What is a TACACS Server?

What is a TACACS server?

TACACS (Terminal Access Controller Access-Control System) is a network security protocol commonly used to provide centralized authentication, authorization, and accounting (AAA) for devices on a network. TACACS allows administrators to control access to network devices and resources by managing user authentication and authorization.

The TACACS protocol separates the authentication, authorization, and accounting processes into three distinct components:

  • Authentication: Verifies the identity of a user trying to access a network device. This process ensures that the user is who they claim to be.
  • Authorization: Determines the level of access or permissions granted to an authenticated user. It defines what actions a user is allowed to perform on the network device.
  • Accounting: Records and logs the actions of users on the network. This includes details such as who accessed the network device, what commands were executed, and when the access occurred. Accounting helps in auditing and tracking user activities.

A TACACS server is a server that implements the TACACS protocol and is responsible for managing authentication, authorization, and accounting for network devices. When a user attempts to access a network device, the device communicates with the TACACS server to verify the user's credentials, determine their level of access, and log the activity.

TACACS is often used in network environments where security and centralized control are crucial, such as in enterprise networks, data centers, and telecommunications infrastructure. There are different versions of TACACS, with TACACS+ (TACACS Plus) being a more advanced and secure version compared to the original TACACS. TACACS+ is widely used in modern network environments.

How does a TACACS server work?

The operation of a TACACS (Terminal Access Controller Access-Control System) server involves three main components: authentication, authorization, and accounting. Here's an overview of how a TACACS server works in the context of these components:


  • When a user attempts to access a network device, such as a router or switch, the device prompts the user for credentials (username and password).
  • The device sends an authentication request to the TACACS server, including the user's credentials.
  • The TACACS server verifies the user's credentials by checking its authentication database. This database may store usernames, corresponding passwords, and other relevant authentication information.
  • If the credentials are valid, the TACACS server sends an authentication success message back to the network device, allowing the user access. If the credentials are invalid, an authentication failure message is sent.


  • Upon successful authentication, the network device sends an authorization request to the TACACS server.
  • The authorization request typically includes information about the user, such as their identity and the requested action (e.g., executing a specific command).
  • The TACACS server consults its authorization database to determine whether the user is allowed to perform the requested action based on their assigned privileges.
  • The server sends an authorization response to the network device, indicating whether the action is authorized or denied. If authorized, it may also include information about the user's access level.


  • The TACACS server logs details of the user's activities, creating accounting records. This information includes user actions, timestamps, and other relevant data.
  • The accounting records are stored for auditing purposes, allowing administrators to review and analyze user activities on the network.
  • Accounting information can be useful for troubleshooting, compliance, and security monitoring.

It's important to note that TACACS operates over TCP (Transmission Control Protocol) and typically uses separate connections for authentication, authorization, and accounting. This separation enhances security and flexibility in managing user access.

TACACS+ is an improved version of TACACS that provides additional security features, encryption, and support for a wider range of authentication methods. While the original TACACS is largely deprecated, TACACS+ is commonly used in modern network environments.

Why use a TACACS server?

Using a TACACS (Terminal Access Controller Access-Control System) server provides several advantages in network environments, particularly in terms of security, centralized control, and auditing. Here are some reasons why organizations use TACACS servers:

Centralized Authentication:

  • TACACS allows for centralized authentication, meaning that user credentials are verified by a single authentication server. This centralization simplifies the management of user accounts and enhances security.

Authorization Control:

  • TACACS provides fine-grained authorization control. Administrators can define specific access levels and permissions for individual users or groups of users. This ensures that users have only the necessary privileges to perform their tasks, reducing the risk of unauthorized access or misuse of network resources.

Granular Accounting:

  • TACACS enables detailed accounting of user activities. It logs information such as who accessed the network, what actions were performed, and when the access occurred. This accounting data is valuable for auditing, compliance, and troubleshooting purposes.

Enhanced Security:

  • TACACS+ (an improved version of TACACS) encrypts the entire authentication packet, providing a higher level of security compared to some other authentication protocols. This helps protect sensitive information such as usernames and passwords during the authentication process.

Vendor-Specific Attributes:

  • TACACS supports vendor-specific attributes, allowing different vendors to implement their own attributes for additional features or customization. This flexibility is particularly useful in heterogeneous network environments where multiple vendors' equipment is used.

Device-Independent Access Control:

  • TACACS is commonly used to control access to a variety of network devices, including routers, switches, firewalls, and other infrastructure components. This device independence makes it a versatile solution for managing access in diverse network environments.

Audit Trail for Compliance:

  • The detailed accounting provided by TACACS servers helps organizations meet regulatory and compliance requirements. It allows administrators to maintain an audit trail of user activities, which is essential for demonstrating adherence to security policies and compliance standards.

User Accountability:

  • TACACS facilitates user accountability by associating specific actions with individual user accounts. This makes it easier to identify and address security incidents or policy violations.

Efficient Network Management:

  • Centralized authentication and authorization simplify network management tasks. Changes to user privileges or access levels can be implemented centrally on the TACACS server, reducing the complexity of managing access control on individual devices.

Customization and Flexibility:

  • TACACS allows for customization through vendor-specific attributes, providing flexibility to tailor access control policies to the specific needs of an organization.

In summary, using a TACACS server enhances the security, manageability, and accountability of network access. It is particularly valuable in large, complex network environments where centralized control and detailed auditing are essential for maintaining a secure and compliant infrastructure.

What's the difference between a TACACS+ server and RADIUS server?

TACACS+ (Terminal Access Controller Access-Control System Plus) and RADIUS (Remote Authentication Dial-In User Service) are both protocols used for network access control, but they have some key differences in terms of their design, functionality, and use cases. Here are the main distinctions between TACACS+ and RADIUS:

Protocol Purpose:

  • TACACS+: Originally designed by Cisco, TACACS+ is primarily focused on providing separate and detailed authentication, authorization, and accounting (AAA) services. It separates these functions into distinct processes.
  • RADIUS: Developed for dial-up networking scenarios, RADIUS integrates authentication and authorization, treating them as a single process. While it also supports accounting, the level of detail in accounting records may not be as extensive as in TACACS+.

AAA Separation:

  • TACACS+: Separates authentication, authorization, and accounting into three independent and customizable processes. This allows for more granular control over each aspect of access management.
  • RADIUS: Combines authentication and authorization into a single process. While accounting is supported, the level of granularity in authorization might not be as fine-tuned as in TACACS+.


  • TACACS+: Encrypts the entire packet, providing a higher level of security. This includes encrypting the usernames, passwords, and other sensitive information exchanged during the authentication process.
  • RADIUS: Typically encrypts only the password, leaving other parts of the packet unencrypted. RADIUS can use additional protocols like EAP (Extensible Authentication Protocol) for more secure communication.

Vendor Specific Attributes:

  • TACACS+: Supports vendor-specific attributes, allowing different vendors to implement their own attributes for additional features or customization.
  • RADIUS: Also supports vendor-specific attributes, but TACACS+ is often considered more flexible in this regard.

Connection Model:

  • TACACS+: Connection-oriented protocol, typically using TCP (Transmission Control Protocol) for communication.
  • RADIUS: Connectionless protocol, using UDP (User Datagram Protocol) for communication. The connectionless nature can make RADIUS more efficient for certain types of transactions.

Usage and Vendor Support:

  • TACACS+: Historically associated with Cisco devices, but it's not limited to Cisco. TACACS+ is widely used in enterprise environments for managing network devices.
  • RADIUS: More commonly used in a broader range of network scenarios, including dial-up and wireless networks. It has broader industry support and is widely implemented in various networking equipment and systems.

In summary, TACACS+ and RADIUS serve similar purposes in terms of network access control, but TACACS+ is often favored in environments where fine-grained control over AAA processes and encryption of the entire packet are crucial. RADIUS, on the other hand, is more commonly used in a variety of networking scenarios due to its broader support and integration capabilities. The choice between TACACS+ and RADIUS depends on the specific requirements and preferences of the network environment.