Remote Access Authentication Methods

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An important step when implementing a VPN is to ensure only authorized entities can access it. Authorization begins with authentication, and VPNs support multiple methods of authentication. If you’re planning to take the SY0-501 exam, you should have a basic understanding of different remote access authentication mechanisms.

For example, can you answer this practice test question?

Q. You are tasked with configuring authentication services settings on computers in your network. You are entering shared secrets on different servers. Which of the following services are you MOST likely configuring? (Select TWO.)

A. RADIUS

B. Kerberos

C. LDAP

D. EAP-TLS

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation are available at the end of this post.

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RADIUS

Remote Authentication Dial-In User Service (RADIUS) is a centralized authentication service. Instead of each individual VPN server needing a separate database to identify who can authenticate, the VPN servers forward the authentication requests to a central RADIUS server. RADIUS can also be used as an 802.1X server with WPA2 Enterprise mode.

Imagine your company has locations in Virginia Beach, Atlanta, and Chicago. Each location has a VPN server that users can access. Bart is a traveling salesman, and he can connect to any of these VPN servers. When entering sales data, he connects to the Atlanta VPN. When using the company-sponsored always-on VPN for his mobile devices, he connects to the Virginia Beach VPN server. Bart has one account for all company access and today he was prompted to change his password.

If each VPN server has a separate database with Bart’s username and password, each of these databases must be updated. This can be labor intensive and result in needless errors.

However, the company could use a centralized RADIUS server, as shown in the figure, instead. Each VPN server is configured with a shared secret (similar to a password) and the RADIUS server is configured with a matching shared secret for each of the VPN servers.

Remote Access Authentication Methods

RADIUS configuration

This centralized RADIUS server could hold a centralized database of user accounts. However, it is more common for the RADIUS server to access an LDAP server that holds the accounts. For example, in a Microsoft domain, the RADIUS server would pass the credentials to a domain controller. A significant benefit is that there is only one account for the user. If Bart changes his password, the domain controller knows the new password.

RADIUS uses the User Datagram Protocol (UDP), which provides a best-effort delivery mechanism. As a result, RADIUS includes logic to detect communication problems. In contrast, RADIUS alternatives use TCP, which provides guaranteed delivery. These alternatives allow TCP to detect and handle communication issues. Also, RADIUS only encrypts the password, while alternatives encrypt the entire authentication process.

Even though RADIUS was created before Extensible Authentication Protocol (EAP) was developed, RADIUS does work with EAP. RFC 3579 “RADIUS Support for EAP” is an informational RFC and describes how to do so. However, alternatives make it easier to extend the use of EAP.

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TACACS+

Terminal Access Controller Access-Control System Plus (TACACS+) is the Cisco alternative to RADIUS. It provides two important security benefits over RADIUS. First, it encrypts the entire authentication process, whereas RADIUS encrypts only the password. Second, TACACS+ uses multiple challenges and responses between the client and the server.

Although CISCO created TACACS+, it can interact with Kerberos. This allows a Cisco VPN concentrator to interact in a Microsoft Active Directory environment. As a reminder, Microsoft Active Directory uses Kerberos for authentication.

Organizations also use TACACS+ as an authentication service for network devices. In other words, you can use it to authenticate users before they are able to access a configuration page for a router or a switch. The network devices must be TACACS+ enabled, and a TACACS+ server provides the authentication services.

Diameter

Diameter is an extension of RADIUS and many organizations have switched to it due to its extra capabilities. Diameter adds several other commands beyond the capabilities of RADIUS, along with adding new commands that can be used with EAP. Diameter uses TCP instead of UDP used by RADIUS. A key benefit is that it is backwards compatible with RADIUS and provides an upgrade path from RADIUS to Diameter.

In geometry, the diameter of a circle is a straight line between the two edges of a circle, whereas the radius is a straight line from the center to an edge. In other words, the diameter of a circle is twice as long as the radius. The designers considered this when naming Diameter to indicate indirectly that it is twice as good as RADIUS.

AAA Protocols

AAA protocols provide authentication, authorization, and accounting. Authentication verifies a user’s identification. Authorization determines if a user should have access. Accounting tracks user access with logs.

As an example, RADIUS, TACACS+, and Diameter are considered AAA protocols because they provide all three services. They authenticate users who attempt remote access, determine if the user is authorized for remote access by checking a database, and then record the user’s activity. TACACS+ uses multiple challenges and responses during a session. Kerberos is sometimes referred to as an AAA protocol, but it does not provide any accounting services.


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Q. You are tasked with configuring authentication services settings on computers in your network. You are entering shared secrets on different servers. Which of the following services are you MOST likely configuring? (Select TWO.)

A. RADIUS

B. Kerberos

C. LDAP

D. EAP-TLS

Answers are A, C. Remote Authentication Dial-in User Service (RADIUS) servers use shared secrets. You can configure them to interact with Lightweight Directory Access Protocol (LDAP)–based systems by entering the same shared secret on both a RADIUS server and an LDAP server. A shared secret is basically just an identical password on both systems.

Kerberos uses tickets for authentication, not shared secrets.

Extensible Authentication Protocol-Transport Layer Security (EAP-TLS) is an authentication protocol that requires the use of certificates on both clients and servers, not shared secrets.

See Chapter 4 of the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide for more information on using VPNs for remote access.

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