OpenBSD Line Printer Spooler Manual Ralph Campbell Computer Systems Research Group Computer Science Division Department of Electrical Engineering and Computer Science University of California, Berkeley Berkeley, CA 94720 ABSTRACT This document describes the structure and installation procedure for the line printer spool- ing system included with the OpenBSD operating system. Revised May 31, 2002 1. Overview The line printer system supports: + multiple printers, + multiple spooling queues, + both local and remote printers, and + printers attached via serial lines that require line ini- tialization such as the baud rate. Raster output devices such as a Varian or Versatec, and laser printers such as an Imagen, are also supported by the line printer system. The line printer system consists mainly of the follow- ing files and commands: SMM:7-2 OpenBSD Line Printer Spooler Manual /etc/printcap printer configuration and capability data base /usr/sbin/lpd line printer daemon, does all the real work /usr/bin/lpr program to enter a job in a printer queue /usr/bin/lpq spooling queue examination program /usr/bin/lprm program to delete jobs from a queue /usr/sbin/lpc program to administer printers and spooling queues /var/run/printer socket on which lpd listens The file /etc/printcap is a master data base describing line printers directly attached to a machine and, also, printers accessible across a network. The manual page entry printcap(5) provides the authoritative definition of the format of this data base, as well as specifying default values for important items such as the directory in which spooling is performed. This document introduces some of the information that may be placed printcap. 2. Commands 2.1. lpd - line printer daemon The program lpd(8), usually invoked at boot time from the /etc/rc file, acts as a master server for coordinating and controlling the spooling queues configured in the printcap file. When lpd is started it makes a single pass through the printcap database restarting any printers that have jobs. In normal operation lpd listens for service requests on multiple sockets, one in the LOCAL domain (named ``/var/run/printer'') for local requests, and one in the Internet domain (under the ``printer'' service specifica- tion) for requests for printer access from off machine; see socket(2) and services(5) for more information on sockets and service specifications, respectively. Lpd spawns a copy of itself to process the request; the master daemon contin- ues to listen for new requests. Clients communicate with lpd using a simple transaction oriented protocol. Authentication of remote clients is done based on the ``privilege port'' scheme employed by rshd(8) and rcmd(3). The following table shows the requests under- stood by lpd. In each request the first byte indicates the ``meaning'' of the request, followed by the name of the printer to which it should be applied. Additional qualif- iers may follow, depending on the request. OpenBSD Line Printer Spooler Manual SMM:7-3 Request Interpretation ________________________________________________________________________________________ ^Aprinter\n check the queue for jobs and print any found ^Bprinter\n receive and queue a job from another machine ^Cprinter [users ...] [jobs ...]\n return short list of current queue state ^Dprinter [users ...] [jobs ...]\n return long list of current queue state ^Eprinter person [users ...] [jobs ...]\n remove jobs from a queue The lpr(1) command is used by users to enter a print job in a local queue and to notify the local lpd that there are new jobs in the spooling area. Lpd either schedules the job to be printed locally, or if printing remotely, attempts to forward the job to the appropriate machine. If the printer cannot be opened or the destination machine is unreachable, the job will remain queued until it is possible to complete the work. 2.2. lpq - show line printer queue The lpq(1) program works recursively backwards display- ing the queue of the machine with the printer and then the queue(s) of the machine(s) that lead to it. Lpq has two forms of output: in the default, short, format it gives a single line of output per queued job; in the long format it shows the list of files, and their sizes, that comprise a job. 2.3. lprm - remove jobs from a queue The lprm(1) command deletes jobs from a spooling queue. If necessary, lprm will first kill off a running daemon that is servicing the queue and restart it after the required files are removed. When removing jobs destined for a remote printer, lprm acts similarly to lpq except it first checks locally for jobs to remove and then tries to remove files in queues off-machine. 2.4. lpc - line printer control program The lpc(8) program is used by the system administrator to control the operation of the line printer system. For each line printer configured in /etc/printcap, lpc may be used to: + disable or enable a printer, + disable or enable a printer's spooling queue, + rearrange the order of jobs in a spooling queue, SMM:7-4 OpenBSD Line Printer Spooler Manual + find the status of printers, and their associated spooling queues and printer daemons. 3. Access control The printer system maintains protected spooling areas so that users cannot circumvent printer accounting or remove files other than their own. The strategy used to maintain protected spooling areas is as follows: + The spooling area is writable only by root and and the daemon group. + The lpr and lprm programs run set-user-id to user daemon and set-group-id to group daemon. + The lpc and lpq programs run set-group-id to group daemon to access spool files. + Control and data files in a spooling area are made with daemon ownership and group ownership daemon. Their mode is 0660. This ensures control files are not modified by a user and that no user can remove files except through lprm. + The printer server, lpd, runs as root but spends most of its time with the effective user-id set to daemon and the effective group-id set to daemon. As a result, spool files it creates belong to user and group daemon. Lpd uses the same verification procedures as rshd(8) in authenticating remote clients. The host on which a client resides must be present in the file /etc/hosts.equiv or /etc/hosts.lpd and the request mes- sage must come from a reserved port number. 4. Setting up OpenBSD comes with the necessary programs installed and with the default line printer queue created. If the system must be modified, the Makefile in the directory /usr/src/usr.sbin/lpr should be used in recompiling and reinstalling the necessary programs. The real work in setting up is to create the printcap file and any printer filters for printers not supported in the distribution system. 4.1. Creating a printcap file The printcap database contains one or more entries per printer. A printer should have a separate spooling direc- tory; otherwise, jobs will be printed on different printers depending on which printer daemon starts first. This section describes how to create entries for printers that do not OpenBSD Line Printer Spooler Manual SMM:7-5 conform to the default printer description (an LP-11 style interface to a standard, band printer). 4.1.1. Printers on serial lines When a printer is connected via a serial communication line it must have the proper baud rate and terminal modes set. The following example is for a DecWriter III printer connected locally via a 9600 baud serial line. lp|LA-180 DecWriter III:\ :lp=/dev/lp:br#9600:ms=onlcr,oxtabs,-parity:\ :tr=\f:of=/usr/libexec/lpr/lpf:lf=/var/log/lpd-errs: The lp entry specifies the file name to open for output. Here it could be left out since ``/dev/lp'' is the default. The br entry sets the baud rate for the tty line and the ms entry sets NL to CR-NL mapping, expansion of tabs to spaces, and disables parity (see stty(1)). The tr entry indicates that a form-feed should be printed when the queue empties so the paper can be torn off without turning the printer off- line and pressing form feed. The of entry specifies the filter program lpf should be used for printing the files; more will be said about filters later. The last entry causes errors to be written to the file ``/var/log/lpd-errs'' instead of the console. Most errors from lpd are logged using syslogd(8) and will not be logged in the specified file. The filters should use syslogd to report errors; only those that write to standard error output will end up with errors in the lf file. (Occasionally errors sent to standard error output have not appeared in the log file; the use of syslogd is highly recommended.) 4.1.2. Remote printers Printers that reside on remote hosts should have an empty lp entry. For example, the following printcap entry would send output to the printer named ``lp'' on the machine ``ucbvax''. lp|default line printer:\ :lp=:rm=ucbvax:rp=lp:sd=/var/spool/output/vaxlpd: The rm entry is the name of the remote machine to connect to; this name must be a known host name for a machine on the network. The rp capability indicates the name of the printer on the remote machine is ``lp''; here it could be left out since this is the default value. The sd entry specifies ``/var/spool/output/vaxlpd'' as the spooling directory instead of the default value of ``/var/spool/lpd''. 4.2. Output filters Filters are used to handle device dependencies and to SMM:7-6 OpenBSD Line Printer Spooler Manual do accounting functions. The output filtering of of is used when accounting is not being done or when all text data must be passed through a filter. It is not intended to do accounting since it is started only once, all text files are filtered through it, and no provision is made for passing owners' login name, identifying the beginning and ending of jobs, etc. The other filters (if specified) are started for each file printed and do accounting if there is an af entry. If entries for both of and other filters are specified, the output filter is used only to print the banner page; it is then stopped to allow other filters access to the printer. An example of a printer that requires output filters is the Benson-Varian. va|varian|Benson-Varian:\ :lp=/dev/va0:sd=/var/spool/output/vad:of=/usr/libexec/lpr/vpf:\ :tf=/usr/libexec/lpr/rvcat:mx#2000:pl#58:px=2112:py=1700:tr=\f: The tf entry specifies ``/usr/libexec/lpr/rvcat'' as the filter to be used in printing troff(1) output. This filter is needed to set the device into print mode for text, and plot mode for printing troff files and raster images (see va(4)). Note that the page length is set to 58 lines by the pl entry for 8.5" by 11" fan-fold paper. To enable account- ing, the varian entry would be augmented with an af filter as shown below. va|varian|Benson-Varian:\ :lp=/dev/va0:sd=/var/spool/output/vad:of=/usr/libexec/lpr/vpf:\ :if=/usr/libexec/lpr/vpf:tf=/usr/libexec/lpr/rvcat:\ :af=/var/log/vaacct:mx#2000:pl#58:px=2112:py=1700:tr=\f: 4.3. Access Control Local access to printer queues is controlled with the rg printcap entry. :rg=lprgroup: Users must be in the group lprgroup to submit jobs to the specified printer. The default is to allow all users access. Note that once the files are in the local queue, they can be printed locally or forwarded to another host depending on the configuration. Remote access is controlled by listing the hosts in either the file /etc/hosts.equiv or /etc/hosts.lpd, one host per line. Note that rsh(1) and rlogin(1) use /etc/hosts.equiv to determine which hosts are equivalent for allowing logins without passwords. The file /etc/hosts.lpd is only used to control which hosts have line printer access. Remote access can be further restricted to only allow remote users with accounts on the local host to print OpenBSD Line Printer Spooler Manual SMM:7-7 jobs by using the rs printcap entry. :rs: 5. Output filter specifications The filters supplied with OpenBSD handle printing and accounting for most common line printers, the Benson-Varian, the wide (36") and narrow (11") Versatec printer/plotters. For other devices or accounting methods, it may be necessary to create a new filter. Filters are spawned by lpd with their standard input the data to be printed, and standard output the printer. The standard error is attached to the lf file for logging errors or syslogd may be used for logging errors. A filter must return a 0 exit code if there were no errors, 1 if the job should be reprinted, and 2 if the job should be thrown away. When lprm sends a kill signal to the lpd process con- trolling printing, it sends a SIGINT signal to all filters and descendents of filters. This signal can be trapped by filters that need to do cleanup operations such as deleting temporary files. Arguments passed to a filter depend on its type. The of filter is called with the following arguments. filter -wwidth -llength The width and length values come from the pw and pl entries in the printcap database. The if filter is passed the fol- lowing parameters. filter [-c] -wwidth -llength -iindent -n login -j jobname -h host accounting_file The -c flag is optional, and only supplied when control characters are to be passed uninterpreted to the printer (when using the -l option of lpr to print the file). The -w and -l parameters are the same as for the of filter. The -n and -h parameters specify the login name and host name of the job owner. The last argument is the name of the account- ing file from printcap. The -j parameter is optional and specifies the name of the print job if available. All other filters are called with the following argu- ments: filter -xwidth -ylength -n login -j jobname -h host accounting_file The -x and -y options specify the horizontal and vertical page size in pixels (from the px and py entries in the printcap file). The rest of the arguments are the same as for the if filter. SMM:7-8 OpenBSD Line Printer Spooler Manual 6. Line printer Administration The lpc program provides local control over line printer activity. The major commands and their intended use will be described. The command format and remaining commands are described in lpc(8). abort and start Abort terminates an active spooling daemon on the local host immediately and then disables printing (preventing new daemons from being started by lpr). This is nor- mally used to forcibly restart a hung line printer dae- mon (i.e., lpq reports that there is a daemon present but nothing is happening). It does not remove any jobs from the queue (use the lprm command instead). Start enables printing and requests lpd to start printing jobs. enable and disable Enable and disable allow spooling in the local queue to be turned on/off. This will allow/prevent lpr from put- ting new jobs in the spool queue. It is frequently convenient to turn spooling off while testing new line printer filters since the root user can still use lpr to put jobs in the queue but no one else can. The other main use is to prevent users from putting jobs in the queue when the printer is expected to be unavailable for a long time. restart Restart allows ordinary users to restart printer dae- mons when lpq reports that there is no daemon present. stop Stop halts a spooling daemon after the current job com- pletes; this also disables printing. This is a clean way to shutdown a printer to do maintenance, etc. Note that users can still enter jobs in a spool queue while a printer is stopped. topq Topq places jobs at the top of a printer queue. This can be used to reorder high priority jobs since lpr only provides first-come-first-serve ordering of jobs. up and down Up and down combine the functionality of enable and start with start and stop. Up is equivalent to issuing OpenBSD Line Printer Spooler Manual SMM:7-9 the start and enable commands, whereas down is equivalent to issuing the stop and disable commands. Down also takes an optional message that will be writ- ten to the printer's status file. This allows the administrator to indicate to users why the printer is out of service. 7. Troubleshooting There are several messages that may be generated by the the line printer system. This section categorizes the most common and explains the cause for their generation. Where the message implies a failure, directions are given to remedy the problem. In the examples below, the name printer is the name of the printer from the printcap database. 7.1. LPR lpr: printer: unknown printer The printer was not found in the printcap database. Usually this is a typing mistake; however, it may indi- cate a missing or incorrect entry in the /etc/printcap file. lpr: printer: jobs queued, but cannot start daemon. The connection to lpd on the local machine failed. This usually means the printer server started at boot time has died or is hung. Check the local socket /var/run/printer to be sure it still exists (if it does not exist, there is no lpd process running). Usually it is enough to get a super-user to type the following to restart lpd. % /usr/sbin/lpd You can also check the state of the master printer dae- mon with the following. % ps l`cat /var/run/lpd.pid` Another possibility is that the lpr program is not set-user-id to daemon, set-group-id to group daemon. This can be checked with % ls -l /usr/bin/lpr SMM:7-10 OpenBSD Line Printer Spooler Manual lpr: printer: printer queue is disabled This means the queue was turned off with % lpc disable printer to prevent lpr from putting files in the queue. This is normally done by the system manager when a printer is going to be down for a long time. The printer can be turned back on by a super-user with lpc. 7.2. LPQ waiting for printer to become ready (offline ?) The printer device could not be opened by the daemon. This can happen for several reasons, the most common is that the printer is turned off-line. This message can also be generated if the printer is out of paper, the paper is jammed, etc. The actual reason is dependent on the meaning of error codes returned by system device driver. Not all printers supply enough information to distinguish when a printer is off-line or having trou- ble (e.g. a printer connected through a serial line). Another possible cause of this message is some other process, such as an output filter, has an exclusive open on the device. Your only recourse here is to kill off the offending program(s) and restart the printer with lpc. printer is ready and printing The lpq program checks to see if a daemon process exists for printer and prints the file status located in the spooling directory. If the daemon is hung, a super user can use lpc to abort the current daemon and start a new one. waiting for host to come up This implies there is a daemon trying to connect to the remote machine named host to send the files in the local queue. If the remote machine is up, lpd on the remote machine is probably dead or hung and should be restarted as mentioned for lpr. sending to host The files should be in the process of being transferred to the remote host. If not, the local daemon should be aborted and started with lpc. OpenBSD Line Printer Spooler Manual SMM:7-11 Warning: printer is down The printer has been marked as being unavailable with lpc. Warning: no daemon present The lpd process overseeing the spooling queue, as specified in the ``lock'' file in that directory, does not exist. This normally occurs only when the daemon has unexpectedly died. The error log file for the printer and the syslogd logs should be checked for a diagnostic from the deceased process. To restart an lpd, use % lpc restart printer no space on remote; waiting for queue to drain This implies that there is insufficient disk space on the remote. If the file is large enough, there will never be enough space on the remote (even after the queue on the remote is empty). The solution here is to move the spooling queue or make more free space on the remote. 7.3. LPRM lprm: printer: cannot restart printer daemon This case is the same as when lpr prints that the dae- mon cannot be started. 7.4. LPD The lpd program can log many different messages using syslogd(8). Most of these messages are about files that can not be opened and usually imply that the printcap file or the protection modes of the files are incorrect. Files may also be inaccessible if people manually manipulate the line printer system (i.e. they bypass the lpr program). In addition to messages generated by lpd, any of the filters that lpd spawns may log messages using syslogd or to the error log file (the file specified in the lf entry in printcap). 7.5. LPC couldn't start printer This case is the same as when lpr reports that the SMM:7-12 OpenBSD Line Printer Spooler Manual daemon cannot be started. cannot examine spool directory Error messages beginning with ``cannot ...'' are usu- ally because of incorrect ownership or protection mode of the lock file, spooling directory or the lpc pro- gram.
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