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Keywords: SUN-NFS
Network Working Group Sun Microsystems, Inc.
Request for Comments: 1094 March 1989
NFS: Network File System Protocol Specification
STATUS OF THIS MEMO
This RFC describes a protocol that Sun Microsystems, Inc., and others
are using. A new version of the protocol is under development, but
others may benefit from the descriptions of the current protocol, and
discussion of some of the design issues. Distribution of this memo
is unlimited.
1. INTRODUCTION
The Sun Network Filesystem (NFS) protocol provides transparent remote
access to shared files across networks. The NFS protocol is designed
to be portable across different machines, operating systems, network
architectures, and transport protocols. This portability is achieved
through the use of Remote Procedure Call (RPC) primitives built on
top of an eXternal Data Representation (XDR). Implementations
already exist for a variety of machines, from personal computers to
supercomputers.
The supporting mount protocol allows the server to hand out remote
access privileges to a restricted set of clients. It performs the
operating system-specific functions that allow, for example, to
attach remote directory trees to some local file system.
1.1. Remote Procedure Call
Sun's Remote Procedure Call specification provides a procedure-
oriented interface to remote services. Each server supplies a
"program" that is a set of procedures. NFS is one such program. The
combination of host address, program number, and procedure number
specifies one remote procedure. A goal of NFS was to not require any
specific level of reliability from its lower levels, so it could
potentially be used on many underlying transport protocols, or even
another remote procedure call implementation. For ease of
discussion, the rest of this document will assume NFS is implemented
on top of Sun RPC, described in RFC 1057, "RPC: Remote Procedure
Call Protocol Specification".
1.2. External Data Representation
The eXternal Data Representation (XDR) standard provides a common way
of representing a set of data types over a network. The NFS Protocol
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RFC 1094 NFS: Network File System March 1989
Specification is written using the RPC data description language.
For more information, see RFC 1014, "XDR: External Data
Representation Standard". Although automated RPC/XDR compilers exist
to generate server and client "stubs", NFS does not require their
use. Any software that provides equivalent functionality can be
used, and if the encoding is exactly the same it can interoperate
with other implementations of NFS.
1.3. Stateless Servers
The NFS protocol was intended to be as stateless as possible. That
is, a server should not need to maintain any protocol state
information about any of its clients in order to function correctly.
Stateless servers have a distinct advantage over stateful servers in
the event of a failure. With stateless servers, a client need only
retry a request until the server responds; it does not even need to
know that the server has crashed, or the network temporarily went
down. The client of a stateful server, on the other hand, needs to
either detect a server failure and rebuild the server's state when it
comes back up, or cause client operations to fail.
This may not sound like an important issue, but it affects the
protocol in some unexpected ways. We feel that it may be worth a bit
of extra complexity in the protocol to be able to write very simple
servers that do not require fancy crash recovery. Note that even if
a so-called "reliable" transport protocol such as TCP is used, the
client must still be able to handle interruptions of service by re-
opening connections when they time out. Thus, a stateless protocol
may actually simplify the implementation.
On the other hand, NFS deals with objects such as files and
directories that inherently have state -- what good would a file be
if it did not keep its contents intact? The goal was to not
introduce any extra state in the protocol itself. Inherently
stateful operations such as file or record locking, and remote
execution, were implemented as separate services, not described in
this document.
The basic way to simplify recovery was to make operations as
"idempotent" as possible (so that they can potentially be repeated).
Some operations in this version of the protocol did not attain this
goal; luckily most of the operations (such as Read and Write) are
idempotent. Also, most server failures occur between operations, not
between the receipt of an operation and the response. Finally,
although actual server failures may be rare, in complex networks,
failures of any network, router, or bridge may be indistinguishable
from a server failure.
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2. NFS PROTOCOL DEFINITION
Servers change over time, and so can the protocol that they use. RPC
provides a version number with each RPC request. This RFC describes
version two of the NFS protocol. Even in the second version, there
are a few obsolete procedures and parameters, which will be removed
in later versions. An RFC for version three of the NFS protocol is
currently under preparation.
2.1. File System Model
NFS assumes a file system that is hierarchical, with directories as
all but the bottom level of files. Each entry in a directory (file,
directory, device, etc.) has a string name. Different operating
systems may have restrictions on the depth of the tree or the names
used, as well as using different syntax to represent the "pathname",
which is the concatenation of all the "components" (directory and
file names) in the name. A "file system" is a tree on a single
server (usually a single disk or physical partition) with a specified
"root". Some operating systems provide a "mount" operation to make
all file systems appear as a single tree, while others maintain a
"forest" of file systems. Files are unstructured streams of
uninterpreted bytes. Version 3 of NFS uses slightly more general
file system model.
NFS looks up one component of a pathname at a time. It may not be
obvious why it does not just take the whole pathname, traipse down
the directories, and return a file handle when it is done. There are
several good reasons not to do this. First, pathnames need
separators between the directory components, and different operating
systems use different separators. We could define a Network Standard
Pathname Representation, but then every pathname would have to be
parsed and converted at each end. Other issues are discussed in
section 3, NFS Implementation Issues.
Although files and directories are similar objects in many ways,
different procedures are used to read directories and files. This
provides a network standard format for representing directories. The
same argument as above could have been used to justify a procedure
that returns only one directory entry per call. The problem is
efficiency. Directories can contain many entries, and a remote call
to return each would be just too slow.
2.2. Server Procedures
The protocol definition is given as a set of procedures with
arguments and results defined using the RPC language (XDR language
extended with program, version, and procedure declarations). A brief
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description of the function of each procedure should provide enough
information to allow implementation. Section 2.3 describes the basic
data types in more detail.
All of the procedures in the NFS protocol are assumed to be
synchronous. When a procedure returns to the client, the client can
assume that the operation has completed and any data associated with
the request is now on stable storage. For example, a client WRITE
request may cause the server to update data blocks, filesystem
information blocks (such as indirect blocks), and file attribute
information (size and modify times). When the WRITE returns to the
client, it can assume that the write is safe, even in case of a
server crash, and it can discard the data written. This is a very
important part of the statelessness of the server. If the server
waited to flush data from remote requests, the client would have to
save those requests so that it could resend them in case of a server
crash.
/*
* Remote file service routines
*/
program NFS_PROGRAM {
version NFS_VERSION {
void
NFSPROC_NULL(void) = 0;
attrstat
NFSPROC_GETATTR(fhandle) = 1;
attrstat
NFSPROC_SETATTR(sattrargs) = 2;
void
NFSPROC_ROOT(void) = 3;
diropres
NFSPROC_LOOKUP(diropargs) = 4;
readlinkres
NFSPROC_READLINK(fhandle) = 5;
readres
NFSPROC_READ(readargs) = 6;
void
NFSPROC_WRITECACHE(void) = 7;
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attrstat
NFSPROC_WRITE(writeargs) = 8;
diropres
NFSPROC_CREATE(createargs) = 9;
stat
NFSPROC_REMOVE(diropargs) = 10;
stat
NFSPROC_RENAME(renameargs) = 11;
stat
NFSPROC_LINK(linkargs) = 12;
stat
NFSPROC_SYMLINK(symlinkargs) = 13;
diropres
NFSPROC_MKDIR(createargs) = 14;
stat
NFSPROC_RMDIR(diropargs) = 15;
readdirres
NFSPROC_READDIR(readdirargs) = 16;
statfsres
NFSPROC_STATFS(fhandle) = 17;
} = 2;
} = 100003;
2.2.1. Do Nothing
void
NFSPROC_NULL(void) = 0;
This procedure does no work. It is made available in all RPC
services to allow server response testing and timing.
2.2.2. Get File Attributes
attrstat
NFSPROC_GETATTR (fhandle) = 1;
If the reply status is NFS_OK, then the reply attributes contains the
attributes for the file given by the input fhandle.
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2.2.3. Set File Attributes
struct sattrargs {
fhandle file;
sattr attributes;
};
attrstat
NFSPROC_SETATTR (sattrargs) = 2;
The "attributes" argument contains fields which are either -1 or are
the new value for the attributes of "file". If the reply status is
NFS_OK, then the reply attributes have the attributes of the file
after the "SETATTR" operation has completed.
Notes: The use of -1 to indicate an unused field in "attributes" is
changed in the next version of the protocol.
2.2.4. Get Filesystem Root
void
NFSPROC_ROOT(void) = 3;
Obsolete. This procedure is no longer used because finding the root
file handle of a filesystem requires moving pathnames between client
and server. To do this right, we would have to define a network
standard representation of pathnames. Instead, the function of
looking up the root file handle is done by the MNTPROC_MNT procedure.
(See Appendix A, "Mount Protocol Definition", for details).
2.2.5. Look Up File Name
diropres
NFSPROC_LOOKUP(diropargs) = 4;
If the reply "status" is NFS_OK, then the reply "file" and reply
"attributes" are the file handle and attributes for the file "name"
in the directory given by "dir" in the argument.
2.2.6. Read From Symbolic Link
union readlinkres switch (stat status) {
case NFS_OK:
path data;
default:
void;
};
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readlinkres
NFSPROC_READLINK(fhandle) = 5;
If "status" has the value NFS_OK, then the reply "data" is the data
in the symbolic link given by the file referred to by the fhandle
argument.
Notes: Since NFS always parses pathnames on the client, the pathname
in a symbolic link may mean something different (or be meaningless)
on a different client or on the server if a different pathname syntax
is used.
2.2.7. Read From File
struct readargs {
fhandle file;
unsigned offset;
unsigned count;
unsigned totalcount;
};
union readres switch (stat status) {
case NFS_OK:
fattr attributes;
nfsdata data;
default:
void;
};
readres
NFSPROC_READ(readargs) = 6;
Returns up to "count" bytes of "data" from the file given by "file",
starting at "offset" bytes from the beginning of the file. The first
byte of the file is at offset zero. The file attributes after the
read takes place are returned in "attributes".
Notes: The argument "totalcount" is unused, and is removed in the
next protocol revision.
2.2.8. Write to Cache
void
NFSPROC_WRITECACHE(void) = 7;
To be used in the next protocol revision.
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2.2.9. Write to File
struct writeargs {
fhandle file;
unsigned beginoffset;
unsigned offset;
unsigned totalcount;
nfsdata data;
};
attrstat
NFSPROC_WRITE(writeargs) = 8;
Writes "data" beginning "offset" bytes from the beginning of "file".
The first byte of the file is at offset zero. If the reply "status"
is NFS_OK, then the reply "attributes" contains the attributes of the
file after the write has completed. The write operation is atomic.
Data from this "WRITE" will not be mixed with data from another
client's "WRITE".
Notes: The arguments "beginoffset" and "totalcount" are ignored and
are removed in the next protocol revision.
2.2.10. Create File
struct createargs {
diropargs where;
sattr attributes;
};
diropres
NFSPROC_CREATE(createargs) = 9;
The file "name" is created in the directory given by "dir". The
initial attributes of the new file are given by "attributes". A
reply "status" of NFS_OK indicates that the file was created, and
reply "file" and reply "attributes" are its file handle and
attributes. Any other reply "status" means that the operation failed
and no file was created.
Notes: This routine should pass an exclusive create flag, meaning
"create the file only if it is not already there".
2.2.11. Remove File
stat
NFSPROC_REMOVE(diropargs) = 10;
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The file "name" is removed from the directory given by "dir". A
reply of NFS_OK means the directory entry was removed.
Notes: possibly non-idempotent operation.
2.2.12. Rename File
struct renameargs {
diropargs from;
diropargs to;
};
stat
NFSPROC_RENAME(renameargs) = 11;
The existing file "from.name" in the directory given by "from.dir" is
renamed to "to.name" in the directory given by "to.dir". If the
reply is NFS_OK, the file was renamed. The RENAME operation is
atomic on the server; it cannot be interrupted in the middle.
Notes: possibly non-idempotent operation.
2.2.13. Create Link to File
Procedure 12, Version 2.
struct linkargs {
fhandle from;
diropargs to;
};
stat
NFSPROC_LINK(linkargs) = 12;
Creates the file "to.name" in the directory given by "to.dir", which
is a hard link to the existing file given by "from". If the return
value is NFS_OK, a link was created. Any other return value
indicates an error, and the link was not created.
A hard link should have the property that changes to either of the
linked files are reflected in both files. When a hard link is made
to a file, the attributes for the file should have a value for
"nlink" that is one greater than the value before the link.
Notes: possibly non-idempotent operation.
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2.2.14. Create Symbolic Link
struct symlinkargs {
diropargs from;
path to;
sattr attributes;
};
stat
NFSPROC_SYMLINK(symlinkargs) = 13;
Creates the file "from.name" with ftype NFLNK in the directory given
by "from.dir". The new file contains the pathname "to" and has
initial attributes given by "attributes". If the return value is
NFS_OK, a link was created. Any other return value indicates an
error, and the link was not created.
A symbolic link is a pointer to another file. The name given in "to"
is not interpreted by the server, only stored in the newly created
file. When the client references a file that is a symbolic link, the
contents of the symbolic link are normally transparently
reinterpreted as a pathname to substitute. A READLINK operation
returns the data to the client for interpretation.
Notes: On UNIX servers the attributes are never used, since symbolic
links always have mode 0777.
2.2.15. Create Directory
diropres
NFSPROC_MKDIR (createargs) = 14;
The new directory "where.name" is created in the directory given by
"where.dir". The initial attributes of the new directory are given
by "attributes". A reply "status" of NFS_OK indicates that the new
directory was created, and reply "file" and reply "attributes" are
its file handle and attributes. Any other reply "status" means that
the operation failed and no directory was created.
Notes: possibly non-idempotent operation.
2.2.16. Remove Directory
stat
NFSPROC_RMDIR(diropargs) = 15;
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The existing empty directory "name" in the directory given by "dir"
is removed. If the reply is NFS_OK, the directory was removed.
Notes: possibly non-idempotent operation.
2.2.17. Read From Directory
struct readdirargs {
fhandle dir;
nfscookie cookie;
unsigned count;
};
struct entry {
unsigned fileid;
filename name;
nfscookie cookie;
entry *nextentry;
};
union readdirres switch (stat status) {
case NFS_OK:
struct {
entry *entries;
bool eof;
} readdirok;
default:
void;
};
readdirres
NFSPROC_READDIR (readdirargs) = 16;
Returns a variable number of directory entries, with a total size of
up to "count" bytes, from the directory given by "dir". If the
returned value of "status" is NFS_OK, then it is followed by a
variable number of "entry"s. Each "entry" contains a "fileid" which
consists of a unique number to identify the file within a filesystem,
the "name" of the file, and a "cookie" which is an opaque pointer to
the next entry in the directory. The cookie is used in the next
READDIR call to get more entries starting at a given point in the
directory. The special cookie zero (all bits zero) can be used to
get the entries starting at the beginning of the directory. The
"fileid" field should be the same number as the "fileid" in the the
attributes of the file. (See section "2.3.5. fattr" under "Basic
Data Types".) The "eof" flag has a value of TRUE if there are no
more entries in the directory.
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2.2.18. Get Filesystem Attributes
union statfsres (stat status) {
case NFS_OK:
struct {
unsigned tsize;
unsigned bsize;
unsigned blocks;
unsigned bfree;
unsigned bavail;
} info;
default:
void;
};
statfsres
NFSPROC_STATFS(fhandle) = 17;
If the reply "status" is NFS_OK, then the reply "info" gives the
attributes for the filesystem that contains file referred to by the
input fhandle. The attribute fields contain the following values:
tsize The optimum transfer size of the server in bytes. This is
the number of bytes the server would like to have in the
data part of READ and WRITE requests.
bsize The block size in bytes of the filesystem.
blocks The total number of "bsize" blocks on the filesystem.
bfree The number of free "bsize" blocks on the filesystem.
bavail The number of "bsize" blocks available to non-privileged
users.
Notes: This call does not work well if a filesystem has variable
size blocks.
2.3. Basic Data Types
The following XDR definitions are basic structures and types used in
other structures described further on.
2.3.1. stat
enum stat {
NFS_OK = 0,
NFSERR_PERM=1,
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NFSERR_NOENT=2,
NFSERR_IO=5,
NFSERR_NXIO=6,
NFSERR_ACCES=13,
NFSERR_EXIST=17,
NFSERR_NODEV=19,
NFSERR_NOTDIR=20,
NFSERR_ISDIR=21,
NFSERR_FBIG=27,
NFSERR_NOSPC=28,
NFSERR_ROFS=30,
NFSERR_NAMETOOLONG=63,
NFSERR_NOTEMPTY=66,
NFSERR_DQUOT=69,
NFSERR_STALE=70,
NFSERR_WFLUSH=99
};
The "stat" type is returned with every procedure's results. A value
of NFS_OK indicates that the call completed successfully and the
results are valid. The other values indicate some kind of error
occurred on the server side during the servicing of the procedure.
The error values are derived from UNIX error numbers.
NFSERR_PERM
Not owner. The caller does not have correct ownership to perform
the requested operation.
NFSERR_NOENT
No such file or directory. The file or directory specified does
not exist.
NFSERR_IO
Some sort of hard error occurred when the operation was in
progress. This could be a disk error, for example.
NFSERR_NXIO
No such device or address.
NFSERR_ACCES
Permission denied. The caller does not have the correct
permission to perform the requested operation.
NFSERR_EXIST
File exists. The file specified already exists.
NFSERR_NODEV
No such device.
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NFSERR_NOTDIR
Not a directory. The caller specified a non-directory in a
directory operation.
NFSERR_ISDIR
Is a directory. The caller specified a directory in a non-
directory operation.
NFSERR_FBIG
File too large. The operation caused a file to grow beyond the
server's limit.
NFSERR_NOSPC
No space left on device. The operation caused the server's
filesystem to reach its limit.
NFSERR_ROFS
Read-only filesystem. Write attempted on a read-only filesystem.
NFSERR_NAMETOOLONG
File name too long. The file name in an operation was too long.
NFSERR_NOTEMPTY
Directory not empty. Attempted to remove a directory that was not
empty.
NFSERR_DQUOT
Disk quota exceeded. The client's disk quota on the server has
been exceeded.
NFSERR_STALE
The "fhandle" given in the arguments was invalid. That is, the
file referred to by that file handle no longer exists, or access
to it has been revoked.
NFSERR_WFLUSH
The server's write cache used in the "WRITECACHE" call got flushed
to disk.
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2.3.2. ftype
enum ftype {
NFNON = 0,
NFREG = 1,
NFDIR = 2,
NFBLK = 3,
NFCHR = 4,
NFLNK = 5
};
The enumeration "ftype" gives the type of a file. The type NFNON
indicates a non-file, NFREG is a regular file, NFDIR is a
directory, NFBLK is a block-special device, NFCHR is a character-
special device, and NFLNK is a symbolic link.
2.3.3. fhandle
typedef opaque fhandle[FHSIZE];
The "fhandle" is the file handle passed between the server and the
client. All file operations are done using file handles to refer
to a file or directory. The file handle can contain whatever
information the server needs to distinguish an individual file.
2.3.4. timeval
struct timeval {
unsigned int seconds;
unsigned int useconds;
};
The "timeval" structure is the number of seconds and microseconds
since midnight January 1, 1970, Greenwich Mean Time. It is used
to pass time and date information.
2.3.5. fattr
struct fattr {
ftype type;
unsigned int mode;
unsigned int nlink;
unsigned int uid;
unsigned int gid;
unsigned int size;
unsigned int blocksize;
unsigned int rdev;
unsigned int blocks;
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unsigned int fsid;
unsigned int fileid;
timeval atime;
timeval mtime;
timeval ctime;
};
The "fattr" structure contains the attributes of a file; "type" is
the type of the file; "nlink" is the number of hard links to the
file (the number of different names for the same file); "uid" is
the user identification number of the owner of the file; "gid" is
the group identification number of the group of the file; "size"
is the size in bytes of the file; "blocksize" is the size in bytes
of a block of the file; "rdev" is the device number of the file if
it is type NFCHR or NFBLK; "blocks" is the number of blocks the
file takes up on disk; "fsid" is the file system identifier for
the filesystem containing the file; "fileid" is a number that
uniquely identifies the file within its filesystem; "atime" is the
time when the file was last accessed for either read or write;
"mtime" is the time when the file data was last modified
(written); and "ctime" is the time when the status of the file was
last changed. Writing to the file also changes "ctime" if the
size of the file changes.
"Mode" is the access mode encoded as a set of bits. Notice that
the file type is specified both in the mode bits and in the file
type. This is really a bug in the protocol and will be fixed in
future versions. The descriptions given below specify the bit
positions using octal numbers.
0040000 This is a directory; "type" field should be NFDIR.
0020000 This is a character special file; "type" field should
be NFCHR.
0060000 This is a block special file; "type" field should be
NFBLK.
0100000 This is a regular file; "type" field should be NFREG.
0120000 This is a symbolic link file; "type" field should be
NFLNK.
0140000 This is a named socket; "type" field should be NFNON.
0004000 Set user id on execution.
0002000 Set group id on execution.
0001000 Save swapped text even after use.
0000400 Read permission for owner.
0000200 Write permission for owner.
0000100 Execute and search permission for owner.
0000040 Read permission for group.
0000020 Write permission for group.
0000010 Execute and search permission for group.
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0000004 Read permission for others.
0000002 Write permission for others.
0000001 Execute and search permission for others.
Notes: The bits are the same as the mode bits returned by the
stat(2) system call in UNIX. The file type is specified both in
the mode bits and in the file type. This is fixed in future
versions.
The "rdev" field in the attributes structure is an operating
system specific device specifier. It will be removed and
generalized in the next revision of the protocol.
2.3.6. sattr
struct sattr {
unsigned int mode;
unsigned int uid;
unsigned int gid;
unsigned int size;
timeval atime;
timeval mtime;
};
The "sattr" structure contains the file attributes which can be
set from the client. The fields are the same as for "fattr"
above. A "size" of zero means the file should be truncated. A
value of -1 indicates a field that should be ignored.
2.3.7. filename
typedef string filename<MAXNAMLEN>;
The type "filename" is used for passing file names or pathname
components.
2.3.8. path
typedef string path<MAXPATHLEN>;
The type "path" is a pathname. The server considers it as a
string with no internal structure, but to the client it is the
name of a node in a filesystem tree.
2.3.9. attrstat
union attrstat switch (stat status) {
case NFS_OK:
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fattr attributes;
default:
void;
};
The "attrstat" structure is a common procedure result. It
contains a "status" and, if the call succeeded, it also contains
the attributes of the file on which the operation was done.
2.3.10. diropargs
struct diropargs {
fhandle dir;
filename name;
};
The "diropargs" structure is used in directory operations. The
"fhandle" "dir" is the directory in which to find the file "name".
A directory operation is one in which the directory is affected.
2.3.11. diropres
union diropres switch (stat status) {
case NFS_OK:
struct {
fhandle file;
fattr attributes;
} diropok;
default:
void;
};
The results of a directory operation are returned in a "diropres"
structure. If the call succeeded, a new file handle "file" and
the "attributes" associated with that file are returned along with
the "status".
3. NFS IMPLEMENTATION ISSUES
The NFS protocol was designed to allow different operating systems to
share files. However, since it was designed in a UNIX environment,
many operations have semantics similar to the operations of the UNIX
file system. This section discusses some of the implementation-
specific details and semantic issues.
3.1. Server/Client Relationship
The NFS protocol is designed to allow servers to be as simple and
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general as possible. Sometimes the simplicity of the server can be a
problem, if the client wants to implement complicated filesystem
semantics.
For example, some operating systems allow removal of open files. A
process can open a file and, while it is open, remove it from the
directory. The file can be read and written as long as the process
keeps it open, even though the file has no name in the filesystem.
It is impossible for a stateless server to implement these semantics.
The client can do some tricks such as renaming the file on remove,
and only removing it on close. We believe that the server provides
enough functionality to implement most file system semantics on the
client.
Every NFS client can also potentially be a server, and remote and
local mounted filesystems can be freely intermixed. This leads to
some interesting problems when a client travels down the directory
tree of a remote filesystem and reaches the mount point on the server
for another remote filesystem. Allowing the server to follow the
second remote mount would require loop detection, server lookup, and
user revalidation. Instead, we decided not to let clients cross a
server's mount point. When a client does a LOOKUP on a directory on
which the server has mounted a filesystem, the client sees the
underlying directory instead of the mounted directory.
For example, if a server has a file system called "/usr" and mounts
another file system on "/usr/src", if a client mounts "/usr", it
does NOT see the mounted version of "/usr/src". A client could do
remote mounts that match the server's mount points to maintain the
server's view. In this example, the client would also have to mount
"/usr/src" in addition to "/usr", even if they are from the same
server.
3.2. Pathname Interpretation
There are a few complications to the rule that pathnames are always
parsed on the client. For example, symbolic links could have
different interpretations on different clients. Another common
problem for non-UNIX implementations is the special interpretation of
the pathname ".." to mean the parent of a given directory. The next
revision of the protocol uses an explicit flag to indicate the parent
instead.
3.3. Permission Issues
The NFS protocol, strictly speaking, does not define the permission
checking used by servers. However, it is expected that a server will
do normal operating system permission checking using AUTH_UNIX style
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authentication as the basis of its protection mechanism. The server
gets the client's effective "uid", effective "gid", and groups on
each call and uses them to check permission. There are various
problems with this method that can been resolved in interesting ways.
Using "uid" and "gid" implies that the client and server share the
same "uid" list. Every server and client pair must have the same
mapping from user to "uid" and from group to "gid". Since every
client can also be a server, this tends to imply that the whole
network shares the same "uid/gid" space. AUTH_DES (and the next
revision of the NFS protocol) uses string names instead of numbers,
but there are still complex problems to be solved.
Another problem arises due to the usually stateful open operation.
Most operating systems check permission at open time, and then check
that the file is open on each read and write request. With stateless
servers, the server has no idea that the file is open and must do
permission checking on each read and write call. On a local
filesystem, a user can open a file and then change the permissions so
that no one is allowed to touch it, but will still be able to write
to the file because it is open. On a remote filesystem, by contrast,
the write would fail. To get around this problem, the server's
permission checking algorithm should allow the owner of a file to
access it regardless of the permission setting.
A similar problem has to do with paging in from a file over the
network. The operating system usually checks for execute permission
before opening a file for demand paging, and then reads blocks from
the open file. The file may not have read permission, but after it
is opened it does not matter. An NFS server can not tell the
difference between a normal file read and a demand page-in read. To
make this work, the server allows reading of files if the "uid" given
in the call has either execute or read permission on the file.
In most operating systems, a particular user (on UNIX, the user ID
zero) has access to all files no matter what permission and ownership
they have. This "super-user" permission may not be allowed on the
server, since anyone who can become super-user on their workstation
could gain access to all remote files. The UNIX server by default
maps user id 0 to -2 before doing its access checking. This works
except for NFS root filesystems, where super-user access cannot be
avoided.
3.4. RPC Information
Authentication
The NFS service uses AUTH_UNIX, AUTH_DES, or AUTH_SHORT style
authentication, except in the NULL procedure where AUTH_NONE is
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also allowed.
Transport Protocols
NFS is supported normally on UDP.
Port Number
The NFS protocol currently uses the UDP port number 2049. This is
not an officially assigned port, so later versions of the protocol
use the "Portmapping" facility of RPC.
3.5. Sizes of XDR Structures
These are the sizes, given in decimal bytes, of various XDR
structures used in the protocol:
/*
* The maximum number of bytes of data in a READ or WRITE
* request.
*/
const MAXDATA = 8192;
/* The maximum number of bytes in a pathname argument. */
const MAXPATHLEN = 1024;
/* The maximum number of bytes in a file name argument. */
const MAXNAMLEN = 255;
/* The size in bytes of the opaque "cookie" passed by READDIR. */
const COOKIESIZE = 4;
/* The size in bytes of the opaque file handle. */
const FHSIZE = 32;
3.6. Setting RPC Parameters
Various file system parameters and options should be set at mount
time. The mount protocol is described in the appendix below. For
example, "Soft" mounts as well as "Hard" mounts are usually both
provided. Soft mounted file systems return errors when RPC
operations fail (after a given number of optional retransmissions),
while hard mounted file systems continue to retransmit forever. The
maximum transfer sizes are implementation dependent. For efficient
operation over a local network, 8192 bytes of data are normally used.
This may result in lower-level fragmentation (such as at the IP
level). Since some network interfaces may not allow such packets,
for operation over slower-speed networks or hosts, or through
gateways, transfer sizes of 512 or 1024 bytes often provide better
results.
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Clients and servers may need to keep caches of recent operations to
help avoid problems with non-idempotent operations. For example, if
the transport protocol drops the response for a Remove File
operation, upon retransmission the server may return an error code of
NFSERR_NOENT instead of NFS_OK. But if the server keeps around the
last operation requested and its result, it could return the proper
success code. Of course, the server could be crashed and rebooted
between retransmissions, but a small cache (even a single entry)
would solve most problems.
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Appendix A. MOUNT PROTOCOL DEFINITION
A.1. Introduction
The mount protocol is separate from, but related to, the NFS
protocol. It provides operating system specific services to get the
NFS off the ground -- looking up server path names, validating user
identity, and checking access permissions. Clients use the mount
protocol to get the first file handle, which allows them entry into a
remote filesystem.
The mount protocol is kept separate from the NFS protocol to make it
easy to plug in new access checking and validation methods without
changing the NFS server protocol.
Notice that the protocol definition implies stateful servers because
the server maintains a list of client's mount requests. The mount
list information is not critical for the correct functioning of
either the client or the server. It is intended for advisory use
only, for example, to warn possible clients when a server is going
down.
Version one of the mount protocol is used with version two of the NFS
protocol. The only information communicated between these two
protocols is the "fhandle" structure.
A.2. RPC Information
Authentication
The mount service uses AUTH_UNIX and AUTH_NONE style
authentication only.
Transport Protocols
The mount service is supported on both UDP and TCP.
Port Number
Consult the server's portmapper, described in RFC 1057, "RPC:
Remote Procedure Call Protocol Specification", to find the port
number on which the mount service is registered.
A.3. Sizes of XDR Structures
These are the sizes, given in decimal bytes, of various XDR
structures used in the protocol:
/* The maximum number of bytes in a pathname argument. */
const MNTPATHLEN = 1024;
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/* The maximum number of bytes in a name argument. */
const MNTNAMLEN = 255;
/* The size in bytes of the opaque file handle. */
const FHSIZE = 32;
A.4. Basic Data Types
This section presents the data types used by the mount protocol. In
many cases they are similar to the types used in NFS.
A.4.1. fhandle
typedef opaque fhandle[FHSIZE];
The type "fhandle" is the file handle that the server passes to the
client. All file operations are done using file handles to refer to
a file or directory. The file handle can contain whatever
information the server needs to distinguish an individual file.
This is the same as the "fhandle" XDR definition in version 2 of the
NFS protocol; see section "2.3.3. fhandle" under "Basic Data Types".
A.4.2. fhstatus
union fhstatus switch (unsigned status) {
case 0:
fhandle directory;
default:
void;
}
The type "fhstatus" is a union. If a "status" of zero is returned,
the call completed successfully, and a file handle for the
"directory" follows. A non-zero status indicates some sort of error.
In this case, the status is a UNIX error number.
A.4.3. dirpath
typedef string dirpath<MNTPATHLEN>;
The type "dirpath" is a server pathname of a directory.
A.4.4. name
typedef string name<MNTNAMLEN>;
The type "name" is an arbitrary string used for various names.
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A.5. Server Procedures
The following sections define the RPC procedures supplied by a mount
server.
/*
* Protocol description for the mount program
*/
program MOUNTPROG {
/*
* Version 1 of the mount protocol used with
* version 2 of the NFS protocol.
*/
version MOUNTVERS {
void
MOUNTPROC_NULL(void) = 0;
fhstatus
MOUNTPROC_MNT(dirpath) = 1;
mountlist
MOUNTPROC_DUMP(void) = 2;
void
MOUNTPROC_UMNT(dirpath) = 3;
void
MOUNTPROC_UMNTALL(void) = 4;
exportlist
MOUNTPROC_EXPORT(void) = 5;
} = 1;
} = 100005;
A.5.1. Do Nothing
void
MNTPROC_NULL(void) = 0;
This procedure does no work. It is made available in all RPC
services to allow server response testing and timing.
A.5.2. Add Mount Entry
fhstatus
MNTPROC_MNT(dirpath) = 1;
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If the reply "status" is 0, then the reply "directory" contains the
file handle for the directory "dirname". This file handle may be
used in the NFS protocol. This procedure also adds a new entry to
the mount list for this client mounting "dirname".
A.5.3. Return Mount Entries
struct *mountlist {
name hostname;
dirpath directory;
mountlist nextentry;
};
mountlist
MNTPROC_DUMP(void) = 2;
Returns the list of remote mounted filesystems. The "mountlist"
contains one entry for each "hostname" and "directory" pair.
A.5.4. Remove Mount Entry
void
MNTPROC_UMNT(dirpath) = 3;
Removes the mount list entry for the input "dirpath".
A.5.5. Remove All Mount Entries
void
MNTPROC_UMNTALL(void) = 4;
Removes all of the mount list entries for this client.
A.5.6. Return Export List
struct *groups {
name grname;
groups grnext;
};
struct *exportlist {
dirpath filesys;
groups groups;
exportlist next;
};
exportlist
MNTPROC_EXPORT(void) = 5;
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Returns a variable number of export list entries. Each entry
contains a filesystem name and a list of groups that are allowed to
import it. The filesystem name is in "filesys", and the group name
is in the list "groups".
Notes: The exportlist should contain more information about the
status of the filesystem, such as a read-only flag.
Author's Address:
Bill Nowicki
Sun Microsystems, Inc.
Mail Stop 1-40
2550 Garcia Avenue
Mountain View, CA 94043
Phone: (415) 336-7278
Email: nowicki@SUN.COM
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