The goal is to replace a pair of Linux firewalls with a pair of ASA5520's. The problem is the way that the ISP hands us our connection. They handoff the connection using a private subnet (172.16.x.x). The Linux firewall then sends any packets destined to the servers using public IP addresses that the ISP supplies. It is like a reverse firewall in the sense that the "inside" interface is configured with a Public IP address and the "outside" interface is configured with a private IP address. The ISP routers know to forward the public IP's via their private subnet. A straight swap should work but it is the NAT that is the issue. NAT is tking place behind the firewalls via Load balancers. curently the ASA is running in parralel and is providing NAT to a few servers. If I were to replace the Linux boxes with the ASA's, I don;t think the current NAT setup will work. I would need to NAT private address (10.x.x.x) to it's public address (217.x.x.x) via the ISP's subnet (172.16.x.x). Is this possible?
With the ASA, it doesn't matter what the "outside" address is. You can NAT to any address that the ISP can route to you. The easiest configuration would be something like this:
nat 1 (inside) 10.0.0.0 255.0.0.0
global (outside) 1 126.96.36.199
You can also make is more restrictive by picking subnets to nat to IP's, for example:
nat 1 (inside) 10.1.1.0 255.255.255.0
nat 1 (inside) 10.1.2.0 255.255.255.0
nat 2 (inside) 10.2.0.0 255.255.0.0
global (outside) 1 188.8.131.52
global (outside) 2 184.108.40.206 - 220.127.116.11
The NAT command identifies the interface and addresses of the "private" ip addressed. The Global command identifies the "public" interface and IP address that will be used as the "source address" of the packet. The number after each is used to "tie" them together. In my example, nat 1 and global 1 and nat 2 and global 2.
You can use this with "static NAT" also, as the other user pointed out. Take a look at the configuration guide for other examples:
I will give it a try. I have never configured a firewall with a public inside address and
private outside address. It is counterintuitive. The problem is that this is a production environment and I need to minimalize downtime. What about terminating the site-to-sites? Can that be done on an intrerface other than the external?
I've never tried to terminate a VPN tunnel on the inside interface. But, I have created a VPN tunnel to the outside interface from a computer on the inside. So, I'm "guessing" that if you configure the VPN tunnel using the inside interface, it should work through the outside.
Not something that I would recommend trying in a production network, but if that's all you've got there might not be another option.
Hopefully someone thats tried that before can tell you more details. If I had an ASA handy I'd give it a shot.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...