wndwFirstEdition/Chapter8

Indice

  1. Troubleshooting
    1. Building your team
    2. Proper troubleshooting technique
    3. Common network problems
      1. Locally hosted websites
      2. Open proxies
      3. Open relay hosts
      4. Peer-to-peer networking
      5. Programs that install themselves (from the Internet)
      6. Windows updates
      7. Programs that assume a high bandwidth link
      8. Windows traffic on the Internet link
      9. Worms and viruses
      10. Email forwarding loops
      11. Large downloads
      12. Sending large files
      13. Users sending each other files

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Troubleshooting

How you establish the support infrastructure for your network is as important as what type of equipment you use. Unlike wired connections, problems with a wireless network are often invisible, and can require more skill and more time to diagnose and remedy. Interference, wind, and new physical obstructions can cause a long-running network to fail. This chapter details a series of strategies to help you build a team that can support your network effectively.

Building your team

Every village, company or family has individuals who are intrigued by technology. They are the ones found splicing the television cable, re-wiring a broken television or welding a new piece to a bicycle. These people will take interest in your network and want to learn as much about it as possible. Though these people are invaluable resources, you must avoid imparting all of the specialized knowledge of wireless networking to only one person. If your only specialist loses interest or finds better paying work somewhere else, they take the knowledge with them when they go.

There may also be many young and ambitious teenagers or young adults who will be interested and have the time to listen, help, and learn about the network. Again, they are very helpful and will learn quickly, but the project team must focus their attention on those who are best placed to support the network in the coming months and years. Young adults and teenagers will go off to university or find employment, especially the ambitious youth who tend to want to be involved. These youth also have little influence in the community, where an older individual is likely to be more capable of making decisions that positively affect the network as a whole. Even though these individuals might have less time to learn and might appear to be less interested, their involvement and proper education about the system can be critical.

Therefore, a key strategy in building a support team is to balance and to distribute the knowledge among those who are best placed to support the network for the long term. You should involve the youth, but do not let them capitalize use or knowledge of these systems. Find people who are committed to the community, who have roots in the community, who can be motivated, and teach them. A complementary strategy is to compartmentalize functions and duties, and to document all methodology and procedures. In this way, people can be trained easily, and substituted with little effort.

For example, in one project site the training team selected a bright young university graduate who had returned to his village. He was very motivated and learned quickly. Because he learned so quickly, he was taught more than had been foreseen, and he was able to deal with a variety of problems, from fixing a PC to rewiring Ethernet cable. Unfortunately, two months after the project launch he was offered a government job and left the community. Even a better salary could not keep him, since the prospect of a stable government job was too appealing. All of the knowledge about the network and how to support it left with him. The training team had to return and begin the training again. The next strategy was to divide functions, and to train people who were permanently rooted in the community: people who had houses and children, and were already employed. It took three times as long to teach three people as it took to train the young university grad, but the community will retain this knowledge for much longer.

Though this might seem to suggest that you should hand-pick who is to be involved, that is not often the best approach. It is often best to find a local partner organization or a local manager, and work with them to find the right technical team. Values, history, local politics, and many other factors will be important to them, while remaining completely unfathomable to people who are not from that community. The best approach is to coach your local partner, to provide them sound criteria, make sure that they understand that criteria, and to set firm boundaries. Such boundaries should include rules about nepotism and patronage, though these rules must consider the local situation. It may be impossible to say that you cannot hire kin, but it is best to provide a means of checks and balances. Where a candidate is kin, there should be clear criteria and a second authority in deciding upon their candidacy. It is also important that the local partner is given this authority and is not undermined by the project organizers, thus compromising their ability to manage. They will be best able to judge who will work best with them. If they are well educated in this process, then your requirements should be satisfied.

Troubleshooting and support of technology is an abstract art. The first time you look at an abstract painting, it may just look to you like a bunch of random paint splatters. After reflecting on the composition for a time, you may come to appreciate the work as a whole, and the "invisible" coherence becomes very real. The neophyte looking at a wireless network may see the antennas and wires and computers, but it can take a while for them to appreciate the point of the "invisible" network. In rural areas, it can often take a huge leap of understanding before locals will appreciate an invisible network that is simply dropped into their village. Therefore, a phased approach is needed to ease people into supporting technology systems. The best method is involvement. Once the participants are chosen and committed to the project, involve them as much as possible. Let them "drive". Give them the cable crimper or keyboard and show them how to do the work. Even if you do not have time to explain every detail and even if it will take longer, they need to be involved physically and see not only what has been done, but how much work was done.

The scientific method is taught in virtually all western schools. Many people learn about it by the time they reach high-school science class. Simply put, you take a set of variables, then slowly eliminate those variables through binary tests until you are left with one or only a few possibilities. With those possibilities in mind, you complete the experiment. You then test to see if the experiment yields something similar to the expected result. If it did not, you re-calculate your expected result and try again. The typical agrarian villager may have been introduced to the concept, but likely will not have had the opportunity to troubleshoot complex problems. Even if they are familiar with the scientific method, they might not think to apply it to resolving real problems.

This method is very effective, although time consuming. It can be sped up by making logical assumptions. For example, if a long-running access point suddenly stops working after a storm, you might suspect a power supply related problem and thus skip most of the procedure. People charged with supporting technology should be taught how to troubleshoot using this method, as there will be times when the problem is neither known nor evident. Simple decision trees or flow charts can be made that test these variables, and try to eliminate the variables to isolate the problem. Of course, these charts should not be followed blindly.

It is often easier to teach this method using a non technological problem first. For example, have your student develop a problem resolution procedure on something simple and familiar, like a battery powered television. Start by sabotaging the television. Give them a battery that is not charged. Disconnect the aerial. Insert a broken fuse. Test the student, making it clear that each problem will show specific symptoms, and point the way as to how to proceed. Once they have fixed the television, have them apply this procedure to a more complicated problem. In a network, you can change an IP address, switch or damage cables, use the wrong SSID, or orient the antenna in the wrong direction. It is important that they develop a methodology and procedure to resolve these problems.

Proper troubleshooting technique

No troubleshooting methodology can completely cover all problems you will encounter when working with wireless networks. But often, problems come down to one of a few common mistakes. Here are a few simple points to keep in mind that can get your troubleshooting effort working in the right direction.

It is unlikely that the people who design your network will be on call twenty-four hours per day to fix problems when they arise. Your troubleshooting team will need to have good troubleshooting skills, but may not be competent enough to configure a router from scratch or crimp a piece of LMR-400. It is often much more efficient to have a number of backup components on-hand, and train your team to be able to swap out the entire broken part. This could mean having an access point or router pre-configured and sitting in a locked cabinet, plainly labeled and stored with backup cables and power supplies. Your team can swap out the failed component, and either send the broken part to an expert for repair, or arrange to have another backup sent in. Assuming that the backups are kept secure and are replaced when used, this can save a lot of time for everyone.

Common network problems

Often, connectivity problems come from failed components, adverse weather, or simple misconfiguration. Once your network is connected to the Internet or opened up to the general public, considerable threats will come from the network users themselves. These threats can range from the benign to the outright malevolent, but all will have impact on your network if it is not properly configured. This section looks at some common problems found once your network is used by actual human beings.

Locally hosted websites

If a university hosts its website locally, visitors to the website from outside the campus and the rest of the world will compete with the university's staff for Internet bandwidth. This includes automated access from search engines that periodically spider your entire site. One solution to this problem is to use split DNS and mirroring. The university mirrors a copy of its websites to a server at, say, a European hosting company, and uses split DNS to direct all users from outside the university network to the mirror site, while users on the university network access the same site locally. Details about how to set this up are provided in chapter three.

http://wiki.wndw.net/images/figures/en/figure-8.1.png

Figure 8.1: In Example 1, all website traffic coming from the Internet must traverse the VSAT. In Example 2, the public web site is hosted on a fast European service, while a copy is kept on an internal server for very fast local access. This improves the VSAT connection and reduces load times for web site users.

Open proxies

A proxy server should be configured to accept only connections from the university network, not from the rest of the Internet. This is because people elsewhere will connect and use open proxies for a variety of reasons, such as to avoid paying for international bandwidth. The way to configure this depends on the proxy server you are using. For example, you can specify the IP address range of the campus network in your

Open relay hosts

An incorrectly configured mail server will be found by unscrupulous people on the Internet, and be used as a relay host to send bulk email and spam. They do this to hide the true source of the spam, and avoid getting caught. To test for an open relay host, the following test should be carried out on your mail server (or on the SMTP server that acts as a relay host on the perimeter of the campus network). Use telnet to open a connection to port 25 of the server in question (with some Windows versions of telnet, it may be necessary to type 'set local_echo' before the text is visible): 

telnet mail.uzz.ac.zz 25

Then, if an interactive command-line conversation can take place (for example, as follows), the server is an open relay host: 

MAIL FROM: spammer@waste.com 
250 OK - mail from <spammer@waste.com> 
RCPT TO: innocent@university.ac.zz 
250 OK - rcpt to spammer@waste.com

Instead, the reply after the first MAIL FROM should be something like: 

550 Relaying is prohibited.

An online tester is available at sites such as http://www.ordb.org/. There is also information about the problem at this site. Since bulk emailers have automated methods to find such open relay hosts, an institution that does not protect its mail systems is almost guaranteed to be found and abused. Configuring the mail server not to be an open relay consists of specifying the networks and hosts that are allowed to relay mail through them in the MTA (eg., Sendmail, Postfix, Exim, or Exchange). This will likely be the IP address range of the campus network.

Peer-to-peer networking

Bandwidth abuse through peer-to-peer (P2P) file-sharing programs such as Kazaa, Morpheus, WinMX and BearShare can be prevented in the following ways:

Programs that install themselves (from the Internet)

There are programs that automatically install themselves and then keep on using bandwidth - for example, the so-called Bonzi-Buddy, the Microsoft Network, and some kinds of worms. Some programs are spyware, which keep sending information about a user's browsing habits to a company somewhere on the Internet. These programs are preventable to some extent by user education and locking down PCs to prevent administrative access for normal users. In other cases, there are software solutions to find and remove these problem programs, such as Spychecker (http://www.spychecker.com/), Ad-Aware (http://www.lavasoft.de/), or xp-antispy (http://www.xp-antispy.de/).

Windows updates

The latest Microsoft Windows operating systems assume that a computer with a LAN connection has a good link to the Internet, and automatically downloads security patches, bug fixes and feature enhancements from the Microsoft Web site. This can consume massive amounts of bandwidth on an expensive Internet link. The two possible approaches to this problem are:

Blocking the Windows updates site on the proxy server is not a good solution because the Windows update service (Automatic Updates) keeps retrying more aggressively, and if all workstations do that, it places a heavy load on the proxy server. The extract below is from the proxy log (Squid access log) where this was done by blocking Microsoft's cabinet (.cab) files.

Much of the Squid log looks like this: 

2003.4.2 13:24:17 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab  *DENIED* Banned extension .cab GET 0 
2003.4.2 13:24:18 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab GET 0 
2003.4.2 13:24:18 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab HEAD 0 
2003.4.2 13:24:19 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab GET 0 
2003.4.2 13:24:19 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab GET 0 
2003.4.2 13:24:20 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab GET 0 
2003.4.2 13:24:21 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab GET 0 
2003.4.2 13:24:21 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab GET 0 
2003.4.2 13:24:21 192.168.1.21 http://windowsupdate.microsoft.com/ident.cab *DENIED* Banned extension .cab HEAD 0

While this may be tolerable for a few PC clients, the problem grows significantly as hosts are added to the network. Rather than forcing the proxy server to serve requests that will always fail, it makes more sense to redirect the Software Update clients to a local update server.

In addition to Windows updates, many other programs and services assume that bandwidth is not a problem, and therefore consume bandwidth for reasons the user might not predict. For example, anti-virus packages (such as Norton AntiVirus) periodically update themselves automatically and directly from the Internet. It is better if these updates are distributed from a local server.

Other programs, such as the RealNetworks video player, automatically download updates and advertisements, as well as upload usage patterns back to a site on the Internet. Innocuous looking applets (like Konfabulator and Dashboard widgets) continually poll Internet hosts for updated information. These can be low bandwidth requests (like weather or news updates), or very high bandwidth requests (such as webcams). These applications may need to be throttled or blocked altogether.

The latest versions of Windows and Mac OS X also have a time synchronization service. This keeps the computer clock accurate by connecting to time servers on the Internet. It is better to install a local time server and distribute accurate time from there, rather than to tie up the Internet link with these requests.

Windows computers communicate with each other via NetBIOS and Server Message Block (SMB). These protocols work on top of TCP/IP or other transport protocols. It is a protocol that works by holding elections to determine which computer will be the master browser. The master browser is a computer that keeps a list of all the computers, shares and printers that you can see in Network Neighbourhood or My Network Places. Information about available shares are also broadcast at regular intervals.

The SMB protocol is designed for LANs and causes problems when the Windows computer is connected to the Internet. Unless SMB traffic is filtered, it will also tend to spread to the Internet link, wasting the organization's bandwidth. The following steps might be taken to prevent this:

Worms and viruses

Worms and viruses can generate enormous amounts of traffic. The W32/Opaserv worm, for example, is still prevalent, even though it is an old one. It spreads through Windows shares and is detected by other people on the Internet because it attempts to spread further. It is therefore essential that anti-virus protection is installed on all PCs. Furthermore, user education about executing attachments and responding to unsolicited email is essential. In fact, it should be a policy that no workstation or server should run unused services. A PC should not have shares unless it is a file server; and a server should not run unnecessary services either. For example, Windows and Unix servers typically run a web server service by default. This should be disabled if that server has a different function; the fewer services a computer runs, the less there is to exploit.

Email forwarding loops

Occasionally, a single user making a mistake can cause a problem. For example, a user whose university account is configured to forward all mail to her Yahoo account. The user goes on holiday. All emails sent to her in her absence are still forwarded to her Yahoo account, which can grow to only 2 MB. When the Yahoo account becomes full, it starts bouncing the emails back to the university account, which immediately forwards it back to the Yahoo account. An email loop is formed that might send hundreds of thousands of eemailmails back and forth, generating massive traffic and crashing mail servers.

There are features of mail server programs that can recognize loops. These should be turned on by default. Administrators must also take care that they do not turn this feature off by mistake, or install an SMTP forwarder that modifies mail headers in such a way that the mail server does not recognize the mail loop.

Large downloads

A user may start several simultaneous downloads, or download large files such as 650MB ISO images. In this way, a single user can use up most of the bandwidth. The solutions to this kind of problem lie in training, offline downloading, and monitoring (including real-time monitoring, as outlined in chapter six). Offline downloading can be implemented in at least two ways:

Sending large files

When users need to transfer large files to collaborators elsewhere on the Internet, they should be shown how to schedule the upload. In Windows, an upload to a remote FTP server can be done using an FTP script file, which is a text file containing FTP commands, similar to the following (saved as c:\ftpscript.txt): 

open ftp.ed.ac.uk 

gventer 
mysecretword 
delete data.zip 
binary 
put data.zip 
quit

To execute, type this from the command prompt: 

ftp -s:c:\ftpscript.txt

On Windows NT, 2000 and XP computers, the command can be saved into a file such as transfer.cmd, and scheduled to run at night using the Scheduled Tasks (Start -> Settings -> Control Panel -> Scheduled Tasks). In Unix, the same can be achieved by using at or cron.

Users sending each other files

Users often need to send each other large files. It is a waste of bandwidth to send these via the Internet if the recipient is local. A file share should be created on the local Windows / Samba /web Novell server, where a user can put the large file for others to access.

Alternatively, a web front-end can be written for a local web server to accept a large file and place it in a download area. After uploading it to the web server, the user receives a URL for the file. He can then give that URL to his local or international collaborators, and when they access that URL they can download it. This is what the University of Bristol has done with their FLUFF system. The University offers a facility for the upload of large files (FLUFF) available from http://www.bristol.ac.uk/fluff/. These files can then be accessed by anyone who has been given their location. The advantage of this approach is that users can give external users access to their files, whereas the file share method can work only for users within the campus network. A system like this can easily be implemented as a CGI script using Python and Apache.

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