lpd
is the line printer daemon (spool area handler) and is normally invoked
at boot time from the
rc(8)
file.
It makes a single pass through the
printcap(5)
file to find out about the existing printers and prints any files
left after a crash.
It then uses the system calls
listen(2)
and
accept(2)
to receive requests to print files in the queue, transfer files to
the spooling area, display the queue, or remove jobs from the queue.
In each case, it forks a child to handle the request so the parent
can continue to listen for more requests.
The options are as follows:
-b bind-address
Normally, if the
-s
option is not specified,
lpd
will listen on all network interfaces for incoming TCP connections.
The
-b
option, followed by a
bind-address
specifies that
lpd
should listen on that address instead of INADDR_ANY.
Multiple
-b
options are permitted, allowing a list of addresses to be specified.
Use of this option silently overrides the
-s
option if it is also present on the command line.
bind-address
can be a numeric host name in IPV4 or IPV6 notation, or a symbolic host
name which will be looked up in the normal way.
-d
The
-d
option turns on the
SO_DEBUGsocket(2)
option.
See
setsockopt(2)
for more details.
-l
The
-l
flag causes
lpd
to log valid requests received from the network.
This can be useful for debugging purposes.
-n maxchild
The
-n
flag sets
maxchild
as the maximum number of child processes that
lpd
will spawn.
The default is 32.
-r
The
-r
flag allows the
“of”
filter to be used if specified for a remote
printer.
Traditionally,
lpd
would not use the output filter for remote printers.
-s
The
-s
flag selects
“secure”
mode, in which
lpd
does not listen on a TCP socket but only takes commands from a
.Ux
domain socket.
This is valuable when the machine on which
lpd
runs is subject to attack over the network and it is desired that the
machine be protected from attempts to remotely fill spools and similar
attacks.
-w maxwait
The
-w
flag sets
maxwait
as the wait time (in seconds) for dead remote server detection.
If no response is returned from a connected server within this period,
the connection is closed and a message logged.
The default is 300 seconds.
If the
[port]
parameter is passed,
lpd
listens on this port instead of the usual
“printer/tcp”
port from
/etc/services.
Access control is provided by two means.
First, all requests must come from one of the machines listed in the file
/etc/hosts.equiv
or
/etc/hosts.lpd
(which follows the same syntax as
hosts.equiv(5/)).
Second, if the
“rs”
capability is specified in the
printcap(5)
entry for the printer being accessed,
lpr
requests will only be honored for those users with accounts on the
machine with the printer.
lpd
performs reverse DNS lookups on network clients.
If a client hostname cannot be determined from its IP address,
the print request will be silently dropped.
This is important to note when debugging print problems
in dynamic address environments.
The file
minfree
in each spool directory contains the number of disk blocks to leave free
so that the line printer queue won't completely fill the disk.
The
minfree
file can be edited with your favorite text editor.
The daemon begins processing files
after it has successfully set the lock for exclusive
access (described a bit later),
and scans the spool directory
for files beginning with
cf.
Lines in each
cf
file specify files to be printed or non-printing actions to be performed.
Each such line begins with a key character to specify what to do
with the remainder of the line.
J
Job Name.
String to be used for the job name on the burst page.
C
Classification.
String to be used for the classification line on the burst page.
L
Literal.
The line contains identification info from the password file and
causes the banner page to be printed.
Troff File.
The file contains
troff(1)
output (cat phototypesetter commands).
n
Ditroff File.
The file contains device independent troff output.
r
DVI File.
The file contains
Tex l
output
DVI format from Stanford.
g
Graph File.
The file contains data produced by
plot.
c
Cifplot File.
The file contains data produced by
cifplot.
v
The file contains a raster image.
r
The file contains text data with
FORTRAN carriage control characters.
\&1
Troff Font R.
Name of the font file to use instead of the default.
\&2
Troff Font I.
Name of the font file to use instead of the default.
\&3
Troff Font B.
Name of the font file to use instead of the default.
\&4
Troff Font S.
Name of the font file to use instead of the default.
W
Width.
Changes the page width (in characters) used by
pr(1)
and the text filters.
I
Indent.
The number of characters to indent the output by (in ASCII).
U
Unlink.
Name of file to remove upon completion of printing.
N
File name.
The name of the file which is being printed, or a blank for the
standard input (when
lpr(1)
is invoked in a pipeline).
If a file cannot be opened, a message will be logged via
syslog(3)
using the
LOG_LPR
facility.
lpd
will try up to 20 times to reopen a file it expects to be there,
after which it will skip the file to be printed.
lpd
uses
flock(2)
to provide exclusive access to the lock file and to prevent multiple
daemons from becoming active simultaneously.
If the daemon should be killed or die unexpectedly, the lock file
need not be removed.
The lock file is kept in a readable
ASCII
form
and contains two lines.
The first is the process ID of the daemon and the second is the control
file name of the current job being printed.
The second line is updated to reflect the current status of
lpd
for the programs
lpq(1)
and
lprm(1).
FILES
/etc/printcap
printer description file
/var/run/lpd.pid
lock file for
lpd
/var/spool/output/*
spool directories
/var/spool/output/*/minfree
minimum free space to leave
/dev/lp*
line printer devices
/var/run/printer
socket for local requests
/etc/hosts.equiv
lists machine names allowed printer access
/etc/hosts.lpd
lists machine names allowed printer access,
but not under same administrative control.
An
lpd
daemon appeared in
Version 6 AT&T UNIX.
lpd
previously required that clients connected using a privileged port
(below 1024).
This restriction was removed because it does not provide additional
security and also because many modern clients connect using an
unprivileged port.