Cisco IOS Firewall (CBAC-Context Based Access Control)

Cisco IOS Firewall(CBAC-Context Base Access Control)

Introduction

• Basic Definition

Benefits

• Monitor Traffic

• (NAT)

Methods of Attack

• Port Scans, ping sweeps• Packet Sniffers• IP spoofing• Application Level Attacks• Denial of Service Attacks

Types of Firewalls

• Basic Router Security

• Packet Filtering Firewalls

• Stateful Inspection Firewalls

Cisco IOS Feature Set

• Security specific option for IOS software.

• Version of the Cisco IOS with an add-on feature set that can be run on several router platforms.

• Affordability

Router Requirements

• Cisco IOS software release IOS 11.2(11)P and above.

• Generally requires more memory, both in terms of flash and RAM.

Secure Cisco Router

• Ability to configure a Cisco router in a fairly secure fashion using plain old ACLs

• ACLs provide granular packet filtering at layers 2, 3, and 4 only.

• IOS firewall provides this level of traffic filtering and more.

What CBAC Does

• Traffic Filtering• Traffic Inspection• Alerts and Audit Trails• Intrusion detection

Traffic Filtering

• Context-Based Access Control intelligently filters TCP and UDP packets.

• Without CBAC, traffic filtering is limited to access-list iplementations that examine packets at the network layer.

• CBAC examines not only network and transport layer info, but also examines appication layer protocol info, such as FTP connection info.

Traffic Inpsection

• CBAC inspects traffic that travels through the firewall to discover and manage state information for TCP and UDP sessions.

• This creates temporary openings in the firewall ACLs to allow return traffic originating from within the internal network.

• Prevents SYN-flood and DoS attacks.

Alerts and Audit Trails

• Generates real-time alerts and audit trails on events tracked by the firewall.

• Uses SYSLOG to track all network transactions

Intrusion Detection

• Cisco IOS IDS identifies 59 of the most common attacks using signatures to detect patterns of misuse in network traffic.

CBAC Opens Temporary Holes in Firewall Access

Lists

Configuring CBAC

• A common setup is to configure ACLs and CBAC inbound on the external interface of an internet router to protect a private network from harmful traffic initiated from the internet.

• You’re main concern is to allow in only return traffic from sessions initiated internally.

Cont. Config

• Router (config)# Access-list 105 deny tcp any any

• Router (config)# Access-list 105 deny udp any any

• Router (config)# Interface serial 0/0• Router (config)# Ip address-group

105 in

• The previous statements block all TCP and UDP traffic when applied inbound on the external interface. This provides a blanket form of inspection across all TCP and UDP traffic. By applying access list 105 to the external interface, we ensured that Internet traffic was intercepted as soon as it reached the Internet router. we could also exercise a more granular level of control by specifying certain application protocols, as this example demonstrates:

• Router (config)# Access-list 105 deny tcp any any eq smtp

• This statement blocks all SMTP traffic to the internal network. It would need to occur in the access list before the previous TCP blanket statements or it would have little effect.

• The next step in this process is to define the timeout and threshold values for CBAC to use when tracking sessions. You can configure several values to enhance CBAC’s ability to defend against network attacks. Most of the timeout and threshold settings have default values that will generally suffice in a startup scenario. Many of the timeouts and thresholds control how the router responds to DoS attacks. (we’ll save a more in-depth discussion of timer/threshold configuration for another time.)

• Keep in mind that CBAC does not inspect ICMP, only TCP and UDP. Accordingly, you’ll need to add inbound ACL entries for appropriate ICMP restrictions. Consider adding these ICMP entries to your ACL. They’ll make it possible for those inside your network to ping hosts on the Internet, as well as allow your router to respond to proper ICMP traffic.

• Up to this point, we’ve shown you how to configure entries for the extended access list and apply that configuration to the inbound traffic on the external interface. The ACL has entries to block all the traffic we want to inspect with CBAC. Rather than modify the timeout and threshold settings, we went with the defaults. we recommend starting with the defaults and tuning these as you go. It’s not a good idea to make changes to these settings if you don’t understand how those changes will affect firewall operation. Next, we defined the actual inspection rule that governs which application layer protocols are examined.

• Inspection rule command structure

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

• This is a global config mode command. It requires that you specify a name, protocol, alert setting, auditing, and the timeout value in seconds

• Applying the inspect command

– Router (config)# ip inspect name myfw tcp alert on audit-trail on

• We’ve named the rule myfw, specified TCP as the protocol to inspect, and activated the alert and auditing options. Notice the alert and audit-trail options. This requires a Syslog system to send the information to. Although that configuration is beyond the scope of this article, I do recommend using auditing for logging all firewall activity. At this point, I’ll apply the rule to the external interface, Serial0, with the following:

Router (config)# Interface serial0/0

Router (config-if)# ip inspect myfw out

• Notice that we have applied the inspection rule outbound on the external interface. It will track sessions started internally and heading out through the external interface, bound for the Internet or some other external network.

• If you have difficulty during CBAC configuration, you can disable and reset all related settings using the following global mode command. This won’t remove your extended access list configured on the outside interface. If you turn off inspection, keep in mind that it will most likely halt all traffic entering your private network because the access list is filtering most, if not all, inbound traffic at the external interface. Turning off inspection is as simple as:

•Router (config)# no ip inspect

• That command will remove all the inspection information from the configuration, including the filter statements and command line that applies it to the interface.

• Now that the basic configuration details are out of the way, let’s look at an

• Configuration with ACLs and CBAC inspection activated !generic inspection of UDP and TCP, and application inspection for http, smtp, ftpip inspect name myfw tcp alert on audit-

trail onip inspect name myfw udp alert on

audit-trail onip inspect name myfw httpip inspect name myfw smtpip inspect name myfw ftp

access list applied inbound to external interfaceaccess-list 105 deny tcp any anyaccess-list 105 deny udp any anyaccess-list 105 permit icmp any any

unreachableaccess-list 105 permit icmp any any echo-replyaccess-list 105 permit icmp any any packet-too-

bigaccess-list 105 permit icmp any any time-

exceededaccess-list 105 permit icmp any any tracerouteaccess-list 105 permit icmp any any

administratively-prohibited

access-list 105 permit icmp any any echo

access-list 105 deny icmp any any

!external interface interface serial0ip access-group 105 inip inspect myfw out

• This basic CBAC configuration will allow only limited ICMP information through the firewall router because access list 101 is applied incoming to the external interface. The inspection rule, filter1, will allow internal users to start outbound WWW sessions via HTTP and track the sessions, opening return points in the static, extended access list. This is also true for FTP and SMTP. If, in the future, I choose to allow users RealAudio or NetMeeting access, I would simply add ip inspect name statements, using filter1 as the name.

• To change the inspection rule, you can easily add or remove line items. To add statements, simply use the ip inspect name command, using the same user-defined rule name. If you need to remove a line, use the no form of the ip inspect name command, as follows:

Router (config)# ip inspect myfw tcpRouter (config)# no ip inspect myfw

tcp

• If at any point you want to check the configuration, you can get CBAC setup details using the show ip inspect command, as in the following:

Router# show ip inspect all

• The all parameter will display information such as current configuration of inspection, as well as current sessions traversing the firewall.

General firewall configuration

recommendations

• Simply installing the IOS firewall software does not fully ensure a secure network. The router and CBAC must be configured properly to secure the private network from unwanted access. With this in mind, you’ll want to apply other well-known protections to the firewall router. Among these are broadcast protection and antispoofing measures. Here are some recommended measures:

General firewall configuration

recommendations• No ip directed-broadcast• No icmp redirect• No ip redirect• No service finger• No cdp run• No ip source-route• Access-list 100 deny ip {internal

network range} any

General firewall configuration

recommendations• Don’t consider this to be a complete list of

precautions. This is merely a sample of the types of settings to implement for a secure network environment. You’ll also want to check the Cisco support Web site for current security recommendations for edge routers. For instance, Cisco recently acknowledged a rather serious security issue for routers running the ip http service. I recommend checking the security area of the Cisco support Web site regularly to evaluate any potential vulnerability in your router/IOS combination and configuration attributes.

Summary

• Although the IOS firewall provides a higher level of security than the standard access list approach, like other firewalls, it shouldn’t be considered invulnerable. A determined hacker may be able to find holes in the most secure of systems. I’ve demonstrated the strong security features of CBAC, but you may want to consider a dual firewall approach if your security needs are highly demanding.

Summary

• When implementing CBAC, consider its limitations carefully. For instance, it will handle inspection of FTP data channels only in the range of 1024 to 65535. Also, if you’re using IPSec, carefully plan how it will interact with the IOS firewall router. Keep in mind the strengths and configuration options, which are plentiful. For example, CBAC can be used as an extranet protection method when your network is connected to a business partner’s network. In this manner, it would inspect traffic in both directions, protecting both networks from unapproved access. You can also guard against traffic leaving the network by applying inspection to outbound traffic