CLI Book 2: Cisco ASA Series Firewall CLI Configuration Guide, 9.1

General Information About Rules

This section describes information for both access rules and EtherType rules, and it includes the following topics:

• Implicit Permits
• Information About Interface Access Rules and Global Access Rules
• Using Access Rules and EtherType Rules on the Same Interface
• Implicit Deny
• Inbound and Outbound Rules
• Transactional-Commit Model

Implicit Permits

For routed mode, the following types of traffic are allowed through by default:

• Unicast IPv4 traffic from a higher security interface to a lower security interface.
• Unicast IPv6 traffic from a higher security interface to a lower security interface.

For transparent mode, the following types of traffic are allowed through by default:

• Unicast IPv4 traffic from a higher security interface to a lower security interface.
• Unicast IPv6 traffic from a higher security interface to a lower security interface.
• ARPs in both directions.

Note ARP traffic can be controlled by ARP inspection, but cannot be controlled by an access rule.

• BPDUs in both directions.

For other traffic, you need to use either an extended access rule (IPv4 and IPv6) or an EtherType rule (non-IPv4/IPv6).

Information About Interface Access Rules and Global Access Rules

You can apply an access rule to a specific interface, or you can apply an access rule globally to all interfaces. You can configure global access rules in conjunction with interface access rules, in which case, the specific interface access rules are always processed before the general global access rules.

Note Global access rules apply only to inbound traffic. See the “Inbound and Outbound Rules” section.

Using Access Rules and EtherType Rules on the Same Interface

You can apply one access rule and one EtherType rule to each direction of an interface.

Implicit Deny

ACLs have an implicit deny at the end of the list, so unless you explicitly permit it, traffic cannot pass. For example, if you want to allow all users to access a network through the ASA except for particular addresses, then you need to deny the particular addresses and then permit all others.

For EtherType ACLs, the implicit deny at the end of the ACL does not affect IP traffic or ARPs; for example, if you allow EtherType 8037, the implicit deny at the end of the ACL does not now block any IP traffic that you previously allowed with an extended ACL (or implicitly allowed from a high security interface to a low security interface). However, if you explicitly deny all traffic with an EtherType ACE, then IP and ARP traffic is denied.

If you configure a global access rule, then the implicit deny comes after the global rule is processed. See the following order of operations:

1. Interface access rule.

2. Global access rule.

3. Implicit deny.

Inbound and Outbound Rules

The ASA supports two types of ACLs:

• Inbound—Inbound access rules apply to traffic as it enters an interface. Global access rules are always inbound.
• Outbound—Outbound ACLs apply to traffic as it exits an interface.

Note “Inbound” and “outbound” refer to the application of an ACL on an interface, either to traffic entering the ASA on an interface or traffic exiting the ASA on an interface. These terms do not refer to the movement of traffic from a lower security interface to a higher security interface, commonly known as inbound, or from a higher to lower interface, commonly known as outbound.

An outbound ACL is useful, for example, if you want to allow only certain hosts on the inside networks to access a web server on the outside network. Rather than creating multiple inbound ACLs to restrict access, you can create a single outbound ACL that allows only the specified hosts. (See Figure 6-1.) The outbound ACL prevents any other hosts from reaching the outside network.

Figure 6-1 Outbound ACL

See the following commands for this example:

Transactional-Commit Model

The ASA rule-engine supports a new feature for rule updation called the Transactional-Commit Model. When this feature is enabled, a rule update is applied after the rule compilation is completed; without affecting the rule matching performance. With the legacy model, rule updates take effect immediately but rule matching slows down during the rule compilation period. This feature is useful to prevent potential packet drops during large compilation of rules under high traffic conditions. This feature is also useful to reduce the rule compilation time under two specific patterns of configurations:

• Preventing packet drops while compiling large rules during high traffic rates.
• Reducing rule compilation time while updating a large number of similar rules.

Guidelines and Limitations

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent firewall mode.

IPv6 Guidelines

Supports IPv6.

Additional Guidelines and Limitations

Evaluate the following alternatives before using the transactional commit model:

• While using large rules, try to optimize the number of rules by using the Object Group Search setting in Advanced Access Rule Configuration settings.
• Perform an incremental rule update instead of a bulk rule update. If a bulk update is necessary perform the bulk update during the maintenance window, when traffic is low.

Information About Extended Access Rules

This section describes information about extended access rules and includes the following topics:

• Access Rules for Returning Traffic
• Allowing Broadcast and Multicast Traffic through the Transparent Firewall Using Access Rules
• Management Access Rules

Access Rules for Returning Traffic

For TCP and UDP connections for both routed and transparent mode, you do not need an access rule to allow returning traffic because the ASA allows all returning traffic for established, bidirectional connections.

For connectionless protocols such as ICMP, however, the ASA establishes unidirectional sessions, so you either need access rules to allow ICMP in both directions (by applying ACLs to the source and destination interfaces), or you need to enable the ICMP inspection engine. The ICMP inspection engine treats ICMP sessions as bidirectional connections. To control ping, specify echo-reply (0) (ASA to host) or echo (8) (host to ASA).

Allowing Broadcast and Multicast Traffic through the Transparent Firewall Using Access Rules

In routed firewall mode, broadcast and multicast traffic is blocked even if you allow it in an access rule, including unsupported dynamic routing protocols and DHCP (unless you configure DHCP relay). Transparent firewall mode can allow any IP traffic through.

Note Because these special types of traffic are connectionless, you need to apply an access rule to both interfaces, so returning traffic is allowed through.

Table 6-1 lists common traffic types that you can allow through the transparent firewall.

Management Access Rules

You can configure access rules that control management traffic destined to the ASA. Access control rules for to-the-box management traffic (defined by such commands as http, ssh, or telnet) have higher precedence than an management access rule applied with the control-plane option. Therefore, such permitted management traffic will be allowed to come in even if explicitly denied by the to-the-box ACL.

Information About EtherType Rules

This section describes EtherType rules and includes the following topics:

• Supported EtherTypes and Other Traffic
• Access Rules for Returning Traffic
• Allowing MPLS

Supported EtherTypes and Other Traffic

An EtherType rule controls the following:

• EtherType identified by a 16-bit hexadecimal number, including common types IPX and MPLS unicast or multicast.
• Ethernet V2 frames.
• BPDUs, which are permitted by default. BPDUs are SNAP-encapsulated, and the ASA is designed to specifically handle BPDUs.
• Trunk port (Cisco proprietary) BPDUs. Trunk BPDUs have VLAN information inside the payload, so the ASA modifies the payload with the outgoing VLAN if you allow BPDUs.
• IS-IS.

The following types of traffic are not supported:

• 802.3-formatted frames—These frames are not handled by the rule because they use a length field as opposed to a type field.

Access Rules for Returning Traffic

Because EtherTypes are connectionless, you need to apply the rule to both interfaces if you want traffic to pass in both directions.

Allowing MPLS

If you allow MPLS, ensure that Label Distribution Protocol and Tag Distribution Protocol TCP connections are established through the ASA by configuring both MPLS routers connected to the ASA to use the IP address on the ASA interface as the router-id for LDP or TDP sessions. (LDP and TDP allow MPLS routers to negotiate the labels (addresses) used to forward packets.)

On Cisco IOS routers, enter the appropriate command for your protocol, LDP or TDP. The interface is the interface connected to the ASA.

Or

Default Settings

See the “Implicit Permits” section.

Configuring Access Rules

To apply an access rule, perform the following steps.

Detailed Steps

Examples

The following example shows how to use the access-group command:

The access-list command lets any host access the global address using port 80. The access-group command specifies that the access-list command applies to traffic entering the outside interface.