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 9.16(4)

Released: October 13, 2022

There are no new features in this release.

New Features in ASA 9.16(3)

Released: April 6, 2022

There are no new features in this release.

New Features in ASA 9.16(2)

Released: August 18, 2021

There are no new features in this release.

New Features in ASA 9.16(1)

Released: May 26, 2021

Feature

Description

Firewall Features

New Section 0 for system-defined NAT rules.

A new Section 0 has been added to the NAT rule table. This section is exclusively for the use of the system. Any NAT rules that the system needs for normal functioning are added to this section, and these rules take priority over any rules you create. Previously, system-defined rules were added to Section 1, and user-defined rules could interfere with proper system functioning. You cannot add, edit, or delete Section 0 rules, but you will see them in show nat detail command output.

The default SIP inspection policy map drops non-SIP traffic.

For SIP-inspected traffic, the default is now to drop non-SIP traffic. The previous default was to allow non-SIP traffic on ports inspected for SIP.

We changed the default SIP policy map to include the no traffic-non-sip command.

Ability to specify the IMSI prefixes to be dropped in GTP inspection.

GTP inspection lets you configure IMSI prefix filtering, to identify the Mobile Country Code/Mobile Network Code (MCC/MNC) combinations to allow. You can now do IMSI filtering on the MCC/MNC combinations that you want to drop. This way, you can list out the unwanted combinations, and default to allowing all other combinations.

We added the following command: drop mcc .

Configure the maximum segment size (MSS) for embryonic connections

You can configure a service policy to set the server maximum segment size (MSS) for SYN-cookie generation for embryonic connections upon reaching the embryonic connections limit. This is meaningful for service policies where you are also setting embryonic connection maximums.

New/Modified commands: set connection syn-cookie-mss .

Improved CPU usage and performance for many-to-one and one-to-many connections.

The system no longer creates local host objects and locks them when creating connections, except for connections that involve dynamic NAT/PAT and scanning threat detection and host statistics. This improves performance and CPU usage in situations where many connections are going to the same server (such as a load balancer or web server), or one endpoint is making connections to many remote hosts.

We changed the following commands: clear local-host (deprecated), show local-host

Platform Features

ASAv support for VMware ESXi 7.0

The ASAv virtual platform supports hosts running on VMware ESXi 7.0. New VMware hardware versions have been added to the vi.ovf and esxi.ovf files to enable optimal performance and usability of the ASAv on ESXi 7.0.

No modified commands.

No modified screens.

Intel QuickAssist Technology (QAT) on ASAv

The ASAv supports hardware crypto acceleration for ASAv deployments that use the Intel QuickAssist (QAT) 8970 PCI adapter. Hardware crypto acceleration for the ASAv using QAT is supported on VMware ESXi and KVM only.

No modified commands.

No modified screens.

ASAv on OpenStack

The ASAv virtual platform has added support for OpenStack.

No modified commands.

No modified screens.

High Availability and Scalability Features

Improved PAT port block allocation for clustering on the Firepower 4100/9300

The improved PAT port block allocation ensures that the control unit keeps ports in reserve for joining nodes, and proactively reclaims unused ports. To best optimize the allocation, you can set the maximum nodes you plan to have in the cluster using the cluster-member-limit command. The control unit can then allocate port blocks to the planned number of nodes, and it will not have to reserve ports for extra nodes you don't plan to use. The default is 16 nodes. You can also monitor syslog 747046 to ensure that there are enough ports available for a new node.

New/Modified commands: cluster-member-limit , show nat pool cluster [summary] , show nat pool ip detail

show cluster history command improvements

We have added additional outputs for the show cluster history command.

New/Modified commands: show cluster history brief , show cluster history latest , show cluster history reverse , show cluster history time

Firepower 1140 maximum contexts increased from 5 to 10

The Firepower 1140 now supports up to 10 contexts.

Certificate Features

Enrollment over Secure Transport (EST) for certification

ASA supports certificate enrollment using the Enrollment over Secure Transport (EST). However, you can configure to use EST enrollments only with RSA and ECDSA keys. You cannot use EdDSA keypair for a trustpoint configured for EST enrollment.

New/Modified commands: enrollment protocol , crypto ca authenticate , and crypto ca enroll

Support for new EdDSA key

The new key option, EdDSA, was added to the existing RSA and ECDSA options.

New/Modified commands: crypto key generate , crypto key zeroize , show crypto key mypubkey

Command to override restrictions on certificate keys

Support to use SHA1with RSA Encryption algorithm for certification and support for certificates with RSA key sizes smaller than 2048 were removed. You can use crypto ca permit-weak-crypto command to override these restrictions.

New/Modified commands: crypto ca permit-weak-crypto

Administrative and Troubleshooting Features

SSH security improvements

SSH now supports the following security improvements:

  • Host key format—crypto key generate {eddsa | ecdsa} . In addition to RSA, we added support for the EdDSA and ECDSA host keys. The ASA tries to use keys in the following order if they exist: EdDSA, ECDSA, and then RSA. If you explicitly configure the ASA to use the RSA key with the ssh key-exchange hostkey rsa command, you must generate a key that is 2048 bits or higher. For upgrade compatibility, the ASA will use smaller RSA host keys only when the default host key setting is used. RSA support will be removed in a later release.

  • Key exchange algorithms—ssh key-exchange group {ecdh-sha2-nistp256 | curve25519-sha256}

  • Encryption algorithms—ssh cipher encryption chacha20-poly1305@openssh.com

  • SSH version 1 is no longer supported—The ssh version command is removed.

New/Modified commands: crypto key generate eddsa , crypto key zeroize eddsa , show crypto key mypubkey, ssh cipher encryption chacha20-poly1305@openssh.com , ssh key-exchange group {ecdh-sha2-nistp256 | curve25519-sha256} , ssh key-exchange hostkey , ssh version

Monitoring Features

SNMPv3 Authentication

You can now use SHA-224 and SHA-384 for user authentication. You can no longer use MD5 for user authentication.

You can no longer use DES for encryption.

New/Modified commands: snmp-server user

VPN Features

Support for IPv6 on Static VTI

ASA supports IPv6 addresses in Virtual Tunnel Interfaces (VTI) configurations.

A VTI tunnel source interface can have an IPv6 address, which you can configure to use as the tunnel endpoint. If the tunnel source interface has multiple IPv6 addresses, you can specify which address to be used, else the first IPv6 global address in the list is used by default.

The tunnel mode can be either IPv4 or IPv6, but it must be the same as IP address type configured on VTI for the tunnel to be active. An IPv6 address can be assigned to the tunnel source or the tunnel destination interface in a VTI.

New/Modified commands: tunnel source interface , tunnel destination , tunnel mode

Support for 1024 VTI interfaces per device

The number of maximum VTIs to be configured on a device has been increased from 100 to 1024.

Even if a platform supports more than 1024 interfaces, the VTI count is limited to the number of VLANs configurable on that platform. For example, ASA 5510 supports 100 VLANs, the tunnel count would be 100 minus the number of physical interfaces configured.

New/Modified commands: None

Support for DH group 15 in SSL

Support has been added for DH group 15 for SSL encryption.

New/Modified commands: ssl dh-group group15

Support for DH group 31 for IPsec encryption

Support has been added for DH group 31 for IPsec encryption.

New/Modified commands: set pfs

Support to limit the SA in IKEv2 queue

Support has been added to limit the number of queues in SA-INIT packets.

New/Modified commands: crypto ikev2 limit queue sa_init

Option to clear IPsec statistics

CLIs have been introduced to clear and reset IPsec statistics.

New/Modified commands: clear crypto ipsec stats and clear ipsec stats

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