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TCP/IP Network Administration, 3rd Edition

TCP/IP Network Administration, 3rd Edition

By Craig Hunt
Third Edition April 2002
0-596-00297-1, Order Number: 2971
746 pages, $44.95 US $69.95 CA £31.95 UK

Chapter 9
Local Network Services

In this chapter:
The Network File System
Sharing Unix Printers
Using Samba to Share Resources with Windows
Network Information Service
DHCP
Managing Distributed Servers
Post Office Servers

Now our attention turns to configuring local network servers. As with name service, these servers are not strictly required for the network to operate, but they provide services that are central to the network's purpose.

There are many network services--many more than can be covered in this chapter. Here we concentrate on servers that provide essential services for local clients. The services covered in this chapter are:

All of these software packages are designed to provide service to systems within your organization and are not intended to service outsiders. Essential services that are as important to external users as they are to in-house users, such as email, web service, and name service, are covered in separate chapters.

We begin our discussion of local network services with NFS, which is the server that provides file sharing on Unix networks.

The Network File System

The Network File System (NFS) allows directories and files to be shared across a network. It was originally developed by Sun Microsystems but is now supported by virtually all Unix and many non-Unix operating systems. Through NFS, users and programs can access files located on remote systems as if they were local files. In a perfect NFS environment, the user neither knows nor cares where files are actually stored.

NFS has several benefits:

There are two sides to NFS: a client side and a server side. The client is the system that uses the remote directories as if they were part of its local filesystem. The server is the system that makes the directories available for use. Attaching a remote directory to the local filesystem (a client function) is called mounting a directory. Offering a directory for remote access (a server function) is called sharing or exporting a directory.[1] Frequently, a system runs both the client and the server NFS software. In this section we'll look at how to configure a system to export and mount directories using NFS.

If you're responsible for an NFS server for a large site, you should take care in planning and implementing the NFS environment. This chapter describes how NFS is configured to run on a client and a server, but you may want more details to design an optimal NFS environment. For a comprehensive treatment, see Managing NFS and NIS by Hal Stern (O'Reilly & Associates).

NFS Daemons

The Network File System is run by several daemons, some performing client functions and some performing server functions. Before we discuss the NFS configuration, let's look at the function of the daemons that run NFS on a Solaris 8 system:

nfsd [nservers]
The NFS daemon, nfsd, runs on NFS servers. This daemon services the client's NFS requests. The nservers option specifies how many daemons should be started.

mountd
The NFS mount daemon, mountd, processes the clients' mount requests. NFS servers run the mount daemon.

nfslogd
The NFS logging daemon, nfslogd, logs activity for exported filesystems. NFS servers run the logging daemon.

rquotad
The remote quota server, rquotad, provides information about user quotas on remote filesystems that is then displayed by the quota command. The remote quota server is run on both clients and servers.

lockd
The lock daemon, lockd, handles file lock requests. Both clients and servers run the lock daemon. Clients request file locks, and servers grant them.

statd
The network status monitor daemon, statd, is required by lockd to provide monitoring services. In particular, it allows locks to be reset properly after a crash. Both clients and servers run statd.

On a Solaris 8 system, the daemons necessary to run NFS are found in the /usr/lib/nfs directory. Most of these daemons are started at boot time by two scripts located in the /etc/init.d directory, nfs.client and nfs.server. The nfs.client script starts the statd and lockd programs.[2] NFS server systems run those two daemons, plus the NFS server daemon (nfsd), the NFS logging daemon (nfslogd), and the mount server daemon (mountd). On Solaris systems, the nfs.server script starts mountd, nfslogd, and 16 copies of nfsd. Solaris systems do not normally start rquotad at boot time. Instead, rquotad is started by inetd, as this grep of the /etc/inetd.conf file shows:

$ grep rquotad /etc/inetd.conf
rquotad/1   tli   rpc/datagram_v  wait root /usr/lib/nfs/rquotad  rquotad

Each system has its own technique for starting these daemons. If some of the daemons aren't starting, ensure your startup scripts and your inetd.conf file are correct.

Sharing Unix Filesystems

The first step in configuring a server is deciding which filesystems will be shared and what restrictions will be placed on them. Only filesystems that provide a benefit to the client should be shared. Before you share a filesystem, think about what purpose it will serve. Some common reasons for sharing filesystems are:

Once you've selected the filesystems you'll share, you must configure them for sharing using the appropriate commands for your system. The following section emphasizes the way this is done on Solaris systems. It is very different on Linux systems, which are covered later. Check your system's documentation to find out exactly how it implements NFS file sharing.

The share command

On Solaris systems, directories are exported using the share command.

A simplified syntax for the share command is:

 share -F nfs [-o options] pathname

where pathname is the path of the directory the server is offering to share with its clients, and options are the access controls for that directory. The options are:

rw
The rw option grants read and write access to the shared filesystem. It can be specified in the form rw=accesslist to identify the systems that are granted this access. When used in this way, only the systems identified in the list are given access to the filesystem. If the access list is not provided with the rw option, all hosts are given read/write access to the filesystem.

ro
This option limits access to read-only. It also can be specified with an access list, e.g., ro=accesslist. When the access list is included, only the systems on the list have access and that access is limited to read-only. If the access list is not provided with the ro option, all hosts are given read-only access to the filesystem, which is the default if no options are specified.

aclok
This option grants full access to all clients, which could open up a security hole. This option is documented on the Solaris system, but it should never be used. It is intended to provide backward compatibility with a version of NFS that no longer exists.

anon=uid
Defines the UID used for users who do not provide a valid user ID.

index=file
Tells NFS to use a web-style index file instead of a directory listing for this filesystem.

log[=tag]
Enable logging. If an optional tag is specified, it must match a tag defined in the /etc/nfs/nfslog.conf file.

nosub
Do not allow clients to mount subdirectories. The default is sub, which allows subdirectories to be mounted.

nosuid
Do not allow clients to create setuid or setgid files on this filesystem. The default is suid, which allows clients to create setuid and setgid files.

public
Use the public file handle for this filesystem.

root=accesslist
This option allows the root users from the systems specified by the access list to have root access to the filesystem.

sec=type
Defines the type of authentication used for accessing this filesystem. type is a colon-separated list of NFS security modes. For access to be successful, the client must support at least one of the security modes identified in the type list. The possible type values are:

sys
Use clear-text user IDs and group IDs to control access to the filesystem. This is the same as traditional Unix file permissions, which are granted based on UID and GID, with the exception that the UID and GID are passed over the network and the server must trust the remote source.

dh
Use Diffie-Hellman public key cryptography for authentication.

krb4
Use the Kerberos Version 4 for authentication.

none
Do not use authentication. When no authentication is used, all users access the filesystem as user nobody.

window=seconds
Defines the maximum lifetime in seconds that the NFS server will permit for a dh or krb4 authentication. The server rejects any security credentials that have a longer lifetime value. seconds defaults to 30000.

A few of the options contain an access list. The access list is a colon-separated list that identifies computers by individual hostnames, individual IP addresses, or by the domain, network, or NIS netgroup to which the hosts belong. The syntax of these list elements is:

hostname
This is any hostname that resolves to an IP address. It can be a fully qualified name or just the hostname as long as the name as written will resolve to an IP address. If the hostname can be found in the local host table, the short name can be used. If the name must be resolved by DNS, the fully qualified hostname, with its domain name attached, should be specified. However, fully qualified names should not be used if your system does not use DNS, i.e., if your system relies exclusively on NIS.

address
An IP address in dotted decimal format can be used.

netgroup
If an NIS netgroup name is used, the option applies to every system within that netgroup. Netgroup names look identical to unqualified hostnames and are easy to confuse with hostnames. Netgroup names should only be used if your system uses NIS.

.domain
A domain name is used to apply the option to every system within that domain. When a domain name is used, it is preceded by a dot (.). Thus .wrotethebook.com applies to every system in the wrotethebook.com domain. Domain names should be used only if your server uses DNS.

@network[/prefix]
A network address is used to apply an option to every system within the network. When a network address is used, it must be preceded by an at-sign (@). An optional network prefix can be used with the address to clearly define the network mask.

The rw and ro options can be combined to grant different levels of access to different clients. For example: 

share -F nfs -o rw=crab:horseshoe ro  /usr/man
share -F nfs -o rw=rodent:crab:horseshoe:jerboas  /export/home/research

The first share command grants read and write access to crab and rodent, and read-only access to all other clients. On the other hand, the second share command grants read/write access to rodent, crab, horseshoe, and jerboas, and no access of any kind to any other client.

The share command does not survive a boot. Put the share commands in the /etc/dfs/dfstab file to make sure that the filesystems continue to be offered to your clients even if the system reboots. Here is a sample dfstab file containing our two share commands: 

% cat /etc/dfs/dfstab 
#   place share(1M) commands here for automatic execution 
#   on entering init state 3. 
# 
#   share [-F fstype] [ -o options] [-d "<text>"] <pathname> [resource] 
#   .e.g., 
#   share  -F nfs  -o rw=engineering  -d "home dirs"  /export/home2 
share -F nfs -o rw=crab:horseshoe ro  /usr/man
share -F nfs -o rw=rodent:crab:horseshoe:jerboas  /export/home/research

The share command, the dfstab file, and even the terminology "share" are Solaris-specific. Most Unix systems say that they are exporting files, instead of sharing files, when they are offering files to NFS clients. Furthermore, they do not use the share command or the dfstab file; instead, they offer filesystems through the /etc/exports file. Linux is an example of such a system.

The /etc/exports file

The /etc/exports file is the NFS server configuration file for Linux systems. It controls which files and directories are exported, which hosts can access them, and what kinds of access are allowed. A sample /etc/exports file might contain these entries: 

/usr/man        crab(rw) horseshoe(rw)  (ro) 
/usr/local      (ro)
/home/research  rodent(rw) crab(rw) horseshoe(rw) jerboas(rw)

This sample file says that:

The options used in each of the entries in the /etc/exports file determine what kinds of access are allowed. The information derived from the sample file is based on the options specified on each line in the file. The general format of the entries is as follows:

 directory [host(option)]...

directory names the directory or file that is available for export. The host is the name of the client granted access to the exported directory, while the option specifies the type of access being granted.

In the sample /etc/exports file shown above, the host value is either the name of a single client or it is blank. When a single hostname is used, access is granted to the individual client. If no host value is specified, the directory is exported to everyone. Like Solaris, Linux also accepts values for domains, networks, and netgroups, although the syntax is slightly different. Valid host values are:

Notice that in Linux, domain names begin with an asterisk (*), instead of the dot used in Solaris. Also note that the at-sign begins a netgroup name, whereas in Solaris the at-sign is used at the beginning of a network address.

The options used in the sample /etc/exports file are:

ro
Read-only prevents NFS clients from writing to this directory. Attempts by clients to write to a read-only directory fail with the message "Read-only filesystem" or "Permission denied." If ro is specified without a client hostname, all clients are granted read-only access.

rw
Read/write permits clients to read and write to this directory. When specified without hostname, all clients are granted read/write access. If a hostname is specified, only the named host is given read/write permission.

Although specific hosts are granted read/write access to some of these directories, the access granted to individual users of those systems is controlled by standard Unix user, group, and world file permissions based on the user's user ID (UID) and group ID (GID). NFS trusts that a remote host has authenticated its users and assigned them valid UIDs and GIDs. Exporting files grants the client system's users the same access to the files they would have if they directly logged into the server. This assumes, of course, that both the client and the server have assigned exactly the same UIDs and GIDs to the same users, which is not always the case. If both the client and the server assign the same UID to a given user, for example, if Craig is assigned 501 on both systems, then both systems properly identify Craig and grant him appropriate access to his files. On the other hand, if the client assigns Craig a UID of 501 and the server has assigned that UID to Michael, the server will grant Craig access to Michael's files as if Craig owned those files. NFS provides several tools to deal with the problems that arise because of mismatched UIDs and GIDs.

One obvious problem is dealing with the root account. It is very unlikely that you want people with root access to your clients to also have root access to your server. By default, NFS prevents this with the root_squash setting, which maps requests that contain the root UID and GID to the nobody UID and GID. Thus if someone is logged into a client as root, they are only granted world permissions on the server. You can undo this with the no_root_squash setting, but no_root_squash opens a potential security hole.

Map other UIDs and GIDs to nobody with the squash_uids, squash_gids, and all_squash options. all_squash maps every user of a client system to the user nobody. squash_uids and squash_gids map specific UIDs and GIDs. For example:

/pub           (ro,all_squash)
/usr/local/pub (squash_uids=0-50,squash_gids=0-50)

The first entry exports the /pub directory with read-only access to every client. It limits every user of those clients to the world permissions granted to nobody, meaning that the only files the users can read are those that have world read permission.

The second entry exports /usr/local/pub to every client with default read/write permission. The squash_uid and squash_gid options in the example show that a range of UIDs and GIDs can be specified in some options.[3] A single UID or GID can be defined with these options, but it is frequently useful to affect a range of values with a single command. In the example we prevent users from accessing the directory with a UID or GID that is 50 or less. These low numbers are usually assigned to non-user accounts. For example, on our Linux system, UID 10 is assigned to uucp. Attempting to write a file as uucp would cause the file to be written with the owner mapped to nobody. Thus the user uucp would be able to write to the /usr/local/pub directory only if that directory had world write permission.

It is also possible to map every user from a client to a specific user ID or group ID. The anonuid and anongid options provide this capability. These options are most useful when the client has only one user and does not assign that user a UID or GID, for example, in the case of a Microsoft Windows PC running NFS. PCs generally have only one user and they don't use UIDs or GIDs. To map the user of a PC to a valid user ID and group ID, enter a line like this in the /etc/exports file:

/home/alana  giant(all_squash,anonuid=1001,anongid=1001)

In this example, the hostname of Alana's PC is giant. The entry grants that client read/write access to the directory /home/alana. The all_squash option maps every request from that client to a specific UID, but this time, instead of nobody, it maps to the UID and the GID defined by the anonuid and anongid options. Of course, for this to work correctly, 1001:1001 should be the UID and GID pair assigned to alana in the /etc/passwd file.

A single mapping is sufficient for a PC, but it might not handle all of the mapping needed for a Unix client. Unix clients assign their users UIDs and GIDs. Problems occur if those differ from the UIDs and GIDs assigned to those same users on the NFS server. Use the map_static option to point to a file that maps the UIDs and GIDs for a specific client. For example:

/export/oscon oscon(map_static=/etc/nfs/oscon.map)

This entry says that the /export/oscon directory is exported to the client oscon with read/write permission. The map_static option points to a file on the server named /etc/nfs/oscon.map that maps the UIDs and GIDs used on oscon to those used on the server. The oscon.map file might contain the following entries:

# UID/GID mapping for client oscon
# remote    local    comment
uid 0-50    -    #squash these
gid 0-50    -    #squash these
uid 100-200    1000    #map 100-200 to 1000-1100
gid 100-200    1000    #map 100-200 to 1000-1100
uid 501    2001    #map individual user
gid 501    2001    #map individual user

The first two lines map the UIDs and GIDs from 0 to 50 to the user nobody. The next two lines map all of the client UIDs and GIDs in the range of 100 to 200 to corresponding numbers in the range of 1000 to 1100 on the server. In other words, 105 on the client maps to 1005 on the server. This is the most common type of entry. On most systems, existing UIDs and GIDs have been assigned sequentially. Often, several systems have assigned the UIDs and GIDs sequentially from 101 to different users in a completely uncoordinated manner. This entry maps the users on oscon to UIDs and GIDs starting at 1000. Another file might map the 100 to 200 entries of another client to UIDs and GIDs starting at 2000. A third file might map yet another client to 3000. This type of entry allows the server to coordinate UIDs and GIDs where no coordination exists. The last two lines map an individual user's UID and GID. This is less commonly required, but it is possible.

The exportfs command

After defining the directories in the /etc/exports file, run the exportfs command to process the exports file and to build /var/lib/nfs/xtab. The xtab file contains information about the currently exported directories, and it is the file that mountd reads when processing client mount requests. To process all of the entries in the /etc/exports file, run exportfs with the -a command-line option:

# exportfs -a

This command, which exports everything in the exports file, is normally run during the boot from a startup script. To force changes in the /etc/exports file to take effect without rebooting the system, use the -r argument:

# exportfs -r

The -r switch synchronizes the contents of the exports file and the xtab file. Items that have been added to the exports file are added to the xtab file, and items that have been deleted are removed from xtab.

The exportfs command can export a directory that is not listed in the /etc/exports file. For example, to temporarily export /usr/local to the client fox with read/write permission, enter this command:

# exportfs fox:/usr/local -o rw

After the client has completed its work with the temporarily exported filesystem, the directory is removed from the export list with the -u option, as shown:

# exportfs -u fox:/usr/local

The -u option can be combined with the -a option to completely shut down all exports without terminating the NFS daemon:

# exportfs -ua

Once the server exports or shares the appropriate filesystems, the clients can mount and use those filesystems. The next section looks at how an NFS client system is configured.

Mounting Remote Filesystems

Some basic information is required before you can decide which NFS directories to mount on your system. You need to know which servers are connected to your network and which directories are available from those servers. A directory cannot be mounted unless it is first exported by a server.

Your network administrator is a good source for this information. The administrator can tell you what systems are providing NFS service, what directories they are exporting, and what these directories contain. If you are the administrator of an NFS server, you should develop this type of information for your users. See Chapter 4 for advice on planning and distributing network information.

On Solaris and Linux systems, you can also obtain information about the shared directories directly from the servers by using the showmount command. The NFS servers are usually the same centrally supported systems that provide other services such as mail and DNS. Select a likely server and query it with the command showmount -e hostname. In response to this command, the server lists the directories that it exports and the conditions applied to their export.

For example, a showmount -e query to jerboas produces the following output: 

% showmount -e jerboas 
export list for jerboas: 
/usr/man           (everyone) 
/home/research     rodent,crab,limulus,horseshoe
/usr/local         (everyone)

The export list shows the NFS directories exported by jerboas as well as who is allowed to access those directories. From this list, rodent's administrator may decide to mount any of the directories offered by jerboas. Our imaginary administrator decides to:

  1. Mount /usr/man from jerboas instead of maintaining the manpages locally.

  2. Mount /home/research to more easily share files with other systems in the research group.

  3. Mount the centrally maintained programs in /usr/local.

These selections represent some of the most common motivations for mounting NFS directories:

The extent to which you use NFS is a personal choice. Some people prefer the greater personal control you get from keeping files locally, while others prefer the convenience offered by NFS. Your site may have guidelines for how NFS should be used, which directories should be mounted, and which files should be centrally maintained. Check with your network administrator if you're unsure about how NFS is used at your site.

The mount command

A client must mount a shared directory before using it. "Mounting" the directory attaches it to the client's filesystem hierarchy. Only directories offered by the servers can be mounted, but any part of the offered directory, such as a subdirectory or a file, can be mounted.

NFS directories are mounted using the mount command. The general structure of the mount command is:

 mount hostname:remote-directory local-directory

The hostname identifies an NFS server, and the remote-directory identifies all or part of a directory offered by that server. The mount command attaches that remote directory to the client's filesystem using the directory name provided for local-directory. The client's local directory, called the mount point, must be created before mount is executed. Once the mount is completed, files located in the remote directory can be accessed through the local directory exactly as if they were local files.

For example, assume that jerboas.wrotethebook.com is an NFS server and that it shares the files shown in the previous section. Further assume that the administrator of rodent wants to access the /home/research directory. The administrator simply creates a local /home/research directory and mounts the remote /home/research directory offered by jerboas on this newly created mount point: 

# mkdir /home/research
# mount jerboas:/home/research /home/research

In this example, the local system knows to mount an NFS filesystem because the remote directory is preceded by a hostname and NFS is the default network filesystem for this client. NFS is the most common default network filesystem. If your client system does not default to NFS, specify NFS directly on the mount command line. On a Solaris 8 system, the -F switch is used to identify the filesystem type:

# mount -F nfs jerboas:/home/research /home/research

On a Linux system the -t switch is used:

# mount -t nfs jerboas:/home/research /home/research

Once a remote directory is mounted, it stays attached to the local filesystem until it is explicitly dismounted or the local system reboots. To dismount a directory, use the umount command. On the umount command line, specify either the local or remote name of the directory that is to be dismounted. For example, the administrator of rodent can dismount the remote jerboas:/home/research filesystem from the local /home/research mount point, with either:

# umount /home/research

or:

# umount jerboas:/home/research

Booting also dismounts NFS directories. Because systems frequently wish to mount the same filesystems every time they boot, Unix provides a system for automatically remounting after a boot.

The vfstab and fstab files

Unix systems use the information provided in a special table to remount all types of filesystems, including NFS directories, after a system reboot. The table is a critical part of providing users consistent access to software and files, so care should be taken whenever it is modified. Two different files with two different formats are used for this purpose by the different flavors of Unix. Linux and BSD systems use the /etc/fstab file, and Solaris, our System V example, uses the /etc/vfstab file.

The format of the NFS entries in the Solaris vfstab file is:

filesystem - mountpoint nfs - yes options

The various fields in the entry must appear in the order shown and be separated by whitespace. The items not in italics (both dashes and the words nfs and yes) are keywords that must appear exactly as shown. filesystem is the name of the directory offered by the server, mountpoint is the pathname of the local mount point, and options are the mount options discussed below. A sample NFS vfstab entry is:

jerboas:/home/research  -  /home/research  nfs  -  yes  rw,soft

This entry mounts the NFS filesystem jerboas:/home/research on the local mount point /home/research. The filesystem is mounted with the rw and soft options set. We previously discussed the commonly used read/write (rw) and read-only (ro) options, and there are many more NFS options. The NFS mount options available on Solaris systems are:

remount
If the filesystem is already mounted read-only, remount the filesystem as read/write.

soft
If the server fails to respond, return an error and don't retry the request.

timeo=n
Defines the number of seconds to wait for a timeout before issuing an error.

hard
If the server fails to respond, retry until it does respond. This is the default.

bg
Do the retries in background mode, which allows the boot to proceed.

fg
Do the retries in foreground mode. This option could hang the boot process while the mount is being retried. For this reason, fg is used primarily for debugging.

intr
Allow a keyboard interrupt to kill a process that is hung waiting for the server to respond. Hard-mounted filesystems can become hung because the client retries forever, even if the server is down. This is a default.

nointr
Don't allow keyboard interrupts. In general, this is a bad idea.

nosuid
Do not allow an executable stored on the mounted filesystem to run setuid. This improves security but may limit utility.

acdirmax=n
Cache directory attributes for no more than n seconds. The default is to hold cache values for a maximum of 60 seconds. Repeated requests for filesystem attributes is one of the leading contributors to NFS traffic. Caching this information helps to reduce the traffic.

acdirmin=n
Cache directory attributes for at least n seconds. The default is 30 seconds.

acregmax=n
Cache file attributes for no more than n seconds. The default is 60 seconds.

acregmin=n
Cache file attributes for at least n seconds. The default is 3 seconds.

actimeo=n
Sets a single value for acdirmax, acdirmin, acregmax, and acregmin.

grpid
Use the group ID of the parent directory when creating new files. If this option is not set, the effective GID of the calling process is used.

noac
Do not cache information. The default is to use caching, which can be specified with the ac option.

port=n
Identifies the IP port number used by the server.

posix
Use POSIX standards for the filesystem. POSIX is a wide-ranging Unix interoperability standard that includes many standards for filesystems, such as the maximum length of filenames and how file locks are set.

proto=protocol
Specifies the transport protocol that will be used by NFS.

public
Use the public file handle when connecting to the NFS server.

quota
Use quota to enforce user quotas on this filesystem.

noquota
Do not use quota to enforce user quotas on this filesystem.

retrans=n
Defines the number of times NFS will retransmit when a connectionless transport protocol is used.

retry=n
Defines the number of times a mount attempt will be retried. The default is to retry 10,000 times.

rsize=n
Defines the size of the read buffer as n bytes. The default for NFS version 3 is 32,768 bytes.

sec=type
Specifies the type of security used for NFS transactions. The type values supported on the Solaris 8 mount command are the same as those listed for the share command: sys, dh, krb4, or none.

wsize=n
Sets the size of the write buffer to n bytes. The default is 32768 bytes for NFS version 3.

vers=version
Specifies the version of NFS that should be used for this mount. By default, the system automatically selects the latest version supported by both the client and the server.

On the Solaris system, the filesystems defined in the vfstab file are mounted by a mountall command located in a startup file. On Linux systems, the startup file contains a mount command with the -a flag set, which causes Linux to mount all filesystems listed in fstab.[4] The format of NFS entries in the /etc/fstab file is:

 filesystem mountpoint nfs options

The fields must appear in the order shown and must be separated by whitespace. The keyword nfs is required for NFS filesystems. filesystem is the name of the directory being mounted. mountpoint is the pathname of the local mount point. options are any of the Linux mount options.

Linux uses most of the same NFS mount options as Solaris. rsize, wsize, timeo, retrans, acregmin, acregmax, acdirmin, acdirmax, actimeo, retry, port, bg, fg, soft, hard, intr, nointr, ac, noac, and posix are all options that Linux has in common with Solaris. In addition to these, Linux uses:

mountport=n
Defines the port to be used by mountd.

mounthost=name
Identifies the server running mountd.

mountprog=n
Defines the RPC program number used by mountd on the remote host.

mountvers=n
Defines the RPC version number used by mountd on the remote host.

nfsprog=n
Defines the RPC program number used by nfsd on the remote host.

nfsvers=n
Defines the RPC version number used by nfsd on the remote host.

namlen=n
Defines the maximum length of the filenames supported by the remote server.

nocto
Do not retrieve attributes when creating a file. The default is to retrieve the attributes, which can be specified with the cto option.

tcp
Specifies that NFS should use TCP as its transport protocol.

udp
Specifies that NFS should use UDP as its transport protocol.

nolock
Prevents the system from starting lockd. The default is to run lockd, which can be requested with the lock option.

Finally, there are several options that are not specific to NFS and can be used on the mount command for any type of filesystem. Table 9-1 lists the common mount options used on Linux systems.

Table 9-1: Common mount options

Option

Function

async

Use asynchronous file I/O, which acknowledges writes as soon as they are received to improve performance.

auto

Mount when the -a option is used.

dev

Allow character and block special devices on the filesystem.

exec

Permit execution of files from the filesystem.

noauto

Don't mount with the -a option.

nodev

Don't allow character and block special devices on the filesystem.

noexec

Don't allow execution of files from the filesystem.

nosuid

Don't allow programs stored on the filesystem to run setuid or setgid.

nouser

Only root can mount the filesystem.

remount

Remount a mounted filesystem with new options.

ro

Mount the filesystem read-only.

rw

Mount the filesystem read/write.

suid

Allow programs to run setuid or setgid.

sync

Use synchronous filesystem I/O, which acknowledges writes only after they are written to disk to improve reliability.

user

Permit ordinary users to mount the filesystem.

atime

Update inode access time for every access.

noatime

Do not update inode access time.

defaults

Set the rw, suid, dev, exec, auto, nouser, and async options.

A grep of fstab shows sample NFS entries.[5] 

% grep nfs /etc/fstab 
jerboas:/usr/spool/mail   /usr/spool/mail   nfs rw     0 0 
jerboas:/usr/man          /usr/man          nfs rw     0 0
jerboas:/home/research    /home/research    nfs rw     0 0

The grep shows that there are three NFS filesystems contained in the /etc/fstab file. The mount -a command in the boot script remounts these three directories every time the system boots.

The vfstab and fstab files are the most common methods used for mounting filesystems at boot time. There is another technique that automatically mounts NFS filesystems, but only when they are actually needed. It is called automounter.

NFS Automounter

An automounter is a feature available on most NFS clients. Two varieties of automounters are in widespread use: autofs and amd. The Automounter Filesystem (autofs) is the automounter implementation that comes with Solaris and Linux, and it is the implementation we cover in this section. Automounter Daemon (amd) is available for many Unix versions and is included with Linux but not with Solaris. To find out more about amd, see Linux NFS and Automounter Administration written by Erez Zadok, the amd maintainer. In this section, automounter and automounter daemon refer to the version of autofs that comes with Solaris 8.

The automounter configuration files are called maps. Three basic map types are used to define the automounter filesystem:

Master map
The configuration file read by automount. It lists all of the other maps that are used to define the autofs filesystem.

Direct map
A configuration file that lists the mount points, pathnames, and options of filesystems that are to be mounted by the automounter daemon (automountd).

Indirect map
A configuration file that contains pathnames and "relative" mount points. The mount points are relative to a directory path declared in the master map. How indirect maps are used will become clear in the examples.

On Solaris systems the automounter daemon (automountd) and the automount command are started by the /etc/init.d/autofs script. The script is run with the start option to start automounter, i.e., autofs start. It is run with the stop option to shut down automounter. automount and automountd are two distinct, separate programs. automountd runs as a daemon and dynamically mounts filesystems when they are needed. automount processes the auto_master file to determine the filesystems that can be dynamically mounted.

To use automounter, first configure the /etc/auto_master file. Entries in the auto_master file have this format:

mount-point     map-name        options

The Solaris system comes with a default auto_master file preconfigured. Customize the file for your configuration. Comment out the +auto_master entry unless you run NIS+ or NIS and your servers offer a centrally maintained auto_master map. Also ignore the /xfn entry, which is for creating a federated (composite) name service. Add an entry for your direct map. In the example, this is called auto_direct. Here is /etc/auto_master after our modifications: 

# Master map for automounter 
# 
#+auto_master 
#/xfn           -xfn 
/net            -hosts          -nosuid 
/home           auto_home
/-              auto_direct

All lines that begin with a sharp sign (#) are comments, including the +auto_master and /xfn lines we commented out. The first real entry in the file specifies that the shared filesystems offered by every NFS server listed in the /etc/hosts file are automatically mounted under the /net directory. A subdirectory is created for each server under /net using the server's hostname. For example, assume that jerboas is listed in the hosts file and that it exports the /usr/local directory. This auto_master entry automatically makes that remote directory available on the local host as /net/jerboas/usr/local.

The second entry automatically mounts the home directories listed in the /etc/auto_home map under the /home directory. A default /etc/auto_home file is provided with the Solaris system. Comment out the +auto_home entry found in the default file. It is used only if you run NIS+ or NIS and your servers offer a centrally maintained auto_home map. Add entries for individual user home directories or for all home directories from specific servers. Here is a modified auto_home map: 

# Home directory map for automounter 
# 
#+auto_home 
craig           crab:/export/home/craig
*               horseshoe:/export/home/&

The first entry mounts the /export/home/craig filesystem shared by crab on the local mount point /home/craig. The auto_home map is an indirect map, so the mount point specified in the map (craig) is relative to the /home mount point defined in the auto_master map. The second entry mounts every home directory found in the /export/home filesystem offered by horseshoe to a "like-named" mount point on the local host. For example, assume that horseshoe has two home directories, /export/home/daniel and /export/home/kristin. Automounter makes them both available on the local host as /home/daniel and /home/kristin. The asterisk (*) and the ampersand (&) are wildcard characters used specifically for this purpose in autofs maps.

That's it for the auto_home map. Refer back to the auto_master map. The third and final entry in the /etc/auto_master file is:

/-              auto_direct

We added this entry for our direct map. The special mount point /- means that the map name refers to a direct map. Therefore the real mount points are found in the direct map file. We named our direct map file /etc/auto_direct. There is no default direct map file; you must create it from scratch. The file we created is: 

# Direct map for automounter 
# 
/home/research  -rw       jerboas:/home/research
/usr/man        -ro,soft  horseshoe,crab,jerboas:/usr/share/man

The format of entries in a direct map file is:

mount-point     options   remote filesystem

Our sample file contains two typical entries. The first entry mounts the remote filesystem /home/research offered by the server jerboas on the local mount point /home/research. It is mounted read/write. The second entry mounts the manpages read-only with a "soft" timeout.[6] Note that three servers are specified for the manpages in a comma-separated list. If a server is unavailable or fails to respond within the soft timeout period, the client asks the next server in the list. This is one of the nice features of automounter.

Automounter has four key features: the -hosts map, wildcarding, automounting, and multiple servers. The -hosts map makes every exported filesystem from every server listed in the /etc/hosts file available to the local user. The wildcard characters make it very easy to mount every directory from a remote server to a like-named directory on the local system. Automounting goes hand-in-glove with these two features because only the filesystems that are actually used are mounted. While -hosts and wildcards make a very large number of filesystems available to the local host, automounting limits the filesystems that are actually mounted to those that are needed. The last feature, multiple servers, improves the reliability of NFS by removing the dependence on a single server.

Sharing Unix Printers

Unix uses NFS to share files; it uses two different tools, the Line Printer Daemon (lpd) and the Line Printer (lp) printer service, to provide printer services for local and remote users. lpd is used on BSD systems and most Linux systems. lp is used on System V systems, including Solaris 8. In the following sections we discuss both tools.

Line Printer Daemon

lpd manages the printer spool area and the print queues. lpd is started at boot time from a startup script. It is generally included in the startup of Linux and BSD systems by default, so you might not need to add it to your startup script. For example, it is started by the /etc/init.d/lpd script on a Red Hat Linux system.

The printcap file

When lpd starts, it reads the /etc/printcap file to find out about the printers available for its use. The printcap file defines the printers and their characteristics. Configuring a printcap file is the scariest part of setting up a Unix print server. (Don't worry. As we'll see later with the Solaris admintool, most systems provide a GUI tool for configuring printers.) The printcap file scares system administrators because the parser that reads the file is very finicky, and the syntax of the parameters in the file is terse and arcane. Most parser problems can be avoided by following these rules:

The configuration parameters used in a printcap file describe the characteristics of the printer. These characteristics are called "capabilities" in the printcap documentation, but really they are the printer characteristics that lpd needs to know in order to communicate with the printer. Parameters are identified by names that are two characters long and are usually assigned a value. The syntax of the parameters varies slightly depending on the type of value they are assigned. Parameters come in three different flavors:

Boolean
All printcap Boolean values default to "false." Specifying a Boolean enables its function. Booleans are specified simply by entering the two-character parameter name in the file. For example, :rs: enables security for remote users.

Numeric
Some parameters are assigned numeric values. For example, :br#9600: sets the baud rate for a serial printer.

String
Some parameters use string values. For example, :rp=laser: defines the name of a remote printer.

A glance at the manpage shows that there are many printcap parameters. Thankfully, you'll never need to use most of them. Most printer definitions are fairly simple, and most printcap files are small.

Print servers usually have only one or two directly attached printers; any other printers defined in printcap are probably remote printers. Most, if not all, of the printers defined in a client's printcap are remote printers. 

# 
# Remote LaserWriter 
# 
lw:\ 
    :lf=/var/adm/lpd-errs:\ 
    :lp=:rm=horseshoe:rp=lw:\
    :sd=/var/spool/lpd-lw:

The lw printer in this sample printcap file is a remote printer. The lf parameter points to the log file used to log status and error messages. The remote machine to which the printer is attached is defined by the :rm=horseshoe: parameter, and the name of the remote printer on that machine is defined by the :rp=lw: parameter. Multiple printers can use the same log file. The final parameter, sd, defines the spool directory. Each printer has its own unique spool directory. Defining the remote printer in the client's printcap file is all that is needed to configure an LPD client.

Writing a printcap from scratch is unnecessary. At most, you will need to tweak the printcap configuration for your own special needs. All modern Unix systems have printer configuration tools that will build the basic printcap for you. The Red Hat printconf-gui tool is an example.

Launch the printer configuration tool on a Red Hat 7.2 system running the Gnome desktop by selecting Printer Configuration from the System menu. When the printconf-gui window opens, click the New button to add a printer to the printcap file. The New button launches a printer configuration wizard. Use the first page of the wizard to define the printer name and the queue type. Enter the printer name, for example lw, in the Queue Name box. Then select the Queue Type. Red Hat 7.2 offers five choices:

Local Printer
Use this type to define a directly attached printer. When Local Printer is selected, the wizard asks you to select the printer port to which the printer is attached. The default is /dev/lp0.

Unix Printer
Use this type to define a printer located on a remote server that is accessed via the LPD protocol. When Unix Printer is selected, the wizard asks for the name of the remote server and the name of the printer on the remote server.

Windows Printer
Use this type to define a remote printer that is accessed via the SMB protocol. When Windows Printer is selected, the wizard asks for the IP address of the remote server, the SMB workgroup, and the name of the remote printer, which it calls a share name. The wizard also allows a username and password to be input in case they are required for access to the printer. Printer sharing through SMB is is covered in detail later in this chapter.

Novell Printer
Use this type to define a remote printer accessed via the NetWare protocols. When Novell Printer is selected, the wizard asks for the name of the server and the printer on that server. A username and password can also be entered if they are required for printer access. To communicate with a Novell printer you must have the NetWare protocols installed on your system.

JetDirect Printer
Use this type to define a network-attached printer that uses the JetDirect protocol. Primarily, this protocol is used on HP printers that contain a built-in Ethernet interface and that connect directly to the Ethernet cable without going through a server. When JetDirect is selected, the wizard asks for the IP address of the printer and gives you a chance to enter a port number in case the printer is not configured to use the standard JetDirect port.

Finally, the wizard presents you with a selection of hundreds of printer drivers. Most Unix systems use standard PostScript printers. Linux systems, however, are built on commodity PC hardware. PCs use a hodgepodge of different printers. The wizard lets you select the correct driver for your printer by selecting the printer make and then the printer model. Once you select the drive, the configuration is finished and the new printer is installed.

The Red Hat tool is just an example. Future versions of Red Hat will have a newer tool, and other Unix systems have their own tools. The point is not the details of the tool, but the fact that the printcap file is not usually written by hand. It is created by a configuration tool.

Using LPD

Once the printer is configured, print jobs are sent to the line printer daemon using the Line Printer Remote (lpr) program. The lpr program creates a control file and sends it and the print file to lpd. There are many possible lpr command-line arguments, but in general the command simply identifies the printer and the file to be printed, as in:

% lpr -Plj ch09

This command sends a file called ch09 to a printer called lj. The printer can be local or remote. It doesn't matter as long as the printer is defined in the printcap file and therefore known to lpd.

The client software provides commands to allow the user to check the status of the print job. Table 9-2 lists these commands, their syntax, and their meaning.

Table 9-2: Line printer commands

Command

Function

lpc start [printer]

Starts a new printer daemon.

lpc status [printer]

Displays printer and queue status.

lpq -Pprinter [user] [job]

Lists the jobs in the printer's queue.

lprm -Pprinter job

Removes a print job from the queue.

In this syntax, printer is the name of the printer as defined in the /etc/printcap file, user is the username of the owner of a print job, and job is the job number associated with the print job while it is waiting in the queue. The keyword all can be used in place of a printer name in any lpc command to refer to all printers.

While lpc is primarily for the system administrator, the status and start commands can be used by anyone. All the commands shown in Table 9-2 are available to users.

The lpq command displays a list of jobs queued for a printer. Command-line arguments permit the user to select which printer queue is displayed and to limit the display from that queue to a specific user's jobs or even to a specific job. Here's an example of displaying the queue for the printer lp: 

$ lpq -Plp
Printer: lp@crab 'Canon'
 Queue: 4 printable jobs
 Server: pid 1459 active
 Unspooler: pid 1460 active
 Status: waiting for subserver to exit at 14:17:47.120
 Rank   Owner/ID             Class Job Files                 Size Time
active alana@crab+458          A   458 /usr/share/printconf 18043 14:16:53
2      micheal@crab+477        A   477 /usr/share/printconf/t 193 14:17:38
3      james@crab+479          A   479 /usr/share/printconf 18259 14:17:43
4      daniel@crab+481         A   481 /usr/share/printconf 18043 14:17:46

A queued print job can be removed by the owner of the job with the lprm command. Assume that daniel wants to remove print job number 481. He enters the following command: 

$ lprm -Plp 481
Printer lp@crab:
  checking perms 'daniel@crab+481'
  dequeued 'daniel@crab+481'

lpd and lpr were among the first commands created for Unix to exploit the power of TCP/IP networking. Managing printers is primarily a system administration task. Only those aspects of lpd related to remote printing are covered here.

Line Printer Service

The Line Printer (LP) print service is used by most System V Unix systems. LP offers the same type of service as LPD.

Traditionally on System V Unix systems, the LP configuration files are located in the /etc/lp directory. These files perform the same basic function that the /etc/printcap file does for LPD. However, the /etc/lp files are not directly edited by the system administrator. The files are created and modified using the System V lpadmin and lpsystem commands.

The lpadmin command adds local printers to the /etc/lp/printers directory, which makes the printers available to LP. The basic syntax of adding a new printer is simple. The -p switch defines the local name of the printer. The -p switch is used with either the -v switch that defines the interface for a local printer or the -s switch that defines the server and printer name for a remote printer. For example, the following command adds a local printer named lp connected to the parallel printer port /dev/lp1:

# lpadmin -plp -v /dev/lp1

This command adds a printer locally known as laser that is the lj printer on the remote server crab:

# lpadmin -llaser -s crab!lj

The specific characteristics of a printer added by lpadmin are controlled by the terminfo file. terminfo is a file that is almost identical to the printcap file. Like printcap, it has a great many possible parameters. For more information on terminfo, see the manpage.

The lpsystem command manages printer access on System V systems. By default, most System V systems share all local printers. Remote printer access settings are defined in the /etc/lp/Systems file, which comes preconfigured with the following entry:

+:x:-:s5:-:n:10:-:-:Allow all connections

As the comment at its end makes clear, this entry grants all remote systems access to the local printers. The first field defines the name of the host being granted access. When a plus sign (+) is used in this field, it means all hosts.

The fields in an /etc/lp/Systems entry are separated by colons (:). The field containing an x and all of the fields containing a dash (-) can be ignored. These fields are unused.

The fourth field identifies the type of operating system used on the remote client. It contains either s5 for System V computers that use LP to print jobs, or bsd for BSD systems that use LPD.

The n in the sixth field indicates that this "connection" should never be timed out and removed from the system. A timeout period in minutes could be entered in this field, but this is not usually done. Keep the connection available as long as the local server is up. The 10 is a related value. It indicates that if a connection to a remote system fails, it should be retried after 10 minutes. This is a good value: it is long enough to give the remote system a chance to restart after a crash. Both n and 10 are the defaults and don't usually need to be changed.

Don't directly edit the /etc/lp/Systems file. Modify it with the lpsystem command. To remove a system from the Systems file, use lpsystem with the -r hostname command-line argument, where hostname is the value in the first field of the entry you wish to delete. For example, to remove the plus sign (+) entry from the default /etc/lp/Systems file, type:

# lpsystem -r +

To add an entry to the Systems file, use the lpsystem command without the -r option. For example, to add a BSD system named clock, enter:

# lpsystem -t bsd -y "Linux PC in room 820" clock

The command adds the following entry to the Systems file:

clock:x:-:bsd:-:n:10:-:-:Linux PC in room 820

The -t command-line option defines the operating system type. The -y option defines the comment; clock is, of course, the hostname. We accepted the default values for the timeout and the retry intervals. These could have been modified from the command line using the -T timeout and the -R retry options. See the manpage for lpsystem for more information.

The lpadmin and lpsystem commands are found on most System V systems, including Solaris. Solaris 8, however, does not rely solely on these commands and the /etc/lp directory to configure LP. On a Solaris system, printers are configured through the /etc/printers.conf file. The lpadmin command will add new printers to the /etc/printers.conf file, but printers are usually configured through the Printer Manager window of the admintool. Figure 9-1 shows the Printer Manager window.

Figure 9-1. Printer Manager

 

Clients select Add, then Access to Printer from the Edit menu, and enter the name of the remote printer and its server in the window that appears. Servers share printers simply by selecting Add, then Local Printer in the same menu and configuring a local printer.

All Unix systems provide some technique for sharing printers. The network administrator's task is to ensure that the printers are accessible via the network and that they are properly secured.

Using Samba to Share Resources with Windows

NFS and lpd are file and print sharing services for Unix systems, and are both native TCP/IP applications. Microsoft Windows printer and file sharing applications are based on NetBIOS (Network Basic Input Output System). Samba bridges these two worlds, providing file and print sharing for Unix and Windows systems. Samba is the key to integrating Unix and Windows because it allows a Unix system to be a file and print server for Windows clients, or to be a client of a Windows server.

The protocol used between NetBIOS clients and servers is Server Message Block Protocol (SMB). Originally, NetBIOS was a monolithic protocol that took data all the way from the application to the physical network. Today, NetBIOS runs over TCP/IP, which allows NetBIOS applications to run on Unix systems that use TCP/IP.

Two things are needed to run NetBIOS on a TCP/IP network: a protocol to carry NetBIOS data over TCP/IP and a technique to map NetBIOS addresses to TCP/IP addresses. The protocol that transports NetBIOS is NetBIOS over TCP/IP (NBT), which is defined by RFCs 1001 and 1002. Address mapping is handled by a special NetBIOS name server. Samba provides both of these services.

Samba services are implemented as two daemons. The SMB daemon (smbd), the heart of Samba, provides the file and printer sharing services. The NetBIOS name server daemon (nmbd) provides NetBIOS-to-IP-address name service. NBT requires some method for mapping NetBIOS computer names, which are the addresses of a NetBIOS network, to the IP addresses of a TCP/IP network.

Samba is included in most Linux distributions and is installed during the initial system installation. On a Red Hat system, the /etc/rc.d/init.d/smb script runs at boot time, and starts both smbd and nmbd. Samba is not included in Solaris 8, but the software is available for download from the Internet. Go to http://www.samba.org to select your nearest download site.

Configuring a Samba Server

The Samba server is configured by the smb.conf file. Look in the startup script to see where smbd expects to find the configuration file. On a Red Hat system, it is /etc/samba/smb.conf. On a Caldera system, it is /etc/samba.d/smb.conf. The default used in most Samba documentation is /usr/local/samba/lib/smb.conf. Use find or check the startup script so you know where it is on your system.

The smb.conf file is divided into sections. Except for the global section, which defines configuration parameters for the entire server, the sections are named after shares. A share is a resource offered by the server to the clients. It can be either a filesystem or a shared printer.

The best way to learn about the smb.conf file is to look at one. Minus the printers share, which is covered later, the Red Hat smb.conf file contains these active lines:

[global]
   workgroup = MYGROUP
   server string = Samba Server
   printcap name = /etc/printcap
   load printers = yes
   printing = lprng
   log file = /var/log/samba/%m.log
   max log size = 0
   security = user
   encrypt passwords = yes
   smb passwd file = /etc/samba/smbpasswd
   socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
  dns proxy = no
[homes]
   comment = Home Directories
   browseable = no
   writable = yes
   valid users = %S
   create mode = 0664
   directory mode = 0775

Two sections of the Red Hat configuration file, global and homes, are listed above. The global section defines parameters that affect the entire server:

workgroup
Defines the hierarchical grouping of hosts, called a workgroup, of which this server is a member. Replace the MYGROUP name in the example with a meaningful workgroup name of 15 characters or less. Make sure you use a meaningful name. Never use the name MYGROUP or WORKGROUP.

server string
Defines the descriptive comment for this server that is displayed by the net view command on DOS clients. Change the string in the example to something meaningful for your system.

printcap name
Defines the location of the printcap file. The printcap file is used to identify the printers that are available to share. The default path is /etc/printcap.

load printers
Specifies whether or not all the printers in the printcap file are to be shared. The default is yes, use all the printers defined in the printcap file. no means don't read the printcap file at all. If no is specified, all shared printers must be defined individually.

printing
Identifies the Unix printing system used by the server. In the example, it is LPR Next Generation (lprng), which is an implementation of the standard LPR/LPD system described earlier in this chapter.

log file
Defines the location of the log file. The example contains the %m variable,[7] which varies according to the client's NetBIOS name. This creates a different log file for each client with a file extension that is the client's NetBIOS name. If the NetBIOS name of the client is crab, the log file is named /var/log/samba/log.crab. If the client's NetBIOS name is rodent, the log file is /var/log/samba/log.rodent.

max log size
Defines the maximum size of a log file in kilobytes. The default is 5 MB, or 5000 KB. (If the maximum size is exceeded, smbd closes the log and renames it with the extension .old.) In the sample configuration, this is set to 0, which means that no maximum size is set for log files.

security
Defines the type of security used. There are four possible settings:

share
Requests share-level security. This is the lowest level of security. The resource is shared with everyone. It is possible to associate a password with a share, but the password is the same for everyone.

user
Requests user-level security. Every user is required to enter a username and an associated password. By default, this is the username and password defined in /etc/passwd. The default values for passwords can be changed. See the discussion of passwords later in this section.

server
Defines server-level security. This is similar to user-level security, but an external server is used to authenticate the username and password. The external server must be defined by the password server option.

domain
Defines domain-level security. In this scheme, the Linux server joins a Windows NT/2000 domain and uses the Windows NT/2000 domain controller as the server that approves usernames and passwords. Use the password server option to point to the Windows NT/2000 Primary Domain Controller (PDC). Log into the PDC and create an account for the Linux system. Finally, add these lines to the global section on the Linux system:

domain master = no

local master = no

preferred master = no

os level = 0

encrypt passwords
Setting this option to yes causes Samba to encrypt passwords before they are sent across the network. This makes the server more compatible with Windows clients from Windows 98 on, which default to encrypted passwords, and makes it harder for intruders to sniff passwords from the network. By default, Samba uses clear-text Unix passwords.

smb passwd file
This option points to the location of the smbpasswd file, where encrypted Samba passwords are stored. When encrypted passwords are used, the Samba server must maintain two password files: passwd and smbpasswd. Use the mksmbpasswd.sh script to build the initial smbpasswd file from the passwd file.

socket options
Defines performance tuning parameters. This option is not required, although setting the send and receive buffers to 8 KB may slightly increase performance. In the case of this sample Red Hat configuration, the TCP_NODELAY setting, which causes Samba to send multiple packets with each transfer, has no effect because it is the default for versions of Samba 2.0.4 or higher. See Appendix B of Using Samba, by Kelly, Eckstein, and Collier-Brown (O'Reilly) for a good discussion of Samba performance tuning.

dns proxy
Specifies whether or not nmbd should forward unresolved NBNS queries to DNS.

In addition to the options described above, several other parameters are commonly used in the global section; they are shown in Table 9-3.

Table 9-3: Other global section parameters

Option

Function

deadtime

Defines the timeout for inactive connections.

debug level

Sets the level of messages written to the log.

keepalive

Uses keepalives to check on the state of the clients.

lock directory

Defines the path of the directory where wins.dat, status files, and lock files are stored.

message command

Defines how smbd handles WinPopup messages.

name resolve order

Defines the order in which services are queried to resolve NetBIOS names. Possible values are: lmhosts, hosts, wins, and bcast.

netbios aliases

Defines other names the server will answer to.

netbios name

Defines the server's NetBIOS name.

syslog

Maps debug levels to syslog levels.

syslog only

Uses syslog instead of Samba log files.

time server

Tells the server to advertise itself as a Windows time server.

wins support

Enables the WINS name server.

As the Red Hat sample configuration demonstrates, many servers come preconfigured with reasonable global parameters to begin running a simple server system. In addition to a preconfigured global section, the Red Hat configuration comes with a preconfigured homes section.

The smb.conf homes section

The homes section is a special share section. It tells smbd to permit users to access their home directories through SMB. Unlike other share sections, which we cover later, this section does not tell smbd the specific path of the directory being shared. Instead, smbd uses the home directory from the /etc/passwd file based on the username of the user requesting the share. The configuration parameters in the Red Hat homes section are:

comment
Provides a description of the share that is displayed in the comment field of the Network Neighborhood window when this share is viewed on a Windows PC.

browseable
Specifies whether or not all users may browse the contents of this share. no means that only users with the correct user ID are allowed to browse this share. yes means all users, regardless of UID, can browse the share. This parameter controls only browsing; actual access to the contents of the share is controlled by standard Linux file permissions.

writable
Specifies whether or not files can be written to this share. If yes, the share can be written to. If no, the share is read-only. This parameter defines the actions permitted by Samba. Actual permission to write to the directory defined by the share is still controlled by standard Linux file permissions.

valid users
This option lists the users who are allowed to use this share. In this example, %S contains the name of the user allowed to access this share.

create mode
This option defines the file permissions used when a client creates a file within the homes share.

directory mode
This option defines the permissions used when a client creates a directory within the homes share.

Sharing directories through Samba

To share a directory through Samba, create a share section in smb.conf that describes the directory and the conditions under which it will be shared. To share a new directory named /usr/doc/pcdocs and the /home/research directory used in the NFS examples, add the following two share sections to the sample smb.conf file:

[pcdocs]
     comment = PC Documentation
     path = /usr/doc/pcdocs
     browseable = yes
     writable = no
     public = yes
 
[research]
     comment = Research Deptment Shared Directory
     path = /home/research
     browseable = no
     writable = yes
     create mode = 0750
     hosts allow = horseshoe,jerboas,crab,rodent

Each share section is labeled with a meaningful name. This name is displayed as a folder in the Network Neighborhood window on client PCs. The example contains some commands we have already covered and a few new commands. The first new command is path, which defines the path of the directory being offered by this share.

The pcdocs share also contains the command public, which grants everyone access, even if they don't have a valid username or password. These public users are granted "guest account" access to the share. On a Linux system, this means they run as user nobody and group nobody and are limited to world permissions.

Files may be written to the research share. The create mode command controls the Unix permissions used when a client writes a file to the share. In the example, the permission 0750 specifies that files will be created as read/write/execute for the owner, read/execute for the group, and no permissions for the world. A related command, directory mode, defines the permission used when a client creates a directory within a share. For example:

directory mode = 0744

This sets the permissions for new directories to read/write/execute for the owner, read/execute for the group, and read/execute for the world. This is a reasonable setting that allows cd and ls to work as expected.

The research share section also contains a hosts allow command, which defines the clients that are allowed to access this share. Even if a user has the correct username and password, that user is allowed to access this share only from the specified hosts. By default, all hosts are granted access, and specific access is controlled by the username and password.

In addition to the hosts allow command, there is a hosts deny command that defines computers that are explicitly denied access to the share. Its syntax is similar to that of the hosts allow command.

Combining these two new share sections with the section that came with the Red Hat configuration creates a server that provides access to user home directories, to public directories, and to private directories limited to members of a group. This provides the same services as NFS in a manner that is simpler for Microsoft Windows clients to use. Samba can also be used to share printers with Windows clients.

Sharing printers through Samba

Shared printers are configured through the smb.conf file. The Red Hat system comes with a smb.conf file that is preconfigured for sharing printers. The following lines occur right after the global and homes sections in the Red Hat smb.conf file:

[printers]
   comment = All Printers
   path = /var/spool/samba
   browseable = no
   guest ok = no
   writable = no
   printable = yes

The printcap and load printers lines in the global section prepare the server to share the printers defined in the printcap file. This printers section makes those printers available to the clients in a manner similar to the way the homes section makes every home directory available to the appropriate user. The Red Hat printers share section contains five parameters.

Three of the parameters, comment, browseable, and path, were explained previously. Here, however, path does not define the path of a shared file. Instead, it defines the path of the spool directory for the SMB printers.

We introduce two new parameters in this configuration, the first of which is printable, which identifies this share as a printer. The default for this option is no, meaning that by default, shares are considered to be file shares instead of printer shares. To create a printer share, set this option to yes. Setting printable = yes permits clients to write printer files to the spool directory defined by the path option. Use a create mode command to limit the permissions of the files created by clients in the spool directory. For example, create mode = 0700.

The other new line, guest ok, defines whether or not guest accounts are permitted access to the resource. This is exactly the same as the public option discussed earlier, so these two options are used interchangeably. no means that the user nobody cannot send a print job to the printer. A user must have a valid user account to use the printer. This is designed to prevent guest users from abusing the printer, but it is also useful to have a valid username for sorting out print jobs if you use banner pages and accounting on the server.

Generally, a print server offers all of its printers to all of its clients. However, individual share sections can be created for each printer in the same way that they are created for file sharing. If you don't want to share every printer, remove the printers section, set the load printers option to no, and add individual share sections for just those printers that you want to share.

An smb.conf file with a share section for a specific printer might contain:

[global]
   workgroup = BOOKS
   server string = Print Server
   load printers = no  
   security = user
[homes]
   comment = Home Directories
   browseable = no
   writable = yes
[hp5m]
   comment = PostScript Laser Printer
   path = /var/spool/samba
   browseable = no
   public = no
   create mode = 0700
   printable = yes
   printer = lp

This sample file has no printers section. Instead, a share section named hp5m is added that shares a printer named lp. The printer name must be found in the printcap file for this to work. The printcap option is allowed to default to /etc/printcap.

smbd is the component of Samba that provides file and printer sharing. The other component of Samba is nmbd.

NetBIOS Name Service

The NetBIOS name server daemon (nmbd) is the part of the basic Samba software distribution that turns a Unix server into a NetBIOS name server (NBNS). nmbd can handle queries from LanManager clients, and it can be configured to act as a Windows Internet Name Server (WINS).

nmbd is configured in the global section of the smb.conf file. The options that relate to running WINS are:

wins support
Set to yes or no. This option determines whether or not nmbd runs as a WINS server. no is the default, so by default, nmbd provides browsing controls but does not provide WINS service.

dns proxy
Set to yes or no. This option tells nmbd to use DNS to resolve WINS queries that it cannot resolve any other way. This is significant only if nmbd is running as a WINS server. The default is yes. DNS can help with NetBIOS name resolution only if NetBIOS names and DNS hostnames are the same.

wins server
Set to the IP address of an external WINS server. This option is useful only if you're not running a WINS server on your Linux system. This option tells Samba the address of the external WINS server to which it should send NetBIOS name queries.

wins proxy
Set to yes or no. The default is no. When set to yes, nmbd resolves broadcast NetBIOS name queries by turning them into unicast queries and sending them directly to the WINS server. If wins support = yes is set, these queries are handled by nmbd itself. If instead wins server is set, these queries are sent to the external server. The wins proxy option is needed only if clients don't know the address of the server or don't understand the WINS protocol.

The NetBIOS name server is generally started at boot time with the following command:

nmbd -D

When started with the -D option, nmbd runs continuously as a daemon listening for NetBIOS name service requests on port 137. The server answers requests using registration data collected from its clients and the NetBIOS name-to-address mappings it has learned from other servers.

The lmhosts file is used to manually map addresses when that is necessary. Most WINS servers do not need an lmhosts file because the servers learn address mappings dynamically from clients and other servers. NetBIOS names are self-registered; clients register their NetBIOS names with the server when they boot. The addresses and names are stored in the WINS database, wins.dat. The lmhosts file is only a small part of the total database.

The lmhosts file is similar to the hosts file described in Chapter 4. Each entry begins with an IP address that is followed by a hostname. However, this time, the hostname is the NetBIOS name. Here is a sample lmhosts file:

$ cat /etc/lmhosts
172.16.12.3       horseshoe
172.16.12.1       crab
172.16.12.2       rodent
172.16.12.4       jerboas

Given this lmhosts file, the NetBIOS name rodent maps to IP address 172.16.12.2. Notice that these NetBIOS names are the same as the TCP/IP hostnames assigned to these clients. Use the same hostnames for both NetBIOS and TCP/IP. Doing otherwise limits configuration choices and creates confusion.

Network Information Service

The Network Information Service (NIS)[8] is an administrative database that provides central control and automatic dissemination of important administrative files. NIS converts several standard Unix files into databases that can be queried over the network. The databases are called NIS maps. Some maps are created from files that you're familiar with from system administration, such as the password file (/etc/passwd) and the groups file (/etc/group). Others are derived from files related to network administration:

/etc/ethers
Creates the NIS maps ethers.byaddr and ethers.byname. The /etc/ethers file is used by RARP (see Chapter 2).

/etc/hosts
Creates the maps hosts.byname and hosts.byaddr (see Chapter 3).

/etc/networks
Creates the maps networks.byname and networks.byaddr (see Chapter 3).

/etc/protocols
Creates the maps protocols.byname and protocols.byaddr (see Chapter 2).

/etc/services
Creates a single map called services.byname (see Chapter 2).

/etc/aliases
Defines electronic mail aliases and creates the maps mail.aliases and mail.byaddr (see Chapter 10).

Check the maps available on your server with the ypcat -x command. This command produced the same map list on both our Solaris and Linux sample systems. Your server may display a longer list. Here is the list from a Linux system: 

% ypcat -x 
Use "passwd"    for map "passwd.byname" 
Use "group"     for map "group.byname" 
Use "networks"  for map "networks.byaddr" 
Use "hosts"     for map "hosts.byname" 
Use "protocols" for map "protocols.bynumber" 
Use "services"  for map "services.byname" 
Use "aliases"   for map "mail.aliases"
Use "ethers"    for map "ethers.byname"

NIS allows these important administrative files to be maintained on a central server yet remain completely accessible to every workstation on the network. All of the maps are stored on a master server that runs the NIS server process ypserv. The maps are queried remotely by client systems. Clients run ypbind to locate the server.

The NIS server and its clients are a NIS domain, a term NIS shares with DNS. The NIS domain is identified by a NIS domain name. The only requirement for the name is that different NIS domains accessible through the same local network must have different names. Although NIS domains and DNS domains are distinct entities, Sun recommends using the DNS domain name as the NIS domain name to simplify administration and reduce confusion.

NIS uses its domain name to create a directory within /var/yp where the NIS maps are stored. For example, the DNS domain of our imaginary network is wrotethebook.com, so we also use this as our NIS domain name. NIS creates a directory named /var/yp/wrotethebook.com and stores the NIS maps in it.

While the NIS protocols and commands were originally defined by Sun Microsystems, the service is now widely implemented. To illustrate this, the majority of examples in this section come from Linux, not from Solaris. The syntax of the commands is very similar from system to system.

The command domainname checks or sets the NIS domain name. The superuser can make wrotethebook.com the NIS domain name by entering:

 # domainname wrotethebook.com

The NIS domain name is normally configured at startup by placing the domainname command in one of the startup files. On many systems, the NIS domain name that is used as input to a domainname command is placed in a second file. For example, on Solaris systems, the value for the NIS domain name is taken from the /etc/defaultdomain file. As shown here, defaultdomain contains only the name of the NIS domain: 

% cat /etc/defaultdomain
wrotethebook.com

On Red Hat Linux systems, the NIS domain name is just one of the values in the /etc/sysconfig/network file:

$ cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=jerboas.wrotethebook.com
NISDOMAIN=wrotethebook.com

Initialize the NIS server and build the initial maps with make. The /var/yp/Makefile contains the instructions needed to build the maps. As noted above, it creates a directory using the NIS domain name. The Makefile reads the files in the /etc directory and places maps created from them in the new directory. To initialize a Linux system as a NIS server: 

# domainname wrotethebook.com 
# cd /var/yp  
# make 
make[1]: Entering directory '/var/yp/wrotethebook.com' 
Updating hosts.byname... 
Updating hosts.byaddr... 
Updating networks.byaddr... 
Updating networks.byname... 
Updating protocols.bynumber... 
Updating protocols.byname... 
Updating rpc.byname... 
Updating rpc.bynumber... 
Updating services.byname... 
Updating passwd.byname... 
Updating passwd.byuid... 
Updating group.byname... 
Updating group.bygid... 
Updating netid.byname...
make[1]: Leaving directory '/var/yp/wrotethebook.com'

After initializing the maps, start the NIS server process ypserv and the NIS binder process ypbind:[9] 

# ypserv
# ypbind

Our system is now running as both a NIS server and a NIS client. A quick test with ypwhich shows that we are bound to the correct server. Use ypcat or ypmatch to test that you can retrieve data from the server. We use ypcat in the following example: 

# ypwhich 
localhost 
# ypcat hosts 
172.16.55.105            cow cow.wrotethebook.com 
172.16.55.106            pig pig.wrotethebook.com 
172.16.26.36             island.wrotethebook.com island
127.0.0.1                localhost

The clients need only to define the correct domain name and to run the binder software ypbind: 

# domainname wrotethebook.com
# ypbind

Most NIS clients use ypbind to locate the server. Using the NIS domain name, ypbind broadcasts a request for a server for that domain. The first server that responds is the server to which the client "binds." The theory is that the server that responds quickest is the server with the least workload. Generally this works well. However, it is possible for the client to bind to an inappropriate system, e.g., a system that was accidentally configured to run ypserv or one that was maliciously configured to be a false server. Because of this possibility, some systems allow you to explicitly configure the server to which the client binds. Linux provides the /etc/yp.conf file for this purpose. The syntax of the entries in different versions of this file varies, so see your system documentation before attempting to use it.

Place the NIS domain name in the appropriate startup file so that the NIS setup survives the boot. The ypbind and ypserv commands are probably already in a startup file. On a Red Hat Linux NIS system, ypbind and ypserv have their own scripts in the /etc/init.d directory. In addition to putting a value for NISDOMAIN in /etc/sysconfig/network, use the chkconfig command to make sure the ypbind and the ypserv scripts run at boot time.

NIS is a possible alternative to DNS, but most systems use both NIS and DNS. Hostnames can be converted to IP addresses by DNS, NIS, and the host file. The order in which the various sources are queried is defined in the nsswitch.conf file.

The nsswitch.conf file

The Name Service Switch file (nsswitch.conf) defines the order in which the sources of information are searched. Despite its name, it applies to more than just name service. All of the databases handled by NIS are covered by the nsswitch.conf file, as shown in this example: 

hosts:      dns  nis  files 
networks:   nis  [NOTFOUND=return]  files 
services:   nis  files
protocols:  nis  files

The first entry in the file says that a hostname lookup is first passed to DNS for resolution; if DNS fails to find a match, the lookup is then passed to NIS and finally looked up in the hosts file. The second entry says that network names are looked up through NIS. The [NOTFOUND=return] string says to use the networks file only if NIS fails to respond, that is, if NIS is down. In this case, if NIS answers that it cannot find the requested network name, terminate the search. The last two entries search for services port and protocol numbers through NIS and then in the files in the /etc directory.

NIS+

Before leaving the topic of NIS, I should say a word about NIS+. It will be a short discussion, because I do not use NIS+ and do not know much about it.

NIS+ replaces NIS on Sun systems. It is not a new version of NIS, but a completely new software product that provides all the functionality of NIS and some new features. The new features are:

Clearly, NIS+ has some excellent new features and advantages over NIS. So why don't I use it? Good question! The hierarchical architecture and enhanced data structures are important if you have a very large network and lots of data in your namespace. However, many sites evolved using NIS on local subnets and do not see the need to move the entire enterprise under NIS+. Improved security seems like a real winner, but sites with low security requirements don't see the need for additional security, and sites with high security requirements may already be behind a firewall that blocks external NIS queries. Additionally, NIS+ is not available for as many operating systems as NIS. And finally, other directory services, such as LDAP, that provide similar services and are more widely available have overtaken NIS+. Taken together, these reasons have slowed the move to NIS+.

To learn more about NIS+ and how to install it on your system, read the NIS+ Transition Guide, the Name Service Configuration Guide, and the Name Service Administration Guide. All of these are available from Sun as part of the Solaris System and Network Administration manual set.

NIS and NIS+ provide a wide range of system configuration information to their clients. However, they cannot provide all the information needed to configure a TCP/IP system. In the next two sections, we look at configuration servers that can do the entire job.

DHCP

Bootstrap Protocol (BOOTP) was the first comprehensive configuration protocol. It provides all of the information commonly used to configure TCP/IP, from the client's IP address to what print server the client should use. BOOTP was simple and effective; so effective, in fact, that it became the basis for Dynamic Host Configuration Protocol (DHCP). DHCP operates over the same UDP ports, 67 and 68, as BOOTP. It provides all of the services of BOOTP as well as some important extensions. Dynamic Host Configuration Protocol provides three important features:

Backward compatibility with Bootstrap Protocol
A DHCP server can support BOOTP clients. Properly configured, a DHCP server can support all of your clients.

Full configurations
A DHCP server provides a complete set of TCP/IP configuration parameters. (See Appendix D for a full list.) The network administrator can handle the entire configuration for the users.

Dynamic address assignments
A DHCP server can provide permanent addresses manually, permanent addresses automatically, and temporary addresses dynamically. The network administrator can tailor the type of address to the needs of the network and the client system.

In this section we configure a DHCP server that supports BOOTP clients, performs dynamic address allocation, and provides a wide range of configuration parameters for its clients.

Several implementations of DHCP are available for Unix systems. Some are commercial packages and some run on a specific version of Unix. We use the Internet Software Consortium (ISC) Dynamic Host Configuration Protocol Daemon (dhcpd). It is freely available over the Internet and runs on a wide variety of Unix systems, including both our Linux and Solaris sample systems. (See Appendix D for information on downloading and compiling dhcpd.) If you use different DHCP server software, it will have different configuration commands, but it will probably perform the same basic functions.

dhcpd.conf

dhcpd reads its configuration from the /etc/dhcpd.conf file. The configuration file contains the instructions that tell the server what subnets and hosts it services and what configuration information it should provide them. dhcpd.conf is an ASCII text file that is similar to a C language source file. The easiest way to learn about the dhcpd.conf file is to look at a sample: 

# Define global values that apply to all systems. 
  
default-lease-time 86400; 
max-lease-time 604800; 
get-lease-hostnames true; 
option subnet-mask 255.255.255.0; 
option domain-name "wrotethebook.com"; 
option domain-name-servers 172.16.12.1, 172.16.3.5; 
option lpr-servers 172.16.12.1; 
option interface-mtu 1500; 
  
# Identify the subnet served, the options related 
# to the subnet, and the range of addresses that 
# are available for dynamic allocation. 
  
subnet 172.16.3.0 netmask 255.255.255.0 { 
    option routers 172.16.3.25; 
    option broadcast-address 172.16.3.255; 
    range 172.16.3.50 172.16.3.250; 
} 
  
subnet 172.16.12.0 netmask 255.255.255.0 { 
    option routers 172.16.12.1; 
    option broadcast-address 172.16.12.255; 
    range 172.16.12.64 172.16.12.192; 
    range 172.16.12.200 172.16.12.250; 
} 
  
# Identify each BOOTP client with a host statement 
  
group { 
    use-host-decl-names true; 
    host 24seven { 
        hardware ethernet 00:80:c7:aa:a8:04; 
        fixed-address 172.16.3.4; 
    } 
    host rodent { 
        hardware ethernet 08:80:20:01:59:c3; 
        fixed-address 172.16.12.2; 
    } 
    host ring { 
        hardware ethernet 00:00:c0:a1:5e:10; 
        fixed-address 172.16.3.16; 
    }
}

This sample configuration file defines a server that is connecting to and serving two separate subnets. It assigns IP addresses dynamically to the DHCP clients on each subnet and supports a few BOOTP clients. All of the lines that begin with a sharp sign (#) are comments. The first few real configuration lines in the file specify a set of parameters and options that apply to all of the subnets and clients served. The first three lines are parameters, which provide direction to the server. All three of the sample parameters define some aspect of how dhcpd should handle dynamic address assignments.

default-lease-time
Tells the server how many seconds long a default address lease should be. The client can request that the address be leased for a specific period of time. If it does, it is assigned the address for that period of time, given some restrictions. Frequently, clients do not request a specific lifetime for an address lease. When that happens, the default-lease-time is used. In the example, the default lease is set to one day (86400 seconds).

max-lease-time
Sets the upper limit for how long an address can be leased. Regardless of the length of time requested by the client, this is the longest address lease that dhcpd will grant. The life of the lease is specified in seconds. In the example here, it is one week.

get-lease-hostnames
Directs dhcpd to provide a hostname to each client that is assigned a dynamic address. Further, the hostname is to be obtained from DNS. This parameter is a Boolean. If it is set to false, which is the default, the client receives an address but no hostname. Looking up the hostname for every possible dynamic address adds substantial time to the startup. Set this to false. Set it to true only if the server handles a very small number of dynamic addresses.

The configuration file uses a few more parameters that will be explained as we go. For a complete list of all DHCP parameters, see Appendix D.

The next four lines are options. The options all start with the keyword option. The keyword is followed by the name of the option and the value assigned to the option. Options define configuration values that are used by the client.

The meanings of the sample options are easy to deduce. The option names are very descriptive. We are providing the clients with the subnet mask, domain name, domain name server addresses, and print server address. These values are similar to those that could have been provided with the old BOOTP service.

DHCP, however, can do more than BOOTP. For sake of illustration, we also define the maximum transmission unit (MTU). The sample interface-mtu option tells the client that the MTU is 1500 bytes. In this case, the option is not needed because 1500 bytes is the default for Ethernet. However, it illustrates the point that DHCP can provide a very complete set of configuration information.

The subnet statements define the networks that dhcpd serves. The identity of each network is determined from the address and the address mask, both of which are required by the subnet statement. dhcpd provides configuration services only to clients that are attached to one of these networks. There must be a subnet statement for every subnet to which the server physically connects, even if some subnets do not contain any clients. dhcpd requires the subnet information to complete its startup.

The options and parameters defined in a subnet statement apply only to the subnet and its clients. The meanings of the sample options are clear. They tell the clients what router and what broadcast address to use. The range parameter is more interesting, as it goes to the heart of one of DHCP's key features.

The range parameter defines the scope of addresses that are available for dynamic address allocation. It always occurs in association with a subnet statement, and the range of addresses must fall within the address space of the subnet. The scope of the range parameter is defined by the two addresses it contains. The first address is the lowest address that can be automatically assigned, and the second is the highest address that can be assigned. The first range parameter in the example identifies a contiguous group of addresses from 172.16.12.50 to 172.16.12.250 that are available for dynamic assignment. Notice that the second subnet statement has two range parameters. This creates two separate groups of dynamic addresses. The reason for this might be that some addresses were already manually assigned before the DHCP server was installed. Regardless of the reason, the point is that we can define a noncontiguous dynamic address space with multiple range statements.

If a range parameter is defined in a subnet statement, any DHCP client on the subnet that requests an address is granted one as long as addresses are available. If a range parameter is not defined, dynamic addressing is not enabled.

To provide automatic address assignment for BOOTP clients, add the dynamic-bootp argument to the range parameter. For example:

 range dynamic-bootp 172.16.8.10 172.16.8.50;

By default, BOOTP clients are assigned permanent addresses. It is possible to override this default behavior with either the dynamic-bootp-lease-cutoff or the dynamic-bootp-lease-length parameter. However, BOOTP clients do not understand address leases and do not know that they should renew an address. Therefore the dynamic-bootp-lease-cutoff and the dynamic-bootp-lease-length parameters are used only in special circumstances. If you're interested in these parameters, see Appendix D.

Each BOOTP client should have an associated host statement that is used to assign the client configuration parameters and options. It can be used to manually assign the client a permanent, fixed address. The sample configuration file ends with three host statements: one for 24seven, one for rodent, and one for ring. Each host statement contains a hardware parameter that defines the type of network hardware (ethernet) and the physical network address (e.g., 08:80:20:01:59:c3) used by the client. The hardware parameter is required in host statements for BOOTP clients. The Ethernet address is used by dhcpd to identify the BOOTP client. DHCP clients can also have associated host statements. For DHCP clients, the hardware parameter is optional because a DHCP client can be identified by the dhcp-client-identifier option. However, it is simpler for a DHCP client connected via Ethernet to be identified by its Ethernet address.

A wide variety of parameters and options can be defined in the host statement. For example, adding to each host statement an option similar to the following assigns each client a hostname:

option host-name 24seven;

It is often easier, however, to define options and parameters at a higher level. Global options apply to all systems. Subnet options apply to every client on the subnet, but the options defined inside a host statement apply to only a single host. The host-name option shown above would need to be repeated with a different hostname in every host statement. An easier way to define a parameter or option for a group of hosts is to use a group statement.

A group statement groups together any other statements. The sole purpose of the group statement is to apply parameters and options to all members of the group. That is exactly what we do in the example. The group statement groups all of the host statements together. The use-host-decl-names parameter in the group statement applies to every host in the group. This particular parameter tells dhcpd to assign each client the hostname that is declared on the host statement associated with that client, which makes the host-name option unnecessary for this configuration.

Given the sample dhcpd.conf file shown earlier, when dhcpd receives a request packet from a client with the Ethernet address 08:80:20:01:59:c3, it sends that client:

The client receives all global values, all subnet values, and all host values that are appropriate. Clearly, DHCP can provide a complete configuration.

Your DHCP configuration, though larger in the number of systems supported, probably is simpler than the example. Some commands appear in the sample primarily for the purpose of illustration. The biggest difference is that most sites do not serve more than one subnet with a single configuration server. Servers are normally placed on each subnet. This reduces the burden on the server, particularly the burden that can be caused by a network-wide power outage. It eliminates the need to move boot packets through routers. Also, the fact that addresses are assigned at the subnet level makes placing the assigning system at the subnet level as well somehow more logical. DHCP servers are not the only servers that work best when located close to the clients. In the next section we look at how to keep distributed servers updated.

Managing Distributed Servers

Large networks have multiple servers. As noted earlier, the servers are often distributed around the network with a server on every subnet. This improves network efficiency, but it conflicts with the goal of central configuration control. The more servers you have, the more dispersed the control, and the more likely that a configuration error will occur. Implementing distributed servers requires a technique for maintaining central control and coordinating configuration information among the servers. TCP/IP offers several techniques for doing this.

Any file transfer protocol can be used to move configuration data or any other kind of data from a central system to a group of distributed systems. Either FTP or TFTP will work, but both present difficulties when used in this way. FTP and TFTP are interactive protocols, and require multiple commands to retrieve a file, making them difficult to script. Additionally, FTP requires password authentication before it grants access to a file, and most security experts frown on storing passwords in scripts. For these reasons, we don't concentrate on using these protocols to distribute the configuration file. Besides, if you know how to use FTP (and you should!), you know how to use it to send a configuration file.

Another possibility is to use NFS to distribute the information. NFS allows files on the server to be used by clients as if they are local files. It is a powerful tool, but it does have limitations when used to distribute configuration information to boot servers. The same power outage that affects the distributed servers can cause the central server to crash. The distributed servers and their clients can be delayed in booting while waiting for the central server to come back online. Sharing a single copy of the configuration file conflicts with the effort to distribute boot services because it puts too much reliance on the central server.

One way to avoid this problem is for the distributed servers to periodically copy the configuration file from the mounted filesystem to a local disk. This is very simple to script, but it creates the possibility that the servers will be "out of sync" at certain times--the distributed servers copy the configuration file on a periodic schedule without knowing if, in the interim, the master file has been updated. Of course, it is possible for all of the remote servers to export filesystems that the central server mounts. The central server can then copy the configuration file directly to the remote filesystems whenever the master file is updated. However, there are easier ways to do this.

The Unix r-commands rcp and rdist provide the most popular methods for distributing the configuration file.

rcp

Remote copy (rcp) is simply a file transfer protocol. It has two advantages over FTP for this particular application: it is easy to script and it does not require a password. rcp is easy to script because only a single line is needed to complete a transfer. An example of transferring the file dhcpd.conf from the master server to a remote server named arthropod.wrotethebook.com is:

# rcp /etc/dhcpd.conf arthropod.wrotethebook.com:/etc/dhcpd.conf

For every remote server that the file is sent to, add a line like this one to the procedure that updates the master configuration file.

rcp is only one choice for distributing the central configuration file. rdist, while a little harder to use, is often a better choice because it has several features that make it particularly well suited for this application.

rdist

The Remote File Distribution Program (rdist) is designed to maintain identical copies of files on multiple hosts. A single rdist command can distribute several different files to many different hosts. It does this by following the instructions stored in an rdist configuration file called a Distfile.

The function of a Distfile is similar to that of the Makefile used by the make command, and it has a similar syntax and structure. Now, don't panic! It's not that bad. The initial configuration of an rdist command is more difficult than the straightforward syntax of an rcp command, but the rdist command provides much more control and is much easier to maintain in the long run.

A Distfile is composed of macros and primitives. Macros can be assigned a single value or a list of values. If a list of values is used, the list is enclosed in parentheses, e.g., macro = ( value value ). Once assigned a value, the macro is referenced using the syntax ${macro}, where macro is the name of the macro. The other components of a Distfile, the primitives, are explained in Table 9-4.[10]

Table 9-4: rdist primitives

Primitive

Function

install

Recursively updates files and directories.

notify address

Sends error/status mail messages to address.

except file

Omits file from the update.

except_pat pattern

Omits filenames that match the pattern.

special "command "

Executes command after each file update.

The simplest way to understand how the primitives and macros are combined to make a functioning Distfile is to look at a sample. The following configuration file distributes the current version of dhcpd and the latest dhcpd.conf configuration file to the remote boot servers horseshoe, arthropod, and limulus: 

HOSTS = ( horseshoe root@limulus arthropod ) 
FILES = ( /usr/sbin/dhcpd /etc/dhcpd.conf ) 
  
${FILES} -> ${HOSTS} 
       install ;
       notify craig@crab.wrotethebook.com

Let's look at each line of the file:

HOSTS = ( horseshoe root@limulus arthropod )
This line defines HOSTS, a macro that contains the hostname of each of the remote servers. Notice the entry for limulus. It tells rdist to log in as root on limulus to perform the update. On horseshoe and arthropod, rdist will run under the same username it has on the local host.

FILES = ( /usr/sbin/dhcpd /etc/dhcpd.conf )
This macro, FILES, defines the two files that will be sent.

${FILES} -> ${HOSTS}
The -> symbol has a special meaning to rdist. It tells rdist to copy the files named at the left of the symbol to the hosts named at the right. In this case, FILES is a macro that contains the file names /usr/sbin/dhcpd and /etc/dhcpd.conf, and HOSTS is a macro that contains the hostnames horseshoe, limulus, and arthropod. Therefore this command tells rdist to copy two files to three different hosts. Any primitives that follow apply to this file-to-host mapping.

install ;
The install primitive explicitly tells rdist to copy the specified files to the specified hosts if the corresponding file is out of date on the remote host. A file is considered out of date if the creation date or the size is not the same as the master file. The semicolon at the end of this line indicates that another primitive follows.

notify craig@crab.wrotethebook.com
Status and error messages are to be mailed to .

Additional files and hosts can be easily added to this file. In the long run, most people find rdist the simplest way to distribute multiple files to multiple hosts.

One final note: the configuration file does not have to be called Distfile. Any filename can be specified on the rdist command line using the -f option. For example, the Distfile shown above could be saved under the name dhcp.dist and invoked with the following command:

% rdist -f dhcp.dist

Post Office Servers

In this section we configure a system to act as a post office server. A post office server, or mailbox server, is a computer that holds mail for a client computer until the client is ready to download it for the mail reader. This service is essential to support mobile users and small systems that are frequently offline and thus not able to receive mail in real time. We look at two techniques for creating a post office server: Post Office Protocol (POP), which is the original protocol for this purpose, and Internet Message Access Protocol (IMAP), which is a popular alternative. We start with POP.

POP Server

A Unix host turns into a Post Office Protocol server when it runs a POP daemon. Check your system's documentation to see if a POP daemon is included in the system software. If it isn't clear from the documentation, check the inetd.conf or xinetd.conf file, or try the simple telnet test from Chapter 4. If the server responds to the telnet test, not only is the daemon available on your system, it is installed and ready to run. 

% telnet localhost 110 
Trying 127.0.0.1 ... 
Connected to localhost. 
Escape character is ' ]'. 
+OK POP3 crab Server (Version 1.004) ready 
quit 
+OK POP3 crab Server (Version 1.001) shutdown
Connection closed by foreign host.

This example is from a system that comes with POP3 ready to run. The Red Hat Linux system includes POP3, although it must be enabled in the /etc/xinetd.d/pop3 file before it can be used. The Solaris system, on the other hand, does not ship with POP2 or POP3. Don't worry if your system doesn't include this software. POP3 software is available from several sites on the Internet where it is stored in both the popper17.tar and the pop3d.tar files. I have used them both, and both work fine.

If you don't have POP3 on your system, download the source code. Extract it using the Unix tar command. pop3d.tar creates a directory called pop3d under the current directory, but popper17.tar does not. If you decide to use popper, create a new directory before extracting it with tar. Edit the Makefile to configure it for your system and do a make to compile the POP3 daemon. If it compiles without errors, install the daemon in a system directory.

On a Solaris system, POP3 is started by the Internet daemon, inetd. Start POP3 from inetd by placing the following in the inetd.conf file:

pop3   stream  tcp     nowait  root    /usr/sbin/pop3d              pop3d

This entry assumes that you are using pop3d, that you placed the executable in the /usr/sbin directory, and that the port for this daemon is identified in the /etc/services file by the name pop3. If these things aren't true, adjust the entry accordingly.

Make sure that POP3 is actually defined in /etc/services. If it isn't, add the following line to that file:

pop3         110/tcp              # Post Office Version 3

Once the lines are added to the services file and the inetd.conf file, send a SIGHUP to inetd to force it to read the new configuration, as in this example: 

# ps -ef | grep inetd 
  root  109  1  0   Jun 09 ?   0:01 /usr/sbin/inetd -s
# kill -HUP 109

Now that POP3 is installed, rerun the test using telnet localhost pop3. If the POP3 daemon answers, you're in business. All users who have a valid user account on the system are now able to download mail via POP3 or read the mail directly on the server.

IMAP Server

Internet Message Access Protocol (IMAP) is an alternative to POP. It provides the same basic service as POP and adds features to support mailbox synchronization, which is the ability to read mail on a client or directly on the server while keeping the mailboxes on both systems completely up to date. On an average POP server, the entire contents of the mailbox are moved to the client and either deleted from the server or retained as if never read. Deletion of individual messages on the client is not reflected on the server because all the messages are treated as a single unit that is either deleted or retained after the initial transfer of data to the client. IMAP provides the ability to manipulate individual messages on either the client or the server and to have those changes reflected in the mailboxes of both systems.

IMAP is not a new protocol; it is about as old as POP3. There have been four distinct versions: IMAP, IMAP2, IMAP3, and the current version, IMAP4, which is defined in RFC 2060. IMAP is popular because of the importance of email as a means of communicating, even when people are out of the office, and the need for a mailbox that can be read and maintained from anywhere.

Solaris 8 does not include IMAP. IMAP binaries for Solaris are available from http://sunfreeware.com. IMAP source code can be obtained via anonymous FTP from ftp.cac.washington.edu. Download /mail/imap.tar.Z from ftp.cac.washington.edu as a binary image. Uncompress and untar the file. This creates a directory containing the source code and Makefile needed to build IMAP.[11] Read the Makefile carefully. It supports many versions of Unix. If you find yours listed, use the three-character operating system type listed there. For a Solaris system using the gcc compiler, enter: 

# make gso

If it compiles without error, as it does on our Solaris system, it produces three daemons: ipop2d, ipop3d, and imapd. We are familiar with installing POP3. The new one is imapd. Install it in /etc/services: 

imap      143/tcp       # IMAP version 4

Also add it to /etc/inetd:

imap  stream  tcp  nowait  root  /usr/sbin/imapd  imapd

Now basic IMAP service is available to every user with an account on the server.

A nice feature of the University of Washington package is that it provides implementations of POP2 and POP3 as well as IMAP. This is important because many email clients run POP3. The IMAP server can be accessed only by an IMAP client. Installing POP3 along with IMAP gives you the chance to support the full range of clients.

Most Linux systems include IMAP, so compiling the source code is not a requirement. Simply make sure that the service is listed in the /etc/services file and available through inetd or xinetd. On Red Hat Linux 7, the /etc/xinetd.d/imap file is disabled by default and must be enabled to allow clients access to the service.

POP and IMAP are important components of a mail service. However, there is a great deal more to configuring a complete email system, as we will see in the next chapter.

Summary

This chapter covered several important TCP/IP network services.

Network File System (NFS) is the leading TCP/IP file-sharing protocol for Unix systems. It allows server systems to share directories with clients that are then used by the clients as if they were local disk drives. NFS uses trusted hosts and Unix UIDs and GIDs for authentication and authorization.

Unix printer sharing is available on a TCP/IP network through the use of the Line Printer Daemon (LPD) or the Line Printer (LP) server. The lpd software is originally from BSD Unix but is widely available. The lpd program reads the printer definitions from the printcap file. The LP software is originally from System V. It uses terminfo for printer capabilities and the /etc/lp directory to configure individual printers. Solaris 8 printer sharing is based on the LP software but it configures printers in a single file, /etc/printers.conf.

Windows PCs use NetBIOS and Server Message Block (SMB) protocol for file and printer sharing. Unix systems can act as SMB servers by using the Samba software package. Samba provides file and printer sharing in a single package that is configured through the smb.conf file.

Network Information Service (NIS) is a server that distributes several system administration databases. It allows central control and automatic distribution of important system configuration information.

Dynamic Host Configuration Protocol (DHCP) extends BOOTP to provide the full set of configuration parameters defined in the Requirements for Internet Hosts RFC. It also provides for dynamic address allocation, which allows a network to make maximum use of a limited set of addresses.

Large networks use distributed boot servers to avoid overloading a single server and to avoid sending boot parameters through IP routers. The configuration files on distributed boot servers are kept synchronized through file transfer, NFS file sharing, or the Remote File Distribution Program (rdist).

Post Office Protocol (POP) and Internet Message Access Protocol (IMAP) servers allow email to be stored on the mail server until the user is ready to read it. In the next chapter, we take a closer look at configuring an electronic mail system as we explore sendmail.

1. Solaris uses the term sharing. Most other systems use the term exporting.

2. Alternatively, the prefix rpc. may be used on the daemon names. For example, the Slackware Linux system uses the filename rpc.nfsd for the NFS daemon. Check your system's documentation.

3. Of the eight options discussed in this section, three, squash_uid, squash_gid, and map_static, map a range of UIDs and GIDs. These three options are not available in the kernel-level NFS (knfsd) used on some Linux systems. Mapping for knfsd must be done with the other options.

4. Red Hat Linux uses a special script, /etc/init.d/netfs, just for mounting all of the different networked filesystems, which include NFS.

5. grep is used because the fstab file contains other information not related to NFS.

6. See the description of NFS mount options earlier in this chapter.

7. Samba has about 20 different variables. See the manpage for a full list.

8. NIS was formerly called the "Yellow Pages," or yp. Although the name has changed, the abbreviation yp is still used.

9. If, during the initial build of the NIS maps, make complains that ypserv is not registered, run ypserv before running make.

10. For more details, see the rdist manpage.

11. The name of the directory tells you the current release level of the software. At this writing, it is imap-2001.

Back to: TCP/IP Network Administration, 3rd Edition

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