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Alternate Formats: rfc1094.txt | rfc1094.txt.pdf

Network Working Group                             Sun Microsystems, Inc.
Request for Comments: 1094                                    March 1989

            NFS: Network File System Protocol Specification


   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.


   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

   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

   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.


   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

   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

            * Remote file service routines
           program NFS_PROGRAM {
                   version NFS_VERSION {
                           NFSPROC_NULL(void)              = 0;

                           NFSPROC_GETATTR(fhandle)        = 1;

                           NFSPROC_SETATTR(sattrargs)      = 2;

                           NFSPROC_ROOT(void)              = 3;

                           NFSPROC_LOOKUP(diropargs)       = 4;

                           NFSPROC_READLINK(fhandle)       = 5;

                           NFSPROC_READ(readargs)          = 6;

                           NFSPROC_WRITECACHE(void)        = 7;

                           NFSPROC_WRITE(writeargs)        = 8;

                           NFSPROC_CREATE(createargs)      = 9;

                           NFSPROC_REMOVE(diropargs)       = 10;

                           NFSPROC_RENAME(renameargs)      = 11;

                           NFSPROC_LINK(linkargs)          = 12;

                           NFSPROC_SYMLINK(symlinkargs)    = 13;

                           NFSPROC_MKDIR(createargs)       = 14;

                           NFSPROC_RMDIR(diropargs)        = 15;

                           NFSPROC_READDIR(readdirargs)    = 16;

                           NFSPROC_STATFS(fhandle)         = 17;
                   } = 2;
           } = 100003;

2.2.1.  Do Nothing

           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

           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.

2.2.3.  Set File Attributes

           struct sattrargs {
                   fhandle file;
                   sattr attributes;

           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

           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

           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;

           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

   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;

           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

           NFSPROC_WRITECACHE(void) = 7;

   To be used in the next protocol revision.

2.2.9.  Write to File

           struct writeargs {
                   fhandle file;
                   unsigned beginoffset;
                   unsigned offset;
                   unsigned totalcount;
                   nfsdata data;

           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;

           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

           NFSPROC_REMOVE(diropargs) = 10;

   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;

           NFSPROC_RENAME(renameargs) = 11;

   The existing file "" in the directory given by "from.dir" is
   renamed to "" 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;

           NFSPROC_LINK(linkargs) = 12;

   Creates the file "" 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.

2.2.14.  Create Symbolic Link

           struct symlinkargs {
                   diropargs from;
                   path to;
                   sattr attributes;

           NFSPROC_SYMLINK(symlinkargs) = 13;

   Creates the file "" 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

           NFSPROC_MKDIR (createargs) = 14;

   The new directory "" 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

           NFSPROC_RMDIR(diropargs) = 15;

   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;

           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.

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;

           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

   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,


   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.

      Not owner.  The caller does not have correct ownership to perform
      the requested operation.

      No such file or directory.  The file or directory specified does
      not exist.

      Some sort of hard error occurred when the operation was in
      progress.  This could be a disk error, for example.

      No such device or address.

      Permission denied.  The caller does not have the correct
      permission to perform the requested operation.

      File exists.  The file specified already exists.

      No such device.

      Not a directory.  The caller specified a non-directory in a
      directory operation.

      Is a directory.  The caller specified a directory in a non-
      directory operation.

      File too large.  The operation caused a file to grow beyond the
      server's limit.

      No space left on device.  The operation caused the server's
      filesystem to reach its limit.

      Read-only filesystem.  Write attempted on a read-only filesystem.

      File name too long.  The file name in an operation was too long.

      Directory not empty.  Attempted to remove a directory that was not

      Disk quota exceeded.  The client's disk quota on the server has
      been exceeded.

      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.

      The server's write cache used in the "WRITECACHE" call got flushed
      to disk.

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;

              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
      0100000 This is a regular file; "type" field should be NFREG.
      0120000 This is a symbolic link file;  "type" field should be
      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.

      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

      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

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:

              fattr attributes;

      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;

      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".


   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

   general as possible.  Sometimes the simplicity of the server can be a
   problem, if the client wants to implement complicated filesystem

   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

   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

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

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

   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

3.4.  RPC Information

      The NFS service uses AUTH_UNIX,  AUTH_DES, or AUTH_SHORT style
      authentication, except in the NULL procedure where AUTH_NONE is

      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

   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.

                   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

   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

      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;

           /* 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;

   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.

A.5.  Server Procedures

   The following sections define the RPC procedures supplied by a mount

            * Protocol description for the mount program
           program MOUNTPROG {
                    * Version 1 of the mount protocol used with
                    * version 2 of the NFS protocol.
                   version MOUNTVERS {

                           MOUNTPROC_NULL(void) = 0;

                           MOUNTPROC_MNT(dirpath) = 1;

                           MOUNTPROC_DUMP(void) = 2;

                           MOUNTPROC_UMNT(dirpath) = 3;

                           MOUNTPROC_UMNTALL(void) = 4;

                           MOUNTPROC_EXPORT(void)  = 5;
                   } = 1;
           } = 100005;

A.5.1.  Do Nothing

           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

           MNTPROC_MNT(dirpath) = 1;

   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;

           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

           MNTPROC_UMNT(dirpath) = 3;

   Removes the mount list entry for the input "dirpath".

A.5.5.  Remove All Mount Entries

           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;

           MNTPROC_EXPORT(void) = 5;

   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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