Transparent Cisco IOS Firewall

The Transparent Cisco IOS Firewall feature allows users to "drop" a Cisco IOS Firewall in front of their existing network without changing the statically defined IP addresses of their network-connected devices. Thus, users can allow selected devices from a subnet to traverse the firewall while access to other devices on the same subnet is denied.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Transparent Cisco IOS Firewall" section.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Restrictions for Transparent Cisco IOS Firewall

•Information About Transparent Cisco IOS Firewall

•How to Configure a Transparent Cisco IOS Firewall

•Configuration Examples for Transparent Cisco IOS Firewall

•Additional References

•Feature Information for Transparent Cisco IOS Firewall

Restrictions for Transparent Cisco IOS Firewall

Layer 3 IP Packet Support Only

Only IP packets (TCP, User Datagram Protocol [UDP] , and Internet Control Message Protocol [ICMP]) are subjected to inspection by the transparent firewall. Non-IP traffic is bridged as usual without interference from the transparent firewall. However, if users wish to block non-IP traffic, the MAC access control lists (ACLs) can be applied on interfaces to filter out non-IP traffic and allow only IP traffic.

The following example shows how to configure an ACL that permits all IP packets (0x0800) into the Ethernet interface but denies all Internetwork Packet Exchange (IPX) packets (0x8137): 

Router(config)# access-list 201 permit 0x0800

Router(config)# access-list 201 deny 0x8137

Router(config)# interface ethernet 0

Router(config-if)# bridge-group 1 input-type-list 201

VLAN Trunk Bridging

Bridging between VLAN trunks works only for dot1q encapsulation; Inter-Switch Link (ISL) encapsulation will not work. (However, ISL VLANs will work if subinterfaces are created and placed in a bridge group.)

Information About Transparent Cisco IOS Firewall

•Benefit of the Transparent Firewall

•Transparent Firewall Overview

•Transparent Bridging Overview

•Layer 2 and Layer 3 Firewalls Configured on the Same Router

Benefit of the Transparent Firewall

Added Security with Minimum Configuration

Users can simply drop a transparent Cisco IOS Firewall into an existing network without having to reconfigure their statically defined devices. Thus, the tedious and costly overhead that is required to renumber devices on the trusted network is eliminated.

Transparent Firewall Overview

A typical Cisco IOS Firewall is a Layer 3 device with trusted and untrusted interfaces on different IP subnets. A Layer 3 firewall works well with Cisco IOS devices that function as routers with preexisting subnet separations. However, when a Layer 3 firewall is placed in an existing network, the network IP addresses must be reconfigured to accommodate the firewall.

A transparent Cisco IOS firewall acts as a Layer 2 transparent bridge with context-based access control (CBAC) and ACLs configured on the bridged interface. Because the Layer 2 firewall intercepts packets at Layer 2 and is "transparent" to the existing network, Layer 3 firewall limitations are not applicable.

Transparent Bridging Overview

If configured for bridging, a Cisco IOS device can bridge any number of interfaces. The device can complete basic bridging tasks such as learning MAC addresses on ports to restrict collision domains and running Spanning Tree Protocol (STP) to prevent looping in the topology.

Within bridging, a user can configure Integrated Routed Bridging (IRB), which allows a device to bridge on some interfaces while a Layer 3 Bridged Virtual Interface (BVI) is presented for routing. The bridge can determine whether the packet is to be bridged or routed on the basis of the destination of the Layer 2 or Layer 3 IP address in the packet. Configured with an IP address, the BVI can manage the device even if there is no interface configured for routing.

Layer 2 and Layer 3 Firewalls Configured on the Same Router

A transparent firewall supports a BVI for routing, so a packet that comes in on a bridged interface can be bridged or routed out of the BVI. This functionality allows a Layer 2 (transparent) firewall and a Layer 3 firewall to be configured on the same router: The transparent firewall operates on the bridged packets while the "normal" firewall operates on the routed packets. For example, if you have six interfaces on your router and two of them are in a bridge group, you can simultaneously configure and run normal inspection on the remaining four interfaces.

How to Configure a Transparent Cisco IOS Firewall

You configure a transparent firewall just as you would configure a Layer 3 firewall (via the ip inspect command, which can be configured on any of the bridged interfaces for the transparent firewall). Also, you configure transparent bridging for a firewall just as you would for any other Cisco IOS device.

•Configuring a Bridge Group (required)

•Configuring Inspection and ACLs (required)

•Forwarding DHCP Traffic (optional)

•Monitoring Transparent Firewall Events (optional)

Configuring a Bridge Group

Perform this task to configure a bridge group and to associate interfaces or subinterfaces in the configured bridge group.

Prerequisites for BVI Configuration

•If a BVI is not configured, you must disable IP routing (via the no ip routing command) for the bridging operation to take effect.

•If configured, a BVI must be configured with an IP address in the same subnet.

•You must configure a BVI if more than two interfaces are placed in a bridge group.

Restrictions

•If more than two interfaces are assigned to a bridge group, any routers that are acting as first-hop gateways to hosts that are in the bridged network (the bridge group) must allow ICMP time-to-live (TTL) exceeded messages to pass.

•Spanning Tree Bridge Protocol Data Units (BPDU) and packets that are to be routed out of the bridge, if IRB is configured, are not inspected.

SUMMARY STEPS

1. enable

2. configure terminal

3. bridge bridge-group protocol {dec | ibm | ieee | vlan-bridge}

4. interface type number

5. bridge-group bridge-group

6. exit

7. bridge irb

8. bridge bridge-group route protocol

9. interface type number

10. ip address ip-address mask

11. no shutdown

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

bridge bridge-group protocol {dec | ibm | ieee | vlan-bridge}

Example:

Router(config)# bridge 1 protocol ieee

Defines the type of Spanning Tree Protocol (STP).

Step 4 

interface type number

Example:

Router(config)# interface Ethernet0

Configures an interface type and enters interface configuration mode.

Step 5 

bridge-group bridge-group

Example:

Router(config-if)# bridge-group 1

Assigns each network interface to a bridge group.

Note Complete Step 4 and Step 5 for each interface you want to assign to a bridge group.

Note You can also assign subinterfaces to a bridge group to control bridging between VLANs.

Step 6 

exit

Exits interface configuration mode.

Step 7 

bridge irb

Example:

Router(config)# bridge irb

Enables the Cisco IOS software to route a given protocol between routed interfaces and bridge groups or to route a given protocol between bridge groups.

Note Step 7 through Step 11 are necessary only if you want to configure a BVI.

Step 8 

bridge bridge-group route protocol

Example:

Router(config)# bridge 1 route ip

Enables the routing of a specified protocol in a specified bridge group.

Step 9 

interface type number

Example:

Router(config)# interface BVI1

Configures a BVI and enters interface configuration mode.

Step 10 

ip address ip-address mask

Example:

Router(config-if) ip address 10.1.1.1 255.255.255.0

Sets a primary IP address for an interface.

Step 11 

no shutdown

Example:

Router(config-if)# no shutdown

Restarts a disabled interface.

Examples

The following example shows how to configure interfaces "ethernet0" and "ethernet1" in a bridge group. These interfaces are associated with the BVI interface "BVI1," which can be reached from any host on either of the interfaces via the IP address 10.1.1.1. 

Router(config)# bridge 1 protocol ieee

Router(config)# interface ethernet0

Router(config-if)# bridge-group 1

Router(config-if)# interface ethernet1

Router(config-if)# bridge-group 1

Router(config-if)# exit

! Configure the BVI.

Router(config)# bridge irb

Router(config)# bridge 1 route ip

Router(config)# interface BVI1

Router(config-if)# ip address 10.1.1.1 255.255.255.0

Router(config-if)# no shutdown

Troubleshooting Tips

To display the status of each bridge group, use the show bridge-group command or to display entries in the bridge table, use the show bridge command.

What to Do Next

After you have configured the bridge group, you must configure an inspection rule and at least one IP ACL. To complete this task, refer to the following section, "Configuring Inspection and ACLs."

Note If inspection is not configured on any interface in the bridge group, IP ACLs on bridged interfaces will not be active.

Configuring Inspection and ACLs

Use this task to configure an inspection rule and apply it on the appropriate interface. Also, use this task to configure at least one ACL and apply it on one or more of the interfaces that you configured in the bridge group.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip inspect name protocol [alert {on | off}] [audit-trail {on | off}] [timeout seconds]

4. interface type number

5. ip inspect inspection-name {in | out}

6. exit

7. access-list access-list-number {permit | deny} {type-code wild-mask | address mask}

8. interface type number

9. ip access-group {access-list-number | access-list-name} {in | out}

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip inspect name protocol [alert {on | off}] [audit-trail {on | off}]

[timeout seconds]

Example:

Router(config)# ip inspect name test tcp

Defines a set of inspection rules.

Step 4 

interface type number

Example:

Router(config)# interface Ethernet0

Configures an interface type and enters interface configuration mode.

Step 5 

ip inspect inspection-name {in | out}

Example:

Router(config-if)# ip inspect test in

Applies a set of inspection rules to an interface.

Step 6 

exit

Exits interface configuration mode.

Step 7 

access-list access-list-number {permit | deny} {type-code wild-mask | address mask}

Example:

Router(config)# access-list 156 permit 10.1.1.0 0.0.0.255 any

Configures the ACL.

Note Repeat this step for each ACL that you want to configure.

Step 8 

interface type number

Example:

Router(config)# interface Ethernet0

Configures an interface type and enters interface configuration mode.

Note Repeat Steps 8 and 9 for each ACL that you want to apply to inbound packets from a specific interface.

Step 9 

ip access-group {access-list-number | access-list-name} {in | out}

Example:

Router(config-if) ip access-group 156 in

Controls access to an interface.

Examples

The following example shows how to configure an inspection rule on interface "ethernet0," which is the inside interface. Policies can be specified via ACL 156 or 101; for example, ACL 156 can be used to specify that rlogin and rsh are not allowed for the internal users, and ACL 101 can be used to specify that an external host requires connectivity to a particular host in the internal domain. 

Router(config)# ip inspect name test tcp

Router(config)# interface ethernet0

Router(config-if)# ip inspect test in

Router(config-if)# exit

!

! Configure the ACLs.

Router(config)# access-list 101 deny ip any any

Router(config)# access-list 156 permit 10.1.1.0 0.0.0.255 any

Router(config)# access-list 156 deny ip any any


Router(config)# interface ethernet0

Router(config-if) ip access-group 156 in

Router(config)# interface ethernet1

Router(config-if) ip access-group 101 in

Forwarding DHCP Traffic

Use this task to enable a transparent firewall to forward DHCP packets across the bridge without inspection; that is, the ip inspect L2-transparent dhcp-passthrough command overrides the ACL for DHCP packets, so DHCP packets will be forwarded even if the ACL is configured to deny all IP packets. Thus, clients on one side of the bridge can get an IP address from a DHCP server on the opposite side of the bridge.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip inspect L2-transparent dhcp-passthrough

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip inspect L2-transparent dhcp-passthrough

Example:

Router#(config) ip inspect L2-transparent dhcp-passthrough

Allows a transparent firewall to forward DHCP passthrough traffic.

Monitoring Transparent Firewall Events

Use either of these optional steps to monitor the activity of the transparent firewall.

Note Effective with Cisco IOS Release 12.4(20)T, the debug ip inspect command is replaced by the debug policy-firewall command. See the Cisco IOS Debug Command Reference for more information.

SUMMARY STEPS

1. enable

2. debug ip inspect L2-transparent {packet | dhcp-passthrough}

3. show ip inspect {name inspection-name | config | interfaces | session [detail] | all}

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

debug ip inspect L2-transparent {packet | dhcp-passthrough}

Example:

Router# debug ip inspect L2-transparent dhcp-passthrough

Enables debugging messages for transparent firewall events.

•packet—Displays messages for all debug packets that are inspected by the transparent firewall.

•dhcp-passthrough—Displays debug messages only for DHCP pass-through traffic that the transparent firewall forwards across the bridge.

Step 3 

show ip inspect {name inspection-name | config | interfaces | session [detail] | all}

Example:

Router# show ip inspect all

Displays Cisco IOS Firewall configuration and session information.

•If the transparent firewall is configured, use the all keyword to display the bridging interface in the interface configuration section of the output.

Examples

The following sample output is a portion of the show ip inspect all command that shows the bridging interface: 

Router# show ip inspect all

.

.

.

Interface Configuration

! Below is the bridging interface.

Interface Ethernet1

Inbound inspection rule is test

tcp alert is on audit-trail is off timeout 3600

ftp alert is on audit-trail is off timeout 3600

Outgoing inspection rule is not set

Inbound access list is not set

Outgoing access list is 156

.

.

.

Configuration Examples for Transparent Cisco IOS Firewall

•Example: Comprehensive Transparent Firewall Configuration

•Example: Monitoring Telnet Connections via debug and show Output

•Examples: Configuring and Verifying DHCP Pass-Through Traffic

Example: Comprehensive Transparent Firewall Configuration

The following example and sample topology (see Figure 1) illustrate how to configure and debug a transparent Cisco IOS Firewall configuration between a client, a firewall, and a server. This example also includes show command output for additional configuration verification. After you have configured a transparent firewall, you can Telnet from the client to the server through the firewall. (See the section "Example: Monitoring Telnet Connections via debug and show Output."

Figure 1 Sample Topology for Transparent Firewall Configuration

Note Effective with Cisco IOS Release 12.4(20)T, the debug ip inspect command is replaced by the debug policy-firewall command. See the Cisco IOS Debug Command Reference for more information. 

 
! Enable debug commands.

Router# debug ip inspect L2-transparent packet

INSPECT L2 firewall debugging is on

Router# debug ip inspect object-creation

INSPECT Object Creations debugging is on

Router# debug ip inspect object-deletion

INSPECT Object Deletions debugging is on

! Start the transparent firewall configuration process

Router# config terminal

Enter configuration commands, one per line.  End with CNTL/Z.

! Configure bridging 

Router(config)# bridge 1 protocol ieee

Router(config)# bridge irb

Router(config)# bridge 1 route ip

Router(config)# interface bvi1

*Mar  1 00:06:42.511:%LINK-3-UPDOWN:Interface BVI1, changed state to down.

Router(config-if)# ip address 209.165.200.225 255.255.255.254

! Configure inspection 

Router(config)# ip inspect name test tcp

! Following debugs show the memory allocated for CBAC rules.

*Mar  1 00:07:21.127:CBAC OBJ_CREATE:create irc 817F04F0 (test)

*Mar  1 00:07:21.127:CBAC OBJ_CREATE:create irt 818AED20 Protocol:tcp Inactivity time:0 
test 

Router(config)# ip inspect name test icmp

Router(config)#

*Mar  1 00:07:39.211:CBAC OBJ_CREATE:create irt 818AEDCC Protocol:icmp Inactivity time:0

! Configure Bridging on ethernet0 interface 

Router(config)# interface ethernet0

Router(config-if)# bridge-group 1

*Mar  1 00:07:49.071:%LINK-3-UPDOWN:Interface BVI1, changed state to up

*Mar  1 00:07:50.071:%LINEPROTO-5-UPDOWN:Line protocol on Interface BVI1, changed state to 
up

! Configure inspection on ethernet0 interface 

Router(config-if)# ip inspect test in

Router(config-if)# 

*Mar  1 00:07:57.543:CBAC OBJ_CREATE:create idbsb 8189CBFC (Ethernet0)


! Incremented the number of bridging interfaces configured for inspection 

*Mar  1 00:07:57.543:L2FW:Incrementing L2FW i/f count

Router(config-if)# interface ethernet1

! Configure bridging and ACL on interface ethernet1 

Router(config-if)# bridge-group 1

Router(config-if)# ip access-group 101 in

*Mar  1 00:08:26.711:%LINEPROTO-5-UPDOWN:Line protocol on Interface Ethernet1, changed 
state to up

Router(config-if)# end

Router(config)# end

!

! Issue the show running-config command to verify the complete transparent firewall 
! configuration. 

Router# show running-config

Building configuration...


Current configuration :1126 bytes

!

version 12.3

no service pad

service timestamps debug datetime msec

service timestamps log datetime msec

no service password-encryption

!

hostname Firewall

!

logging buffered 12000 debugging

no logging console

!

no aaa new-model

ip subnet-zero

no ip domain lookup

!

!

ip inspect name test tcp

ip inspect name test icmp

ip audit notify log

ip audit po max-events 100

no ftp-server write-enable

!

!

! 

no crypto isakmp enable

!

!

bridge irb

!

!

interface Ethernet0

 no ip address

 no ip proxy-arp

 ip inspect test in

 bridge-group 1

 hold-queue 100 out

!

interface Ethernet1

 no ip address

 ip access-group 101 in

 no ip unreachables

 no ip proxy-arp

 duplex auto

 bridge-group 1

!

interface BVI1

 ip address 209.165.200.225 255.255.255.254

!

ip classless

ip route 9.1.0.0 255.255.0.0 9.4.0.1

no ip http server

no ip http secure-server

!

!

ip access-list log-update threshold 1

access-list 101 permit icmp any any log

access-list 101 deny   ip any any log

!

control-plane

!

bridge 1 protocol ieee

bridge 1 route ip

!

line con 0

 no modem enable

 stopbits 1

line aux 0

 stopbits 1

line vty 0 4

 login

!

scheduler max-task-time 5000

!

end

!

! Issue show brige commands to check the tables.

Router# show bridge


Total of 300 station blocks, 300 free

Codes:P - permanent, S - self


Bridge Group 1:


! The bridge table is empty because no traffic has been seen 

!

Router# show bridge group


Bridge Group 1 is running the IEEE compatible Spanning Tree protocol


Port 2 (Ethernet0) of bridge group 1 is forwarding

Port 3 (Ethernet1) of bridge group 1 is forwarding

! Note that the interfaces are in a "forwarding" state. The interfaces move from  
! a listening state to a learning state and finally to a forwarding state. It takes 
! approximately 30 seconds to move to a forwarding after "bridge-group 1" is configured.

Example: Monitoring Telnet Connections via debug and show Output

The following examples shows how to monitor established Telnet connections from the client to the server through the firewall (see Figure 1) and from the server to the client. In these example, the debug ip inspect L2-transparent packet command has been issued to generate the debug messages. Relevant show commands are also issued for additional verification.

Note Effective with Cisco IOS Release 12.4(20)T, the debug ip inspect command is replaced by the debug policy-firewall command. See the Cisco IOS Debug Command Reference for more information.

•Telnet Connection from the Client (97.0.0.2) to the Server (97.0.0.23)

•Telnet Connection from the Server (97.0.0.23) to the Client (97.0.0.2)

Telnet Connection from the Client (97.0.0.2) to the Server (97.0.0.23)

The following example is output from the initial Telnet connection between the client and the server. A subsequent connection is established to highlight differences in the debug output. Explanations are given inline. 

! A packet is received by the firewall in the flood path because the bridge-table is 
! initially empty. However, the client seems to have the server's mac-address in its ARP 
! cache, so the bridge floods the packet and it appears in the firewall's "flood" path. 

*Mar  1 00:17:32.119:L2FW:insp_l2_flood:input is Ethernet0 output is Ethernet1

! Source and destination IP addresses and the L4 protocol of the packet 

*Mar  1 00:17:32.123:L2FW*:Src 97.0.0.2 dst 97.0.0.23 protocol tcp

! ACL processing status. An ACL is not configured in this direction; that is, from the 
! client to the server. 

*Mar  1 00:17:32.123:L2FW:Input ACL not configured or the ACL is bypassed

*Mar  1 00:17:32.123:L2FW:Output ACL is not configured or ACL is bypassed

! If there are exactly two interfaces in the bridge-group and the packet is in flood path, 
! the firewall invokes inspection directly, skipping the Unicast flood algorithm. If there 
! are more than 2 interfaces, the firewall "drops" the packet and issues the algorithm. 

*Mar  1 00:17:32.123:L2FW:FLOOD number of i/fs in bridge-group is exactly 2. Calling 
Inspection

! The packet is being inspected.

*Mar  1 00:17:32.123:L2FW:insp_l2_inspection:input is Ethernet0 output is Ethernet1

*Mar  1 00:17:32.123:L2FW*:Src 97.0.0.2 dst 97.0.0.23 protocol tcp

*Mar  1 00:17:32.123:L2FW:Input ACL not configured or the ACL is bypassed

*Mar  1 00:17:32.123:L2FW:Output ACL is not configured or ACL is bypassed


! Memory is allocated for the transparent firewall attributes in the session structure 

*Mar  1 00:17:32.123:L2FW:allocating L2 extension for sis

! CBAC-related debug messages: The packet has been passed to the existing CBAC code.

*Mar  1 00:17:32.123:CBAC Pak 814635DC sis 816C9C24 initiator_addr (97.0.0.2:11016) 
responder_addr (97.0.0.23:23)

initiator_alt_addr (97.0.0.2:11016) responder_alt_addr (97.0.0.23:23)

! CBAC session structure has been allocated 

*Mar  1 00:17:32.127:CBAC OBJ_CREATE:create sis 816C9C24

*Mar  1 00:17:32.127:CBAC OBJ-CREATE:sid 816D69D8 acl 101 Prot:tcp

*Mar  1 00:17:32.127: Src 97.0.0.23 Port [23:23]

*Mar  1 00:17:32.127: Dst 97.0.0.2 Port [11016:11016]

! The Layer 2 header length is being computed for caching the L2 header, which will be 
! used if a TCP RST should be sent in the future to tear down the connection. 

*Mar  1 00:17:32.127:L2FW:L2 header length(initiator->responder) is 14

! Checks to see if the header is 802.3, SNAP, SAP. (This header is 802.3.)

*Mar  1 00:17:32.127:L2FW:info_start is NULL for init->rsp

*Mar  1 00:17:32.127:CBAC OBJ_CREATE:create host entry 816D4018 addr 97.0.0.23 bucket 118

! CBAC has indicated that the packet should be passed

*Mar  1 00:17:32.127:L2FW:insp_inspection returned FALSE. PASS


! The next packet in the flow has arrived on the interrupt path. This packet is from the 
! server (ethernet1) to the client (ethernet0). 

*Mar  1 00:17:32.131:L2FW*:insp_l2_fast_inspection:pak 812C9084, input-interface 
Ethernet1, output-interface Ethernet0

*Mar  1 00:17:32.131:L2FW*:Src 97.0.0.23 dst 97.0.0.2 protocol tcp

*Mar  1 00:17:32.131:L2FW:Input ACL not configured or the ACL is bypassed

*Mar  1 00:17:32.131:L2FW:Output ACL is not configured or ACL is bypassed

! The Layer 2 header length is computed and will be cached 

*Mar  1 00:17:32.131:L2FW:L2 header length is 14 (rsp->init)

*Mar  1 00:17:32.131:L2FW:info_start is NULL rsp->init

! CBAC has indicated that the packet should be forwarded 

*Mar  1 00:17:32.131:L2FW*:insp_l2_fast_inspection returning INSP_L2_OK

! A new packet has arrived from the client. The following trace repeats for each packet 
received by the firewall 

*Mar  1 00:17:32.135:L2FW*:insp_l2_fast_inspection:pak 81462FB4, input-interface 
Ethernet0, output-interface Ethernet1

*Mar  1 00:17:32.135:L2FW*:Src 97.0.0.2 dst 97.0.0.23 protocol tcp

*Mar  1 00:17:32.135:L2FW:Input ACL not configured or the ACL is bypassed

*Mar  1 00:17:32.135:L2FW:Output ACL is not configured or ACL is bypassed

*Mar  1 00:17:32.135:L2FW*:insp_l2_fast_inspection returning INSP_L2_OK

       ...<more packets >...

! The host entry for the server is deleted. 

*Mar  1 00:17:32.263:CBAC OBJ_DELETE:delete host entry 816D4018 addr 97.0.0.23


! Issue the show ip inspect command to verify that a CBAC session has been established 

Router# show ip inspect session detailed


Established Sessions

 Session 816C9C24 (97.0.0.2:11016)=>(97.0.0.23:23) tcp SIS_OPEN

  Created 00:00:28, Last heard 00:00:09

  Bytes sent (initiator:responder) [38:75]

  In  SID 97.0.0.23[23:23]=>97.0.0.2[11016:11016] on ACL 101 (12 matches)

Router#

!

! Issue the show bridge command to verify that entries for the client and server have been 
! created in the bridge-table.

Router# show bridge


Total of 300 station blocks, 298 free

Codes:P - permanent, S - self


Bridge Group 1:


    Address       Action   Interface       Age   RX count   TX count

0008.a3b6.b603   forward   Ethernet0         2         14         12

0007.0d97.308f   forward   Ethernet1         2         12         13

Router#

!

! Close the TCP connection (by typing exit at the client). 

*Mar  1 00:21:26.259:CBAC OBJ_DELETE:delete sis 816C9C24

*Mar  1 00:21:26.259:CBAC OBJ-DELETE:sid 816D69D8 on acl 101 Prot:tcp

*Mar  1 00:21:26.259: Src 97.0.0.23 Port [23:23]

*Mar  1 00:21:26.259: Dst 97.0.0.2 Port [11016:11016]

! The data structures pertaining to the Layer 2 firewall have been deleted from the 
! session. The session has also been deleted. 

*Mar  1 00:21:26.259:L2FW:Deleting L2FW data structures

A New Telnet Connection from the Client (97.0.0.2) to the Server (97.0.0.23) 

! The initial SYN packet from the client has arrived in the interrupt path. Note that the 
! corresponding packet from the previous telnet session came in on the flood path because 
! the bridge-table was empty so the bridge was forced to flood the packet. Since the 
! bridge-table is now populated, the packet does not not to be flooded. This is the only 
! difference between the previous telnet session and this session. Subsequent packets will 
! follow the same path (and generate the same debugs) as the previous session. 

*Mar  1 00:23:31.883:L2FW*:insp_l2_fast_inspection:pak 81465190, input-interface 
Ethernet0, output-interface Ethernet1

*Mar  1 00:23:31.883:L2FW*:Src 97.0.0.2 dst 97.0.0.23 protocol tcp

*Mar  1 00:23:31.883:L2FW:Input ACL not configured or the ACL is bypassed

*Mar  1 00:23:31.883:L2FW:Output ACL is not configured or ACL is bypassed

! CBAC has indicated that the packet should be punted to the process path since memory 
! allocation and the control-plane is involved 

*Mar  1 00:23:31.883:L2FW*:insp_l2_fast_inspection returning INSP_L2_PUNT


! After being punted from the interrupt path, the packet has arrived at the process level 
! for inspection. Moving forward, the debug messages are similar to the flood case in the 
! previous session.

*Mar  1 00:23:31.883:L2FW:insp_l2_inspection:input is Ethernet0 output is Ethernet1

*Mar  1 00:23:31.883:L2FW*:Src 97.0.0.2 dst 97.0.0.23 protocol tcp

*Mar  1 00:23:31.883:L2FW:Input ACL not configured or the ACL is bypassed

*Mar  1 00:23:31.883:L2FW:Output ACL is not configured or ACL is bypassed

*Mar  1 00:23:31.887:L2FW:allocating L2 extension for sis

*Mar  1 00:23:31.887:CBAC Pak 81465190 sis 816C9C24 initiator_addr (97.0.0.2:11017) 
responder_addr (97.0.0.23:23)

initiator_alt_addr (97.0.0.2:11017) responder_alt_addr (97.0.0.23:23)

*Mar  1 00:23:31.887:CBAC OBJ_CREATE:create sis 816C9C24

*Mar  1 00:23:31.887:CBAC OBJ-CREATE:sid 816D69D8 acl 101 Prot:tcp

*Mar  1 00:23:31.887: Src 97.0.0.23 Port [23:23]

*Mar  1 00:23:31.887: Dst 97.0.0.2 Port [11017:11017]

*Mar  1 00:23:31.887:L2FW:L2 header length(initiator->responder) is 14

*Mar  1 00:23:31.887:L2FW:info_start is NULL for init->rsp

*Mar  1 00:23:31.887:CBAC OBJ_CREATE:create host entry 816D4018 addr 97.0.0.23 bucket 118

! CBAC has indicated that the packet should be Passed 

*Mar  1 00:23:31.891:L2FW:insp_inspection returned FALSE. PASS

!

! Issue the show ip inspect command to verify the newly created inspect session 

Router# show ip inspect session details

Established Sessions

 Session 816C9C24 (97.0.0.2:11017)=>(97.0.0.23:23) tcp SIS_OPEN

  Created 00:00:52, Last heard 00:00:37

  Bytes sent (initiator:responder) [38:75]

  In  SID 97.0.0.23[23:23]=>97.0.0.2[11017:11017] on ACL 101 (10 matches)

Router#

Telnet Connection from the Server (97.0.0.23) to the Client (97.0.0.2)

The following sample output is from a Telnet connection that was initiated from the server to the client. This connection will not go through because "ACL 101" is configured to allow only ICMP packets and deny all other packets. Note that inspection is not configured from the server to the client. This example is shown to display the debug messages that are associated with dropped packets. 

! The first packet from the server comes in on ethernet1 interface 

*Mar  1 00:26:12.367:L2FW*:insp_l2_fast_inspection:pak 815C89FC, input-interface 
Ethernet1, output-interface Ethernet0

*Mar  1 00:26:12.367:L2FW*:Src 97.0.0.23 dst 97.0.0.2 protocol tcp

! This packet is punted up since ACL 101 is configured for logging. Logging happens in the 
process path. If logging was not configured, the packet would have been dropped instead of 
being punted to process level 

*Mar  1 00:26:12.367:L2FW:Packet punted up by Input ACL for logging

! The packet arrives at process level 

*Mar  1 00:26:12.367:L2FW:insp_l2_inspection:input is Ethernet1 output is Ethernet0

*Mar  1 00:26:12.371:L2FW*:Src 97.0.0.23 dst 97.0.0.2 protocol tcp

! The ACL log is generated 

*Mar  1 00:26:12.371:%SEC-6-IPACCESSLOGP:list 101 denied tcp 97.0.0.23(11045) -> 
97.0.0.2(23), 1 packet

! The packet is dropped by the ACL 

*Mar  1 00:26:12.371:L2FW:Packet processed and dropped by Input ACL

! The packet is dropped by the ACL and is therefore NOT sent to CBAC for inspection 

*Mar  1 00:26:12.371:L2FW:Packet is dropped in insp_l2_inspection

Examples: Configuring and Verifying DHCP Pass-Through Traffic

The following examples show how to verify (via debug messages) DHCP pass-through that has been allowed and traffic that has not been allowed.

•Example: Allowing DHCP Pass-Through Traffic

•Example: Denying DHCP Pass-Through Traffic

Example: Allowing DHCP Pass-Through Traffic

In this example, the static IP address of the client is removed and the address is acquired via DHCP using the ip address dhcp command on the interface that is connected to the transparent firewall. 


Router# show debug

ARP:

  ARP packet debugging is on

L2 Inspection:

  INSPECT L2 firewall debugging is on

  INSPECT L2 firewall DHCP debugging is on

Router#

Router#

! Configure DHCP passthrough 

Router(config)# ip insp L2-transparent dhcp-passthrough

! The DHCP discover broadcast packet arrives from the client. Since this packet is a 
! broadcast (255.255.255.255), it arrives in the flood path 

*Mar  1 00:35:01.299:L2FW:insp_l2_flood:input is Ethernet0 output is Ethernet1

*Mar  1 00:35:01.299:L2FW*:Src 0.0.0.0 dst 255.255.255.255 protocol udp

*Mar  1 00:35:01.299:L2FW:udp ports src 68 dst 67

*Mar  1 00:35:01.299:L2FW:src 0.0.0.0 dst 255.255.255.255

! The DHCP pass through flag is checked and the packet is allowed 

*Mar  1 00:35:01.299:L2FW:DHCP packet seen. Pass-through flag allows the packet

! The packet is a broadcast packet and therefore not sent to CBAC 

*Mar  1 00:35:01.299:L2FW*:Packet is broadcast or multicast.PASS

! The DHCP server 97.0.0.23 responds to the client's request 

*Mar  1 00:35:01.303:L2FW:insp_l2_flood:input is Ethernet1 output is Ethernet0

*Mar  1 00:35:01.303:L2FW*:Src 97.0.0.23 dst 255.255.255.255 protocol udp

*Mar  1 00:35:01.307:L2FW:udp ports src 67 dst 68

*Mar  1 00:35:01.307:L2FW:src 97.0.0.23 dst 255.255.255.255

*Mar  1 00:35:01.307:L2FW:DHCP packet seen. Pass-through flag allows the packet

*Mar  1 00:35:01.307:L2FW*:Packet is broadcast or multicast.PASS

*Mar  1 00:35:01.311:L2FW:insp_l2_flood:input is Ethernet0 output is Ethernet1

*Mar  1 00:35:01.311:L2FW*:Src 0.0.0.0 dst 255.255.255.255 protocol udp

*Mar  1 00:35:01.311:L2FW:udp ports src 68 dst 67

*Mar  1 00:35:01.311:L2FW:src 0.0.0.0 dst 255.255.255.255

*Mar  1 00:35:01.315:L2FW:DHCP packet seen. Pass-through flag allows the packet

*Mar  1 00:35:01.315:L2FW*:Packet is broadcast or multicast.PASS

*Mar  1 00:35:01.315:L2FW:insp_l2_flood:input is Ethernet1 output is Ethernet0

*Mar  1 00:35:01.323:L2FW*:Src 97.0.0.23 dst 255.255.255.255 protocol udp

*Mar  1 00:35:01.323:L2FW:udp ports src 67 dst 68

*Mar  1 00:35:01.323:L2FW:src 97.0.0.23 dst 255.255.255.255

*Mar  1 00:35:01.323:L2FW:DHCP packet seen. Pass-through flag allows the packet

*Mar  1 00:35:01.323:L2FW*:Packet is broadcast or multicast.PASS

! The client has an IP address (97.0.0.5) and has issued a G-ARP to let everyone know it's 
address 

*Mar  1 00:35:01.327:IP ARP:rcvd rep src 97.0.0.5 0008.a3b6.b603, dst 97.0.0.5 BVI1

Router#

Example: Denying DHCP Pass-Through Traffic

In this example, DHCP pass-through traffic is not allowed (via the no ip inspect L2-transparent dhcp-passthrough command). The client is denied when it attempts to acquire a DHCP address from the server. 

! Deny DHCP pass-through traffic

Router(config)# no ip inspect L2-transparent dhcp-passthrough 

! The DHCP discover broadcast packet arrives from the client 

*Mar  1 00:36:40.003:L2FW:insp_l2_flood:input is Ethernet0 output is Ethernet1

*Mar  1 00:36:40.003:L2FW*:Src 0.0.0.0 dst 255.255.255.255 protocol udp

*Mar  1 00:36:40.003:L2FW:udp ports src 68 dst 67

*Mar  1 00:36:40.007:L2FW:src 0.0.0.0 dst 255.255.255.255

! The pass-through flag is checked 

*Mar  1 00:36:40.007:L2FW:DHCP packet seen. Pass-through flag denies the packet

! The packet is dropped because the flag does not allow DHCP passthrough traffic. Thus, 
! the client cannot acquire an address, and it times out 

*Mar  1 00:36:40.007:L2FW:FLOOD Dropping the packet after ACL check.

Router#

Additional References

Related Documents

Related Topic
Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Cisco IOS Firewall commands

Cisco IOS Security Command Reference

Bridging commands

Cisco IOS Bridging and IBM Networking Command Reference, Volume 1 of 2: Bridging

Additional bridging configuration information

The section "Bridging" of the Cisco IOS Bridging and IBM Networking Configuration Guide

DHCP configuration information

The chapter "Configuring DHCP" in the Cisco IOS IP Configuration Guide


Standards

Standards
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

—


MIBs

MIBs
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFCs
Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

—


Technical Assistance

Description
Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


Feature Information for Transparent Cisco IOS Firewall

Table 1 lists the features in this module and provides links to specific configuration information.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Table 1 Feature Information for Transparent Cisco IOS Firewall 

Feature Name
Releases
Feature Information

Transparent Cisco IOS Firewall

12.3(7)T

The Transparent Cisco IOS Firewall feature allows users to "drop" a Cisco IOS Firewall in front of their existing network without changing the statically defined IP addresses of their network-connected devices. Thus, users can allow selected devices from a subnet to traverse the firewall while access to other devices on the same subnet is denied.

The following commands were introduced or modified: debug ip inspect L2-transparent, ip inspect L2-transparent dhcp-passthrough.


Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

© 2009 Cisco Systems, Inc. All rights reserved.