Home > Articles > Security > Network Security

Rules, Rulesets, and Rulebases

Generically, a rule is a set of criteria to match against a packet's properties and an action to take if the criteria and properties match. The criteria might be as vague and all-encompassing as "all packets," or it might be as specific as "any inbound fragmented TCP packet arriving on interface eth0, with the IP source routing option and TCP flag FIN set, destined for port 143 on host, originating from network"

The table of actions of a rule generally are composed of one of the possibilities shown in Table 2.

Table 2: Generic Firewall Rule Actions


Action Description


Accept the packet, and pass or forward it on to its final destination.


Drop the packet on the floor. Do not pass go. Do not even acknowledge the fact that the packet ever existed. Don't let the sending machine know the packet has been discarded. What packet?


Do not accept the packet, but alert the sending machine that the packet has been rejected. This is normally done for TCP by sending a packet with the reset (RST) flag set. For UDP, because it has no concept of state, an ICMP is returned with the type destination port unreachable.

Action Modifiers

Modifier Description


Log the action applied to the packet and all pertinent information. This could be date/time, source/destination IP addresses, source/destination ports, rule number that triggered the action, and the action itself. Some log modifiers allow you to log part or all the packet itself for later inspection.


Rewrite the packet address and possibly port information. This is normally used to do Network Address Translation (NAT), or Network Address Port Translation (otherwise known as IP Masquerading).


Redirect the packet to reroute it from one IP address and port to another. This is useful for implementing transparent proxies.

Rule Order

In most firewalls, the packets are inspected and matched against the rules in the rulebase in a sequential order. This means that the packet is compared to the first rule, and then the second rule, and then the third rule, and so on until a match is made. After the packet is matched to a rule, no additional actions or comparisons of the packet to the rulebase are made. One notable exception to this rule is the IP Filter firewall that is native to OpenBSD, FreeBSD, and NetBSD, but also available for Linux and several other UNIX systems. IP Filter normally processes packets in rule groups, and maintains status of the last matching rule. You have the flexibility in IP Filter to stop the process of matching further rules by using a special command in the rule. However, unless configured differently, the decision to drop or pass the packet is not made until the last rule in the group has been compared to the packet.

To better illustrate the difference, take the following sample rulebase:

block all inbound traffic
pass all inbound traffic

For most firewalls, including CheckPoint's FireWall-1 and native Linux ipchains firewalling, the first rule to match applies to the packet. In the sample rulebase, an incoming packet matches rule #1. The packet is then immediately dropped by rule #1, and no further comparison takes place.

IP Filter and firewalls like it apply the action on the status of last matching rule. In the sample rulebase, the incoming packet matches rule #1. At this point, IP Filter sets the state for that packet to "matched rule #1, drop packet," but continues to compare the packet to the remaining rules in the ruleset group. The incoming packet in the example also matches rule #2, so the state of the packet is now "matched rule #2, pass packet." Because there is no rule #3, the packet is actually passed rather than blocked by IP Filter.

If you want to make IP Filter act more like other firewalls, you can apply the quick modifier to an individual rule. The quick modifier halts the packet comparison process of a ruleset on a match. The following IP Filter rules then mimic the actions of ipchains and FireWall-1:

block in quick all
pass in all

Performance-Tuning Your Firewall

Knowing what traffic is passing through your firewall and how your firewall applies rulesets in a rulebase will help you tune your firewall for performance. For example, you can increase the performance of a firewall that processes a packet on the first matching rule by moving your most commonly matched rules to the beginning. In effect, this causes the firewall to do less work per packet. Firewall performance tuning should only be attempted after you have a working rulebase, and special attention should be made to the order. Do not misconfigure your firewall by placing accept rules before rules that would otherwise deny those packets. The importance of ordering rules in a rulebase is illustrated better in the examples in the following sections.

Remote Administration of Firewalls and DMZ Servers

SSH is a suite of applications that includes the ssh program, which replaces rlogin and telnet; and scp, which replaces rcp and ftp. Also included is sshd, which is the server side of the package. SSH encrypts all traffic (including passwords) to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks. Additionally, SSH provides many secure tunneling capabilities. There are several implementations of the SSH suite and protocols. SSH is available in many implementations and formats:

  • SSH is a commercial product from SSH Communications Security (http://www.ssh.com/) that supports the Windows Win32 platforms (Windows 9x/NT/2000), Linux, and several UNIX flavors including MacOS X.

  • F-Secure SSH is another commercial implementation for cross-platform environments available from F-Secure Corporation. F-Secure SSH supports Windows Win32 platforms (Windows9x/NT/2000), Macintosh, and most major UNIX platforms. F-Secure SSH can be found at http://www.datafellows.com/products/ssh/.

  • OpenSSH is a free implementation maintained by the OpenBSD team at http://www.openssh.com. It supports SSH protocol versions 1.3, 1.5, and 2.0. It runs on many versions of UNIX and UNIX variants. Versions for other platforms can also be found there.

  • NiftyTelnet is a free software implementation of SSH available for MacOS. It includes support for scp and SSH protocol version 1.5. NiftyTelnet can be found at http://www.lysator.liu.se/~jonasw/freeware/niftyssh/.

  • MindTerm is a 100% pure Java implementation of SSH 1.5 from MindBright, an IT consulting company in Sweden. MindTerm runs on just about any platform that has a recent Java Runtime Environment. It can be executed on the command line or launched from within a browser. MindTerm can be found at http://www.mindbright.se/mindterm/.

With the various commercial and free implementations of SSH available, there is no reason not to install it on your servers. Unless you are using hardwired serial consoles to manage your servers, it would be otherwise irresponsible for an administrator to not be using SSH for remote administration.

Managing your Windows NT/2000 servers securely is slightly more difficult because of the primary "ease-of-use" feature: the GUI. There are a plethora of "remote desktop" utility programs available. The main difficulty you will have is in ensuring the security of the product, and gauging the time-to-fix from the vendor for any exposed exploits.

Windows NT 4.0 has a Terminal Server Edition, which allows remote clients to "natively" control desktop sessions. Unfortunately, the release of both service packs and hotfixes from Microsoft are delayed weeks to months behind the normal Windows NT 4.0 releases. The additional risk assumed during the delayed release window for intruders to take advantage of known exploits is high enough that you should not consider this option.

pcAnywhere from Network Associates is a commonly implemented remote control utility. It allows network traffic to be encrypted (through the Microsoft Crypto API), and file transfer so you can also replace FTP.

Obviously, you will have to evaluate your current remote management strategy and ensure that it makes sense to control your DMZ servers or remote servers at the colocation facility. Criteria to compare include the following:

  • File transfer capabilities. Transfer of files both to and from the remote server. Compression of files is useful, especially over lower bandwidth links. Delta file transfer will only transmit the changes to files, which can save time and bandwidth as well.

  • Data encryption. Encrypting the data over the wire is important; otherwise, intruders may be able to sniff account, password, or other critical information. Strong encryption of 3DES, TwoFish, Rijndael, Serpent, or CAST is preferred.

  • Configurability. Security through obscurity isn't security at all. But it certainly may help keep out the script kiddies. Look for the ability to reconfigure the listening ports for the remote administration service.

Many intruders use a remote administration tool known as Back Orifice 2000, found at http://www.bo2k.com/. It is normal for management and most administrators to feel uneasy about using a tool created by a group of hackers known as the Cult of the Dead Cow. However, the tool is quite flexible, easy on resources, and is open source. Utilizing a plug-in architecture, it can easily be expanded to support additional encryption technologies or authentication methods, for example. Because the source is available, it can easily be audited and patched for discovered vulnerabilities.

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