Use of NFS Considered Harmful  

First of all usage of 'considered harmful" usually signify primitive fundamentalist stance of the critique. Also this critique is applicable only to older versions of protocols. NFS v.4 contains some improvements

Following are a few known problems with NFS and suggested workarounds.

a. Time Synchronization

NFS does not synchronize time between client and server, and offers no mechanism for the client to determine what time the server thinks it is. What this means is that a client can update a file, and have the timestamp on the file be either some time long in the past, or even in the future, from its point of view.

While this is generally not an issue if clocks are a few seconds or even a few minutes off, it can be confusing and misleading to humans. Of even greater importance is the affect on programs. Programs often do not expect time difference like this, and may end abnormally or behave strangely, as various tasks timeout instantly, or take extraordinarily long while to timeout.

Poor time synchronization also makes debugging problems difficult, because there is no easy way to establish a chronology of events. This is especially problematic when investigating security issues, such as break in attempts.

Workaround: Use the Network Time Protocol (NTP) religiously. Use of NTP can result in machines that have extremely small time differences.

Note: The NFS protocol version 3 does have support for the client specifying the time when updating a file, but this is not widely implemented. Additionally, it does not help in the case where two clients are accessing the same file from machines with drifting clocks.

b. File Locking Semantics

Programs use file locking to insure that concurrent access to files does not occur except when guaranteed to be safe. This prevents data corruption, and allows handshaking between cooperative processes.

In Unix, the kernel handles file locking. This is required so that if a program is terminated, any locks that it has are released. It also allows the operations to be atomic, meaning that a lock cannot be obtained by multiple processes.

Because NFS is stateless, there is no way for the server to keep track of file locks - it simply does not know what clients there are or what files they are using. In an effort to solve this, a separate server, the lock daemon, was added. Typically, each NFS server will run a lock daemon.

The combination of lock daemon and NFS server yields a solution that is almost like Unix file locking. Unfortunately, file locking is extremely slow, compared to NFS traffic without file locking (or file locking on a local Unix disk). Of greater concern is the behaviour of NFS locking on failure.

In the event of server failure (e.g. server reboot or lock daemon restart), all client locks are lost. However, the clients are not informed of this, and because the other operations (read, write, and so on) are not visibly interrupted, they have no reliable way to prevent other clients from obtaining a lock on a file they think they have locked.

In the event of client failure, the locks are not immediately freed. Nor is there a timeout. If the client process terminates, the client OS kernel will notify the server, and the lock will be freed. However, if the client system shuts down abnormally (e.g. power failure or kernel panic), then the server will not be notified. When the client reboots and remounts the NFS exports, the server is notified and any client locks are freed.

If the client does not reboot, for example if a frustrated user hits the power switch and goes home for the weekend, or if a computer has had a hardware failure and must wait for replacement parts, then the locks are never freed! In this unfortunate scenario, the server lock daemon must be restarted, with the same effects as a server failure.

Workaround: If possible (given program source and skill with code modification), remove locking and insure no inconsistency occurs via other mechanisms, possibly using atomic file creation (see below) or some other mechanism for synchronization. Otherwise, build platforms never fail and have a staff trained on the implications of NFS file locking failure. If NFS is used only for files that are never accessed by more than a single client, locking is not an issue.

Note: A status monitor mechanism exists to monitor client status, and free client locks if a client is unavailable. However, clients may chose not to use this mechanism, and in many implementations do not.

c. File Locking API

In Unix, there are two flavours of file locking, flock() from BSD and lockf() from System V. It varies from system to system which of these mechanisms work with NFS. In Solaris, Sun's Unix variant, lockf() works with NFS, and flock() is implemented via lockf(). On other systems, the results are less consistent. For example, on some systems, lockf() is not implemented at all, and flock() does not support NFS; while on other systems, lockf() supports NFS but flock() does not.

Regardless of the system specifics, programs often assume that if they are unable to obtain a lock, it is because another program has the lock. This can cause problems as programs wait for the lock to be freed. Since the reason the lock fails is because locking is unsupported, the attempt to obtain a lock will never work. This results in either the applications waiting forever, or aborting their operation.

These results will also vary with the support of the server. While typically the NFS server runs an accompanying lock daemon, this is not guaranteed.

Workaround: Upgrade to the latest versions of all operating systems, as they usually have improved and more consistent locking support. Also, use the lock daemon. Additionally, try to use only programs written to handle NFS locking properly, veified either by code review or a vendor compliance statement.

d. Exclusive File Creation

In Unix, when a program creates a file, it may ask for the operation to fail if the file already exists (as opposed to the default behaviour of using the existing file). This allows programs to know that, for example, they have a unique file name for a temporary file. It is also used by various daemons for locking various operations, e.g. modifying mail folders or print queues.

Unfortunately, NFS does not properly implement this behaviour. A file creation will sometimes return success even if the file already exists. Programs written to work on a local file system will experience strange results when they attempt to update a file after using file creation to lock it, only to discover another file is modifying it (I have personally seen mailboxes with hundreds of mail messages corrupted because of this), because it also "locked" the file via the same mechanism.

Workaround: If possible (given program source and skill with code modification), use the following method, as documented in the Linux open() manual page:

The solution for performing atomic file locking using a lockfile is to create a unique file on the same fs (e.g., incorporating hostname and pid), use link(2) to make a link to the lockfile and use stat(2) on the unique file to check if its link count has increased to 2. Do not use the return value of the link() call.

This still leaves the issue of client failure unanswered. The suggested solution for this is to pick a timeout value and assume if a lock is older than a certain application-specific age that it has been abandoned.

e. Delayed Write Caching

In an effort to improve efficiency, many NFS clients cache writes. This means that they delay sending small writes to the server, with the idea that if the client makes another small write in a short amount of time, the client need only send a single message to the server.

Unix servers typically cache disk writes to local disks the same way. The difference is that Unix servers also keep track of the state of the file in the cache memory versus the state on disk, so programs are all presented with a single view of the file.

In NFS caching, all applications on a single client will typically see the same file contents. However, applications accessing the file from different clients will not see the same file for several seconds.

Workaround: It is often possible to disable client write caching. Unfortunately, this frequently causes unacceptably slow performance, depending on the application. (Applications that perform I/O of large chunks of data should be unaffected, but applications that perform lots of small I/O operations will be severely punished.) If locking is employed, applications can explicitly cooperate and flush files from the local cache to the server, but see the previous sections on locking when employing this solution.

f. Read Caching and File Access Time

Unix file systems typically have three times associated with a file: the time of last modification (file creation or write), the time of last "change" (write or change of inode information), or the time of last access (file execution or read). NFS file systems also report this information.

NFS clients perform attribute caching for efficiency reasons. Reading small amounts of data does not update the access time on the server. This means a server may report a file has been unaccessed for a much longer time than is accurate.

This can cause problems as administrators and automatic cleanup software may delete files that have remained unused for a long time, expecting them to be stale lock files, abandoned temporary files and so on.

Workaround: Attribute caching may be disabled on the client, but this is usually not a good idea for performance reasons. Administrators should be trained to understand the behaviour of NFS regarding file access time. Any programs that rely on access time information should be modified to use another mechanism.

g. Indestructible Files

In Unix, when a file is opened, the data of that file is accessible to the process that opened it, even if the file is deleted. The disk blocks the file uses are freed only when the last process which has it open has closed it.

An NFS server, being stateless, has no way to know what clients have a file open. Indeed, in NFS clients never really "open" or "close" files. So when a file is deleted, the server merely frees the space. Woe be unto any client that was expecting the file contents to be accessible as before, as in the Unix world!

In an effort to minimize this as much as possible, when a client deletes a file, the operating systems checks if any process on the same client box has it open. If it does, the client renames the file to a "hidden" file. Any read or write requests from processes on the client that were to the now-deleted file go to the new file.

This file is named in the form .nfsXXXX, where the XXXX value is determined by the inode of the deleted file - basically a random value. If a process (such as rm) attempts to delete this new file from the client, it is replaced by a new .nfsXXXX file, until the process with the file open closes it.

These files are difficult to get rid of, as the process with the file open needs to be killed, and it is not easy to determine what that process is. These files may have unpleasant side effects such as preventing directories from being removed.

If the server or client crashes while a .nfsXXXX file is in use, they will never be deleted. There is no way for the server or a client to know whether a .nfsXXXX file is currently being used by a client or not.

Workaround: One should be able to delete .nfsXXXX files from another client, however if a process writes to the file, it will be created at that time. It would be best to exit or kill processes using an NFS file before deleting it. Unfortunately, there is no way to know if an uncooperative process has a file open.

h. User and Group Names and Numbers

NFS uses user and group numbers, rather than names. This means that each machine that accesses an NFS export needs (or at least should) have the same user and group identifiers as the NFS export has. Note that this problem is not unique to NFS, and also applies, for instance, to removable media and archives. It is most frequently an issue with NFS, however.

Workaround: Either the /etc/passwd and /etc/group files must be synchronized, or something like NIS needs to be used for this purpose.

i. Superuser Account

NFS has special handling of the superuser account (also known as the root account). By default, the root user may not update files on an NFS mount.

Normally on a Unix system, root may do anything to any file. When an NFS drive has been mounted, this is no longer the case. This can confuse scripts and administrators alike.

To clarify: a normal user (for example "shane" or "billg") can update files that the superuser ("root") cannot.

Workaround: Enable root access to specific clients for NFS exports, but only in a trusted environment since NFS is insecure. Therefore, this does not guarantee that unauthorized client will be unable to access the mount as root.