EXPORTS(5) Linux File Formats Manual EXPORTS(5)
NAME exports - NFS file systems being exported (for Kernel based NFS)
DESCRIPTION The file /etc/exports serves as the access control list for file sys- tems which may be exported to NFS clients. It is used by exportfs(8) to give information to mountd(8) and to the kernel based NFS file server daemon nfsd(8).
The file format is similar to the SunOS exports file. Each line con- tains an export point and a whitespace-separated list of clients allowed to mount the file system at that point. Each listed client may be immediately followed by a parenthesized, comma-separated list of export options for that client. No whitespace is permitted between a client and its option list.
Blank lines are ignored. A pound sign ("#") introduces a comment to the end of the line. Entries may be continued across newlines using a backslash. If an export name contains spaces it should be quoted using double quotes. You can also specify spaces or other unusual character in the export name using a backslash followed by the character code as three octal digits.
Machine Name Formats NFS clients may be specified in a number of ways:
single host This is the most common format. You may specify a host either by an abbreviated name recognized be the resolver, the fully quali- fied domain name, or an IP address.
netgroups NIS netgroups may be given as @group. Only the host part of each netgroup members is consider in checking for membership. Empty host parts or those containing a single dash (-) are ignored.
wildcards Machine names may contain the wildcard characters * and ?. This can be used to make the exports file more compact; for instance, *.cs.foo.edu matches all hosts in the domain cs.foo.edu. As these characters also match the dots in a domain name, the given pattern will also match all hosts within any subdomain of cs.foo.edu.
IP networks You can also export directories to all hosts on an IP (sub-) network simultaneously. This is done by specifying an IP address and netmask pair as address/netmask where the netmask can be specified in dotted-decimal format, or as a contiguous mask length (for example, either /255.255.252.0 or /22 appended to the network base address result in identical subnetworks with 10 bits of host). Wildcard characters generally do not work on IP addresses, though they may work by accident when reverse DNS lookups fail.
General Options exportfs understands the following export 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.
rw Allow both read and write requests on this NFS volume. The default is to disallow any request which changes the filesystem. This can also be made explicit by using the ro option.
async This option allows the NFS server to violate the NFS protocol and reply to requests before any changes made by that request have been committed to stable storage (e.g. disc drive).
Using this option might improve performance with version 2 only, but at the cost that an unclean server restart (i.e. a crash) can cause data to be lost or corrupted.
sync Reply to requests only after the changes have been committed to stable storage (see async above).
no_wdelay This option has no effect if async is also set. The NFS server will normally delay committing a write request to disc slightly if it suspects that another related write request may be in progress or may arrive soon. This allows multiple write requests to be committed to disc with the one operation which can improve performance. If an NFS server received mainly small unrelated requests, this behaviour could actually reduce perfor- mance, so no_wdelay is available to turn it off. The default can be explicitly requested with the wdelay option.
nohide This option is based on the option of the same name provided in IRIX NFS. Normally, if a server exports two filesystems one of which is mounted on the other, then the client will have to mount both filesystems explicitly to get access to them. If it just mounts the parent, it will see an empty directory at the place where the other filesystem is mounted. That filesystem is "hidden".
Setting the nohide option on a filesystem causes it not to be hidden, and an appropriately authorised client will be able to move from the parent to that filesystem without noticing the change.
However, some NFS clients do not cope well with this situation as, for instance, it is then possible for two files in the one apparent filesystem to have the same inode number.
The nohide option is currently only effective on single host exports. It does not work reliably with netgroup, subnet, or wildcard exports.
This option can be very useful in some situations, but it should be used with due care, and only after confirming that the client system copes with the situation effectively.
The option can be explicitly disabled with hide.
crossmnt This option is similar to nohide but it makes it possible for clients to move from the filesystem marked with crossmnt to exported filesystems mounted on it. Thus when a child filesys- tem "B" is mounted on a parent "A", setting crossmnt on "A" has the same effect as setting "nohide" on B.
subtree_check This option enables subtree checking, which does add another level of security, but can be unreliability in some circum- stances.
If a subdirectory of a filesystem is exported, but the whole filesystem isn t then whenever a NFS request arrives, the server must check not only that the accessed file is in the appropriate filesystem (which is easy) but also that it is in the exported tree (which is harder). This check is called the subtree_check.
In order to perform this check, the server must include some information about the location of the file in the "filehandle" that is given to the client. This can cause problems with accessing files that are renamed while a client has them open (though in many simple cases it will still work).
subtree checking is also used to make sure that files inside directories to which only root has access can only be accessed if the filesystem is exported with no_root_squash (see below), even if the file itself allows more general access.
As a general guide, a home directory filesystem, which is nor- mally exported at the root and may see lots of file renames, should be exported with subtree checking disabled. A filesystem which is mostly readonly, and at least doesn t see many file renames (e.g. /usr or /var) and for which subdirectories may be exported, should probably be exported with subtree checks enabled.
This type of subtree checking is disabled by default.
no_auth_nlm This option (the two names are synonymous) tells the NFS server not to require authentication of locking requests (i.e. requests which use the NLM protocol). Normally the NFS server will require a lock request to hold a credential for a user who has read access to the file. With this flag no access checks will be performed.
Early NFS client implementations did not send credentials with lock requests, and many current NFS clients still exist which are based on the old implementations. Use this flag if you find that you can only lock files which are world readable.
The default behaviour of requiring authentication for NLM requests can be explicitly requested with either of the synony- mous auth_nlm, or secure_locks.
no_acl On some specially patched kernels, and when exporting filesys- tems that support ACLs, this option tells nfsd not to reveal ACLs to clients, so they will see only a subset of actual per- missions on the given file system. This option is safe for filesystems used by NFSv2 clients and old NFSv3 clients that perform access decisions locally. Current NFSv3 clients use the ACCESS RPC to perform all access decisions on the server. Note that the no_acl option only has effect on kernels specially patched to support it, and when exporting filesystems with ACL support. The default is to export with ACL support (i.e. by default, no_acl is off).
mp This option makes it possible to only export a directory if it has successfully been mounted. If no path is given (e.g. mountpoint or mp) then the export point must also be a mount point. If it isnt then the export point is not exported. This allows you to be sure that the directory underneath a mountpoint will never be exported by accident if, for example, the filesys- tem failed to mount due to a disc error.
If a path is given (e.g. mountpoint=/path or mp=/path) then the nominted path must be a mountpoint for the exportpoint to be exported.
fsid=num This option forces the filesystem identification portion of the file handle and file attributes used on the wire to be num instead of a number derived from the major and minor number of the block device on which the filesystem is mounted. Any 32 bit number can be used, but it must be unique amongst all the exported filesystems.
This can be useful for NFS failover, to ensure that both servers of the failover pair use the same NFS file handles for the shared filesystem thus avoiding stale file handles after failover.
Some Linux filesystems are not mounted on a block device; exporting these via NFS requires the use of the fsid option (although that may still not be enough).
The value 0 has a special meaning when use with NFSv4. NFSv4 has a concept of a root of the overall exported filesystem. The export point exported with fsid=0 will be used as this root.
refer=path@host[+host][:path@host[+host]] A client referencing the export point will be directed to choose from the given list an alternative location for the filesystem. (Note that the server currently needs to have a filesystem mounted here, generally using mount --bind, although it is not actually exported.)
sec=flavor[:flavor] The sec option, followed by a colon-delimited list of security flavors, restricts the export to clients using those flavors. Available security flavors include:
none (no cryptographic security) sys (no cryptographic security) krb5 (authentication only) krb5i (integrity protection) krb5p (privacy protection)
For the purposes of security flavor negotiation, order counts: preferred flavors should be listed first. The order of the sec= option with respect to the other options does not matter, unless you want some options to be enforced differently depend- ing on flavor. In that case you may include multiple sec= options, and following options will be enforced only for access using flavors listed in the immediately preceding sec= option. The only options that are permitted to vary in this way are ro, rw, no_root_squash, root_squash, and all_squash.
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 file system. 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 anony- mous or nobody uid. This mode of operation (called root squashing) is the default, and can be turned off with no_root_squash.
By default, exportfs chooses a uid and gid of 65534 for squashed access. These values can also be overridden by the anonuid and anongid options. Finally, you can map all user requests to the anonymous uid by specifying the all_squash option.
Heres 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 sen- sitive, such as user bin.
no_root_squash Turn off root squashing. This option is mainly useful for disk- less clients.
all_squash Map all uids and gids to the anonymous user. Useful for NFS- exported public FTP directories, news spool directories, etc. The opposite option is no_all_squash, which is the default set- ting.
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 exam- ple section below, 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 above. Line 5 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 dont use a reserved port for NFS.
SEE ALSO exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
Linux 4 March 2005 EXPORTS(5)