The nfsd filesystem is a special filesystem which provides access to
the Linux NFS server. The filesystem consists of a single directory
which contains a number of files. These files are actually gateways
into the NFS server. Writing to them can affect the server. Reading
from them can provide information about the server.
This file system is only available in Linux 2.6 and later series
kernels (and in the later parts of the 2.5 development series leading
up to 2.6). This man page does not apply to 2.4 and earlier.
As well as this filesystem, there are a collection of files in the
procfs filesystem (normally mounted at /proc) which are used to
control the NFS server. This manual page describes all of these
The exportfs and mountd programs (part of the nfs-utils package)
expect to find this filesystem mounted at /proc/fs/nfsd or
/proc/fs/nfs. If it is not mounted, they will fall-back on 2.4 style
functionality. This involves accessing the NFS server via a
systemcall. This systemcall is scheduled to be removed after the 2.6
The three files in the nfsd filesystem are:
This file contains a list of filesystems that are currently
exported and clients that each filesystem is exported to,
together with a list of export options for that
client/filesystem pair. This is similar to the
/proc/fs/nfs/exports file in 2.4. One difference is that a
client doesn't necessarily correspond to just one host. It
can respond to a large collection of hosts that are being
Each line of the file contains a path name, a client name, and
a number of options in parentheses. Any space, tab, newline
or back-slash character in the path name or client name will
be replaced by a backslash followed by the octal ASCII code
for that character.
This file represents the number of nfsd thread currently
running. Reading it will show the number of threads. Writing
an ASCII decimal number will cause the number of threads to be
changed (increased or decreased as necessary) to achieve that
This is a somewhat unusual file in that what is read from it
depends on what was just written to it. It provides a
transactional interface where a program can open the file,
write a request, and read a response. If two separate
programs open, write, and read at the same time, their
requests will not be mixed up.
The request written to filehandle should be a client name, a
path name, and a number of bytes. This should be followed by
a newline, with white-space separating the fields, and octal
quoting of special characters.
On writing this, the program will be able to read back a
filehandle for that path as exported to the given client. The
filehandle's length will be at most the number of bytes given.
The filehandle will be represented in hex with a leading '\x'.
The directory /proc/net/rpc in the procfs filesystem contains a
number of files and directories. The files contain statistics that
can be display using the nfsstat program. The directories contain
information about various caches that the NFS server maintains to
keep track of access permissions that different clients have for
different filesystems. The caches are:
This cache maps the name of a client (or domain) to an
internal data structure. The only access that is possible is
to flush the cache.
This cache contains a mapping from IP address to the name of
the authentication domain that the ipaddress should be treated
as part of.
This cache contains a mapping from directory and domain to
This cache contains a mapping from domain and a filesystem
identifier to a directory. The filesystem identifier is
stored in the filehandles and consists of a number indicating
the type of identifier and a number of hex bytes indicating
the content of the identifier.
Each directory representing a cache can hold from 1 to 3 files. They
flush When a number of seconds since epoch (1 Jan 1970) is written
to this file, all entries in the cache that were last updated
before that file become invalidated and will be flushed out.
Writing 1 will flush everything. This is the only file that
will always be present.
This file, if present, contains a textual representation of
ever entry in the cache, one per line. If an entry is still
in the cache (because it is actively being used) but has
expired or is otherwise invalid, it will be presented as a
comment (with a leading hash character).
This file, if present, acts a channel for request from the
kernel-based nfs server to be passed to a user-space program
When the kernel needs some information which isn't in the
cache, it makes a line appear in the channel file giving the
key for the information. A user-space program should read
this, find the answer, and write a line containing the key, an
expiry time, and the content. For example the kernel might
appear in the auth.unix.ip/content file. The user-space
program might then write
nfsd 127.0.0.1 1057206953 localhost
to indicate that 127.0.0.1 should map to localhost, at least
If the program uses select(2) or poll(2) to discover if it can
read from the channel then it will never see and end-of-file
but when all requests have been answered, it will block until
another request appears.
In the /proc filesystem there are 4 files that can be used to enabled
extra tracing of nfsd and related code. They are:
They control tracing for the NFS client, the NFS server, the Network
Lock Manager (lockd) and the underlying RPC layer respectively.
Decimal numbers can be read from or written to these files. Each
number represents a bit-pattern where bits that are set cause certain
classes of tracing to be enabled. Consult the kernel header files to
find out what number correspond to what tracing.