Distributed File Systems

1. 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

2. 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

3. Overview of NFS • Developed by Sun Microsystems 1984 • Independent of operating system, network, and transport protocols. • Now 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

4. mountd biod nfsd statdlockd statdlockd The NFS Protocol Stack MOUNT NFS server client XDR RPC TRANSPORT, NETWORK, LINK & PHYSICAL LAYERS

5. The NFS Client • NFS clients access network shared directories using the mount command • NFS mount 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 signals • Use /etc/fstab to make NFS mounts permanent a02:/tmp  /mnt/nethome   nfs    soft      0      0 • Manually mounting /tmp as /mnt/nethome on local host from a02: # hostname a01 # mount –o rw,soft -t nfs a02:/tmp /mnt/nethome

6. 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: # # #

7. The NFS Server Solaris type’s • The share 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 • Share options: ro/rw share readonly or read/write (default) for all clients rw=host[:host]... allow only the named hosts to write, no access to others ro=host:[host]... readonly access to the named hosts, no access to others root=host:[host]... allow root access from named hostsonly anon=-1 deny access to unknown users # share -F nfs -o ro=rosies /home/hawkeye

8. More on Shared Directories • Add a line to /etc/dfs/dfstab to share a directory permanent • each line in this file is simply the share command required • commands in this file are executed by the nfs script in /etc/init.d • Use unshare to stop sharing a directory • Use dfshares to look at your shared resources • can specify other hosts on command line • Use dfmounts to look at who has mounted your shares • can specify other hosts on command line # dfshares RESOURCE SERVER ACCESS TRANSPORT /home/hawkeye mash4077 - - # dfmounts RESOURCE SERVER PATHNAME CLIENTS /home/hawkeye mash4077 /home/hawkeye rosies

9. Exercise - Sharing Directories • Write down the commands to do the following? # share /usr/share readonly for all clients # share /etc readonly for rosies and tokyo and read/write for seoul # list the files 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

10. NFS under BSD systems • NFS uses different commands for BSD systems • Mount directories using mount as for SVR4 • Share directories using exportfs • Permanently share directories using /etc/exports • one line per shared directory • use access=host instead of ro=host • Look at network shares using showmount # cat /etc/exports /home/hawkeye ro=rosies /etc rw=seoul,access=rosies,tokyo # exportfs -a # showmount -e export list for mash4077: /home/hawkeye rosies /etc seoul,rosies,tokyo

11. Installing NFS, RedHat Linux • Check if NFS is installed with rpm • Check if RPC portmap package installed rpm # rpm -qa | grep nfs redhat-config-nfs-1.1.3-1 nfs-utils-1.0.1-3.9 # rpm -qa | grep portmap portmap-4.0-57

12. How To Get NFS Started • Activate the 3 nessesary servers for NFS at boot • NFS server demon • NFS file locking • RPC portmap • Start the NFS server • With sysconfig • Check that ther services for NFS is running with rpcinfo # chkconfig --level 35 nfs on # chkconfig --level 35 nfslock on # chkconfig --level 35 portmap on # service nfs 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     100021    1   udp   1024  nlockmgr     100021    3   udp   1024  nlockmgr     100021    4   udp   1024  nlockmgr     100005    1   udp   1042  mountd     100005    1   tcp   2342  mountd     100005    2   udp   1042  mountd     100005    2   tcp   2342  mountd     100005    3   udp   1042  mountd     100005    3   tcp   2342  mountd

13. The /etc/exports File, static shares • Sample exports file • Some options in exports file • Squash changes remote identity to selectable local identity • RedHat uses another format in /etc/exports than BSD system’s # cat /etc/exports /data/files           *(ro,sync) /home                 192.168.0.0/24(rw,sync) /data/test            *.my-site.com(rw,sync) /data/database        192.168.0.203/32(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

14. Activating Modifications The 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 192.168.0.10 # exportfs -a # exportfs -r # exportfs -ua # exportfs -a # exportfs 192.168.0.10/usr/src –o rw

15. Configuring The NFS Client • Ensure Portmap Is Running • Clients need portmap only to be running • If not, start portmap • Temporary mount nfs shares on client • umount temporaty mounted nfs shares on client • Show exported shares on server # rpcinfo -p localhost #/etc/init.d/portmap start # mkdir /mnt/nethome # mount –t nfs 192.168.0.10:/home /mnt/nethome # umount /mnt/nethome # showmount --exports 192.168.0.10

16. 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: #/etc/fstab #Directory MountPoint Type   Options   Dump   FSCK 192.168.0.10:/data/files  /mnt/nfs   nfs    soft      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 retry=minutes bg/fg retry mounting in background or foreground mount –a

17. 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 • 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=555(hawkeye) gid=501(hawkeye) # touch /tmp/hawkeye # ssh mail ls -l /tmp/hawkeye -rwxr-xr-x 2 soonlee sonlee 0 Jan 11 11:21 /share/hawkeye

18. 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

19. 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

20. 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

21. Summary • 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