How ACLs work

How ACLs work 
11.1.2 An ACL is made up of statements that define whether packets are accepted or rejected at inbound and outbound interfaces. This page will explain how these statements are edited and added to an ACL. These decisions are made by matching a condition statement in an access list and then performing the accept or reject action defined in the statement.

The order in which ACL statements are placed is important. The Cisco IOS software tests the packet against each condition statement in order from the top of the list to the bottom. Once a match is found in the list, the accept or reject action is performed and no other ACL statements are checked. If a condition statement that permits all traffic is located at the top of the list, no statements added below that will ever be checked.

If additional condition statements are needed in an access list, the entire ACL must be deleted and recreated with the new condition statements. To make the process of revising an ACL simpler it is a good idea to use a text editor such as Notepad and paste the ACL into the router configuration.

The beginning of the router process is the same, whether ACLs are used or not. As a frame enters an interface, the router checks to see whether the Layer 2 address matches or if it is a broadcast frame. If the frame address is accepted, the frame information is stripped off and the router checks for an ACL on the inbound interface. If an ACL exists, the packet is now tested against the statements in the list. If the packet matches a statement, the packet is either accepted or rejected. If the packet is accepted in the interface, it will then be checked against routing table entries to determine the destination interface and switched to that interface. Next, the router checks whether the destination interface has an ACL. If an ACL exists, the packet is tested against the statements in the list. If the packet matches a statement, it is either accepted or rejected. If there is no ACL or the packet is accepted, the packet is encapsulated in the new Layer 2 protocol and forwarded out the interface to the next device.

As a review, ACL statements operate in sequential, logical order. If a condition match is true, the packet is permitted or denied and the rest of the ACL statements are not checked. If all the ACL statements are unmatched, an implicit deny any statement is placed at the end of the list by default. The invisible deny any statement at the end of the ACL will not allow unmatched packets to be accepted. When first learning how to create ACLs, it is a good idea to add the deny any at the end of ACLs to reinforce the dynamic presence of the implicit deny.

The next page will describe how ACLs are created

Access Control List Fundamentals / Introduction to ACLs

Access Control List Fundamentals 
Introduction to ACLs
11.1.1 This page will explain what ACLs are and how they are used.

ACLs are lists of conditions used to test network traffic that tries to travel across a router interface. These lists tell the router what types of packets to accept or deny. Acceptance and denial can be based on specified conditions. ACLs enable management of traffic and secure access to and from a network.

ACLs can be created for all routed network protocols such as IP and Internetwork Packet Exchange (IPX). ACLs can be configured at the router to control access to a network or subnet.

To filter network traffic, ACLs determine if routed packets are forwarded or blocked at the router interfaces. The router examines each packet and will forward or discard it based on the conditions specified in the ACL. An ACL makes routing decisions based on source address, destination address, protocols, and upper-layer port numbers.

ACLs must be defined on a per protocol, per direction, or per port basis. To control traffic flow on an interface, an ACL must be defined for each protocol enabled on the interface. ACLs control traffic in one direction at a time on an interface. Two separate ACLs must be created to control inbound and outbound traffic. Every interface can have multiple protocols and directions defined. If the router has two interfaces configured for IP, AppleTalk, and IPX, 12 separate ACLs would be needed. There would be one ACL for each protocol, times two for each direction, times two for the number of ports.

ACLs can be used to perform the following tasks:

• Limit network traffic and increase network performance. For example, ACLs that restrict video traffic could greatly reduce the network load and increase network performance.
• Provide traffic flow control. ACLs can restrict the delivery of routing updates. If updates are not required because of network conditions, bandwidth is preserved.
• Provide a basic level of security for network access. ACLs can allow one host to access a part of the network and prevent another host from accessing the same area. For example, Host A is allowed to access the Human Resources network and Host B is prevented from accessing it.
• Decide which types of traffic are forwarded or blocked at the router interfaces. ACLs can permit e-mail traffic to be routed, but block all Telnet traffic.
• Control which areas a client can access on a network.
• Screen hosts to permit or deny access to a network segment. ACLs can be used to permit or deny a user to access file types such as FTP or HTTP.

If ACLs are not configured on the router, all packets that pass through the router will be permitted to access the entire network.
The next page will describe how ACLs work.

Module 11: Access Control Lists (ACLs) / Overview

Overview

Network administrators must be able to deny unwanted access to a network and allow authorized users to access necessary services. Security tools such as passwords, callback equipment, and physical security devices are helpful. However, they often lack the flexibility of basic traffic filters and the specific controls that most administrators prefer. For example, a network administrator may want to allow users access to the Internet, but not permit external users Telnet access into the LAN.

Routers provide the capability to filter traffic, such as blocking Internet traffic, with access control lists (ACLs). An ACL is a sequential list of permit or deny statements that apply to addresses or upper-layer protocols. This module will introduce standard and extended ACLs as a way to control network traffic and explain how they are used as part of a security solution.

This module includes tips, considerations, recommendations, and general guidelines on how to use ACLs. It also includes the commands and configurations needed to create ACLs. Finally, this module provides examples of standard and extended ACLs and describes ACL placement on router interfaces.

An ACL can be as simple as a single line that permits packets from a specific host or it can be a complex set of rules and conditions that defines network traffic and determines the router processes. While many of the advanced uses of ACLs are beyond the scope of this course, this module provides details about standard and extended ACLs, the proper placement of ACLs, and some special applications of ACLs.

This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams. 
Students who complete this module should be able to perform the following tasks: 

• Describe the differences between standard and extended ACLs
• Explain the rules for placement of ACLs
• Create and apply named ACLs
• Describe the function of firewalls

Use ACLs to restrict virtual terminal access 

Summary of Module 10

Summary

This page summarizes the topics discussed in this module. The transport layer of the OSI model is responsible for the reliable transport and regulation of data flow from a source to a destination. TCP makes sure that each host on the network is ready and willing to communicate. A three-way handshake is a process that ensures that each side is ready for data transmission and allows each device to determine the initial sequence number. A three-way handshake starts with a host initiating a connection. The other host receives a packet, records a sequence number and then replies with an ACK. The initiating host then responds back and finalizes the connection. DoS attacks are designed to deny services to legitimate hosts that attempt to establish connections. It is used by hackers to halt system response. SYN flooding is one type of DoS attack. It exploits the normal three-way handshake and causes targeted devices to ACK to source addresses that will not complete the handshake. Spoofing occurs when a receiving device replies to a non-existent, unreachable IP address and is placed in a wait state until it receives the final ACK from the initiator. In addition to software specifically created as a defense against these kinds of attacks, an administrator can decrease the connection timeout period and increase the connection queue size. Breaking data into smaller pieces is called segmenting and is done with TCP. Once the data is segmented, it must be transmitted to the destination device. TCP applies sequence numbers to the data segments so that the receiver can reassemble the bytes properly and the sender knows when all the segments have been received. Windowing is the process of flow control that regulates how much data is sent during a given transmission period. TCP uses a sliding window when determining transmission size. A sliding window allows for devices to negotiate a window size to allow for more than one byte to be sent during a single transmission. Many protocols use PAR to provide reliability. With PAR, the source sends a packet, starts a timer, and waits for an ACK before it sends the next packet. If the timer expires before the source receives an ACK, the source retransmits the packet and resets the timer. TCP uses expectational ACKs in which the Acknowledgment Number refers to the next octet that is expected. UDP provides connectionless, non-guaranteed transmission of packets at Layer 4 of the OSI model. Since UDP does not use windowing or acknowledgments, application layer protocols must provide error detection. A port number must be associated with the conversation between hosts to ensure that the packet reaches the appropriate service on the server. Port numbers have the following assigned ranges: The Well Known Ports are those from 0 through 1023 The Registered Ports are those from 1024 through 49151 The Dynamic and/or Private Ports are those from 49152 through 65535 The three methods of addressing include port numbers, which are located at the transport layer and serviced by the network layer. The network layer assigns the logical or IP address and the data link layer assigns the physical or MAC address.

Comparison of MAC addresses, IP addresses, and port numbers

Comparison of MAC addresses, IP addresses, and port numbers 
10.2.6 This page will describe the three types of addresses in reference to the OSI model. Port numbers are located at the transport layer and are serviced by the network layer. The network layer assigns the logical address, or IP address, and is then serviced by the data link layer, which assigns the physical address, or MAC address.
A good analogy can be made with a normal letter. The address on a letter consists of a name, street, city, and state. These can be compared to the port, MAC, and IP address used for network data. The name on the envelope would be equivalent to a port number, the street address is the MAC, and the city and state is the IP address. Multiple letters can be mailed to the same street address, city and state, but contain different names on the letters. For instance, two letters could be mailed to the same house with one addressed to “John Doe” and the other to “Jane Doe”. This is analogous to multiple sessions with different port numbers. 
This page concludes this lesson. The next page will summarize the main points from this module. 

Port numbering and well-known port numbers / Example of multiple sessions between hosts

Port numbering and well-known port numbers

10.2.4 This page will discuss the three categories of port numbers.

Port numbers are represented by 2 bytes in the header of a TCP or UDP segment. This 16-bit value can result in port numbers ranging from 0 to 65535. The three categories of port numbers are well-known ports, registered ports, and dynamic or private ports. The first 1023 ports are well-known ports. These ports are used for well-known network services such as FTP, Telnet, or DNS. 

Registered ports range from 1024 to 49151. Ports between 49152 and 65535 are defined as dynamic or private ports.

The Interactive Media Activity will help students become more familiar with port numbers.

The next page will give an example of multiple sessions.

Example of multiple sessions between hosts 

10.2.5 This page will explain how port numbers are used to track multiple sessions that can occur between hosts. The source and destination port numbers combine with the network address to form a socket. A pair of sockets, one on each host, forms a unique connection. For instance, a host might have a Telnet connection through port 23 and an Internet connection through port 80. The IP and the MAC addresses would be the same because the packets are received from the same host. Therefore, each conversation on the source side needs its own port number, and each service requested needs its own port number.

In the Lab Activity, students will enable HTTP on a router and observe well-known ports.
The next page will discuss the three types of addresses.

Ports for clients

Ports for clients

10.2.3 This page will discuss source ports, which are set by clients.

Whenever a client connects to a service on a server, a source and destination port must be specified. TCP and UDP segments contain fields for source and destination ports. Destination ports, or ports for services, are normally defined using the well-known ports. Source ports set by the client are determined dynamically.

In general, a client determines the source port by randomly assigning a number above 1023. For example, a client that attempts to communicate with a Web server will use TCP and assign the destination port as 80 and the source port as 1045. When the packet arrives at the server, it moves up to the transport layer and eventually to the HTTP service, which operates at port 80. The HTTP server responds to the clients request with a segment that uses port 80 as the source and 1045 as the destination. Clients and servers use ports to distinguish which process each segment is associated with.

The next page will teach students about the three categories of port numbers.