COMPUTER AND NETWORK
SYSTEM ADMINISTRATION
Summer 1996 - Lesson 29
Miscellanous Topics
I. Cron
- Used to schedule execution of shell scripts and programs
- Managed by "/usr/etc/cron" (SunOS) or "/usr/sbin/crond" (Linux)
- Typical cron daemon has a 60-second clock resolution for timing
- The cron daemon checks various files for the list of commands
to run. Location is cron-dependent:
BSD-based: /usr/lib/crontab or /etc/crontab
AT&T-based: /usr/lib/crontabs, /var/spool/cron/crontabs, etc.
Vixie cron: a nice AT&T cron clone (Linux)
- Luckily, the format of the crontab(5) file is universal (p. 173):
Six-field version:
minute hour day month day-of-week command string
0-59 0-23 1-31 1-12 1=Mon, 2=Tues
Seven-field version:
minute hour day month day-of-week user command string
0-59 0-23 1-31 1-12 1=Mon, 2=Tues
Wildcards and ranges are permitted:
* = any time
single integer = exact time
comma list integers = list of exact times
integer dash integer = range of times
Examples:
15 3 * * * find / -name .nfs\* -mtime +7 -exec rm -f {} \; -o -fstype nfs -prune
5 4 * * 6 /usr/lib/newsyslog >/dev/null 2>&1
45 10 * * 1-5 wall "it is 10:45 AM on a weekday"
45 * * * * /usr/local/bin/killidle -t720 > /dev/null
30 5 * * * /bin/calendar -
- To edit BSD-style cron jobs, edit the file directly then SIGHUP
the cron daemon.
- On an AT&T-style cron, use the "crontab" command:
crontab -l # list a user's crontab file
crontab -e # edits a user's crontab file using $VISUAL or $EDITOR
crontab -r # remove a user's crontab file
"crontab -e" brings up your current crontab file; you edit a copy;
it replaces the system copy after you exit -- the cron daemon
is sent a signal on a named pipe to re-read your file.
- AT&T has separate crontab files per user in the "crontabs" directory.
Permission to use "crontab" is specified in "cron.allow" and
"cron.deny" (book notes the files can be squirreled away in a
number of places, such as /var/spool/cron). "Out of the box"
UNIXes typically only permit "root" to have crontab entries.
- A useful "watered down" version of background tasking can be
done with the "at" commands.
- "at" gives you a one-time shot at running a shell script (or job).
Linux has a series of commands that permit management of these
jobs (at, batch, atq, atrm). Scheduling of the "at" jobs
occurs using the same cron daemon.
- Files "at.deny" and "at.allow" usually exist as well, to give
people the ability to use the "at" commands.
- Common cron tasks:
o Unattended backups
o Environmental monitors (network, A/C, power, etc.)
o Network management (sharing system files via "rdist")
o Skulker scripts
o Idle reapers
o Accounting tasks
o Log management (syslog, web logs, etc.)
o Calendar feature: /bin/calendar
- For full-featured batch subsystems, install either public domain
or commercial packages, such as:
Network Queuing System (NQS)
Distributed Queuing System (DQS) -- Used & developed at SCRI
LoadLeveler (IBM)
II. USEnet News
- "The BBS system of the Internet" - undoubtably the largest bulletin
board system in the world
o Requires a news server to be set up with "news feeds"
from other news servers
o Notes are distributed in a fan-out fashion, with a
protocol for discarding already-received notes
o A busy news server can push 100s of megabytes a day
Example : news.fsu.edu, news.scri.fsu.edu
Try : "telnet news.fsu.edu nntp"
(nntp = port 119)
o The dominate method for mass-scale communication prior
to the Web (still has a healthy existance).
- Terms:
o newsgroup = collection of notes of a particular topic
o article = note body
o post = submit an article to a set of newsgroups
o NNTP = Network News Transfer Protocol
o FAQ = Frequently Asked Questions
o moderation = unmoderated newsgroups permit anybody to post;
moderated newsgroups send email to the
moderator instead, who then posts
o subscribe = tell your client software to grab articles
only for a subset of the thousands of newsgroups.
o expired = articles that are automatically removed after
some period of time, to free up disk space for
newer articles
o flame = emotional overreaction to a notes article :)
o smileys = ASCII-based graphics :) :O ;)
- Newsgroup hierarchy
o Not unlike the DNS hierarchy, but backwards :)
(larger domain is on the left:
comp.os.ms-windows.networking.windows)
o Table 24.1, page 565 lists the top level newsgroup
classifications (alt, bionet, biz, clari, comp, k12, etc.).
Local newsgroups: "fsu.*" (fsu.jobs, fsu.networking, etc.)
Some fun ones: see table 24.5, page 579
Some of my favorites:
rec.humor.funny
comp.infosystems.*
comp.protocols.time.ntp
comp.sys.*
comp.unix.*
comp.sources.*
comp.arch
comp.security.announce
- News readers (USEnet client software)
o UNIX-based: readnews (ancient), rn, nn, trn, tin, xrn
+ Use "~/.newsrc" to keep track of which newsgroups
you are subscribed to and what articles you've
already read
o PC/Mac based: many :)
o Netscape Navigator has a built-in news reader
- Server software
o C-news (successor to B-news :)
o NNTP (add-on to C-news for TCP-based news delivery)
o InterNetNews (INN) - has built-in NNTP support
Most recent & popular; ACNS and SCRI run INN
- A true JOY to install
- Requires having network neighbors that agree to
provide you with a news feed
- Requires 1 GB of disk or more for a full feed
III. Printing
- Two main branches (as usual):
o BSD-based (lpd, lpc, lpr, lprm)
o AT&T-based (lpsched, lpadmin, lp, etc.)
Table 25.1, page 581 gives a mapping of some O/S names
to their style of printing (Linux uses BSD-based)
- Terms:
o spooler - system that manages the print files separately
from users (/var/spool/lpd, typically)
o dpi = dots per inch; a measurement of printer quality
o PDL = Page Description Language; example is PostScript
o bitmap = a variety of binary mappings for image data
o RIP = Raster Image Processor; converts between PDL and
a bitmap
o PostScript = most common PDL (from Adobe Systems, Inc.); a
text file that usually starts with "%!"
o HPGL = Hewlett-Packard Graphics Language; another PDL
from another vendor
o PCL = HP's Printer Command Language
o QuickDraw = Macintosh format; PICT is the bitmap format
o roff = run off; ancient formatting package
- BSD printing
o All user commands (lpr, lpq, lprm) go through a line
printer daemon (lpd), which handles the spooling and
communication with the actual printers
o User submits prints via "lpr" command
o "lpr" interrogates the file /etc/printcap for printer
names and attributes (printcap = printer capabilities)
o Can handle local printers and remote (printer on a different
computer or a printer directly connected to the network)
o "lpq" used to check status of printer
o "lprm" used to remove a print queue entry
o "lpc" is a system-admin tool for managing the print queues
(permits you to software-disable and re-enable a queue,
get printer status, etc.) -- See pages 596 through 598
o The file "/etc/hosts.lpd" provides host-level control
over who gets to use your printers.
o BSD line printing is an "all or none" system -- if you
want to restrict certain printers from certain people or
enforce print quotas you must build your own system
using /etc/printcap fields.
- AT&T printing
o More flexible than BSD, but wasn't designed with native
networking like BSD; more difficult to initially set up
o More commands: See table 25.5, page 600
o Submit prints via "lp" command
o Show status with "lpstat"
o "lpsched" is the main daemon that supports the spooling
of the print jobs
o "lpadmin" is a tool that is used to manage the various
printer configuration files. There is no central
"printcap" file with all the printing information, rather
a collection of directories and files in places
like /usr/spool/lp
o After much effort, you can usually finagle an AT&T
print system to interact with a BSD print system
- Nice table (25.8) on page 612 that compares the rough equivalences
between the BSD and AT&T print commands.
- Cute Exhibit A (page 613) that "grades" the two systems.
- No matter which system you get, the real work involves getting
the vendor-supplied printing software to work with the spooling
system. See a short list on pages 613 and 614.
- Note that samba permits you to use UNIX-defined printers on
Windows systems (and vice versa), while packages such as
CAP (the Columbia Appletalk Package) and netatalk permit you
to share UNIX and Macintosh printers. In many cases, the
maturity and flexibility of Linux machines make them ideal
gateway machines between such disparate worlds as Windows and
Macintosh -- the biggest problem is having the sysadmin knowledge
to know how to connect it all together.
IV. Hardware Maintenance
- Usually the SysAdmin has to handle all aspects of the computer
system, including those that involve (*gasp*) plain old paperwork
and phone calls.
- Shop around for warranty periods as well as price.
- Don't always buy your equipment from one vendor, especially
when it comes to additional memory and peripherals.
- Practice good grounding techniques when handling static-sensitive
parts.
- Sometimes a glitch will free up if you "reseat" a board, connector,
or a chip.
- Some equipment may require PM (Preventative Maintenance) time;
cleaning/adjusting equipment before it breaks.
- You should buy a hardware maintenance contract whenever you have
a fairly expensive piece of equipment where it is too expensive
to maintain your own spare parts and/or repair knowledge. You
often have choices on the level of maintenance:
o On-site 7 by 24: most expensive!
o On-site 9-5, weekdays
o Next-day swap
- Have your vendor contact information handy (the larger ones
typically have 1-800 numbers - know the serial numbers of your
machines and your customer identification number).