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Understanding Central Network Access Using RADIUS and TACACS+

Remote Access Dial In User Service (RADIUS) and Terminal Access Controller Access-Control System Plus (TACACS+) are two common security protocols used to provide centralized access into networks. RADIUS was designed to authenticate and log remote network users, while TACACS+ is most commonly used for administrator access to network devices like routers and switches. Both protocols provide centralized Authentication, Authorization, and Accounting (AAA) management for computers that connect and use a network service.

  • Authentication - Who is allowed to gain access to the network? Traditionally authorized users provide a username and password to verify their identity for both RADIUS and TACACS+.

  • Authorization - What services can a user access once they are authenticated? It is unlikely that you want your finance people to have access to the developer database. Visitors may have access only to the Internet, while only IT staff can access the entire passwords database.

  • Accounting - What services did each user access and for how long? Accounting records record the user's identification, network address, point of attachment and a unique session identifier—these statistics are tracked and added to the user’s record. This is useful when time on the system is billed to individuals or departments.

Why Do I Want Remote Authentication ?

Remote authentication enables you to keep your username and passwords in one place, on a central server. The advantage to using RADIUS or TACACS+ on this central server is that you don't configure changes on each separate network device when a user is added or deleted, or when a user changes a password. You only make one change to the configuration on the server and then devices continue to access the server for authentication. Although authentication is the most well known function of RADIUS and TACACS+, there are two additional functions provided, authorization and accounting.


Instead of using a flat database on the RADIUS server, you can refer to external sources such as SQL, Kerberos, LDAP, or Active Directory servers to verify user credentials.

Why Not Just Rely on Firewalls and Filters for Access Control?

Routers and firewalls usually control access to services using filters based on source and/or destination IP addresses and ports. This means that restrictions are applied to devices and not to individual clients. For example if I enable traffic from to access a particular web server, then anyone who is sitting at the machine with the address of automatically has access to this server. Using RADIUS or TACACS+, that same person sitting at the machine with the address of also has to provide a username and password to access a service.

What About Using LDAP For Authentication?

Lightweight Directory Access Protocol (LDAP) is a client/server protocol used to access and manage directory information. It reads and edits directories over IP networks and runs directly over TCP/IP using simple string formats for data transfer. Directory servers include information about various entities on your network, such as user names, passwords, rights associated with user names, metadata associated with user names, devices connected to the network, and device configuration.

Use LDAP to obtain directory information, such as email addresses and public keys. If you want to make directory information available over the Internet, this is the way to do it. LDAP works well for captive portal authentication. However, LDAP does not implement 802.1X security easily. 802.1X was essentially designed with RADIUS in mind, so 802.1X challenge/response protocols like MSCHAPv2 work well with RADIUS.

Where Is RADIUS Installed on the Network?

RADIUS includes three components: an authentication server, client protocols, and an accounting server. The RADIUS server portion of the protocol is usually a background process running on a UNIX or Microsoft Windows server.

With RADIUS, the term client refers to a network access device (NAD) that provides the client part of the RADIUS service—a modem pool, a switch, a network firewall, or any other device that needs to authenticate users can be configured as a NAD to recognize and process connection requests from outside the network edge. When a NAD receives a user's connection request, it may perform an initial access negotiation with the user to obtain identity/password information. Then the NAD passes this information to the RADIUS server as part of an authentication/authorization request.


RADIUS requires that each network client device be configured.

How Is TACACS+ Installed on the Network?

TACACS+ logon authentication protocol uses software running on a central server to control access by TACACS-aware devices on the network. The server communicates with switches or other TACACS-aware devices automatically—these devices do not require further configuration if they are TACACS-aware. The TACACS+ protocol is supported by most enterprise and carrier-grade devices.

Install the TACACS+ Service as close as possible to the user database, preferably on the same server. TACACS+ needs to be closely synchronized with your Domain, and any network connection issues, DNS problems, or even time discrepancies can cause a critical service failure. Installing TACACS+ on the same server as the user database can also improve performance.

TACACS+ servers should be deployed in a fully trusted internal network. If you keep your TACACS+ service within your trusted network, you need to open only one port, TCP 49. There should not be any direct access from untrusted or semi-trusted networks.


RADIUS is typically deployed in a semi-trusted network, and TACACS+ uses internal administrative logins, so combining these services on the same server could potentially compromise your network security.

A Comparison of RADIUS and TACACS+

Table 1: RADIUS and TACACS+



Primary Use

Authenticate and log remote network users

Provide administrator access to network devices like routers and switches

Authentication and Authorization

Authentication and Authorization checking are bundled together. When the client device requests authentication from the server, the server replies with both authentication attributes and authorization attributes. These functions can not be performed separately.

All three AAA functions (authentication, authorization, and accounting) can be used independently. Therefore, one method such as kerberos can be used for authentication, and a separate method such as TACACS+ can be used for authorization.


The accounting features of the RADIUS protocol can be used independently of RADIUS authentication or authorization.


User Datagram Protocol (UDP)/IP with best-effort is used for delivery on ports 1645/1646, 1812/1813

TCP used for delivery on port 49. Also has multiprotocol support for AppleTalk Remote Access (ARA) protocol, NetBIOS Frame Protocol Control protocol, Novell Asynchronous Services Interface (NASI), and X.25 PAD connection.

Encryption applied to


Username and password

802.1X Security

If you want to use 802.1x port-based network access control, you have to use the RADIUS client because the TACACS+ client does not support that feature.




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