2004.05_Configuring a Cups Print Server.pdf

COVER STORY

CUPS network printing

Printing by numbers

Sharing a printer among multiple

users can help save money and

resources. Connect your printers to a

CUPS server to give access to Apple or

even Microsoft clients.

BY TIM SCHÜRMANN

been replaced by newer desk-

tops make ideal print servers on

a network. A print server not only allows

multiple users to share a printer, but a

dedicated print PC also offloads some of

the work from your own desktop, allow-

ing it to proceed with its own tasks at top

speed. On some home networks, differ-

ent users will send output to different

printers. While dad is busy printing out a

mail merge document on a laser printer,

the kids might be printing the pics from

the last party on their inkjet. This sce-

nario is not far removed from the

printing environments in most enter-

prises, where the Common Unix Printing

System (CUPS), gives Linux, Microsoft

and Apple clients access to any printer

on the network.

your distribution does not have CUPS, or

if you want to update the existing ver-

sion, you can download the source code

from the CUPS website [1].

Prior to CUPS, distributions used to

work with less powerful BSD or LPRng

print systems. Both of these are difficult

to integrate in a networked environment.

If you have an older distribution, you

should consider replacing the older

printing system with CUPS. The CUPS

documentation provides more informa-

tion on this subject [2].

It is not necessary to modify your

applications to use CUPS. Your programs

will either interface with CUPS natively

(this is the case for KDE applications), or

they will use the so-called System V, or

Berkley, printing commands. These are

the lp and lpr command-line tools,

which are called by the applications.

CUPS provides compatible versions of

these programs. These simple com-

mands are useful if your need to send a

file to a (remote) using only the com-

mand line. The CUPS Software Users

Manual [2] provides details on these

commands.

Printing Protocol ( IPP , [3]). IPP is an

extension of the Hyptertext Transport

Protocol (HTTP), which is used for

transferring Web pages across an Inter-

net. This also explains why IPP uses a

similar paradigm to data communica-

tions across an Internet. A computer,

known as a client in CUPS-speak, sends

printing data to a CUPS server. The dae-

mon running in the background on the

server, cupsd , accepts this data, and per-

forms some additional processing, before

sending the data to the printer for out-

put. In other words, cupsd is the core of

the CUPS system.

You can access the daemon in your

Web browser, by typing http://

localhost:631 . Simply replace localhost in

this URL with the hostname of your

CUPS computer. localhost is a place-

holder for the local machine. CUPS does

not permit external access in the default

configuration, so you will need to launch

the Web browser locally on the system

you are configuring. If you want to per-

mit external access, you will need to edit

the /etc/cups/cupsd.conf configuration

file. Look for the sections starting with

<Location /…> and ending with

</Location> (see Figure 1).

The location tags contain details on

how CUPS should control access to the

various sections. <Location /> refers to

First Steps

Tw o steps are required to run CUPS on a

network. First of all, you need to set up

the computers to which your printers are

attached. These machines can be normal

desktops or dedicated servers that do not

need a GUI. Today’s distros will typically

install CUPS as part of the original setup,

or provide configuration tools that you

can call after completing the setup. The

latter category typically sets up CUPS to

launch when you boot the system. If

Browser-Based Configuration

In contrast to the older LPRng system,

CUPS is perfectly adapted to life on the

network, as it is based on the Internet

May 2004

www.linux-magazine.com

Configuring a CUPS Print Server

O utmoded computers that have

CUPS network printing

COVER STORY

Figure 1: The Allow entry, permits the computer with the IP address

192.168.0.16 to access the Web interface remotely.

Figure 2: The welcome page of the CUPS daemon’s Web interface. This

assumes correct access privilege configuration.

the main menu, and <Location

/admin> to the Administration item.

Adding Allow From 192.168.0.16 allows

the computer with the IP address

192.168.0.16 to access the interface. For

security reasons it is not recommended

to allow cleartext passwords to cross the

wire. The CUPS FAQ [4] provides an

overview of access privileges.

The Web interface allows users to set

up and manage CUPS (see Figure 2).

Users can also view the jobs in the print

queues. The lpstat command provides

the same functionality. If your distribu-

tion has a configuration program for

setting up printers, it is preferable to use

that program. For example, Suse Linux

has the YaST tool. There is a command

line based alternative, lpadmin , which

has a few additional options compared

with the approaches mentioned so far

[2]. If you do not like any of these meth-

ods, you can still opt for one of the many

third party configuration programs.

Newer KDE versions have a component

for this task. You will find it in the Con-

trol Center under Hardware|Printers (see

Figure 3).

We will be using the Web interface in

the following examples. The entry for

Printers in the interface gives you access

to a list of print queues (see Figure 4).

Each queue has a name and individual

settings. You can assign multiple queues

to a printer or interface. The interface

that the documents in a print queue will

be sent to is defined by a Uniform

Resource Identifier (URI). The notation

looks similar to a Web page name, for

example parallel:/dev/lp0 for the first

parallel port, or usb:/dev/usb/lp0 for the

first USB printer. The lpinfo -v command

provides a list of available interfaces. We

will be returning to the subject of URIs

later on.

To create a new print queue, select

Administration in your browser, and

click on Add Printer . Then enter the

name of the print queue in the Name

field. The maximum length of the name

is 127 characters. The screens that follow

prompt you to select the interface to

which your printer is attached, the

printer model, and finally the appropri-

ate GhostScript filter.

The following steps assume that CUPS

has been set up correctly on any com-

puter to which a printer is attached

(servers). You can print a test page in the

browser window by clicking on Print-

ers|Print Test Page .

Cups Revealed

Printing is a simple thing, when you come to

think of it. A user selects Print in the menu

and the program sends the data to the

printer, which then creates a pile of paper

with the results. In fact, printing is not as

trivial as it sounds.

The first problem is the fact that the com-

puter can freeze while the print job is being

processed. Most printers simply do not have

enough memory to load a whole document.

To avoid this problem, the print system uses

a separate program, called a spooler, that

provides caching in the form of the so-called

print queue. The spooler monitors the

printer and passes on the next job, as soon

as the printer is ready for it. On a CUPS sys-

tem, cupsd , the CUPS daemon (also known

as a scheduler) takes care of this. The dae-

mon is typically launched when a machine

boots, and hangs around in the background

waiting for new jobs.

The huge variety of printer models is

another issue. Each printer uses different

control sequences, that is, it speaks a differ-

ent language. An application wanting to

send a document to the printer needs to for-

mat the data in a way that the printer

understands. Unfortunately, to do this, the

application would need to speak the same

language as all of today’s printers. As this is

unrealistic, filters translate between the

spooler and the printer. These tools translate

the incoming data into the printer language

before the job is printed. In other words,

applications output data in a standard for-

mat and pass the output on to the spooler.

Linux uses the PostScript format. Adobe [6]

developed PostScript as a special program-

ming language for graphics. PostScript

capable printers can interpret this kind of

data directly and do not need a separate fil-

ter.

Open Source programmers invented Ghost-

Script, a program that allows applications to

output the PostScript format on non-Post-

Script printers. GhostScript uses a printer

driver to convert PostScript documents to

the target language for the printer. When

faced with a printer that can’t interpret

PostScript, the CUPS spooler simply calls

GhostScript, which converts the data as

needed.

Back-ends are the last link in the chain. A

back-end represents a port such as a USB or

parallel port. Back-end definitions allow us

to add new interface types, which do not

currently exist.

www.linux-magazine.com

May 2004

COVER STORY

CUPS network printing

Figure 3: The CUPS configuration item in the KDE Control Center. Click the

empty space to display more useful features, such as a function for re-start-

ing the CUPS daemon.

Figure 4: The “Printers” entry lists the print queues that CUPS recognizes,

including their URIs (Device URI).

Client Setup

The second step is to configure the com-

puters that will be sharing your printer

(clients). There are several possible

approaches, the common denominator

being the fact that the CUPS daemon,

cupsd , needs to be running as a back-

ground task. Some distributions only

launch the daemon automatically on

booting a machine if a printer has been

set up on the machine. YaST on Suse

Linux 9 has an option to set up the

server at a later stage. To do so, select

Change|Advanced and then CUPS Server .

If you have a distro where the configura-

tion program does not allow this, write a

CUPS init script, and add this to run-

levels 3 and 5 in your /etc/init.d/

directory.

Introducing the Printer

You need to enter the details of a printer

on the client to allow the client to find

that printer on the network. To do so,

launch the Web interface as described

previously. Select the Internet Printing

Protocol (IPP) as the ( Device ). Then enter

the URI for the remote printer, for exam-

ple: ipp://myserver/printers/myprinter .

Replace myserver with the name. or IP

address, of your print server, and

myprinter with the name of your print

queue. The configuration programs pro-

vided by most distributions have similar

options (Figures 5a and 5b show Suse

Linux). Repeat this procedure for each

external print queue. Admittedly, this

approach is quite convoluted, if you

have a large number of printers.

It makes sense to configure CUPS to

use a remote print server for every job,

to avoid running the CUPS daemon on

the clients. This means modifying the

configuration file, /etc/cups/client.conf .

Look for the line that starts with Server-

Name . You may need to remove the

number sign (#). Type a space, and then

the name, or preferably the IP address, of

the server that will be processing your

print jobs after the ServerName keyword

(see Figure 6).

The third approach is probably the

most convenient. Every CUPS daemon

periodically broadcasts its configuration

across the network. Clients that receive

the signal can immediately start using

the queues. The advantage of this

approach is the extremely simple config-

Figure 5a: Setting up remote printers on Suse Linux 8.2. First, tell YaST that

you want to access a remote printer that uses IPP …

Figure 5b: … then enter the hostname and the remainder of the URI to com-

plete the configuration.

May 2004

www.linux-magazine.com

CUPS network printing

COVER STORY

uration. You simply set up the new

printer on the machine to which it is

attached to allow any machine on the

network to access that printer. Of course,

broadcasting is not the most secure

approach. Any computer that can receive

the signal can access the printer. At

worst, this would allow the whole Inter-

net to print to your printer. Browsing is

enabled in the old CUPS package, newer

versions do not broadcast by default.

Configuration tools allow you to define

broadcast options. Again, this means

using YaST on Suse Linux. The menu

items you need are located in

Change/Advanced/CUPS Server Settings

(see Figure 7). To set up a server for

broadcasting manually, modify the

/etc/cups/cupsd.conf configuration file.

Lines that start with a number sign (#)

are comments that CUPS will ignore.

Each setting occupies a single line and

consists of a keyword with a value. Refer

to [2] for an overview of the values. The

file provides short explanations and sam-

ples.

If you are configuring broadcasting,

you need the lines that start with Browse .

For example, the value for BrowseInter-

val defines the interval in seconds at

which a server will broadcast its configu-

ration data. The BrowseAddress entry is

followed by the broadcast address. For

example, BrowseAddress 192.168.0.255

sends the settings to any computers on

the local subnet 192.168.0 .

Polling is a useful alternative to brows-

ing. In this case, the client retrieves its

configuration from the server. In the

client-side cupsd.conf file, set the value

for the BrowsePoll keyword to the name

or IP address of the server. You can have

multiple BrowsePoll entries, allowing the

client to query multiple servers. Enabling

BrowseRelay tells the computer to relay

any information it has learned by polling

to all the computers on the subnet.

the daemon. The KDE Print dialog will

have a list of printers from this point

onward (see Figure 8).

If browsing does not seem to work,

check your hostname configurations.

You may need to add them to /etc/hosts

for broadcasts to work.

Integrating Apple and

Microsoft Systems

You can integrate any operating system

that supports IPP. This includes MacOS

X version 10.2 or later. If you have

MacOS, simply check Printer Sharing in

the system settings below Services . This

will display the printers as Shared print-

ers in the print dialog. Older MacOS

versions need the netatalk package. Add

an entry for each printer to the papd.conf

configuration file. Refer to [5] for more

details. The important thing here is to

ensure that you have the correct PPD file

from /etc/cups/ppd for each printer:

CUPS Reloaded

You need to restart the CUPS daemon

after changing the cupsd.conf file. The

command for doing this depends on

your distribution. Users with Suse Linux

can type /etc/init.d/cups restart . If you

used YaST to change your configuration,

the YaST tool also takes care of restarting

Description:MyPrinter@MyServer:\

:pr=|/usr/bin/lp -d MyPrinter:\

:op=deamon:\

:pd=/etc/cups/ppd/MyPrinter.ppd:

www.linux-magazine.com

May 2004

COVER STORY

CUPS network printing

Figure 6: After enabling forwarding, only the computer at IP 192.168.

0.20 will need a print daemon – any print jobs will be forwarded to

this machine.

names must be capital-

ized.

Then modify your

Samba configuration to

allow the fileserver to

export printer drivers.

The Samba documenta-

tion provides details on

this. cupsaddsmb -U

root -a will export your

printers. This will put

the drivers on your

Samba server. Note that

cupsaddsmb uses the

root account to copy the

files with the smbclient

program. Ensure that

Samba permits access to this account.

Samba has a new CUPS back-end

called smbspool that allows Linux com-

puters to access printers shared by

Windows machines. Your distribution

should set up the back-end correctly. If

not, su to root, and enter the following

command: ln -s ‘which smbspool’

/usr/lib/cups/backend/smb (backtick).

This will allow you to set up the printer

just like a remote printer on another

CUPS server. Instead of an address like

ipp://… , use a device such as smb://

workgroup/server/sharename . If the prin

ter is attached to a computer with NT, or

a Windows 9x machine with passwords,

you will also need to supply the pass-

word: smb://user:password@workgroup/

server/sharename .

be to set up access and security func-

tions. For example, CUPS allows you to

assign printer quotas and passwords.

This would not typically make sense on

your home network, unless you happen

to have an expensive color laser, that is.

Admins are well-advised to check out the

documentation.

Classes can also provide useful

options. A class is a set of printers.

When a print job is sent to a class, CUPS

prints the job on the first idle printer in

the class. Again, the Web interface pro-

vides a convenient simple approach to

setting up classes via the Classes menu

entry.

The documentation provided by your

distribution may contain some useful

troubleshooting information; if not,

check out the detailed online documen-

tation for CUPS at [2]. The protocol files

below /var/log/cups can also provide

useful hints.

Windows 2000/XP also has native IPP

support. Any other Windows versions

need a working Samba version 2.0.6 or

later. Add the following two lines to the

[Global] section of the smb.conf configu-

ration file: printing = cups and printcap

name = cups . Current distributions

should have this configuration by

default. The Windows client needs a

printer driver capable of generating Post-

Script output, such as the Apple

LaserWriter.

If you prefer to use the original printer

driver, you will need to set up a new

print queue on your CUPS server. Select

the raw device as the printer model. This

tells CUPS to send incoming data directly

to the output port. Note that computers

without the original driver will not be

able to produce useful results on your

printer.

Samba 2.2 or later automatically

exports the appropriate printer driver

when a Windows client

attempts to print. cup-

saddsmb, as used in the

following examples,

can only handle the

Adobe [6] PostScript

drivers, or the drivers

from its own home-

page. The Adobe dri-

vers are available as

EXE files only. You will

need to run a program

such as Winzip to

extract them on a Win-

dows machine first.

Store the driver files in

the /usr/share/cups/dri-

vers directory. The file

■

Figure 8: After completing the setup, the KDE

print dialog should have a list of remote printers

and their details.

Glossary IPP

Expert Options

This article only deals with setting up

CUPS on a network. The next step would

The Internet Printing Protocol was created

by the Printer Working Group. Although it is

based on HTTP 1.1, it listens on port 631

(rather than port 80). In fact, the two proto-

cols are so closely related that IPP

implementations like cups can be accessed

natively via HTTP. The printer URI would be

HTTP://servername:631/... rather than IPP://

servername/... in this case.

INFO

Figure 7: Enable browsing in Suse’s YaST. In our example, details of the

attached printers are broadcast to the 192.168.0 subnet.

[1] CUPS project: http://www.cups.org

[2] CUPS documentation:

http://localhost:631

[3] IPP: http://www.pwg.org/ipp/

[4] CUPS FAQ:

http://www.danka.de/printpro/faq.html

[5] Netatalk: http://netatalk.sourceforge.net

[6] Adobe: http://www.adobe.com

May 2004

www.linux-magazine.com

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: