



Internet Engineering Task Force (IETF)                           T. Dahm
Request for Comments: 9887                                              
Updates: 8907                                                 J. Heasley
Category: Standards Track                                            NTT
ISSN: 2070-1721                                         D.C. Medway Gash
                                                     Cisco Systems, Inc.
                                                                  A. Ota
                                                             Google Inc.
                                                           December 2025


    Terminal Access Controller Access-Control System Plus (TACACS+)
                              over TLS 1.3

Abstract

   This document specifies the use of Transport Layer Security (TLS)
   version 1.3 to secure the communication channel between a Terminal
   Access Controller Access-Control System Plus (TACACS+) client and
   server.  TACACS+ is a protocol used for Authentication,
   Authorization, and Accounting (AAA) in networked environments.  The
   original TACACS+ protocol does not mandate the use of encryption or
   secure transport.  This specification defines a profile for using TLS
   1.3 with TACACS+, including guidance on authentication, connection
   establishment, and operational considerations.  The goal is to
   enhance the confidentiality, integrity, and authenticity of TACACS+
   traffic, aligning the protocol with modern security best practices.

   This document updates RFC 8907.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9887.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Technical Definitions
     2.1.  Requirements Language
   3.  TACACS+ over TLS
     3.1.  Separating TLS Connections
     3.2.  TLS Connection
     3.3.  TLS Authentication Options
     3.4.  TLS Certificate-Based Authentication
       3.4.1.  TLS Certificate Path Verification
       3.4.2.  TLS Certificate Identification
       3.4.3.  Cipher Suites Requirements
     3.5.  TLS PSK Authentication
     3.6.  TLS Resumption
   4.  Obsolescence of TACACS+ Obfuscation
   5.  Security Considerations
     5.1.  TLS
       5.1.1.  TLS Use
       5.1.2.  TLS 0-RTT
       5.1.3.  TLS Options
       5.1.4.  Unreachable Certificate Authority (CA)
       5.1.5.  TLS Server Name Indicator (SNI)
       5.1.6.  Server Identity Wildcards
     5.2.  TACACS+ Configuration
     5.3.  Well-Known TCP/IP Port Number
   6.  Operational Considerations
     6.1.  Migration
     6.2.  Maintaining Non-TLS TACACS+ Clients
     6.3.  YANG Model for TACACS+ Clients
   7.  IANA Considerations
   8.  Acknowledgments
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Authors' Addresses

1.  Introduction

   "The Terminal Access Controller Access-Control System Plus (TACACS+)
   Protocol" [RFC8907] provides device administration for routers,
   network access servers, and other networked computing devices via one
   or more centralized TACACS+ servers.  The protocol provides
   authentication, authorization, and accounting services (AAA) for
   TACACS+ clients within the device administration use case.

   The content of the protocol is highly sensitive and requires secure
   transport to safeguard a deployment.  However, TACACS+ lacks
   effective confidentiality, integrity, and authentication of the
   connection and network traffic between the TACACS+ server and client.
   The security mechanisms as described in Section 4.5 of [RFC8907] are
   extremely weak.

   To address these deficiencies, this document updates the TACACS+
   protocol to use TLS 1.3 authentication and encryption [RFC8446], and
   obsoletes the use of TACACS+ obfuscation mechanisms.  The maturity of
   TLS in version 1.3 and above makes it a suitable choice for the
   TACACS+ protocol.

2.  Technical Definitions

   The terms defined in Section 3 of [RFC8907] are fully applicable here
   and will not be repeated.  The following terms are also used in this
   document.

   Obfuscation:  TACACS+ was originally intended to incorporate a
      mechanism for securing the body of its packets.  The algorithm is
      categorized as obfuscation in Section 10.5.2 of [RFC8907].  The
      term is used to ensure that the algorithm is not mistaken for
      encryption.  It should not be considered secure.

   Non-TLS connection:  This term refers to the connection defined in
      [RFC8907].  It is a connection without TLS, using the unsecure
      TACACS+ authentication and obfuscation (or the unobfuscated option
      for testing).  The use of well-known TCP/IP host port number 49 is
      specified as the default for non-TLS connections.

   TLS connection:  A TLS connection is a TCP/IP connection with TLS
      authentication and encryption used by TACACS+ for transport.  A
      TLS connection for TACACS+ is always between one TACACS+ client
      and one TACACS+ server.

   TLS TACACS+ server:  This document describes a variant of the TACACS+
      server, introduced in Section 3.2 of [RFC8907], which utilizes TLS
      for transport, and makes some associated protocol optimizations.
      Both server variants respond to TACACS+ traffic, but this document
      specifically defines a TACACS+ server (whether TLS or non-TLS) as
      being bound to a specific port number on a particular IP address
      or hostname.  This definition is important in the context of the
      configuration of TACACS+ clients to ensure they direct their
      traffic to the correct TACACS+ servers.

   Peer:  The peer of a TACACS+ client (or server) in the context of a
      TACACS+ connection, is a TACACS+ server (or client).  Together,
      the ends of a TACACS+ connection are referred to as peers.

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  TACACS+ over TLS

   TACACS+ over TLS takes the protocol defined in [RFC8907], removes the
   option for obfuscation, and specifies that TLS 1.3 be used for
   transport (Section 3.1 elaborates on TLS version support).  A new
   well-known default host port number is used.  The next sections
   provide further details and guidance.

   TLS is introduced into TACACS+ to fulfill the following requirements:

   1.  Confidentiality and Integrity: The MD5 algorithm underlying the
       obfuscation mechanism specified in [RFC8907] has been shown to be
       insecure [RFC6151] when used for encryption.  This prevents
       TACACS+ from being used in a deployment compliant with
       [FIPS-140-3].  Securing the TACACS+ protocol with TLS is intended
       to provide confidentiality and integrity without requiring the
       provision of a secured network.

   2.  Peer authentication: The authentication capabilities of TLS
       replace the shared secrets of obfuscation for mutual
       authentication.

   This document adheres to the recommendations in [REQ-TLS13].

3.1.  Separating TLS Connections

   Peers implementing the TACACS+ protocol variant defined in this
   document MUST apply mutual authentication and encrypt all data
   exchanged between them.  Therefore, when a TCP connection is
   established for the service, a TLS handshake begins immediately.
   Options that upgrade an initial non-TLS connection MUST NOT be used;
   see Section 5.3.

   To ensure clear separation between TACACS+ traffic using TLS and that
   which does not (see Section 5.3), servers supporting TACACS+ over TLS
   MUST listen on a TCP/IP port distinct from that used by non-TLS
   TACACS+ servers.  It is further RECOMMENDED to deploy the TLS and
   non-TLS services on separate hosts, as discussed in Section 5.1.1.

   Given the prevalence of default port usage in existing TACACS+ client
   implementations, this specification assigns the well-known TCP port
   number 300 for TACACS+ over TLS (see Section 7).

   While the use of the designated port number is strongly encouraged,
   deployments with specific requirements MAY use alternative TCP port
   numbers.  In such cases, operators must carefully consider the
   operational implications described in Section 5.3.

3.2.  TLS Connection

   A TACACS+ client initiates a TLS connection by making a TCP
   connection to a configured TLS TACACS+ server on the TACACS+ TLS port
   number.  Once the TCP connection is established, the client MUST
   immediately begin the TLS negotiation before sending any TACACS+
   protocol data.

   A minimum of TLS 1.3 [RFC8446] MUST be used for transport.  It is
   expected that TACACS+, as described in this document, will work with
   future versions of TLS.  Earlier versions of TLS MUST NOT be used.

   Once the TLS connection has been successfully established, the
   exchange of TACACS+ data MUST proceed in accordance with the
   procedures defined in [RFC8907].  However, all TACACS+ messages SHALL
   be transmitted as TLS application data.  The TACACS+ obfuscation
   mechanism defined in [RFC8907] MUST NOT be applied when operating
   over TLS (Section 4).

   TLS TACACS+ connections are generally not long-lived.  The connection
   will be closed by either a peer if it encounters an error or an
   inactivity timeout.

   For connections not operating in Single Connection Mode (as defined
   in Section 4.3 of [RFC8907]), the TCP session SHALL be closed upon
   completion of the associated TACACS+ session.  Connections operating
   in Single Connection Mode MAY persist for a longer duration but are
   typically subject to timeout and closure after a brief period of
   inactivity.  Consequently, support for transport-layer keepalive
   mechanisms is not required.

   Why a connection is closed has no bearing on TLS resumption, unless
   closed by a TLS error, in which case it is possible that the ticket
   has been invalidated.

   TACACS+ clients and servers widely support IPv6 configuration in
   addition to IPv4.  This document makes no changes to recommendations
   in this area.

3.3.  TLS Authentication Options

   Implementations MUST support certificate-based mutual authentication,
   to provide a core option for interoperability between deployments.
   This authentication option is specified in Section 3.4.

   In addition to certificate-based TLS authentication, implementations
   MAY support the following alternative authentication mechanisms:

   *  Pre-Shared Keys (PSKs) (Section 3.5), also known as external PSKs
      in TLS 1.3.

   *  Raw Public Keys (RPKs).  The details of RPKs are considered out of
      scope for this document.  Refer to [RFC7250] and Section 4.4.2 of
      [RFC8446] for implementation, deployment, and security
      considerations.

3.4.  TLS Certificate-Based Authentication

   TLS certificate authentication is the primary authentication option
   for TACACS+ over TLS.  This section covers certificate-based
   authentication only.

   Deploying TLS certificate-based authentication correctly will
   considerably improve the security of TACACS+ deployments.  It is
   essential for implementers and operators to understand the
   implications of a TLS certificate-based authentication solution,
   including the correct handling of certificates, Certificate
   Authorities (CAs), and all elements of TLS configuration.  For
   guidance, start with [BCP195].

   Each peer MUST validate the certificate path of its remote peer,
   including revocation checking, as described in Section 3.4.1.

   If the verification succeeds, the authentication is successful and
   the connection is permitted.  Policy may impose further constraints
   upon the peer, allowing or denying the connection based on
   certificate fields or any other parameters exposed by the
   implementation.

   Unless disabled by configuration, a peer MUST NOT permit connection
   of any peer that presents an invalid TLS certificate.

3.4.1.  TLS Certificate Path Verification

   The implementation of certificate-based mutual authentication MUST
   support certificate path validation as described in Section 6 of
   [RFC5280].

   In some deployments, a peer may be isolated from a remote peer's CA.
   Implementations for these deployments MUST support certificate chains
   (aka bundles or chains of trust), where the entire chain of the
   remote peer's certificate is stored on the local peer.

   TLS Cached Information Extension [RFC7924] SHOULD be implemented.
   This MAY be augmented with RPKs [RFC7250], though revocation must be
   handled as it is not part of that specification.

   Other approaches may be used for loading the intermediate
   certificates onto the client, but they MUST include support for
   revocation checking.  For example, [RFC5280] details the Authority
   Information Access (AIA) extension to provide information about the
   issuer of the certificate in which the extension appears.  It can be
   used to provide the address of the Online Certificate Status Protocol
   (OCSP) responder from where the revocation status of the certificate
   (which includes the extension) can be checked.

3.4.2.  TLS Certificate Identification

   For the client-side validation of presented TLS TACACS+ server
   identities, implementations MUST follow the validation techniques
   defined in [RFC9525].  Identifier types DNS-ID, IP-ID, or SRV-ID are
   applicable for use with the TLS TACACS+ protocol; they are selected
   by operators depending upon the deployment design.  TLS TACACS+ does
   not use URI-IDs for TLS TACACS+ server identity verification.

   Wildcards in TLS TACACS+ server identities simplify certificate
   management by allowing a single certificate to secure multiple
   servers in a deployment.  However, this introduces security risks, as
   compromising the private key of a wildcard certificate impacts all
   servers using it.  To address these risks, the guidelines in
   Section 6.3 of [RFC9525] MUST be followed, and the wildcard SHOULD be
   confined to a subdomain dedicated solely to TACACS+ servers.

   For the TLS TACACS+ server-side validation of client identities,
   implementations MUST support the ability to configure which fields of
   a certificate are used for client identification to verify that the
   client is a valid source for the received certificate and that it is
   permitted access to TACACS+. Implementations MUST support either:

   *  Network-address-based validation methods as described in
      Section 5.2 of [RFC5425] or

   *  Client Identity validation of a shared identity in the certificate
      subjectAltName.  This is applicable in deployments where the
      client securely supports an identity which is shared with the TLS
      TACACS+ server.  Matching of dNSName and iPAddress MUST be
      supported.  Other options defined in Section 4.2.1.6 of [RFC5280]
      MAY be supported.  This approach allows a client's network
      location to be reconfigured without issuing a new client
      certificate.

   Implementations MUST support the TLS Server Name Indication (SNI)
   extension (Section 3 of [RFC6066]).  TLS TACACS+ clients MUST support
   the ability to configure the TLS TACACS+ server's domain name, so
   that it may be included in the SNI "server_name" extension of the
   client hello (This is distinct from the IP Address or hostname
   configuration used for the TCP connection).  Refer to Section 5.1.5
   for security related operator considerations.

   Certificate provisioning is out of scope of this document.

3.4.3.  Cipher Suites Requirements

   Implementations MUST support the TLS 1.3 mandatory cipher suites
   (Section 9.1 of [RFC8446]).  Readers should refer to [BCP195].  The
   cipher suites offered or accepted SHOULD be configurable so that
   operators can adapt.

3.5.  TLS PSK Authentication

   As an alternative to certificate-based authentication,
   implementations MAY support PSKs, also known as external PSKs in TLS
   1.3 [RFC8446].  These should not be confused with resumption PSKs.

   The use of external PSKs is less well established than certificate-
   based authentication.  It is RECOMMENDED that systems follow the
   directions of [RFC9257] and Section 4 of [RFC8446].

   Where PSK authentication is implemented, PSK lengths of at least 16
   octets MUST be supported.

   PSK identity MUST follow recommendations of Section 6.1 of [RFC9257].
   Implementations MUST support PSK identities of at least 16 octets.

   Although this document removes the option of obfuscation (Section 4),
   it is still possible that the TLS and non-TLS versions of TACACS+
   exist in an organization, for example, during migration
   (Section 6.1).  In such cases, the shared secrets configured for
   TACACS+ obfuscation clients MUST NOT be the same as the PSKs
   configured for TLS clients.


3.6.  TLS Resumption

   TLS Resumption [RFC8446] can minimize the number of round trips
   required during the handshake process.  If a TLS client holds a
   ticket previously extracted from a NewSessionTicket message from the
   TLS TACACS+ server, it can use the PSK identity tied to that ticket.
   If the TLS TACACS+ server consents, the resumed session is
   acknowledged as authenticated and securely linked to the initial
   session.

   The client SHOULD use resumption when it holds a valid unused ticket
   from the TLS TACACS+ server, as each ticket is intended for a single
   use only and will be refreshed during resumption.  The TLS TACACS+
   server can reject a resumption request, but the TLS TACACS+ server
   SHOULD allow resumption if the ticket in question has not expired and
   has not been used before.

   When a TLS TACACS+ server is presented with a resumption request from
   the TLS client, it MAY still choose to require a full handshake.  In
   this case, the negotiation proceeds as if the session was a new
   authentication, and the resumption attempt is ignored.  As described
   in Appendix C.4 of [RFC8446], reuse of a ticket allows passive
   observers to correlate different connections.  TLS TACACS+ clients
   and servers SHOULD follow the client tracking preventions in
   Appendix C.4 of [RFC8446].

   When processing TLS resumption, certificates must be verified to
   check for revocation during the period since the last
   NewSessionTicket Message.

   The resumption ticket_lifetime SHOULD be configurable, including a
   zero seconds lifetime.  Refer to Section 4.6.1 of [RFC8446] for
   guidance on ticket lifetime.

4.  Obsolescence of TACACS+ Obfuscation

   The obfuscation mechanism documented in Section 4.5 of [RFC8907] is
   weak.

   The introduction of TLS authentication and encryption to TACACS+
   replaces this former mechanism, so obfuscation is hereby obsoleted.
   This section describes how the TACACS+ client and servers MUST
   operate regarding the obfuscation mechanism.

   Peers MUST NOT use obfuscation with TLS.

   A TACACS+ client initiating a TACACS+ TLS connection MUST set the
   TAC_PLUS_UNENCRYPTED_FLAG bit, thereby asserting that obfuscation is
   not used for the session.  All subsequent packets MUST have the
   TAC_PLUS_UNENCRYPTED_FLAG bit set to 1.

   A TLS TACACS+ server that receives a packet with the
   TAC_PLUS_UNENCRYPTED_FLAG bit not set to 1 over a TLS connection MUST
   return an error of TAC_PLUS_AUTHEN_STATUS_ERROR,
   TAC_PLUS_AUTHOR_STATUS_ERROR, or TAC_PLUS_ACCT_STATUS_ERROR as
   appropriate for the TACACS+ message type, with the
   TAC_PLUS_UNENCRYPTED_FLAG bit set to 1, and terminate the session.
   This behavior corresponds to that defined in Section 4.5 of [RFC8907]
   regarding data obfuscation for TAC_PLUS_UNENCRYPTED_FLAG or key
   mismatches.

   A TACACS+ client that receives a packet with the
   TAC_PLUS_UNENCRYPTED_FLAG bit not set to 1 MUST terminate the
   session, and SHOULD log this error.

5.  Security Considerations

5.1.  TLS

   This document improves the confidentiality, integrity, and
   authentication of the connection and network traffic between TACACS+
   peers by adding TLS support.

   Simply adding TLS support to the protocol does not guarantee the
   protection of the TLS TACACS+ server and clients.  It is essential
   for the operators and equipment vendors to adhere to the latest best
   practices for ensuring the integrity of network devices and selecting
   secure TLS key and encryption algorithms.

   [BCP195] offers substantial guidance for implementing and deploying
   protocols that use TLS.  Those implementing and deploying Secure
   TACACS+ must adhere to the recommendations relevant to TLS 1.3
   outlined in [BCP195] or its subsequent versions.

   This document outlines additional restrictions permissible under
   [BCP195].  For example, any recommendations referring to TLS 1.2,
   including the mandatory support, are not relevant for Secure TACACS+,
   as TLS 1.3 or above is mandated.

   This document concerns the use of TLS as transport for TACACS+ and
   does not make any changes to the core TACACS+ protocol, other than
   the direct implications of deprecating obfuscation.  Operators MUST
   be cognizant of the security implications of the TACACS+ protocol
   itself.  Further documents are planned, for example, to address the
   security implications of password-based authentication and enhance
   the protocol to accommodate alternative schemes.

5.1.1.  TLS Use

   New TACACS+ production deployments SHOULD use TLS authentication and
   encryption.  Also see [RFC3365].

   TLS TACACS+ servers (as defined in Section 2) MUST NOT allow non-TLS
   connections, because of the threat of downgrade attacks or
   misconfiguration described in Section 5.3.  Instead, separate non-TLS
   TACACS+ servers SHOULD be set up to cater for these clients.

   It is NOT RECOMMENDED that TLS TACACS+ servers and non-TLS TACACS+
   servers be deployed on the same host, for reasons discussed in
   Section 5.3.  Non-TLS connections would be better served by deploying
   the required non-TLS TACACS+ servers on separate hosts.

   TACACS+ clients MUST NOT fail back to a non-TLS connection if a TLS
   connection fails.  This prohibition includes during the migration of
   a deployment (Section 6.1).

5.1.2.  TLS 0-RTT

   TLS 1.3 resumption and PSK techniques make it possible to send early
   data, aka 0-RTT data, data that is sent before the TLS handshake
   completes.  Replay of this data is a risk.  Given the sensitivity of
   TACACS+ data, clients MUST NOT send data until the full TLS handshake
   completes; that is, clients MUST NOT send 0-RTT data and TLS TACACS+
   servers MUST abruptly disconnect clients that do.

   TLS TACACS+ clients and servers MUST NOT include the "early_data"
   extension.  See Sections 2.3 and 4.2.10 of [RFC8446] for security
   concerns.

5.1.3.  TLS Options

   Recommendations in [BCP195] MUST be followed to determine which TLS
   versions and algorithms should be supported, deprecated, obsoleted,
   or abandoned.

   Also, Section 9 of [RFC8446] prescribes mandatory supported options.

5.1.4.  Unreachable Certificate Authority (CA)

   Operators should be cognizant of the potential of TLS TACACS+ server
   and/or client isolation from their peer's CA by network failures.
   Isolation from a public key certificate's CA will cause the
   verification of the certificate to fail and thus TLS authentication
   of the peer to fail.  The approach mentioned in Section 3.4.1 can
   help address this and should be considered.

5.1.5.  TLS Server Name Indicator (SNI)

   Operators should be aware that the TLS SNI extension is part of the
   TLS client hello, which is sent in cleartext.  It is, therefore,
   subject to eavesdropping.  Also see Section 11.1 of [RFC6066].

5.1.6.  Server Identity Wildcards

   The use of wildcards in TLS server identities creates a single point
   of failure: a compromised private key of a wildcard certificate
   impacts all servers using it.  Their use MUST follow the
   recommendations of Section 7.1 of [RFC9525].  Operators MUST ensure
   that the wildcard is limited to a subdomain dedicated solely to TLS
   TACACS+ servers.  Further, operators MUST ensure that the TLS TACACS+
   servers covered by a wildcard certificate are impervious to
   redirection of traffic to a different server (for example, due to on-
   path attacks or DNS cache poisoning).

5.2.  TACACS+ Configuration

   Implementors must ensure that the configuration scheme introduced for
   enabling TLS is straightforward and leaves no room for ambiguity
   regarding whether TLS or non-TLS will be used between the TACACS+
   client and the TACACS+ server.

   This document recommends the use of a separate port number that TLS
   TACACS+ servers will listen to.  Where deployments have not
   overridden the defaults explicitly, TACACS+ client implementations
   MUST use the correct port values:

   *  49: for non-TLS connection TACACS+

   *  300: for TLS connection TACACS+

   Implementors may offer a single option for TACACS+ clients and
   servers to disable all non-TLS TACACS+ operations.  When enabled on a
   TACACS+ server, it will not respond to any requests from non-TLS
   TACACS+ client connections.  When enabled on a TACACS+ client, it
   will not establish any non-TLS TACACS+ server connections.

5.3.  Well-Known TCP/IP Port Number

   A new port number is considered appropriate (rather than a mechanism
   that negotiates an upgrade from an initial non-TLS TACACS+
   connection) because it allows:

   *  ease of blocking the unobfuscated or obfuscated connections by the
      TCP/IP port number,

   *  passive Intrusion Detection Systems (IDSs) monitoring the
      unobfuscated to be unaffected by the introduction of TLS,

   *  avoidance of on-path attacks that can interfere with upgrade, and

   *  prevention of the accidental exposure of sensitive information due
      to misconfiguration.

   However, the coexistence of inferior authentication and obfuscation,
   whether a non-TLS connection or deprecated parts that compose TLS,
   also presents an opportunity for downgrade attacks.  Causing failure
   of connections to the TLS-enabled service or the negotiation of
   shared algorithm support are two such downgrade attacks.

   The simplest mitigation exposure from non-TLS connection methods is
   to refuse non-TLS connections at the host entirely, perhaps using
   separate hosts for non-TLS connections and TLS.

   Another approach is mutual configuration that requires TLS.  TACACS+
   clients and servers SHOULD support configuration that requires peers,
   globally and individually, to use TLS.  Furthermore, peers SHOULD be
   configurable to limit offered or recognized TLS versions and
   algorithms to those recommended by standards bodies and implementers.

6.  Operational Considerations

   Operational and deployment considerations are spread throughout the
   document.  While avoiding repetition, it is useful for the impatient
   to direct particular attention to Sections 5.2 and 5.1.5.  However,
   it is important that the entire Section 5 is observed.

   It is essential for operators to understand the implications of a TLS
   certificate-based authentication solution, including the correct
   handling of certificates, CAs, and all elements of TLS configuration.
   Refer to [BCP195] for guidance.  Attention is drawn to the
   provisioning of certificates to all peers, including TACACS+ TLS
   clients, to permit the mandatory mutual authentication.

6.1.  Migration

   Section 5.2 mentions that for an optimal deployment of TLS TACACS+,
   TLS should be universally applied throughout the deployment.
   However, during the migration process from a non-TLS TACACS+
   deployment, operators may need to support both TLS and non-TLS
   TACACS+ servers.  This migration phase allows operators to gradually
   transition their deployments from an insecure state to a more secure
   one, but it is important to note that it is vulnerable to downgrade
   attacks.  Therefore, the migration phase should be considered
   insecure until it is fully completed.  To mitigate this hazard:

   *  The period where any client is configured with both TLS and non-
      TLS TACACS+ servers should be minimized.

   *  The operator must consider the security impact of supporting both
      TLS and non-TLS connections, as mentioned above.

6.2.  Maintaining Non-TLS TACACS+ Clients

   Some TACACS+ client devices in a deployment may not implement TLS.
   These devices will require access to non-TLS TACACS+ servers.
   Operators must follow the recommendation of Section 5.1.1 and deploy
   separate non-TLS TACACS+ servers for these non-TLS clients from those
   used for the TLS clients.

6.3.  YANG Model for TACACS+ Clients

   [TACACS-YANG] specifies a YANG model for managing TACACS+ clients,
   including TLS support.

7.  IANA Considerations

   IANA has allocated the following new well-known system in the
   "Service Name and Transport Protocol Port Number Registry" (see
   <https://www.iana.org/assignments/service-names-port-numbers>).  The
   service name "tacacss" follows the common practice of appending an
   "s" to the name given to the non-TLS well-known port name.  See the
   justification for the allocation in Section 5.3.

   Service Name:  tacacss
   Port Number:  300
   Transport Protocol:  TCP
   Description:  TLS Secure Login Host Protocol (TACACSS)
   Assignee:  IESG
   Contact:  IETF Chair
   Reference:  RFC 9887

   Considerations about service discovery are out of scope of this
   document.

8.  Acknowledgments

   The author(s) would like to thank Russ Housley, Steven M. Bellovin,
   Stephen Farrell, Alan DeKok, Warren Kumari, Tom Petch, Tirumal Reddy,
   Valery Smyslov, and Mohamed Boucadair for their support, insightful
   review, and/or comments.  [RFC5425] was also used as a basis for the
   general approach to TLS.  [RFC9190] was used as a basis for TLS
   resumption recommendations.  Although still in draft form at the time
   of writing, [RFC9813] was used as a model for PSK recommendations.

9.  References

9.1.  Normative References

   [BCP195]   Best Current Practice 195,
              <https://www.rfc-editor.org/info/bcp195>.
              At the time of writing, this BCP comprises the following:

              Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
              <https://www.rfc-editor.org/info/rfc8996>.

              Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5425]  Miao, F., Ed., Ma, Y., Ed., and J. Salowey, Ed.,
              "Transport Layer Security (TLS) Transport Mapping for
              Syslog", RFC 5425, DOI 10.17487/RFC5425, March 2009,
              <https://www.rfc-editor.org/info/rfc5425>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <https://www.rfc-editor.org/info/rfc7924>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8907]  Dahm, T., Ota, A., Medway Gash, D.C., Carrel, D., and L.
              Grant, "The Terminal Access Controller Access-Control
              System Plus (TACACS+) Protocol", RFC 8907,
              DOI 10.17487/RFC8907, September 2020,
              <https://www.rfc-editor.org/info/rfc8907>.

   [RFC9525]  Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              RFC 9525, DOI 10.17487/RFC9525, November 2023,
              <https://www.rfc-editor.org/info/rfc9525>.

9.2.  Informative References

   [FIPS-140-3]
              NIST, "Security Requirements for Cryptographic Modules",
              NIST FIPS 140-3, DOI 10.6028/NIST.FIPS.140-3, March 2019,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.140-3.pdf>.

   [REQ-TLS13]
              Salz, R. and N. Aviram, "New Protocols Using TLS Must
              Require TLS 1.3", Work in Progress, Internet-Draft, draft-
              ietf-uta-require-tls13-12, 14 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              require-tls13-12>.

   [RFC3365]  Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61,
              RFC 3365, DOI 10.17487/RFC3365, August 2002,
              <https://www.rfc-editor.org/info/rfc3365>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

   [RFC9190]  Preuß Mattsson, J. and M. Sethi, "EAP-TLS 1.3: Using the
              Extensible Authentication Protocol with TLS 1.3",
              RFC 9190, DOI 10.17487/RFC9190, February 2022,
              <https://www.rfc-editor.org/info/rfc9190>.

   [RFC9257]  Housley, R., Hoyland, J., Sethi, M., and C. A. Wood,
              "Guidance for External Pre-Shared Key (PSK) Usage in TLS",
              RFC 9257, DOI 10.17487/RFC9257, July 2022,
              <https://www.rfc-editor.org/info/rfc9257>.

   [RFC9813]  DeKok, A., "Operational Considerations for Using TLS Pre-
              Shared Keys (TLS-PSKs) with RADIUS", BCP 243, RFC 9813,
              DOI 10.17487/RFC9813, July 2025,
              <https://www.rfc-editor.org/info/rfc9813>.

   [TACACS-YANG]
              Boucadair, M. and B. Wu, "A YANG Data Model for Terminal
              Access Controller Access-Control System Plus (TACACS+)",
              Work in Progress, Internet-Draft, draft-ietf-opsawg-
              secure-tacacs-yang-13, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-
              secure-tacacs-yang-13>.

Authors' Addresses

   Thorsten Dahm
   Email: thorsten.dahm@gmail.com


   John Heasley
   NTT
   Email: heas@shrubbery.net


   Douglas C. Medway Gash
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA 95134
   United States of America
   Email: dcmgash@cisco.com


   Andrej Ota
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America
   Email: andrej@ota.si
