EXPORTS(5) BSD File Formats Manual EXPORTS(5)NAMEexports — define remote mount points for NFS mount requests
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 white‐
space. 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). The third form has the string ``V4:''
followed by a single absolute path name, to specify the NFSv4 tree root.
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
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 sys‐
tem 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 spec‐
ify 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 com‐
patible 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 fla‐
vors 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
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 subnet‐
work. 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
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 oper‐
ations (SetClientID, SetClientIDConfirm, Renew, DelegPurge and Release‐
LockOnwer) 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 mean‐
ingless 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.
/etc/exports the default remote mount-point file
/usr /usr/local -maproot=0:10 friends
/usr -maproot=daemon grumpy.cis.uoguelph.ca 18.104.22.168
/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
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 cre‐
dentials 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 22.214.171.124 and grumpy.cis.uoguelph.ca with users mapped
to their remote credentials and root mapped to the user and groups asso‐
ciated 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
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
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
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 nota‐
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 sup‐
press the error message for this condition that would normally be sys‐
logged. 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
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 pro‐
vided. The machine grumpy.cis.uoguelph.ca is permitted to perform NFSv4
state operations on the server using AUTH_SYS credentials, as well as
SEE ALSOnetgroup(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.
BSD June 30, 2008 BSD