NETWORK FILE SYSTEM (NFS)

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NETWORK FILE SYSTEM (NFS)

• By Ameeta.Jakate
  

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NFS

• NFS was introduced in 1985 as a means of
  providing transparent access to remote file
  systems.
• NFS Architecture is based on a client-server
  model.
• Clients and Servers communicate via remote
  procedure calls.
• Servers export set of files while clients access
  such files .

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NFS- Design Goals

• NFS should not be restricted to UNIX.
• Protocol should not be dependant on any
  particular hardware.
• There should be simple recovery mechanisms from
  server or client crashes.
• Applications should be able to access remote
  files transparently.
• NFS performance must be comparable to that of a
  local disk.

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NFS Components

• The NFS protocol
• The Remote Procedure Call (RPC protocol) defines
  the format of all interactions between client
  and server.
• The Extended Data Representation (XDR) provides a
  machine independent method of encoding data to
  send over the network.
• The NFS server code is responsible for processing
  all client requests and providing access to to
  the exported file systems.
• NFS client code implements all client system
  calls on remote files by sending one or more RPC
  requests to the server.

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NFS Components

• The Mount protocol defines the semantics for
  mounting and unmounting NFS file systems.
• Daemon processes.
• On the server a set of nfsd daemons listen for
  and respond to the client NFS requests and mountd
  daemon handles the mount requests.
• On the client a set of biod daemons handle
  asynchronous I/O for blocks of NFS files.
• Lockd and Statd though not in base protocol are
  used to provide facilities for locking files over
  a network.

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NFS Security

• NFS performs access control when file system is
  mounted and also on every NFS request.
• Servers maintain a exports list .
• When a client tries to mount a file system ,the
  server mountd checks the list and denies access
  to ineligible users.
• On each NFS request client sends authentication
  information.
• Any given UID must belong to the same person on
  all machines sharing NFS file system.

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NFS Security

• To prevent intrusions into NFS we use UID
  remapping.
• The server maintains a translation map for for
  each client.
• Map defines translation from credentials received
  over the network to an identity to be used on a
  server.
• Root remapping is used as it is dangerous to
  allow superusers on all clients to have root
  access to files on the server.
• Root remapping means the server maps the
  superuser from any client to the user nobody.

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Server-Side NFS.

• A server is usually said to export a directory
  when it makes the directory available for use by
  other other machines.
• Process to mount a file system and the process to
  access the files is totally different.
• Mountd-handles mount requests
• Nfsd-handles actual file service
• Access control databse is a binary file
  calledxtab
• Exportfs is the helper command used .
• /etc/exports is the human-readable format of
  xtab.

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Server-Side NFS

• Whenever we make changes to the exports
  file,for the changes to take effect we should use
  the command exportfs a.
• After a clients mount request has been validated
  by mountd the various file operation requests
  are handled by nfsd.
• Nfsd takes a numeric argument that specifies the
  number of copies of itself that it should fork.

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Client-Side NFS

• mount command is used to establish temporary
  network mounts.
• Mounts that are part of a systems permanent
  configuration should be listed in /etc/fstab
  so that they are mounted automatically at boot
  time.
• When we add entries to /etc/fstab we must
  create mount point directories with mkdir.
• Alternatively mounts can be handled by automatic
  mounting service such as automount or amd.

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Client-Side NFS

• File systems can be either mounted hard or soft.
• If the file system is hard-mounted then the
  client keeps retrying the request until a reply
  is received.
• If the file system is soft-mounted then the
  client gives up after a while and returns an
  error.
• biod/nfsiod provide client-side caching .

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NFS

• nfsstat is a command used to display the
  various statistics kept by the NFS system.
• nfsstat -s displays statistics for NFS server
  processes.
• nfsstat c shows information related to the
  client-side operations.