CLI Book 1: Cisco ASA Series General Operations CLI Configuration Guide, 9.2

Information About Routed Firewall Mode

In routed mode, the ASA is considered to be a router hop in the network. Routed mode supports many interfaces. Each interface is on a different subnet. You can share interfaces between contexts.

The ASA acts as a router between connected networks, and each interface requires an IP address on a different subnet. The ASA supports multiple dynamic routing protocols. However, we recommend using the advanced routing capabilities of the upstream and downstream routers instead of relying on the ASA for extensive routing needs.

Information About Transparent Firewall Mode

Traditionally, a firewall is a routed hop and acts as a default gateway for hosts that connect to one of its screened subnets. A transparent firewall, on the other hand, is a Layer 2 firewall that acts like a “bump in the wire,” or a “stealth firewall,” and is not seen as a router hop to connected devices.

• Using the Transparent Firewall in Your Network
• Bridge Groups
• Management Interface (ASA 5512-X and Higher)
• Allowing Layer 3 Traffic
• Allowed MAC Addresses
• Passing Traffic Not Allowed in Routed Mode
• BPDU Handling
• MAC Address vs. Route Lookups
• ARP Inspection
• MAC Address Table

Using the Transparent Firewall in Your Network

The ASA connects the same network between its interfaces. Because the firewall is not a routed hop, you can easily introduce a transparent firewall into an existing network.

Figure 6-1 shows a typical transparent firewall network where the outside devices are on the same subnet as the inside devices. The inside router and hosts appear to be directly connected to the outside router.

Figure 6-1 Transparent Firewall Network

Bridge Groups

If you do not want the overhead of security contexts, or want to maximize your use of security contexts, you can group interfaces together in a bridge group, and then configure multiple bridge groups, one for each network. Bridge group traffic is isolated from other bridge groups; traffic is not routed to another bridge group within the ASA, and traffic must exit the ASA before it is routed by an external router back to another bridge group in the ASA. Although the bridging functions are separate for each bridge group, many other functions are shared between all bridge groups. For example, all bridge groups share a syslog server or AAA server configuration. For complete security policy separation, use security contexts with one bridge group in each context.

Figure 6-2 shows two networks connected to the ASA, which has two bridge groups.

Figure 6-2 Transparent Firewall Network with Two Bridge Groups

Note Each bridge group requires a management IP address. The ASA uses this IP address as the source address for packets originating from the bridge group. The management IP address must be on the same subnet as the connected network. For another method of management, see Management Interface (ASA 5512-X and Higher).

The ASA does not support traffic on secondary networks; only traffic on the same network as the management IP address is supported.

Management Interface (ASA 5512-X and Higher)

In addition to each bridge group management IP address, you can add a separate Management slot / port interface that is not part of any bridge group, and that allows only management traffic to the ASA. For more information, see Management Interface.

Allowing Layer 3 Traffic

• Unicast IPv4 and IPv6 traffic is allowed through the transparent firewall automatically from a higher security interface to a lower security interface, without an ACL.

Note Broadcast and multicast traffic can be passed using access rules. See the firewall configuration guidefor more information.

• ARPs are allowed through the transparent firewall in both directions without an ACL. ARP traffic can be controlled by ARP inspection.
• For Layer 3 traffic travelling from a low to a high security interface, an extended ACL is required on the low security interface. See the firewall configuration guide for more information.

Allowed MAC Addresses

The following destination MAC addresses are allowed through the transparent firewall. Any MAC address not on this list is dropped.

• TRUE broadcast destination MAC address equal to FFFF.FFFF.FFFF
• IPv4 multicast MAC addresses from 0100.5E00.0000 to 0100.5EFE.FFFF
• IPv6 multicast MAC addresses from 3333.0000.0000 to 3333.FFFF.FFFF
• BPDU multicast address equal to 0100.0CCC.CCCD
• AppleTalk multicast MAC addresses from 0900.0700.0000 to 0900.07FF.FFFF

Passing Traffic Not Allowed in Routed Mode

In routed mode, some types of traffic cannot pass through the ASA even if you allow it in an ACL. The transparent firewall, however, can allow almost any traffic through using either an extended ACL (for IP traffic) or an EtherType ACL (for non-IP traffic).

Non-IP traffic (for example AppleTalk, IPX, BPDUs, and MPLS) can be configured to go through using an EtherType ACL.

Note The transparent mode ASA does not pass CDP packets packets, or any packets that do not have a valid EtherType greater than or equal to 0x600. An exception is made for BPDUs and IS-IS, which are supported.

Passing Traffic For Routed-Mode Features

For features that are not directly supported on the transparent firewall, you can allow traffic to pass through so that upstream and downstream routers can support the functionality. For example, by using an extended ACL, you can allow DHCP traffic (instead of the unsupported DHCP relay feature) or multicast traffic such as that created by IP/TV. You can also establish routing protocol adjacencies through a transparent firewall; you can allow OSPF, RIP, EIGRP, or BGP traffic through based on an extended ACL. Likewise, protocols like HSRP or VRRP can pass through the ASA.

BPDU Handling

To prevent loops using the Spanning Tree Protocol, BPDUs are passed by default. To block BPDUs, you need to configure an EtherType ACL to deny them. If you are using failover, you might want to block BPDUs to prevent the switch port from going into a blocking state when the topology changes. See Transparent Firewall Mode Requirements for more information.

MAC Address vs. Route Lookups

When the ASA runs in transparent mode, the outgoing interface of a packet is determined by performing a MAC address lookup instead of a route lookup.

Route lookups, however, are necessary for the following traffic types:

• Traffic originating on the ASA—For example, if your syslog server is located on a remote network, you must use a static route so the ASA can reach that subnet.
• Traffic that is at least one hop away from the ASA with NAT enabled—The ASA needs to perform a route lookup to find the next hop gateway; you need to add a static route on the ASA for the real host address.
• Voice over IP (VoIP) and DNS traffic with inspection enabled, and the endpoint is at least one hop away from the ASA—For example, if you use the transparent firewall between a CCM and an H.323 gateway, and there is a router between the transparent firewall and the H.323 gateway, then you need to add a static route on the ASA for the H.323 gateway for successful call completion. If you enable NAT for the inspected traffic, a static route is required to determine the egress interface for the real host address that is embedded in the packet. Affected applications include:

– CTIQBE

– DNS

– GTP

– H.323

– MGCP

– RTSP

– SIP

– Skinny (SCCP)

ARP Inspection

By default, all ARP packets are allowed through the ASA. You can control the flow of ARP packets by enabling ARP inspection.

When you enable ARP inspection, the ASA compares the MAC address, IP address, and source interface in all ARP packets to static entries in the ARP table, and takes the following actions:

• If the IP address, MAC address, and source interface match an ARP entry, the packet is passed through.
• If there is a mismatch between the MAC address, the IP address, or the interface, then the ASA drops the packet.
• If the ARP packet does not match any entries in the static ARP table, then you can set the ASA to either forward the packet out all interfaces (flood), or to drop the packet.

Note The dedicated management interface, if present, never floods packets even if this parameter is set to flood.

ARP inspection prevents malicious users from impersonating other hosts or routers (known as ARP spoofing). ARP spoofing can enable a “man-in-the-middle” attack. For example, a host sends an ARP request to the gateway router; the gateway router responds with the gateway router MAC address. The attacker, however, sends another ARP response to the host with the attacker MAC address instead of the router MAC address. The attacker can now intercept all the host traffic before forwarding it on to the router.

ARP inspection ensures that an attacker cannot send an ARP response with the attacker MAC address, so long as the correct MAC address and the associated IP address are in the static ARP table.

MAC Address Table

The ASA learns and builds a MAC address table in a similar way as a normal bridge or switch: when a device sends a packet through the ASA, the ASA adds the MAC address to its table. The table associates the MAC address with the source interface so that the ASA knows to send any packets addressed to the device out the correct interface.

The ASA 5505 includes a built-in switch; the switch MAC address table maintains the MAC address-to-switch port mapping for traffic within each VLAN. This section only discusses the bridge MAC address table, which maintains the MAC address-to-VLAN interface mapping for traffic that passes between VLANs.

Because the ASA is a firewall, if the destination MAC address of a packet is not in the table, the ASA does not flood the original packet on all interfaces as a normal bridge does. Instead, it generates the following packets for directly connected devices or for remote devices:

• Packets for directly connected devices—The ASA generates an ARP request for the destination IP address, so that the ASA can learn which interface receives the ARP response.
• Packets for remote devices—The ASA generates a ping to the destination IP address so that the ASA can learn which interface receives the ping reply.

The original packet is dropped.

Prerequisites

When you change modes, the ASA clears the running configuration (see Guidelines and Limitations for more information).

• If you already have a populated configuration, be sure to back up your configuration before changing the mode; you can use this backup for reference when creating your new configuration. See Backing Up Configurations or Other Files.
• Use the CLI at the console port to change the mode. If you use any other type of session, including the ASDM Command Line Interface tool or SSH, you will be disconnected when the configuration is cleared, and you will have to reconnect to the ASA using the console port in any case.
• Set the mode within the context.

Detailed Steps

Note To set the firewall mode to transparent and also configure ASDM management access after the configuration is cleared, see Customizing ASDM Access (ASA 5505) or Customizing ASDM Access (ASA 5512-X and Higher, ASAv).

Task Flow for Configuring ARP Inspection

To configure ARP Inspection, perform the following steps:

Step 1 Add static ARP entries according to the Adding a Static ARP Entry. ARP inspection compares ARP packets with static ARP entries in the ARP table, so static ARP entries are required for this feature.

Step 2 Enable ARP inspection according to the Enabling ARP Inspection.

Adding a Static ARP Entry

ARP inspection compares ARP packets with static ARP entries in the ARP table. Although hosts identify a packet destination by an IP address, the actual delivery of the packet on Ethernet relies on the Ethernet MAC address. When a router or host wants to deliver a packet on a directly connected network, it sends an ARP request asking for the MAC address associated with the IP address, and then delivers the packet to the MAC address according to the ARP response. The host or router keeps an ARP table so it does not have to send ARP requests for every packet it needs to deliver. The ARP table is dynamically updated whenever ARP responses are sent on the network, and if an entry is not used for a period of time, it times out. If an entry is incorrect (for example, the MAC address changes for a given IP address), the entry times out before it can be updated.

Note The transparent firewall uses dynamic ARP entries in the ARP table for traffic to and from the ASA, such as management traffic.

Detailed Steps

Examples

For example, to allow ARP responses from the router at 10.1.1.1 with the MAC address 0009.7cbe.2100 on the outside interface, enter the following command:

What to Do Next

Enable ARP inspection according to the Enabling ARP Inspection.

Enabling ARP Inspection

This section describes how to enable ARP inspection.

Detailed Steps

Examples

For example, to enable ARP inspection on the outside interface, and to drop all non-matching ARP packets, enter the following command:

Adding a Static MAC Address

Normally, MAC addresses are added to the MAC address table dynamically as traffic from a particular MAC address enters an interface. You can add static MAC addresses to the MAC address table if desired. One benefit to adding static entries is to guard against MAC spoofing. If a client with the same MAC address as a static entry attempts to send traffic to an interface that does not match the static entry, then the ASA drops the traffic and generates a system message. When you add a static ARP entry (see Adding a Static ARP Entry), a static MAC address entry is automatically added to the MAC address table.

To add a static MAC address to the MAC address table, enter the following command:

Setting the MAC Address Timeout

The default timeout value for dynamic MAC address table entries is 5 minutes, but you can change the timeout. To change the timeout, enter the following command:

Disabling MAC Address Learning

By default, each interface automatically learns the MAC addresses of entering traffic, and the ASA adds corresponding entries to the MAC address table. You can disable MAC address learning if desired, however, unless you statically add MAC addresses to the table, no traffic can pass through the ASA.

To disable MAC address learning, enter the following command:

Monitoring ARP Inspection

To monitor ARP inspection, perform the following task:

Monitoring the MAC Address Table

You can view the entire MAC address table (including static and dynamic entries for both interfaces), or you can view the MAC address table for an interface. To view the MAC address table, enter the following command:

Examples

The following is sample output from the show mac-address-table command that shows the entire table:

The following is sample output from the show mac-address-table command that shows the table for the inside interface:

How Data Moves Through the ASA in Routed Firewall Mode

This section describes how data moves through the ASA in routed firewall mode and includes the following topics:

• An Inside User Visits a Web Server
• An Outside User Visits a Web Server on the DMZ
• An Inside User Visits a Web Server on the DMZ
• An Outside User Attempts to Access an Inside Host
• A DMZ User Attempts to Access an Inside Host

An Inside User Visits a Web Server

Figure 6-3 shows an inside user accessing an outside web server.

Figure 6-3 Inside to Outside

The following steps describe how data moves through the ASA (see Figure 6-3):

1. The user on the inside network requests a web page from www.example.com.

2. The ASA receives the packet and because it is a new session, the ASA verifies that the packet is allowed according to the terms of the security policy (access lists, filters, AAA).

For multiple context mode, the ASA first classifies the packet to a context.

3. The ASA translates the local source address (10.1.2.27) to the global address 209.165.201.10, which is on the outside interface subnet.

The global address could be on any subnet, but routing is simplified when it is on the outside interface subnet.

4. The ASA then records that a session is established and forwards the packet from the outside interface.

5. When www.example.com responds to the request, the packet goes through the ASA, and because the session is already established, the packet bypasses the many lookups associated with a new connection. The ASA performs NAT by untranslating the global destination address to the local user address, 10.1.2.27.

6. The ASA forwards the packet to the inside user.

An Outside User Visits a Web Server on the DMZ

Figure 6-4 shows an outside user accessing the DMZ web server.

Figure 6-4 Outside to DMZ

The following steps describe how data moves through the ASA (see Figure 6-4):

1. A user on the outside network requests a web page from the DMZ web server using the global destination address of 209.165.201.3, which is on the outside interface subnet.

2. The ASA receives the packet and untranslates the destination address to the local address 10.1.1.3.

3. Because it is a new session, the ASA verifies that the packet is allowed according to the terms of the security policy (access lists, filters, AAA).

For multiple context mode, the ASA first classifies the packet to a context.

4. The ASA then adds a session entry to the fast path and forwards the packet from the DMZ interface.

5. When the DMZ web server responds to the request, the packet goes through the ASA and because the session is already established, the packet bypasses the many lookups associated with a new connection. The ASA performs NAT by translating the local source address to 209.165.201.3.

6. The ASA forwards the packet to the outside user.

An Inside User Visits a Web Server on the DMZ

Figure 6-5 shows an inside user accessing the DMZ web server.

Figure 6-5 Inside to DMZ

The following steps describe how data moves through the ASA (see Figure 6-5):

1. A user on the inside network requests a web page from the DMZ web server using the destination address of 10.1.1.3.

2. The ASA receives the packet and because it is a new session, the ASA verifies that the packet is allowed according to the terms of the security policy (access lists, filters, AAA).

For multiple context mode, the ASA first classifies the packet to a context.

3. The ASA then records that a session is established and forwards the packet out of the DMZ interface.

4. When the DMZ web server responds to the request, the packet goes through the fast path, which lets the packet bypass the many lookups associated with a new connection.

5. The ASA forwards the packet to the inside user.

An Outside User Attempts to Access an Inside Host

Figure 6-6 shows an outside user attempting to access the inside network.

Figure 6-6 Outside to Inside

The following steps describe how data moves through the ASA (see Figure 6-6):

1. A user on the outside network attempts to reach an inside host (assuming the host has a routable IP address).

If the inside network uses private addresses, no outside user can reach the inside network without NAT. The outside user might attempt to reach an inside user by using an existing NAT session.

2. The ASA receives the packet and because it is a new session, the ASA verifies if the packet is allowed according to the security policy (access lists, filters, AAA).

3. The packet is denied, and the ASA drops the packet and logs the connection attempt.

If the outside user is attempting to attack the inside network, the ASA employs many technologies to determine if a packet is valid for an already established session.

A DMZ User Attempts to Access an Inside Host

Figure 6-7 shows a user in the DMZ attempting to access the inside network.

Figure 6-7 DMZ to Inside

The following steps describe how data moves through the ASA (see Figure 6-7):

1. A user on the DMZ network attempts to reach an inside host. Because the DMZ does not have to route the traffic on the Internet, the private addressing scheme does not prevent routing.

2. The ASA receives the packet and because it is a new session, the ASA verifies if the packet is allowed according to the security policy (access lists, filters, AAA).

The packet is denied, and the ASA drops the packet and logs the connection attempt.

How Data Moves Through the Transparent Firewall

Figure 6-8 shows a typical transparent firewall implementation with an inside network that contains a public web server. The ASA has an access list so that the inside users can access Internet resources. Another access list lets the outside users access only the web server on the inside network.

Figure 6-8 Typical Transparent Firewall Data Path

This section describes how data moves through the ASA and includes the following topics:

• An Inside User Visits a Web Server
• An Inside User Visits a Web Server Using NAT
• An Outside User Visits a Web Server on the Inside Network
• An Outside User Attempts to Access an Inside Host

An Inside User Visits a Web Server

Figure 6-9 shows an inside user accessing an outside web server.

Figure 6-9 Inside to Outside

The following steps describe how data moves through the ASA (see Figure 6-9):

1. The user on the inside network requests a web page from www.example.com.

2. The ASA receives the packet and adds the source MAC address to the MAC address table, if required. Because it is a new session, it verifies that the packet is allowed according to the terms of the security policy (access lists, filters, AAA).

For multiple context mode, the ASA first classifies the packet to a context.

3. The ASA records that a session is established.

4. If the destination MAC address is in its table, the ASA forwards the packet out of the outside interface. The destination MAC address is that of the upstream router, 209.165.201.2.

If the destination MAC address is not in the ASA table, the ASA attempts to discover the MAC address by sending an ARP request or a ping. The first packet is dropped.

5. The web server responds to the request; because the session is already established, the packet bypasses the many lookups associated with a new connection.

6. The ASA forwards the packet to the inside user.

An Inside User Visits a Web Server Using NAT

Figure 6-10 shows an inside user accessing an outside web server.

Figure 6-10 Inside to Outside with NAT

The following steps describe how data moves through the ASA (see Figure 6-10):

1. The user on the inside network requests a web page from www.example.com.

2. The ASA receives the packet and adds the source MAC address to the MAC address table, if required. Because it is a new session, it verifies that the packet is allowed according to the terms of the security policy (access lists, filters, AAA).

For multiple context mode, the ASA first classifies the packet according to a unique interface.

3. The ASA translates the real address (10.1.2.27) to the mapped address 209.165.201.10.

Because the mapped address is not on the same network as the outside interface, then be sure the upstream router has a static route to the mapped network that points to the ASA.

4. The ASA then records that a session is established and forwards the packet from the outside interface.

5. If the destination MAC address is in its table, the ASA forwards the packet out of the outside interface. The destination MAC address is that of the upstream router, 10.1.2.1.

If the destination MAC address is not in the ASA table, the ASA attempts to discover the MAC address by sending an ARP request and a ping. The first packet is dropped.

6. The web server responds to the request; because the session is already established, the packet bypasses the many lookups associated with a new connection.

7. The ASA performs NAT by untranslating the mapped address to the real address, 10.1.2.27.

An Outside User Visits a Web Server on the Inside Network

Figure 6-11 shows an outside user accessing the inside web server.

Figure 6-11 Outside to Inside

The following steps describe how data moves through the ASA (see Figure 6-11):

1. A user on the outside network requests a web page from the inside web server.

2. The ASA receives the packet and adds the source MAC address to the MAC address table, if required. Because it is a new session, it verifies that the packet is allowed according to the terms of the security policy (access lists, filters, AAA).

For multiple context mode, the ASA first classifies the packet to a context.

3. The ASA records that a session is established.

4. If the destination MAC address is in its table, the ASA forwards the packet out of the inside interface. The destination MAC address is that of the downstream router, 209.165.201.1.

If the destination MAC address is not in the ASA table, the ASA attempts to discover the MAC address by sending an ARP request and a ping. The first packet is dropped.

5. The web server responds to the request; because the session is already established, the packet bypasses the many lookups associated with a new connection.

6. The ASA forwards the packet to the outside user.

An Outside User Attempts to Access an Inside Host

Figure 6-12 shows an outside user attempting to access a host on the inside network.

Figure 6-12 Outside to Inside

The following steps describe how data moves through the ASA (see Figure 6-12):

1. A user on the outside network attempts to reach an inside host.

2. The ASA receives the packet and adds the source MAC address to the MAC address table, if required. Because it is a new session, it verifies if the packet is allowed according to the terms of the security policy (access lists, filters, AAA).

For multiple context mode, the ASA first classifies the packet to a context.

3. The packet is denied because there is no access list permitting the outside host, and the ASA drops the packet.

4. If the outside user is attempting to attack the inside network, the ASA employs many technologies to determine if a packet is valid for an already established session.