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rfc: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

Sun Microsystems, Inc. [Page 1] 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.

Sun Microsystems, Inc. [Page 2] RFC 1094 NFS: Network File System March 1989

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

Sun Microsystems, Inc. [Page 3] RFC 1094 NFS: Network File System March 1989

 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;

Sun Microsystems, Inc. [Page 4] RFC 1094 NFS: Network File System March 1989

                         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.

Sun Microsystems, Inc. [Page 5] RFC 1094 NFS: Network File System March 1989

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;
         };

Sun Microsystems, Inc. [Page 6] RFC 1094 NFS: Network File System March 1989

         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.

Sun Microsystems, Inc. [Page 7] RFC 1094 NFS: Network File System March 1989

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;

Sun Microsystems, Inc. [Page 8] RFC 1094 NFS: Network File System March 1989

 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.

Sun Microsystems, Inc. [Page 9] RFC 1094 NFS: Network File System March 1989

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;

Sun Microsystems, Inc. [Page 10] RFC 1094 NFS: Network File System March 1989

 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.

Sun Microsystems, Inc. [Page 11] RFC 1094 NFS: Network File System March 1989

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,

Sun Microsystems, Inc. [Page 12] RFC 1094 NFS: Network File System March 1989

         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.

Sun Microsystems, Inc. [Page 13] RFC 1094 NFS: Network File System March 1989

 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.

Sun Microsystems, Inc. [Page 14] RFC 1094 NFS: Network File System March 1989

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;

Sun Microsystems, Inc. [Page 15] RFC 1094 NFS: Network File System March 1989

            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.

Sun Microsystems, Inc. [Page 16] RFC 1094 NFS: Network File System March 1989

    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:

Sun Microsystems, Inc. [Page 17] RFC 1094 NFS: Network File System March 1989

            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

Sun Microsystems, Inc. [Page 18] RFC 1094 NFS: Network File System March 1989

 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

Sun Microsystems, Inc. [Page 19] RFC 1094 NFS: Network File System March 1989

 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

Sun Microsystems, Inc. [Page 20] RFC 1094 NFS: Network File System March 1989

    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.

Sun Microsystems, Inc. [Page 21] RFC 1094 NFS: Network File System March 1989

 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.

Sun Microsystems, Inc. [Page 22] RFC 1094 NFS: Network File System March 1989

                 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;

Sun Microsystems, Inc. [Page 23] RFC 1094 NFS: Network File System March 1989

         /* 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.

Sun Microsystems, Inc. [Page 24] RFC 1094 NFS: Network File System March 1989

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;

Sun Microsystems, Inc. [Page 25] RFC 1094 NFS: Network File System March 1989

 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;

Sun Microsystems, Inc. [Page 26] RFC 1094 NFS: Network File System March 1989

 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

Sun Microsystems, Inc. [Page 27]

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