RFC1094

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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  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: [email protected]

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