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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.