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Versions: 00

Internet-Draft                                                  D. Bider
Updates: 4254 (if approved)                              Bitvise Limited
Intended status: Standards Track                       December 18, 2018
Expires: May 18, 2019


     Sending and Handling of Global Requests in Secure Shell (SSH)
                   draft-ssh-global-requests-ok-00.txt


Abstract

  This memo updates RFC 4254 to clarify when global requests can be sent
  or received and provides a mechanism for SSH software to indicate it
  will accept global requests according to this requirement.

Status

  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), its areas, and its working groups.  Note that other
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Copyright

  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
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  carefully, as they describe your rights and restrictions with respect
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  described in the Simplified BSD License.




Bider                                                           [Page 1]

Internet-Draft           Global Requests in SSH            December 2018


1.  Overview and Rationale

  Secure Shell (SSH) is a common protocol for secure communication on
  the Internet. [RFC4254] requires both clients and servers to correctly
  handle messages of type SSH_MSG_GLOBAL_REQUEST received at any time.
  In practice, several client implementations and some servers mishandle
  this requirement. This discourages implementations from deploying
  protocol enhancements including host key synchronization and active
  keep-alives. Software that uses such enhancements must rely on remote
  version information to decide if global requests are safe to use.
  However, this is not accurate as to the remote party's capabilities.

  This memo updates RFC 4254 to clarify when software may send and must
  accept global requests. An [RFC8308] extension is defined allowing
  SSH software to indicate it complies with this requirement.

1.1.  Requirements Terminology

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in [RFC2119].

1.2.  Wire Encoding Terminology

  The wire encoding types in this document - "string", "byte" and
  "boolean" - have meanings as described in [RFC4251].


2.  Global Request Sending and Handling

  The requirement in [RFC4254], which states that both a client and a
  server must correctly handle global requests at any time, is replaced
  as defined in this section.

  A server MAY send a message of type SSH_MSG_GLOBAL_REQUEST at any time
  after it has sent the message SSH_MSG_USERAUTH_SUCCESS (defined in
  [RFC4252]), including immediately following that message. A server
  MUST NOT send SSH_MSG_GLOBAL_REQUEST before it has sent
  SSH_MSG_USERAUTH_SUCCESS.

  A client MAY send a message of type SSH_MSG_GLOBAL_REQUEST at any time
  after it has received SSH_MSG_USERAUTH_SUCCESS from the server. A
  client MUST NOT send SSH_MSG_GLOBAL_REQUEST before it has received
  SSH_MSG_USERAUTH_SUCCESS.

  A server MUST handle correctly - as defined in [RFC4254] - any message
  of type SSH_MSG_GLOBAL_REQUEST received after the server has sent
  SSH_MSG_USERAUTH_SUCCESS. A server MAY implement arbitrary behavior
  for global requests received before this. However, see Section 2.1
  (Security Consideration).

  A client MUST handle correctly - as defined in [RFC4254] - any message
  of type SSH_MSG_GLOBAL_REQUEST received after it has received
  SSH_MSG_USERAUTH_SUCCESS from the server. A client MAY implement
  arbitrary behavior for global requests received before this.

  Implementations MUST correctly handle SSH_MSG_GLOBAL_REQUEST messages
  received during SSH key re-exchange. When implementations receive
  global requests during key re-exchange, they MAY defer processing them
  and responding until key re-exchange has completed.

2.1.  Security Consideration

  A server that chooses to handle SSH_MSG_GLOBAL_REQUEST before it has
  sent SSH_MSG_USERAUTH_SUCCESS MUST apply precautions which take into
  account that the client has not yet authenticated.


3.  "global-requests-ok" Extension

  SSH software that implements [RFC8308] MAY include the following
  extension when sending an SSH_MSG_EXT_INFO message:

    string  extension-name  = "global-requests-ok"
    string  extension-value = ""

  The sender MUST send an empty extension value. A receiver that does
  not expect an extension value MUST ignore it. A receiver MUST tolerate
  and ignore non-printable binary characters in the extension value.
  Future specifications MAY define meanings for this value.

  A receiver SHOULD assume, if the remote party includes this extension
  in its SSH_MSG_EXT_INFO, that the remote will handle global requests
  as required by this document, regardless of any heuristic knowledge
  the receiver may have about the remote party's software and version.
  The receiver SHOULD enable any functionality that relies on global
  requests if this extension is received.


4.  Practical Uses of Global Requests

  The following are some uses of the SSH_MSG_GLOBAL_REQUEST message
  which are prevented or made difficult by software which incorrectly
  disconnects when receiving a global request:

4.1.  Active Keep-Alive

  Network connections can terminate in ways that prevent SSH software
  from immediately detecting the disconnection. The TCP stack might not
  report the disconnection for minutes. Meanwhile resources used by the
  previous session, such as port numbers for TCP forwarding, may remain
  in use so that a reconnected client cannot resume its functions.

  A common strategy to detect if the remote party is still connected is
  to send a global request which the remote does not have to recognize,
  only reply to. For example:

    byte     SSH_MSG_GLOBAL_REQUEST
    string   request-name = "keep-alive@implementation.example.com"
    boolean  want-reply   = true

  This requires the remote party to reply with SSH_MSG_REQUEST_FAILURE,
  which is sufficient to confirm the connection is still active.

  This strategy cannot be used if the remote party might disconnect on
  receiving a global request.

4.2.  Host Key Synchronization

  A practical deficiency of SSH as standardized and widely used is that
  it provides no mechanism for host key rollover. A server that wishes
  to migrate its host key from e.g. DSA to RSA, or from RSA to
  Curve25519, or from 1024-bit RSA to 3072-bit RSA, has no automated
  way of informing clients of the intended new host key. Instead, server
  administrators must contact all clients - which sometimes number in
  thousands - where host key information must be updated manually. The
  common result is that servers rarely change host keys until forced.

  OpenSSH supports and documents an extension ([OPENSSH]) which uses a
  global request named "hostkeys-00@openssh.com" to synchronize host
  keys. After successful authentication, the server sends this request
  to the client, listing all of the server's host keys. The client can
  respond with a further request for the server to prove possession of
  those host keys.

  This mechanism cannot be used if the remote party might disconnect on
  receiving a global request.


5.  IANA Considerations

  IANA is requested to update the "Secure Shell (SSH) Protocol
  Parameters" registry established with [RFC4250], adding the following
  entry in the table Extension Names [IANA-EXT]:

    Extension Name          Reference           Note
    global-requests-ok      [this document]     Section 3


6.  Security Considerations

  Security considerations appear where applicable in the document.

  The security considerations of [RFC4251] also apply to this document.


7.  References

7.1.  Normative References

  [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

  [RFC4251]   Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

  [RFC4252]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, January 2006.

  [RFC4254]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, January 2006.

  [RFC8308]   Bider, D., "Extension Negotiation in the Secure Shell
              (SSH) Protocol", RFC 8308, March 2018.

7.2.  Informative References

  [RFC4250]   Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250, January 2006.

  [OPENSSH]   "OpenSSH deviations and extensions to the published SSH
              protocol", <https://cvsweb.openbsd.org/src/usr.bin/ssh/
              PROTOCOL?annotate=HEAD>.

  [IANA-EXT]  "Secure Shell (SSH) Protocol Parameters",
              <https://www.iana.org/assignments/ssh-parameters/
              ssh-parameters.xhtml#extension-names>.


Author's Address

  Denis Bider
  Bitvise Limited
  4105 Lombardy Court
  Colleyville, Texas  76034
  United States of America

  Email: ietf-ssh3@denisbider.com
  URI:   https://www.bitvise.com/


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