Introduction to the ASA

The 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 9.15(1)

Released: November 2, 2020

Feature

Description

Platform Features

ASAv for the Public Cloud

We introduced the ASAv for the following Public Cloud offerings:

  • Oracle Cloud Infrastrucure (OCI)

  • Google Cloud Platform (GCP)

No modified commands.

ASAv support for Autoscale

The ASAv now supports Autoscale for the following Public Could offerings:

  • Amazon Web Services (AWS)

  • Miscrosoft Azure

Autoscaling increases or decreases the number of ASAv application instances based on capacity requirements.

No modified commands.

ASAv for Microsoft Azure support for Accelerated Networking (SR-IOV).

The ASAv on the Microsoft Azure Public Cloud now supports Azure's Accelerated Networking (AN), which enables single root I/O virtualization (SR-IOV) to a VM, greatly improving its networking performance.

No modified commands.

Firewall Features

Changes to PAT address allocation in clustering. The PAT pool flat option is now enabled by default and it is not configurable.

The way PAT addresses are distributed to the members of a cluster is changed. Previously, addresses were distributed to the members of the cluster, so your PAT pool would need a minimum of one address per cluster member. Now, the master instead divides each PAT pool address into equal-sized port blocks and distributes them across cluster members. Each member has port blocks for the same PAT addresses. Thus, you can reduce the size of the PAT pool, even to as few as one IP address, depending on the amount of connections you typically need to PAT. Port blocks are allocated in 512-port blocks from the 1024-65535 range. You can optionally included the reserved ports, 1-1023, in this block allocation when you configure PAT pool rules. For example, in a 4-node cluster, each node gets 32 blocks with which it will be able to handle 16384 connections per PAT pool IP address compared to a single node handling all 65535 connections per PAT pool IP address.

As part of this change, PAT pools for all systems, whether standalone or operating in a cluster, now use a flat port range of 1023 - 65535. Previously, you could optionally use a flat range by including the flat keyword in a PAT pool rule. The flat keyword is no longer supported: the PAT pool is now always flat. The include-reserve keyword, which was previously a sub-keyword to flat , is now an independent keyword within the PAT pool configuration. With this option, you can include the 1 - 1023 port range within the PAT pool.

Note that if you configure port block allocation (the block-allocation PAT pool option), your block allocation size is used rather than the default 512-port block. In addition, you cannot configure extended PAT for a PAT pool for systems in a cluster.

New/Modified commands: nat , show nat pool

XDMCP inspection disabled by default in new installations.

Previously, XDMCP inspection was enabled by default for all traffic. Now, on new installations, which includes new systems and reimaged systems, XDMCP is off by default. If you need this inspection, please enable it. Note that on upgrades, your current settings for XDMCP inspection are retained, even if you simply had it enabled by way of the default inspection settings.

High Availability and Scalability Features

Disable failover delay

When you use bridge groups or IPv6 DAD, when a failover occurs the new active unit waits up to 3000 ms for the standby unit to finish networking tasks and transition to the standby state. Then the active unit can start passing traffic. To avoid this delay, you can disable the waiting time, and the active unit will start passing traffic before the standby unit transitions.

New/Modified commands: failover wait-disable

Routing Features

Multicast IGMP interface state limit raised from 500 to 5000

The multicast IGMP state limit per interface was raised from 500 to 5000.

New/Modified commands: igmp limit

Also in 9.12(4).

Interface Features

DDNS support for the web update method

You can now configure an interface to use DDNS with the web update method.

New/Modified commands: show ddns update interface , show ddns update method , web update-url , web update-type

Certificate Features

Modifications to Match Certificate commands to support static CRL Distribution Point URL

The static CDP URL configuration commands allowed CDPs to be mapped uniquely to each certificate in a chain that is being validated. However, only one such mapping was supported for each certificate. This modification allows statically configured CDPs to be mapped to a chain of certificates for authentication.

New/Modified commands: match certificate override cdp ,

Administrative and Troubleshooting Features

Manual import of node secret file from the RSA Authentication Manager for SDI AAA server groups.

You can import the node secret file that you export from the RSA Authentication Manager for use with SDI AAA server groups.

We added the following commands: aaa sdi import-node-secret , clear aaa sdi node-secret , show aaa sdi node-secrets .

show fragment command output enhanced

The output for show fragment command was enhanced to include IP fragment related drops and error counters.

No modified commands.

show tech-support command output enhanced

The output for show tech-support command was enhanced to include the bias that is configured for the crypto accelerator. The bias value can be ssl, ipsec, or balanced.

No modified commands.

Monitoring Features

Support to configure cplane keepalive holdtime values

Due to communication delays caused by high CPU usage, the response to the keepalive event fails to reach ASA, resulting in trigerring failover due to card failure. You can now configure the keepalive timeout period and the maximum keepalive counter value to ensure sufficient time and retries are given.

New/Modified commands: service-module

VPN Features

Support for configuring the maximum in-negotiation SAs as an absolute value

You can now configure the maximum in-negotiation SAs as an absolute value up to 15000 or a maximum value derived from the maximum device capacity; formerly, only a percentage was allowed.

New/Modified commands: crypto ikev2 limit max-in-negotiation-sa value

Also in 9.12(4).
Cross-Site Request Forgery (CSRF) Vulnerabilities Prevention for WebVPN Handlers

ASA provides protection against CSRF attacks for WebVPN handlers. If a CSRF attack is detected, a user is notified by warning messages. This feature is enabled by default.

Kerberos server validation for Kerberos Constrained Delegation (KCD).

When configured for KCD, the ASA initiates an AD domain join with the configured server in order to acquire Kerberos keys. These keys are required for the ASA to request service tickets on behalf of clientless SSL VPN users. You can optionally configure the ASA to validate the identity of the server during domain join.

We modified the kcd-server command to add the validate-server-certificate keyword.

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