Linux with Operating System Concepts

Syntax
Meaning
-D
Delete any previously defined rules
-b 
#
# is a number, establish 
#
buffers, for example, -b 1024
-f 
#
Set failure flag to 
#
(0 is silent, 1 is print failure messages, 2 is panic or halt the system)
-w 
directory
Log attempts to access the 
directory
-w 
filename
Log attempts to access the 
file
-w 
filename
–p 
[rwxa]*
Log attempts to read 
file
(r), write to 
file
(w), execute 
file
(x), or change 
file
’s attributes 
(a). The * indicates that any combination of the options r, w, x, and a can be listed.
-a 
action,list
–S 
syscall
–F 
field
=
value
Log system calls; 
action
is either 
always
or 
never
, 
list
is one of 
task
, 
entry
, 
exit
, 
user
or 
exclude
. The –S option allows you to specify a Linux operation 
such as 
chmod
, 
mkdir
, or 
mount
. The –F option allow you to fine-tune the match 
by testing some system or user parameters such as EUID

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by separating the different recurrences with a slash (/). Below are some examples of 
crontab entries. We cover crontab in more detail in Chapter 14.
• 
0 12 * 15 * ./foobar
—run the script foobar at 12 noon on the 15th of 
every month
• 
0 0 1 1 * ./foobar
—run the script foobar on January 1 at midnight (once 
per year)
• 
30 15 * * 6 ./foobar
—run the script foobar every Saturday at 3:30 pm 
(15 
=
3 pm in military time)
• 
0/15/30/45 * 1 1 * ./foobar
—run the script foobar on January 1 every 
15 min of every hour
As stated earlier, a scheduled job missed because of downtime does not get exe-
cuted when the system is brought back up. Instead, if the job is critical enough, it 
should be scheduled with anacron.
• cups—the common Unix printing system is used to control print jobs on the printers 
connected to a Linux computer. The printers may be directly connected to the com-
puter or available via a network even if the network consists of computers running 
different versions of Linux or Unix. In Chapter 8, we briefly looked at how to install a 
printer on a Linux computer, the result of which is a configuration file storing infor-
mation about the installed printer. In addition, installing a printer requires that the 
proper device driver for the printer is installed. When the user issues a printer com-
mand (
lp
, 
lpr
, 
lpq
, 
lprm
), cups takes over to handle the command by accessing 
the appropriate printer. An additional command, 
lpc
, allows you to alter a printer’s 
properties (this is also available via the Printer Configuration GUI).
• dnsmasq—this service is something like a mini-DNS server for Linux (DNS was 
introduced in Chapter 5 and is also explored in Chapters 12 and 15). The role of 
dnsmasq is threefold, all of which revolve around caching DNS information. First, 
it caches responses from DNS server access so that further repeated accesses can 
be handled locally while the entries remain stored in cache. Second, it uses the /etc/
hosts file, which contains specific IP alias to address mapping as set up by the sys-
tem administrator to bypass DNS server access. Third, it responds to requests from 
DHCP servers regarding IP alias to IP address mappings. With dnsmasq, we reduce 
the amount of access to the DNS server and thus reduce waiting time in Internet-
based communication. The service uses /
etc/dnsmasq.conf
as a configuration 
file to specify details such as a DNS server to query in case the local cache does not 
contain the relevant information, what network interface(s) to use (e.g., eth1 for any 
DHCP requests), and the amount of time that items should remain cached.
• iptables and ip6tables—these are the Linux firewall for IPv4 and IPv6 messages, 
respectively. The names of the services are also the names of a file of firewall rules, 
which are located in /etc/sysconfig. In addition to the rules files of 
iptables
and 

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ip6tables
, the configuration files are 
iptables-config
and 
ip6tables-
config
. We explore specific firewall rules in Chapter 12.
• logrotate—this service performs operations on log files, including rotating logs files, 
compressing log files, and emailing log files. Log rotation renames the current log file 
(usually stored under /var/log) and creates a new log file so that no log file grows too 
large. Typically, log files are rotated automatically based on some rotation rate such as 
weekly or monthly. Often, log files’ names are affixed with either a date or a number 
indicated their age, for instance, mylog (the current log file), mylog.1, mylog.2, and 
mylog.3 (older log files of one, two, and three rotations in the past). The subdirectory 
/
etc/logrotate.d
contains numerous configuration files, one per type of log file. 
Each of these configuration files lists specific log files (e.g., /
var/log/messages
) 
followed by a series of directives for handling that group of log files such as the maxi-
mum size of a log file, the rotation rate, the number of log files to retain, and so forth.
• nfs—permits the exportation of file systems for remote mounting. The system admin-
istrator places file system information in the /
etc/exports
file. For each file system 
that can be remotely mounted, the entry must include the file system’s local mount 
point, the location(s) where the file system can be remotely mounted (this might be * 
to indicate anywhere, or specific or partial URLs or IP addresses as in 10.11.12.0/24 
meaning any computer in the network 10.11.12) and any options for the file system 
such as ro (read only) and sync. We examined NFS in Chapter 10.
• syslogd—there are two other logging services of note, syslogd, and klogd. The klogd 
service logs kernel messages and is not configurable. The syslogd service logs nonker-
nel operating system messages. It is configurable and can also be set up to log kernel 
messages and application software messages. The syslogd daemon has a configuration 
file, /
etc/syslog
. We explore how to configure syslogd in Section 11.7. It should be 
noted that the most recent versions of Linux have renamed the daemon to rsyslogd 
and the configuration file to
/etc/rsyslog
.
Another service of note is called 
oddjobd
. Service requests are made by the user or 
system administrator, the operating system, external messages coming in from the net-
work, or other software. In some cases, software will not have permissions to execute a 
given service. The oddjobd service provides a mapping so that specific software can make 
requests of specific services.
The above list describes only some of the many services available. Missing from the 
above discussion, among others, are services dealing with the computer network, port 
mapping, and discovering the network router. Many of these network services are explored 
in Chapter 12.
11.5.3 Starting and Stopping Services
As a system administrator, it will be your task to handle services. Your options are to 
start or stop services, and to reconfigure them. Decisions include whether a service 

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should be running at all times or on demand, whether a service should start at system 
initialization time (recall the rc.conf script from Section 11.4 that started and stopped 
services based on runlevel), or whether a service should not be run. And then, you will 
have to determine whether to use the default configuration or alter the configuration of 
a service. We examine how to start and stop services here, saving configuration for the 
next two sections.
There are usually four different things you can do to a service: start, stop, restart, or 
obtain the service’s status. You can control the service from either a GUI program or from 
the command line. The service configuration tool allows you to select any service and view 
its status, start, stop, or restart it. Although it is called a