NAME¶
exports —
define remote mount points
for NFS mount requests
SYNOPSIS¶
DESCRIPTION¶
The
exports file specifies remote mount points for the NFS
mount protocol per the NFS server specification; see
Network
File System Protocol Specification, RFC1094, Appendix A and
NFS: Network File System Version 3 Specification,
Appendix I.
Each line in the file (other than comment lines that begin with a #) specifies
the mount point(s) and export flags within one local server file system or the
NFSv4 tree root for one or more hosts. A long line may be split over several
lines by ending all but the last line with a backslash
(‘
\
’). A host may be specified only once
for each local file or the NFSv4 tree root on the server and there may be only
one default entry for each server file system that applies to all other hosts.
The latter exports the file system to the “world” and should be
used only when the file system contains public information.
In a mount entry, the first field(s) specify the directory path(s) within a
server file system that can be mounted on by the corresponding client(s).
There are three forms of this specification. The first is to list all mount
points as absolute directory paths separated by whitespace. This list of
directory paths should be considered an “administrative control”,
since it is only enforced by the
mountd(8) daemon and not
the kernel. As such, it only applies to NFSv2 and NFSv3 mounts and only with
respect to the client's use of the mount protocol. The second is to specify
the pathname of the root of the file system followed by the
-alldirs flag; this form allows the host(s) to mount at any
point within the file system, including regular files if the
-r option is used on
mountd(8). Because
NFSv4 does not use the mount protocol, the “administrative
controls” are not applied. Thus, all the above export line(s) should be
considered to have the
-alldirs flag, even if the line is
specified without it. The third form has the string ``V4:'' followed by a
single absolute path name, to specify the NFSv4 tree root. This line does not
export any file system, but simply marks where the root of the server's
directory tree is for NFSv4 clients. The exported file systems for NFSv4 are
specified via the other lines in the
exports(5) file in the
same way as for NFSv2 and NFSv3. The pathnames must not have any symbolic
links in them and should not have any “
.” or
“
..” components. Mount points for a file system
may appear on multiple lines each with different sets of hosts and export
options.
The second component of a line specifies how the file system is to be exported
to the host set. The option flags specify whether the file system is exported
read-only or read-write and how the client UID is mapped to user credentials
on the server. For the NFSv4 tree root, the only option that can be specified
in this section is
-sec.
Export options are specified as follows:
-maproot=
user The
credential of the specified user is used for remote access by root. The
credential includes all the groups to which the user is a member on the local
machine (see
id(1)). The user may be specified by name or
number.
-maproot=
user:group1:group2:...
The colon separated list is used to specify the precise credential to be used
for remote access by root. The elements of the list may be either names or
numbers. Note that user: should be used to distinguish a credential containing
no groups from a complete credential for that user.
-mapall=
user or
-mapall=
user:group1:group2:...
specifies a mapping for all client UIDs (including root) using the same
semantics as
-maproot.
The option
-r is a synonym for
-maproot in
an effort to be backward compatible with older export file formats.
In the absence of
-maproot and
-mapall
options, remote accesses by root will result in using a credential of -2:-2.
All other users will be mapped to their remote credential. If a
-maproot option is given, remote access by root will be
mapped to that credential instead of -2:-2. If a
-mapall
option is given, all users (including root) will be mapped to that credential
in place of their own.
-sec=
flavor1:flavor2...
specifies a colon separated list of acceptable security flavors to be used for
remote access. Supported security flavors are sys, krb5, krb5i and krb5p. If
multiple flavors are listed, they should be ordered with the most preferred
flavor first. If this option is not present, the default security flavor list
of just sys is used.
The
-ro option specifies that the file system should be
exported read-only (default read/write). The option
-o is a
synonym for
-ro in an effort to be backward compatible with
older export file formats.
WebNFS exports strictly according to the spec (RFC 2054 and RFC 2055) can be
done with the
-public flag. However, this flag in itself
allows r/w access to all files in the file system, not requiring reserved
ports and not remapping UIDs. It is only provided to conform to the spec, and
should normally not be used. For a WebNFS export, use the
-webnfs flag, which implies
-public,
-mapall=nobody and
-ro. Note that only one file system can be WebNFS exported
on a server.
A
-index=file option
can be used to specify a file whose handle will be returned if a directory is
looked up using the public filehandle (WebNFS). This is to mimic the behavior
of URLs. If no
-index option is specified, a directory
filehandle will be returned as usual. The
-index option only
makes sense in combination with the
-public or
-webnfs flags.
Specifying the
-quiet option will inhibit some of the syslog
diagnostics for bad lines in
/etc/exports. This can be
useful to avoid annoying error messages for known possible problems (see
EXAMPLES below).
The third component of a line specifies the host set to which the line applies.
The set may be specified in three ways. The first way is to list the host
name(s) separated by white space. (Standard Internet “dot”
addresses may be used in place of names.) The second way is to specify a
“netgroup” as defined in the
netgroup file (see
netgroup(5)). The third way is to specify an Internet
subnetwork using a network and network mask that is defined as the set of all
hosts with addresses within the subnetwork. This latter approach requires less
overhead within the kernel and is recommended for cases where the export line
refers to a large number of clients within an administrative subnet.
The first two cases are specified by simply listing the name(s) separated by
whitespace. All names are checked to see if they are “netgroup”
names first and are assumed to be hostnames otherwise. Using the full domain
specification for a hostname can normally circumvent the problem of a host
that has the same name as a netgroup. The third case is specified by the flag
-network=
netname[
/
prefixlength]
and optionally
-mask=netmask. The
netmask may be specified either by attaching a
prefixlength to the
-network option,
or by using a separate
-mask option. If the mask is not
specified, it will default to the mask for that network class (A, B or C; see
inet(4)). See the
EXAMPLES section below.
Scoped IPv6 address must carry scope identifier as documented in
inet6(4). For example,
“
fe80::%re2/10
” is used to specify
fe80::/10
on
re2
interface.
For the third form which specifies the NFSv4 tree root, the directory path
specifies the location within the server's file system tree which is the root
of the NFSv4 tree. All entries of this form must specify the same directory
path. This location can be any directory and does not need to be within an
exported file system. If it is not in an exported file system, a very limited
set of operations are permitted, so that an NFSv4 client can traverse the tree
to an exported file system. Although parts of the NFSv4 tree can be
non-exported, the entire NFSv4 tree must consist of local file systems capable
of being exported via NFS. NFSv4 does not use the mount protocol and does
permit clients to cross server mount point boundaries, although not all
clients are capable of crossing the mount points.
The
-sec option on these line(s) specifies what security
flavors may be used for NFSv4 operations that do not use file handles. Since
these operations (SetClientID, SetClientIDConfirm, Renew, DelegPurge and
ReleaseLockOnwer) allocate/modify state in the server, it is possible to
restrict some clients to the use of the krb5[ip] security flavors, via this
option. See the
EXAMPLES section below.
This third form is meaningless for NFSv2 and NFSv3 and is ignored for them.
The
mountd(8) utility can be made to re-read the
exports file by sending it a hangup signal as follows:
After sending the
SIGHUP
, check the
syslogd(8) output to see whether
mountd(8)
logged any parsing errors in the
exports file.
FILES¶
- /etc/exports
- the default remote mount-point file
EXAMPLES¶
/usr /usr/local -maproot=0:10 friends
/usr -maproot=daemon grumpy.cis.uoguelph.ca 131.104.48.16
/usr -ro -mapall=nobody
/u -maproot=bin: -network 131.104.48 -mask 255.255.255.0
/a -network 192.168.0/24
/a -network 3ffe:1ce1:1:fe80::/64
/u2 -maproot=root friends
/u2 -alldirs -network cis-net -mask cis-mask
/cdrom -alldirs,quiet,ro -network 192.168.33.0 -mask 255.255.255.0
/private -sec=krb5i
/secret -sec=krb5p
V4: / -sec=krb5:krb5i:krb5p -network 131.104.48 -mask 255.255.255.0
V4: / -sec=sys:krb5:krb5i:krb5p grumpy.cis.uoguelph.ca
Given that
/usr,
/u,
/a
and
/u2 are local file system mount points, the above
example specifies the following:
The file system rooted at
/usr is exported to hosts
friends where friends is specified in the netgroup file with
users mapped to their remote credentials and root mapped to UID 0 and group
10. It is exported read-write and the hosts in “friends” can mount
either
/usr or
/usr/local. It is exported
to
131.104.48.16 and
grumpy.cis.uoguelph.ca with users mapped to their remote
credentials and root mapped to the user and groups associated with
“daemon”; it is exported to the rest of the world as read-only
with all users mapped to the user and groups associated with
“nobody”.
The file system rooted at
/u is exported to all hosts on the
subnetwork
131.104.48 with root mapped to the UID for
“bin” and with no group access.
The file system rooted at
/u2 is exported to the hosts in
“friends” with root mapped to UID and groups associated with
“root”; it is exported to all hosts on network
“cis-net” allowing mounts at any directory within /u2.
The file system rooted at
/a is exported to the network
192.168.0.0, with a netmask of 255.255.255.0. However, the netmask length in
the entry for
/a is not specified through a
-mask option, but through the
/
prefix notation.
The file system rooted at
/a is also exported to the IPv6
network
3ffe:1ce1:1:fe80::
address, using the upper 64
bits as the prefix. Note that, unlike with IPv4 network addresses, the
specified network address must be complete, and not just contain the upper
bits. With IPv6 addresses, the
-mask option must not be
used.
The file system rooted at
/cdrom will be exported read-only to
the entire network 192.168.33.0/24, including all its subdirectories. Since
/cdrom is the conventional mountpoint for a CD-ROM device,
this export will fail if no CD-ROM medium is currently mounted there since
that line would then attempt to export a subdirectory of the root file system
with the
-alldirs option which is not allowed. The
-quiet option will then suppress the error message for this
condition that would normally be syslogged. As soon as an actual CD-ROM is
going to be mounted,
mount(8) will notify
mountd(8) about this situation, and the
/cdrom file system will be exported as intended. Note that
without using the
-alldirs option, the export would always
succeed. While there is no CD-ROM medium mounted under
/cdrom, it would export the (normally empty) directory
/cdrom of the root file system instead.
The file system rooted at
/private will be exported using
Kerberos 5 authentication and will require integrity protected messages for
all accesses. The file system rooted at
/secret will also be
exported using Kerberos 5 authentication and all messages used to access it
will be encrypted.
For the experimental server, the NFSv4 tree is rooted at ``/'', and any client
within the 131.104.48 subnet is permitted to perform NFSv4 state operations on
the server, so long as valid Kerberos credentials are provided. The machine
grumpy.cis.uoguelph.ca is permitted to perform NFSv4 state operations on the
server using AUTH_SYS credentials, as well as Kerberos ones.
SEE ALSO¶
nfsv4(4),
netgroup(5),
mountd(8),
nfsd(8),
showmount(8)
BUGS¶
The export options are tied to the local mount points in the kernel and must be
non-contradictory for any exported subdirectory of the local server mount
point. It is recommended that all exported directories within the same server
file system be specified on adjacent lines going down the tree. You cannot
specify a hostname that is also the name of a netgroup. Specifying the full
domain specification for a hostname can normally circumvent the problem.