Operations and Management Area Working Group
Internet Engineering Task Force (IETF) T. Dahm
Internet-Draft
Request for Comments: 9887
Updates: 8907 (if approved) J. Heasley
Intended status:
Category: Standards Track NTT
Expires: 10 January 2026
ISSN: 2070-1721 D.C. Medway Gash
Cisco Systems, Inc.
A. Ota
Google Inc.
9 July
October 2025
Terminal Access Controller Access-Control System Plus (TACACS+)
over TLS 1.3
(TACACS+ over TLS)
draft-ietf-opsawg-tacacs-tls13-24
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, 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.
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 10 January 2026.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Technical Definitions . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language
3. TACACS+ over TLS . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Separating TLS Connections . . . . . . . . . . . . . . . 5
3.2. TLS Connection . . . . . . . . . . . . . . . . . . . . . 5
3.3. TLS Authentication Options . . . . . . . . . . . . . . . 6
3.4. TLS Certificate-Based Authentication . . . . . . . . . . 6
3.4.1. TLS Certificate Path Verification . . . . . . . . . . 7
3.4.2. TLS Certificate Identification . . . . . . . . . . . 8
3.4.3. Cipher Suites Requirements . . . . . . . . . . . . . 9
3.5. TLS PSK Authentication . . . . . . . . . . . . . . . . . 9
3.6. TLS Resumption . . . . . . . . . . . . . . . . . . . . . 9
4. Obsolescence of TACACS+ Obfuscation . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1. TLS Use . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.2. TLS 0-RTT . . . . . . . . . . . . . . . . . . . . . . 12
5.1.3. TLS Options . . . . . . . . . . . . . . . . . . . . . 12
5.1.4. Unreachable Certification Authority (CA) . . . . . . 12
5.1.5. TLS Server Name Indicator (SNI) . . . . . . . . . . . 12
5.1.6. Server Identity Wildcards . . . . . . . . . . . . . . 12
5.2. TACACS+ Configuration . . . . . . . . . . . . . . . . . . 13
5.3. Well-Known TCP/IP Port Number . . . . . . . . . . . . . . 13
6. Operational Considerations . . . . . . . . . . . . . . . . . 14
6.1. Migration . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Maintaining Non-TLS TACACS+ Clients . . . . . . . . . . . 15
6.3. YANG Model for TACACS+ Clients . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9. References
9.1. Normative References . . . . . . . . . . . . . . . . . . . . 16
10.
9.2. Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
The
"The Terminal Access Controller Access-Control System Plus (TACACS+)
Protocol
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 authorization, and accounting services (AAA) for
TACACS+ clients within the device administration use case.
While the content of the protocol is highly sensitive, TACACS+ lacks
effective confidentiality, integrity, and authentication of the
connection and network traffic between the TACACS+ server and client,
requiring secure transport to safeguard a deployment. The security
mechanisms as described in Section 10 of [RFC8907] are extremely
weak.
To address these deficiencies, this document updates the TACACS+
protocol to use TLS 1.3 [RFC8446] authentication and encryption, encryption [RFC8446], and
obsoletes the use of TACACS+ obfuscation mechanisms (Section 10.5 of
[RFC8907]). 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 test). The use of well-known TCP/IP host port number 49 is
specified as the default for Non-TLS 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) 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, 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 MD5 obfuscation, and specifies that TLS 1.3 be used for
transport (Section 3.1 elaborates on TLS version support). A new well-
known
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 [FIPS-140-3] - deployment compliant deployment. 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
[I-D.ietf-uta-require-tls13]. [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 which that upgrade an initial Non-TLS connection, non-TLS connection MUST NOT be
used, 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 a well-known TCP port
number for TACACS+ over TLS: [TBD] (Section 7), 300, with the associated service name
"tacacss" (see Section 7. 7). This allows clients to unambiguously
distinguish between TLS and non-TLS connections, even in the absence
of an explicitly configured port number.
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.
Minimum
A minimum of TLS 1.3 [RFC8446] MUST be used for transport, it transport. It is
expected that TACACS+ TACACS+, as described in this document 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], [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).
The connection persists until the TLS TACACS+ peer closes it, either
due to an error, or at the conclusion of the TACACS+ session, or, if
Single Connection Mode (Section 4.3 of [RFC8907]) has been
negotiated, when an inactivity timeout occurs. Why it 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+ connections are generally not long-lived. For connections
not operating in Single Connection Mode (as defined in Section 4.3 of
[RFC8907])
[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.
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 RPK RPKs are considered out-of- 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 verification 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
(a.k.a.
(aka bundles or chains of trust), where the entire chain of the
remote's
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 the standard. 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 [RFC9525] the validation
techniques. techniques
defined in [RFC9525]. Identifier types DNS-ID, IP-ID, or SRV-ID are
applicable for use with the TLS TACACS+ protocol, protocol; they are selected
by operators depending upon the deployment design. TLS TACACS+ does
not use URI-
IDs 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, 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
* Network-address-based validation methods as described in
Section 5.2 of [RFC5425]. [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 extension (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 external PSKs in TLS
1.3 [RFC8446]. These should not be confused with resumption PSKs.
The use of External 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 authentication is implemented, PSK lengths of at least 16
octets MUST be supported.
PSK Identity 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 MD5 obfuscation
(Section 4), it is still possible that the TLS and Non-TLS non-TLS versions
of TACACS+ may 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
The TLS Resumption protocol, detailed in [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
[RFC8907] describes the obfuscation mechanism, documented in
Section 5.2 of [RFC5425]. Such a method is weak.
The introduction of TLS authentication and encryption to TACACS+
replaces this former mechanism and 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, 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 protocols that
use TLS and their deployment. 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]
[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+, 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 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 non-TLS
connections, because of the threat of downgrade attacks or
misconfiguration described in Section 5.3. Instead, separate Non-TLS non-TLS
TACACS+ servers SHOULD be set up to cater for these clients.
It is NOT RECOMMENDED that TLS TACACS+ servers and Non-TLS 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 non-TLS TACACS+ servers on separate hosts.
TACACS+ Clients clients MUST NOT fail back to a Non-TLS 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. 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 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 Certification 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 where implemented.
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 MUST be impervious to
redirection of traffic to a different server (for example, due to on-path 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 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 non-TLS connection TACACS+: Port number 49. TACACS+
* 300: for TLS connection TACACS+: (TBD). TACACS+
Implementors may offer a single option for TACACS+ clients and
servers to disable all Non-TLS non-TLS TACACS+ operations. When enabled on a
TACACS+ server, it will not respond to any requests from Non-TLS non-TLS
TACACS+ client connections. When enabled on a TACACS+ client, it
will not establish any Non-TLS 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 non-TLS TACACS+
Connection)
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, co-existence the coexistence of inferior authentication and obfuscated, obfuscation,
whether a Non-TLS non-TLS connection or deprecated parts that compose TLS,
also presents an opportunity for down-grade downgrade attacks. Causing failure
of connections to the TLS-enabled service or the negotiation of
shared algorithm support are two such down-grade downgrade attacks.
The simplest mitigation exposure from Non-TLS non-TLS connection methods is
to refuse Non-TLS non-TLS connections at the host entirely, perhaps using
separate hosts for Non-TLS 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 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 non-TLS TACACS+
deployment, operators may need to support both TLS and Non-TLS 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- non-
TLS TACACS+ servers should be minimized.
* The operator must consider the impact of mixed TLS and Non-TLS non-TLS on
security, 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 non-TLS TACACS+ servers.
Operators must follow the recommendation of Section 5.1.1 and deploy
separate Non-TLS non-TLS TACACS+ servers for these Non-TLS non-TLS clients from those
used for the TLS clients.
6.3. YANG Model for TACACS+ Clients
[ietf-opsawg-secure-tacacs-yang]
[TACACS-YANG] specifies a YANG model for managing TACACS+ clients,
including TLS support.
7. IANA Considerations
IANA (has allocated) is requested to allocate has allocated a new well-known system TCP/IP port number ([TBD]) (300)
for the service name "tacacss", described as "TACACS+ over TLS". The
service name "tacacss" follows the common practice of appending an
"s" to the name given to the Non-TLS well-
known non-TLS well-known port name. This
allocation is justified in Section 5.3.
IANA (has added) is requested to add tacacss as a new has added the following entry to the "Service name Name and Transport
Protocol Port Number Registry" available
at https://www.iana.org/assignments/service-names-port-numbers/ (see
<https://www.iana.org/assignments/service-names-port-numbers>).
Service Name: tacacss
Port Number: [TBD] 300
Transport Protocol: TCP
Description: TLS Secure Login Host Protocol (TACACSS)
Assignee: IESG
Contact: IETF Chair
Reference: [TBD] (This Document) RFC EDITOR: this port number should replace "[TBD]" within this
document. 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.
resumption recommendations. Although still in draft form at the time
of writing, [I-D.ietf-radext-tls-psk] [RFC9813] was used as a model for PSK
Recommendations. 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>.
10.
9.2. Informative References
[FIPS-140-3]
National Institute of Standards and Technology, U.S.
Department of Commerce, "NIST Federal Information
Processing Standards (FIPS) Publication 140-3",
<https://csrc.nist.gov/pubs/fips/140-3/final>.
[I-D.ietf-radext-tls-psk]
DeKok, A., "Operational Considerations
NIST, "Security Requirements for RADIUS and TLS-
PSK", Work in Progress, Internet-Draft, draft-ietf-radext-
tls-psk-12, 21 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-radext-
tls-psk-12>.
[I-D.ietf-uta-require-tls13] 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>.
[ietf-opsawg-secure-tacacs-yang]
Boucadair, M., Ed., Wu, B., Zheng, G., and M. Wang, "A
YANG Data Model for Terminal Access Controller Access-
Control System Plus (TACACS+)",
<https://datatracker.ietf.org/doc/draft-ietf-opsawg-
secure-tacacs-yang/>.
[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