Implementing Network Security ( Version 2.0) – CCNAS Chapter 4 Exam Online

This ACL is denying all TCP/IP traffic coming into the outside interface. Because the source address matches the any parameter and because the access list line is filtering based on denying access (deny), the packet is dropped.

By allowing the ICMP echo reply message inbound to the organization, internal users are allowed to ping external addresses (and the reply message allowed to return).

For the purpose of applying an access list to a particular interface, the ipv6 traffic-filter IPv6 command is equivalent to the access-group IPv4 command. The direction in which the traffic is examined (in or out) is also required.

With a three interface firewall design that has internal, external, and DMZ connections, typical configurations include the following:Traffic originating from DMZ destined for the internal network is normally blocked.
Traffic originating from the DMZ destined for external networks is typically permitted based on what services are being used in the DMZ.
Traffic originating from the internal network destined from the DMZ is normally inspected and allowed to return.
Traffic originating from external networks (the public network) is typically allowed in the DMZ only for specific services.

The ICMP protocol is used for neighbor discovery. The two permit statements allow neighbor advertisement and neighbor solicitation messages between IPv6 devices.

Common ACEs to assist with antispoofing include blocking packets that have a source address in the 127.0.0.0/8 range, any private address, or any multicast addresses. Furthermore, the administrator should not allow any outbound packets with a source address other than a valid address that is used in the internal networks of the organization.

The classic firewall provides stateful inspection including protocols that require multiple channels for communication such as FTP and H.323. Protocol numbers, port numbers, and source and destination IP addresses are all standard filters for extended ACLs.

The traffic flow from the internal network to the public network is commonly inspected. The traffic flows cause dynamic entries to be added to the external interface for inbound traffic so that traffic that originates from the internal network going to the public network is allowed to return to the internal source.

A best practice for configuring an extended ACL is to ensure that the most specific ACE is placed higher in the ACL. Consider the two permit UDP statements. If both of these were in an ACL, the SNMP ACE is more specific than the UDP statement that permits a range of 10,001 UDP port numbers. The SNMP ACE would be entered before the other UDP ACE. The ACEs from most specific to least specific are as follows:

• permit udp 172.16.0.0 0.0.255.255 host 172.16.1.5 eq snmptrap
• deny udp any host 172.16.1.5 eq snmptrap
• permit tcp 172.16.0.0 0.0.3.255 any established
• deny tcp any any eq telnet
• permit udp any any range 10000 20000
• permit ip any any

Most traffic within an organization originates from a private IP address. The amount of inspection done to that traffic depends on its destination or whether traffic that is going to that private IP address originated the connection. The demilitarized zone typically holds servers. Traffic that is destined to those servers is filtered based on what services are being provided by the server (HTTP, HTTPS, DNS, etc.).

The extended access list in the exhibit is permitting SSH (TCP port 22) traffic that is sourced from the 192.168.1.0/24 network and traveling to the 192.168.2.0/24 network. The packets meeting this criteria are logged to the local logging buffer (the default), a syslog server, or both depending on how the router is configured for syslog settings. All other traffic is denied because of the implicit deny at the end of every ACL.

The first ACE allows the 192.168.10.1 device to do any TCP/IP-based transactions with any other destination. The second ACE stops devices on the 192.168.10.0/24 network from issuing any pings to any other location. Everything else is permitted by the third ACE. Therefore, a Telnet/SSH session or ping reply is allowed from a device on the 192.168.10.0/24 network.

A stateful firewall performs better than a proxy server. A stateful firewall cannot authenticate users or prevent Layer 7 attacks. Both a stateful firewall and a proxy server can filter packets.​

Limitations of stateful firewalls include the following:

• Stateful firewalls cannot prevent application layer attacks.
• Protocols such as UDP and ICMP are not stateful and do not generate information needed for a state table.
• An entire range of ports must sometimes be opened in order to support specific applications that open multiple ports.
• Stateful firewalls lack user authentication.

The steps for configuring zones in a Zone-Based Policy Firewall are as follows:

Step 1. Determine the zones.
Step 2. Establish policies between zones.
Step 3. Design the physical infrastructure.
Step 4. Identify subsets within zones and merge traffic requirements.​

Both a Classic Firewall and Zone-Based Firewall can be implemented concurrently on a router, but they cannot both be configured on a single interface.

The rules for traffic transiting through the router are as follows:

• If neither interface is a zone member, then the resulting action is to pass the traffic.
• If both interfaces are members of the same zone, then the resulting action is to pass the traffic.
• If one interface is a zone member, but the other is not, then the resulting action is to drop the traffic regardless of whether a zone-pair exists.
• If both interfaces belong to the same zone-pair and a policy exists, then the resulting action is inspect, allow, or drop as defined by the policy.

Traffic that originates within a router such as pings from a command prompt, remote access from a router to another device, or routing updates are not affected by outbound access lists. The traffic must flow through the router in order for the router to apply the ACEs.

The three actions that can be applied are inspect, drop,and pass.​
Inspect – This action offers state-based traffic control.
Drop – This is the default action for all traffic. Similar to the implicit deny any at the end of every ACL, there is an explicit drop applied by the IOS to the end of every policy map.
Pass – This action allows the router to forward traffic from one zone to another.

A stateful firewall filters packets based on state information maintained in a state table. Because it uses state information, the stateful firewall can analyze traffic at OSI Layers 4 and 5.

The pass action performed by Cisco IOS ZPF permits forwarding of traffic in a manner similar to the permit statement in an access control list.