usr.sbin/mountd/exports.5

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.Dd October 08, 2024
.Dt EXPORTS 5
.Os
.Sh NAME
.Nm exports
.Nd define remote mount points for
.Tn NFS
mount requests
.Sh SYNOPSIS
.Nm
.Sh DESCRIPTION
The
.Nm
file specifies remote mount points for the
.Tn NFS
mount protocol per the
.Tn NFS
server specification; see
.%T "Network File System Protocol Specification" ,
RFC1094, Appendix A and
.%T "NFS: Network File System Version 3 Specification" ,
Appendix I.
.Pp
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
.Pq Ql \e .
A host may be specified only once for each local file system 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
.Dq world
and should
be used only when the file system contains public information.
.Pp
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).
Note well that exporting a directory on the server does not guarantee that only
files below the exported directory will be accessible.
This is true even in the absence of the
.Fl alldirs
flag.
To provide this guarantee, the exported directories must be local file system
mount points on the server.
For example, if one exports
.Pa /home ,
and
.Pa /home
is not a file system mount point, then clients will be able to access arbitrary
files on the root file system.
As such, to avoid confusion with respect to what is exported, it may be prudent
to limit exported directories to server local file system mount points.
When exporting ZFS datasets with the
.Sy sharenfs
property, this is auomatically the case.
.Pp
There are three forms of the directory path 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
.Dq administrative control ,
since it is only enforced by the
.Xr 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
.Fl alldirs
flag;
this form allows the host(s) to mount at any point within the file system,
including regular files if the
.Fl r
option is used on
.Xr mountd 8 .
Because NFSv4 does not use the mount protocol,
the
.Dq administrative controls
are not applied and all directories within this server
file system are mountable via NFSv4 even if the
.Fl alldirs
flag has not been specified.
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
.Nm
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
.Dq Pa \&.
or
.Dq Pa ..
components.
Pathnames are decoded by
.Xr strunvis 3
allowing special characters to be included in the directory name(s).
In particular, whitespace, such as embedded blanks in directory names
can be handled.
For example, a blank can be encoded as \(rs040.
.Xr vis 1
with the
.Fl M
option may be used to encode directory name(s) with embedded special
characters.
Mount points for a file system may appear on multiple lines each with
different sets of hosts and export options.
.Pp
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 options that can be specified in this
section are ones related to security:
.Fl sec ,
.Fl tls ,
.Fl tlscert
and
.Fl tlscertuser .
.Pp
Export options are specified as follows:
.Pp
.Sm off
.Fl maproot Li = Sy 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 (see
.Xr id 1 ) .
The user may be specified by name or number.
The user string may be quoted, or use backslash escaping.
.Pp
.Sm off
.Fl maproot Li = Sy user:group1:group2:...
.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
.Cm user:
should be used to specify a credential containing no groups, in which case the
established credential will use
.Ql nogroup ,
else 65533
.Pq Dv GID_NOGROUP ,
as the fallback group
.Pq a credential object must have at least one group internally .
Using just
.Cm user
.Pq without colon at end
falls into the
.Sm off
.Fl maproot Li = Sy user
.Sm on
case described above.
The group names may be quoted, or use backslash escaping.
.Pp
.Sm off
.Fl mapall Li = Sy user
.Sm on
or
.Sm off
.Fl mapall Li = Sy user:group1:group2:...
.Sm on
specifies a mapping for all client UIDs (including root)
using the same semantics as
.Fl maproot .
.Pp
The option
.Fl r
is a synonym for
.Fl maproot
in an effort to be backward compatible with older export file formats.
.Pp
In the absence of
.Fl maproot
and
.Fl mapall
options, remote accesses by root will result in using a credential of 65534:65533.
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 65534:65533.
If a
.Fl mapall
option is given,
all users (including root) will be mapped to that credential in
place of their own.
.Pp
.Sm off
.Fl sec Li = Sy flavor1:flavor2...
.Sm on
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.
.Pp
The
.Fl ro
option specifies that the file system should be exported read-only
(default read/write).
The option
.Fl o
is a synonym for
.Fl ro
in an effort to be backward compatible with older export file formats.
.Pp
.Tn WebNFS
exports strictly according to the spec (RFC 2054 and RFC 2055) can
be done with the
.Fl 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
.Tn WebNFS
export,
use the
.Fl webnfs
flag, which implies
.Fl public ,
.Sm off
.Fl mapall No = Sy nobody
.Sm on
and
.Fl ro .
Note that only one file system can be
.Tn WebNFS
exported on a server.
.Pp
A
.Sm off
.Fl index No = Pa file
.Sm on
option can be used to specify a file whose handle will be returned if
a directory is looked up using the public filehandle
.Pq Tn WebNFS .
This is to mimic the behavior of URLs.
If no
.Fl index
option is specified, a directory filehandle will be returned as usual.
The
.Fl index
option only makes sense in combination with the
.Fl public
or
.Fl webnfs
flags.
.Pp
The
.Fl tls ,
.Fl tlscert
and
.Fl tlscertuser
export options are used to require the client to use TLS for the mount(s)
per RFC 9289.
For NFS mounts using TLS to work,
.Xr rpc.tlsservd 8
must be running on the server.
.Bd -filled -offset indent
.Fl tls
requires that the client use TLS.
.br
.Fl tlscert
requires that the client use TLS and provide a verifiable X.509 certificate
during TLS handshake.
.br
.Fl tlscertuser
requires that the client use TLS and provide a verifiable X.509 certificate.
The otherName component of the certificate's subjAltName must have a
an OID of 1.3.6.1.4.1.2238.1.1.1 and a UTF8 string of the form
.Dq user@domain .
.Dq user@domain
will be translated to the credentials of the specified user in the same
manner as
.Xr nfsuserd 8 ,
where
.Dq user
is normally a username is the server's password database and
.Dq domain
is the DNS domain name for the server.
All RPCs will be performed using these credentials instead of the
ones in the RPC header in a manner similar to
.Sm off
.Fl mapall Li = Sy user .
.Sm on
.Ed
.Pp
If none of these three flags are specified, TLS mounts are permitted but
not required.
.Pp
Specifying the
.Fl quiet
option will inhibit some of the syslog diagnostics for bad lines in
.Pa /etc/exports .
This can be useful to avoid annoying error messages for known possible
problems (see
.Sx EXAMPLES
below).
.Pp
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
.Dq dot
addresses may be used in place of names.)
The second way is to specify a
.Dq netgroup
as defined in the
.Pa netgroup
file (see
.Xr 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.
.Pp
The first two cases are specified by simply listing the name(s) separated
by whitespace.
All names are checked to see if they are
.Dq 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
.Sm off
.Fl network Li = Sy netname Op Li / Ar prefixlength
.Sm on
and optionally
.Sm off
.Fl mask No = Sy netmask .
.Sm on
The netmask may be specified either by attaching a
.Ar prefixlength
to the
.Fl network
option, or by using a separate
.Fl mask
option.
If the mask is not specified, it will default to the historical mask
for that network class (A, B, or C; see
.Xr inet 4 ) .
This usage is deprecated, and will elicit a warning log message.
See the
.Sx EXAMPLES
section below.
.Pp
Scoped IPv6 address must carry scope identifier as documented in
.Xr inet6 4 .
For example,
.Dq Li fe80::%re2/10
is used to specify
.Li fe80::/10
on
.Li re2
interface.
.Pp
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.
There can only be one NFSv4 root directory per server.
As such, all entries of this form must specify the same directory path.
For file systems other than ZFS,
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.
All ZFS file systems in the subtree below the NFSv4 tree root must be
exported.
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.
.Pp
The
.Fl 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
.Sx EXAMPLES
section below.
This third form is meaningless for NFSv2 and NFSv3 and is ignored for them.
.Pp
The
.Xr mountd 8
utility can be made to re-read the
.Nm
file by sending it a hangup signal as follows:
.Bd -literal -offset indent
service mountd reload
.Ed
.Pp
After sending the
.Dv SIGHUP ,
check the
.Xr syslogd 8
output to see whether
.Xr mountd 8
logged any parsing errors in the
.Nm
file.
.Sh FILES
.Bl -tag -width /etc/exports -compact
.It Pa /etc/exports
the default remote mount-point file
.El
.Sh EXAMPLES
Given that
.Pa /usr , /u , /a
and
.Pa /u2
are
local file system mount points, let's consider the following example:
.Pp
.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 -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
.Ed
.Pp
The file systems rooted at
.Pa /usr
and
.Pa /usr/local
are exported to hosts within the
.Dq friends
network group
with users mapped to their remote credentials and
root mapped to UID 0 and group 10.
They are exported read-write and the hosts in
.Dq friends .
.Pp
The file system rooted at
.Pa /usr
is exported to
.Em 131.104.48.16
and
.Em grumpy.cis.uoguelph.ca
with users mapped to their remote credentials and
root mapped to the user and groups associated with
.Dq daemon ;
it is exported to the rest of the world as read-only with
all users mapped to the user and groups associated with
.Dq nobody .
.Pp
The file system rooted at
.Pa /u
is exported to all hosts on the subnetwork
.Em 131.104.48
with root mapped to the UID for
.Dq bin
and with no group access.
.Pp
The file system rooted at
.Pa /u2
is exported to the hosts in
.Dq friends
with root mapped to UID and groups
associated with
.Dq root ;
it is exported to all hosts on network
.Dq cis-net
allowing mounts at any
directory within /u2.
.Pp
The file system rooted at
.Pa /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
.Pa /a
is not specified through a
.Fl mask
option, but through the
.Li / Ns Ar prefix
notation.
.Pp
The file system rooted at
.Pa /a
is also exported to the IPv6 network
.Li 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.
.Pp
The file system rooted at
.Pa /cdrom
will be exported read-only to the entire network 192.168.33.0/24, including
all its subdirectories.
Since
.Pa /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
.Fl alldirs
option which is not allowed.
The
.Fl 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,
.Xr mount 8
will notify
.Xr mountd 8
about this situation, and the
.Pa /cdrom
file system will be exported as intended.
Note that without using the
.Fl alldirs
option, the export would always succeed.
While there is no CD-ROM medium mounted under
.Pa /cdrom ,
it would export the (normally empty) directory
.Pa /cdrom
of the root file system instead.
.Pp
The file system rooted at
.Pa /private
will be exported using Kerberos 5 authentication and will require
integrity protected messages for all accesses.
The file system rooted at
.Pa /secret
will also be exported using Kerberos 5 authentication and all messages
used to access it will be encrypted.
.Pp
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.
.Pp
In the following example some directories are exported as NFSv3 and NFSv4:
.Bd -literal -offset indent
V4: /wingsdl/nfsv4
/wingsdl/nfsv4/usr-ports -maproot=root -network 172.16.0.0 -mask 255.255.0.0
/wingsdl/nfsv4/clasper   -maproot=root clasper
.Ed
.Pp
Only one V4: line is needed or allowed to declare where NFSv4 is
rooted.
The other lines declare specific exported directories with
their absolute paths given in /etc/exports.
.Pp
The exported directories' paths are used for both v3 and v4.
However, they are interpreted differently for v3 and v4.
A client mount command for usr-ports would use the server-absolute name when
using nfsv3:
.Bd -literal -offset indent
mount server:/wingsdl/nfsv4/usr-ports /mnt/tmp
.Ed
.Pp
A mount command using NFSv4 would use the path relative to the NFSv4
root:
.Bd -literal -offset indent
mount server:/usr-ports /mnt/tmp
.Ed
.Pp
This also differentiates which version you want if the client can do
both v3 and v4.
The former will only ever do a v3 mount and the latter will only ever
do a v4 mount.
.Pp
Note that due to different mount behavior between NFSv3 and NFSv4 a
NFSv4 mount request for a directory that the client does not have
permission for will succeed and read/write access will fail
afterwards, whereas NFSv3 rejects the mount request.
.Sh SEE ALSO
.Xr vis 1 ,
.Xr strunvis 3 ,
.Xr nfsv4 4 ,
.Xr netgroup 5 ,
.Xr zfsprops 7 ,
.Xr mountd 8 ,
.Xr nfsd 8 ,
.Xr rpc.tlsservd 8 ,
.Xr service 8 ,
.Xr showmount 8
.Sh STANDARDS
The implementation is based on the following documents:
.Bl -dash
.It
.Rs
.%T "Network File System Protocol Specification, Appendix A, RFC 1094"
.Re
.It
.Rs
.%T "NFS: Network File System Version 3, Appendix I, RFC 1813"
.Re
.It
.Rs
.%T "Towards Remote Procedure Call Encryption by Default, RFC 9289"
.Re
.El
.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.
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.