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8

Configuring a Simple Firewall

The Cisco 850 and Cisco 870 series routers support network traffic filtering by means of access lists.

The routers also support packet inspection and dynamic temporary access lists by means of

Context-Based Access Control (CBAC).

Basic traffic filtering is limited to configured access list implementations that examine packets at the

network layer or, at most, the transport layer, permitting or denying the passage of each packet through

the firewall. However, the use of inspection rules in CBAC allows the creation and use of dynamic

temporary access lists. These dynamic lists allow temporary openings in the configured access lists at

firewall interfaces. These openings are created when traffic for a specified user session exits the internal

network through the firewall. The openings allow returning traffic for the specified session (that would

normally be blocked) back through the firewall.

See the

Cisco IOS Security Configuration Guide, Release 12.3

, for more detailed information on traffic

filtering and firewalls.

Figure 8-1

shows a network deployment using PPPoE or PPPoA with NAT and a firewall.

Figure 8-1

Router with Firewall Configured

2

3

7

5

6

1

4

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Chapter 8

Configuring a Simple Firewall

In the configuration example that follows, the firewall is applied to the outside WAN interface (FE4) on

the Cisco 851 or Cisco 871 and protects the Fast Ethernet LAN on FE0 by filtering and inspecting all

traffic entering the router on the Fast Ethernet WAN interface FE4. Note that in this example, the network

traffic originating from the corporate network, network address 10.1.1.0, is considered safe traffic and

is not filtered.

Configuration Tasks

Perform the following tasks to configure this network scenario:

•

Configure Access Lists

•

Configure Inspection Rules

•

Apply Access Lists and Inspection Rules to Interfaces

A configuration example that shows the results of these configuration tasks is provided in the

“Configuration Example” section on page 8-5

.

Note

The procedures in this chapter assume that you have already configured basic router features as well as

PPPoE or PPPoA with NAT. If you have not performed these configurations tasks, see

Chapter 1, “Basic

Router Configuration,” Chapter 3, “Configuring PPP over Ethernet with NAT,”

and

Chapter 4,

“Configuring PPP over ATM with NAT,”

as appropriate for your router. You may have also configured

DHCP, VLANs, and secure tunnels.

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Chapter 8

Configuring a Simple Firewall

Configure Access Lists

Configure Access Lists

Perform these steps to create access lists for use by the firewall, beginning in global configuration mode:

Command

Purpose

Step 1

access-list

access-list-number

{

deny

|

permit

}

protocol source source-wildcard

[

operator

[

port

]]

destination

Example:

Router(config)#

access-list 103 deny ip any

any

Router(config)#

access-list 103 permit host

200.1.1.1 eq isakmp any

Router(config)#

Creates an access list which prevents Internet-

initiated traffic from reaching the local (inside)

network of the router, and which compares

source and destination ports.

See the

Cisco IOS IP Command Reference,

Volume 1 of 4: Addressing and Services

for

details about this command.

Configure Inspection Rules

Perform these steps to configure firewall inspection rules for all TCP and UDP traffic, as well as specific

application protocols as defined by the security policy, beginning in global configuration mode:

Command or Action

Purpose

Step 1

ip inspect name

inspection-name protocol

Example:

Router(config)#

ip inspect name firewall tcp

Router(config)#

Defines an inspection rule for a particular

protocol.

Step 2

ip inspect name

inspection-name protocol

Example:

Router(config)#

ip inspect name firewall rtsp

Router(config)#

ip inspect name firewall h323

Router(config)#

ip inspect name firewall

netshow

Router(config)#

ip inspect name firewall ftp

Router(config)#

ip inspect name firewall

sqlnet

Router(config)#

Repeat this command for each inspection rule

that you wish to use.

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Chapter 8

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Apply Access Lists and Inspection Rules to Interfaces

Apply Access Lists and Inspection Rules to Interfaces

Perform these steps to apply the ACLs and inspection rules to the network interfaces, beginning in global

configuration mode:

Command

Purpose

Step 1

interface

type number

Example:

Router(config)#

interface vlan 1

Router(config-if)#

Enters interface configuration mode for the

inside network interface on your router.

Step 2

ip inspect

inspection-name

{

in

|

out

}

Example:

Router(config-if)#

ip inspect firewall in

Router(config-if)#

Assigns the set of firewall inspection rules to the

inside interface on the router.

Step 3

exit

Example:

Router(config-if)#

exit

Router(config)#

Returns to global configuration mode.

Step 4

interface

type number

Example:

Router(config)#

interface fastethernet 4

Router(config-if)#

Enters interface configuration mode for the

outside network interface on your router.

Step 5

ip access-group

{

access-list-number

|

access-list-name

}{

in

|

out

}

Example:

Router(config-if)#

ip access-group 103 in

Router(config-if)#

Assigns the defined ACLs to the outside

interface on the router.

Step 6

exit

Example:

Router(config-if)#

exit

Router(config)#

Returns to global configuration mode.

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Configuration Example

Configuration Example

A telecommuter is granted secure access to a corporate network, using IPSec tunneling. Security to the

home network is accomplished through firewall inspection. The protocols that are allowed are all TCP,

UDP, RTSP, H.323, NetShow, FTP, and SQLNet. There are no servers on the home network; therefore,

no traffic is allowed that is initiated from outside. IPSec tunneling secures the connection from the home

LAN to the corporate network.

Like the Internet Firewall Policy, HTTP need not be specified because Java blocking is not necessary.

Specifying TCP inspection allows for single-channel protocols such as Telnet and HTTP. UDP is

specified for DNS.

The following configuration example shows a portion of the configuration file for the simple firewall

scenario described in the preceding sections.

!

! Firewall inspection is set up for all TCP and UDP traffic as well as

! specific application protocols as defined by the security policy.

ip inspect name firewall tcp

ip inspect name firewall udp

ip inspect name firewall rtsp

ip inspect name firewall h323

ip inspect name firewall netshow

ip inspect name firewall ftp

ip inspect name firewall sqlnet

!

interface vlan 1! This is the internal home network.

ip inspect firewall in ! Inspection examines outbound traffic.

no cdp enable

!

interface fastethernet 4! FE4 is the outside or Internet-exposed interface.

! acl 103 permits IPSec traffic from the corp. router

! as well as denies Internet-initiated traffic inbound.

ip access-group 103 in

ip nat outside

no cdp enable

!

! acl 103 defines traffic allowed from the peer for the IPSec tunnel.

access-list 103 permit udp host 200.1.1.1 any eq isakmp

access-list 103 permit udp host 200.1.1.1 eq isakmp any

access-list 103 permit esp host 200.1.1.1 any

! Allow ICMP for debugging but should be disabled because of security implications.

access-list 103 permit icmp any any

access-list 103 deny ip any any ! Prevents Internet-initiated traffic inbound.

! acl 105 matches addresses for the ipsec tunnel to or from the corporate network.

access-list 105 permit ip 10.1.1.0 0.0.0.255 192.168.0.0 0.0.255.255

no cdp run

!