INTERNET-DRAFT Christopher R. Hertel
draft-crhertel-smb-url-11.txt Samba Team
Expires January 8, 2007 July 8, 2006
SMB File Sharing URI Scheme
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Abstract
The Server Message Block (SMB) protocol is one of the most widely
used network file system protocols in existence. This document
describes a scheme for an SMB Uniform Resource Indicator (SMB URI)
which can be used to identify SMB workgroups, servers, server shares,
directories, files, inter-process communications ports (named pipes),
and devices--the objects in the SMB network file system space.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Purpose. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SMB URI Scheme Overview . . . . . . . . . . . . . . . . . . . . . . 4
3. NetBIOS, NBT, and NBT Transport Semantics . . . . . . . . . . . . . 4
3.1. The NetBIOS Name Service . . . . . . . . . . . . . . . . . . . 5
3.2. The NetBIOS Datagram Service . . . . . . . . . . . . . . . . . 5
3.3. The NetBIOS Session Service. . . . . . . . . . . . . . . . . . 5
3.4 Accommodating NBT in the SMB URI . . . . . . . . . . . . . . . 5
4. NetBIOS-Based Workgroups. . . . . . . . . . . . . . . . . . . . . . 6
5. SMB URI Definition. . . . . . . . . . . . . . . . . . . . . . . . . 9
6. SMB URI Syntax Elements . . . . . . . . . . . . . . . . . . . . . .10
6.1. scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
6.2. smb-service. . . . . . . . . . . . . . . . . . . . . . . . . .10
6.3. auth-domain. . . . . . . . . . . . . . . . . . . . . . . . . .11
6.4. smb-srv-name . . . . . . . . . . . . . . . . . . . . . . . . .11
6.5. port . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
6.6. scope-id . . . . . . . . . . . . . . . . . . . . . . . . . . .12
6.7. nbt-context. . . . . . . . . . . . . . . . . . . . . . . . . .13
7. The Relationship Between the SMB URI and the UNC Format . . . . . .15
8. Authentication. . . . . . . . . . . . . . . . . . . . . . . . . . .16
9. Security Considerations . . . . . . . . . . . . . . . . . . . . . .16
10. Character Encoding Issues. . . . . . . . . . . . . . . . . . . . .16
11. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . .17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .18
14. Copyright Notice . . . . . . . . . . . . . . . . . . . . . . . . .19
15. Disclaimer of Validity . . . . . . . . . . . . . . . . . . . . . .19
Appendix A. Working with NetBIOS Names (Implementation Notes). . . . .20
A.1. NetBIOS Names. . . . . . . . . . . . . . . . . . . . . . . . .20
A.2. SMB Sessions via NBT . . . . . . . . . . . . . . . . . . . . .20
A.3. Resolving DNS names and IP addresses to SMB server names . . .21
A.4. Determining the Namespace of the smb-srv-name. . . . . . . . .22
A.5. Workgroup vs. SMB Server Names . . . . . . . . . . . . . . . .24
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1. Introduction
The Server Message Block protocol (SMB) was created in the 1980's by
Dr. Barry Feigenbaum at IBM Corporation. It was later extended by
various contributors at 3Com, IBM, Intel, and Microsoft. During the
mid 1990's SMB was renamed CIFS (Common Internet File System). Both
names are in general use today. In common parlance, however, the
term "SMB" is typically used when referring to the core filesharing
protocol itself, while "CIFS" includes the set of sub-protocols and
extensions used to create the complete filesharing suite.
SMB was originally carried via a proprietary network transport, the
interface to which was called NetBIOS (Network Basic Input Output
System). Two Internet RFCs ([RFC1001], [RFC1002]) were published
which describe a mechanism for implementing the NetBIOS API on top
of TCP and UDP, thus allowing SMB to be transported over IP inter-
networks. Those RFCs are now known collectively as Internet
Standard #19, and the transport protocol that they describe is
commonly called NBT (or, sometimes, NetBT) for "NetBIOS over
TCP/IP."
In addition to transport via NBT, newer implementations of SMB
typically support SMB over TCP/IP without the intervening NetBIOS
emulation layer. This is known as SMB over "native TCP" or "naked
transport".
Several attempts have been made to document and even standardize the
CIFS suite ([XOPENSMB], [ONET], [SNIACIFS], [IMPCIFS]), yet the
further development of CIFS remains under the control of Microsoft.
Despite its proprietary nature, the workings of SMB are sufficiently
well known that SMB file sharing has been successfully implemented
by several third-party commercial vendors and in Open Source. SMB
server and client software is available for a wide variety of
operating system platforms. The very large number of systems which
support this form of file sharing make an SMB URI scheme both
practical and desirable.
1.1. Purpose
This document does not attempt to detail the proper implementation of
SMB/CIFS or the workings of the NBT transport layer. The goal is to
present the syntax of the SMB URI and to describe how that syntax is
mapped to the semantics of the CIFS suite. A limited set of
implementation guidelines is provided in the appendices.
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2. SMB URI Scheme Overview
An SMB URI is identified by one of two scheme names: "smb" or "cifs".
There are existing implementations that support one or the other or
both of these names, so new and updated implementations must,
unfortunately, provide support for both. As of this writing, the
"smb" name appears to be the more popular of the two and is,
therefore, the preferred form to use when transcribing SMB URIs.
The SMB URI conforms to the general syntax of URI described in
[RFC3986]. The scheme is comparable to other URI schemes that are in
common use, and should be familiar to anyone who uses common user
agents such as web browsers. For example, consider the following
URIs:
ftp://server/dirpath/file.name
smb://server/dirpath/file.name
Both strings identify the same file, differing only in the protocol
used to access the file.
As with other URI schemes, servers (hosts) may be identified by DNS
name, IPv4 address, or IPv6 address. The SMB URI scheme also
supports the use of NetBIOS names to identify SMB services. NetBIOS
names are resolved via the NBT Name Service, as described in
[RFC1001] and detailed in [RFC1002].
3. NetBIOS, NBT, and NBT Transport Semantics
NBT was created so that applications and services that made use of
the NetBIOS API could communicate over IP internetworks. The NBT
transport maps the semantics of the NetBIOS API onto TCP and UDP,
hiding the fact that the underlying transport mechanism has changed.
In order to make this work, RFC1001/1002 define three key services:
- The NetBIOS Name Service
- The NetBIOS Datagram Service
- The NetBIOS Session Service
These three services collaborate to create a "virtual NetBIOS LAN"
on top of IP networks.
The use of NBT as a transport for SMB is widespread, so the syntax
and semantics of the SMB URI scheme have been adapted to accommodate
NetBIOS naming conventions and NBT context requirements.
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3.1. The NetBIOS Name Service
The NetBIOS API uses names (strings of 16 octets) to identify
communications endpoints. These names are the addresses used in
"NetBIOS LANs". The NetBIOS Name Service is responsible for
transparently mapping these names to the IP addresses at which they
are registered, so that NetBIOS applications and services can find
one another.
The set of nodes that are connected to the same virtual NetBIOS LAN
is known as the "scope" of the LAN. NBT scopes are always given a
name, or Scope Identifier (Scope ID), though this normally goes
unnoticed because the default Scope ID is the empty string ("").
Multiple scopes, each having a different Scope ID, may intersect
across the same IP internetwork without conflict.
The SMB URI scheme provides a mechanism for specifying the NBT
Scope ID in the URI string.
3.2. The NetBIOS Datagram Service
The SMB URI scheme does not make direct use of the Datagram Service,
so there is no special syntax to support the handling of NetBIOS
datagram messages.
3.3. The NetBIOS Session Service
The Session Service provides a one-to-one mapping of NetBIOS Sessions
to TCP sessions. NetBIOS sessions over TCP are initiated by sending
an NBT SESSION REQUEST message. An NBT POSITIVE SESSION RESPONSE
indicates that the session has been established successfully. (See
[RFC1001], section 16.2.)
The SMB URI scheme supports the exchange of SMB messages over either
NBT sessions or "naked" TCP sessions. Other than the SESSION
REQUEST/POSITIVE SESSION RESPONSE, there is no difference between
the two transport types.
3.4 Accommodating NBT in the SMB URI
The SMB URI scheme supports both NBT and native TCP transport of SMB
messages. The syntax of the scheme provides for the inclusion of NBT
context information so that NetBIOS names can be properly resolved
and NetBIOS sessions established.
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The scheme provides the ability to specify the following NBT context
information:
- Broadcast Address for NBT broadcast name resolution (B mode)
- NBNS Address for point-to-point name resolution (P mode)
- Name resolution mode selection (B, P, M, or H mode)
- NBT Scope ID
- NetBIOS CALLED name (destination address)
- NetBIOS CALLING name (source address)
Most SMB implementations use configuration files or DHCP to
establish an initial NBT context. The starting context is referred
to as the "base context" in the remainder of this document. NBT
context information given in absolute URIs is applied against the
base context to create the "current context". NBT context
information given in relative URIs is applied against the current
context to modify the current context.
The current NBT context is always used to interpret the meaning of a
given URI string. A relative URI containing updated NBT context
information will cause the resulting URI to be re-evaluated.
4. NetBIOS-Based Workgroups
The "workgroup" system is a part of the SMB/CIFS protocol suite.
Workgroups are built on top of NetBIOS, and are identified by their
NetBIOS names. The workgroup system allows SMB file servers to be
organized into named groups, with the goal of making it easier to
locate resources by categorizing them.
Within each workgroup, a list of member servers is maintained. In
addition to the server list, each workgroup maintains a list of all
other known workgroups. The combined list is known as the "browse
list". A copy of the browse list may be obtained by sending a
specific query via SMB.
The SMB URI scheme views the SMB file sharing environment
hierarchically. Conceptually, the hierarchy is arranged as follows:
List of workgroups (from the browse list)
+ List of servers within a given workgroup (from the browse list)
+ List of shares (shared objects) offered by a server
+ Directories, files, etc. within a share
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That hierarchy is mapped to the SMB URI scheme as follows:
URI Format Indicates
========================= =======================================
smb:// List of known workgroups
smb://smb-wrkgrp/ + SMB servers within the workgroup
smb://smb-server/ + Shares offered by an SMB server
smb://smb-server/abs-path + Directories, files, etc.
As shown above, the SMB URI provides syntax that indicates requests
for subsets of the browse list. In particular, the form:
smb://
represents a request for the list of all known workgroups, while the
form:
smb://smb-wrkgrp/
represents a request for the list of servers that are members of a
particular workgroup.
A problem arises, however, because the syntax used for requesting the
list of servers in a workgroup is indistinguishable from that of a
request for the list of shares (shared objects, such as exported
filesystems) offered by an SMB file server. Thus, the two requests
must be differentiated semantically. Consider the following example:
smb://corgi/
If the name "corgi" is a NetBIOS name and it resolves (via the NBT
Name Service) to a workgroup name, then a user agent would return a
list of servers in the CORGI workgroup. If, however, the name
resolves to a server name, then the user agent would return a list
of shares offered by the SMB server named CORGI.
It is rare, but possible (in a misconfigured NBT network), that a
NetBIOS name will represent both a workgroup and an SMB file server.
In this situation, SMB file services take precedence. Some user
agents may be capable of returning both the list of servers in the
workgroup and the list of shares provided by the SMB file server,
allowing the user to determine which is the desired result. This
is the recommended approach.
There is a special case to be considered when using relative
references to move between a workgroup reference and a reference to a
server in the workgroup. Consider a workgroup named "corgis" and a
server named "cue" that is a member of that workgroup.
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Presented with the URI string
smb://corgis/
a user agent may return a list of servers that are members of the
CORGIS workgroup, including node CUE, and allow the user to select
one of those SMB servers.
The relative reference used to transition from
smb://corgis/
to
smb://cue/
would be "../cue".
When moving upward in the hierarchy, one might might expect:
"smb://cue/" + ".." ==> "smb://"
In this example, however, the user agent is aware that node "cue" is
a member of the "corgis" workgroup, so:
"smb://cue/" + ".." ==> "smb://corgis/"
The NBT workgroup membership of an SMB server may be determined
either by sending a Node Status Request query to the server (see
[RFC1001], section 15.1.4) or by maintaining a local cache of
workgroup information, or both. Obviously, the choice is
implementation dependent. If the server's workgroup membership is
not available via either of these methods, then it is acceptable to
move directly to the top of the hierarchy (smb://).
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5. SMB URI Definition
The following grammar defines the syntax of the SMB URI. It is
based upon the grammar given in Appendix A of [RFC3986]. Refer to
that RFC for token definitions missing from the grammar below.
smb-URI = ( smb-absURI / smb-relURI )
smb-absURI = scheme "://" smb-service [ "?" [ nbt-context ] ]
smb-relURI = ( path-absolute / path-rootless )
[ "?" [ nbt-context ] ]
scheme = "smb" / "cifs"
smb-service = ( smb-wrkgrp / smb-net-path )
smb-wrkgrp = [ smb-userinfo "@" ] [ smb-srv-name ]
[ ":" port ] [ "/" ]
smb-net-path = smb-server [ path-absolute ]
smb-server = [ smb-userinfo "@" ] smb-srv-name [ ":" port ]
smb-srv-name = nbt-name / host
nbt-name = netbiosname [ "." scope-id ]
netbiosname = netbiosnamec 1*14( netbiosnamec / "*" )
netbiosnamec = ( ALPHA / DIGIT / pct-encoded
/ "-" / "_" / "~"
/ "!" / "$" / "&" / "'" / "(" / ")"
/ "+" / "," / ";" / "="
)
scope-id = [ scope-label *( "." scope-label ) ]
scope-label = 1*63( scope-char )
scope-char = ALPHA / DIGIT / "-" / "_" / "~"
/ sub-delims / pct-encoded
smb-userinfo = [ auth-domain ";" ] userinfo-nosem
auth-domain = smb-srv-name
userinfo-nosem = *( unreserved / pct-encoded
/ "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / "=" / ":"
)
nbt-context = nbt-param *(";" nbt-param )
nbt-param = ( "BROADCAST=" IPv4address [ ":" port ]
/ "CALLED=" netbiosname
/ "CALLING=" netbiosname
/ ( "NBNS=" / "WINS=" ) host [ ":" port ]
/ "NODETYPE=" ("B" / "P" / "M" / "H")
/ ( "SCOPEID=" / "SCOPE=" ) scope-id
)
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6. SMB URI Syntax Elements
The SMB URI scheme is more or less comparable to other URI schemes
used for remote filesystem access. It differs primarily in its
support for the NBT transport and NBT workgroups. This section
provides further explanation and description of those syntax
elements that are most likely to require clarification.
6.1. scheme
As described in section 2, above, an SMB URI is identified by one of
two scheme names: "smb" or "cifs". These are equivalent, but the
former is preferred.
6.2. smb-service
The SMB URI can be used to access workgroup information or SMB file
server services. There are minor differences in SMB URI syntax
depending up on which of these service types is being accessed.
It is possible, for instance, to request workgroup information
without specifying a destination server name. In particular,
the URI:
smb://
represents a request for the list of locally available NBT
workgroups.
In some situations the workgroup list may not be available to
unauthenticated users, so the SMB URI scheme allows inclusion of
smb-userinfo information without the need to specify an smb-srv-name
(a workgroup name). Thus, the following is permitted:
smb://user@/
In the above, the username "user" is being supplied. (The user agent
should prompt for a password to prevent the password from being
exposed.)
As with the smb-userinfo field, an SMB URI may include a port
reference without an smb-srv-name, as in the following example:
smb://:4220/
(This is an example of an attempt to retrieve an NBT workgroup list
via SMB using destination TCP port 4220.)
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Another difference between workgroup and SMB file server references
is that workgroup references can not be followed by a path. The
browse list does not offer shares, directories, or files so an SMB
URI string such as the following cannot represent a workgroup query:
smb://corgis/puppies/
6.3. auth-domain
The auth-domain field is passed to the underlying SMB layer for
interpretation. It is used to specify the SMB authentication
authority (typically a "Domain Controller"). The interpretation of
this field is specific to the workings of the SMB protocol and should
be handled by the underlying SMB implementation.
6.4. smb-srv-name
The SMB URI supports the use of NetBIOS names and Scope IDs to
identify SMB servers and services. When included as part of an SMB
URI, the syntax of the NetBIOS name is a superset of the syntax of a
DNS domain name label. For example:
smb://jcifs/
Syntactically, the string "jcifs" in the smb-srv-name field of the
above string may be seen as either a DNS host name (unqualified), or
as a NetBIOS name. The underlying SMB implementation must determine
the namespace of the name. (This is a common problem in SMB
implementations and is typically solved by first attempting to
resolve the name as a NetBIOS name and then, if that fails, as a DNS
host name.)
Likewise, given:
smb://jcifs.samba.org/
the string "jcifs.samba.org" may be interpreted either as a qualified
DNS name, or as a NetBIOS name with appended Scope ID.
A NetBIOS name is simply a string of octets with a maximum length of
15 octets. (The actual maximum length of the NetBIOS name is
16-octets, but the 16th is reserved.) In practice, the only
restriction on the syntax of a NetBIOS name is that it may not begin
with an ASCII asterisk character (0x2A). Octet values that are
permitted by NetBIOS name syntax but excluded by the SMB URI syntax
must be escaped. Note, in particular, that the dot character (0x2E)
must be escaped if used in a NetBIOS name.
The resolution of NetBIOS names to IP addresses is described in
[RFC1001] and [RFC1002].
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6.5. port
RFC1001/1002 includes a mechanism for retargeting Session Service
connections to alternate ports (see [RFC1001], section 16.1.1.)
which means that non-standard ports may be used for SMB over NBT
transport. There may be other valid reasons for providing SMB
services on on-standard ports.
The URI port field may be used to specify an alternate service
port for SMB over either NBT or native TCP transport. (The
transport type must be detected by the underlying SMB
implementation.)
6.6. scope-id
The correct syntax of an NBT Scope Identifier is described in section
9 of [RFC1001] as
"...a character string meeting the requirements of the domain name
system for domain names."
The intent was that the Scope ID should match the "preferred syntax"
for domain names, as given in Appendix 1 of [RFC883] (which has since
been superceded). Specifically:
"The labels must follow the rules for ARPANET host names. They
must start with a letter, end with a letter or digit, and have as
interior characters only letters, digits, and hyphen. There are
also some restrictions on the length. Labels must be 63
characters or less."
Several implementations of NBT now exist that disregard the character
set restrictions described above. The SMB URI is, therefore,
a bit more flexible with regard to the octet values that may be
used. In fact, the grammar given above allows the use of pct-encoded
values, so any octet value may be specified. Note, however, that
implementations MUST discard zero values ("%00") in the Scope ID
because many implementations interpret the Scope ID as being a
nul-terminated string.
Although the syntax allows Scope IDs that do not match the
original preferred syntax, the use of nonconforming Scope IDs is
generally considered unwise because the interpretation of
nonconforming Scope IDs is likely to be inconsistent.
The requirement that the Scope ID consist of dot-delimited labels of
one to sixty-three octets is enforced by the on-the-wire encoding
used by NBT.
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The SMB URI scheme provides two mechanisms for specifying an NBT
Scope ID. The first, as shown in the grammar above, is to append the
Scope ID to the NetBIOS name as part of the smb-srv-name field, using
a dot (".") as a delimiter. This mechanism is included to support
existing implementations.
The other mechanism is to specify the Scope ID as part of the
nbt-context. The two examples given below are equivalent:
smb://netbios.scope.id/
smb://netbios/?SCOPE=scope.id
The latter format is less ambiguous and, therefore, preferred. User
agents that rewrite URI strings for display purposes should rewrite
SMB URI strings that contain a Scope ID to conform to the nbt-context
format.
Note also that the scope-id syntax specifically permits a Scope ID
that is the empty string (""). The empty string is a valid Scope
ID and is, in fact, the default on all known implementations.
6.7. nbt-context
NBT context information is appended to the tail end of an SMB URI
string in the form of a URI query string. Context information is
specified using key/value pairs. Multiple context elements may be
specified by separating the key/value pairs with semi-colons.
The nbt-context may be used to provide information about the NBT
transport layer and related support servers. Information provided
in the nbt-context overrides the current NBT context maintained by
the user agent. The nbt-context is interpreted locally by the user
agent.
The nbt-context is made up of zero or more nbt-params fields, which
are specified as key/value pairs. For example:
smb://jcifs/?CALLED=VIRTSERV;NBNS=172.24.19.18
In the above example, the CALLED parameter is assigned a value of
"VIRTSERV", and the NBNS parameter is assigned a value of
"172.24.19.18".
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The following keywords are defined:
BROADCAST: The IPv4 broadcast address to which to send NBT
broadcast name queries. This may, for example, be
used on multi-homed hosts to specify a target subnet.
The value assigned to the BROADCAST keyword may
optionally include a port number (delimited by
a colon). The standard port for NBT name resolution
is UDP/137. It is rare that a different port will
be used for broadcast name resolution.
CALLED: Specifies the NetBIOS name of the SMB server (the
NetBIOS destination address.) A CALLED name is
required by the NBT Session Request message (see
[RFC1002], Section 4.3.2).
If NBT transport is used and the CALLED name is not
specified within the URI string, the underlying SMB
implementation must deduce the CALLED name from
available information. (See Appendix A, below.)
CALLING: Specifies the NetBIOS name of the client (the NetBIOS
source address.) This value is only used with NBT
transport. It is required by the NBT Session Request
message (see [RFC1002], Section 4.3.2).
If NBT transport is used and the CALLING name is not
specified in the current NBT context, the underlying
SMB implementation must generate a suitable name.
(Typically, this will be based on the system's host
name.)
NBNS: Specifies the NetBIOS Name Server (NBNS) to be used
for point-to-point NBT Name Resolution. The NBNS may
be specified using a DNS name or an IP address. See
[RFC1001] for information on the NBNS.
The value assigned to this parameter may, optionally,
include a port number (delimited by a colon). The
standard port for NBT name resolution is UDP/137.
The use of a non-standard port for point-to-point NBT
name resolution is rare (but less so than it is for
broadcast name resolution).
NODETYPE: One of B, P, M, H, or the empty string. These
represent the different mechanisms by which a NetBIOS
name may be resolved to an IP address on an NBT
network. The first three types are defined in RFC
1001/1002. H mode is the inverse of M mode (in H mode
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the NBNS is queried before a broadcast query is sent).
An empty NODETYPE indicates that NBT name resolution
should not be attempted (use DNS name resolution
instead). Some examples:
smb://smedley/?NBNS=172.24.19.18;NODETYPE=H
smb://corgis/?NODETYPE=B
smb://jcifs.samba.org/?NODETYPE=;CALLED=SMBSERV
SCOPE: Specifies the NBT Scope Identifier. Use of the SCOPE
keyword is preferred over inclusion of the Scope ID in
the nbt-name field. User agents must support both
mechanisms.
The default Scope ID is the empty string. This can be
specified in the SMB URI by assigning an empty value
to the SCOPE keyword. For example:
smb://bran/SCOPE=
smb://marika/SCOPE=;NODETYPE=B
SCOPEID: A synonym for SCOPE.
WINS: A synonym for NBNS.
Although all of the keywords and values are shown in upper case, case
is not significant.
The client implementation should provide a means for setting the base
context. The nbt-context is used to override default values or to
supply values missing from the local configuration. Most of all, the
nbt-context makes it possible for an SMB URI string to maintain a
consistent interpretation as it travels from one NBT scope to
another.
7. The Relationship Between the SMB URI and the UNC Format
Some operating systems support a format known as Universal Naming
Convention (UNC). UNC is a means for identifying network resources.
SMB is one of the protocols supported by UNC.
In general, a UNC string specifying a resource available via SMB
protocol can be converted into an SMB URI string by simply adding
the "smb:" (or "cifs:") prefix and reversing the direction of all
of the separating slashes. For example:
UNC form URI form
------------------------------- -----------------------------------
\\corgis\docs\ smb://corgis/docs/
\\corgis\docs\jolyon\ smb://corgis/docs/jolyon/
\\corgis\docs\jolyon\rabbit.txt smb://corgis/docs/jolyon/rabbit.txt
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8. Authentication
SMB authentication can be divided into the following categories:
o None
o Share-based
o User-based
o Authentication-Server-based (NT Domain, AD Domain, or Kerberos)
The authentication mechanism to be used is negotiated during
client/server session setup. Client applications, therefore, are
aware of the server's authentication requirements and may prompt for
appropriate input (username, password, authentication domain). By
prompting for authentication information, an application ensures that
such information is entered by the user in a controlled manner, and
that security measures (if any), such as password encryption or
password hash generation, are applied by the SMB protocol handler
before the data are transmitted.
Some authentication values may also be provided within the SMB URI
string. In particular, the following fields may optionally be
included in the URI:
auth-domain - The authentication domain (single-signon database
server) to use for authorization
userinfo - User account identifier (username)
9. Security Considerations
All of the implementations of the SMB URI that are known to exist at
the time of this writing support inclusion of a password in the
URI string. This is generally considered to be dangerous, since
it may encourage exposure of the plaintext password. User agents
should provide a mechanism for prompting the user to enter passwords,
separate from the URI itself, in a reasonably secure fashion.
All other general security considerations relate to the workings of
the SMB/CIFS protocol suite and are beyond the scope of this
document.
10. Character Encoding Issues
The only restriction that RFC1001/1002 places on the octet values
that may be used in a NetBIOS name is that the name may not begin
with an asterisk ('*', ASCII value 0x2A). No other values are
excluded by those RFCs. For historical reasons, however, some
implementations disallow the use of a nul byte (0x00) within a
NetBIOS name. NetBIOS names are interpreted as fixed-length
strings of octets, so common mutli-byte character sets may cause
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problems with older implementations.
Octet values less than 128 (0x80) in a NetBIOS name are typically
interpreted as US-ASCII characters. The interpretation of octet
values above 127 is dependent upon host configuration; there is no
protocol mechanism to specify which codepage or character set is in
use. In general, URI escape sequences should be used to represent
characters with octet values above 127.
NetBIOS names, share names, and the directory paths and filenames
offered by an SMB server may all contain characters from outside the
7-bit US-ASCII character set. Applications MUST support the use of
the URI escape sequence as described in [RFC3986] to accommodate
octet values that represent non-US-ASCII characters.
The SMB protocol has evolved over time to include support for various
character encoding schemes. A complete discussion of SMB and NBT
character encoding issues is way beyond the scope of this document.
11. Acknowledgments
The creation of this document would not have been possible without
the help and guidance of
Michael B. Allen
David Farmer
Roy T. Fielding
Steven French
Larry Masinter
Richard Sharpe
and the aggregate knowledge and wisdom of
The jCIFS Team
The Samba Team
The Samba-TNG Team
The SNIA CIFS Working Group
The samba-technical and jCIFS mailing list participants
The IETF URI-review and W3C URI mailing list participants
Funding for the RFC Editor function is currently provided by the
Internet Society.
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12. References
[RFC883] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION and
SPECIFICATION", RFC 883, November 1983.
[RFC1001] Karl Auerbach, et. al., "Protocol Standard For a NetBIOS
Service on a TCP/UDP Transport: Concepts and Methods", RFC
1001, March 1987.
[RFC1002] Karl Auerbach, et. al., "Protocol Standard For a NetBIOS
Service on a TCP/UDP Transport: Detailed Specifications",
RFC 1002, March 1987.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986,
January 2005.
[XOPENSMB] "Protocols for X/Open PC Interworking: SMB, Version 2",
ISBN 1-872630-45-6, The Open Group, October 1992.
[ONET] Microsoft Corporation, Intel Corporation, "Microsoft
Networks/OpenNET Filesharing Protocol", Document Version
2, Intel Part No. 138446, November 7, 1988.
[SNIACIFS] Storage Network Industry Association CIFS Documentation
Work Group, "Common Internet File System (CIFS) Technical
Reference", Version: CIFS-TR 1.0, March 1, 2002.
[IMPCIFS] Hertel, Christopher R., "Implementing CIFS -- the Common
Internet File System", ISBN 0-13-047116-X, Prentice Hall
PTR, August 2003
(http://ubiqx.org/cifs/)
13. Author's Address
Christopher R. Hertel
Alvarri, Inc.
15 South Fifth Street
Suite 1010
Minneapolis MN 55402
E'mail: crh@samba.org
crh@ubiqx.mn.org
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14. Copyright Notice
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
15. Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Appendix A. Working with NetBIOS Names (Implementation Notes)
The information presented in this section is intended as a guide for
implementors.
Name resolution, particularly with the inclusion of support for
RFC1001/1002 NBT naming, may result in ambiguous meaning for some SMB
URI strings. This problem is reduced if correct NBT context
information is included in URI strings, and can be eliminated if all
implementations follow the same basic sequence when resolving server
names to addresses.
A.1. NetBIOS Names
NetBIOS names are addresses. They represent communication end-points
within a NetBIOS LAN. [RFC1001] and [RFC1002] provide a mechanism
for creating virtual NetBIOS LANs over TCP and UDP transport. The
core of that mechanism is the NetBIOS Name Service, which provides
for mapping between NetBIOS names and the IP addresses at which
they are registered. A given host system may register several
NetBIOS names, each representing an application or service that can
communicate with other applications or services through the NetBIOS
API.
A.2. SMB Sessions via NBT
SMB sessions are established and messages transferred via the NetBIOS
session service (see [RFC[1001], section 5.3 and [RFC1002] section
4.3). The system that originates the connection is the "calling"
node, and the target node is the "called" node. In order to
establish an SMB session, a TCP connection must be established
between the calling and called nodes.
Before a NetBIOS session can be established, the calling node must
discover the IP address of the called node. This is done using the
NetBIOS Name Service (see [RFC1001] section 5.2 and [RFC1002] section
4.2). NetBIOS names are always 16 octets, padded with spaces (0x20)
if necessary, as specified in the RFCs. By convention, however, the
16th octet is reserved for use as a service type indicator. This
field is known as the "suffix".
The suffix byte is NEVER specified in an SMB URI string, but is
appended by the client implementation.
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A.3. Resolving DNS names and IP addresses to SMB server names
The NetBIOS Session Service requires that the client provide the
NetBIOS names of both the calling and called nodes in the NBT
SESSION REQUEST. When connecting to an SMB server, the calling name
(source address) is typically the default NetBIOS name of the client,
space padded as described, with a suffix byte value of 0x00. The
called name (destination address) is the NetBIOS name of the server
with a suffix byte value of 0x20.
Applications which support the SMB URI must include support for the
use of DNS names or IP addresses in addition to NetBIOS names when
initiating SMB connections via NetBIOS over TCP/IP (NBT) transport.
This functionality is an extension to the NetBIOS over TCP/IP
behavior specified in RFC 1001 and RFC 1002, and is not part of that
standard. It is, however, a common extension and must be supported
for compatibility reasons, and to provide access to SMB shares in
situations in which the NetBIOS name space cannot be guaranteed to be
consistent.
As stated above, the Session Request packet requires a called and a
calling name, both of which are NetBIOS names. In order to create an
NBT Session Request packet, the DNS name or IP address of the server
must be reverse-mapped to the server's NetBIOS name. Mechanisms for
doing so include:
- Issuing a NetBIOS Adapter Status Query
A NetBIOS Adapter Status Query is sent to the target IP address.
(See [RFC1001] section 15.1.4 and [RFC1002] sections 4.2.17 &
4.2.18.) If a response is received, and if the target node is
running an SMB server service, then the response will include a
NetBIOS name with a suffix byte value of 0x20. This NetBIOS name
may be used as the called name in a Session Request packet.
It is possible that multiple entries will have a suffix byte of
0x20. If this is the case each name may be tried in turn, or one
of the other methods must be used to discover the name of the SMB
server service.
- Generic Server Name
This method is not supported by all SMB server implementations.
Some SMB servers will accept the generic SMB server name
"*SMBSERVER". A client can simply use the name "*SMBSERVER" as
the called name in a Session Request packet. As with all SMB
server NetBIOS names, the "*SMBSERVER" name must be space padded
and terminated with a suffix byte value of 0x20.
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The "*SMBSERVER" name begins with an asterisk character, so it is
an illegal NetBIOS name (see [RFC1001], section 5.2). Thus, it is
never registered with the NetBIOS Name Service and will not be
returned in a NetBIOS Adapter Status Response.
If the target does not support the "*SMBSERVER" generic name, or
if it is not running SMB services, it will return a CALLED NAME
NOT PRESENT error.
Some SMB servers are capable of providing multiple SMB file
services, each under a different NetBIOS name. In order to support
the generic server name, these servers must designate one service
as a default that will answer to "*SMBSERVER".
- Parsing the DNS Name or IP address (guessing)
This is the least reliable method for discovering an SMB server
name.
Systems which support STD 19 transport will often use the same
base host name within the DNS and NetBIOS name spaces. Thus, the
first label of the DNS name is a good guess at the NetBIOS name of
the target. If the input is an IP address rather than a DNS name,
the a reverse lookup against the DNS may be performed to determine
the DNS name.
The first label of the DNS name consists of the initial portion of
the DNS name string up to but not including the first dot
character ('.'). If the label is greater than 15 bytes in length,
it cannot be a NetBIOS name. The label must be space padded to a
total of 15 bytes, with a suffix value 0x20 (space) added. This
forms a valid NetBIOS name that may be used as a called name in a
Session Request packet.
If the target returns a CALLED NAME NOT PRESENT error, then the
DNS name guess is incorrect.
Any of the above may be tried in any order.
A.4. Determining the Namespace of the smb-srv-name
NetBIOS names, DNS names, and IP addresses can not be easily
distinguished syntactically. Unfortunately, given the nature and
history of the SMB/CIFS suite, the appropriate mechanism for
distinguishing between these server specifier types is the
trial-and-error method.
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Implementations should begin with the assumption that the specifier
is a NetBIOS name. The following process is used to test this
assumption:
If the NODETYPE (which is part of the NBT context) is the empty
string, then no NetBIOS name resolution mechanism has been
selected and the name cannot be resolved as a NetBIOS name. Exit.
If the name string contains dot characters ('.', ASCII 0x2E), then
separate the name into NetBIOS name and Scope ID at the first dot.
Otherwise use the entire string as the NetBIOS name, and assume an
Scope ID of "" unless the Scope ID is specified in the
nbt-context.
REPEAT
If the resulting NetBIOS name is greater than 15 octets in
length, then the name is not a NetBIOS name. Exit.
Issue STD 19 Name Queries using the NetBIOS name and Scope ID.
Suffix values of 0x20, 0x1B, and/or 0x1D should be used. (See
section A.5., below.)
If a Positive Name Query Response is received, then the name is
a NetBIOS name. Exit, indicating success and returning the
NetBIOS name and scope ID as parsed.
END
If the server specifier is not a NetBIOS name, then it is either a
DNS name or an IP address. These are semantically equivalent.
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A.5. Workgroup vs. SMB Server Names
If the URI string is of the form
smb://smb-srv-name/
then the smb-srv-name may represent either an SMB server name or a
workgroup name. The name MUST NOT be interpreted as a workgroup name
if:
- There is path information following the trailing slash.
Workgroups do not make shares or directories available.
- The server field is entered as a DNS name or an IP address.
A workgroup name is a NetBIOS group name. Workgroups,
conceptually, represent a group of servers rather than an
individual server, and the browse list may be retrieved
from one or more browse servers.
In these cases, the smb_srv_name is interpreted as a reference to an
SMB server only. Workgroups may only be accessed via their NetBIOS
names.
When testing the name using the algorithm presented in section A.4, a
NetBIOS name suffix value of 0x20 is used to find an SMB server, and
a suffix value of 0x1D or 0x1B is used to find a workgroup browse
server.
A system operating in B mode will use the 0x1D suffix to search for a
Local Master Browser operating on the same subnet. A system
operating in P mode must use the 0x1B suffix to query the NBNS for
the Domain Master Browser. An M mode system will first send a
broadcast query for the 0x1D name and, if that fails, query the NBNS
for the 0x1B name. H mode behavior is the opposite of M mode.
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