


Network Working Group                                         J. Salowey
Internet-Draft                                             Cisco Systems
Expires: August 5, 2006                                      S. Suehring
                                                        February 1, 2006


   Uniform Resource Identifier (URI) Scheme for Secure File Transfer
                 Protocol (SFTP) and Secure Shell (SSH)
                draft-ietf-secsh-scp-sftp-ssh-uri-04.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes the Uniform Resource Identifiers used to
   locate resources for the Secure File Transfer Protocol (SFTP) and the
   Secure Shell (SSH) protocols.  The document describes the generic
   syntax involved in URI definitions as well as specific definitions
   for each protocol.  These specific definitions may include user
   credentials such as username and also may include other parameters
   such as host key fingerprint.  In addition, security considerations



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   and examples are also provided within this document.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   General Syntax . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Secure Shell (SSH) URI . . . . . . . . . . . . . . . . . . .   3
     3.1  Scheme Name  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2  Status . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.3  URI Scheme Syntax  . . . . . . . . . . . . . . . . . . . .   3
     3.4  URI Semantics  . . . . . . . . . . . . . . . . . . . . . .   4
     3.5  Encoding Considerations  . . . . . . . . . . . . . . . . .   4
     3.6  Protocols using this URI scheme  . . . . . . . . . . . . .   4
     3.7  Security Considerations  . . . . . . . . . . . . . . . . .   5
     3.8  Contact  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.   Secure File Transfer Protocol (SFTP) URI . . . . . . . . . .   5
     4.1  Scheme Name  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2  Status . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.3  URI Scheme Syntax  . . . . . . . . . . . . . . . . . . . .   5
     4.4  URI Semantics  . . . . . . . . . . . . . . . . . . . . . .   6
     4.5  Encoding Considerations  . . . . . . . . . . . . . . . . .   6
     4.6  Protocols using this URI scheme  . . . . . . . . . . . . .   6
     4.7  Security Considerations  . . . . . . . . . . . . . . . . .   6
     4.8  Contact  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.   Parameters . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1  SSH connection parameters  . . . . . . . . . . . . . . . .   7
     5.2  SFTP Parameters  . . . . . . . . . . . . . . . . . . . . .   7
   6.   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .   8
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .   9
   9.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .   9
   10.  References . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1   Normative References . . . . . . . . . . . . . . . . . .   9
     10.2   Informative References . . . . . . . . . . . . . . . . .  10
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  10
        Intellectual Property and Copyright Statements . . . . . . .  11















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1.  Introduction

   This document describes the Uniform Resource Identifiers (URIs) to be
   used with the Secure File Transfer Protocol (SFTP) [I-D.ietf-secsh-
   filexfer] and Secure Shell (SSH) [RFC4251] protocols.

2.  General Syntax

   A hierarchical URI shall consist of the scheme and the scheme
   specific portion separated by a colon ":" followed by the
   hierarchical part, as discussed in [RFC3986].  This specification
   uses the definitions "port", "host", "scheme", "userinfo", "path-
   empty", "path-abempty" and "authority" from [RFC3986].  This document
   follows the ABNF notation defined in [RFC4234].

3.  Secure Shell (SSH) URI

   This section describes the SSH URI and contains the information
   necessary to register the URI according to the template in
   [I-D.hansen-2717bis-2718bis-uri-guidelines].

3.1  Scheme Name

   The Secure Shell scheme name is "ssh".

3.2  Status

   The requested status of the SSH URI is "permanent".

3.3  URI Scheme Syntax

   The Secure Shell (SSH) scheme shall consist of the scheme name "ssh"
   followed by a colon ":" followed by hier-part defined in [RFC3986].
   The SSH URI ABNF definition follows.


   sshURI        =  "ssh:" hier-part
   hier-part     =  "//" authority path-abempty
   authority     =  [ [ ssh-info ] "@" ] host [ ":" port ]
   host          =  <as specified in [RFC3986]>
   port          =  <as specified in [RFC3986]>
   path-abempty  =  <as specified in [RFC3986]>
   ssh-info      =  [ userinfo ] [";" c-param *("," c-param)]
   userinfo      =  <as specified in [RFC3986]>
   c-param       =  paramname "=" paramvalue
   paramname     =  *( ALPHA / DIGIT / "-" )
   paramvalue    =  *( ALPHA / DIGIT / "-" )




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   The following reserved characters from [RFC3986] are used as
   delimiters within the SSH URI: ";", ",", ":", and "=" .  They must
   not be escaped when used as delimiters and must be escaped when the
   appear in other uses.

3.4  URI Semantics

   The intended usage of the SSH URI is to establish an interactive SSH
   terminal session with the host defined in the authority portion of
   the URI.  The only operation defined for the URI is to establish an
   SSH terminal session with a remote host.

   If the userinfo or connection parameters are present the at-sign "@"
   shall precede the authority section of the URI.  Optionally, the
   authority section MAY also include the port preceded by a colon ":".
   The host SHOULD be a non-empty string.  If the port is not included,
   the default port is assumed.

   The ssh-info portion of the URI MAY include credentials consisting of
   a username followed by optional parameters.  The convention of
   including the password separated from the username by a ":" in the
   URI is NOT RECOMMENDED and is deprecated in accordance with
   [RFC3986].

   One or more optional connection parameters (c-param) may be specified
   within the userinfo section of the URI.  These conn-parameters are
   separated from the userinfo by a semi-colon ";".  The only connection
   parameter defined in this document is for the host-key fingerprint
   described in Section 5.1.  It is possible that additional parameters
   be defined in the future.  If a connection parameter is not
   understood it SHOULD be ignored.

   The SSH URI does not define a usage for a non-empty path element.  If
   a non-empty path element is included in an SSH URI then it SHOULD be
   ignored.

3.5  Encoding Considerations

   The encoding of the "host" portion of the URI is as defined in
   [RFC3986].  The encoding of the connection parameters is described in
   Section 5.1

3.6  Protocols using this URI scheme

   This URI scheme is used by the SSH protocol version 2 defined in
   [RFC4251].





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3.7  Security Considerations

   See Section 8.

3.8  Contact

   This document is a product of the SSH working group.

4.  Secure File Transfer Protocol (SFTP) URI

   This section describes the Secure File Transfer protocol URI and
   contains the information necessary to register the URI according to
   the template in [I-D.hansen-2717bis-2718bis-uri-guidelines].

4.1  Scheme Name

   The Secure File Transfer Protocol (SFTP) scheme name is "sftp".

4.2  Status

   The requested status of the SFTP URI is "permanent".

4.3  URI Scheme Syntax

   The SFTP URI scheme shall consist of the scheme name "sftp" followed
   by a colon ":" followed by hier-part defined in [RFC3986].  The SFTP
   URI ABNF definition follows.


   sftpURI       =  "sftp:" hier-part
   hier-part     =  "//" authority path [";" s-param *("," s-param)]
   path          =  path-abempty
   path-abempty  =  <as specified in [RFC3986]>
   authority     =  [ ssh-info "@" ] host [ ":" port ]
   host          =  <as specified in [RFC3986]>
   port          =  <as specified in [RFC3986]>
   ssh-info      =  [ userinfo ] [";" c-param *("," c-param)]
   userinfo      =  <as specified in [RFC3986]>
   c-param       =  paramname "=" paramvalue
   paramname     =  *( ALPHA / DIGIT / "-" )
   paramvalue    =  *( ALPHA / DIGIT / "-" )
   s-param       =  paramname "=" paramvalue

   The authority part is the same as that defined in the SSH scheme.
   The following reserved characters from [RFC3986] are used as
   delimiters within the SFTP URI: ";", ",", ":", "=" and "/".  They
   must not be escaped when used as delimiters and must be escaped when
   the appear in other uses.



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4.4  URI Semantics

   The intended usage of the SFTP URI is to retrieve the contents of a
   file or directory listing.  The only operation defined for the URI is
   this "GET" operation.

   The authority portion of the SFTP URL is the same as for the SSH URL
   defined in Section 3.4.  The URIs for SFTP are hierarchical URIs
   where each component of the path consists of path elements separated
   by a '/'.  This formatting is meant to represent the path information
   as in Section 5 of [I-D.ietf-secsh-filexfer].  The SFTP
   implementation determines where the root of the path in the URI is.
   It is RECOMMENDED that path be interpreted as an absolute path from
   the root of the file system.  An implementation MAY use the tilde
   ("~") character as the first path element in the path to denote a
   path relative to the user's home directory.  Note that dot segments
   "." and ".." are only interpreted within the URI path hierarchy and
   are removed as part of the URL resolution process defined in
   [RFC3986].

   Following the path additional sftp specific parameters may be
   specified.  These are described in Section 5.2.  It is possible that
   additional parameters be defined in the future.  If a sftp parameter
   is not understood it SHOULD be ignored.

4.5  Encoding Considerations

   Path segments SHOULD be represented in the UTF-8 [RFC3629] character
   set and clients SHOULD NOT disable UTF-8 translation on the server
   with the filename-translation-control extension.  The shortest valid
   UTF-8 encoding of the UNICODE data MUST be used.  The encoding of the
   "host" portion of the URI is as defined in [RFC3986].  The encoding
   of the connection parameters is described in Section 5.1 and the
   encoding of SFTP parameters is described in Section 5.2.

4.6  Protocols using this URI scheme

   This URI scheme is used by the SFTP protocol defined in [I-D.ietf-
   secsh-filexfer].

4.7  Security Considerations

   The SFTP URI retrieves data from a remote host.  Even though the
   connection is secured by SFTP care should be taken in handling and
   processing data retrieved from potentially unknown sources to avoid
   malicious content from an attacker that may have been placed on the
   host.  For additional security considerations see Section 8.




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4.8  Contact

   This document is a product of the SSH working group.

5.  Parameters


5.1   SSH connection parameters

   The following parameters are associated with an SSH connection and
   are applicable to SSH and SFTP.  All parameters are optional and MUST
   NOT overwrite configured defaults.  Individual parameters are
   separated by a comma (",").

   fingerprint

      The fingerprint parameter contains the fingerprint of the host key
      for the host specified in the URL.  The fingerprint is encoded as
      host-key-alg-fingerprint.  Host-key-alg is host public key
      algorithm defined in [RFC4253] and the fingerprint format is
      [I-D.ietf-secsh-publickeyfile].  For use in a URI, the fingerprint
      shall use a single dash "-" as a separator instead of the colon
      ":" as described in [I-D.ietf-secsh-publickeyfile].  This
      parameter MUST NOT overwrite a key that is already configured for
      the host.  The fingerprint MAY be used to validate the
      authenticity of the host key if the URL was obtained from an
      authenticated source with its integrity protected.  If this
      parameter is not included then the host key is validated using
      another method.  See Security Considerations section for
      additional considerations.  There MUST be only one fingerprint
      parameter present in a given URL.

5.2  SFTP Parameters

   The SFTP parameters determine how to handle the file transfer
   character translation.  Additional parameters MAY be used.

   typecode

      The typecode identifies the type of file which determines how it
      will be treated.  The name for the typecode attribute is "type".
      The value "i" indicates that a file should be transmitted without
      character conversion performed.  The value "a" indicates that the
      file should be opened with the SSH_FXF_ACCESS_TEXT_MODE flag set
      so it is converted to the canonical newline convention in use.
      The value "d" indicates that the path is a directory and should be
      opened using SSH_FXP_OPENDIR.




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6.  Examples

   The following section shows basic examples of URLs for each protocol.
   This section should not be considered to include all possible
   combinations of URLs for each protocol.

   An SSH connection to the host host.example.com on the standard port
   using username user.

        ssh://user@host.example.com

   An SSH connection to the host host.example.com on port 2222 using
   username user.

        ssh://user@host.example.com:2222

   An SSH connection to the host having the specified host-key
   fingerprint at host.example.com on the standard port using username
   user.

        ssh://user;fingerprint=ssh-dss-c1-b1-30-29-d7-b8-de-6c-97-
             77-10-d7-46-41-63-87@host.example.com

   Retrieve file.txt from the user's home directory on the host at
   host.example.com using SFTP using username user.  This example
   assumes that the implementation supports the indication of a path
   relative to the home directory using a leading tilde.

        sftp://user@host.example.com/~/file.txt

   Retrieve file.txt from the absolute path /dir/path on the host at
   host.example.com using SFTP using username user.

        sftp://user@host.example.com/dir/path/file.txt

   Retrieve the directory listing of user's home directory on the host
   having the specified host-key fingerprint at host.example.com using
   SFTP.

        sftp://user;fingerprint=ssh-dss-c1-b1-30-29-d7-b8-de-6c-97-
              77-10-d7-46-41-63-87@host.example.com:2222/;type=d

7.  IANA Considerations

   Section 3 and Section 4 provide the information required in the URL
   registration template in accordance with [I-D.hansen-2717bis-2718bis-
   uri-guidelines].




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8.  Security Considerations

   Passwords SHOULD NOT be included within the URI as doing so poses a
   security risk.  URIs are usually sent in the clear with no encryption
   or other security, any password or other credentials included in the
   userinfo could be seen by a potential attacker.

   Although the host-key fingerprint is not confidential information,
   care must be taken in handling fingerprints associated with URIs
   because URIs transmitted or stored without protection may be modified
   by an attacker.  In general an implementation cannot determine the
   source of a URI so a fingerprint received in a URI should have no
   more trust associated with it than a raw public key received in the
   SSH protocol itself.  If a locally configured key exists for the
   server already it MUST NOT be automatically overwritten with
   information from the URI.  If the host is unknown then the
   implementation should treat the fingerprint received with the same
   caution that it does with any unknown public key.  The client MAY
   offer the fingerprint and URI for external validation before allowing
   a connection based on this information.  If the client chooses to
   make a connection based on the URI information and it finds that the
   fingerprint in the URI and the public key offered by the server do
   not match then it SHOULD provide a warning and provide a means to
   abort the connection.  Sections 4.1 and 9.2.4 of [RFC4251] provide a
   good discussion of handling public keys received in the SSH protocol.

9.  Acknowledgements

   Ben Harris, Tom Petch and the members of the SSH working group have
   provided much useful feedback in the preparation of this document.

10.  References

10.1  Normative References

   [I-D.ietf-secsh-filexfer]
              Galbraith, J. and O. Saarenmaa, "SSH File Transfer
              Protocol", draft-ietf-secsh-filexfer-12 (work in
              progress), January 2006.

   [I-D.ietf-secsh-publickeyfile]
              Galbraith, J. and R. Thayer, "SSH Public Key File Format",
              draft-ietf-secsh-publickeyfile-11 (work in progress),
              January 2006.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.




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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

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

   [RFC4253]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, January 2006.

10.2  Informative References

   [I-D.hansen-2717bis-2718bis-uri-guidelines]
              Hansen, T., "Guidelines and Registration Procedures for
              new URI Schemes",
              draft-hansen-2717bis-2718bis-uri-guidelines-06 (work in
              progress), October 2005.


Authors' Addresses

   Joseph Salowey
   Cisco Systems
   2901 3rd Ave
   Seattle, WA  98121
   US

   Email: jsalowey@cisco.com


   Steve Suehring
   PO BOX 1033
   Stevens Point, WI  54481
   US

   Email: suehring@braingia.com












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