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HISTORY
Written by Rich $alz (rsalz@uunet.uu.net) for InterNetNews.
SEE ALSO
expire(8), wildmat(3)
exports—NFS filesystems being exported.
SYNOPSIS
/etc/exports
DESCRIPTION
The file /etc/exports serves as the access control list for filesystems that can be exported to NFS clients. It is used by both the NFS mount daemon mountd(8) and the NFS file server daemon nfsd(8).
The file format is similar to the SunOS exports file, except that several additional options are permitted. Each line contains a mount point and a list of machine or netgroup names allowed to mount the filesystem at that point. An optional parenthesized list of mount parameters may follow each machine name. Blank lines are ignored, and a # introduces a comment to the end of the line.
General Options
| secure |
This option requires that requests originate on an Internet port less than IPPORT_RESERVED (1024). This option is on by default. To turn it off, specify insecure. |
| ro |
Allow only read-only requests on this NFS volume. The default is to allow write requests as well, which can also be made explicit by using the rw option. |
| link_relative |
Convert absolute symbolic links (where the link contents start with a slash) into relative links by prepending the necessary number of ../s to get from the directory containing the link to the root on the server. This has subtle, perhaps questionable, semantics when the file hierarchy is not mounted at its root. |
| ink_absolute | Leave all symbolic links as they are. This is the default operation. |
USER ID MAPPING
nfsd bases its access control to files on the server machine on the UID and GID provided in each NFS RPC request. The normal behavior a user would expect is that she can access her files on the server just as she would on a normal filesystem. This requires that the same UIDs and GIDs are used on the client and the server machine. This is not always true, nor is it always desirable.
Very often, it is not desirable that the root user on a client machine is also treated as root when accessing files on the NFS server. To this end, UID 0 is normally mapped to a different ID: the so-called anonymous or nobody UID. This mode of operation (called root squashing) is the default and can be turned off with no_root_squash.
By default, nfsd tries to obtain the anonymous UID and GID by looking up user nobody in the password file at startup time. If it isn't found, a UID and GID of -2 (65534) is used. These values can also be overridden by the anonuid and anongid options.
In addition to this, nfsd lets you specify arbitrary UIDs and GIDs that should be mapped to user nobody as well. Finally, you can map all user requests to the anonymous UID by specifying the all_squash option.
For the benefit of installations where UIDs differ between different machines, nfsd provides a way to dynamically map server UIDs to client UIDs and vice versa. This is enabled with the map daemon option and uses the UGID RPC protocol. For this to work, you have to run the ugidd(8) mapping daemon on the client host.
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Here's the complete list of mapping options:
| root_squash | Map requests from UID/GID 0 to the anonymous UID/GID. Note that this does not apply to any other UIDs that might be equally sensitive, such as user bin. |
| no_root_squash | Turn off root squashing. This option is mainly useful for diskless clients. |
| squash_uids and squash_gids |
This option specifies a list of UIDs or GIDs that should be subject to anonymous mapping. A valid list of IDs looks like this: |
| squash_uids=0-15,20,25-50 | |
| Usually, your squash lists will look a lot simpler, such as | |
| squash_uids=0-100 | |
| all_squash |
Map all UIDs and GIDs to the anonymous user. Useful for NFS-exported public FTP directories, newsspool directories, and so on. The opposite option is no_all_squash, which is the default setting. |
| map_daemon |
This option turns on dynamic UID/GID mapping. Each UID in an NFS request will be translated to the equivalent server UID, and each UID in an NFS reply will be mapped the other way round. This option requires that rpc.ugidd(8) runs on the client host. The default setting is map_identity, which leaves all UIDs untouched. The normal squash options apply regardless of whether dynamic mapping is requested. |
| anonuid and anongid |
These options explicitly set the UID and GID of the anonymous account. This option is primarily useful for PC/NFS clients, where you might want all requests appear to be from one user. As an example, consider the export entry for /home/joe in the section "Example," which maps all requests to UID 150 (which is supposedly that of user joe). |
EXAMPLE
# sample /etc/exports file / master(rw) trusty(rw,no_root_squash) /projects proj*.local.domain(rw) /usr *.local.domain(ro) @trusted(rw) /home/joe pc001(rw,all_squash,anonuid=150,anongid=100) /pub (ro,insecure,all_squash)
The first line exports the entire filesystem to machines master and trusty. In addition to write access, all UID squashing is turned off for host trusty. The second and third entry show examples for wildcard hostnames and netgroups (this is the entry @trusted). The fourth line shows the entry for the PC/NFS client discussed previously. The last line exports the public FTP directory to every host in the world, executing all requests under the nobody account. The insecure option in this entry also allows clients with NFS implementations that don't use a reserved port for NFS.
CAVEATS
Unlike other NFS server implementations, this nfsd allows you to export both a directory and a subdirectory thereof to the same host, for instance /usr and /usr/X11R6. In this case, the mount options of the most specific entry apply. For instance, when a user on the client host accesses a file in /usr/X11R6, the mount options given in the /usr/X11R6 entry apply. This is also true when the latter is a wildcard or netgroup entry.
FILES
| /etc/exports | Configuration file for nfsd(8) |
| /etc/passwd | The password file |
DIAGNOSTICS
An error parsing the file is reported using syslogd(8) as level NOTICE from a DAEMON whenever nfsd(8) or mountd(8) is started. Any unknown host is reported at that time, but often not all hosts are not yet known to named(8) at boot time, so as hosts are found, they are reported with the same syslogd(8) parameters.