CBAC (Context-Based Access Conrol)
CBAC provides statefull application layer filtering, including support for unorthodox protocols and multimedia applications. It can examine supported connections for embedded NAT and PAT information and perform the necessary translations. In addition, it can open additional statefull connections for supported applications, such as FTP and H.323.
Features offered by CBAC
Port mapping:- Allows the mapping of ports so that CBAC can perform its application inspection correctly, such as assigning FTP to port 1024 if your FTP server is processing traffic on this port.
Filtering of Java applets:- Filters embedded Java applets on HTTP connections, allowing you to block known malicious sites.
DoS protection:- Detects and prevents Denial of Service (DoS) attacks by limiting the number of connections that a device can set up.
CBAC also provides real time alerts and audit trails.
NOTE:- All the inspection features are applied globally while using CBAC, we don’t have flexibility to do inspection for certain network or interested traffic flow with respect to ip addressing scheme.In order to accomplish the same we can use advance level of IOSFW feature set known as ZBFW(Zone Based Firewall).
Here are the steps to configure CBAC
Step 1. Identify the interfaces as internal and external on your router.
Step 2. Configure the IP ACL rules to filter traffic based on your requiremnet.
Step 3. Now we can change the global timeout values for connections as per requirement.
Step 4. In case if the application is using a nonstandard port number, such as FTP with 1024.Configure Port Application Mapping (PAM).
Step 5. As a next step Configure inspection rules. These rules define what entries are added to the state table and CBAC will open up pin holes for the returning traffic in the ACL rules applied in the opposite direction with respect to inspection rules.
Step 6. Now apply the inspection rules to the respective interface of the router.
Step 7. Final step would be to test the CBAC configuration by passing some interested traffic through the router running CBAC in order to validate the configuration is fine.
Scenario below shows how we can enable IOSFW(CBAC) on a Router.
Router outside interface 184.108.40.206 Router inside interface 192.168.10.1 Inside Host 192.168.10.2 Natted ip address for Inside Host 220.127.116.11
ip inspect name IOSFW icmp router-traffic ip inspect name IOSFW tcp router-traffic ip inspect name IOSFW udp router-traffic
access-list 151 deny ip any any
interface FastEthernet0/0 description **WAN** ip address 18.104.22.168 255.255.255.0 ip access-group 151 in ip nat outside no shut
interface FastEthernet0/1 description **LAN** ip address 192.168.10.1 255.255.255.0 ip nat inside ip inspect IOSFW in no shut
CBAC(config)#ip inspect audit-trail CBAC(config)#logging buffered debugging CBAC(config)#logging on CBAC#
- Mar 1 00:38:30.851: %FW-6-SESS_AUDIT_TRAIL_START: Start tcp session: initiator (192.168.10.2:56162) -- responder (22.214.171.124:23)
CBAC#sh ip inspect sessions Established Sessions
Session 669F632C (192.168.10.2:56162)=>(126.96.36.199:23) tcp SIS_OPEN CBAC#sh ip inspect sessions detailEstablished Sessions Session 669F632C (192.168.10.2:56162)=>(188.8.131.52:23) tcp SIS_OPEN Created 00:00:11, Last heard 00:00:05 Bytes sent (initiator:responder) [42:90] In SID 184.108.40.206[23:23]=>220.127.116.11[56162:56162] on ACL INBOUND (18 matches) Note:-
>>Turn off inspect for unused protocols(Only use what you require). >>By using the command router-traffic along with the desired protocol,Enables inspection of sessions to/from the router.
Related show Commands
This section provides information you can use to confirm your configuration is working properly.
Other Show Commands
show ip inspect config
show ip inspect interfaces
show ip inspect stat
debug ip inspect detail
debug ip inspect tcp
debug ip inspect object-cre
debug ip inspect object-del
debug ip inspect event
Certain show commands are supported by the Output Interpreter Tool (registered customers only), which allows you to view an analysis of show command output.
Add show running config of your device