ASDM Book 2: Cisco ASA Series Firewall ASDM Configuration Guide, 7.2

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
• Rule Order
• Implicit Deny
• Using Remarks
• NAT and Access Rules
• Inbound and Outbound Rules
• Information About Access Rules

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 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 Inbound and Outbound Rules.

Using Access Rules and EtherType Rules on the Same Interface

You can apply both access rules and EtherType rules to each direction of an interface.

Rule Order

The order of rules is important. When the ASA decides whether to forward or drop a packet, the ASA tests the packet against each rule in the order in which the rules are listed. After a match is found, no more rules are checked. For example, if you create an access rule at the beginning that explicitly permits all traffic for an interface, no further rules are ever checked. For more information, see Implicit Deny.

You can disable a rule by making it inactive.

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.

Using Remarks

In the ASDM access rule window, a remark that displays next to the rule is the one that was configured before the rule, so when you configure a remark from the CLI and then view it in an ASDM access rule window, the remark displays next to the rule that was configured after the remark in the CLI. However, the packet tracer in ASDM matches the remark that is configured after the matching rule in the CLI.

NAT and Access Rules

Access rules always use the real IP addresses when determining an access rule match, even if you configure NAT. For example, if you configure NAT for an inside server, 10.1.1.5, so that it has a publicly routable IP address on the outside, 209.165.201.5, then the access rule to allow the outside traffic to access the inside server needs to reference the server’s real IP address (10.1.1.5), and not the mapped address (209.165.201.5).

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 3-1.) The outbound ACL prevents any other hosts from reaching the outside network.

Figure 3-1 Outbound ACL

Information About Access Rules

This section describes information about 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.

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 3-1 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 (such as HTTP, Telnet, and SSH) have higher precedence than an management access rule. 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

Adding an Access Rule

To apply an access rule, perform the following steps.

Detailed Steps

Step 1 Choose Configuration > Firewall > Access Rules.

Step 2 Click Add, and choose one of the following options:

The Add Access Rule dialog box appears.

Step 3 From the Interface drop-down list, choose the interface on which to apply the rule. Choose Any to apply a global rule.

Step 4 In the Action field, click one of the following radio buttons next to the desired action:

• Permit —Permits access if the conditions are matched.
• Deny —Denies access if the conditions are matched.

Step 5 In the Source field, enter an IP address that specifies the network, interface IP, or any address from which traffic is permitted or denied to the specified destination. You may use either an IPv4 or IPv6 address.

For more information about enabling IPv6 on an interface, see the general operations configuration guide.

Step 6 In the User field, enter a user name or group to the ACL. Enter the user name in the format domain_NetBIOS_name \ user _ name. Enter the group name in the format domain _ NetBIOS_name \ group _ name.

You can configure access rules based on user names and user group names rather than through source IP addresses. The ASA applies the security policies based on an association of IP addresses to Windows Active Directory login information and reports events based on the mapped user names instead of network IP addresses.

See the general operations configuration guide for more information.

Step 7 To browse for a user name or user group, click the ellipsis (...) button. The Browse User dialog box appears.

Step 8 In the Destination field, enter an IP address that specifies the network, interface IP, any address to which traffic is permitted or denied from the source specified in the Source field. You may use either an IPv4 or IPv6 address.

Step 9 Select the service type.

Step 10 (Optional) To add a time range to your access rule that specifies when traffic can be allowed or denied, click More Options to expand the list.

a. To the right of the Time Range drop down list, click the browse button.

The Browse Time Range dialog box appears.

b. Click Add.

The Add Time Range dialog box appears.

c. In the Time Range Name field, enter a time range name, with no spaces.

d. Choose the Start Time and the End Time.

e. To specify additional time constraints for the time range, such as specifying the days of the week or the recurring weekly interval in which the time range will be active, click Add, and choose the specifications.

f. Click OK to apply the optional time range specifications.

Step 11 (Optional) In the Description field, add a text description about the access rule.

The description can contain multiple lines; however, each line can be no more than 100 characters in length.

Step 12 (Optional) Logging is enabled by default. You can disable logging by unchecking the check box, or you can change the logging level from the drop-down list. The default logging level is Informational.

Step 13 Click OK. The access rule appears with the newly configured access rules.

Step 14 Click Apply to save the access rule to your configuration.

You can edit or delete a particular access rule by selecting the rule and then clicking Edit or Delete.

Adding an EtherType Rule (Transparent Mode Only)

The EtherType Rules window shows access rules based on packet EtherTypes. EtherType rules are used to configure non-IP related traffic policies through the ASA when operating in transparent mode. In transparent mode, you can apply both extended and EtherType access rules to an interface. EtherType rules take precedence over the extended access rules.

For more information about EtherType rules, see Information About Access Rules.

To add an EtherType rule, perform the following steps:

Step 1 Choose Configuration > Device Management > Management Access > EtherType Rules.

Step 2 Click Add.

The Add EtherType rules window appears.

Step 3 (Optional) To specify the placement of the new EtherType rule, select an existing rule, and click Insert... to add the EtherType rule before the selected rule, or click Insert After... to add the EtherType rule after the selected rule.

Step 4 From the Interface drop-down list, choose the interface on which to apply the rule. Choose Any to apply a global rule.

Step 5 In the Action field, click one of the following radio buttons next to the desired action:

• Permit —Permits access if the conditions are matched.
• Deny —Denies access if the conditions are matched.

Step 6 In the EtherType field, choose an EtherType value from the drop-down list.

Step 7 (Optional) In the Description field, add a test description about the rule.

The description can contain multiple lines; however, each line can be no more than 100 characters in length.

Step 8 (Optional) To specify the direction for this rule, click More Options to expand the list, and then specify the direction by clicking one of the following radio buttons:

• In —Incoming traffic
• Out —Outgoing traffic

Step 9 Click OK.

Configuring Management Access Rules

You can configure an interface ACL that supports access control for to-the-box management traffic from a specific peer (or set of peers) to the security appliance. One scenario in which this type of ACL would be useful is when you want to block IKE Denial of Service attacks.

To configure an extended ACL that permits or denies packets for to-the-box traffic, perform the following steps:

Step 1 Choose Configuration > Device Management > Management Access > Management Access Rules.

Step 2 Click Add, and choose one of the following actions:

The Add Management Access Rule dialog box appears.

Step 3 From the Interface drop-down list, choose an interface on which to apply the rule. Choose Any to apply a global rule.

Step 4 In the Action field, click one of the following radio buttons to choose the action:

• Permit —Permits access if the conditions are matched.
• Deny —Denies access if the conditions are matched.

Step 5 In the Source field, enter an IP address that specifies the network object group, interface IP, or any address from which traffic is permitted or denied. You may use either an IPv4 or IPv6 address.

Note IPv6 must be enabled on at least one interface before you can configure an extended ACL with an IPv6 address. For more information about enabling IPv6 on an interface, see the general operations configuration guide.

Step 6 In the Service field, add a service name for rule traffic, or click the ellipsis (...) to browse for a service.

Step 7 (Optional) In the Description field, add a description for this management access rule.

The description can contain multiple lines; however, each line can be no more than 100 characters in length.

Step 8 (Optional) Logging is enabled by default. You can disable logging by unchecking the check box, or you can change the logging level from the drop-down list. The default logging level is Informational.

Step 9 (Optional) To add a source service (TCP, UDP, and TCP-UDP only) and a time range to your access rule that specifies when traffic can be allowed or denied, click More Options to expand the list.If you want to turn off this Management Access Rule, uncheck Enable Rule.

• Add a source service in the Source Service field, or click the ellipsis (...) to browse for a service.

The destination service and source service must be the same. Copy and paste the destination Service field to the Source Service field.

• To configure the logging interval (if you enable logging and choose a non-default setting), enter a value in seconds in the Logging Interval field.
• To select a predefined time range for this rule, from the Time Range drop-down list, choose a time range; or click the ellipsis (...) to browse for a time range. You can also specify additional time constraints for the time range, such as specifying the days of the week or the recurring weekly interval in which the time range will be active.

Step 10 Click OK. The dialog box closes, and the Management Access rule is added.

Step 11 Click Apply. The rule is saved in the running configuration.

Advanced Access Rule Configuration

The Advanced Access Rule Configuration dialog box lets you to set global access rule logging options.

When you enable logging, if a packet matches the access rule, the ASA creates a flow entry to track the number of packets received within a specific interval. The ASA generates a system log message at the first hit and at the end of each interval, identifying the total number of hits during the interval and reporting the time of the last hit.

Note The ASApane displays the hit count information in the “last rule hit” row. To view the rule hit count and timestamp, choose Configuration > Firewall > Advanced > ACL Manager, and hover the mouse pointer over a cell in the ACL Manager table.

At the end of each interval, the ASA resets the hit count to 0. If no packets match the access rule during an interval, the ASA deletes the flow entry.

A large number of flows can exist concurrently at any point of time. To prevent unlimited consumption of memory and CPU resources, the ASA places a limit on the number of concurrent deny flows; the limit is placed only on deny flows (and not permit flows) because they can indicate an attack. When the limit is reached, the ASA does not create a new deny flow until the existing flows expire. If someone initiates a denial of service attack, the ASA can create a very large number of deny flows in a very short period of time. Restricting the number of deny-flows prevents unlimited consumption of memory and CPU resources.

Prerequisites

These settings only apply if you enable the newer logging mechanism for the access rule.

Fields

• Maximum Deny-flows—The maximum number of deny flows permitted before the ASA stops logging, between 1 and the default value. The default is 4096.
• Alert Interval—The amount of time (1-3600 seconds) between system log messages (number 106101) that identify that the maximum number of deny flows was reached. The default is 300 seconds.
• Per User Override table—Specifies the state of the per user override feature. If the per user override feature is enabled on the inbound access rule, the access rule provided by a RADIUS server replaces the access rule configured on that interface. If the per user override feature is disabled, the access rule provided by the RADIUS server is combined with the access rule configured on that interface. If the inbound access rule is not configured for the interface, per user override cannot be configured.

By default, VPN remote access traffic is not matched against interface ACLs. However, if you deselect the Enable inbound VPN sessions to bypass interface access lists setting on the Configuration > Remote Access VPN > Network (Client) Access > AnyConnect Connection Profiles pane), the behavior depends on whether there is a VPN filter applied in the group policy (see the Configuration > Remote Access VPN > Network (Client) Access > Group Policies > Add/Edit > General > More Options > Filter field) and whether you set the Per User Override option:

– No Per User Override, no VPN filter —Traffic is matched against the interface ACL.

– No Per User Override, VPN filter —Traffic is matched first against the interface ACL, then against the VPN filter.

– Per User Override, VPN filter —Traffic is matched against the VPN filter only.

• Object Group Search Setting—Reduces the amount of memory used to store service rules, but lengthens the amount of time to search for a matching access rule.

Access Rule Explosion

The security appliance allows you to turn off the expansion of access rules that contain certain object groups. When expansion is turned off, an object group search is used for lookup, which lowers the memory requirements for storing expanded rules but decreases the lookup performance. Because of the trade-off of performance for memory utilization, you can turn on and turn off the search.

To configure the option of turning off the expansion of access rules that contain s, perform the following steps:

Step 1 Choose Configuration > Firewall > Access Rules.

Step 2 Click the Advanced button.

Step 3 Check the Enable Object Group Search Algorithm check box.

Configuring HTTP Redirect

The HTTP Redirect table displays each interface on the ASA, shows whether it is configured to redirect HTTP connections to HTTPS, and the port number from which it redirects those connections.

Note To redirect HTTP, the interface requires an ACL that permits HTTP. Otherwise, the interface cannot listen to the HTTP port.

The Configuration > Device Management > Advanced > HTTP Redirect > Edit pane lets you change the HTTP redirect setting of an interface or the port from which it redirects HTTP connections. Select the interface in the table and click Edit. You can also double-click an interface. The Edit HTTP/HTTPS Settings dialog box opens.

Edit HTTP/HTTPS Settings

The Edit HTTP/HTTPS Settings dialog box lets you change the HTTP redirect setting of an interface or the port number.

Fields

The Edit HTTP/HTTPS Settings dialog box includes the following fields:

• Interface—Identifies the interface on which the ASA redirects or does not redirect HTTP requests to HTTPS.
• Redirect HTTP to HTTPS—Check to redirect HTTP requests to HTTPS, or uncheck to not redirect HTTP requests to HTTPS.
• HTTP Port—Identifies the port from which the interface redirects HTTP connections. By default it listens to port 80.

For more information about access rules, see Information About Access Rules.