What is the difference between RADIUS and Tacacs+ authentication?

The main difference between RADIUS and TACACS+ authentication lies in their primary function: RADIUS is predominantly a network access protocol designed for user authentication, while TACACS+ is mainly used for administrating network devices like routers and switches, offering granular control over administrative commands. Both are critical components of a comprehensive AAA (Authentication, Authorization, and Accounting) strategy, yet they serve distinct purposes with differing technical implementations.

RADIUS vs. TACACS+: Key Distinctions

While both protocols facilitate centralized AAA, their architectural and functional differences make them suitable for different use cases. The table below highlights their core distinctions:

Feature	RADIUS (Remote Authentication Dial-In User Service)	TACACS+ (Terminal Access Access Control System Plus)
Primary Use Case	User authentication for network access (Wi-Fi, VPN, 802.1X for wired networks).	Device administration and command authorization for network devices (routers, switches, firewalls, servers).
Transport Protocol	UDP (User Datagram Protocol)	TCP (Transmission Control Protocol)
Port Numbers	1812 (Authentication), 1813 (Accounting). Older implementations may use 1645/1646.	49 (single port for Authentication, Authorization, Accounting).
Encryption	Only the password in the access-request packet is encrypted. Other attributes are visible.	The entire packet payload is encrypted, providing end-to-end security for all data exchanged.
AAA Separation	Authentication and Authorization are typically combined, making granular authorization challenging.	Authentication, Authorization, and Accounting are handled as separate, distinct processes.
Protocol Type	Open standard (defined by RFCs like 2865 and 2866).	Cisco proprietary, although widely supported by other network vendors.
Command Authorization	Limited or no per-command authorization capabilities, primarily focused on network access.	Highly granular command authorization, allowing precise control over which commands users can execute.
Reliability	Less reliable due to UDP's connectionless nature; retransmission logic handled by the application layer.	More reliable due to TCP's connection-oriented, guaranteed delivery and retransmission.
Accounting	Records basic user session data (start/stop times, data usage).	Provides detailed command-level accounting, recording every command executed by an administrator.

Understanding RADIUS

RADIUS is a client-server protocol primarily used for centralized Authentication, Authorization, and Accounting (AAA) management for users connecting to a network. It's an open standard, making it widely compatible across various vendors.

How RADIUS Works

User Initiates Connection: A user attempts to connect to a network resource (e.g., Wi-Fi access point, VPN concentrator, 802.1X switch port).
NAS (Network Access Server) Request: The network device (acting as a RADIUS client) receives the user's credentials and forwards an Access-Request message to the RADIUS server.
RADIUS Server Authentication: The RADIUS server authenticates the user against an internal database or external directory service (like Active Directory, LDAP).
Authorization Response: Based on the authentication result, the RADIUS server sends an Access-Accept (with authorization attributes like VLAN assignment or bandwidth limits) or Access-Reject message back to the NAS.
Network Access: If accepted, the NAS grants the user access according to the specified authorization.
Accounting: The NAS also sends Accounting-Start and Accounting-Stop messages to the RADIUS server, logging session duration and data usage.

Common Use Cases for RADIUS

Wireless Network Authentication: Securing Wi-Fi access using WPA2-Enterprise or WPA3.
VPN Access: Authenticating users connecting to corporate VPNs.
802.1X for Wired Networks: Authenticating devices and users on wired LAN ports.
Dial-up and DSL Services: Traditional authentication for Internet access.

Advantages of RADIUS

Open Standard: Broad industry support and interoperability.
Simplicity for Network Access: Well-suited for straightforward user authentication and basic authorization.
Scalability: Can handle a large number of concurrent user sessions.

Understanding TACACS+

TACACS+ (Terminal Access Controller Access Control System Plus) is a Cisco proprietary protocol, though widely adopted by other vendors, that provides centralized AAA services. It shines in environments requiring precise control and auditing over administrative access to network infrastructure.

How TACACS+ Works

Administrator Attempts Access: An administrator tries to log into a network device (e.g., router, switch, firewall) via console, SSH, or Telnet.
Device Request: The network device (TACACS+ client) prompts for credentials and sends an AUTHEN/START request to the TACACS+ server.
TACACS+ Server Authentication: The TACACS+ server authenticates the administrator against a configured user database (e.g., local, LDAP, Active Directory).
Authorization Request: After successful authentication, the device sends AUTHOR/REQUEST to the server to determine what commands the administrator is permitted to run.
Granular Authorization: The TACACS+ server responds with AUTHOR/RESPONSE, granting or denying access to specific commands or command sets.
Command Execution and Accounting: As the administrator executes commands, the device sends ACCT/REQUEST messages to the TACACS+ server, recording each command for auditing purposes.

Common Use Cases for TACACS+

Network Device Administration: Securing management access to routers, switches, firewalls, and load balancers.
Server and Appliance Management: Controlling administrative access to Linux/Unix servers or other network appliances.
Command Control: Implementing granular authorization to restrict which commands specific administrators can execute.

Advantages of TACACS+

Granular Command Authorization: Provides unparalleled control over administrative access, allowing definition of specific command sets.
Enhanced Security: Full packet encryption significantly enhances the confidentiality of AAA data.
Reliability: Utilizes TCP, ensuring reliable delivery of AAA messages.
Separation of AAA: Decoupling Authentication, Authorization, and Accounting allows for greater flexibility and custom policy enforcement.
Detailed Accounting: Offers comprehensive logging of every command executed, crucial for auditing and compliance.

Choosing the Right Protocol: Practical Insights

The decision to use RADIUS, TACACS+, or both often depends on your specific security requirements and the type of access you need to manage.

When to Use RADIUS

User Network Access: Ideal for authenticating end-users to corporate Wi-Fi, VPNs, or wired 802.1X networks.
Large User Bases: Efficient for managing access for many users with relatively simple authorization rules.
Open Standards: Preferred in environments that prioritize open protocols and multi-vendor interoperability for end-user access.
Less Sensitive Access: Suitable for scenarios where full packet encryption is not a paramount requirement for all attributes.

When to Use TACACS+

Network Device Administration: The superior choice for managing access to critical network infrastructure.
High Security Environments: Essential for organizations requiring robust security, full encryption, and granular control over administrative actions.
Compliance and Auditing: Provides detailed command-level accounting logs, vital for meeting regulatory compliance and forensic analysis.
Role-Based Access Control (RBAC): Supports complex RBAC models for administrative functions, ensuring administrators only perform authorized tasks.

Can They Work Together?

Yes, it is common practice for organizations to deploy both RADIUS and TACACS+ concurrently. They complement each other perfectly:

RADIUS handles authentication and authorization for end-users connecting to the network.
TACACS+ manages administrative access and command authorization for IT personnel configuring network devices.

This hybrid approach ensures robust and tailored AAA services across the entire enterprise infrastructure.