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\"     @(#)exports.5    8.3 (Berkeley) 3/29/95
\"
Dd March 27, 2024
Dt EXPORTS 5
Os
Sh NAME
Nm exports
Nd exported filesystem mount points for
Tn NFS
mount requests
Sh DESCRIPTION
The
Nm
file on an
Tn NFS
server lists filesystems to be exported to
Tn NFS
clients.
It is read and applied by
Xr mountd 8
on start and on
Dv SIGHUP .
Pp
Each entry in
Nm
is a line with a list of directories followed by a list of hosts,
netgroups, and options, separated by spaces or tabs:
Pp
D1 Li / Ns Ar dir Li ... Oo Ar host | Ar netgroup | Fl Ar option Oc Li ...
Pp
All directories in a single line must live in the same filesystem,
which is exported to the hosts and netgroups listed, according to the
options specified.
Exported directories must not have pathname components that are
symbolic links,
Ql \&. ,
or
Ql \&.. .
Pp
Bf -symbolic
Warning:
Exporting a directory exposes the
Em entire
contents of the filesystem that the directory lives in to
Tn NFS
clients.
Ef
This happens even if an exported directory is not the root directory of
a filesystem on the server.
Tn NFS
clients are only prevented from access to files and directories on
filesystems that are
Em not
exported at all.
Pp
Bf -symbolic
Warning:
Access control is only by network address.
Ef
Tn NFS
servers with any non-public data should be exposed only to restricted
or firewalled networks with ingress filtering.
There is no authentication or encryption to make it safe for
restricting access on the open internet.
Pp
Blank lines are ignored.
Text beginning with
Ql #
until the end of line is ignored as a comment.
Each line ending with
Ql \e
has the next line appended, without the
Ql \e ,
as a continuation line.
Characters can be escaped with
Ql \e .
Pp
All directories, which begin with
Ql / ,
must come before any hosts, netgroups, or options on a line.
Options begin with
Ql - .
All other items on an export line are interpreted either as netgroups
Po see
Xr netgroup 5
Pc
or as hosts, which can be either names, as in example.com, or numbers,
as in 192.0.2.123 or 2001:db8:1234:abcd::42.
Sets of hosts in a contiguous network range can be specified with the
Fl network
option.
Pp
The same filesystem may be exported on multiple lines with different
options to different sets of hosts, as long as it is exported at most
once to each host, netgroup, or network.
Pp
Export lines with no hosts, netgroups, or
Fl network
options are exported to
Em any
hosts on the network, with no access control.
Pp
Supported export options:
Bl -tag -width Fl
It Fl alldirs
Allow mount requests from clients at any point within the filesystem,
including regular files.
Only the root directory of the filesystem should be specified on the
line.
Pp
Note that omitting the
Fl 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.
Sm off
It Fl maproot Li = Ar user
Sm on
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
Po see
Xr id 1
Pc .
The user may be specified by name or number.
Sm off
It Fl maproot Li = Ar user\^ Li \&: Op Ar group1\^ Li \&: group2\^ Li \&: Ar ...
Sm on
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
Sq Ar user\^ Ns Li \&:
(with the trailing colon)
should be used to distinguish a credential containing no groups from a
complete credential for that user.
Sm off
It Fl mapall Li = Ar user
It Fl mapall Li = Ar user\^ Li \&: Op Ar group1\^ Li \&: Ar group2\^ Li \&: Ar ...
Sm on
Mapping for all client uids (including root)
using the same semantics as
Fl maproot .
It Fl r Ar user
It Fl r Ar user\^ Ns Li \&: Ns Op Ar group1\^ Ns Li \&: Ns Ar group2\^ Ns Li \&: Ns Ar ...
Synonym for
Fl maproot ,
for compatibility with older export file formats.
Pp
Em Note :
Not a synonym for the read-only option
Fl ro .
El
Pp
In the absence of
Fl maproot
and
Fl 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
Fl maproot
option is given,
remote access by root will be mapped to that credential instead of -2:-2.
If a
Fl mapall
option is given,
all users (including root) will be mapped to that credential in
place of their own.
Bl -tag -width Fl
It Fl kerb
Specifies that the Kerberos authentication server should be
used to authenticate and map client credentials.
Sy This option is currently not implemented.
It Fl ro
Export filesystem read-only.
Clients will be forbidden to change or write to anything in the
filesystem
Po
except for named pipes, sockets, and device nodes, where
write semantics is client-side anyway
Pc .
It Fl o
Synonym for
Fl ro
for compatibility with older export file formats.
It Fl noresvport
Allow NFS RPC calls for the filesystem to come from non-reserved
ports.
Normally, clients are required to use reserved ports for operations.
Using this option decreases the security of your system.
\" XXX ^ Not really...
It Fl noresvmnt
Allow mount RPC requests for the filesystem to come from non-reserved
ports.
Normally, clients are required to use reserved ports for mount requests.
Using this option decreases the security of your system.
\" XXX ^ Not really...
It Fl webnfs
(WebNFS)
Enables WebNFS export, equivalent to combining
Fl public ,
Fl mapall=nobody ,
and
Fl ro .
It Fl public
(WebNFS)
Enables WebNFS export strictly according to the spec,
RFC 2054 and RFC 2055.
This implies:
Bl -bullet -compact
It
read/write access to all files in the filesystem
It
not requiring reserved ports
Pq Fl noresvport , Fl noresvmnt
It
not remapping uids
El
Pp
Bf -symbolic
Warning:
Fl public
is only provided to conform to the spec, and should normally
not be used.
Ef
For a WebNFS export,
use the
Fl webnfs
flag.
It Fl index Ns Li = Ns Ar file
(WebNFS)
File whose handle will be returned if
a directory is looked up using the public filehandle.
This is to mimic the behavior of URLs.
If no
Fl index
option is specified, a directory filehandle will be returned as usual.
Pp
The
Fl index
option only makes sense in combination with the
Fl public
or
Fl webnfs
flags.
El
Pp
Bf -symbolic
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.
Ef
\" XXX Isn't this an issue for _all_ read/write exports, not just
\" WebNFS ones?
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.
Bl -tag -width Fl
Sm off
It Fl network Li = Ar netname Op Li / Ar prefixlength
Sm on
Export the filesystem to all hosts in the specified network.
Pp
This approach to identifying hosts 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.
Pp
The netmask may be specified either by
Ar prefixlength ,
or
Pq for IPv4 networks only
by using a separate
Fl mask
option.
If the mask is not specified, it will default to the mask for that network
class
Po
A, B or C; see
Xr inet 4
Pc .
Pp
Scoped IPv6 address must carry a scope identifier as documented in
Xr inet6 4 .
For example,
Ql fe80::%ne2/10
is used to specify
Ql fe80::/10
on
Ql ne2
interface.
Sm off
It Fl mask No = Ar netmask
Sm on
(IPv4-only)
Netmask for
Fl network
options with no
Ar prefixlength .
El
Sh FILES
Bl -tag -width Pa -compact
It Pa /etc/exports
The default remote mount-point file.
El
Pp
If you have modified the
Pa /etc/exports
file, send the mountd process a
Dv SIGHUP
to make it re-read it:
Pp
Dl "kill -HUP $(cat /var/run/mountd.pid)"
Sh EXAMPLES
Bd -literal -offset indent
/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
Ed
Pp
Given that
Pa /usr ,
Pa /u ,
and
Pa /u2
are local filesystem mount points, the above example specifies the
following:
Bl -tag -width ".Pa /usr"
It Pa /usr
is exported to hosts
Ql friends
where
Ql friends
is specified in the
Xr netgroup 5
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
Ql friends
can mount either
Pa /usr
or
Pa /usr/local .
Pp
It is also exported to
Ql 131.104.48.16
and
Ql grumpy.cis.uoguelph.ca
with users mapped to their remote credentials and
root mapped to the user and groups associated with
Ql daemon .
Pp
It is also exported to the rest of the world as read-only with
all users mapped to the user and groups associated with
Ql nobody .
It Pa /u
is exported to all hosts on the subnetwork
Ql 131.104.48
with root mapped to the uid for
Ql bin
and with no group access.
It Pa /u2
is exported to the hosts in
Ql friends
with root mapped to uid and groups associated with
Ql root ;
it is exported to all hosts on network
Ql cis-net
allowing mounts at any
directory within
Pa /u2
and mapping all uids to credentials for the principal
that is authenticated by a Kerberos ticket.
Pq Sy Kerberos not implemented.
It Pa /a
is exported to the network
Ql 192.168.0.0 ,
with a netmask of
Ql 255.255.255.0 .
However, the netmask in the entry for
Pa /a
is not specified through a
Fl mask
option, but through the
Li / Ns Ar prefixlen
notation.
It Pa /a
is also exported to the IPv6 network
Ql 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
Fl mask
option must not be used.
El
Sh SEE ALSO
Xr netgroup 5 ,
Xr mountd 8 ,
Xr nfsd 8 ,
Xr showmount 8
Rs
%T NFS: Network File System Protocol Specification
%R RFC 1094
%I IETF Network Working Group
%O Appendix A
%U https://datatracker.ietf.org/doc/html/rfc1094#appendix-A.1
Re
Rs
%A B. Callaghan
%A B. Pawlowski
%A P. Staubach
%T NFS Version 3 Protocol Specification
%R RFC 1813
%I IETF Network Working Group
%O Appendix I
%U https://datatracker.ietf.org/doc/html/rfc1813#section-5.0
Re
Sh 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.
Pp
Filesystems that provide a namespace for a subtree of another
filesystem such as nullfs
Pq Xr mount_null 8
and umapfs
Pq Xr mount_umap 8
Em do not
restrict
Tn NFS
clients to that namespace, so they cannot be used to securely limit
Tn NFS
clients to a subtree of a filesystem.
If you want to export one subtree and prevent access to other subtrees,
the exported subtree must be on its own filesystem on the server.
Sh 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.
\" XXX Explain what `contradictory' means here and give some positive
\" and negative examples.
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.

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