configuring nfs in centos linux

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NFS is abreviation for network filesystem. It is used in linux unix platform for sharing directories between linux or unix machines over a network. It is more like folder sharing in windows systems. It was originally developed by Sun Microsystems. We will see how to install and configure nfs, How to mount a nfs share. What are the processes associated with nfs, why portpmap is needed for nfs, how to list the nfs shares of a system etc.

Advantages of NFS are:
Local systems needs only less disk space because commonly used data can be stored on a single server system and can be accessed by others over the network usin nfs.
We can mount all removable devices such as dvd, cdrom, floppy etc on one single system and made them available to other systems by sharing those via nfs.

The package name is nfs-utils. We can check whether the nfs package is installed using the following command.
[root@server ~]# rpm -qa | grep -i nfs
nfs-utils-1.0.9-33.el5
[root@server ~]#

Checking the status of the nfs service
[root@server ~]# /etc/init.d/nfs status
rpc.mountd is stopped
nfsd is stopped

Starting the nfs service
[root@server ~]# /etc/init.d/nfs start
Starting NFS services:                                     [  OK  ]
Starting NFS quotas:                                       [  OK  ]
Starting NFS daemon:                                       [  OK  ]
Starting NFS mountd:                                       [  OK  ]

NFS defaultly binds to the tcp port 2048
[root@server ~]# netstat -ntpla | grep 2049
tcp        0      0 0.0.0.0:2049                0.0.0.0:*                   LISTEN      -

You can find all the sub processes and binded ports of nfs by rpcinfo command. NFS takes the ports assigned by portmapped. Soportmapped needs tobe running for nfs to work.
[root@server ~]# rpcinfo -p
   program vers proto   port
    100000    2   tcp    111  portmapper
    100000    2   udp    111  portmapper
    100011    1   udp    832  rquotad
    100011    2   udp    832  rquotad
    100011    1   tcp    835  rquotad
    100011    2   tcp    835  rquotad
    100003    2   udp   2049  nfs
    100003    3   udp   2049  nfs
    100003    4   udp   2049  nfs
    100021    1   udp  32773  nlockmgr
    100021    3   udp  32773  nlockmgr
    100021    4   udp  32773  nlockmgr
    100003    2   tcp   2049  nfs
    100003    3   tcp   2049  nfs
    100003    4   tcp   2049  nfs
    100021    1   tcp  35223  nlockmgr
    100021    3   tcp  35223  nlockmgr
    100021    4   tcp  35223  nlockmgr
    100005    1   udp    872  mountd
    100005    1   tcp    875  mountd
    100005    2   udp    872  mountd
    100005    2   tcp    875  mountd
    100005    3   udp    872  mountd
    100005    3   tcp    875  mountd
[root@server ~]#

/etc/exports is the main file for nfs. We specify the directories to be shared in this file with the information for whom it is shared and with which permissions it is shared.
* - means it is shared to all ip addresses.
ro - means read only
rw - means read write

[root@server ~]# cat /etc/exports
#Directory_path   IP_address(Permissions)
/media/CentOS *(ro)
/kick *()
[root@server ~]#

To activate all shares specified in /etc/exports run the following command
[root@server ~]# exportfs -a

If u made any changes in /etc/exports you can reload it using the following command
[root@server ~]# exportfs -r

You can list the permissions of the shares by running
[root@server ~]# exportfs -v
/media/CentOS   <world>(ro,wdelay,root_squash,no_subtree_check,anonuid=65534,anongid=65534)
/kick           <world>(ro,wdelay,root_squash,no_subtree_check,anonuid=65534,anongid=65534)

For checking the shares in a system with ip address  192.168.137.100
[root@server ~]# showmount -e 192.168.137.100
Export list for 192.168.137.100:
/kick         *
/media/CentOS *
[root@server ~]#

From a remote machine you can mount the share /media/CentOS in the machine 192.168.137.100 to /mnt as
[root@server ~]# mount 192.168.137.100:/media/CentOS /mnt
[root@server ~]# mount
*** OUTPUT TRUNCATED ***
192.168.137.100:/media/CentOS on /mnt type nfs (rw,addr=192.168.137.100)
[root@server ~]#

[root@server ~]# cat /var/lib/nfs/etab
/media/CentOS   *(ro,sync,wdelay,hide,nocrossmnt,secure,root_squash,no_all_squash,no_subtree_check,secure_locks,acl,mapping=identity,anonuid=65534,anongid=65534)
/kick   *(ro,sync,wdelay,hide,nocrossmnt,secure,root_squash,no_all_squash,no_subtree_check,secure_locks,acl,mapping=identity,anonuid=65534,anongid=65534)

Some of the important nfs files are

/var/lib/nfs/etab contains information about what filesystems should be exported to whom at the moment.
/var/lib/nfs/rmtab contains a list of which filesystems actually are mounted by certain clients at the moment.
/proc/fs/nfs/exports contains information about what filesystems are exported to actual client (individual, not subnet or whatever) at the moment.
/var/lib/nfs/xtab is the same information as /proc/fs/nfs/exports but is maintained by nfs-utils instead of directly by the kernel. It is only used if /proc isn't mounted.

[root@server ~]# cat /var/lib/nfs/rmtab
192.168.137.200:/media/CentOS:0x00000002
192.168.137.200:/kick:0x00000002
192.168.137.248:/media/CentOS:0x00000003
192.168.137.20:/media/CentOS:0x00000001
[root@server ~]#