Getting Started With Networking for Linux Part 3; Sharing File Systems

Introduction

Welcome to the third installment of the Getting Started With Networking for Linux series. In the previous articles we covered networking hardware, basic TCP/IP configuration and how to implement DNS (Domain Name System) for your network. While this article doesn't necessarily require knowledge of the previous topics, it would be a good idea to read the other articles first if you are new to networking in general.

Today we're going to see how you can share and mount remote file systems across your network using NFS (Network File System). We will also see how to use Samba to make sharing and accessing Microsoft Window file systems a possibility. If you haven't experimented with this type of service before you will find that having access to the data of all the computers on a network from one single computer is a very handy service. The idea behind this service is that when a file system is shared on a network, any computer with the correct permissions can mount that file system locally and read/write data from it just like you manage data on your local hard drive.

In the Unix world the NFS service is used to export and mount file systems. NFS is a standard Unix service and chances are it's already installed on your machine. Microsoft Windows machines don't use the NFS service and instead they export file systems using the SMB protocol that is common among Microsoft Window machines. To export and mount Microsoft Windows shares we are going to use Samba, which is an open source software package that allows Linux to use the SMB protocol. Samba may or may not be installed by default on your Linux distribution but in any case you can get the software from www.samba.com.

Exporting File Systems With NFS

The NFS configuration file used to export file system directories to other computers is the /etc/exports file. This file contains a table of exported directories and which computers on your network have access to them. Here's a listing an example NFS configuration file:

/mnt/cdrom 192.168.1.4(ro)
/home/josh/mp3 *.domain.lan(rw)
/usr/local/networkbackup 192.168.1.0/255.255.255.0(rw)
/home/josh/work bob(rw) mary(rw) john(rw) simon(ro)
/home/ftp/pub (ro,insecure,all_squash)


As you can see, there are numerous ways to export directories to all or only a few computers. The first column lists the directories that you want to export. The second column lists the hosts that have access to that directory and the permissions they get granted. You can list hosts with IP addresses, DNS domains with wild cards, IP subnets and hostnames.

The last entry in our example file is a special one. It exports the /home/ftp/pub directory to everybody, even to hosts not on part of your network. The permissions for each host if put after its entry in parenthesis with (ro) being read-only and (rw) being read/write. Many other options (like mapping of user and group ids) can also be put inside the parenthesis. For a complete list of options read the exports(5) man page by typing man 5 exports at your shell prompt.

One thing to note is that DNS wild cards don't match the dots in domain names. This means that having *.domain.lan as a host in your /etc/export file won't give access to computers with domains like subnet.domain.lan. To match those domains you need to specifically add them like *.subnet.domain.lan.

Once you have tailored your /etc/exports file to your taste you must restart the NFS daemon for the changes to take effect. On Linux the NFS file systems are handled by the rpc.mountd and rpc.nfsd daemons. You must restart both of these daemons to have your directories exported. You can restart them by typing the following commands:

killall -HUP rpc.mountd
killall -HUP rpc.nfsd


Mounting Remote NFS File Systems

Now that we have our directories exported let's see how to mount them from another machine. The process of mounting NFS file systems is almost the same as mounting local hard drives or cdroms. You must first have a mount point to access the file system once it's mounted. A common place to put mount points is in the /mnt directory. Mount points are simply directories, so all you need to do is mkdir /mnt/mountpoint to create one.

mkdir /mnt/remote_cdrom
mkdir /mnt/josh_mp3
mkdir /mnt/ftp_pub


Typing the above commands would create three directories in /mnt. Now that we've got mount points let's mount the file systems. For example, say that the computer we have exported the directories on is at IP address 192.168.1.98 and has a domain of node98.domain.lan; the following commands would mount the exported directories on our local machine (assuming we have the correct permissions in the server's /etc/export file).

mount 192.168.1.98:/mnt/cdrom /mnt/remote_cdrom
mount node98.domain.lan:/home/josh/mp3 /mnt/josh_mp3
mount 192.168.1.98:/home/ftp/pub /mnt/ftp_pub


Instead of telling mount to use a local device like we usually do when mounting hard drives or cdroms, we put a hostname or IP address followed by a colon and the exported directory on the target computer. After mounting those three directories you can read and write data to them like normal directories on your local file system. If you try to mount a directory on which you have no permissions you will get a permission denied error. If you get this error check the remote computer's /etc/export file to make sure the computer trying to access the directory is listed. If you get a RPC TimeOut error this means that the rpc.mountd and rpc.nfsd daemons might not be running so be sure to check that they are in fact running by typing ps ax on the NFS server. The rpc.nfsd and rpc.mountd also need the rpc.portmap daemon so they can make rpc calls across the network and this might also be the reason you're getting a RPC TimeOut error.