Slackware Linux Essentials
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5.6 -Network File Systems

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At this point, you should have a working TCP/IP connection to your network. You should -be able to ping other computers on your internal network and, if you have configured an -appropriate gateway, you should also be able to ping computers on the Internet itself. As -we know, the whole point in bringing a computer onto a network is to access information. -While some people might bring a computer up on a network just for the fun of it, most -people wish to be able to share files and printers. They wish to be able to access -documents on the Internet or play an online game. Having TCP/IP installed and functional -on your new Slackware system is a means to that end, but with just TCP/IP installed, -functionality will be very rudimentary. To share files, we will have to transfer them -back and forth using either FTP or SCP. We cannot browse files on our new Slackware -computer from the Network Neighborhood or My Network Places icons on Windows computers. -We'd like to be able to access files on other Unix machines seamlessly.

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Ideally, we'd like to be able to use a network file system to allow us transparent access to our -files on other computers. The programs that we use to interact with information stored on -our computers really do not need to know on what computer a given file is stored; they -just need to know that it exists and how to get to it. It is then the responsibility of -the operating system to manage access to that file through the available file systems and -network file systems. The two most commonly used network file systems are SMB (as -implemented by Samba) and NFS.

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5.6.1 SMB/Samba/CIFS

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SMB (for Server Message Block) is a descendant of the older NetBIOS protocol that was -initially used by IBM in their LAN Manager product. Microsoft has always been fairly -interested in NetBIOS and it's successors (NetBEUI, SMB and CIFS). The Samba project has -existed since 1991, when it was originally written to link an IBM PC running NetBIOS with -a Unix server. These days, SMB is the preferred method for sharing file and print -services over a network for virtually the entire civilized world because Windows supports -it.

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Samba's configuration file is /etc/samba/smb.conf; one of -the most well commented and documented configuration files you will find anywhere. Sample -shares have been setup for you to view and modify for your needs. If you need even -tighter control the man page for smb.conf is indispensable. Since Samba is documented so -well in the places I've mentioned above, we will not rewrite the documentation here. We -will, however, quickly cover the basics.

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smb.conf is broken down into multiple sections: one section -per share, and a global section for setting options that are to be used everywhere. Some -options are only valid in the global section; some are only valid outside the global -section. Remember that the global section can be over-ridden by any other section. Refer -to the man pages for more information.

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You will most likely wish to edit your smb.conf file to -reflect the network settings in your LAN. I would suggest modifying the items listed -below:

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-[global]
-# workgroup = NT-Domain-Name or Workgroup-Name, eg: LINUX2
-workgroup = MYGROUP
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Change the workgroup name to reflect the workgroup or domain name that you are using -locally.

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-# server string is the equivalent of the NT Description field
-server string = Samba Server
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This will be the name of your Slackware computer displayed in the Network Neighborhood -(or My Network Places) folder.

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-# Security mode. Most people will want user level security. See
-# security_level.txt for details. NOTE: To get the behaviour of
-# Samba-1.9.18, you'll need to use "security = share".
-security = user
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You'll almost certainly wish to implement user level security on your Slackware -system.

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-# You may wish to use password encryption. Please read
-# ENCRYPTION.txt, Win95.txt and WinNT.txt in the Samba
-# documentation.
-# Do not enable this option unless you have read those documents
-encrypt passwords = yes
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If encrypt passwords is not enabled, you will not be able to use Samba with NT4.0, -Win2k, WinXP, and Win2003. Earlier Windows operating systems did not require encryption -to share files.

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SMB is an authenticated protocol, meaning you must supply a correct username and -password in order to use this service. We tell the samba server what usernames and -passwords are valid with the smbpasswd command. smbpasswd takes a couple of common switches to tell it to either add -traditional users, or add machine users (SMB requires that you add the computers' NETBIOS -names as machine users, restricting what computers one can authenticate from).

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-Adding a user to the /etc/samba/private/smbpasswd file.
-# smbpasswd -a user
-Adding a machine name to the /etc/samba/private/smbpasswd file.
-# smbpasswd -a -m machine
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It's important to note that a given username or machine name must already exist in the -/etc/passwd file. You can accomplish this simply with the adduser command. Note that when using the adduser command to add a machine name one must append a dollar sign -(“$”) to the machine name. This should not however, be done with smbpasswd. smbpasswd appends the dollar -sign on its own. Failing to mangle the machine name this way with adduser will result in an error when adding the machine name to -samba.

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-# adduser machine$
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5.6.2 Network File System (NFS)

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NFS (or Network File System) was originally written by Sun for their Solaris -implementation of Unix. While it is significantly easier to get up and running when -compared to SMB, it is also significantly less secure. The primary insecurity in NFS is -that it is easy to spoof user and group id's from one machine to another. NFS is an -unauthenticated protocol. Future versions of the NFS protocol are being devised that -enhance security, but these are not common at the time of this writing.

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NFS configuration is governed by the /etc/exports file. When -you load the default /etc/exports file into an editor, you'll -see a blank file with a two line comment on top. We'll need to add a line to the exports -file for each directory that we wish to export, with a listing of client workstations -that will be allowed to access that file. For instance, if we wished to export directory -/home/foo to workstation Bar, we would simply add the line:

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-/home/foo Bar(rw)
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to our /etc/exports. Below, you'll find the example from the -man page for the exports file:

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-# sample /etc/exports file
-/               master(rw) trusty(rw,no_root_squash)
-/projects       proj*.local.domain(rw)
-/usr            *.local.domain(ro) @trusted(rw)
-/home/joe       pc001(rw,all_squash,anonuid=150,anongid=100)
-/pub            (ro,insecure,all_squash)
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As you can see, there are various options available, but most should be fairly clear -from this example.

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NFS works under the assumption that a given user on one machine in a network has the -same user ID on all machines across the network. When an attempt is made to read or write -from a NFS client to an NFS server, a UID is passed as part of the read/write request. -This UID is treated the same as if the read/write request originated on the local -machine. As you can see, if one could arbitrarily specify a given UID when accessing -resources on a remote system, Bad Things (tm) could and would happen. As a partial hedge -against this, each directory is mounted with the root_squash -option. This maps the UID for any user claiming to be root to a different UID, thus -preventing root access to the files or folders in the exported directory. root_squash seems to be enabled by default as a security measure, -but the authors recommend specifying it anyway in your /etc/exports file.

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You can also export a directory directly from the command line on the server by using -the exportfs command as follows:

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-# exportfs -o rw,no_root_squash Bar:/home/foo
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This line exports the /home/foo directory to the computer -“Bar” and grants Bar -read/write access. Additionally, the NFS server will not invoke root_squash, which means any user on Bar with a UID of -“0” (root's UID) will have the same privileges as root on the server. The -syntax does look strange (usually when a directory is specified in computer:/directory/file syntax, you are referring to a file in a -directory on a given computer).

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You'll find more information on the man page for the exports file.

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