Home > Articles

This chapter is from the book

This chapter is from the book

Configure Common System Services

This section focuses on configuring common system services, including SSH, NTP, syslog, chrony, and localization.


The Secure Shell (SSH) protocol is designed to replace insecure remote communication operations, such as the telnet, ftp, rlogin, rsh, rcp, and rexec commands/protocols. The primary issue with earlier communication methods is that those methods send data across the network in plaintext rather than in an encrypted format. In some cases, such as with telnet and ftp, this can include sending user account data (such as name and password) across the network in plaintext.

SSH provides a better level of security by encrypting the data sent across the network. SSH has become such a standard in Linux that almost all distributions include both the client and server software by default. In the event that you do not have this software installed on your system, you should install the openssh, openssh-server, openssh-clients, and openssh-askpass software packages.

The /etc/ssh directory is the location where the Secure Shell configuration files are stored. The configuration file for the SSH server is the /etc/ssh/sshd_config file. Don’t confuse this with the /etc/ssh/ssh_config file, which is used to configure client utilities, such as the ssh, scp, and sftp commands.

There are two different SSH protocols that are numbered 1 and 2. These are not versions but rather two separate protocols developed to provide secure data connections. There was a time when both protocols were commonly used, but now almost all SSH clients use only protocol 2. To set the protocol that your SSH server accepts, use the Protocol keyword:

Protocol 2

If you have some older SSH clients that require protocol 1, you can configure your SSH server to accept both protocol connections by using the following keyword setting in the /etc/ssh/sshd_config file:

Protocol 1,2

If you have multiple network cards (or virtual interfaces), you may want to limit the SSH server to listen to only some of the network cards. To do this, use the ListenAddress keyword and specify the IP address assigned to the network cards that SSH should accept connections on:


The standard port number that the SSH server listens to is port 22. You can modify the SSH server to listen to another port by using the Port keyword:

Port 2096

You might need to change what sort of log messages you want the SSH server to record. This can be set by using the LogLevel keyword. The levels available are as follows:

  • arrow.jpg QUIET

  • arrow.jpg FATAL

  • arrow.jpg ERROR

  • arrow.jpg INFO

  • arrow.jpg VERBOSE

  • arrow.jpg DEBUG

  • arrow.jpg DEBUG1 (which is the same as DEBUG)

  • arrow.jpg DEBUG2

  • arrow.jpg DEBUG3

Network Time Protocol (NTP)

The Network Time Protocol daemon (ntpd) is a process that ensures the system clock is in sync with the time provided by remote NTP servers. Most of the configuration for this process is handled via the /etc/ntp.conf file. Table 7.1 shows the important settings of the /etc/ntp.conf file.

TABLE 7.1 /etc/ntp.conf File Settings




Contains a value that represents the typical delta (change) over time from the NTP-reported time and the system clock. This value is used to regularly update the system clock without having to access an NTP server.


Used to indicate restrictions for the daemon, including what machines can access this NTP server when it is used as a service.


Used to list an NTP server for this machine when it is used as an NTP client.

Here is an example of a typical /etc/ntp.conf file:

# For more information about this file, see the man pages
# ntp.conf(5), ntp_acc(5), ntp_auth(5), ntp_clock(5), ntp_misc(5),

driftfile /var/lib/ntp/drift

# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default kod nomodify notrap nopeer noquery

# Permit all access over the loopback interface. This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict ::1

# Hosts on local network are less restricted.
# restrict mask nomodify notrap

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.
server 0.fedora.pool.ntp.org iburst
server 1.fedora.pool.ntp.org iburst
server 2.fedora.pool.ntp.org iburst
server 3.fedora.pool.ntp.org iburst

# Enable public key cryptography.

includefile /etc/ntp/crypto/pw

# Key file containing the keys and key identifiers used when operating
# with symmetric key cryptography.
keys /etc/ntp/keys

The pool.ntp.org address is a link to a cluster of NTP servers that are geographically spread throughout the world. These servers can be freely used within the /etc/ntp.conf file. For example, the following servers are provided by the Fedora project (but note that these are often mirrors, pointing to other systems, so the resulting hostnames for these servers will be different once you have connected to them):


The ntpq command allows you to perform queries on NTP servers. For example, the ntpq command in the following example displays a summary of the status of NTP servers:

[root@onecoursesource ~]# ntpq -p
     remote           refid      st t when poll reach   delay   offset
*propjet.latt.ne    2 u  120 1024  377   98.580    7.067
-services.quadra      3 u  272 1024  377   72.504  -10.689
+mirror    3 u  287 1024  377   20.406   -2.555
+    2 u  741 1024  377   69.403   -3.670

Table 7.2 lists some important options to the ntpq command.

TABLE 7.2 ntpq Command Options




Enables debugging mode.


Lists host IP addresses rather than names.


Prints a list of all peers.


The syslog service has existed since 1980. Although it was advanced at the time it was created, its limitations have grown over time as more complex logging techniques have become required.

In the mid-2000s, the rsyslog service was created as an extension of the traditional syslog service. The rsyslog service extends the capabilities of syslog through the inclusion of modules.

The configuration of syslog and rsyslog services is consistent, with the exception of slightly different naming conventions (for example, rsyslog.conf versus syslog.conf) and additional features available in the log files.

The syslogd or rsyslogd daemon is responsible for logging of application and system events. It determines which events to log and where to place log entries, based on configuration settings in the /etc/syslog.conf file.

Table 7.3 describes some important options to the syslogd and rsyslogd commands.

TABLE 7.3 syslogd and rsyslogd Command Options




Enables debugging mode.


Specifies the configuration file (with /etc/syslog.conf as the default).

-m x

Creates a timestamp in the log files every x minutes. (You can set x to 0 to omit timestamps.)


Enables the syslogd daemon to accept logs from remote systems.


Enables verbose mode.


Disables DNS lookups for IP addresses.

The /etc/rsyslog.conf file is one of the configuration files for the rsyslogd daemon. The following is a typical rsyslog.conf file with the comments and blank lines removed (along with the modules):

[root@OCS ~]# grep -v "^$" /etc/rsyslog.conf | grep -v "^#"
*.info;mail.none;authpriv.none;cron.none /var/log/messages
authpriv.*                               /var/log/secure
mail.*                                   -/var/log/maillog
cron.*                                    /var/log/cron
*.emerg                                    *
uucp,news.crit                            /var/log/spooler
local7.*                                 /var/log/boot.log

Every line represents one logging rule that is broken into two primary parts: the selector (for example, uucp,news.crit) and the action (/var/log/spooler). The selector is also broken into two parts: the facility (uucp,news) and the priority (crit). Note that when a priority is provided, it means “this priority and all priorities of a higher level.”

The following list shows the available facilities in order from lower level to higher level:

  • arrow.jpg auth (or security)

  • arrow.jpg authpriv

  • arrow.jpg cron

  • arrow.jpg daemon

  • arrow.jpg kern

  • arrow.jpg lpr

  • arrow.jpg mail

  • arrow.jpg mark

  • arrow.jpg news

  • arrow.jpg syslog

  • arrow.jpg user

  • arrow.jpg uucp

  • arrow.jpg local0 through local7

The following list shows the available priority levels:

  • arrow.jpg debug

  • arrow.jpg info

  • arrow.jpg notice

  • arrow.jpg warning (or warn)

  • arrow.jpg err (or error)

  • arrow.jpg crit

  • arrow.jpg alert

  • arrow.jpg emerg (or panic)

The following list shows the available “actions”, which are really just where the log entry should be sent:

  • arrow.jpg Regular file (where using - before the filename prevents syncing with every log entry, thus reducing hard drive writes)

  • arrow.jpg Named pipes

  • arrow.jpg Console or terminal devices

  • arrow.jpg Remote hosts

  • arrow.jpg Users, which write to the specified user’s terminal windows (where * specifies all users)


Traditionally, the NTP server on Linux has been the ntpd server (discussed earlier in this chapter, in the “Network Time Protocol [NTP]” section). A newer alternative, chrony, provides some benefits over the ntpd server, including:

  • arrow.jpg Faster synchronization

  • arrow.jpg Typically more accurate time

  • arrow.jpg Improved response to clock frequency changes

  • arrow.jpg No need for periodic polling of other NTP servers

  • arrow.jpg Smaller (less memory and CPU utilization)

Pearson IT Certification Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Pearson IT Certification and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Pearson IT Certification products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.pearsonitcertification.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020