Configuring Context-Based Access Control (CBAC)

作者:admin发表时间:2010-9-12 评论:0 点击 4,189+

Topology Diagram

Addressing Table

Device Interface IP Address Subnet Mask Default Gateway
R1 Fa0/1 192.168.1.1 255.255.255.0 N/A
S0/0/0 10.1.1.1 255.255.255.252 N/A
R2 S0/0/0 10.1.1.2 255.255.255.252 N/A
S0/0/1 10.2.2.2 255.255.255.252 N/A
R3 Fa0/1 192.168.3.1 255.255.255.0 N/A
S0/0/1 10.2.2.1 255.255.255.252 N/A
PC-A NIC 192.168.1.3 255.255.255.0 192.168.1.1
PC-C NIC 192.168.3.3 255.255.255.0 192.168.3.1

Learning Objectives

Verify connectivity among devices before firewall configuration.

Configure an IOS firewall with CBAC on router R3

Verify CBAC functionality using ping, Telnet, and HTTP.

Introduction

Context-Based Access Control (CBAC) is used to create an IOS firewall. In this activity,

you will create a basic CBAC configuration on edge router R3. R3 provides access to

resources outside of the network for hosts on the inside network. R3 blocks external

hosts from accessing internal resources. After the configuration is complete, you will

verify firewall functionality from internal and external hosts.

The routers have been pre-configured with the following:

Enable password: ciscoenpa55

Password for console: ciscoconpa55

Password for vty lines: ciscovtypa55

IP addressing

Static routing

All switch ports are in VLAN 1 for switches S1 and S3.

Task 1: Block Traffic From Outside

S tep 1. V er ify B a s ic N etwo rk C o nnec tiv ity .

Verify network connectivity prior to configuring the IOS firewall.

From the PC-C command prompt, ping the PC-A server.

From the PC-C command prompt, Telnet to the Router R2 S0/0/1 interface: IP address

10.2.2.2. Exit the Telnet session.

From PC-C, open a web browser to the PC-A server to display the web page. Close the

browser on

PC-C.

From the PC-A server command prompt, ping PC-C.

S tep 2. C onfigur e a na med IP A C L on R 3 to bloc k a ll tr a ffic or igina ting fr

om the outs ide networ k .

Use the ip access-list extended command to create a named IP ACL.

R3(config)# ip access-list extended OUT-IN

R3(config-ext-nacl)# deny ip any any

R3(config-ext-nacl)# exit

S tep 3. A pply the A CL to inte rfa c e S e ria l 0/0/1.

R3(config)# interface s0/0/1

R3(config-if)# ip access-group OUT-IN in

S tep 4. C o nfirm tha t tra ffic entering interfa c e S e ria l 0/0/1 is dro pped.

From the PC-C command prompt, ping the PC-A server. The ICMP echo replies are blocked by

the ACL.

Task 2: Create a CBAC Inspection Rule

S tep 1. C rea te a n ins pec tio n ru le to ins pec t IC MP , T elnet, a nd HT T P tra

ffic .

R3(config)# ip inspect name IN-OUT-IN icmp

R3(config)# ip inspect name IN-OUT-IN telnet

R3(config)# ip inspect name IN-OUT-IN http

S tep 2. T u rn o n time-s ta mped lo gging a nd C B A C a u dit tra il mes s a ges .

Use the ip inspect audit-trail command to turn on CBAC audit messages to provide a

record of network access through the firewall, including illegitimate access attempts.

Enable logging to the syslog server,

192.168.1.3, with the logging host command. Make sure that logged messages are

timestamped.

R3(config)# ip inspect audit-trail

R3(config)# service timestamps debug datetime msec

R3(config)# logging host 192.168.1.3

S tep 3. A pply the ins pec tio n ru le to egres s tra ffic o n interfa c e S 0/0/1.

R3(config-if)# ip inspect IN-OUT-IN out

S tep 4. V er ify tha t a u dit tra il me s s a ges a r e being lo gged o n the s y s lo

g s e rv er.

From PC-C, test connectivity to PC-A with ping, Telnet, and HTTP. Ping and HTTP should

be successful. Note that PC-A will reject the Telnet session.

From PC-A, test connectivity to PC-C with ping and Telnet. All should be blocked.

Review the syslog messages on server PC-A: click the Config tab and then click the

SYSLOG

option.

Task 3: Verify Firewall Functionality

S tep 1. O pen a T elnet s es s io n fro m P C -C to R 2.

The Telnet should succeed. While the Telnet session is active, issue the command show ip

inspect sessions

on R3. This command displays the existing sessions that are currently being tracked and

inspected by CBAC.

R3# show ip inspect sessions

Established Sessions

Session 100424296 (192.168.3.3:1031)=>(10.1.1.2:23) telnet SIS_OPEN

What is the source IP address and port number? 192.168.3.3:1031 (port 1031 is random)

What is the destination IP address and port number? 10.1.1.2:23 (Telnet = port 23)

Exit the Telnet session.

S tep 2. F ro m P C -C , o pen a web bro ws er to the P C -A s erv er web pa ge u s ing

the s e rv er IP a ddr es s .

The HTTP session should succeed. While the HTTP session is active, issue the command

show ip inspect sessions on R3.

R3# show ip inspect sessions

Established Sessions

Session 104637440 (192.168.3.3:1032)=>(192.168.1.3:http SIS_OPEN

Note: If the HTTP session times out before you execute the command on R3, you will have

to click the Go

button on PC-C to generate a session between PC-C and PC-A.

What is the source IP address and port number? 192.168.3.3:1027 (port 1032 is random)

What is the destination IP address and port number? 192.168.1.3:80 (HTTP web = port 80)

Close the browser on PC-C.

S tep 3. V iew the interfa c e c o nfig u ra tio n a nd ins pec tio n ru le timer s .

Enter the show ip inspect interfaces command on R3.

The output shows existing sessions that are currently being tracked and inspected by

CBAC.

R3# show ip inspect interfaces

Interface Configuration

Interface Serial0/0/1

Inbound inspection rule is not set

Outgoing inspection rule is IN-OUT-IN

icmp alert is on audit-trail is off timeout 10

telnet alert is on audit-trail is off timeout 3600 http alert is on audit-trail is off

timeout 3600

Inbound access list is OUT-IN Outgoing access list is not set

Task 4: Review CBAC Configuration

S tep 1. Dis pla y C B A C c o nfigu ra tio n.

Enter the show ip inspect config command on R3 to display the complete CBAC inspection

configuration.

R3# show ip inspect config Session audit trail is enabled Session alert is enabled

one-minute (sampling period) thresholds are [unlimited : unlimited]

connections

max-incomplete sessions thresholds are [unlimited : unlimited]

max-incomplete tcp connections per host is unlimited. 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

tcp reassembly queue length 16; timeout 5 sec; memory-limit 1024 kilo bytes

dns-timeout is 5 sec

Inspection Rule Configuration

Inspection name IN-OUT-IN

icmp alert is on audit-trail is off timeout 10 telnet alert is on audit-trail is off

timeout 3600 http alert is on audit-trail is off timeout 3600

S tep 2. Dis pla y re a l-time o u tpu t tha t c a n be u s ed fo r tro u bles ho o

ting.

Enter the debug ip inspect detailed command on R3 to display detailed messages about

CBAC software events, including information about CBAC packet processing.

From PC-C, open a web browser on PC-C; enter the PC-A (server) IP address: 192.168.1.3.

R3# debug ip inspect detailed

INSPECT Detailed Debug debugging is on

*Mar 01, 02:37:28.3737: %FW-6-SESS_AUDIT_TRAIL_START: Start http session:

initiator (192.168.3.3:1039) — responder (192.168.1.3:80)

*Mar 01, 02:37:28.3737: CBAC: Finding pregen session for src_tableid:0,

src_addr:192.168.3.3, src_port:1039, dst_tableid:0, dst_addr:192.168.1.3, dst_port:80

*Mar 01, 02:37:38.3737: %FW-6-SESS_AUDIT_TRAIL_STOP: Stop http session:

initiator (192.168.3.3:1041) — responder (192.168.1.3:80)

S tep 3. C hec k R es u lts .

Your completion percentage should be 100%. Click Check Results to see feedback and

verification of which required components have been completed.

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