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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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