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<FONT SIZE=4 COLOR="#FF0000"><B>The </B><B>lpd</B><B> Printing Daemon</B></FONT></CENTER></H4>

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<P>All printing on the Linux system is handled by the lpd daemon, which is usually started when the system boots. During the startup process, the lpd daemon reads through the file /etc/printcap to identify the sections that apply to any of the printers 
known to be attached to the system. The lpd daemon uses two other processes, called listen and accept, to handle incoming requests for printing and to copy them to a spooling area.

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<P>In most cases, you won't have to modify the lpd daemon. However, there might be times when you have to stop it manually and restart it. The command to load lpd is

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<FONT COLOR="#000080">lpd [-l] [port]</FONT></PRE>

<P>The -l option invokes a logging system that notes each print request. This option can be useful when you're debugging the printer system. The port number allowed in the lpd command line is used to specify the Internet port number if the system 
configuration information is to be overridden. You will probably never have to use it.

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<P>The size of the print spool area is set by an entry in the file minfree in each spool directory (each printer has its own spool directory). The contents of minfree show the number of disk blocks to keep reserved so that spooling large requests doesn't 
fill up the hard drive. The contents of the file can be changed with any editor.

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<P>Access to the lpd daemon to allow printing of a user request must pass a quick validation routine. Two files are involved: /etc/hosts.equiv and /etc/hosts.lpd. If the machine name of the sending user is not in either file, the print requests are 
refused. Because the local machine is always in hosts.equiv (as localhost), users on the Linux machine should always have their print requests granted.

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<FONT SIZE=4 COLOR="#FF0000"><B>Following a Print Request</B></FONT></CENTER></H4>

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<P>To understand how the print daemon works, as well as how print requests are managed by Linux, it is instructive to follow a print request. When a user requests a print job with the lpr command, lpr assembles the data to be printed and copies it into the 
spooling queue, where lpd can find it.

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<NOTE>The lpr program is the only one in the Linux system that can actually queue files for printing. Any other program that offers printing capabilities does so by calling lpr.</NOTE>

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<P>As part of its spooling task, lpr also checks for instructions on how to print the file. It can get the information from three sources: the command line (supplied as arguments), environment variables (set by the shell or the user), or the system's 
default values.

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<P>The lpr program knows which spool to put the print request in because of the destination printer designation. The printer destination can be specified on the lpr command line, or through an environment variable. When the destination printer name has 
been determined, lpr checks the file /etc/printcap to look up the printer's information, including the spool directory. The spool directory is usually of the form /usr/spool/printer_name, such as /usr/spool/lp1.

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<P>Within the spool directory, lpr creates two files. The first has the letters cf (control file) followed by a print ID number. The cf file contains information about the print job, including the owner's name. The second file starts with df (data file) 
and has the actual contents of the file to be printed with it. When lpr has finished creating the df file, it sends a signal to lpd that informs the daemon that a print job is waiting in the spool directory.

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<P>When lpd gets the signal from lpr, it checks the file /etc/printcap to see whether the printer is for a local or remote printer. If the print job is for a remote printer (one attached to another machine on the network), lpd opens a connection to the 
remote machine, transfers both the control and data files, and deletes the local copies.

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<P>If the print job is for a local printer, lpd checks to make sure the printer exists and is active, and then sends the print request to the printing daemon running that queue.

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<FONT SIZE=4 COLOR="#FF0000"><B>The </B><B>/etc/printcap</B><B> File and Spooling Directories</B></FONT></CENTER></H4>

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<P>The /etc/printcap file is consulted by both the user's print command lpr and the lpd print daemon. It contains information about every printer that is accessible from the Linux machine.

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<P>The format of /etc/printcap is straightforward (and similar to the /etc/termcap file for terminal descriptions). The following is an extract from /etc/printcap:

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<FONT COLOR="#000080"># HP Laserjet

lp|hplj|laserjet-acctng|HP LaserJet 4M in Room 425:\

:lp=/dev/lp0:\

:sd=/usr/spool/lp0:\

:lf=/usr/spool/errorlog:\

:mx#0:\

:of=/usr/spool/lp0/hpjlp:\</FONT></PRE>

<P>The first field in each entry is a list of all the allowable names for the printer. These can be used with the environment variables set by a user's shell or by the system, as well as with options on the lpr command line with a destination printer 
specified. Valid names are separated by a vertical bar.

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<P>Usually, each entry includes at least three names: a short name that is four characters or less (such as hplj); a more complete name with an owner, if necessary (such as laserjet-acctng); and a full, descriptive name with any other information necessary 
to identify the printer (such as HP LaserJet 4M in Room 425).

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<NOTE>If a print job is submitted without a destination name, and one can't be determined from environment variable values, it is routed to the printer lp. Therefore, one of the printers (usually the system default printer) should also have the name lp as 
part of its identifier.</NOTE>

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<P>A comment in the file is shown with a pound symbol (sometimes called a hash mark) as the first character. Following the printer name is a set of two-character parameters and values used by the printer. The format of these entries is always one of the 
following:

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NN 

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A Boolean value</FONT>

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NN=string

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Set equal to string</FONT>

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NN#number

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Set not equal to number</FONT>

</TABLE><P>When a Boolean value is used (no assignment follows the two-character identifier), the value is set to True by default. If the value of False was required, the two-character identifier would not be included in the description.

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<P>Most assignments are shown with colons beginning and ending each definition to enhance readability and make the file easier for the print utilities to parse. Null values are valid assignments employed by putting two colons together.

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<P>A few of the parameters in the /etc/printcap file are worth highlighting because they are useful for administration purposes. Not all of these parameters might be present in every printer definition in the /etc/printcap file, but most appear:

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sd

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The spool directory</FONT>

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lf

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The log directory for error messages</FONT>

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af

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Accounting log file</FONT>

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mx

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Determines the type of files that can be printed</FONT>

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of

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Output filter program to be used when printing</FONT>

</TABLE><P>All printers should have their own spool directories, usually under the printer name in /usr/spool, such as /usr/spool/hplj. Spool directories are necessary for both remote and local printers. When a new printer is added to the system, the spool 
directory might have to be created manually (using mkdir). The permissions for the spool directory should be set to 775. The directory must be owned by root or daemon. The group ID should be set to root or daemon, too. In both cases, daemon theoretically 
is the better ID for user and group, although root will work also.

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<P>The error log file can be located anywhere on the system. It can be shared by all printers, if desired, because each log entry includes the name of the printer.

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<P>The accounting log file is used to record printouts for systems in which users are charged. If accounting records are not to be used on the system, ignore the entry entirely in the /etc/printcap file. The file can also be used for generating statistics, 
however. Some heavily used systems might want to have the accounting file for those purposes even when charges are not incurred by the users. An entry is written to the accounting log file after a print job has completed. Account information can be 
displayed with the Linux pac command. (Use the man pac command to display the man pages for more information about pac.)

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<P>The mx character enables you to identify the types of files to be printed. Usually this is set to mx#0, meaning that there are no restrictions on the types of files.

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<P>Output filters modify the format of the outgoing file to the printer to fit its requirements. For example, many laser printers can't handle 66 lines per page, so the output filter repaginates to 60 lines (or whatever the number of lines per page is set 
to). Sometimes, special codes must be added to force line feeds, font changes, or paper bin selections. All these items are part of the output filter. Several other types of filters are available, but the output filter is the one most commonly encountered.


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<P>Within each spool directory, there may be two status files: status and lock. Each file is one line long and can be modified with an editor. These files contain a description of the current state of the printer. They are created and managed by the lpd 
printer daemon and used by several printer commands for status information.

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<FONT SIZE=4 COLOR="#FF0000"><B>Adding Printer Devices with </B><B>mknod</B></FONT></CENTER></H4>

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<P>Linux supports both parallel and serial printer devices. Both parallel and serial printers are character mode devices. Unfortunately, most Linux distributions do not have an easy-to-use printer installation and configuration utilities like many UNIX 
versions. Instead, the printer devices must be created and set up manually.

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<P>Parallel printers are referred to as devices lp0, lp1, or lp2, depending on the address of the parallel port they are used with. (The most common is the single parallel port on a PC, which is /dev/lp0.) Valid parallel port devices, their addresses, and 
their usual equivalents under MS-DOS are as follows:

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/dev/lp0

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0x03bc

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LPT1</FONT>

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/dev/lp1

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0x0378

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LPT2</FONT>

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/dev/lp2

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0x0278

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LPT3</FONT>

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<NOTE>To determine the address of a parallel port, you can use a diagnostic utility (such as DOS's MSD.EXE). Some BIOS versions display port addresses when the system is booting. If you are unsure, try the ports starting with /dev/lp0, and wait to see 
whether a printout is possible. The first parallel port on a PC is typically set to address 0x03bc.</NOTE>

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<P>Linux uses the mknod (make node) command to create a parallel printer device file. After the device has been made, the ownership of the device driver file must be altered to root or daemon.

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<P>The following is a command to make a parallel printer device on the first parallel port (/dev/lp0):

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<FONT COLOR="#000080">mknod -m 620 /dev/lp0 c 6 0

chown root.daemon /dev/lp0</FONT></PRE>

<P>In this example, the file permissions are set to mode 620, the device /dev/lp0 is created, and it is set to be a character mode device with a major device number of 6 and a minor device number of 0. Usually, minor device numbers start at 0 and are 
incremented upward; therefore, because this is the first printer added, the minor device number is set to 0.

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