Red Hat Enterprise Linux 3: System Administration Guide
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# 23.2. Mounting NFS File Systems

Use the `mount` command to mount a shared NFS directory from another machine:

    mount shadowman.example.com:/misc/export /misc/local

| Warning
| The mount point directory on local machine (`/misc/local` in the above example) must exist.

In this command, `shadowman.example.com` is the hostname of the NFS file server, `/misc/export` is the directory that `shadowman` is exporting, and `/misc/local` is the location to mount the file system on the local machine. After the `mount` command runs (and if the client has proper permissions from the `shadowman.example.com` NFS server) the client user can execute the command `ls /misc/local` to display a listing of the files in `/misc/export` on `shadowman.example.com`.

## 23.2.1. Mounting NFS File Systems using `/etc/fstab`

An alternate way to mount an NFS share from another machine is to add a line to the `/etc/fstab` file. The line must state the hostname of the NFS server, the directory on the server being exported, and the directory on the local machine where the NFS share is to be mounted. You must be root to modify the `/etc/fstab` file.

The general syntax for the line in `/etc/fstab` is as follows:

    server:/usr/local/pub    /pub   nfs    rsize=8192,wsize=8192,timeo=14,intr

The mount point `/pub` must exist on the client machine. After adding this line to `/etc/fstab` on the client system, type the command `mount /pub` at a shell prompt, and the mount point `/pub` will be mounted from the server.

## 23.2.2. Mounting NFS File Systems using autofs

A third option for mounting an NFS share is the use of autofs. Autofs uses the automount daemon to manage your mount points by only mounting them dynamically when they are accessed.

Autofs consults the master map configuration file `/etc/auto.master` to determine which mount points are defined. It then starts an automount process with the appropriate parameters for each mount point. Each line in the master map defines a mount point and a separate map file that defines the file systems to be mounted under this mount point. For example, the `/etc/auto.misc` file might define mount points in the `/misc` directory; this relationship would be defined in the `/etc/auto.master` file.

Each entry in `auto.master` has three fields. The first field is the mount point. The second field is the location of the map file, and the third field is optional. The third field can contain information such as a timeout value.

For example, to mount the directory `/proj52` on the remote machine penguin.example.net at the mount point `/misc/myproject` on your machine, add the following line to `auto.master`:

    /misc   /etc/auto.misc --timeout 60

Add the following line to `/etc/auto.misc`:

    myproject  -rw,soft,intr,rsize=8192,wsize=8192 penguin.example.net:/proj52

The first field in `/etc/auto.misc` is the name of the `/misc` subdirectory. This directory is created dynamically by automount. It should not actually exist on the client machine. The second field contains mount options such as `rw` for read and write access. The third field is the location of the NFS export including the hostname and directory.

| Note
| The directory `/misc` must exist on the local file system. There should be no subdirectories in `/misc` on the local file system.

Autofs is a service. To start the service, at a shell prompt, type the following commands:

    /sbin/service autofs restart

To view the active mount points, type the following command at a shell prompt:

    /sbin/service autofs status

If you modify the `/etc/auto.master` configuration file while autofs is running, you must tell the automount daemon(s) to reload by typing the following command at a shell prompt:

    /sbin/service autofs reload

To learn how to configure autofs to start at boot time, refer to Chapter 21 Controlling Access to Services for information on managing services.

## 23.2.3. Using TCP

The default transport protocol for NFS is UDP; however, the Red Hat Enterprise Linux 3 kernel includes support for NFS over TCP. To use NFS over TCP, include the `-o tcp` option to `mount` when mounting the NFS-exported file system on the client system. For example:

    mount -o tcp shadowman.example.com:/misc/export /misc/local

If the NFS mount is specified in `/etc/fstab`:

    server:/usr/local/pub    /pub   nfs    rsize=8192,wsize=8192,timeo=14,intr,tcp

If it is specified in an autofs configuration file:

    myproject  -rw,soft,intr,rsize=8192,wsize=8192,tcp penguin.example.net:/proj52

Since the default is UDP, if the `-o tcp` option is not specified, the NFS-exported file system is accessed via UDP.

The advantages of using TCP include the following:

- Improved connection durability, thus less `NFS stale file handles` messages.

- Performance gain on heavily loaded networks because TCP acknowledges every packet, unlike UDP which only acknowledges completion.

- TCP has better congestion control than UDP (which has none). On a very congested network, UDP packets are the first types of packet that are dropped. Which means if NFS is writing data (in 8K chunks) all of that 8K has to retransmitted. With TCP because of its reliability, one parts of that 8K data is transmitted at a time.

- Error detection. When a tcp connection breaks (due to the server going down) the client stops sending data and starts the reconnection process. With UDP, since its connection-less, the client continue to pound the network with data until server comes up.

The main disadvantage is that there is a very small performance hit due to the overhead associated with the TCP protocol.

## 23.2.4. Preserving ACLs

The Red Hat Enterprise Linux 3 kernel provides ACL support for the ext3 file system and ext3 file systems mounted with the NFS or Samba protocols. Thus, if an ext3 file system has ACLs enabled for it and is NFS exported, if the NFS client can read ACLs, they are used by the NFS client as well.

For more information about mounting NFS file systems with ACLs, refer to Chapter 8 Access Control Lists.

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