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Cisco ASA Security Contexts

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Date: Dec 16, 2005.

Chapter Information

Architectural Overview
Configuration of Security Contexts
Deployment Scenarios
Monitoring and Troubleshooting the Security Contexts
Summary

Chapter Description

The virtual firewall methodology enables a physical firewall to be partitioned into multiple standalone firewalls. Each standalone firewall acts and behaves as an independent entity with its own configuration, interfaces, security policies, routing table, and administrators. In Cisco ASA, these virtual firewalls are known as security contexts. This chapter covers security contexts in detail.

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Deployment Scenarios

The virtual firewall solution is useful in deployments where multiple firewalls are needed to protect traffic to and from the trusted networks. Although virtual firewalls can be deployed in many ways, for ease of understanding, we cover two design scenarios:

Virtual firewall using two customer contexts
Virtual firewall using a shared interface

Virtual Firewall Using Two Customer Contexts

SecureMe has an office in Brussels that provides firewall services to two small companies, Cubs and Bears. SecureMe's office is located in the same building as the offices of these companies. Cubs and Bears have specific requirements that SecureMe is obliged to meet. However, the appliance in Brussels has two active physical interfaces and, as a result, SecureMe wants to use subinterfaces to accommodate these customers. To conserve public addresses on the outside interfaces, the administrator uses a subnet mask of 255.255.255.248. Figure 9-5 shows SecureMe's new topology that will be set up in Brussels.

Figure 9-5 SecureMe Brussels Multimode Topology

The security requirements for SecureMe, along with Cubs and Bears, are as follows:

SecureMe security requirements:

For SSH and Telnet user authentication, use a AAA server.
Log all the system-generated messages to a syslog server.

Cubs security requirements:

All hosts on 192.168.10.0/24 should be able to access the Internet.
The source IP address should be translated to 209.165.200.230 using PAT.
Allow HTTP clients from the Internet to access Cub's web server (192.168.5.10) on the DMZ network. This address should appear as 209.165.200.231 for the Internet users.
Deny and log all other inbound traffic on the outside interface.

Bears security requirements:

Allow hosts on the 192.168.20.0/24 subnet to access www.cisco.com only. All other web traffic should be blocked.
The source IP address should be translated to 209.165.201.10 using interface PAT.
Block and log all inbound traffic on the outside interface.

Example 9-17 shows the relevant configuration to achieve the goals just listed.

Example 9-17. ASA's Relevant Configuration with Multiple Security Contexts

                                
   System Execution Space
Brussels# show run
ASA Version 7.0(1) <system>
! Main GigabitEthernet0/0 interface
interface GigabitEthernet0/0
! Sub-interface assigned to the admin context as the inside interface. A VLAN ID is
   
   !assigned to the interface
interface GigabitEthernet0/0.1
 vlan 5
! Sub-interface assigned to the Cubs context as the inside interface. A VLAN ID is
   
   !assigned to the interface
interface GigabitEthernet0/0.2
 vlan 10
! Sub-interface assigned to the Bears context as the inside interface. A VLAN ID is
   
   !assigned to the interface
interface GigabitEthernet0/0.3
 vlan 20
! Main GigabitEthernet0/1 interface
interface GigabitEthernet0/1

! Sub-interface assigned to the admin context as the outside interface. A VLAN ID
   
   is !assigned to the interface
interface GigabitEthernet0/1.1
 vlan 101
! Sub-interface assigned to the Cubs context as the outside interface. A VLAN ID is
   
   !assigned to the interface
interface GigabitEthernet0/1.2
 vlan 110
! Sub-interface assigned to the Bears context as the outside interface. A VLAN ID
   
   is !assigned to the interface
interface GigabitEthernet0/1.3
 vlan 120
! Sub-interface assigned to the Cubs context as the DMZ interface. A VLAN ID is
   
   !assigned to the interface
interface GigabitEthernet0/1.5
 vlan 130
!
hostname Brussels
! context named "admin" is the designated Admin context
admin-context admin
! "admin" context definition along with the allocated interfaces.
context admin
  description admin Context for admin purposes
  allocate-interface GigabitEthernet0/0.1
  allocate-interface GigabitEthernet0/1.1
  config-url disk0:/admin.cfg
! "Cubs" context definition along with the allocated interfaces.
context Cubs
  description Cubs Customer Context
  allocate-interface GigabitEthernet0/0.2
  allocate-interface GigabitEthernet0/1.2
  allocate-interface GigabitEthernet0/1.5
  config-url disk0:/Cubs.cfg
! "Bears" context definition along with the allocated interfaces.
context Bears
  description Bears Customer Context
  allocate-interface GigabitEthernet0/0.3
  allocate-interface GigabitEthernet0/1.3
  config-url disk0:/Bears.cfg
______________________________________________________________________________
                                  Admin Context
Brussels/admin# show running
ASA Version 7.0(1) <context>
!inside interface of the admin context with security level set to 100
interface GigabitEthernet0/0.1
 nameif inside
 security-level 100
 ip address 192.168.1.1 255.255.255.0
!outside interface of the admin context with security level set to 0
interface GigabitEthernet0/1.1
 nameif outside
 security-level 0
 ip address 209.165.202.130 255.255.255.248
!
hostname admin
!configuration of a syslog server with logging level set to emergencies with

      timestamp
logging enable
logging timestamp
logging trap emergencies
logging host inside 192.168.1.10
!
route outside 0.0.0.0 0.0.0.0 209.165.202.129 1
!configuration of a AAA server using RADIUS for authentication
aaa-server uauth protocol radius
aaa-server uauth host 192.168.1.20
 key cisco123
!setting up telnet and SSH authentication
aaa authentication telnet console uauth
aaa authentication ssh console uauth
!Telnet to the admin context is allowed from the inside interface
telnet 192.168.1.0 255.255.255.0 inside
telnet timeout 5
!SSH to the admin context is allowed from the outside interface
ssh 0.0.0.0 0.0.0.0 outside
ssh timeout 5
______________________________________________________________________________
                                   Cubs Context
Brussels/Cubs# show running
ASA Version 7.0(1) <context>
!inside interface of the Cubs context with security level set to 100
interface GigabitEthernet0/0.2
 nameif inside
 security-level 100
 ip address 192.168.10.1 255.255.255.0
!outside interface of the Cubs context with security level set to 0
interface GigabitEthernet0/1.2
 nameif outside
 security-level 0
 ip address 209.165.200.225 255.255.255.248
!DMZ interface of the Cubs context with security level set to 50
interface GigabitEthernet0/1.5
 nameif dmz
 security-level 50
 ip address 192.168.5.1 255.255.255.0
!
hostname Cubs
!Access-list configuration to allow web traffic. The access-list is applied to the

      outside interface.
access-list outside-in extended permit tcp any host 209.165.200.231 eq www
access-list outside-in extended deny ip any any log
access-group outside-in in interface outside
!NAT configuration to allow inside hosts to get Internet connectivity
global (outside) 1 209.165.200.230
nat (inside) 1 192.168.10.0 255.255.255.0

!Static address translation for the Web-Server
static (dmz,outside) 209.165.200.231 192.168.5.10 netmask 255.255.255.255
!
route outside 0.0.0.0 0.0.0.0 209.165.200.226 1
______________________________________________________________________________
                                  Bears Context
Brussels/Bears# show running
ASA Version 7.0(1) <context>
!inside interface of the Bears context with security level set to 100
interface GigabitEthernet0/0.3
 nameif inside
 security-level 100
 ip address 192.168.20.1 255.255.255.0
!outside interface of the Bears context with security level set to 0
interface GigabitEthernet0/1.3
 nameif outside
 security-level 0
 ip address 209.165.201.2 255.255.255.224
!
enable password 8Ry2YjIyt7RRXU24 encrypted
passwd 2KFQnbNIdI.2KYOU encrypted
hostname Bears
!Access-list configuration to permit web traffic initiated from the inside host and
   
   destined to 198.133.219.25. Deny all other traffic. The access-list is applied
   to the inside interface.
access-list inside-in extended permit tcp 192.168.20.0 255.255.255.0 host
198.133.219.25   eq 80
access-group inside-in in interface inside
!Access-list configuration to deny and log all inbound traffic. The access-list is

      applied to the outside interface
access-list outside-in extended deny ip any any log
access-group outside-in in interface outside
!NAT configuration to allow inside hosts to get Internet connectivity
global (outside) 1 interface
nat (inside) 1 192.168.20.0 255.255.255.0
!
route outside 0.0.0.0 0.0.0.0 209.165.201.1 1

Virtual Firewall Using a Shared Interface

An educational institute contacts SecureMe to provide firewall services for two of its departments—faculty and students—over a shared outside interface. The hosts in the student context are allowed to access a web server in the faculty context. Additionally, they are allowed to check their e-mail messages from 209.165.202.130. The faculty context, on the other hand, does not restrict anything going out to the Internet.

The SecureMe global policy restricts access of the security appliance to the valid and authorized users on the AAA servers. SecureMe does not have many public addresses available, so it is using interface PAT for address translation. Additionally, SecureMe does not want the administrators of the individual security contexts to be able to determine the interface assignment for their contexts. Figure 9-6 shows SecureMe's proposed topology for this institute.

Figure 9-6 Security Contexts Using a Shared Interface

Example 9-18 shows the relevant configuration for the Cisco ASA used in this deployment.

Example 9-18. ASA's Relevant Configuration Using a Shared Outside Interface

                              
   System Execution Space
SecuremeInstitute# show run
ASA Version 7.0(1) <system>
! Main GigabitEthernet0/0 interface
interface GigabitEthernet0/0

! Sub-interface assigned to the admin context as the inside interface. A VLAN ID is

      !assigned to the interface
interface GigabitEthernet0/0.1
 vlan 5
! Sub-interface assigned to the Students context as the inside interface. A VLAN ID

      is !assigned to the interface
interface GigabitEthernet0/0.2
 vlan 10
! Sub-interface assigned to the Faculty context as the inside interface. A VLAN ID

      is !assigned to the interface
interface GigabitEthernet0/0.3
 vlan 20
! Main GigabitEthernet0/1 interface to be used as the shared interface
interface GigabitEthernet0/1
!
hostname SecuremeInstitute
! context named "admin" is the designated Admin context
admin-context admin
! "admin" context definition along with the allocated interfaces.
context admin
  description admin Context for admin purposes
  allocate-interface GigabitEthernet0/0.1 inside invisible
  allocate-interface GigabitEthernet0/1 outside invisible
  config-url disk0:/admin.cfg
! "Students" context definition along with the allocated interfaces.
context Students
  description Students Customer Context
  allocate-interface GigabitEthernet0/0.2 S_inside invisible
  allocate-interface GigabitEthernet0/1 S_outside invisible
  config-url disk0:/Students.cfg
! "Faculty" context definition along with the allocated interfaces.
context Faculty
  description Faculty Customer Context
  allocate-interface GigabitEthernet0/0.3 F_inside invisible
  allocate-interface GigabitEthernet0/1 F_outside invisible
  config-url disk0:/Faculty.cfg
________________________________________________________________________
                                 Admin Context
SecuremeInstitute/admin# show running
ASA Version 7.0(1) <context>
!inside interface of the admin context with security level set to 100
interface inside
 nameif inside
 security-level 100
 ip address 192.168.1.1 255.255.255.0
!outside interface of the admin context with security level set to 0
interface outside
 nameif outside
 security-level 0
 ip address 209.165.200.225 255.255.255.224
!
hostname admin
!
route outside 0.0.0.0 0.0.0.0 209.165.200.230 1
!configuration of a AAA server using RADIUS for authentication
aaa-server uauth protocol radius
aaa-server uauth host 192.168.1.20
 key cisco123
aaa authentication telnet console uauth
aaa authentication ssh console uauth
!Telnet to the admin context is allowed from the inside interface
telnet 192.168.1.0 255.255.255.0 inside
telnet timeout 5
!SSH to the admin context is allowed from the outside interface
ssh 0.0.0.0 0.0.0.0 outside
ssh timeout 5
route outside 0.0.0.0 0.0.0.0 209.165.200.225 1
_________________________________________________________________________
                                 Students Context
SecuremeInstitute/Students# show running
ASA Version 7.0(1) <context>
!inside interface of the Students context with security level set to 100
interface S_inside
 nameif inside
 security-level 100
 ip address 192.168.10.1 255.255.255.0
!outside interface of the Students context with security level set to 0
interface S_outside
 nameif outside
 security-level 0
 ip address 209.165.200.226 255.255.255.224
!Access-list configuration to allow email and web traffic. The access-list is

      applied to the inside interface.
access-list inside-in extended permit tcp 192.168.10.0 255.255.255.0 host
   209.165.202.130 eq smtp
access-list inside-in extended permit tcp 192.168.10.0 255.255.255.0 host
   209.165.200.230 eq www
access-group inside-in in interface S_inside
!
hostname Students
!NAT configuration to allow inside hosts to get Internet connectivity
global (S_outside) 1 interface
nat (S_inside) 1 192.168.10.0 255.255.255.0
!
route S_outside 0.0.0.0 0.0.0.0 209.165.200.225 1
___________________________________________________________________________
                                 Faculty Context
SecuremeInstitute/Faculty# show running
ASA Version 7.0(1) <context>
!inside interface of the Faculty context with security level set to 100
interface F_inside
  nameif inside
  security-level 100
  ip address 192.168.20.1 255.255.255.0
!outside interface of the Faculty context with security level set to 0
interface F_outside
  nameif outside
  security-level 0
  ip address 209.165.200.227 255.255.255.224
!
enable password 8Ry2YjIyt7RRXU24 encrypted
passwd 2KFQnbNIdI.2KYOU encrypted
hostname Faculty
!Access-list configuration to allow web traffic. The access-list is applied to the

      outside interface.
access-list outside-in extended permit tcp host 209.165.200.226 host 209.165.200.230
eq www
access-group outside-in in interface F_outside
!NAT configuration to allow inside hosts to get Internet connectivity
global (F_outside) 1 interface
nat (F_inside) 1 192.168.20.0 255.255.255.0
!Static address translation for the Web-Server
static (F_inside,F_outside) 209.165.200.230 192.168.20.20 netmask 255.255.255.255
!
route F_outside 0.0.0.0 0.0.0.0 209.165.200.225 1

4. Monitoring and Troubleshooting the Security Contexts | Next Section Previous Section

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