Context-Based Access Control (CBAC)
Introduction
The Context-Based Access Control (CBAC) feature of the Cisco IOS® Firewall Feature Set actively inspects the activity behind a firewall. CBAC specifies what traffic needs to be let in and what traffic needs to be let out by using access lists (in the same way that Cisco IOS uses access lists). However, CBAC access lists include ip inspect statements that allow the inspection of the protocol to make sure that it is not tampered with before the protocol goes to the systems behind the firewall.Prerequisites
Requirements
There are no specific requirements for this document.Components Used
This document is not restricted to specific software and hardware versions.Conventions
For more information on document conventions, refer to the Cisco Technical Tips Conventions.Background Information
CBAC can also be used with Network Address Translation (NAT), but the configuration in this document deals primarily with pure inspection. If you perform NAT, your access lists need to reflect the global addresses, not the real addresses.Prior to configuration, consider these questions.
- What traffic do you want to let out?
- What traffic do you want to let in?
- What traffic do you want to inspect?
What Traffic Do You Want to Let Out?
What traffic you want to let out depends on your site security policy, but in this general example everything is permitted outbound. If your access list denies everything, then no traffic can leave. Specify outbound traffic with this extended access list:access-list 101 permit ip [source-network] [source-mask] any access-list 101 deny ip any any
What Traffic Do You Want to Let In?
What traffic you want to let in depends on your site security policy. However, the logical answer is anything that does not damage your network.In this example, there is a list of traffic that seems logical to let in. Internet Control Message Protocol (ICMP) traffic is generally acceptable, but it can allow some possibilities for DOS attacks. This is a sample access list for incoming traffic:
Extended IP Access List 101
permit tcp 10.10.10.0 0.0.0.255 any (84 matches) permit udp 10.10.10.0 0.0.0.255 any permit icmp 10.10.10.0 0.0.0.255 any (3 matches) deny ip any any
Extended IP Access List 102
Access list 101 is for the outbound traffic. Access list 102 is for the inbound traffic. The access lists permit only a routing protocol, Enhanced Interior Gateway Routing Protocol (EIGRP), and specified ICMP inbound traffic.permit eigrp any any (486 matches) permit icmp any 10.10.10.0 0.0.0.255 echo-reply (1 match) permit icmp any 10.10.10.0 0.0.0.255 unreachable permit icmp any 10.10.10.0 0.0.0.255 administratively-prohibited permit icmp any 10.10.10.0 0.0.0.255 packet-too-big permit icmp any 10.10.10.0 0.0.0.255 echo (1 match) permit icmp any 10.10.10.0 0.0.0.255 time-exceeded deny ip any any (62 matches) access-list 101 permit tcp 10.10.10.0 0.0.0.255 any access-list 101 permit udp 10.10.10.0 0.0.0.255 any access-list 101 permit icmp 10.10.10.0 0.0.0.255 any access-list 101 deny ip any any access-list 102 permit eigrp any any access-list 102 permit icmp any 10.10.10.0 0.0.0.255 echo-reply access-list 102 permit icmp any 10.10.10.0 0.0.0.255 unreachable access-list 102 permit icmp any 10.10.10.0 0.0.0.255 administratively-prohibited access-list 102 permit icmp any 10.10.10.0 0.0.0.255 packet-too-big access-list 102 permit icmp any 10.10.10.0 0.0.0.255 echo access-list 102 permit icmp any 10.10.10.0 0.0.0.255 time-exceeded access-list 102 deny ip any any
In the example, a server on the Ethernet side of the router is not accessible from the Internet. The access list blocks it from establishing a session. To make it accessible, the access list needs to be modified to allow the conversation to occur. To change an access list, remove the access list, edit it, and reapply the updated access list.
Note: The reason that you remove the access-list 102 before edit and reapply, is due to the "deny ip any any" at the end of the access list. In this case, if you were to add a new entry before you remove the access-list, the new entry appears after the deny. Therefore, it is never checked.
This example adds the Simple Mail Transfer Protocol (SMTP) for 10.10.10.1 only.
Extended IP Access List 102
permit eigrp any any (385 matches)
permit icmp any 10.10.10.0 0.0.0.255 echo-reply
permit icmp any 10.10.10.0 0.0.0.255 unreachable
permit icmp any 10.10.10.0 0.0.0.255 administratively-prohibited
permit icmp any 10.10.10.0 0.0.0.255 packet-too-big
permit icmp any 10.10.10.0 0.0.0.255 echo
permit icmp any 10.10.10.0 0.0.0.255 time-exceeded
permit tcp any host 10.10.10.1 eq smtp (142 matches)
!--- In this example, you inspect traffic that has been
!--- initiated from the inside network.
What Traffic Do You Want to Inspect?
The CBAC within Cisco IOS supports:| Keyword Name | Protocol |
|---|---|
| cuseeme | CUSeeMe Protocol |
| ftp | File Transfer Protocol |
| h323 | H.323 Protocol (for example Microsoft NetMeeting or Intel Video Phone) |
| http | HTTP Protocol |
| rcmd | R commands (r-exec, r-login, r-sh) |
| realaudio | Real Audio Protocol |
| rpc | Remote Procedure Call Protocol |
| smtp | Simple Mail Transfer Protocol |
| sqlnet | SQL Net Protocol |
| streamworks | StreamWorks Protocol |
| tcp | Transmission Control Protocol |
| tftp | TFTP Protocol |
| udp | User Datagram Protocol |
| vdolive | VDOLive Protocol |
Each protocol is tied to a keyword name. Apply the keyword name to an interface that you want to inspect. For example, this configuration inspects FTP, SMTP, and Telnet:
This document addresses what traffic you want to let out, what traffic you want to let in, and what traffic you want to inspect. Now that you are prepared to configure CBAC, complete these steps:router1#configure Configuring from terminal, memory, or network [terminal]? Enter configuration commands, one per line. End with CNTL/Z. router1(config)#ip inspect name mysite ftp router1(config)#ip inspect name mysite smtp router1(config)#ip inspect name mysite tcp router1#show ip inspect config Session audit trail is disabled one-minute (sampling period) thresholds are [400:500]connections max-incomplete sessions thresholds are [400:500] max-incomplete tcp connections per host is 50. Block-time 0 minute. tcp synwait-time is 30 sec -- tcp finwait-time is 5 sec tcp idle-time is 3600 sec -- udp idle-time is 30 sec dns-timeout is 5 sec Inspection Rule Configuration Inspection name mysite ftp timeout 3600 smtp timeout 3600 tcp timeout 3600
- Apply the configuration.
- Enter the access lists as configured above.
- Configure the inspection statements.
- Apply the access lists to the interfaces.
| Context-Based Access Control Configuration |
|---|
! version 11.2 no service password-encryption service udp-small-servers service tcp-small-servers ! hostname router1 ! ! no ip domain-lookup ip inspect name mysite ftp ip inspect name mysite smtp ip inspect name mysite tcp ! interface Ethernet0 ip address 10.10.10.2 255.255.255.0 ip access-group 101 in ip inspect mysite in no keepalive ! interface Serial0 no ip address encapsulation frame-relay no fair-queue ! interface Serial0.1 point-to-point ip address 10.10.11.2 255.255.255.252 ip access-group 102 in frame-relay interface-dlci 200 IETF ! router eigrp 69 network 10.0.0.0 no auto-summary ! ip default-gateway 10.10.11.1 no ip classless ip route 0.0.0.0 0.0.0.0 10.10.11.1 access-list 101 permit tcp 10.10.10.0 0.0.0.255 any access-list 101 permit udp 10.10.10.0 0.0.0.255 any access-list 101 permit icmp 10.10.10.0 0.0.0.255 any access-list 101 deny ip any any access-list 102 permit eigrp any any access-list 102 permit icmp any 10.10.10.0 0.0.0.255 echo-reply access-list 102 permit icmp any 10.10.10.0 0.0.0.255 unreachable access-list 102 permit icmp any 10.10.10.0 0.0.0.255 administratively-prohibited access-list 102 permit icmp any 10.10.10.0 0.0.0.255 packet-too-big access-list 102 permit icmp any 10.10.10.0 0.0.0.255 echo access-list 102 permit icmp any 10.10.10.0 0.0.0.255 time-exceeded access-list 102 permit tcp any host 10.10.10.1 eq smtp access-list 102 deny ip any any ! line con 0 line vty 0 4 login ! end |
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