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EXPORTS(5) File Formats Manual EXPORTS(5)

exports — 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, RFC 1094, Appendix A and NFS: Network File System Version 3 Specification, RFC 1813, 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.

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. 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 -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 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 whitespace. (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 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 and optionally -mask=netmask. If the mask is not specified, it will default to the mask for that network class (A, B or C; see inet_addr(3)).

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
/u2 -maproot=root friends
/u2 -alldirs -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 GID 10. It is exported read-write and the hosts in “friends” can mount either /usr or /usr/local. It is exported to and 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 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.

/etc/exports
Default remote mount point file.
/etc/examples/exports
Example remote mount point file.

netgroup(5), mountd(8), nfsd(8), showmount(8)

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.

Regarding -alldirs, because NFS mount filehandles are filesystem wide the -alldirs option applies to exports of the entire filesystem — even mount points that are higher up elsewhere in the directory hierarchy. Hence if the server has a filesystem /export and you wished to export the sub-directory

/export/root/client -alldirs client.foo.com

you must realize that this also allows mounts to be requested against other locations in the /export filesystem; thus the host client.foo.com is also permitted to mount the directory /export/root/client2 if it exists.

May 16, 2020 OpenBSD-7.6