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.TH EXPORTS 5 "4 March 2005" "Linux" "Linux File Formats Manual"
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exports \- NFS file systems being exported (for Kernel based NFS)
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serves as the access control list for file systems which may be
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exported to NFS clients. It is used by
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to give information to
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and to the kernel based NFS file server daemon
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The file format is similar to the SunOS
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file. Each line contains an export point and a whitespace-separated list
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of clients allowed to mount the file system at that point. Each listed
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client may be immediately followed by a parenthesized, comma-separated
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list of export options for that client. No whitespace is permitted
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between a client and its option list.
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Also, each line may have one or more specifications for default options
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after the path name, in the form of a dash ("\-") followed by an option
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list. The option list is used for all subsequent exports on that line
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Blank lines are ignored. A pound sign ("#") introduces a comment to the
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end of the line. Entries may be continued across newlines using a
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backslash. If an export name contains spaces it should be quoted using
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double quotes. You can also specify spaces or other unusual character in
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the export name using a backslash followed by the character code as three
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To apply changes to this file, run exportfs \-ra or restart the NFS server.
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.SS Machine Name Formats
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NFS clients may be specified in a number of ways:
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This is the most common format. You may specify a host either by an
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abbreviated name recognized be the resolver, the fully qualified domain
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name, or an IP address.
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NIS netgroups may be given as
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Only the host part of each
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netgroup members is consider in checking for membership. Empty host
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parts or those containing a single dash (\-) are ignored.
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Machine names may contain the wildcard characters \fI*\fR and \fI?\fR.
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This can be used to make the \fIexports\fR file more compact; for instance,
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\fI*.cs.foo.edu\fR matches all hosts in the domain
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\fIcs.foo.edu\fR. As these characters also match the dots in a domain
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name, the given pattern will also match all hosts within any subdomain
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You can also export directories to all hosts on an IP (sub-) network
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simultaneously. This is done by specifying an IP address and netmask pair
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where the netmask can be specified in dotted-decimal format, or as a
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contiguous mask length (for example, either `/255.255.252.0' or `/22' appended
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to the network base address result in identical subnetworks with 10 bits of
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host). Wildcard characters generally do not work on IP addresses, though they
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may work by accident when reverse DNS lookups fail.
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'''This is a special ``hostname'' that identifies the given directory name
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'''as the public root directory (see the section on WebNFS in
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'''for a discussion of WebNFS and the public root handle). When using this
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'''must be the only entry on this line, and must have no export options
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'''associated with it. Note that this does
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'''actually export the named directory; you still have to set the exports
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'''options in a separate entry.
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'''The public root path can also be specified by invoking
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'''.B \-\-public\-root
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'''option. Multiple specifications of a public root will be ignored.
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.SS RPCSEC_GSS security
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You may use the special strings "gss/krb5", "gss/krb5i", or "gss/krb5p"
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to restrict access to clients using rpcsec_gss security. However, this
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syntax is deprecated; on linux kernels since 2.6.23, you should instead
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use the "sec=" export option:
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The sec= option, followed by a colon-delimited list of security flavors,
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restricts the export to clients using those flavors. Available security
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flavors include sys (the default--no cryptographic security), krb5
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(authentication only), krb5i (integrity protection), and krb5p (privacy
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protection). For the purposes of security flavor negotiation, order
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counts: preferred flavors should be listed first. The order of the sec=
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option with respect to the other options does not matter, unless you
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want some options to be enforced differently depending on flavor.
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In that case you may include multiple sec= options, and following options
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will be enforced only for access using flavors listed in the immediately
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preceding sec= option. The only options that are permitted to vary in
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this way are ro, rw, no_root_squash, root_squash, and all_squash.
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understands the following export options:
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This option requires that requests originate on an Internet port less
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than IPPORT_RESERVED (1024). This option is on by default. To turn it
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Allow both read and write requests on this NFS volume. The
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default is to disallow any request which changes the filesystem.
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This can also be made explicit by using
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This option allows the NFS server to violate the NFS protocol and
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reply to requests before any changes made by that request have been
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committed to stable storage (e.g. disc drive).
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Using this option usually improves performance, but at the cost that
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an unclean server restart (i.e. a crash) can cause data to be lost or
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Reply to requests only after the changes have been committed to stable
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In releases of nfs-utils up to and including 1.0.0, this option was the
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default. In all releases after 1.0.0,
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must be explicitly requested if needed.
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To help make system administrators aware of this change, 'exportfs'
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will issue a warning if neither
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This option has no effect if
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is also set. The NFS server will normally delay committing a write request
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to disc slightly if it suspects that another related write request may be in
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progress or may arrive soon. This allows multiple write requests to
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be committed to disc with the one operation which can improve
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performance. If an NFS server received mainly small unrelated
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requests, this behaviour could actually reduce performance, so
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is available to turn it off.
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The default can be explicitly requested with the
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This option is based on the option of the same name provided in IRIX
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NFS. Normally, if a server exports two filesystems one of which is
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mounted on the other, then the client will have to mount both
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filesystems explicitly to get access to them. If it just mounts the
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parent, it will see an empty directory at the place where the other
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filesystem is mounted. That filesystem is "hidden".
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option on a filesystem causes it not to be hidden, and an
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appropriately authorised client will be able to move from the parent to
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that filesystem without noticing the change.
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However, some NFS clients do not cope well with this situation as, for
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instance, it is then possible for two files in the one apparent
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filesystem to have the same inode number.
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option is currently only effective on
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exports. It does not work reliably with netgroup, subnet, or wildcard
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This option can be very useful in some situations, but it should be
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used with due care, and only after confirming that the client system
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copes with the situation effectively.
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The option can be explicitly disabled with
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This option is similar to
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but it makes it possible for clients to move from the filesystem marked
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with crossmnt to exported filesystems mounted on it. Thus when a child
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filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
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the same effect as setting "nohide" on B.
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This option disables subtree checking, which has mild security
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implications, but can improve reliability in some circumstances.
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If a subdirectory of a filesystem is exported, but the whole
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filesystem isn't then whenever a NFS request arrives, the server must
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check not only that the accessed file is in the appropriate filesystem
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(which is easy) but also that it is in the exported tree (which is
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harder). This check is called the
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In order to perform this check, the server must include some
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information about the location of the file in the "filehandle" that is
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given to the client. This can cause problems with accessing files that
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are renamed while a client has them open (though in many simple cases
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subtree checking is also used to make sure that files inside
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directories to which only root has access can only be accessed if the
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filesystem is exported with
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(see below), even if the file itself allows more general access.
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As a general guide, a home directory filesystem, which is normally
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exported at the root and may see lots of file renames, should be
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exported with subtree checking disabled. A filesystem which is mostly
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readonly, and at least doesn't see many file renames (e.g. /usr or
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/var) and for which subdirectories may be exported, should probably be
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exported with subtree checks enabled.
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The default of having subtree checks enabled, can be explicitly
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From release 1.1.0 of nfs-utils onwards, the default will be
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as subtree_checking tends to cause more problems than it is worth.
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If you genuinely require subtree checking, you should explicitly put
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file. If you put neither option,
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will warn you that the change is pending.
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This option (the two names are synonymous) tells the NFS server not to require authentication of
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locking requests (i.e. requests which use the NLM protocol). Normally
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the NFS server will require a lock request to hold a credential for a
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user who has read access to the file. With this flag no access checks
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Early NFS client implementations did not send credentials with lock
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requests, and many current NFS clients still exist which are based on
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the old implementations. Use this flag if you find that you can only
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lock files which are world readable.
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The default behaviour of requiring authentication for NLM requests can
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be explicitly requested with either of the synonymous
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On some specially patched kernels, and when exporting filesystems that
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support ACLs, this option tells nfsd not to reveal ACLs to clients, so
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they will see only a subset of actual permissions on the given file
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system. This option is safe for filesystems used by NFSv2 clients and
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old NFSv3 clients that perform access decisions locally. Current
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NFSv3 clients use the ACCESS RPC to perform all access decisions on
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the server. Note that the
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option only has effect on kernels specially patched to support it, and
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when exporting filesystems with ACL support. The default is to export
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with ACL support (i.e. by default,
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'''This makes everything below the directory inaccessible for the named
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'''client. This is useful when you want to export a directory hierarchy to
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'''a client, but exclude certain subdirectories. The client's view of a
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'''directory flagged with noaccess is very limited; it is allowed to read
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'''its attributes, and lookup `.' and `..'. These are also the only entries
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'''returned by a readdir.
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'''Convert absolute symbolic links (where the link contents start with a
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'''slash) into relative links by prepending the necessary number of ../'s
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'''to get from the directory containing the link to the root on the
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'''server. This has subtle, perhaps questionable, semantics when the file
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'''hierarchy is not mounted at its root.
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'''Leave all symbolic link as they are. This is the default operation.
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This option makes it possible to only export a directory if it has
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successfully been mounted.
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If no path is given (e.g.
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.IR mountpoint " or " mp )
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then the export point must also be a mount point. If it isn't then
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the export point is not exported. This allows you to be sure that the
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directory underneath a mountpoint will never be exported by accident
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if, for example, the filesystem failed to mount due to a disc error.
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If a path is given (e.g.
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.IR mountpoint= "/path or " mp= /path)
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then the nominated path must be a mountpoint for the exportpoint to be
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.IR fsid= num|root|uuid
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NFS needs to be able to identify each filesystem that it exports.
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Normally it will use a UUID for the filesystem (if the filesystem has
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such a thing) or the device number of the device holding the
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filesystem (if the filesystem is stored on the device).
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As not all filesystems are stored on devices, and not all filesystems
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have UUIDs, it is sometimes necessary to explicitly tell NFS how to
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identify a filesystem. This is done with the
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For NFSv4, there is a distinguished filesystem which is the root of
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all exported filesystem. This is specified with
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both of which mean exactly the same thing.
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Other filesystems can be identified with a small integer, or a UUID
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which should contain 32 hex digits and arbitrary punctuation.
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Linux kernels version 2.6.20 and earlier do not understand the UUID
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setting so a small integer must be used if an fsid option needs to be
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set for such kernels. Setting both a small number and a UUID is
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supported so the same configuration can be made to work on old and new
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.IR refer= path@host[+host][:path@host[+host]]
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A client referencing the export point will be directed to choose from
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the given list an alternative location for the filesystem.
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(Note that the server must have a mountpoint here, though a different
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filesystem is not required; so, for example,
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.IR "mount --bind" " /path /path"
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.IR replicas= path@host[+host][:path@host[+host]]
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If the client asks for alternative locations for the export point, it
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will be given this list of alternatives. (Note that actual replication
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of the filesystem must be handled elsewhere.)
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.IR refer= path@host[+host][:path@host[+host]]
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A client referencing the export point will be directed to choose from
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the given list an alternative location for the filesystem.
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(Note that the server must have a mountpoint here, though a different
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filesystem is not required; so, for example,
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.IR "mount --bind" " /path /path"
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.IR replicas= path@host[+host][:path@host[+host]]
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If the client asks for alternative locations for the export point, it
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will be given this list of alternatives. (Note that actual replication
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of the filesystem must be handled elsewhere.)
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bases its access control to files on the server machine on the uid and
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gid provided in each NFS RPC request. The normal behavior a user would
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expect is that she can access her files on the server just as she would
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on a normal file system. This requires that the same uids and gids are
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used on the client and the server machine. This is not always true, nor
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is it always desirable.
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Very often, it is not desirable that the root user on a client machine
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is also treated as root when accessing files on the NFS server. To this
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end, uid 0 is normally mapped to a different id: the so-called
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uid. This mode of operation (called `root squashing') is the default,
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and can be turned off with
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'''tries to obtain the anonymous uid and gid by looking up user
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'''in the password file at startup time. If it isn't found, a uid and gid
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chooses a uid and gid
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of 65534 for squashed access. These values can also be overridden by
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.IR anonuid " and " anongid
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'''In addition to this,
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'''lets you specify arbitrary uids and gids that should be mapped to user
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Finally, you can map all user requests to the
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anonymous uid by specifying the
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.IR all_squash " option.
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Here's the complete list of mapping options:
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Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
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not apply to any other uids or gids that might be equally sensitive, such as
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Turn off root squashing. This option is mainly useful for diskless clients.
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Map all uids and gids to the anonymous user. Useful for NFS-exported
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public FTP directories, news spool directories, etc. The opposite option
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which is the default setting.
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.IR anonuid " and " anongid
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These options explicitly set the uid and gid of the anonymous account.
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This option is primarily useful for PC/NFS clients, where you might want
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all requests appear to be from one user. As an example, consider the
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in the example section below, which maps all requests to uid 150 (which
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is supposedly that of user joe).
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# sample /etc/exports file
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/ master(rw) trusty(rw,no_root_squash)
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/projects proj*.local.domain(rw)
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/usr *.local.domain(ro) @trusted(rw)
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/home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
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/pub *(ro,insecure,all_squash)
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/srv/www \-sync,rw server @trusted @external(ro)
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'''/pub/private (noaccess)
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The first line exports the entire filesystem to machines master and trusty.
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In addition to write access, all uid squashing is turned off for host
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trusty. The second and third entry show examples for wildcard hostnames
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and netgroups (this is the entry `@trusted'). The fourth line shows the
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entry for the PC/NFS client discussed above. Line 5 exports the
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public FTP directory to every host in the world, executing all requests
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under the nobody account. The
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option in this entry also allows clients with NFS implementations that
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don't use a reserved port for NFS.
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The sixth line exports a directory read-write to the machine 'server'
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as well as the `@trusted' netgroup, and read-only to netgroup `@external',
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all three mounts with the `sync' option enabled.
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''' The last line denies all NFS clients
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'''access to the private directory.
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'''Unlike other NFS server implementations, this
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'''allows you to export both a directory and a subdirectory thereof to
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'''the same host, for instance
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'''.IR /usr " and " /usr/X11R6 .
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'''In this case, the mount options of the most specific entry apply. For
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'''instance, when a user on the client host accesses a file in
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'''the mount options given in the
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'''entry apply. This is also true when the latter is a wildcard or netgroup
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'''An error parsing the file is reported using syslogd(8) as level NOTICE from
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'''a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
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'''host is reported at that time, but often not all hosts are not yet known
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'''to named(8) at boot time, thus as hosts are found they are reported
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'''with the same syslogd(8) parameters.
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.pc/07-remove-duplicate-exports-paragraphs.patch/utils/exportfs/exports.man
