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What is level 0 security?

Published in ASA Security Levels 3 mins read

Level 0 security is the lowest security level on a Cisco ASA (Adaptive Security Appliance) firewall, typically assigned to the "outside" interface. This means that by default, traffic originating from the outside network cannot reach any internal interfaces without explicit permission defined in access lists.

Understanding Security Levels on ASA Firewalls

ASA firewalls use security levels to control traffic flow between different interfaces. These levels range from 0 to 100, with higher numbers representing more trusted networks.

Security Level Description
0 The lowest security level, typically assigned to the outside (untrusted) network. Traffic from this interface is blocked by default.
1-99 Intermediate security levels, used to define varying levels of trust within the network.
100 The highest security level, usually assigned to the inside (most trusted) network.

Implications of Level 0 Security

Because level 0 is the lowest, it's crucial to understand the implications for network security:

  • Default Deny: The default behavior for traffic originating from an interface with security level 0 is to be denied access to other interfaces. This "default deny" policy is a fundamental security principle.

  • Access Lists are Essential: To allow any traffic from the outside (level 0) to the inside, you must configure access lists that explicitly permit the desired traffic.

Example: Allowing Web Traffic from the Outside

To allow web traffic (port 80 and 443) from the internet to a web server on your internal network, you would need to create an access list on the ASA's outside interface:

access-list outside_in extended permit tcp any host <web_server_ip> eq 80
access-list outside_in extended permit tcp any host <web_server_ip> eq 443
access-group outside_in in interface outside

In this example:

  • access-list outside_in: Defines the name of the access list.
  • extended permit tcp any host <web_server_ip> eq 80: Allows TCP traffic from any source (any) to the specified web server's IP address (<web_server_ip>) on port 80 (HTTP).
  • extended permit tcp any host <web_server_ip> eq 443: Allows TCP traffic to port 443 (HTTPS).
  • access-group outside_in in interface outside: Applies the access list to the "outside" interface in the inbound direction.

Without these access lists, no web traffic would be able to reach the web server from the internet.

Conclusion

Level 0 security on an ASA firewall provides a crucial layer of protection by enforcing a default deny policy for traffic originating from untrusted networks. It's a cornerstone of network security, ensuring that only explicitly permitted traffic can enter the internal network.

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