Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

  • Print
  • + Share This

Networking Stack Enhancements

Now we finally come to a topic near and dear to my heart. Initially this article was going to focus only on the improvements to the networking stack, but on consideration I decided that it wouldn't be fair to members of the audience who weren't using Linux as a router, firewall, packet-shaper, what-have-you… (not to mention the fact that the other new features are important in their own right). That being said, get ready for some very interesting developments.

Packet Filtering

First, the packet-filtering and masquerading code has been changed again. For the third time in as many kernel series, administrators will have to learn yet another interface to configure their firewall rules and masquerading. The new mechanism is called netfilter and uses the tool iptables to administer the rulesets, as compared to ipfw functionality (using the tool ipfwadm) in the 2.0 kernel and ipchains functionality (using the tool ipchains) in the 2.2 kernel. Frankly, I was a bit skeptical at first; not too long after taking the time to convert all my ipfwadm commands over to ipchains (taking advantage of a few of the improvements along the way), everything has to be redone just so that I can upgrade to the 2.4 kernel!

Well, that's not entirely true. One of the first features I encountered is that netfilter (I will use the term netfilter to refer to the kernel facility and iptables to refer to the explicit command) is that it provides modules (or tables) that implement the ipfwadm or ipchains functionality. The only caveat is that you can't mix the three facilities together, but this shouldn't pose much of a problem. The backward compatibility is a nice option for those who want to take advantage of some of the previously mentioned features of the 2.4 kernel, but can't yet risk reconfiguring the firewall to use netfilter. Because the facilities are implemented as modules, you can switch between them simply by loading and unloading modules and using the appropriate command-line tool for the enabled facility. This certainly eases the migration to netfilter, although it's unlikely once you get a taste of netfilter's features that you'll stick with ipchains for long.

Network Address Translation (NAT)

The first major netfilter feature is a biggie: Linux finally supports true Network Address Translation (NAT), just like Cisco PIX or any other NAT solution. The difference between this and just masquerading is subtle, but fantastic. With masquerading, all client packets being forwarded by the router are massaged to carry the source address of the masquerading router, and this massaging process involves remapping the source port number to a free (normally different) port on the masquerading router. When the destination responds, it sends the packet to the router at the remapped port, and the router then "unmasquerades" the packet and forwards it to the client.

This works perfectly well for standard (really, I should say "simple") client/server protocols where the client chooses any port (typically above 1024) and talks to a single, well-known server port. This scheme shows its limitations, however, when the client doesn't always initiate the connection, or when the server would like to stream UDP packets back to the client. (Examples include IRC, FTP, X, and RealAudio.) The 2.2 world has workarounds for given protocols—kernel modules and user-space daemons—but they all have the limitation that only one of any given well-known port is available on the masquerading machine. Imagine configuring H.323 for a family of workstations behind a masquerading firewall. This protocol can be described as "peer-to-peer," meaning that each side of the connection has both client and server components. Once you map the server port to one of your workstations, you're all out of server ports. X windows suffers the same fate, but fortunately OpenSSH comes to the rescue.

NAT takes care of this issue for you, given that you have a pool of addresses for the network on the external side of the router. For each client, the NAT module can allocate a "real" address to it from this pool, meaning that neither the system on the other end nor the client on your network has any idea that address translation is occurring. Of course, this isn't limited to just clients; you can use this in conjunction with packet filters to enforce very strict firewalling policies by routing all traffic for, say, your SMTP server to the NAT address on the firewall, and then filtering out everything except TCP traffic to and from port 25. To do this, you would merely need to advertise the MX record for that SMTP server to be the address on the NAT, and then configure the NAT to forward this traffic to the mail server. It doesn't take much imagination to conceive a plan for load balancing based on this feature (similar to the Big-IP products from F5 Networks).

What Else Can Netfilter Do?

Let's move on from NAT and talk about the rest of netfilter. One nice thing about netfilter is that it's conceptually more similar to ipchains than ipchains is to ipfwadm. There is still the concept of chains of rules that a packet traverses, as well as the idea of both kernel chains and user-defined chains. Depending on matching criteria, a packet will either match and thus traverse a chain, or pass by it. At the end of the chain, it will be passed on to another chain, dropped, etc. The details of the kernel chains have changed somewhat, so be sure to read the relevant documentation.

The architecture of netfilter is such that it's extensible; extensions for a particular protocol or to add a given functionality are available as modules that be loaded into the kernel. Some of these extensions are familiar to ipchains users, such as the TCP and UDP match extensions. Others are brand new, such as the limit module, which can be used to set the maximum rate at which packets can traverse a particular chain. This can be used to suppress duplicate log entries, prevent DoS (Denial of Service) attacks, and limit bursts of a given type of service. Another extension allows matching based on the owner or group of locally generated packets. As an example, this could be used on multi-user machines to prevent users from sending mail directly out to an external host, forcing them to use the local MTA (Mail Transfer Agent), which would ensure logging as well as prevent spoofing and internally generated spam.

Before moving on, let me mention a couple of other goodies. A LOG extension allows logging of rule matches via the standard syslog facility, and a QUEUE extension sends packets up to user space to be massaged, reviewed, and so on before further traversing the stack. Both of these features promise very sophisticated security tools, such as adaptive packet filters that change their configuration based on traffic volume or type of attack.


If netfilter doesn't inspire you to rush out and upgrade, perhaps your interests lie more at the heart of the network than on the periphery. Quality of Service (QoS) was all the buzz at the last Networld+INTEROP I attended. It deals with one of the core limitations of TCP/IP—the sticky problem of needing to prioritize network traffic based on interactive versus batch, and important customers versus Joe Nobody. The kernel does this through the QoS facility, which offers a variety of algorithms for the packet scheduler. Some of these algorithms allow special prioritization for real-time scheduling; others can be used to throttle certain types of traffic (on certain interfaces, if desired). Of significance is the fact that the standards-based diffserv (Differentiated Services) and RSVP (Resource Reservation Protocol) protocols are supported, which means that Linux can coordinate with other non–Linux network infrastructures to provide quality of service features to your clients.

Various other gizmos and doodads make the Linux 2.4 kernel attractive. Policy-based routing replaces the iproute2 functionality in version 2.2 to allow routing decisions to be made based on packet source address, TOS field, or a netfilter rule (based on the mark feature). An experimental HTTP server is implemented directly in kernel space for the ultimate in Web server performance, and an Ethernet "frame diverter" provides transparent proxying/redirection all the way down at layer 2. Of all the goodies, I still feel that netfilter is the first and foremost. NAT is vitally important to Linux's future as a network appliance, so I'm glad to see it in there. As Rusty Russell (the Linux IP Firewall maintainer and author of netfilter) states in his Linux 2.4 Packet Filtering HOWTO, "I predicted 6 months, and it took 12, but I felt by the end that it had been done Right." I'm glad that he took the time. You can read up on netfilter at http://netfilter.kernelnotes.org/.

  • + 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