What is TACACS+ Protocol? Features & RADIUS Comparison

Do you manage a complex network where controlling who accesses which devices and what they can do is a challenge? Many organizations grapple with ensuring only the right people perform the right actions on their critical infrastructure. This is where a powerful security framework known as TACACS+ steps in.

TACACS+ is nothing but a robust network security protocol. It separates your network access control into distinct, manageable steps. This allows you to centralise authentication, authorization, and accounting (AAA) for your network devices. By using this protocol, you can significantly enhance your network's security and auditability.

In this comprehensive guide, we will explore this vital protocol, understand what TACACS+ stands for, how it works, and its primary advantages over other protocols. We will also perform a detailed comparison between TACACS vs RADIUS.

 What Does TACACS Stand For? The TACACS Full Form

The term TACACS stands for Terminal Access Controller Access-Control System. It is a protocol initially developed by the company BBN for the U.S. Department of Defense.

It is important to note that when network professionals talk about TACACS today, they almost always refer to TACACS+. TACACS+ is the current, enhanced version that addresses many limitations of its predecessors (original TACACS and Extended TACACS - XTACACS).

The initial TACACS full form refers to the basic security method. The plus sign (+) in TACACS+ signifies the major improvements and architectural changes that make it the industry-standard enterprise protocol for AAA services.

Secure Admin Access

What is TACACS+ Used For?

TACACS+ plays a vital role in managing administrative access to network devices, such as routers, switches, firewalls, and access servers. It provides a centralized control plane for all network administrators and operators.

This protocol aims at solving three fundamental security questions whenever a user attempts to access a device:

1. Authentication:Who are you? (Verifying the user's identity.)
2. Authorization:What are you allowed to do? (Determining the user's permissible commands and resources.)
3. Accounting:What did you do? (Tracking all user actions for auditing.)

A TACACS+ server serves as the central repository for user credentials and access policies. Therefore, every time a network device needs to verify a user, it sends the request to the central TACACS+ server.

Also Read: What Is Application Security? Tools, Testing & Best Practices

 How TACACS Works?

Understanding how TACACS works is essential for securing your network. The system operates on a client-server principle and uses the TCP protocol, typically on port 49.

Let us explore the core components and the sequential process.

Core Components of the TACACS System

The overall TACACS system comprises of three main elements:

1. Network Access Server (NAS) or Client: This is the network device—like a router or a switch—where the user tries to log in. This device sends the user's request to the server.
2. TACACS+ Server: This central server holds the database of all user accounts, passwords, and the specific access rights (policies) for each user.
3. Client/User: The individual attempting to gain access to the network device.

The Three A's: Authentication, Authorization, and Accounting

The operation of TACACS+ is based on a modular, three-step process:

1. Authentication:
   • The user enters their username and password on the NAS (router).
   • The NAS sends these credentials to the TACACS+ server using a secure TCP connection.
   • The TACACS+ server validates the user's identity against its internal database or an external database (like LDAP).
   • If the user is valid, the server sends back an "ACCESS ACCEPT" response.
2. Authorization:
   • After a successful login, the NAS sends a separate request to the server for authorization.
   • The server determines the specific commands, resources, or privilege levels the authenticated user can access. For example, some users may only run "show" commands, while others can access "configure terminal."
   • The server returns a list of permissible commands or attributes. This is a key difference from other protocols.
3. Accounting:
   • The NAS sends detailed start and stop records to the server.
   • The TACACS+ server logs exactly what the user did, including all commands executed, their login time, and their logout time.
   • This detailed logging ensures comprehensive auditability and non-repudiation.

Also Read: What is Network Traffic Analysis (NTA) in Cybersecurity?

TACACS Configuration and Key Features

A proper TACACS configuration is vital for network security. Network devices must be configured to point to the correct TACACS+ server address and use a shared secret key for secure communication.

TACACS+ offers several critical features that make it the preferred choice in many enterprise environments:

• Full Encryption: The protocol encrypts the entire body of the packet, not just the password. This significantly enhances security by preventing eavesdropping on the session, including the commands a user executes.
• Decoupled AAA: It allows the separation of the Authentication, Authorization, and Accounting functions. You could potentially use one database for authentication and a different one for authorization.
• Extensive Authorization: The authorization stage is highly granular. It permits the administrator to control which specific commands a user can execute on a particular device. For instance, a user can be restricted to only run shutdown on one router but not on another.
• TCP Reliance: It relies on the Transmission Control Protocol (TCP), which provides connection-oriented, reliable transport. This ensures that packets arrive in order and that the communication is stable.

Difference Between TACACS and RADIUS: TACACS vs RADIUS

When discussing network access protocols, the comparison between TACACS vs RADIUS is unavoidable. Both protocols serve the purpose of AAA, but their architecture and approach differ significantly. This comparison chart highlights the core distinctions.

Why TACACS+ Often Wins for Administrative Access?

While RADIUS is excellent for network access (like a Wi-Fi login), TACACS+ is primarily chosen for controlling administrative access to core network devices. The main reason is nothing but its superior and more granular authorization capabilities and its full packet encryption.

• Granular Control: The separate authorization stage enables a network engineer to define policy like: "This user can run debug commands on this router but nothing else."
• Enhanced Security: The full-body encryption protects the command history. Put it another way, an attacker cannot see the commands being executed, even if they intercept the packet.

Also Read: What is Phishing Simulation? Benefits & Best Practices

What is the Current Version of TACACS?

As we know, the original TACACS protocol is essentially obsolete. The current version of TACACS is nothing but TACACS+.

The TACACS+ protocol definition has not had a major version change in many years, which indicates its stability and functional completeness. All modern network devices and AAA servers implement this current standard, making it the industry go-to for centralized device management. When you hear the term TACACS in networking, it refers to this mature, secure, and reliable TACACS+ version.

Conclusion

The TACACS+ protocol is an essential tool for any organization that needs strict, auditable control over its network infrastructure. Its modular AAA functions, full packet encryption, and highly granular authorization make it the definitive standard for securing administrative access.

By understanding and properly implementing TACACS configuration, you ensure that only authorized personnel can access and modify your critical network devices, and that you maintain a full, secure record of all their actions.

We focus on helping you implement robust, high-availability security protocols like TACACS+ so you can focus on building your network's future. Contact us today to review your current network access control methods and begin your journey to a fully secured, centrally managed system.

 Key Takeaways on TACACS+

• TACACS full form is Terminal Access Controller Access-Control System. TACACS+is the enhanced, current version.
• What TACACS is used for is primarily centralizing the AAA services (Authentication, Authorization, and Accounting) for network device administration.
• How TACACS works is based on the reliable TCP protocol on port 49, which ensures a stable communication session.
• The main difference between TACACS and RADIUS is that TACACS+ offers superior, more granular authorization and full packet encryption, making it ideal for administrative access control.

 Frequently Asked Questions (FAQs) About TACACS+

1. What is a TACACS Server Directed Request?

A TACACS Server Directed Request occurs when a client device (NAS) sends the username to the TACACS+ server, and the server determines which specific authentication method or external service (e.g., another server or token) should be used for that particular user. This allows for flexible, per-user authentication policies.

2. Can I use TACACS+ and RADIUS at the same time?

Yes, you can use both TACACS+ and RADIUS simultaneously. Many enterprises implement TACACS+ for administrator logins (device configuration) and RADIUS for general user network access (VPN, Wi-Fi). This hybrid approach provides an optimal security model.

3. What is the security advantage of TACACS+ using TCP?

TACACS+ uses TCP, which provides reliability. If a packet is lost, TCP automatically handles re-transmission. Furthermore, the connection-oriented nature of TCP makes it harder for an attacker to spoof session initiation compared to connectionless UDP used by RADIUS.

4. How does TACACS+ handle command authorization?

The TACACS+ server sends a list of explicitly permitted or denied commands to the NAS after a user authenticates. When the user enters a command, the NAS checks this list before executing the command. This determines the specific actions the user can perform.