Zone-Based Firewall and Firewall concepts

Cisco IOS Classic Firewall stateful inspection (formerly known as Context-Based Access Control, or CBAC) employed an interface-based configuration model, in which a stateful inspection policy was applied to an interface. All traffic passing through that interface received the same inspection policy. This configuration model limited the granularity of the firewall policies and caused confusion of the proper application of firewall policies, particularly in scenarios when firewall policies must be applied between multiple interfaces.

Zone-Based Policy Firewall (also known as Zone-Policy Firewall, or ZFW) changes the firewall configuration from the older interface-based model to a more flexible, more easily understood zone-based model. Interfaces are assigned to zones, and inspection policy is applied to traffic moving between the zones. Inter-zone policies offer considerable flexibility and granularity, so different inspection policies can be applied to multiple host groups connected to the same router interface.

Zones establish the security borders of your network. A zone defines a boundary where traffic is subjected to policy restrictions as it crosses to another region of your network. ZFW’s default policy between zones is deny all. If no policy is explicitly configured, all traffic moving between zones is blocked. This is a significant departure from stateful inspection’s model where traffic was implicitly allowed until explicitly blocked with an access control list (ACL). 

Rules For Applying Zone-Based Policy Firewall

Router network interfaces’ membership in zones is subject to several rules that govern interface behavior, as is the traffic moving between zone member interfaces:

• A zone must be configured before interfaces can be assigned to the zone.
  
• An interface can be assigned to only one security zone.
  
• All traffic to and from a given interface is implicitly blocked when the interface is assigned to a zone, except traffic to and from other interfaces in the same zone, and traffic to any interface on the router.
  
• Traffic is implicitly allowed to flow by default among interfaces that are members of the same zone.
  
• In order to permit traffic to and from a zone member interface, a policy allowing or inspecting traffic must be configured between that zone and any other zone.
  
• The self zone is the only exception to the default deny all policy. All traffic to any router interface is allowed until traffic is explicitly denied.
  
• Traffic cannot flow between a zone member interface and any interface that is not a zone member. Pass, inspect, and drop actions can only be applied between two zones.
  
• Interfaces that have not been assigned to a zone function as classical router ports and might still use classical stateful inspection/CBAC configuration.
  
• If it is required that an interface on the box not be part of the zoning/firewall policy. It might still be necessary to put that interface in a zone and configure a pass all policy (sort of a dummy policy) between that zone and any other zone to which traffic flow is desired.
  
• From the preceding it follows that, if traffic is to flow among all the interfaces in a router, all the interfaces must be part of the zoning model (each interface must be a member of one zone or another).
  
• The only exception to the preceding deny by default approach is the traffic to and from the router, which will be permitted by default. An explicit policy can be configured to restrict such traffic.

A security zone should be configured for each region of relative security within the network, so that all interfaces that are assigned to the same zone will be protected with a similar level of security. For example, consider an access router with three interfaces:

• One interface connected to the public Internet
  
• One interface connected to a private LAN that must not be accessible from the public Internet
  
• One interface connected to an Internet service demilitarized zone (DMZ), where a Web server, Domain Name System (DNS) server, and e-mail server must be accessible to the public Internet
  

In this example, each zone holds only one interface. If an additional interface is added to the private zone, the hosts connected to the new interface in the zone can pass traffic to all hosts on the existing interface in the same zone. Additionally, the hosts’ traffic to hosts in other zones is similarly affected by existing policies.
Typically, the example network will have three main policies:

• Private zone connectivity to the Internet
  
• Private zone connectivity to DMZ hosts
  
• Internet zone connectivity to DMZ hosts
  

Because the DMZ is exposed to the public Internet, the DMZ hosts might be subjected to undesired activity from malicious individuals who might succeed at compromising one or more DMZ hosts. If no access policy is provided for DMZ hosts to reach either private zone hosts or Internet zone hosts, then the individuals who compromised the DMZ hosts cannot use the DMZ hosts to carry out further attack against private or Internet hosts. ZFW imposes a prohibitive default security posture. Therefore, unless the DMZ hosts are specifically provided access to other networks, other networks are safeguarded against any connections from the DMZ hosts. Similarly, no access is provided for Internet hosts to access the private zone hosts, so private zone hosts are safe from unwanted access by Internet hosts.

Steps to configure ZFW:

1. Define zones.
   
2. Define zone-pairs.
   
3. Define class-maps that describe traffic that must have policy applied as it crosses a zone-pair.
   
4. Define policy-maps to apply action to your class-maps’ traffic.
   
5. Apply policy-maps to zone-pairs.
   
6. Assign interfaces to zones. 
   
   

Configuring Zone-Based Policy Firewall Class-Maps:

Class-maps define the traffic that the firewall selects for policy application. Layer 4 class-maps sort the traffic based on these criteria listed here. These criteria are specified using the match command in a class-map:

• Access-group—A standard, extended, or named ACL can filter traffic based on source and destination IP address and source and destination port.
  
• Protocol—The Layer 4 protocols (TCP, UDP, and ICMP) and application services such as HTTP, SMTP, DNS, etc. Any well-known or user-defined service known to Port-Application Mapping can be specified.
  
• Class-map—A subordinate class-map that provides additional match criteria can be nested inside another class-map.
  
• Not—The not criterion specifies that any traffic that does not match a specified service (protocol), access-group or subordinate class-map will be selected for the class-map.
  

Class-maps can apply match-any or match-all operators to determine how to apply the match criteria. If match-any is specified, traffic must meet only one of the match criteria in the class-map. If match-all is specified, traffic must match all of the class-map’s criteria in order to belong to that particular class.

class-map type inspect match-any my-test-cmap
 match protocol http
 match protocol tcp 

HTTP traffic must encounter the match protocol http first to make sure the traffic is handled by the service-specific capabilities of HTTP inspection. If the match lines are reversed, so traffic encounters the match protocol tcp statement before it compares it to match protocol http, the traffic is simply classified as TCP traffic, and inspected according to the capabilities of the Firewall’s TCP Inspection component. This is a problem for certain services such as FTP, TFTP, and several multimedia and voice signaling services such as H.323, SIP, Skinny, RTSP, and others. These services require additional inspection capabilities to recognize the more complex activities of these services. So, more specific rules should by positioned before less specific.

Class-maps can apply an ACL as one of the match criteria for policy application. If a class-map’s only match criterion is an ACL and the class-map is associated with a policy-map applying the inspect action, the router applies basic TCP or UDP inspection for all traffic allowed by the ACL, except that which ZFW provides application-aware inspection. This includes (but not limited to) FTP, SIP, Skinny (SCCP), H.323, Sun RPC, and TFTP. If application-specific inspection is available and the ACL allows the primary or control channel, any secondary or media channel associated with the primary/control is allowed, regardless of whether the ACL allows the traffic.
If a class-map applies only ACL 101 as the match criteria, an ACL 101 appears as this:

access-list 101 permit ip any any

All traffic is allowed in the direction of the service-policy applied to a given zone-pair, and corresponding return traffic is allowed in the opposite direction. Therefore, the ACL must apply the restriction to limit traffic to specific desired types. Note that the PAM list includes application services such as HTTP, NetBIOS, H.323, and DNS. However, in spite of PAM’s knowledge of the specific application’s use of a given port, firewall only applies sufficient application-specific capability to accommodate the well-known requirements of the application traffic. Thus, simple application traffic such as telnet, SSH, and other single-channel applications are inspected as TCP, and their statistics are combined together in the show command output. If application-specific visibility into network activity is desired, you need to configure inspection for services by application name (configure match protocol http, match protocol telnet, etc.).

Compare the statistics available in the show policy-map type inspect zone-pair command output from this configuration with the more explicit firewall policy shown further down the page. This configuration is used to inspect traffic from a Cisco IP Phone, as well as several workstations that use a variety of traffic, which includes http, ftp, netbios, ssh, and dns:

class-map type inspect match-all all-private
 match access-group 101
!         
policy-map type inspect priv-pub-pmap
 class type inspect all-private
  inspect
 class class-default
!
zone security private
zone security public
zone-pair security priv-pub source private destination public
 service-policy type inspect priv-pub-pmap
! 
interface FastEthernet4
 ip address 172.16.108.44 255.255.255.0
 zone-member security public
!
interface Vlan1
 ip address 192.168.108.1 255.255.255.0
 zone-member security private
!
access-list 101 permit ip 192.168.108.0 0.0.0.255 any

While this configuration is easy to define and accommodates all traffic that originates in the private zone (as long as the traffic observes the standard, PAM-recognized destination ports), it provides limited visibility into service activity, and does not offer the opportunity to apply ZFW’s bandwidth and session limits for specific types of traffic. This show policy-map type inspect zone-pair priv-pub command output is the result of the previous simple configuration that uses only a permit ip [subnet] any ACL between zone-pairs. As you can see, most of workstation traffic is counted in the basic TCP or UDP statistics:

stg-871-L#show policy-map type insp zone-pair priv-pub
 Zone-pair: priv-pub
 
  Service-policy inspect : priv-pub-pmap
 
    Class-map: all-private (match-all)
      Match: access-group 101
      Inspect
        Packet inspection statistics [process switch:fast switch]
        tcp packets: [413:51589]
        udp packets: [74:28]
        icmp packets: [0:8]
        ftp packets: [23:0]
        tftp packets: [3:0]
        tftp-data packets: [6:28]
        skinny packets: [238:0]
 
        Session creations since subsystem startup or last reset 39
        Current session counts (estab/half-open/terminating) [3:0:0]
        Maxever session counts (estab/half-open/terminating) [3:4:1]
        Last session created 00:00:20
        Last statistic reset never
        Last session creation rate 2
        Maxever session creation rate 7
        Last half-open session total 0
 
    Class-map: class-default (match-any)
      Match: any 
      Drop (default action)
        0 packets, 0 bytes

By contrast, a similar configuration that adds application-specific classes provides more granular application statistics and control, and still accommodates the same breadth of services that was shown in the first example by defining the last-chance class-map matching only the ACL as the last chance in the policy-map:

class-map type inspect match-all all-private
 match access-group 101
class-map type inspect match-all private-ftp
 match protocol ftp
 match access-group 101
class-map type inspect match-any netbios
 match protocol msrpc
 match protocol netbios-dgm
 match protocol netbios-ns
 match protocol netbios-ssn
class-map type inspect match-all private-netbios
 match class-map netbios
 match access-group 101
class-map type inspect match-all private-ssh
 match protocol ssh
 match access-group 101
class-map type inspect match-all private-http
 match protocol http
 match access-group 101
!
policy-map type inspect priv-pub-pmap
 class type inspect private-http
  inspect
 class type inspect private-ftp
  inspect
 class type inspect private-ssh
  inspect
 class type inspect private-netbios
  inspect
 class type inspect all-private
  inspect
 class class-default!
zone security private
zone security public
zone-pair security priv-pub source private destination public
 service-policy type inspect priv-pub-pmap
! 
interface FastEthernet4
 ip address 172.16.108.44 255.255.255.0
 zone-member security public
!
interface Vlan1
 ip address 192.168.108.1 255.255.255.0
 zone-member security private
!
access-list 101 permit ip 192.168.108.0 0.0.0.255 any

The more-specific configuration provides this substantial granular output for the show policy-map type inspect zone-pair priv-pub command:

stg-871-L#sh policy-map type insp zone-pair priv-pub
 Zone-pair: priv-pub

  Service-policy inspect : priv-pub-pmap

   Class-map: private-http (match-all)
    Match: protocol http
    Match: access-group 101
    Inspect
      Packet inspection statistics [process switch:fast switch]
      tcp packets: [0:2193]

      Session creations since subsystem startup or last reset 731
      Current session counts (estab/half-open/terminating) [0:0:0]
      Maxever session counts (estab/half-open/terminating) [0:3:0]
      Last session created 00:29:25
      Last statistic reset never
      Last session creation rate 0
      Maxever session creation rate 4
      Last half-open session total 0

   Class-map: private-ftp (match-all)
    Match: protocol ftp
    Inspect
      Packet inspection statistics [process switch:fast switch]
      tcp packets: [86:167400]
      ftp packets: [43:0]

      Session creations since subsystem startup or last reset 7
      Current session counts (estab/half-open/terminating) [0:0:0]
      Maxever session counts (estab/half-open/terminating) [2:1:1]
      Last session created 00:42:49
      Last statistic reset never
      Last session creation rate 0
      Maxever session creation rate 4
      Last half-open session total 0

   Class-map: private-ssh (match-all)
    Match: protocol ssh
    Inspect
      Packet inspection statistics [process switch:fast switch]
      tcp packets: [0:62]

      Session creations since subsystem startup or last reset 4
      Current session counts (estab/half-open/terminating) [0:0:0]
      Maxever session counts (estab/half-open/terminating) [1:1:1]
      Last session created 00:34:18
      Last statistic reset never
      Last session creation rate 0
      Maxever session creation rate 2
      Last half-open session total 0
 
   Class-map: all-private (match-all)
    Match: access-group 101
    Inspect
      Packet inspection statistics [process switch:fast switch]
      tcp packets: [51725:158156]
      udp packets: [8800:70]
      tftp packets: [8:0]
      tftp-data packets: [15:70]
      skinny packets: [33791:0]

      Session creations since subsystem startup or last reset 2759
      Current session counts (estab/half-open/terminating) [2:0:0]
      Maxever session counts (estab/half-open/terminating) [2:6:1]
      Last session created 00:22:21
      Last statistic reset never
      Last session creation rate 0
      Maxever session creation rate 12
      Last half-open session total 0
 
   Class-map: class-default (match-any)
    Match: any 
    Drop (default action)
      4 packets, 112 bytes

Another added benefit of using a more granular class-map and policy-map configuration, as mentioned earlier, is the possibility of applying class-specific limits on session and rate values and specifically adjusting inspection parameters by applying a parameter-map to adjust each class’s inspection behavior.

Configuring Zone-Based Policy Firewall Policy-Maps

The policy-map applies firewall policy actions to one or more class-maps to define the service-policy that will be applied to a security zone-pair. When an inspect-type policy-map is created, a default class named class class-default is applied at the end of the class. The class class-default’s default policy action is drop, but can be changed to pass. The log option can be added with the drop action. Inspect cannot be applied on class class-default. 

Zone-Based Policy Firewall Actions

ZFW provides three actions for traffic that traverses from one zone to another:

• Drop—This is the default action for all traffic, as applied by the "class class-default" that terminates every inspect-type policy-map. Other class-maps within a policy-map can also be configured to drop unwanted traffic. Traffic that is handled by the drop action is "silently" dropped (i.e., no notification of the drop is sent to the relevant end-host) by the ZFW, as opposed to an ACL's behavior of sending an ICMP “host unreachable” message to the host that sent the denied traffic. Currently, there is not an option to change the "silent drop" behavior. The log option can be added with drop for syslog notification that traffic was dropped by the firewall.
  
• Pass—This action allows the router to forward traffic from one zone to another. The pass action does not track the state of connections or sessions within the traffic. Pass only allows the traffic in one direction. A corresponding policy must be applied to allow return traffic to pass in the opposite direction. The pass action is useful for protocols such as IPSec ESP, IPSec AH, ISAKMP, and other inherently secure protocols with predictable behavior. However, most application traffic is better handled in the ZFW with the inspect action.
  
• Inspect—The inspect action offers state-based traffic control. For example, if traffic from the private zone to the Internet zone in the earlier example network is inspected, the router maintains connection or session information for TCP and User Datagram Protocol (UDP) traffic. Therefore, the router permits return traffic sent from Internet-zone hosts in reply to private zone connection requests. Also, inspect can provide application inspection and control for certain service protocols that might carry vulnerable or sensitive application traffic. Audit-trail can be applied with a parameter-map to record connection/session start, stop, duration, the data volume transferred, and source and destination addresses.

Actions are associated with class-maps in policy-maps:

conf t
 policy-map type inspect z1-z2-pmap
  class type inspect service-cmap
   inspect|drop|allow [service-parameter-map]

Parameter-maps offer options to modify the connection parameters for a given class-map’s inspection policy.

Configuring Zone-Policy Firewall Parameter-Maps

Parameter-maps specify inspection behavior for ZFW, for parameters such as DoS protection, TCP connection/UDP session timers, and audit-trail logging settings. Parameter-maps are also applied with Layer 7 class and policy-maps to define application-specific behavior, such as HTTP objects, POP3 and IMAP authentication requirements, and other application-specific information.
Inspection parameter-maps for ZFW are configured as type inspect, similar to other ZFW class and policy-objects:

stg-871-L(config)#parameter-map type inspect z1-z2-pmap
stg-871-L(config-profile)#?
parameter-map commands:
  alert           Turn on/off alert
  audit-trail     Turn on/off audit trail
  dns-timeout     Specify timeout for DNS
  exit            Exit from parameter-map
  icmp            Config timeout values for icmp
  max-incomplete  Specify maximum number of incomplete connections before
                  clamping
  no              Negate or set default values of a command
  one-minute      Specify one-minute-sample watermarks for clamping
  sessions        Maximum number of inspect sessions
  tcp             Config timeout values for tcp connections
  udp             Config timeout values for udp flows

Specific types of parameter-maps specify parameters applied by Layer 7 application inspection policies. Regex-type parameter-maps define a regular expression for use with HTTP application inspection that filters traffic using a regular expression:

parameter-map type regex [parameter-map-name]

Protocol-info-type parameter-maps define server names for use with instant messaging application inspection:

parameter-map type protocol-info [parameter-map-name]