Home > Articles > Security > Network Security

  • Print
  • + Share This
Like this article? We recommend

Getting to Know Your Enemy

For the serious newbies out there, here's how networks work on a really, really basic level. Your system's master process, the one that got the system going (after you pushed the on switch, that is) is called init. init's process ID is 1. It is always 1. If you want to check it out, find init in your process table using ps.

# ps ax | grep init
   1  ?   S    6:03 init

One of the services that init starts when your system boots is inetd. Its job is to listen for network requests, which it references by way of Internet socket numbers or ports. For instance, when you telnet to your system by typing telnet mysystem, you are actually requesting that inetd on mysystem start an in.telnetd process that handles communication over port 23. Then in.telnetd starts a process that eventually asks for your login name and password and, miraculously, you are logged in. Basically, inetd listens to find out what other daemons should wake up to answer the port request. If you want to see what those service numbers translate to, do a more (or less) on /etc/services, a text file that lists the known TCP service ports.

From a resources perspective, it makes sense to have a single process listening rather than one for each and every service. For those of you who can remember and visualize such things, picture Lily Tomlin as the telephone operator who (eventually) patches people through to the party to whom they wished to speak. She is inetd and the people to whom you wish to speak are the service daemons. You request extension 23 and, eventually, she puts you through.

When inetd starts, it reads a file called inetd.conf. You'll find this one in your /etc directory. Here are a couple of sample lines from inetd.conf.

#
# These are standard services.
#
ftp stream  tcp nowait  root /usr/sbin/tcpd in.ftpd -l -a
telnet  stream tcp  nowait root  /usr/sbin/tcpd   in.telnetd
#
# Shell, login, exec, comsat and talk are BSD protocols.
#
shell   stream  tcp nowait  root /usr/sbin/tcpd in.rshd
login   stream  tcp nowait  root /usr/sbin/tcpd in.rlogind
#exec   stream  tcp nowait  root /usr/sbin/tcpd in.rexecd

When a cracker first visits your site with the intention of breaking in, he will often employ a tool known as a port scanner to find out what inetd is listening for on your system.

One of my favorite port scanners is nmap. You can pick up nmap from http://www.insecure.org/nmap/index.html. The latest version even comes with a nice GUI front end called nmapfe. Let's run nmap against my test system and see what we get.

The options are -sS for TCP SYN, or half-open scan, and -O for OS fingerprinting. OS fingerprinting means that nmap will try to guess the OS version running on the system. A cracker who knows what release of an OS you are running will use that information to decide on the most likely exploits for a successful entry. Here's the nmap command and the output from my test system.

# nmap -sS -O localhost
  
Starting nmap V. 2.3BETA5 by Fyodor (fyodor@dhp.com,
http://www.insecure.org/nmap/)
Interesting ports on localhost (127.0.0.1):
Port  State    Protocol Service
21   open    tcp    ftp           
23   open    tcp    telnet         
25   open    tcp    smtp          
53   open    tcp    domain         
79   open    tcp    finger         
80   open    tcp    http          
98   open    tcp    linuxconf        
111   open    tcp    sunrpc         
113   open    tcp    auth          
139   open    tcp    netbios-ssn       
513   open    tcp    login          
514   open    tcp    shell          
515   open    tcp    printer         

TCP Sequence Prediction: Class=random positive increments
             Difficulty=4360068 (Good luck!)
Remote operating system guess: Linux 2.1.122 - 2.2.12

Nmap run completed -- 1 IP address (1 host up) scanned in 2
seconds

Those open ports are the jumping-off point for your crackers. With this information, they know what to bother with and what to forget about. If there is no daemon listening on a network port, why bother trying to get in that way?

Now go back and look at your /etc/inetd.conf file. Notice that exec is commented out (there's a hash mark, #, or octothorp, at the beginning of the line), but login is not. If you reference that with the output of nmap, you'll see that those services not commented out in inetd.conf are listed, while those with the hash mark at the beginning are not.

This is how you shut down unnecessary ports monitored by inetd. Your TCP wrapper is keeping an eye on those ports, but if no one needs to have access to remote shell, why have inetd listen for it at all? The wrapper's job is to provide access to specific services for specific IP addresses. In the first section, we did the quick lock-down with the wrapper. Now go through your list of services, decide what you need and what you don't, and then disable the don'ts by commenting out those lines.

To activate the changes, you need to restart inetd. Find inetd's process ID and send a SIGHUP to it. That means you do a kill - 1 on the process. Be careful. A kill -1 looks an awful lot like a kill 1. Do you remember what process had ID 1? Kill init, and you kill the whole system. If you are worried and don't mind typing a few extra keystrokes, use kill -SIGHUP instead of kill -1.

Now let's rerun nmap.

Starting nmap V. 2.3BETA5 by Fyodor (fyodor@dhp.com,
http://www.insecure.org/nmap/)
Interesting ports on localhost (127.0.0.1):
Port  State    Protocol Service
21   open    tcp    ftp           
23   open    tcp    telnet         
25   open    tcp    smtp          
53   open    tcp    domain         
80   open    tcp    http          
111   open    tcp    sunrpc         
113   open    tcp    auth          
139   open    tcp    netbios-ssn       
515   open    tcp    printer         

TCP Sequence Prediction: Class=random positive increments
             Difficulty=3487082 (Good luck!)
Remote operating system guess: Linux 2.1.122 - 2.2.12

Nmap run completed -- 1 IP address (1 host up) scanned in 3
seconds

This last run is the same as the one previous from a command standpoint, but finger, linuxconf, shell, and login are gone. I could argue that the smart thing would have been to leave rlogin in place and deactivate telnet, but keep in mind that this is an example. Disabling telnet may not be appropriate for your location. For those services that are run by inetd, disabling them in this manner completely removes them from external access, even beyond your /etc/hosts.allow file (discussed in the first part of this series).

What should you disable? If you are running a single, private machine that does not require anyone in the outside world to access it, then just about everything in the list could go. However, if you have a small network with a couple of PCs, you may still want to run ftp, telnet, or rlogin.

One final note. Use tools like port scanners wisely. Use them only to test the security of your own systems, and never, never use them to scan other people's systems. Remember, just as you are learning to deal with and watch for the cracker, so can others watch you.

  • + 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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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