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 RFC 1094, 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 filesystem for one or more hosts. A host may be specified only
once for each local filesystem on the server and there may be only one default
entry for each server filesystem that applies to all other hosts. The latter
exports the filesystem to the “world” and should be used only when
the filesystem contains public information.
If you have modified the
/etc/exports file, send the mountd a
SIGHUP to make it re-read the
/etc/exports file: “kill
-HUP `cat /var/run/mountd.pid`”.
In a mount entry, the first field(s) specify the directory path(s) within a
server filesystem that can be mounted on by the corresponding client(s). There
are two forms of this specification. The first is to list all mount points as
absolute directory paths separated by whitespace. The second is to specify the
pathname of the root of the filesystem followed by the
-alldirs flag; this form allows the host(s) to mount at any
point within the filesystem, including regular files. Note that the
-alldirs option should not be used as a security measure to
make clients mount only those subdirectories that they should have access to.
A client can still access the whole filesystem via individual RPCs if it
wanted to, even if just one subdirectory has been mounted. The pathnames must
not have any symbolic links in them and should not have any “.” or
“..” components. Mount points for a filesystem may appear on
multiple lines each with different sets of hosts and export options.
The second component of a line specifies how the filesystem is to be exported to
the host set. The option flags specify whether the filesystem is exported
read-only or read-write and how the client uid is mapped to user credentials
on the server.
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.
The
-kerb option specifies that the Kerberos authentication
server should be used to authenticate and map client credentials. This option
is currently not implemented.
The
-ro option specifies that the filesystem 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.
The
-noresvport option specifies that NFS RPC calls for the
filesystem do not have to come from reserved ports. Normally, clients are
required to use reserved ports for operations. Using this option decreases the
security of your system.
The
-noresvmnt option specifies that mount RPC requests for
the filesystem do not have to come from reserved ports. Normally, clients are
required to use reserved ports for mount requests. Using this option decreases
the security of your system.
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 filesystem, 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.
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.
Warning: exporting
a filesystem both using WebNFS and read/write in the normal way to other hosts
should be avoided in an environment that is vulnerable to IP spoofing.
WebNFS enables any client to get filehandles to the exported filesystem. Using
IP spoofing, a client could then pretend to be a host to which the same
filesystem was exported read/write, and use the handle to gain access to that
filesystem.
The third component of a line specifies the host set to which the line applies.
If no host set is specified, the filesystem is exported to everyone. 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)). A netgroup that
contains an item that does have a host entry is treated like an error. 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)).
Scoped IPv6 address must carry scope identifier as documented in
inet6(4). For example,
“fe80::%ne2/10” is used to specify fe80::/10 on ne2 interface.
For example:
/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 -kerb -network cis-net -mask cis-mask
Given that
/usr,
/u, and
/u2 are local filesystem mount points, the above example
specifies the following:
/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”.
/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.
/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 and mapping all uids to credentials for the principal
that is authenticated by a Kerberos ticket.
/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.
/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.
FILES
- /etc/exports
- The default remote mount-point file.
SEE ALSO
netgroup(5),
mountd(8),
nfsd(8),
showmount(8)
CAVEATS
Don't re-export NFS-mounted filesystems unless you are sure of the implications.
NFS has some assumptions about the characteristics of the file systems being
exported, e.g. when timestamps are updated. Re-exporting should work to some
extent and can even be useful in some cases, but don't expect it works as well
as with local file systems.
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
filesystem 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.