NETWORK FILE SYSTEM (NFS) By Ameeta.Jakate

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.

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.

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.

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.

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.

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”.

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 called”xtab” Exportfs is the helper command used. /etc/exports is the human-readable format of xtab.

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.

Client-Side NFS “mount” command is used to establish temporary network mounts. Mounts that are part of a system’s 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”.

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.

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.