Home > Articles > Operating Systems, Server

  • Print
  • + Share This
This chapter is from the book

2.2.2 Open Network Ports (#2)

Just as every account on your system is a potential path for a cracker, every network service is a road to it. Most Linux distributions install "tons" of software and services by default. They deliberately prefer "easy" over "secure." Many of these are not necessary or wanted. Take the time to remove software and services you do not need. Better still—do not install them to begin with.

To find out what services are being run, use the netstat -atuv command or use the ports program discussed in "Turn Off Unneeded Services" on page 86. Either will list all open ports on your system. Even a home system can have dozens or hundreds of ports open. A large Web server could have many more.

If there are services listed that you do not want to be provided by this box, disable them. Many distributions offer a Control panel to do this easily, including Red Hat and Mandrake. You might want to remove the binaries from the disk or chmod them to 0, especially any that are set-UID or set-GID.

NFS, finger, the shell, exec, and login r* services (rsh, rexec, and rlogin), FTP, telnet, sendmail, DNS, and linuxconf are some of the more popular services that get installed by default on many Linux distributions; at least some of these should not be enabled for most systems. Most of these are controlled by the inet daemon, inetd; these can be disabled by editing the /etc/inetd.conf file.

You do not need the FTP or telnet daemons to use the respective clients to connect into other systems. You do not need the sendmail daemon listening on port 25 to send mail out or to send mail to local users or to download mail via POP or IMAP. (You do need to invoke sendmail periodically to de-spool delayed outgoing mail. The techniques are explained in "Hardening for Very High Security" on page 306.) You only need DNS (named, the name daemon) if other systems will be querying yours for this data. Most programs running on your own system will be very happy to read /etc/resolv.conf and query your ISP's or organization's main DNS server instead of contacting a named process running on your system. Coincidentally named's ports are some of the most popular ports that crackers use to break into systems. If you do need to run named, use the recently added facilities that allow it to chroot itself and switch to a nonroot user.

All these services, except the normal installation of NFS,2 DNS, and sendmail, are started on demand by inetd. They may be turned off by commenting out their entries in /etc/inetd.conf. Many distributions offer a Control panel or Linuxconf to do this easily, including Red Hat and Mandrake. The standalone services are turned off by altering their entries under /etc/rc.d.

On Red Hat–based systems, issue the following commands to shut down portmap and prevent it from being restarted on reboot. Even as late as Red Hat 7.3 on a standard non-server install, the evil portmap is invoked.

/etc/rc.d/init.d/nfs stop
/etc/rc.d/init.d/nfslock stop
/etc/rc.d/init.d/portmap stop
chkconfig --del nfs
chkconfig --del nfslock
chkconfig --del portmap

An alternative tool is the ASCII menu-based ntsysv program. Like chkconfig, ntsysv only manipulates the symbolic links under /etc/rc.d/rc[0-6].d so you also will need to explicitly shut down the service. To do both of these issue the commands:

/etc/rc.d/init.d/portmap stop

On other distributions that use the System V-style of startup scripts (/etc/rc.d/rc[0-6].d directories for Red Hat derivations and /etc/rc.[0-b].d for Debian), rename the appropriate script under rcX.d (X usually is 3) that starts with a capital-S and has the service name in it. For example,

cd /etc/rc.d/rc3.d
mv S11portmap K11portmap

Just as only scripts starting with "S" are invoked when entering the respective run level, scripts starting with "K" are invoked when exiting that run level. This is to turn off daemons that should run only in that run level. For example, this mechanism will turn off sshd, the SSH daemon, when switching from run level "3" (multiuser with networking) to run level "s" (single-user mode). Just as a selected Ssomething script can be disabled by renaming to ssomething, one of these latter scripts can be renamed from Ksomething to ksomething to disable it.

On Slackware and similar systems, simply comment out the lines that start them in /etc/rc.d/*. The grep program may be used to find these. Be sure to terminate any of these services that currently are running on your system after altering the configuration files.

If you do not want to bother with kill, a simple reboot will do this and verify that the configuration files were correctly altered. (A set of available rescue disks before this reboot would be a fine idea.)

The most careful SysAdmins will reboot their systems several times after making changes to startup scripts to ensure correct and reliable startup and operation.

To remove these services from your system, you can use your distribution's package manager to delete them. Red Hat–based installations (Red Hat, Mandrake, Caldera, Yellow Dog, TurboLinux) use RPM. Debian-based distributions (Debian and Corel) use dpkg. SuSE uses YAST and Slackware uses pkgtool.

Linux is like the Swiss Army knife of networking—it has one or two tools of mass destruction that get used all the time, others that are used less often, and some that are never used. Unlike the Swiss Army knife, you can slim down Linux to just the services you need, and discard those you do not. I will never use the awl or corkscrew on my knife just like I will never use rsh or finger. Decide which ports you want to have open (such as www and ftp) and close the rest. Closing unnecessary ports makes your system more secure and perform better.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT 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 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 InformIT. 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.informit.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