TACACS+ Security, TLS, and SSH Public Keys
draft-dahm-tacacs-security-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Thorsten Dahm , dcmgash@cisco.com , Andrej Ota , John Heasley | ||
| Last updated | 2022-03-31 | ||
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draft-dahm-tacacs-security-00
Operations and Management Area Working Group T. Dahm
Internet-Draft
Updates: RFC8097 (if approved) D. Gash
Intended status: Standards Track Cisco Systems, Inc.
Expires: 2 October 2022 A. Ota
J. Heasley
NTT
31 March 2022
TACACS+ Security, TLS, and SSH Public Keys
draft-dahm-tacacs-security-00
Abstract
The TACACS+ Protocol [RFC8907] provides device administration for
routers, network access servers and other networked computing devices
via one or more centralized servers. This document, a companion to
the TACACS+ protocol [RFC8907], adds new packet formats to improve
security and function, Transport Layer Security (currently defined by
TLS 1.3 [RFC8446]) support, and support for SSH [RFC4716] public keys
and deprecates former inferior security mechanisms.
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
[BCP14] 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 of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents 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 2 October 2022.
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Copyright Notice
Copyright (c) 2022 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 (https://trustee.ietf.org/
license-info) in effect on the date of 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Technical Definitions . . . . . . . . . . . . . . . . . . . . 3
2.1. AVP . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Empty Value . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Unsecure Connection . . . . . . . . . . . . . . . . . . . 4
2.4. Peer . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.5. TLS Connection . . . . . . . . . . . . . . . . . . . . . 4
3. TLS for TACACS+ . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Well-Known TCP/IP Port . . . . . . . . . . . . . . . . . 5
3.2. TLS Connection . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Cipher Requirements . . . . . . . . . . . . . . . . . 5
3.2.2. TLS Authentication . . . . . . . . . . . . . . . . . 6
3.3. TLS Identification . . . . . . . . . . . . . . . . . . . 6
4. TACACS+ Extended Authentication Packet Types . . . . . . . . 6
4.1. The Extended Authentication START Packet Body . . . . . . 7
4.2. The Extension Authentication REPLY Packet Body . . . . . 9
4.3. The Extended Authentication CONTINUE Packet Body . . . . 10
5. SSH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. New Enumerated TACACS+ Protocol Values and well-known
AVPs . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. SSH Public Key Support . . . . . . . . . . . . . . . . . 13
5.3. SSH Authorization and Accounting . . . . . . . . . . . . 15
6. Deprecation of TACACS+ Encryption . . . . . . . . . . . . . . 16
7. Protocol Deprecations . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1.1. TLS Use . . . . . . . . . . . . . . . . . . . . . . . 17
8.1.2. TLS 0-RTT . . . . . . . . . . . . . . . . . . . . . . 17
8.1.3. TLS PSK . . . . . . . . . . . . . . . . . . . . . . . 17
8.1.4. TLS Options . . . . . . . . . . . . . . . . . . . . . 18
8.1.5. Unreachable TLS CA . . . . . . . . . . . . . . . . . 18
8.2. Well-Known TCP/IP Port . . . . . . . . . . . . . . . . . 18
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8.3. SSH Public Key Caching . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
11. Normative References . . . . . . . . . . . . . . . . . . . . 19
12. Informative References . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The TACACS+ Protocol [RFC8907] provides device administration for
routers, network access servers and other networked computing devices
via one or more centralized servers. The protocol provides
authentication, authorization and accounting services for TACACS+
clients.
While the content of the protocol is highly sensitive, TACACS+ lacks
modern and/or effective confidentiality, integrity, and
authentication of the connection and network traffic between the
server and client. The existing mechanisms of TACACS+ are extremely
weak and the Security Considerations section of the TACACS+ Protocol
[RFC8907] adequately describes this.
To address these deficiencies, this document updates the TACACS+
Protocol [RFC8907] to use TLS 1.3 [RFC8446] authentication and
encryption, and deprecates the use of its former mechanisms.
To support SSH authentication using public keys, highly desired by
the operator community, this document introduces a method to support
sending public keys to a TACACS+ client, allowing centralized
management.
To accomplish these goals and improve security and functionality when
a network proxy is involved in a TACACS+ connection, new and uniform
packet formats are introduced.
2. Technical Definitions
The Technical Definitions section of the TACACS+ Protocol [RFC8907]
is fully applicable here and will not be repeated, though may be
augmented. The following terms are also used in this document.
2.1. AVP
An Attribute-Value Pair or AVP is another name a TACACS+ argument as
defined in [RFC8907] Sections 6.1 and 8.
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2.2. Empty Value
An empty or zero-length value of an AVP as defined in [RFC8907]
Sections 8.1.
2.3. Unsecure Connection
This is another term for a Connection as defined in TACACS+ Protocol
[RFC8907]. It is a Connection without TLS and therefore being
plaintext or possibly using unsecure TACACS+ authentication and
obfuscation.
2.4. Peer
This refers to a TACACS+ Server or Client.
2.5. TLS Connection
A TLS Connection is a TCP/IP connection with TLS authentication and
encryption used by TACACS+ for transport, similar to a Connection as
defined in TACACS+ Protocol [RFC8907].
3. TLS for TACACS+
TACACS+ connections are TCP/IP connections initiated by the Client to
the Server. The well-known TCP/IP port 49 on the Server is used for
unencrypted and encrypted connections as defined in the TACACS+
Protocol [RFC8907]. A connection may be used for only a single
Session or the multiplexing of multiple Sessions in TACACS+ Single
Connection Mode.
TLS is introduced into TACACS+ to fulfill the following requirements:
1. Confidentiality and Integrity: The MD5 obfuscation specified in
the original protocol definition is not fit for purpose,
requiring that TACACS+ be deployed over a secured network.
Securing TACACS+ protocol with TLS is intended to provide
confidentiality and integrity without requiring the provision of
a secured network.
2. Peer authentication: The use of shared keys to add and remove the
MD5 obfuscation was intended to provide a form of Peer
authentication for the TACACS+ protocol. This document
deprecates the MD5 obfuscation, and specifies that the
authentication capabilities of TLS are used to allow the Peers to
authenticate each other.
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3.1. Well-Known TCP/IP Port
All data exchanged by TACACS+ Peers MUST be encrypted, including the
authentication of the Peers. Therefore, TLS Hello MUST be initiated
by the client immediately upon the establishment of the TCP/IP
connection.
This document favors the predictable use of TLS security for a
deployment, see (Section 8.2). TACACS+ TLS will therefore follow
[RFC7605], where a different well-known system TCP/IP port is
assigned by IANA, port [TBD] (Section 9) with the service name [TBDN]
(Section 9), for TLS connections.
3.2. TLS Connection
A TACACS+ Client initiates a TLS connection by making a TCP
connection to a configured Server on the tacacss well-known port
(Section 3.1). Once the TCP connection is established, the Client
MUST immediately begin the TLS negotiation before sending any TACACS+
protocol data.
Implementations MUST support TLS 1.3 [RFC8446] and MAY permit TLS 1.3
session resumption. If resumption is supported, the resumption
ticket_lifetime SHOULD be configurable, including a zero seconds
lifetime.
Once the TLS connection is established, the exchange of TACACS+ data
proceeds as normal, except that it is transmitted over TLS as TLS
application data and without TACACS+ obfuscation (see Section 6)
The connection persists until the Server or Client closes it. It
might be closed due to an error or at the conclusion of the TACACS+
Session. If Single Connection Mode has been negotiated, it might
remain open after a successful Session, until an error or a timeout
occurs. Why it closed has no bearing on TLS resumption, unless
closed by a TLS error, in which case the ticket might be invalidated.
3.2.1. Cipher Requirements
Implementations MUST support the TLS 1.3 mandatory cipher suites (See
RFC8446 Section 9.1). The cipher suites offered or accepted SHOULD
be configurable so that operators can adapt.
This document makes no cipher suite recommendations, but
recommendations can be found in the TLS Cipher Suites section of the
[TLSCSREC].
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3.2.2. TLS Authentication
Implementations MUST support certificate-based TLS authentication and
certificate revocation bi-directionally for authentication, identity
verification and policy purposes. Certificate path verification as
described in Section 3.2.2.1 MUST be supported.
If this 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 disconnect a Peer that
offers an invalid TLS Certificate.
3.2.2.1. TLS Certificate Path Verification
Implementations MUST support certificate Path verification as
described in [RFC5280].
3.3. TLS Identification
In addition to authentication of TLS certificates, implementations
MUST support policy consideration of Peer-identifying certificate
fields and policy used to verify that the Peer is a valid source for
the received certificate and that it is permitted access to TACACS+.
Implementations MUST support either:
Network location based validation methods as described in [RFC5425],
Section 5.2.
or
Device Identity based validation methods where the peer's identity is
used in the certificate subjectName. This is applicable in
deployments where the device securely supports an identity which is
shared with its peer. This approach allows a peer's network location
to be reconfigured without issuing a new client certificate. Only
the local server mapping needs to be updated.
4. TACACS+ Extended Authentication Packet Types
Versions 1 and 2 of the TACACS+ Protocol, as defined in [RFC8907],
specify the TACACS+ Authentication Packets for START, REPLY and
CONTINUE which support the credential validation use case but does
not accommodate any further arguments which may be used to give
context to the request.
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One use-case where this shortcoming inhibits correct operation is for
the TACACS+ proxy. Because the originating client is not encoded in
the regular Authentication START Packet, TACACS+ Servers generally
attempt to determine the client from the TCP connection. This is
effective only for the first step: proxied TACACS+ servers can no
longer securely enforce policy based upon the end client IP-Address.
Further, advanced use cases (such as SSH key distribution) would
otherwise rely on embedding structured information into the single
data fields, obfuscating the content of the protocol.
To support these use cases, and allow clients to add environment
information to the request, the Extended Authentication Packets
brings the Authentication phase of the protocol inline with the
Authorization and Accounting Phase by incorporating extensible
argument s.
The server should expect Extended Authentication Packet Bodies if the
minor version in the Packet Header is: 0x2
4.1. The Extended Authentication START Packet Body
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1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
| action | priv_lvl | authen_type | authen_service |
+----------------+----------------+----------------+----------------+
| user_len | port_len | rem_addr_len | data_len |
+----------------+----------------+----------------+----------------+
| arg_cnt |
+----------------+----------------+----------------+----------------+
| arg_1_len |
+----------------+----------------+----------------+----------------+
| ... |
+----------------+----------------+----------------+----------------+
| arg_N_len |
+----------------+----------------+----------------+----------------+
| user ...
+----------------+----------------+----------------+----------------+
| port ...
+----------------+----------------+----------------+----------------+
| rem_addr ...
+----------------+----------------+----------------+----------------+
| data...
+----------------+----------------+----------------+----------------+
| arg_1 ...
+----------------+----------------+----------------+----------------+
| arg_2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg_N ...
+----------------+----------------+----------------+----------------+
Figure 1
The action, priv_level, authen_type, authen_service, user_len,
port_len, rem_addr_len, data_len, user, port, rem_addr and data
fields are used exactly as defined in the Authentication START Packet
Body in [RFC8907].
The following fields contain the arguments that may be used to extend
the authentication process. These are common to the Extended
Authentication START, Extended Authentication REPLY, and Extended
Authentication CONTINUE packet bodies; these fields represent the
sole update from the previous START, REPLY and CONTINUE packet
bodies.
The new fields are as follows:
arg_cnt
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This represents the number of arguments in the packet.
arg_1_len ... arg_N_len, arg_1 ... arg_N
Each argument is encoded in the packet as a single arg field (arg_1
... arg_N) with a corresponding length field that indicates the
length of each argument in bytes.
The arguments are argument-value pairs. The argument and the value
are in a single string and are separated by either a "=" (0X3D) or a
"*" (0X2A). The equals sign indicates a mandatory argument. The
asterisk indicates an optional one. For the rules regarding optional
and mandatory arguments, refer to [RFC8907]
Multiple arguments with the same name are permitted within a packet,
a common example is cmd-arg. The handling of repeated arguments is
specific to the semantics of the argument and so are documented with
that argument. Order is significant when processing arguments.
The addition of arguments to the authentication packets is intended
to permit the flexibility for the TACACS+ authentication phase that
has been available previously for authorization and accounting.
These fields are intended to be used as needed in deployment, they
are used in this document in the enhancements for SSH (Section 5),
and to support the origin client to enhance TACACS+ Proxy:
origin_client
The IP-Address of the originating TACACS+ client. This is text
encoded in line with the rest of the TACACS+ protocol, and may be
IPv4 or IPv6. This argument is optional. IPv4 addresses are
specified as octet numeric values separated by dots ('.'). IPv6
address text representation is defined in [RFC5952].
4.2. The Extension Authentication REPLY Packet Body
The TACACS+ server sends only one type of extended authentication
packet to the client.
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1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
| status | flags | server_msg_len |
+----------------+----------------+----------------+----------------+
| data_len |
+----------------+----------------+----------------+----------------+
| arg_cnt |
+----------------+----------------+----------------+----------------+
| arg_1_len |
+----------------+----------------+----------------+----------------+
| ... |
+----------------+----------------+----------------+----------------+
| arg_N_len |
+----------------+----------------+----------------+----------------+
| data ...
+----------------+----------------+----------------+----------------+
| server_msg ...
+----------------+----------------+----------------+----------------+
| arg_1 ...
+----------------+----------------+----------------+----------------+
| arg_2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg_N ...
+----------------+----------------+----------------+----------------+
Figure 2
The status, flags, server_msg_len, data_len, server_msg, and data
fields are used exactly as defined in the Authentication REPLY Packet
Body in [RFC8907]].
The new arg_cnt, arg_1 ... arg_N, and arg_1_len .... arg_N_len fields
are used as defined in The Extended Authentication START Packet Body
(Section 4.1).
4.3. The Extended Authentication CONTINUE Packet Body
This packet is sent from the client to the server following the
receipt of an Extended REPLY packet.
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1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
| status | user_msg len |
+----------------+----------------+----------------+----------------+
| data_len |
+----------------+----------------+----------------+----------------+
| arg_cnt |
+----------------+----------------+----------------+----------------+
| arg_1_len |
+----------------+----------------+----------------+----------------+
| ... |
+----------------+----------------+----------------+----------------+
| arg_N_len |
+----------------+----------------+----------------+----------------+
| user_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+----------------+----------------+----------------+
| arg_1 ...
+----------------+----------------+----------------+----------------+
| arg_2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg_N ...
+----------------+----------------+----------------+----------------+
Figure 3
The user_msg len, data_len, user_msg, and data fields are used
exactly as defined in the Authentication REPLY Packet Body in
[RFC8907]. However, the status field replaces the flags field and
has the following enumeration:
* TAC_PLUS_AUTHEN_CONTINUE_STATUS_NONE := 00
* TAC_PLUS_AUTHEN_CONTINUE_STATUS_PASS := 01
* TAC_PLUS_AUTHEN_CONTINUE_STATUS_FAIL := 02
* TAC_PLUS_AUTHEN_CONTINUE_STATUS_FRAGMENT := 03
* TAC_PLUS_AUTHEN_CONTINUE_STATUS_ERROR := 04
* TAC_PLUS_AUTHEN_CONTINUE_STATUS_ABORT := 05
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TAC_PLUS_AUTHEN_CONTINUE_STATUS_NONE or
TAC_PLUS_AUTHEN_CONTINUE_STATUS_ABORT MUST be used when the Extended
Authentication Packets are used for the continuation of
authentication flows documented in [RFC8907].
The client may prematurely terminate a session by setting the
TAC_PLUS_AUTHEN_CONTINUE_STATUS_ABORT or
TAC_PLUS_AUTHEN_CONTINUE_STATUS_ERROR status in the CONTINUE message.
The remainder are detailed in SSH (Section 5).
The new arg_cnt, arg_1 ... arg_N, and arg_1_len .... arg_N_len fields
are used as defined in The Extended Authentication START Packet Body
(Section 4.1).
5. SSH
Most network equipment now support SSH [RFC4251] for Command Line
Interface (CLI) and NETCONF [RFC6242]. Operators SHOULD use SSH
public keys for authentication. Some devices support public keys in
native configuration, but there is desire to centrally manage keys
and SSH subsystem authorization.
5.1. New Enumerated TACACS+ Protocol Values and well-known AVPs
The following new enumerated TACACS+ protocol values and well-known
AVPs are needed to support SSH in the subsequent sections. These new
values augment those in [RFC8907] Sections 5.1 - 5.3, 6.1, and 8.2 as
follows:
TAC_PLUS_AUTHEN_TYPE_SSHPK := 0x07
Extended Authentication START Packet authen_type for SSH pubkeys.
TAC_PLUS_AUTHEN_STATUS_GETSSHPKTYPE := 0x22
Extended Authentication REPLY Packet status to solicit SSH pubkey
type.
TAC_PLUS_AUTHEN_STATUS_SSHPK := 0x23
Extended Authentication REPLY Packet status to provide SSH
pubkeys.
TAC_PLUS_REPLY_FLAG_FRAGMENT := 0x02
Extended Authentication REPLY Packet flag indicating the REPLY is
incomplete.
TAC_PLUS_AUTHEN_CONTINUE_STATUS_PASS := 0x01
Extended Authentication CONTINUE Packet flag indicating
authentication success.
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TAC_PLUS_AUTHEN_CONTINUE_STATUS_FAIL := 0x08
Extended Authentication CONTINUE Packet flag indicating
authentication failure.
TAC_PLUS_AUTHEN_CONTINUE_STATUS_FRAGMENT := 0x03
Extended Authentication CONTINUE Packet flag requesting the next
REPLY packet of an incomplete REPLY.
TAC_PLUS_AUTHEN_CONTINUE_STATUS_ERROR := 0x04
Extended Authentication CONTINUE Packet flag indicating
authentication error.
AVP ssh_pubkey_type (String)
Attribute to carry SSH public key type names.
AVP ssh_pubkey (String)
Attribute to carry SSH public keys.
TAC_PLUS_AUTHEN_METH_SSHPK := 0x21
Authorization REQUEST Packet authen_method for SSH pubkey
authentication.
AVP ssh_subsystem (String)
Attribute to carry SSH subsystem name for authorization
5.2. SSH Public Key Support
To support central management of SSH public keys via TACACS+, the
Authentication sequence of [RFC8907] Section 5.4 is extended using
Extended Authentication Packet (Section 4) sequences to deliver SSH
public keys to devices for local verification.
Besides new header values and flags and AVPs for Extended
Authentication Packets, the SSH public key authentication process
differs from other TACACS+ authentication types in that there may be
more Authentication Reply and Authentication Continue Packets pairs
than previously.
The process follows:
1. The client begins an authentication session with an Extended
Authentication START Packet. The START packet MUST include a
non-zero-length username and the server MUST send an
Authentication REPLY Packet with status
TAC_PLUS_AUTHEN_STATUS_ERROR, if the client fails to do so.
The client MAY include one or more instances of the
ssh_pubkey_type AVP, indicating the SSH public key types that it
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wants. The set of permissible values for this AVP are the SSH
public algorithm names defined in the IANA SSH Protocol
Parameters Registry [SSHALGS], which are case-sensitive as
specified and otherwise constrained by [RFC4250] Section 4.6.1.
Multiple values MUST be separated by a comma, therefore multiple
ssh_pubkey_type AVPs MUST include commas for separation when the
Peer concatenates them and the Peer MUST be prepared to ignore a
leading or trailing comma in the concatenated value. The server
MUST NOT reply with status TAC_PLUS_AUTHEN_STATUS_ERROR if it
receives an algorithm name that it does not recognize. If the
client marks a ssh_pubkey_type AVP as mandatory, the server MUST
reply with at least one key of that type for the given user or
reply with status TAC_PLUS_AUTHEN_STATUS_SSHNOPK with the
relevant ssh_pubkey_type AVP.
The client MAY send an Empty Value for the algorithm name to
request all types available for the given user.
The process ends and the client MUST start a new authentication
session if it receives status SSHNOPK or ERROR.
2. If a ssh_pubkey_type AVP was not provided in the START packet,
the server replies with the status code
TAC_PLUS_AUTHEN_STATUS_GETSSHPKTYPE. The client MUST send a
CONTINUE packet with one or more ssh_pubkey_type AVPs, else the
server sends a REPLY packet with status
TAC_PLUS_AUTHEN_STATUS_ERROR.
3. If the server has none of the requested ssh_pubkey_type(s) or any
of the mandatory ssh_pubkey_types for the user or no pubkeys at
all, the server MUST send a REPLY packet with status
TAC_PLUS_AUTHEN_STATUS_SSHNOPK with the ssh_pubkey_type AVP(s)
that it received.
The process ends and the client MUST start a new authentication
session if it receives status SSHNOPK or ERROR.
4. The server sends REPLY packets with status
TAC_PLUS_AUTHEN_STATUS_SSHPK and includes one or more ssh_pubkey
optional AVPs, each containing one or more keys. The ssh_pubkey
AVPs are formatted according to the rules of SSH Public Key File
Format [RFC4716]. As such, the client MUST be prepared to accept
keys with Key File Markers. To address concatenation of multiple
ssh_pubkey AVPs or multiple keys in a single AVP, the server MUST
terminate each key file End Marker with a Line Termination
sequence as specified in RFC4716 Section 3.1.
Since it is possible to have more ssh_pubkey AVPs than fit in a
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REPLY packet, the server SHOULD set the REPLY packet flag
TAC_PLUS_REPLY_FLAG_FRAGMENT if two or more packets are required,
indicating that the client SHOULD request the remainder.
An AVP SHALL NOT span multiple fragments; each must be contained
entirely in the fragment in which it begins.
5. If the TAC_PLUS_REPLY_FLAG_FRAGMENT flag is set, the client MAY
reply with the same CONTINUE packet as before with the
TAC_PLUS_AUTHEN_CONTINUE_STATUS_FRAGMENT flag set. The server
replies with the next REPLY fragment as before, clearing the
TAC_PLUS_REPLY_FLAG_FRAGMENT flag of the last REPLY fragment.
This repeats until the last REPLY fragment is received, the
client aborts the authentication process, or an error occurs.
The client MUST NOT set TAC_PLUS_AUTHEN_CONTINUE_STATUS_FRAGMENT
if the REPLY packet did not have the TAC_PLUS_REPLY_FLAG_FRAGMENT
flag set and the server MUST reply with
TAC_PLUS_AUTHEN_STATUS_ERROR if it does so.
6. Once the client has all of the pubkeys, it performs the ssh
pubkey authentication with its ssh client. The client MUST then
reply to the server with the status of that authentication by
sending a CONTINUE packet with one of the following new or
existing CONTINUE flags: TAC_PLUS_CONTINUE_FLAG_ABORT,
TAC_PLUS_AUTHEN_CONTINUE_STATUS_PASS,
TAC_PLUS_AUTHEN_CONTINUE_STATUS_FAIL, or
TAC_PLUS_AUTHEN_CONTINUE_STATUS_ERROR.
7. The client MUST give the server the final consent, by waiting for
a REPLY packet with one of the status:
TAC_PLUS_AUTHEN_STATUS_PASS, TAC_PLUS_AUTHEN_STATUS_FAIL, or
TAC_PLUS_AUTHEN_STATUS_ERROR, thus ending the authentication
session.
5.3. SSH Authorization and Accounting
To support central management of SSH and SSH subsystem authorization
and accounting via TACACS+, this document adds a new authen_method to
RFC8907 Section 6.1 Authorization REQUEST [RFC8907] and a well-known
AVP to Section 8.2 Authorization Arguments [RFC8907].
The new authen_method TAC_PLUS_AUTHEN_METH_SSHPUBKEY indicates that
the user was authenticated with a SSH public key.
The well-known ssh_subsystem AVP defines the SSH subsystem for which
the authorization is requested and MUST be present any time the
authorization is for a SSH connection.
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The set of permissible values for this AVP are the SSH Subsystem
Names defined in the IANA SSH Connection Protocol Subsystem Names
Registry [SSHSUBSYS], which are case-sensitive as specified and
otherwise constrained by [RFC4250] Section 4.6.1. The client MAY
send an Empty Value for the subsystem name to indicate no subsystem,
also known as a shell or CLI. The server MUST NOT reply with status
TAC_PLUS_AUTHOR_STATUS_ERROR if it receives a subsystem name whose
syntax is valid but whose value is not recognized. Subsystems might
need additional data for authorization or accounting that will be
particular to that subsystem and are therefore out of scope for this
document.
These new authen_methods and AVPs apply equally to accounting.
6. Deprecation of TACACS+ Encryption
The original draft of TACACS+ described an encryption mechanism built
into the protocol. This is insufficient for modern purposes and the
document TACACS+ Protocol [RFC8907] reclassified the mechanism as one
capable only of obfuscation.
The introduction of TLS PSK and certificate Peer authentication and
TLS encryption to TACACS+ obsolesces these former mechanisms and so
are hereby deprecated. This section describes how the TACACS+ client
and servers MUST operate with regards to the obfuscation mechanism.
The TACACS+ server or client receiving TACACS+ Packets MUST process
the packet as if TAC_PLUS_UNENCRYPTED_FLAG was set. The actual value
of TAC_PLUS_UNENCRYPTED_FLAG flag in the TACACS+ header MUST be
ignored.
Peers SHOULD set the TAC_PLUS_UNENCRYPTED_FLAG flag in the Packet
Header of packets on TLS Connections, indicating that the data
obfuscation is not used.
7. Protocol Deprecations
This section deprecates features from the TACACS+ Protocol.
MS-CHAPv1: has been replaced by MS-CHAPv2 in most deployments, the
intent of this deprecation is to complete the transition. MD4 is
still required to support MS-CHAPv2 so cannot be deprecated at this
point It should be noted that the use of MD4 is purely to allow
compatible MS-CHAPv2 operation and not for security; the TLS
transport is intended to provide that function.
TAC_PLUS_AUTHEN_SENDAUTH: the sendauth mechanism can not be
supported, as it permits the leak of sensitive information.
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8. Security Considerations
8.1. TLS
This document improves the confidentiality, integrity, and
authentication of the connection and network traffic between TACACS+
Peers by adding TLS support. This does not in itself protect the
server nor clients; the operator and equipment vendors have a role.
That role is to diligently follow current best practices for
maintaining the integrity of network devices and selection of TLS key
and encryption algorithms.
8.1.1. TLS Use
TLS encryption SHOULD be used in deployments when both the Clients
and Servers support it. Servers that support TLS encryption MAY be
configured to allow Unsecure Connections when TLS encryption is not
supported by the Client, but this is NOT RECOMMENDED because of the
threat of downgrade attacks, as described in Section 8.2. Unsecure
Connections would be better served by separate Servers from the TLS
Servers.
It is NOT RECOMMENDED to deploy TACACS+ without TLS authentication
and encryption, including TLS using the NULL algorithm, except for
within test and debug environments. Also see [RFC3365].
8.1.2. 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 possible. Given the sensitivity
of TACACS+ data, a Client MUST NOT send data until the full TLS
handshake completes; that is, Clients MUST NOT send 0-RTT data and
Servers MAY abruptly disconnect Clients that do.
8.1.3. TLS PSK
Implementations MAY support TLS authentication with Pre-Shared Keys
(PSKs), also known as external PSKs in TLS 1.3, which are not
resumption PSKs. PSKs SHOULD NOT be shared among Clients or Servers
to limit exposure of a compromised key and to ease key rotation.
Also see [RFC8773] and [I-D.ietf-tls-external-psk-guidance].
PSKs are otherwise considered out-of-scope for this document.
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8.1.4. TLS Options
Unfortunately, no single and timely TLS recommendations document
exists. Therefore, implementers and operators SHOULD make use of the
various RFCs to determine which TLS versions and algorithms should be
supported, deprecated, or abandoned, in the absence of updates to
this document. Useful examples are the TLS specifications themselves
(TLS 1.3 [RFC8446]), which prescribes mandatory support in Section 9,
and TLS Recommendations [RFC7525].
8.1.5. Unreachable TLS CA
Operators SHOULD be cognizant of the potential of Server and/or
Client isolation from their Peer's Certificate Authority (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. Certificate caching and Raw
Public Keys [RFC7250] are methods to address this, but both are out
of scope for this document. Certificate fingerprints are another
option.
8.2. Well-Known TCP/IP Port
A new port is considered appropriate and superior to a "STARTTLS"
command or other negotiation method because it allows:
* ease of blocking the unencrypted or inferiorly encrypted
connections by the TCP/IP port number,
* passive Intrusion Detection Systems (IDSs) monitoring the
unencrypted version to be unaffected by the introduction of TLS,
* avoidance of Man in the Middle (MitM) attacks that can interfere
with STARTTLS,
* and helps prevent the accidental exposure of sensitive information
due to misconfiguration.
However, co-existence of inferior authentication and encryption,
whether an Unsecure Connection or deprecated parts that compose TLS,
also presents opportunity for down-grade attacks. Causing failure of
connections to the TLS-enabled service or the negotiation of shared
algorithm support are two such down-grade attacks. The simplest way
to address the exposure from Unsecure Connection methods is to refuse
Unsecure Connections at the server entirely, perhaps using separate
servers for Unsecure Connections and TLS. Another approach is mutual
configuration that requires TLS. Clients and Servers SHOULD support
configuration that requires Peers, globally and individually, use
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TLS. Furthermore, Peers SHOULD be configurable to limit offered or
recognized TLS versions and algorithms to those recommended by
standards bodies and implementers.
Servers and Clients could also maintain a cache of Peers that have
engaged in TACACS+ TLS connections and demand TLS from that point
forward. However, this has potential to be a Denial of Service (DoS)
vector, whereby an attacker causes a server to believe that a client
that does not support TLS has successfully connected with TLS.
8.3. SSH Public Key Caching
A Client MUST NOT cache SSH public keys received from a Server for
future SSH client authentication. Doing so would deny the Server the
opportunity to deny authentication for other reasons than key
validity or to revoke a key. The Server has no method to revoke a
key, except by not offering the key in future authentication
sessions.
9. IANA Considerations
The authors request that, when this draft is accepted by the working
group, the OPSAWG Chairs submit a request to IANA for an early
allocation, per [RFC4020] and [RFC6335], of a new well-known system
TCP/IP port number 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 of the non-TLS well-
known port. This allocation is justified in Section 8.2.
RFC EDITOR: this port number should replace "[TBD]" and the service
name should replace "[TBDN]" within this document.
10. Acknowledgments
The author(s) would like to thank Russ Housley, Steven M. Bellovin,
Stephen Farrell, Alan DeKok, Warren Kumari, and Tom Petch for their
support, insightful review, and/or comments. [RFC5425] was also used
as a basis for the approach to TLS.
11. Normative References
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017.
<https://www.rfc-editor.org/bcp/bcp14.txt>
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[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250,
DOI 10.17487/RFC4250, January 2006,
<https://www.rfc-editor.org/info/rfc4250>.
[RFC4716] Galbraith, J. and R. Thayer, "The Secure Shell (SSH)
Public Key File Format", RFC 4716, DOI 10.17487/RFC4716,
November 2006, <https://www.rfc-editor.org/info/rfc4716>.
[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>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[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>.
[RFC8773] Housley, R., "TLS 1.3 Extension for Certificate-Based
Authentication with an External Pre-Shared Key", RFC 8773,
DOI 10.17487/RFC8773, March 2020,
<https://www.rfc-editor.org/info/rfc8773>.
[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>.
[SSHALGS] IANA, "Public Key Algorithm Names",
<https://www.iana.org/assignments/ssh-parameters/ssh-
parameters.xhtml#ssh-parameters-19>.
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[SSHSUBSYS]
IANA, "SSH Protocol Subsystem Names",
<https://www.iana.org/assignments/ssh-parameters/ssh-
parameters.xhtml#ssh-parameters-15>.
12. Informative References
[I-D.ietf-tls-external-psk-guidance]
Housley, R., Hoyland, J., Sethi, M., and C. A. Wood,
"Guidance for External PSK Usage in TLS", Work in
Progress, Internet-Draft, draft-ietf-tls-external-psk-
guidance-06, 4 February 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
external-psk-guidance-06>.
[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>.
[RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of
Standards Track Code Points", RFC 4020,
DOI 10.17487/RFC4020, February 2005,
<https://www.rfc-editor.org/info/rfc4020>.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
January 2006, <https://www.rfc-editor.org/info/rfc4251>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
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[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>.
[RFC7605] Touch, J., "Recommendations on Using Assigned Transport
Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
August 2015, <https://www.rfc-editor.org/info/rfc7605>.
[TLSCSREC] IANA, "Transport Layer Security (TLS) Parameters",
<https://www.iana.org/assignments/tls-parameters/tls-
parameters.xhtml#tls-parameters-4>.
Authors' Addresses
Thorsten Dahm
Email: thorsten.dahm@gmail.com
Douglas Gash
Cisco Systems, Inc.
Email: dcmgash@cisco.com
Andrej Ota
Email: andrej@ota.si
John Heasley
NTT
Email: heas@shrubbery.net
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