Extension Negotiation in the Secure Shell (SSH) Protocol
RFC 8308
| Document | Type | RFC - Proposed Standard (March 2018; No errata) | |
|---|---|---|---|
| Author | denis bider | ||
| Last updated | 2018-03-19 | ||
| Replaces | draft-ssh-ext-info | ||
| Stream | IETF | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Daniel Migault | ||
| Shepherd write-up | Show (last changed 2017-06-01) | ||
| IESG | IESG state | RFC 8308 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Yes | ||
| Telechat date | |||
| Responsible AD | Eric Rescorla | ||
| Send notices to | Daniel Migault <daniel.migault@ericsson.com> | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | RFC-Ed-Ack | ||
Internet Engineering Task Force (IETF) D. Bider
Request for Comments: 8308 Bitvise Limited
Updates: 4251, 4252, 4253, 4254 March 2018
Category: Standards Track
ISSN: 2070-1721
Extension Negotiation in the Secure Shell (SSH) Protocol
Abstract
This memo updates RFCs 4251, 4252, 4253, and 4254 by defining a
mechanism for Secure Shell (SSH) clients and servers to exchange
information about supported protocol extensions confidentially after
SSH key exchange.
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/rfc8308.
Copyright Notice
Copyright (c) 2018 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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bider Standards Track [Page 1]
RFC 8308 Extension Negotiation in SSH March 2018
Table of Contents
1. Overview and Rationale ..........................................3
1.1. Requirements Terminology ...................................3
1.2. Wire Encoding Terminology ..................................3
2. Extension Negotiation Mechanism .................................3
2.1. Signaling of Extension Negotiation in SSH_MSG_KEXINIT ......3
2.2. Enabling Criteria ..........................................4
2.3. SSH_MSG_EXT_INFO Message ...................................4
2.4. Message Order ..............................................5
2.5. Interpretation of Extension Names and Values ...............6
3. Initially Defined Extensions ....................................6
3.1. "server-sig-algs" ..........................................6
3.2. "delay-compression" ........................................7
3.2.1. Awkwardly Timed Key Re-Exchange .....................9
3.2.2. Subsequent Re-Exchange ..............................9
3.2.3. Compatibility Note: OpenSSH up to Version 7.5 .......9
3.3. "no-flow-control" .........................................10
3.3.1. Prior "No Flow Control" Practice ...................10
3.4. "elevation" ...............................................11
4. IANA Considerations ............................................12
4.1. Additions to Existing Registries ..........................12
4.2. New Registry: Extension Names .............................12
4.2.1. Future Assignments to Extension Names Registry .....12
5. Security Considerations ........................................12
6. References .....................................................13
6.1. Normative References ......................................13
6.2. Informative References ....................................13
Acknowledgments ...................................................14
Author's Address ..................................................14
Bider Standards Track [Page 2]
RFC 8308 Extension Negotiation in SSH March 2018
1. Overview and Rationale
Secure Shell (SSH) is a common protocol for secure communication on
the Internet. The original design of the SSH transport layer
[RFC4253] lacks proper extension negotiation. Meanwhile, diverse
implementations take steps to ensure that known message types contain
no unrecognized information. This makes it difficult for
implementations to signal capabilities and negotiate extensions
without risking disconnection. This obstacle has been recognized in
the process of updating SSH to support RSA signatures using SHA-256
and SHA-512 [RFC8332]. To avoid trial and error as well as
authentication penalties, a client must be able to discover public
key algorithms a server accepts. This extension mechanism permits
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