Chapter 6 Configuring NAT (ASA 8.2 and Earlier)

Using Dynamic NAT

To configure a dynamic NAT, PAT, or identity NAT rule, perform the following steps.

Step 1 In the Configuration > Firewall > NAT Rules pane, choose Add > Add Dynamic NAT Rule. The Add Dynamic NAT Rule dialog box appears.

Step 2 In the Original area, from the Interface drop-down list, choose the interface that is connected to the hosts with real addresses that you want to translate.

Step 3 Enter the real addresses in the Source field, or click the ... button to select an IP address that you already defined in ASDM.

Specify the address and subnet mask using prefix/length notation, such as 10.1.1.0/24. If you enter an IP address without a mask, it is considered to be a host address, even if it ends with a 0.

Step 4 To choose a global pool, use one of the following options:

•Select an already-defined global pool.

If the pool includes a range of addresses, then the ASA performs dynamic NAT. If the pool includes a single address, then the ASA performs dynamic PAT. If a pool includes both ranges and single addresses, then the ranges are used in order, and then the PAT addresses are used in order. See the “Multiple Addresses in the Same Global Pool” section on page 6-20for more information.

Pools are identified by a pool ID. If multiple global pools on different interfaces share the same pool ID, then they are grouped. If you choose a multi-interface pool ID, then traffic is translated as specified when it accesses any of the interfaces in the pool. For more information about pool IDs, see the “Dynamic NAT Implementation” section on page 6-17.

•Create a new global pool or edit an existing pool by clicking Manage. See the “Managing Global Pools” section on page 6-22.

•Choose identity NAT by selecting global pool 0.

Step 5 (Optional) To enable translation of addresses inside DNS replies, expand the Connection Settings area, and check the Translate the DNS replies that match the translation rule check box.

If your NAT rule includes the real address of a host that has an entry in a DNS server, and the DNS server is on a different interface from a client, then the client and the DNS server need different addresses for the host; one needs the mapped address and one needs the real address. This option rewrites the address in the DNS reply to the client. The mapped host needs to be on the same interface as either the client or the DNS server. Typically, hosts that need to allow access from other interfaces use a static translation, so this option is more likely to be used with a static rule. See the “DNS and NAT” section on page 6-14for more information.

Step 6 (Optional) To enable connection settings, expand the Connection Settings area, and set one or more of the following options:

Note You can also set these values using a security policy rule (see Chapter 22, “Configuring Connection Settings”). If you set them in both places, then the ASA uses the lower limit. For TCP sequence randomization, if it is disabled using either method, then the ASA disables TCP sequence randomization.

•Randomize sequence number—With this check box checked (the default), the ASA randomizes the sequence number of TCP packets. Each TCP connection has two ISNs: one generated by the client and one generated by the server. The ASA randomizes the ISN of the TCP SYN passing in both the inbound and outbound directions.

Randomizing the ISN of the protected host prevents an attacker from predicting the next ISN for a new connection and potentially hijacking the new session.

Cisco ASA Series Firewall ASDM Configuration Guide

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Cisco Systems ASA 5555-X, ASA 5505, ASA 5545-X, ASA 5585-X, ASA 5580 Configuring NAT ASA 8.2 and Earlier Using Dynamic NAT
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ASA Services Module, ASA 5555-X, ASA 5545-X, ASA 5585-X, ASA 5580 specifications

Cisco Systems has long been a leader in the field of network security, and its Adaptive Security Appliance (ASA) series is a testament to this expertise. Within the ASA lineup, models such as the ASA 5505, ASA 5580, ASA 5585-X, ASA 5545-X, and ASA 5555-X stand out for their unique features, capabilities, and technological advancements.

The Cisco ASA 5505 is designed for small businesses or branch offices. It provides essential security features such as firewall protection, flexible VPN capabilities, and intrusion prevention. The ASA 5505 supports a user-friendly interface, allowing for straightforward management. Its built-in threat detection and prevention tools provide a layered defense, and with scalability in mind, it can accommodate various expansion options as organizational needs grow.

Moving up the line, the ASA 5580 delivers greater throughput and advanced security features. This model is suited for medium to large enterprises that require robust protection against increasingly sophisticated threats. Its multi-core architecture allows it to manage high volumes of traffic seamlessly while maintaining excellent performance levels. The ASA 5580 also supports application-layer security and customizable access policies, making it highly adaptable to diverse security environments.

The ASA 5585-X further enhances Cisco's security offerings with advanced malware protection and extensive security intelligence capabilities. It incorporates next-generation firewall features, including context-aware security, and supports advanced threat detection technologies. This model is ideal for large enterprises or data centers that prioritize security while ensuring uninterrupted network performance and availability.

For enterprises requiring a balance of performance and security, the ASA 5545-X presents a compelling option. This model features scalable performance metrics, high availability, and integrated advanced threat protection. Coupled with advanced endpoint protection and detailed monitoring capabilities, the ASA 5545-X enables organizations to manage their security posture effectively.

Lastly, the ASA 5555-X blends cutting-edge technologies with strong security infrastructures. It boasts high throughput and the ability to execute deep packet inspections. Its sophisticated architecture supports threat intelligence feeds that provide real-time security updates, making it a powerful tool against modern threats.

Each of these Cisco ASA models brings specific advantages to varied environments. Their integrative capabilities enable businesses to enhance their security postures while benefiting from seamless scalability and management. As cybersecurity threats evolve, these advanced appliances play a vital role in protecting valuable digital assets.
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