Home > Articles > Operating Systems, Server

Tunneling Linux Routers

Learn about the iproute2 toolset, an IPsec implementation of Linux, and the future Internet protocol IPv6.
This chapter is from the book

Cesium and xenon are the routers that didn't know they were going to be routers. They were yanked out of their retirement jobs as door stops and dust collectors and impressed into service to save us large sums of money. The basic story is that my group is located in a work center amidst service reps and other folks who need, at best, a single network connection. So every cube is wired with a single network drop, and (gasp) it's all token ring. The group was fortunate enough to be allocated some Sun Ultra5 workstations by the budget gods, but the systems didn't include token ring cards (over a thousand-dollar add-on if purchased directly from Sun). There are a few third-party token ring cards that will operate under Solaris, but we still only had one network drop per cube. Wiring costs, if you've never had to pay them or budget for them, are astronomical. You would think that wireless LAN technologies like 802.11b would price themselves out of the market when compared to the cost and performance of CAT5, but wireless technologies suddenly become attractive when you're faced with funding a crew of electricians pulling cable for a week. Furthermore, we didn't want to pour company money into more token ring drops on the principle of the thing. Well, a Sun workstation without a network connection is only fun for about a day, after which it becomes a very expensive screensaver platform. All Sun systems come with Ethernet ports, so we chipped in on a 10Base-T hub and built our own network segment between the machines, but we were geographically limited to a single row of cubes—you cannot have CAT5 cables running across walkways in work centers.

Although we were short on network connections, we had literally stacks of old 486 and older Pentium boxes, and cartons full of token ring cards. (At the risk of dating myself, a 90MHz Pentium was once a wickedly fast computer—probably at about the same time that token ring was a strategic technology.) Enter cesium to tie the rowA segment to the token ring, and not long afterward, xenon for rowB, and so on. Each token ring gateway, for lack of a better term, needed a token ring card to tie to the main network and an Ethernet card for the local workstation segment. Once we were physically tied to both networks, they had to talk to each other. And sometimes, just sometimes, you're not the network administrator. In this instance, it meant that we were not free to request from the local network administrator that a subnet from the local address space be routing to our token ring gateway so that we could route it to our Ethernet-based workstations. In the general case, you may find yourself needing to construct (sometimes elaborate) network topologies solely because you are not in a situation to request a simple change from the network powers that be.

Fortunately, the networking stack in Linux provides several facilities that make these sorts of configurations possible. (I guess I wasn't alone in needing this sort of configuration.) In this chapter we'll briefly revisit masquerading and talk about how we might let a few things from the outside in. We'll also talk about tunnelling, which can be used to connect two or more networks separated by an intervening network with no knowledge of/support for those networks. As part of the discussion on tunnelling we'll introduce iproute2, a powerful replacement for the traditional network-configuration commands like ifconfig and route. Then, we're going to put it all together to build secure tunnels, also known as VPNs, using FreeS/WAN, an IPsec implementation for Linux. Finally, we'll sink our teeth into some IPv6.

6.1 Linux and Token Ring

In terms of networking hardware, cesium and xenon don't look much different from our basic Ethernet router, silicon, save that they have a token ring card in place of the second Ethernet adapter. There's a long story about not being able to boot Compaq Deskpro XL90s directly into Linux because of some craziness about a DOS-based device driver that has to be loaded to map the BIOS into memory. Instead of going into that, I'll summarize by saying that the best way to boot one of these beasts is to boot oN of a DOS diskette, load the silly device driver, and then use syslinux to load the Linux kernel from that same floppy and boot the box into Linux. And find a HOWTO There is another equally long story about getting a PCI-based Madge token ring card running under Linux. At the time, the drivers were not part of the upstream kernel, and I had another relatively discouraging experience with vendor-supplied drivers. Not that I'm ungrateful that Madge oNers Linux support—that's the card I had on hand—and I was able to get things running. (And token ring cards remain expensive. I suppose they never reached critical mass in the marketplace.) But things just seem to take longer with vendor-supplied drivers. The precompiled binary versions are invariably for a kernel other than the one you need to run (and there always seems to be a good reason for that), and the source kits can suNer from odd build dependencies (in this case newt) or general libc or kernel version touchiness. I'll take a driver from the upstream kernel over a vendor driver any day.

Since this is a networking book, token ring itself deserves a little attention. Although much maligned—sysadmins who join my group always react with You're running what?—token ring is a little like the Rodney Dangerfield of LAN link-layer protocols. The protocol is actually much like FDDI, both in its token passing (only the person holding the token is permitted to speak, or transmit) and in its stability during peak-load situations. Token ring has the ability to weather very high load without any nasty drops in throughput as on a bridged Ethernet segment, and without expensive concentrators like Ethernet switches. If you're interested in learning more about the protocol, try http://www.networkuptime.com/faqs/token-ring/, which is a collection of questions and answers from the comp.dcom.lans.token-ring Usenet news-group. You can also go straight to the source and try http://www.8025.org/, but it's a tad dry for my tastes.

If you do happen to be running token ring on Linux, you can expect the same stability from it that you do with Ethernet. You may notice that a few tools lag behind their Ethernet counterparts. For example, older versions of tcpdump won't decode the packets on the wire for you, so you're stuck with a timestamp plus station addresses, flags, and then raw packet data as shown here. Newer versions work as you'd expect. Here is what the old output looks like:

cesium:~$ tcpdump -n -i tr0
22:21:30.643295 0:0:90:0:5a:7d 10:40:c0:0:0:1 609e 352:
                                           c220 aaaa 0300 0000 0800 4510 0148 792d
                                           4000 4006 e5db ab88 23f0 ab88 5f96 0016
                                           03ec 77f0 03e8 7847 968f 8018 7d78 94ec
                                           0000 0101 080a
22:21:30.643295 60:9e:0:6:7c:97 10:40:10:0:5a:7d 7ac1 74:
                                           aaaa 0300 0000 0800 4510 0034 e54e 4000
                                           3e06 7cce ab88 5f96 ab88 23f0 03ec 0016
                                           7847 968f 77f0 03e8 8010 7c70 9bb2 0000
                                           0101 080a 99a9

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