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Distributed File Systems. Objectives to understand Unix network file sharing Contents Installing NFS How To Get NFS Started The /etc/exports File Activating Modifications The Exports File NFS And DNS Configuring The NFS Client Other NFS Considerations Practical

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distributed file systems
Distributed File Systems
  • Objectives
    • to understand Unix network file sharing
  • Contents
    • Installing NFS
    • How To Get NFS Started
    • The /etc/exports File
    • Activating Modifications The Exports File
    • NFS And DNS
    • Configuring The NFS Client
    • Other NFS Considerations
  • Practical
    • to share and mount NFS file systems
  • Summary
nfs dfs an overview
NFS/DFS: An Overview
  • Unix distributed filesystems are used to
    • centralise administration of disks
    • provide transparent file sharing across a network
  • Three main systems:
    • NFS: Network File Systems developed by Sun Microsystems 1984
    • AFS: Andrew Filesystem developed by Carnegie-Mellon University
  • Unix NFS packages usually include client and server components
    • A DFS server shares local files on the network
    • A DFS client mounts shared files locally
    • a Unix system can be a client, server or both depending on which commands are executed
  • Can be fast in comparasion to many other DFS
    • Very little overhead
    • Simple and stable protocols
    • Based on RPC (The R family and S family)
general overview of nfs
General Overview of NFS
  • Developed by Sun Microsystems 1984
  • Independent of operating system, network, and transport protocols.
  • Available on many platforms including:
    • Linux, Windows, OS/2, MVS, VMS, AIX, HP-UX….
  • Restrictions of NFS
    • stateless open architecture
    • Unix filesystem semantics not guaranteed
    • No access to remote special files (devices, etc.)
  • Restricted locking
    • file locking is implemented through a separate lock daemon
  • Industry standard is currently nfsV3 as default in
    • RedHat, SuSE, OpenBSD, FreeBSD, Slackware, Solaris, HP-UX, Gentoo
  • Kernel NFS or UserSpace NFS
three versions of nfs available
Three versions of NFS available
  • Version 2:
    • Supports files up to 4GB long (most common 2GByte)
    • Requires an NFS server to successfully write data to its disks before the write request is considered successful
    • Has a limit of 8KB per read or write request. (1 TCP Window)
  • Version 3 is the industry standard:
    • Supports extremely large file sizes of up to 264 - 1 bytes
    • Supports files up to 8 Exabyte
    • Supports the NFS server data updates as being successful when the data is written to the server's cache
    • Negotiates the data limit per read or write request between the client and server to a mutually decided optimal value.
  • Version 4 is coming:
    • File locking and mounting are integrated in the NFS daemon and operate on a single, well known TCP port, making network security easier
    • Support for the bundling of requests from each client provides more efficient processing by the NFS server.
    • File locking is mandatory, whereas before it was optional
important nfs daemons
Important NFS Daemons
  • Portmap The primary daemon upon which all the RPC rely
    • Manages connections for applications that use the RPC specification
    • Listens to TCP port 111 for initial connection
    • negotiate a range of TCP ports, usually above port 1024, for further comms.
    • You need to run portmap on both the NFS server and client.
  • Nfs (rpc.nfsd)
    • Starts the RPC processes needed to serve shared NFS file systems
    • Listens to TCP or UDP port 2049 (port can vary)
    • The nfs daemon needs to be run on the NFS server only.
  • Nfslock (rpc.mountd)
    • Used to allow NFS clients to lock files on the server via RPC processes.
    • Neogated port UDP/TCP port
    • The nfslock daemon needs to be run on both the NFS server and client
  • netfs
    • Allows RPC processes run on NFS clients to mount NFS filesystems on the server.
    • The nfslock daemon needs to be run on the NFS client only.
the nfs protocol stack aka vfs






The NFS Protocol Stack aka. VFS








RPC depend on PORTMAP which is on both client and server

installing kernelnfs linux
Installing kernelNFS, Linux
  • Check if NFS is installed with rpm
  • Check if RPC portmap package installed rpm
  • If not Install them, allways begin with portmap
  • If you are not running SuSE
    • Install: portmap, nfs-utils,nfs-server (should be implemented in kernel)

suse93:~ # rpm -qa | grep nfs




# rpm -qa | grep portmap


# rpm –ivh http://ftp.sunet.se/pub/os/Linux/distributions/suse/suse/i386/9.3/suse/i586/portmap-5beta-733.i586.rpm

# rpm –ivh http://ftp.sunet.se/pub/os/Linux/distributions/suse/suse/i386/9.3/suse/i586/nfs-utils-1.0.7-3.i586.rpm

how to get kernelnfs server started
How To Get kernelNFS server Started
  • Activate the 3 nessesary servers for NFS at boot
    • NFS server demon
    • NFS file locking
    • RPC portmap
  • Start the PORTMAPPER and NFS server
    • Which starts all dependent services
    • Whatever you do allways start PORTMAP first
  • Check that services for NFS is running with rpcinfo
  • In some Unixes you need to separately start

/etc/init.d/portmap start or shortly portmap(d)

/etc/init.d/nfs start or shortly nfs(d)

/etc/init.d/nfslock start or shortly nfslock(d)

# insserv portmap

# insserv nfsserver

# rcportmap start

# rcnfsserver start

# rpcinfo -p localhost

program vers proto port

100000 2 tcp 111 portmapper

100000 2 udp 111 portmapper

100003 2 udp 2049 nfs

100003 3 udp 2049 nfs

100227 3 udp 2049 nfs_acl

100003 2 tcp 2049 nfs

100003 3 tcp 2049 nfs

100227 3 tcp 2049 nfs_acl

100024 1 udp 1034 status

100021 1 udp 1034 nlockmgr

program vers proto port

100021 4 udp 1034 nlockmgr

100024 1 tcp 1029 status

100021 1 tcp 1029 nlockmgr

100021 3 tcp 1029 nlockmgr

100021 4 tcp 1029 nlockmgr

100005 1 udp 835 mountd

100005 1 tcp 838 mountd

100005 2 udp 835 mountd

100005 2 tcp 838 mountd

100005 3 udp 835 mountd

100005 3 tcp 838 mountd

how to get nfs client started
How To Get NFS client Started
  • Activate the 2 nessesary servers for NFS at boot
    • NFS file locking nfslock
    • RPC portmap
  • Start the PORTMAPPER and NFS server
    • With rc
  • Check that services for NFS is running with rpcinfo
    • Note! There can be more services running dependent on your system setup
  • In some Unixes you need to separately start

/etc/init.d/netfs start or shortly netfs(d)

/etc/init.d/nfslock start or shortly nfslock(d)

  • Allways start portmap first then netfs and last nfslock

# insserv portmap

# rcportmap start

# rpcinfo -p localhost

rpcinfo -p

program vers proto port

100000 2 tcp 111 portmapper

100000 2 udp 111 portmapper

nfs and dns
  • Check FORWARD resolution
  • Check REVERSE resolution
  • Both forward and reverse must be same
    • If not, fix your DNS zonefiles (review netadmin chapter 3)
  • Syncronized /etc/hosts in server and client will also do
  • Some common error messages
    • Lookup: host resolution error
    • Timeout: firewall port setup
    • Not registered: portmap is not running

# host domain name pointer a01.my-site.com.

# host a01.my-site.com

a01.my-site.com has address

forward lookup doesn't exist

RPC: Timeout

RPC: Program not registered

failed: server is down.

the nfs server sharing directories
The NFS Server sharing directories
  • The exportfs command is used to share directories on the network
    • any directory can be exported
    • subdirectories of an exported directory may not be exported unless they are on a different disk
    • parents of an exported directory may not be exported unless they are on a different disk
    • only local filesystems can be exported
  • Some exportfs –o sharing options
  • We share the home directory in –v verbose mode
    • rw = Read Write (default)
    • squash_uids, squash_gids = make user and group id’s specified

to be ”squashed to” user with identity nobody

    • directory is shared to host rosies only

roread only accessrwread and write access

syncwrite when requestedwdelaywait for sync

hide dont show subdirs that is exported of other export

no_all_squash remote uid’s & gid’s become equal of client

root_squash remote root uid become anonymous on the client

no_root_squash remote root equals to local root user

squash_uids remote uid’s & gid’s are threated as identity nobody

# exportfs –v -o rw,squash_uids=0-499,squash_gids=0-499 rosies:/home

exporting rosies:/home

more on shared directories
More on Shared Directories
  • If someone is using the shared directory, you will not be able unshare.
  • Check if someone is accessing RPC, using a share

The first red line show that someone is using

RPC against our server. The second red line

show that someone have accessed /home

  • Unshare a share in –v verbose mode
  • Check what the server is


# showmount –a localhost

All mount points on server:






# exportfs -v -u rosies:/home

unexporting roseis:/home

# exportfs -v






the etc exports file static shares
The /etc/exports File, static shares
  • Sample exports file
  • Some options in exports file (same as exportfs)
  • Squash changes remote identity to selectable local identity
  • Linux uses another format in /etc/exports than BSD system’s

# cat /etc/exports

/data/files           *(ro,sync)

/home       ,sync)

/data/test            *.my-site.com(rw,sync)


roread only access rw read and write access

syncwrite when requestedwdelaywait for sync

hide dont show subdirs that is exported of other export

no_all_squash remote uid’s & gid’s become equal of client

root_squash remote root uid become anonymous on the client

no_root_squash remote root equals to local root user

squash_uids remote uid’s & gid’s are threated as identity nobody

the etc exports file squashing
The /etc/exports File, Squashing
  • Sample exports file using map_static
  • Map_static file =/etc/squash.map
  • Squash changes remote identity to selectable local identity

# cat /etc/exports

/data/files           *(ro,sync)

/home       ,rw,sync)

/data/test            *.my-site.com(rw,sync)


# /etc/squash.map

# remote local comment

uid 0-100 - # squash to user nobody

gid 0-100 - # squash to group nobody

uid 1-200 1000 # map to uid 1000 - 1100

gid 1-200 500 # map to gid 500 - 600

uid 0-100 2001 # map individual user to uid 2001

gid 0-100 2001 # map individual user to gid 2001

activating modifications in exports file
Activating Modifications in Exports File
  • Re-reading all entries in /etc/exports file
    • When no directories have been exported to NFS, then the "exportfs -a" command is used:
  • After adding share(s) to /etc/exports file
    • When adding a share you can use the "exportfs -r" command to export only the new entries:
  • Deleting, Moving Or Modifying A Share
    • In this case it is best to temporarily unmount the NFS directories using the "exportfs -ua" command followed by the "exportfs -a" command.
  • Termporary export /usr/src to host’s on net

# exportfs -a

# exportfs -r

# exportfs -ua

# exportfs -a

# exportfs –o rw

exercise sharing directories
Exercise - Sharing Directories
  • Write down the commands to do the following?

With one command share /usr/share readonly for all clients in your net


Permanently Share /etc readonly for rosies and tokyo and read/write for seoul


list the file containing the permanent shares


two commands showing what your host has shared



check who has mounted your shared directories


check who has mounted directories on rosies


check the server nfs status


From the server, with one command check that the nfs-client has portmapper running


the nfsstat command
The nfsstat Command

# nfsstat -s

  • Server statistics

A large table arrives after command is issued

  • Client statistics
  • Server numbers of filehandlers
    • Usage information on the server's file handle cache, including the total number of lookups, and the number of hits and misses.
    • The server has a limited number of filehandlers that can be tuned

# nfsstat -c

Server nfs v3:

null getattr setattr lookup access readlink

0 0% 15 31% 0 0% 0 0% 0 0% 0 0%

read write create mkdir symlink mknod

0 0% 0 0% 0 0% 0 0% 0 0% 0 0%

remove rmdir rename link readdir readdirplus

0 0% 0 0% 0 0% 0 0% 0 0% 0 0%

fsstat fsinfo pathconf commit

17 35% 16 33% 0 0% 0 0%

# nfsstat -o fh

Server file handle cache:

lookup anon ncachedir ncachedir stale

0 0 0 0 0

the nfs client side
The NFS Client side
  • Ensure Portmap Is Running
    • Clients need portmap only to be running
    • Also check that server is up
  • If not, start portmap
  • Show exported shares on a remote server
  • Temporary mount nfs shares on client with default options
  • umount temporaty mounted nfs shares on client

# rpcinfo -p localhost

# rpcinfo -p

#rcportmap start

# showmount -e

Export list for

/home *

/exports/network-install/SuSE/9.3 *

# mkdir /mnt/nethome

# mount –t nfs /mnt/nethome

# umount /mnt/nethome

to see what is mounted on client side
To see what is mounted on client side
  • Using the df command show disk usage:
  • The mount command is most detailed about mount options
  • The showmount shows all exported shares on a remote server plus all mounts from client
  • Client nfsstat will show statistics

# df –F NFS

Filesystem 1k-blocks Used Available Use% Mounted on 79366688 58235488 21131200 74% /mnt/a

# mount | grep nfs on /mnt/a type nfs (rw,addr=

# showmount -a

All mount points on




# nfsstat –c

Client rpc stats:

calls retrans authrefrsh

129 0 0

mount o options t nfs
mount –o <options> –t nfs
  • NFS clients access network shared directories using the mount command
  • NFS mount –o options:

rw/ro read-write (default) or read-only

hard retry mount operation until server responds (default) or

soft try mount once and allow to timeout

retrans & transmission and timeout parameters for soft mounted operationstimeout

bg after first mount failure, retry mount in the background

intr allow operations on filesystems to be interrupted with kill signals

nfsvers=n The version of NFS the mount command should attempt to use

  • Use /etc/fstab to make NFS mounts permanent

a02:/tmp  /mnt/nethome   nfs    soft,intr,nfsvers=3      0      0

  • Manually mounting /tmp as /mnt/nethome on local host from a02:

# hostname


# mount –o rw,soft -t nfs a02:/tmp /mnt/nethome

mount nfs shares at boot in client
Mount ”nfs-shares” at boot in client
  • Make entries in /etc/fstab
  • Some /etc/fstab mount options
  • Mount all unmounted
    • If you made changes on live system in fstab, you can mount all unmounted filesystem with:


#Directory MountPoint Type   Options   Dump   FSCK  /mnt/nfs   nfs    soft,nfsvers=3     0      0

auto mount this when mount –a is used

defaults (rw suid dev exec auto nouser async)

user allow regular users to mount/umount

sync use syncron I/O most safe

soft skip mount if server not responding

hard try until server responds


bg/fg retry mounting in background or foreground

mount –a

exercise using mount with nfs
Exercise - Using mount with NFS

What command will mount /usr/share from mash4077 on the local mount point /usr/share?

  • How do I check what filesystems are mounted locally?
  • Make a static mount in a01 ”/mnt/nethome” of exported ”a02:/tmp” in /etc/fstab:
  • Manually mount exported a02:/usr/share as read only on a01:
  • How can I show what is nfs exported on the server






nfs security
NFS security
  • NFS is inherently insecure
    • NFS can be run in encrypted mode which encrypts data over the network
    • AFS more appropriate for security conscious sites
  • User IDs must be co-ordinated across all platforms
    • UIDs and not user names are used to control file access (use LDAP or NIS)
    • mismatched user id's cause access and security problems
  • Fortunately root access is denied by default
    • over NFS root is mapped to user nobody

# mount | grep "/share"

mail:/share on /share

# id

uid=318(hawkeye) gid=318(hawkeye)

# touch /share/hawkeye

# ssh mail ls -l /share/hawkeye

-rwxr-xr-x 2 soonlee sonlee 0 Jan 11 11:21 /share/hawkeye

nfs hanging
NFS Hanging
  • Run NFS on a reliable network
  • Avoid having NFS servers that NFS mount each other's filesystems or directories
  • Always use the sync option whenever possible
  • Mission critical computers shouldn't rely on an NFS server to operate
  • Dont have NFS shares in search path
nfs hanging continued
NFS Hanging continued
  • File Locking
    • Known issues exist, test your applications carefullý
  • Nesting Exports
    • NFS doesn't allow you to export directories that are subdirectories of directories that have already been exported unless they are on different partitions.
  • Limiting "root" Access
    • no_root_squash
  • Restricting Access to the NFS server
    • You can add user named "nfsuser" on the NFS client to let this user squash access for all other users on that client
  • Use nfsV3 if possible
nfs firewall considerations
NFS Firewall considerations
  • NFS uses many ports
    • RPC uses TCP port 111
    • NFS server itself uses port 2049
    • MOUNTD listens on neogated UDP/TCP port’s
    • NLOCKMGR listens on neogated UDP / TCP port’s
    • Expect almost any TCP/UDP port over 1023 can be allocated for NFS
  • NFS need a STATEFUL firewall
    • A stateful firewall will be able dealing with traffic that originates from inside a network and block traffic from outside
  • SPI can demolish NFS
    • Stateful packet inspection on cheaper routers/firewalls can missinteprete NFS traffic as DOS attacks and start drop packages
    • This is a hacker tool, it can hack some NFS
    • Invented by Leendert van Doom
  • Use VPN and IPSEC tunnels
    • With complex services like NFS IPSEC or some kind of VPN should be considered if used in untrusted networks.
nfs automounter for clients or servers
NFS Automounter for clients or servers
  • Automatically mount directories from server when needed
  • To activate automount manually and at boot
    • Management of shares centralized on server
    • Increases security and reduces lockup problems with static shares
  • Main configuration sit in /etc/auto.master
    • MOUNT-KEY is local mountpoint, here /doc, /- (from root) and /home
    • MOUNT-OPTIONS is the standard mount options previously described, here -ro
    • LOCATION can be a direct share on a server like server and map file auto.direct and indirect like /etc/auto.home.
  • Common configuration /etc/auto.misc is for floppy/cd/dvd.
  • Centralized administration need to set /etc/nsswitch.conf

# rcautofs start

# insserv autofs

/doc -ro server:/usr/doc

/- /etc/auto.direct

/home /etc/auto.home

automount: files nisldap

direct and indirect map files structure
Direct And Indirect Map Files structure
  • File /etc/auto.master sets the mandatory automount config
    • map files always try to mount in auto.master ”mount key”
  • Direct map file /etc/auto.direct

Direct Maps are used to define NFS filesystems that are mounted on different servers or that all don't start with the same prefix.

  • Indirect map file /etc/auto.home

Indirect Maps define directories that can be mounted under the same mount point. Like users home directories.

/data/sales -rw server:/disk1/data/sales /sql/database -ro,soft snail:/var/mysql/database

peter server:/home/peter

kalle akvarius:/home/bob

walker iss:/home/bunny

wildcards in map files
Wildcards In Map Files
  • Wildcards In Map Files
    • The asterisk (*), which means all
    • the ampersand (&), which instructs automounter to substitute the value of the key for the & character.
  • Using the Ampersand Wildcard /etc/auto.home
    • the key is peter, so the ampersand wildcard is interpreted to mean peter too. This means you'll be mounting the server:/home/peter directory.
  • Using the Asterisk Wildcard /etc/auto.home
    • In the example below, the key is *, meaning that automounter will attempt to mount any attempt to enter the /home directory. But what's the value of the ampersand? It is actually assigned the value of the key that triggered the access to the /etc/auto.home file. If the access was for /home/peter, then the ampersand is interpreted to mean peter, and server:/home/peter is mounted. If access was for /home/kalle, then akvarius:/home/kalle would be mounted.

peter server:/home/&

* bigboy:/home/&

other dfs systems
Other DFS Systems
  • RFS: Remote File Sharing
    • developed by AT&T to address problems with NFS
    • stateful system supporting Unix filesystem semantics
    • uses same SVR4 commands as NFS, just use rfs as file type
    • standard in SVR4 but not found in many other systems
  • AFS: Andrew Filesystem
    • developed as a research project at Carnegie-Mellon University
    • now distributed by a third party (Transarc Corporation)
    • available for most Unix platforms and PCs running DOS, OS/2, Windows
    • uses its own set of commands
    • remote systems access through a common interface (the /afs directory)
    • supports local data caching and enhanced security using Kerberos
    • fast gaining popularity in the Unix community
  • Unix supports file sharing across a network
  • NFS is the most popular system and allows Unix to share files with other O/S
  • Servers share directories across the network using the share command
  • Permanent shared drives can be configured into /etc/fstab
  • Clients use mount to access shared drives
  • Use mount and exportfs to look at distributed files/catalogs