RFC2755 - Security Negotiation for WebNFS

Network Working Group A. Chiu
Request for Comments: 2755 M. Eisler
Category: Informational B. Callaghan
Sun Microsystems
January 2000
Security Negotiation for WebNFS
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes a protocol for a WebNFS client [RFC2054] to
negotiate the desired security mechanism with a WebNFS server
[RFC2055] before the WebNFS client falls back to the MOUNT v3
protocol [RFC1813]. This document is provided so that people can
write compatible implementations.
Table of Contents
1. IntrodUCtion .............................................. 2
2. Security Negotiation Multi-component LOOKUP ............... 3
3 Overloaded Filehandle ..................................... 4
3.1 Overloaded NFS Version 2 Filehandle ..................... 5
3.2 Overloaded NFS Version 3 Filehandle ..................... 6
4. WebNFS Security Negotiation ............................... 6
5. Security Considerations ................................... 10
6. References ................................................ 10
7. Acknowledgements .......................................... 10
8. Authors" Addresses ........................................ 11
9. Full Copyright Statement .................................. 12
1. Introduction
The MOUNT protocol is used by an NFS client to oBTain the necessary
filehandle for data Access. MOUNT versions 1 and 2 [RFC1094] return
NFS version 2 filehandles, whereas MOUNT version 3 [RFC1813] returns
NFS version 3 filehandles.
Among the existing versions of the MOUNT protocol, only the MOUNT v3
provides an RPC procedure (MOUNTPROC3_MNT) which facilitates security
negotiation between an NFS v3 client and an NSF v3 server. When this
RPC procedure succeeds (MNT3_OK) the server returns to the client an
array of security mechanisms it supports for the specified pathname,
in addition to an NFS v3 filehandle.
A security mechanism referred to in this document is a generalized
security flavor which can be an RPC authentication flavor [RFC1831]
or a security flavor referred to in the RPCSEC_GSS protocol
[RFC2203]. A security mechanism is represented as a four-octet
integer.
No RPC procedures are available for security negotiation in versions
1 or 2 of the MOUNT protocol.
The NFS mount command provides a "sec=" option for an NFS client to
specify the desired security mechanism to use for NFS transactions.
If this mount option is not specified, the default action is to use
the default security mechanism over NFS v2 mounts, or to negotiate a
security mechanism via the MOUNTPROC3_MNT procedure of MOUNT v3 and
use it over NFS v3 mounts. In the latter, the client picks the first
security mechanism in the array returned from the server that is also
supported on the client.
As specified in RFC2054, a WebNFS client first assumes that the
server supports WebNFS and uses the publsc filehandle as the initial
filehandle for data access, eliminating the need for the MOUNT
protocol. The WebNFS client falls back to MOUNT if the server does
not support WebNFS.
Since a WebNFS client does not use MOUNT initially, the
MOUNTPROC3_MNT procedure of MOUNT v3 is not available for security
negotiation until the WebNFS client falls back to MOUNT. A viable
protocol needs to be devised for the WebNFS client to negotiate
security mechanisms with the server in the absence of the
MOUNTPROC3_MNT procedure.
The WebNFS security negotiation protocol must meet the following
requirements:
- Must work seamlessly with NFS v2 and v3, and the WebNFS
protocols
- Must be backward compatible with servers that do not support
this negotiation
- Minimum number of network turnarounds (latency)
This document describes the WebNFS security negotiation protocol
developed by Sun Microsystems, Inc. Terminology and definitions from
RFCs 2054 and 2055 are used in this document. The reader is eXPected
to be familiar with them.
2. Security Negotiation Multi-component LOOKUP
The goal of the WebNFS security negotiation is to allow a WebNFS
client to identify a security mechanism which is used by the WebNFS
server to protect a specified path and is also supported by the
client. The WebNFS client initiates the negotiation by sending the
WebNFS server the path. The WebNFS server responds with the array of
security mechanisms it uses to secure the specified path. From the
array of security mechanisms the WebNFS client selects the first one
that it also supports.
Without introducing a new WebNFS request, the WebNFS security
negotiation is achieved by modifying the request and response of the
existing multi-component LOOKUP (MCL) operation [RFC2055]. Note that
the MCL operation is accomplished using the LOOKUP procedure
(NFSPROC3_LOOKUP for NFS v3 and NFSPROC_LOOKUP for NFS v2). This and
the next sections describe how the MCL request and response are
modified to facilitate WebNFS security negotiation.
For ease of reference, the modified MCL request is henceforth
referred to as SNEGO-MCL (security negotiation multi-component
LOOKUP) request.
A multi-component LOOKUP request [RFC2055] is composed of a public
filehandle and a multi-component path:
For Canonical Path:
LOOKUP FH=0x0, "/a/b/c"
For Native Path:
LOOKUP FH=0x0, 0x80 "a:b:c"
A multi-component path is either an ASCII string of slash separated
components or a 0x80 character followed by a native path. Note that
a multi-component LOOKUP implies the use of the public filehandle in
the LOOKUP.
Similar to the MCL request, a SNEGO-MCL request consists of a public
filehandle and a pathname. However, the pathname is uniquely
composed, as described below, to distinguish it from other pathnames.
The pathname used in a SNEGO-MCL is the regular WebNFS multi-
component path prefixed with two octets. The first prefixed octet is
the 0x81 non-ascii character, similar to the 0x80 non-ascii character
for the native paths. This octet represents client"s indication to
negotiate security mechanisms. It is followed by the security index
octet which stores the current value of the index into the array of
security mechanisms to be returned from the server. The security
index always starts with one and gets incremented as negotiation
continues. It is then followed by the pathname, either an ASCII
string of slash separated canonical components or 0x80 and a native
path.
A security negotiation multi-component LOOKUP request looks like
this:
For Canonical Path:
LOOKUP FH=0x0, 0x81 <sec-index> "/a/b/c"
For Native Path:
LOOKUP FH=0x0, 0x81 <sec-index> 0x80 "a:b:c"
In the next section we will see how the MCL response is modified for
WebNFS security negotiation.
3. Overloaded Filehandle
As described in RFC2054, if a multi-component LOOKUP request
succeeds, the server responds with a valid filehandle:
LOOKUP FH=0x0, "a/b/c"
----------->
<-----------
FH=0x3
NFS filehandles are used to uniquely identify a particular file or
Directory on the server and are opaque to the client. The client
neither examines a filehandle nor has any knowledge of its contents.
Thus, filehandles make an ideal repository for the server to return
the array of security mechanisms to the client in response to a
SNEGO-MCL request.
To a successful SNEGO-MCL request the server responds, in place of
the filehandle, with an array of integers that represents the valid
security mechanisms the client must use to access the given path. A
length field is introduced to store the size (in octets) of the array
of integers.
As the filehandles are limited in size (32 octets for NFS v2 and up
to 64 octets for NFS v3), it can happen that there are more security
mechanisms than the filehandles can accommodate. To circumvent this
problem, a one-octet status field is introduced which indicates
whether there are more security mechanisms (1 means yes, 0 means no)
that require the client to perform another SNEGO-MCL to get them.
To summarize, the response to a SNEGO-MCL request contains, in place
of the filehandle, the length field, the status field, and the array
of security mechanisms:
FH: length, status, {sec_1 sec_2 ... sec_n}
The next two sub-sections describe how NFS v2 and v3 filehandles are
"overloaded" to carry the length and status fields and the array of
security mechanisms.
3.1 Overloaded NFS Version 2 Filehandle
A regular NFS v2 filehandle is defined in RFC1094 as an opaque value
occupying 32 octets:
1 2 3 4 32
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+
...
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+
An overloaded NFS v2 filehandle looks like this:
1 2 3 4 5 8 32
+---+---+---+---+---+---+---+---+ +---+---+---+---+ +---+---+
l s sec_1 ... sec_n ...
+---+---+---+---+---+---+---+---+ +---+---+---+---+ +---+---+
Note that the first four octets of an overloaded NFS v2 filehandle
contain the length octet, the status octet, and two padded octets to
make them XDR four-octet aligned. The length octet l = 4 * n, where
n is the number of security mechanisms sent in the current overloaded
filehandle. Apparently, an overloaded NFS v2 filehandle can carry up
to seven security mechanisms.
3.2 Overloaded NFS Version 3 Filehandle
A regular NFS v3 filehandle is defined in RFC1813 as a variable
length opaque value occupying up to 64 octets. The length of the
filehandle is indicated by an integer value contained in a four octet
value which describes the number of valid octets that follow:
1 4
+---+---+---+---+
len
+---+---+---+---+
1 4 up to 64
+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+
...
+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+
An overloaded NFS v3 filehandle looks like the following:
1 4
+---+---+---+---+
len
+---+---+---+---+
1 4 5 8
+---+---+---+---+---+---+---+---+ +---+---+---+---+
s sec_1 ... sec_n
+---+---+---+---+---+---+---+---+ +---+---+---+---+
Here, len = 4 * (n+1). Again, n is the number of security mechanisms
contained in the current overloaded filehandle. Three octets are
padded after the status octet to meet the XDR four-octet alignment
requirement. An overloaded NFS v3 filehandle can carry up to fifteen
security mechanisms.
4. WebNFS Security Negotiation
With the SNEGO-MCL request and the overloaded NFS v2 and v3
filehandles defined above, the following diagram depicts the WebNFS
security negotiation protocol:
Client Server
------ ------
LOOKUP FH=0x0, 0x81 <sec-index> "path"
----------->
<-----------
FH: length, status, {sec_1 sec_2 ... sec_n}
where
0x81 represents client"s indication to negotiate security
mechanisms with the server,
path is either an ASCII string of slash separated components or
0x80 and a native path,
sec-index, one octet, contains the index into the array of
security mechanisms the server uses to protect the specified path,
status, one octet, indicates whether there are more security
mechanisms (1 means yes, 0 means no) that require the client to
perform another SNEGO-MCL to get them,
length (one octet for NFS v2 and four octets for NFS v3) describes
the number of valid octets that follow,
{sec_1 sec_2 ... sec_n} represents the array of security
mechanisms. As noted earlier, each security mechanism is
represented by a four-octet integer.
Here is an example showing the WebNFS security negotiation protocol
with NFS v2. In the example it is assumed the server shares /export
with 10 security mechanisms {0x3900 0x3901 0x3902 ... 0x3909} on the
export, two SNEGO-MCL requests would be needed for the client to get
the complete security information:
LOOKUP FH=0x0, 0x81 0x01 "/export"
----------->
<-----------
0x1c, 0x01, {0x3900 0x3901 0x3902 0x3903 0x3904 0x3905 0x3906}
LOOKUP FH=0x0, 0x81 0x08 "/export"
----------->
<-----------
0x0c, 0x00, {0x3907 0x3908 0x3909}
The order of the security mechanisms returned in an overloaded
filehandle implies preferences, i.e., one is more recommended than
those following it. The ordering is the same as that returned by the
MOUNT v3 protocol.
The following shows a typical scenario which illustrates how the
WebNFS security negotiation is accomplished in the course of
accessing publicly shared filesystems.
Normally, a WebNFS client first makes a regular multi-component
LOOKUP request using the public filehandle to obtain the filehandle
for the specified path. Since the WebNFS client does not have any
prior knowledge as to how the path is protected by the server the
default security mechanism is used in this first multi-component
LOOKUP. If the default security mechanism does not meet server"s
requirements, the server replies with the AUTH_TOOWEAK RPC
authentication error, indicating that the default security mechanism
is not valid and the WebNFS client needs to use a stronger one.
Upon receiving the AUTH_TOOWEAK error, to find out what security
mechanisms are required to access the specified path the WebNFS
client sends a SNEGO-qMCL request, using the default security
mechanism.
If the SNEGO-MCL request succeeds the server responds with the
filehandle overloaded with the array of security mechanisms required
for the specified path. If the server does not support WebNFS
security negotiation, the SNEGO-MCL request fails with NFSERR_IO for
NFS v2 or NFS3ERR_IO for NFS v3 [RFC2055].
Depending on the size of the array of security mechanisms, the WebNFS
client may have to make more SNEGO-MCL requests to get the complete
array.
For successful SNEGO-MCL requests, the WebNFS client retrieves the
array of security mechanisms from the overloaded filehandle, selects
an appropriate one, and issues a regular multi-component LOOKUP using
the selected security mechanism to acquire the filehandle.
All subsequent NFS requests are then made using the selected security
mechanism and the filehandle.
The following depicts the scenario outlined above. It is assumed
that the server shares /export/home as follows:
share -o sec=sec_1:sec_2:sec_3,public /export/home
and AUTH_SYS is the client"s default security mechanism and is not
one of {sec_1, sec_2, sec_3}.
Client Server
------ ------
LOOKUP FH=0x0, "/export/home"
AUTH_SYS
----------->
<-----------
AUTH_TOOWEAK
LOOKUP FH=0x0, 0x81 0x01 "/export/home"
AUTH_SYS
----------->
<-----------
overloaded FH: length, status, {sec_1 sec_2 sec_3}
LOOKUP FH=0x0, "/export/home"
sec_n
----------->
<-----------
FH = 0x01
NFS request with FH=0x01
sec_n
----------->
<-----------
...
In the above scenario, the first request is a regular multi-component
LOOKUP which fails with the AUTH_TOOWEAK error. The client then
issues a SNEGO-MCL request to get the security information.
There are WebNFS implementations that allow the public filehandle to
work with NFS protocol procedures other than LOOKUP. For those
WebNFS implementations, if the first request is not a regular multi-
component LOOKUP and it fails with AUTH_TOOWEAK, the client should
issue a SNEGO-MCL with
0x81 0x01 "."
as the path to get the security information.
5. Security Considerations
The reader may note that no mandatory security mechanisms are
specified in the protocol that the client must use in making SNEGO-
MCL requests. Normally, the client uses the default security
mechanism configured on his system in the first SNEGO-MCL request.
If the default security mechanism is not valid the server replies
with the AUTH_TOOWEAK error. In this case the server does not return
the array of security mechanisms to the client. The client can then
make another SNEGO-MCL request using a stronger security mechanism.
This continues until the client hits a valid one or has exhausted all
the supported security mechanisms.
6. References
[RFC1094] Sun Microsystems, Inc., "NFS: Network File System Protocol
Specification", RFC1094, March 1989.
http://www.ietf.org/rfc/rfc1094.txt
[RFC1813] Callaghan, B., Pawlowski, B. and P. Staubach, "NFS Version
3 Protocol Specification", RFC1813, June 1995.
http://www.ietf.org/rfc/rfc1813.txt
[RFC2054] Callaghan, B., "WebNFS Client Specification", RFC2054,
October 1996. http://www.ietf.org/rfc/rfc2054.txt
[RFC2055] Callaghan, B., "WebNFS Server Specification", RFC2055,
October 1996. http://www.ietf.org/rfc/rfc2055.txt
[RFC2203] Eisler, M., Chiu, A. and Ling, L., "RPCSEC_GSS Protocol
Specification", RFC2203, September 1997.
http://www.ietf.org/rfc/rfc2203.txt
7. Acknowledgements
This specification was extensively brainstormed and reviewed by the
NFS group of Solaris Software Division.
8. Authors" Addresses
Alex Chiu
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
Phone: +1 (650) 786-6465
EMail: alex.chiu@Eng.sun.com
Mike Eisler
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
Phone: +1 (719) 599-9026
EMail: michael.eisler@Eng.sun.com
Brent Callaghan
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
Phone: +1 (650) 786-5067
EMail: brent.callaghan@Eng.sun.com
9. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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