Network Address Translation (NAT) is an essential feature of Cisco ASA Firewalls that allows for the translation of IP addresses between different network interfaces. In this article, we will explore the process of configuring NAT on Cisco ASA Firewalls and understand its various applications.
Contents
Introduction
In this video tutorial, we will delve into the configuration of Access Control Lists (ACLs) and NAT on Cisco ASA Firewalls from the command line. While we have already explored ACLs in a previous video, this time we will focus on NAT. Let’s dive right in!
How the Cisco ASA handles NAT
Similar to an iOS router, the Cisco ASA Firewall has inside and outside interfaces. These interfaces represent the logical concepts of trusted and untrusted networks, respectively. However, unlike a router, the ASA doesn’t require fixed inside and outside statements for interface configuration. Instead, NAT rules define the inside and outside interfaces on a rule-by-rule basis.
On the inside interface, the ASA uses real IP addresses, which are the actual IP addresses of the hosts that need translation. On the outside interface, the ASA defines mapped addresses, which are the IP addresses after translation. This could be a public IP address on the internet, for example.
NAT can work in two directions: traffic initiated from the inside network or traffic initiated from the outside network. If a NAT rule allows translation from either side, it is called bi-directional NAT. On the other hand, if a NAT rule only allows traffic initiation from the inside network, it is called unidirectional NAT. However, once traffic is initiated, unidirectional NAT can handle return traffic from the outside network.
Configuring NAT on Cisco ASA Firewalls
To demonstrate the configuration of NAT, let’s focus on a lab environment with specific goals:
- Prevent NAT between the inside network and the DMZ.
- Assign a separate public IP to the DMZ intranet server and configure a different public IP for general internet use.
- Use a pool of IPs for workstations instead of a single IP.
- Make Workstation 2’s web page available on the internet on port 8080.
- Exclude Workstation 1 from NAT when administering the ISP router via SSH.
- Rewrite DNS responses from the DNS server to the internet.
To achieve these goals, we will start by creating network objects for the mapped IP and the intranet server’s real IP. This allows us to define the inside and outside interfaces for NAT configuration. We will also specify whether the NAT is static or dynamic, depending on the requirements.
Next, we will configure NAT rules to enable internet access from the inside network. This involves defining the inside and outside interfaces, specifying the source addresses (real IP addresses), and choosing the mapped IP object. We will also update the Access Control List (ACL) to allow general internet access.
For more complex scenarios, such as when a single public IP address is insufficient, we can configure NAT using a pool of IPs instead of a single IP. This provides a larger number of translations, allowing for unique public IPs for each host. We will create an object with a pool of IP addresses, define the NAT rule with the Pat pool keyword, and update the ACL accordingly.
To address the requirement of translating Workstation 2’s webserver from port 80 to port 8080, we will configure a static NAT with port translation. This involves creating objects for the mapped IP and the real IP, specifying the source and destination ports, and updating the ACL to allow access.
Additionally, we will need to bypass NAT for Workstation 1 when administering the ISP router via SSH. This can be achieved using identity NAT, where the NAT rule uses the real IP address as the map type. We will define the object for the real IP, configure the NAT rule as static identity NAT, and update the ACL to allow SSH access.
Finally, to handle DNS rewriting, we will edit the network object for the internet server and add the DNS keyword to the NAT configuration. This will enable the ASA to rewrite the DNS response from the internal IP to the public IP, ensuring correct resolution of DNS queries.
FAQs
Q: Can I use object NAT for all scenarios?
A: While object NAT is recommended whenever possible due to its simplicity and ease of understanding, there may be situations where it is not suitable. In such cases, you can use twice NAT, which offers more advanced features and allows for different translations based on specific conditions.
Q: How can I verify the NAT configuration on the ASA?
A: You can use the “show xlate” command to view the active translations on the ASA. This command displays both static and dynamic translations, along with relevant flags indicating the NAT type. Additionally, you can use the “show nat detail” command to view the NAT rules in detail, including the translate hits and untranslated hits.
Q: Why are there three sections for NAT rules on the ASA?
A: The three sections (sections 1, 2, and 3) enable you to control the order of NAT rules and create policies. Object NAT rules go in section 1, while twice NAT rules can be placed in section 1 or section 3. This allows for flexibility in defining the order in which NAT rules are evaluated and applied.
Conclusion
Configuring Network Address Translation (NAT) on Cisco ASA Firewalls is a crucial aspect of network security and connectivity. By understanding the various types of NAT and their applications, you can effectively manage IP address translations to ensure seamless communication between different network interfaces. With the proper configuration and understanding of NAT rules, you can enhance the functionality and security of your network infrastructure.
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