Cisco ASA 5500 Series Configuration Guide using the CLI, 8.2
Configuring the Transparent or Routed Firewall
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Configuring the Transparent or Routed Firewall

Table Of Contents

Configuring the Transparent or Routed Firewall

Configuring the Firewall Mode

Information About the Firewall Mode

Information About Routed Firewall Mode

Information About Transparent Firewall Mode

Licensing Requirements for the Firewall Mode

Default Settings

Guidelines and Limitations

Setting the Firewall Mode

Feature History for Firewall Mode

Configuring ARP Inspection for the Transparent Firewall

Information About ARP Inspection

Licensing Requirements for ARP Inspection

Default Settings

Guidelines and Limitations

Configuring ARP Inspection

Task Flow for Configuring ARP Inspection

Adding a Static ARP Entry

Enabling ARP Inspection

Monitoring ARP Inspection

Feature History for ARP Inspection

Customizing the MAC Address Table for the Transparent Firewall

Information About the MAC Address Table

Licensing Requirements for the MAC Address Table

Default Settings

Guidelines and Limitations

Configuring the MAC Address Table

Adding a Static MAC Address

Setting the MAC Address Timeout

Disabling MAC Address Learning

Monitoring the MAC Address Table

Feature History for the MAC Address Table

Firewall Mode Examples

How Data Moves Through the Security Appliance in Routed Firewall Mode

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

How Data Moves Through the Transparent Firewall

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


Configuring the Transparent or Routed Firewall


This chapter describes how to configure the firewall mode, routed or transparent, and how to customize transparent firewall operation.


Note In multiple context mode, you cannot set the firewall mode separately for each context; you can only set the firewall mode for the entire ASA.


This chapter includes the following sections:

Configuring the Firewall Mode

Configuring ARP Inspection for the Transparent Firewall

Customizing the MAC Address Table for the Transparent Firewall

Firewall Mode Examples

Configuring the Firewall Mode

This section describes routed and transparent firewall mode, and how to set the mode. This section includes the following topics:

Information About the Firewall Mode

Licensing Requirements for the Firewall Mode

Default Settings

Guidelines and Limitations

Setting the Firewall Mode

Feature History for Firewall Mode

Information About the Firewall Mode

This section describes routed and transparent firewall mode, and includes the following topics:

Information About Routed Firewall Mode

Information About Transparent Firewall Mode

Information About Routed Firewall Mode

In routed mode, the ASA is considered to be a router hop in the network. It can use OSPF or RIP (in single context mode). 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. In single context mode, the routed firewall supports OSPF, EIGRP, and RIP. Multiple context mode supports static routes only. 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.

This section describes transparent firewall mode, and includes the following topics:

Transparent Firewall Network

Allowing Layer 3 Traffic

Allowed MAC Addresses

Passing Traffic Not Allowed in Routed Mode

BPDU Handling

MAC Address vs. Route Lookups

Using the Transparent Firewall in Your Network

Transparent Firewall Network

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

Allowing Layer 3 Traffic

IPv4 and IPv6 traffic is allowed through the transparent firewall automatically from a higher security interface to a lower security interface, without an access list. ARPs are allowed through the transparent firewall in both directions without an access list. ARP traffic can be controlled by ARP inspection. For Layer 3 traffic travelling from a low to a high security interface, an extended access list is required on the low security interface. See Chapter 11 "Adding an Extended Access List," or Chapter 15 "Adding an IPv6 Access List," 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 access list. The transparent firewall, however, can allow almost any traffic through using either an extended access list (for IP traffic) or an EtherType access list (for non-IP traffic).


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. For example, you cannot pass IS-IS packets. An exception is made for BPDUs, which are supported.


For example, you can establish routing protocol adjacencies through a transparent firewall; you can allow OSPF, RIP, EIGRP, or BGP traffic through based on an extended access list. Likewise, protocols like HSRP or VRRP can pass through the ASA.

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

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 access list, you can allow DHCP traffic (instead of the unsupported DHCP relay feature) or multicast traffic such as that created by IP/TV.

BPDU Handling

To prevent loops using the spanning tree protocol, BPDUs are passed by default. To block BPDUs, you need to configure an EtherType access list 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 the "Transparent Firewall Mode Requirements" section 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)

Using the Transparent Firewall in Your Network

Figure 4-1 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 4-1 Transparent Firewall Network

Licensing Requirements for the Firewall Mode

The following table shows the licensing requirements for this feature.

Model
License Requirement

All models

Base License.


Default Settings

The default mode is routed mode.

Guidelines and Limitations

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

The firewall mode is set for the entire system and all contexts; you cannot set the mode individually for each context.

For multiple context mode, set the mode in the system execution space.

When you change modes, the ASA clears the running configuration because many commands are not supported for both modes. This action removes any contexts from running. If you then re-add a context that has an existing configuration that was created for the wrong mode, the context configuration might not work correctly. Be sure to recreate your context configurations for the correct mode before you re-add them, or add new contexts with new paths for the new configurations.

Transparent Firewall Guidelines

Follow these guidelines when planning your transparent firewall network:

For IPv4, a management IP address is required for both management traffic and for traffic to pass through the ASA. For multiple context mode, an IP address is required for each context.

Unlike routed mode, which requires an IP address for each interface, a transparent firewall has an IP address assigned to the entire device. The ASA uses this IP address as the source address for packets originating on the ASA, such as system messages or AAA communications.

The management IP address must be on the same subnet as the connected network. You cannot set the subnet to a host subnet (255.255.255.255).

For IPv6, at a minimum you need to configure link-local addresses for each interface for through traffic. For full functionality, including the ability to manage the ASA, you need to configure a global IP address for the device.

You can configure an IP address (both IPv4 and IPv6) for the Management 0/0 or Management 0/1 management-only interface. This IP address can be on a separate subnet from the main management IP address.

The transparent ASA uses an inside interface and an outside interface only. If your platform includes a dedicated management interface, you can also configure the management interface or subinterface for management traffic only.

In single mode, you can only use two data interfaces (and the dedicated management interface, if available) even if your security appliance includes more than two interfaces.


Note In transparent firewall mode, the management interface updates the MAC address table in the same manner as a data interface; therefore you should not connect both a management and a data interface to the same switch unless you configure one of the switch ports as a routed port (by default Cisco Catalyst switches share a MAC address for all VLAN switch ports). Otherwise, if traffic arrives on the management interface from the physically-connected switch, then the ASA updates the MAC address table to use the management interface to access the switch, instead of the data interface. This action causes a temporary traffic interruption; the ASA will not re-update the MAC address table for packets from the switch to the data interface for at least 30 seconds for security reasons.


Each directly connected network must be on the same subnet.

Do not specify the ASA management IP address as the default gateway for connected devices; devices need to specify the router on the other side of the ASA as the default gateway.

For multiple context mode, each context must use different interfaces; you cannot share an interface across contexts.

For multiple context mode, each context typically uses a different subnet. You can use overlapping subnets, but your network topology requires router and NAT configuration to make it possible from a routing standpoint.

IPv6 Guidelines

Supports IPv6.

Additional Guidelines and Limitations

When you change modes, the ASA clears the running configuration because many commands are not supported for both modes. The startup configuration remains unchanged. If you reload without saving, then the startup configuration is loaded, and the mode reverts back to the original setting. See the "Setting the Firewall Mode" section for information about backing up your configuration file.

If you download a text configuration to the ASA that changes the mode with the firewall transparent command, be sure to put the command at the top of the configuration; the ASA changes the mode as soon as it reads the command and then continues reading the configuration you downloaded. If the command appears later in the configuration, the ASA clears all the preceding lines in the configuration. See the "Downloading Software or Configuration Files to Flash Memory" section for information about downloading text files.

Unsupported Features in Transparent Mode

Table 4-1 lists the features are not supported in transparent mode.

Table 4-1 Unsupported Features in Transparent Mode

Feature
Description

Dynamic DNS

DHCP relay

The transparent firewall can act as a DHCP server, but it does not support the DHCP relay commands. DHCP relay is not required because you can allow DHCP traffic to pass through using two extended access lists: one that allows DCHP requests from the inside interface to the outside, and one that allows the replies from the server in the other direction.

Dynamic routing protocols

You can, however, add static routes for traffic originating on the ASA. You can also allow dynamic routing protocols through the ASA using an extended access list.

Multicast IP routing

You can allow multicast traffic through the ASA by allowing it in an extended access list.

QoS

VPN termination for through traffic

The transparent firewall supports site-to-site VPN tunnels for management connections only. It does not terminate VPN connections for traffic through the ASA. You can pass VPN traffic through the security appliance using an extended access list, but it does not terminate non-management connections. SSL VPN is also not supported.


Setting the Firewall Mode

This section describes how to change the firewall mode.


Note We recommend that you set the firewall mode before you perform any other configuration because changing the firewall mode clears the running configuration.


Prerequisites

When you change modes, the ASA clears the running configuration (see the "Guidelines and Limitations" section 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 the "Backing Up Configuration Files" section.

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.

Detailed Steps

Command
Purpose

firewall transparent

Example:

hostname(config)# firewall transparent

Sets the firewall mode to transparent. Enter this command in the system execution space for multiple context mode. To change the mode to routed, enter the no firewall transparent command.

This command also appears in each context configuration for informational purposes only; you cannot enter this command in a context.

Note You are not prompted to confirm the firewall mode change; the change occurs immediately.


Feature History for Firewall Mode

Table 4-2 lists the release history for this feature.

Table 4-2 Feature History for Firewall Mode 

Feature Name
Releases
Feature Information

Transparent firewall mode

7.0(1)

A transparent firewall 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.

The following commands were introduced: firewall transparent, show firewall.


Configuring ARP Inspection for the Transparent Firewall

This section describes ARP inspection and how to enable it, and includes the following topics:

Information About ARP Inspection

Licensing Requirements for ARP Inspection

Default Settings

Guidelines and Limitations

Configuring ARP Inspection

Monitoring ARP Inspection

Feature History for ARP Inspection

Information About 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.

Licensing Requirements for ARP Inspection

The following table shows the licensing requirements for this feature.

Model
License Requirement

All models

Base License.


Default Settings

By default, all ARP packets are allowed through the ASA.

If you enable ARP inspection, the default setting is to flood non-matching packets.

Guidelines and Limitations

Context Mode Guidelines

Supported in single and multiple context mode.

In multiple context mode, configure ARP inspection within each context.

Firewall Mode Guidelines

Supported only in transparent firewall mode. Routed mode is not supported.

Configuring ARP Inspection

This section describes how to configure ARP inspection, and includes the following topics:

Task Flow for Configuring ARP Inspection

Adding a Static ARP Entry

Enabling ARP Inspection

Task Flow for Configuring ARP Inspection

Follow these steps to configure ARP Inspection:


Step 1 Add static ARP entries according to the "Adding a Static ARP Entry" section. 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" section.


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

Command
Purpose

arp interface_name ip_address mac_address

Example:

hostname(config)# arp outside 10.1.1.1 0009.7cbe.2100

Adds a static ARP entry.


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:

hostname(config)# arp outside 10.1.1.1 0009.7cbe.2100
 
   

What to Do Next

Enable ARP inspection according to the "Enabling ARP Inspection" section.

Enabling ARP Inspection

This section describes how to enable ARP inspection.

Detailed Steps

Command
Purpose

arp-inspection interface_name enable [flood | no-flood]

Example:

hostname(config)# arp-inspection outside enable no-flood

Enables ARP inspection.

The flood keyword forwards non-matching ARP packets out all interfaces, and no-flood drops non-matching packets.

Note The default setting is to flood non-matching packets. To restrict ARP through the ASA to only static entries, then set this command to no-flood.


Examples

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

hostname(config)# arp-inspection outside enable no-flood
 
   

Monitoring ARP Inspection

To monitor ARP inspection, perform the following task:

Command
Purpose
show arp-inspection

Shows the current settings for ARP inspection on all interfaces.


Feature History for ARP Inspection

Table 4-2 lists the release history for this feature.

Table 4-3 Feature History for ARP Inspection 

Feature Name
Releases
Feature Information

ARP inspection

7.0(1)

ARP inspection compares the MAC address, IP address, and source interface in all ARP packets to static entries in the ARP table.

The following commands were introduced: arp, arp-inspection, and show arp-inspection.


Customizing the MAC Address Table for the Transparent Firewall

This section describes the MAC address table, and includes the following topics:

Information About the MAC Address Table

Licensing Requirements for the MAC Address Table

Default Settings

Guidelines and Limitations

Configuring the MAC Address Table

Monitoring the MAC Address Table

Feature History for the MAC Address Table

Information About the 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 adaptive security appliance 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 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.

Licensing Requirements for the MAC Address Table

The following table shows the licensing requirements for this feature.

Model
License Requirement

All models

Base License.


Default Settings

The default timeout value for dynamic MAC address table entries is 5 minutes.

By default, each interface automatically learns the MAC addresses of entering traffic, and the ASA adds corresponding entries to the MAC address table.

Guidelines and Limitations

Context Mode Guidelines

Supported in single and multiple context mode.

In multiple context mode, configure the MAC address table within each context.

Firewall Mode Guidelines

Supported only in transparent firewall mode. Routed mode is not supported.

Additional Guidelines

In transparent firewall mode, the management interface updates the MAC address table in the same manner as a data interface; therefore you should not connect both a management and a data interface to the same switch unless you configure one of the switch ports as a routed port (by default Cisco Catalyst switches share a MAC address for all VLAN switch ports). Otherwise, if traffic arrives on the management interface from the physically-connected switch, then the ASA updates the MAC address table to use the management interface to access the switch, instead of the data interface. This action causes a temporary traffic interruption; the ASA will not re-update the MAC address table for packets from the switch to the data interface for at least 30 seconds for security reasons.

Configuring the MAC Address Table

This section describes how you can customize the MAC address table, and includes the following sections:

Adding a Static MAC Address

Setting the MAC Address Timeout

Disabling MAC Address Learning

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 the "Adding a Static ARP Entry" section), 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:

Command
Purpose

mac-address-table static interface_name mac_address

Example:

hostname(config)# mac-address-table static inside 0009.7cbe.2100

Adds a static MAC address entry.

The interface_name is the source interface.


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:

Command
Purpose

mac-address-table aging-time timeout_value

Example:

hostname(config)# mac-address-table aging-time 10

Sets the MAC address entry timeout.

The timeout_value (in minutes) is between 5 and 720 (12 hours). 5 minutes is the default.


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:

Command
Purpose

mac-learn interface_name disable

Example:

hostname(config)# mac-learn inside disable

Disables MAC address learning.

The no form of this command reenables MAC address learning. The clear configure mac-learn command reenables MAC address learning on all interfaces.


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:

Command
Purpose
show mac-address-table [interface_name]

Shows the MAC address table.


Examples

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

hostname# show mac-address-table
interface				    mac address				       type			      Time Left
-----------------------------------------------------------------------
outside					0009.7cbe.2100				   static				-
inside					0010.7cbe.6101				   static				-
inside					0009.7cbe.5101				   dynamic				10
 
   

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

hostname# show mac-address-table inside
interface				    mac address       type			      Time Left
-----------------------------------------------------------------------
inside					0010.7cbe.6101				   static				-
inside					0009.7cbe.5101				   dynamic				10
 
   

Feature History for the MAC Address Table

Table 4-2 lists the release history for this feature.

Table 4-4 Feature History for the MAC Address Table 

Feature Name
Releases
Feature Information

MAC address table

7.0(1)

Transparent firewall mode uses a MAC address table.

The following commands were introduced: mac-address-table static, mac-address-table aging-time, mac-learn disable, and show mac-address-table.


Firewall Mode Examples

This section includes examples of how traffic moves through the ASA, and includes the following topics:

How Data Moves Through the Security Appliance in Routed Firewall Mode

How Data Moves Through the Transparent Firewall

How Data Moves Through the Security Appliance 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 4-2 shows an inside user accessing an outside web server.

Figure 4-2 Inside to Outside

The following steps describe how data moves through the ASA (see Figure 4-2):

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 according to either a unique interface or a unique destination address associated with a context; the destination address is associated by matching an address translation in a context. In this case, the interface would be unique; the www.example.com IP address does not have a current address translation in 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 translating 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 4-3 shows an outside user accessing the DMZ web server.

Figure 4-3 Outside to DMZ

The following steps describe how data moves through the ASA (see Figure 4-3):

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 untranslates the destination address to the local address 10.1.1.3.

3. 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 according to either a unique interface or a unique destination address associated with a context; the destination address is associated by matching an address translation in a context. In this case, the classifier "knows" that the DMZ web server address belongs to a certain context because of the server address translation.

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 4-4 shows an inside user accessing the DMZ web server.

Figure 4-4 Inside to DMZ

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

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 according to either a unique interface or a unique destination address associated with a context; the destination address is associated by matching an address translation in a context. In this case, the interface is unique; the web server IP address does not have a current address translation.

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 4-5 shows an outside user attempting to access the inside network.

Figure 4-5 Outside to Inside

The following steps describe how data moves through the ASA (see Figure 4-5):

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 4-6 shows a user in the DMZ attempting to access the inside network.

Figure 4-6 DMZ to Inside

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

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 4-7 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 4-7 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 4-8 shows an inside user accessing an outside web server.

Figure 4-8 Inside to Outside

The following steps describe how data moves through the ASA (see Figure 4-8):

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 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, 209.186.201.2.

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 4-8 shows an inside user accessing an outside web server.

Figure 4-9 Inside to Outside with NAT

The following steps describe how data moves through the ASA (see Figure 4-8):

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 translating the mapped address to the real address, 10.1.2.27.

An Outside User Visits a Web Server on the Inside Network

Figure 4-10 shows an outside user accessing the inside web server.

Figure 4-10 Outside to Inside

The following steps describe how data moves through the ASA (see Figure 4-10):

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 according to a unique interface.

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 4-11 shows an outside user attempting to access a host on the inside network.

Figure 4-11 Outside to Inside

The following steps describe how data moves through the ASA (see Figure 4-11):

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 according to a unique interface.

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.