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Wednesday, April 16, 2008

Eight Basic Commands to Configure a Cisco ASA Security Appliance

Note:  This post has been updated to reflect changes in NAT/PAT configuration.  View the updated post here.  There is also a video demonstrating this process using the newer commands here.

There are literally thousands of commands and sub-commands available to configure a Cisco security appliance. As you gain knowledge of the appliance, you will use more and more of the commands. Initially, however, there are just a few commands required to configure basic functionality on the appliance. Basic functionality is defined as allowing inside hosts to access outside hosts, but not allowing outside hosts to access the inside hosts. Additionally, management must be allowed from at least one inside host. Here are eight basic commands:

interface
The interface command identifies either the hardware interface or the VLAN interface that will be configured. Once in interface configuration mode, you can assign physical interfaces to switchports and enable them (turn them on) or you can assign names and security levels to VLAN interfaces.

nameif
The nameif command gives the interface a name and assigns a security level. Typical names are outside, inside, or DMZ.

security-level
Security levels are used by the appliance to control traffic flow. Traffic is permitted to flow from interfaces with higher security levels to interfaces with lower security levels, but not the other way. Access-lists must be used to permit traffic to flow from lower security levels to higher security levels. Security levels range from 0 to 100. The default security level for an outside interface is 0. For an inside interface, the default security level is 100.In the following sample configuration, the interface command is first used to name the inside and outside VLAN interfaces, then the DMZ interface is named and a security level of 50 is assigned to it.
ciscoasa(config)# interface vlan1
ciscoasa(config-if)# nameif inside
INFO: Security level for "inside" set to 100 by default.
ciscoasa(config-if)# interface vlan2
ciscoasa(config-if)# nameif outside
INFO: Security level for "outside" set to 0 by default.
ciscoasa(config-if)#interface vlan3
ciscoasa(config-if)# nameif dmz
ciscoasa(config-if)# security-level 50

ip address
The ip address command assigns an IP address to a VLAN interface either statically or by making it a DHCP client. With modern versions of security appliance software, it is not necessary to explicitly configure default subnet masks. If you are using non-standard masks, you must explicitly configure the mask, but otherwise, it's not necessary.In the following sample configuration, an IP address is assigned to VLAN 1, the inside interface.
ciscoasa(config-if)# interface vlan 1
ciscoasa(config-if)# ip address 192.168.1.1

switchport access
The switchport access command on the ASA 5505 security appliance assigns a physical interface to a logical (VLAN) interface. In the next example, the interface command is used to identify physical interfaces, assign them to switchports on the appliance, and enable them (turn them on) through the use of the "no shutdown" statement.
ciscoasa(config-if)# interface ethernet 0/0
ciscoasa(config-if)# switchport access vlan 2
ciscoasa(config-if)# no shutdown
ciscoasa(config-if)# interface ethernet 0/1
ciscoasa(config-if)# switchport access vlan 1
ciscoasa(config-if)# no shutdown

nat
The nat command enables network address translation on the specified interface for the specified subnet.In this sample, configuration, NAT is enabled on the inside interface for hosts on the 192.168.1.0/24 subnet. The number "1" is the NAT I.D. which will be used by the global command to associate a global address or pool with the inside addresses. (Note: NAT 0 is used to prevent the specified group of addresses from being translated.)
ciscoasa(config)# nat (inside) 1 192.168.1.0 255.255.255.0

global
The global command works in tandem with the nat command. It identifies the interface (usually outside) through which traffic from nat'ed hosts (usually inside hosts) must flow. It also identifies the global address which nat'ed hosts will use to connect to the outside world.In the following sample, the hosts associated with NAT I.D. 1 will use the global address 12.3.4.5 on the outside interface.
ciscoasa(config)# global (outside) 1 12.3.4.5

In this additional example of the use of the "global" command, the interface statement tells the firewall that hosts associated with NAT I.D. 1 will use the DHCP-assigned global address on the outside interface.
ciscoasa(config)# global (outside) 1 interface

route
The route command, in its most basic form, assigns a default route for traffic, typically to an ISP's router. It can also be used in conjunction with access-lists to send specific types of traffic to specific hosts on specific subnets.In this sample configuration, the route command is used to configure a default route to the ISP's router at 12.3.4.6. The two zeroes before the ISP's router address are shorthand for an IP address of 0.0.0.0 and a mask of 0.0.0.0. The statement outside identifies the interface through which traffic will flow to reach the default route.
ciscoasa(config-if)# route outside 0 0 12.3.4.6

The above commands create a very basic firewall, but frankly, using a sophisticated device such as a Cisco PIX or ASA security appliance to perform such basic firewall functions is overkill. Other commands to use include hostname to identify the firewall, telnet or SSH to allow remote administration, DHCPD commands to allow the firewall to assign IP addresses to inside hosts, and static route and access-list commands to allow internal hosts such as DMZ Web servers or DMZ mail servers to be accessible to Internet hosts. Obviously, if you're using a device such as an ASA or a PIX, you'll probably be doing a lot more with it than simply setting up a basic firewall, but the above commands will provide a foundation for the more complex configurations.

For more step-by-step guides on the ASA, please check out my book The Accidental Administrator: Cisco ASA Security Appliance: A Step-by-Step Configuration Guide, available on Amazon.com, Barnes and Noble, and other channels.

Based on the two-day workshop Cisco ASA Training: Two-Day Hands-On Workshop from soundtraining.net (http://www.soundtraining.net/cisco-asa-training-101).

1 comment:

Don R. Crawley said...

I have updated this post to reflect the change in NAT/PAT configuration, effective with version 8.3(1) of the ASA software. Since many people will continue to run older versions of the software, I'm going to leave this post intact. The updated post, however, can be found here