15, 2017

Feature

Description

Platform Features

ASAv50 platform

The ASAv virtual platform has added a high-end performance ASAv50 platform that provides 10 Gbps Firewall throughput levels. The ASAv50 requires ixgbe-vf vNICs, which are supported on VMware and KVM only.

SR-IOV on the ASAv platform

The ASAv virtual platform supports Single Root I/O Virtualization (SR-IOV) interfaces, which allows multiple VMs to share a single PCIe network adapter inside a host. ASAv SR-IOV support is available on VMware, KVM, and AWS only.

Automatic ASP load balancing now supported for the ASAv

Formerly, you could only manually enable and disable ASP load balancing.

We modified the following command: asp load-balance per-packet auto

Firewall Features

Support for setting the TLS proxy server SSL cipher suite

You can now set the SSL cipher suite when the ASA acts as a TLS proxy server. Formerly, you could only set global settings for the ASA using the ssl cipher command.

We introduced the following command: server cipher-suite

Global timeout for ICMP errors

You can now set the idle time before the ASA removes an ICMP connection after receiving an ICMP echo-reply packet. When this timeout is disabled (the default), and you enable ICMP inspection, then the ASA removes the ICMP connection as soon as an echo-reply is received; thus any ICMP errors that are generated for the (now closed) connection are dropped. This timeout delays the removal of ICMP connections so you can receive important ICMP errors.

We added the following command: timeout icmp-error

High Availability and Scalability Features

Improved cluster unit health-check failure detection

You can now configure a lower holdtime for the unit health check: .3 seconds minimum. The previous minimum was .8 seconds. This feature changes the unit health check messaging scheme to heartbeats in the data plane from keepalives in the control plane. Using heartbeats improves the reliability and the responsiveness of clustering by not being susceptible to control plane CPU hogging and scheduling delays. Note that configuring a lower holdtime increases cluster control link messaging activity. We suggest that you analyze your network before you configure a low holdtime; for example, make sure a ping from one unit to another over the cluster control link returns within the holdtime/3, because there will be three heartbeat messages during one holdtime interval. If you downgrade your ASA software after setting the hold time to .3 - .7, this setting will revert to the default of 3 seconds because the new setting is unsupported.

We modified the following commands: health-check holdtime, show asp drop cluster counter, show cluster info health details

Configurable debounce time to mark an interface as failed for the Firepower 4100/9300 chassis

You can now configure the debounce time before the ASA considers an interface to be failed, and the unit is removed from the cluster. This feature allows for faster detection of interface failures. Note that configuring a lower debounce time increases the chances of false-positives. When an interface status update occurs, the ASA waits the number of milliseconds specified before marking the interface as failed and the unit is removed from the cluster. The default debounce time is 500 ms, with a range of 300 ms to 9 seconds.

New or modified command: health-check monitor-interface debounce-time

VPN Features

Support for IKEv2, certificate based authentication, and ACL in VTI

Virtual Tunnel Interface (VTI) now supports BGP (static VTI). You can now use IKEv2 in standalone and high availability modes. You can use certificate based authentication by setting up a trustpoint in the IPsec profile. You can also apply access lists on VTI using access-group commands to filter ingress traffic.

We introduced the following command in the IPsec profile configuration mode: set trustpoint.

Mobile IKEv2 (MobIKE) is enabled by default

Mobile devices operating as remote access clients require transparent IP address changes while moving. Supporting MobIKE on ASA allows a current IKE security association (SA) to be updated without deleting the current SA. MobIKE is “always on.”

We introduced the following command: ikev2 mobike-rrc. Used to enable/disable return routability checking.

SAML 2.0 SSO Updates

The default signing method for a signature in a SAML request changed from SHA1 to SHA2, and you can configure which signing method you prefer: rsa-sha1, rsa-sha256, rsa-sha384, or rsa-sha512.

We changed the following command in webvpn mode: saml idp signature can be configured with a value. Disabled is still the default.

Change for tunnelgroup webvpn-attributes

We changed the pre-fill-username and secondary-pre-fill-username value from ssl-client to client .

We changed the following commands in webvpn mode: pre-fill-username and secondary-pre-fill-username can be configured with a client value.

AAA Features

Login history

By default, the login history is saved for 90 days. You can disable this feature or change the duration, up to 365 days. This feature only applies to usernames in the local database when you enable local AAA authentication for one or more of the management methods (SSH, ASDM, Telnet, and so on).

We introduced the following commands: aaa authentication login-history, show aaa login-history

Password policy enforcement to prohibit the reuse of passwords, and prohibit use of a password matching a username

You can now prohibit the reuse of previous passwords for up to 7 generations, and you can also prohibit the use of a password that matches a username.

We introduced the following commands: password-history, password-policy reuse-interval, password-policy username-check

Separate authentication for users with SSH public key authentication and users with passwords

In releases prior to 9.6(2), you could enable SSH public key authentication (ssh authentication ) without also explicitly enabling AAA SSH authentication with the Local user database (aaa authentication ssh console LOCAL ). In 9.6(2), the ASA required you to explicitly enable AAA SSH authentication. In this release, you no longer have to explicitly enable AAA SSH authentication; when you configure the ssh authentication command for a user, local authentication is enabled by default for users with this type of authentication. Moreover, when you explicitly configure AAA SSH authentication, this configuration only applies for usernames with passwords, and you can use any AAA server type (aaa authentication ssh console radius_1 , for example). For example, some users can use public key authentication using the local database, and other users can use passwords with RADIUS.

We did not modify any commands.

Also in Version 9.6(3).

Monitoring and Troubleshooting Features

Saving currently-running packet captures when the ASA crashes

Formerly, active packet captures were lost if the ASA crashed. Now, packet captures are saved to disk 0 at the time of the crash with the filename [context_name.]capture_name.pcap.

We did not modify any commands.

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.

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.

Permitting or Denying Traffic with Access Rules

You can apply access rules to limit traffic from inside to outside, or allow traffic from outside to inside. For bridge group interfaces, you can also apply an EtherType access rule 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.

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.

You can configure Cloud Web Security on the ASA, or install an ASA module that provides URL and other filtering services, such as ASA CX or ASA FirePOWER. You can also use the ASA in conjunction with an external product such as the Cisco Web Security Appliance (WSA).

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 Supported Hardware or Software Modules

Some ASA models allow you to configure software modules, or to insert hardware modules into the chassis, to provide advanced services. These modules provide additional traffic inspection and can block traffic based on your configured policies. You can send traffic to these modules to take advantage of these advanced services.

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 in a "bridge group".

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.

Routed mode supports Integrated Routing and Bridging, so you can also configure bridge groups in routed mode, and route between bridge groups and regular interfaces. In routed mode, you can replicate transparent mode functionality; if you do not need multiple context mode or clustering, you might consider using routed mode instead.

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.

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; however, some features are not supported. See the feature chapters for more information.

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.

ASA Clustering Overview

ASA Clustering lets you group multiple ASAs together as a single logical device. A cluster provides all the convenience of a single device (management, integration into a network) while achieving the increased throughput and redundancy of multiple devices.

You perform all configuration (aside from the bootstrap configuration) on the control unit only; the configuration is then replicated to the member units.

Special and Legacy Services

For some services, documentation is located outside of the main configuration guides and online help.

Special Services Guides

Special services allow the ASA to interoperate with other Cisco products; for example, by providing a security proxy for phone services (Unified Communications), or by providing Botnet traffic filtering in conjunction with the dynamic database from the Cisco update server, or by providing WCCP services for the Cisco Web Security Appliance. Some of these special services are covered in separate guides:

Legacy Services Guide

Legacy services are still supported on the ASA, however there may be better alternative services that you can use instead. Legacy services are covered in a separate guide:

Cisco ASA Legacy Feature Guide

This guide includes the following chapters:

  • Configuring RIP

  • AAA Rules for Network Access

  • Using Protection Tools, which includes Preventing IP Spoofing (ip verify reverse-path), Configuring the Fragment Size (fragment), Blocking Unwanted Connections (shun), Configuring TCP Options (for ASDM), and Configuring IP Audit for Basic IPS Support (ip audit).

  • Configuring Filtering Services