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FIGURE 10-1: Diagram of an open wireless “hotspot” network.
There are many other free, public wireless networks all over the world, including Asia, the
Middle East, Australia, New Zealand, the Pacific region, the Caribbean, and Europe. To get an
idea of the magnitude of this phenomenon, check out the Personal Telco Web site, which lists
hundreds of community wireless groups that are building free and open networks
( The most ambitious
of these envision not just isolated nodes but an interlocking “network of networks” or
Metropolitan Area Network (MAN) designed to allow roaming from site to site within the area
served by the network. Hiccup-free roaming is a technical challenge that has not been entirely
solved at this time, however.
Figure 10-1 shows a common configuration for an open wireless “hotspot” network. Users with
a laptop computer log in, click “I agree” or authenticate in some manner. And then they are
allowed to access the Internet.
This community-based approach to networking can allow you to bring high-speed data to your
neighborhood, even if your local telephone companies or wireless ISPs are moving too slowly,
not moving at all, or have tried and failed due to financial factors. Because noncommercial ser-
vices can be much cheaper in the long run (basically, just the investment in equipment and
expertise), they may garner a wider user base than commercial services could. If even a small
percentage of those users set up their own free wireless hotspots, a “positive feedback” loop is
created, in which the benefits of joining the “free” wireless community increase, bringing more
users and still more hotspots. (The network effect says that the value of a network increases
exponentially with the number of users.)
If you want to be part of this wireless revolution, this chapter is for you!
In this chapter, you™ll build a wireless hotspot that can share your Internet connection with
anyone who comes within range, while giving you the ability to implement a splash screen (the
first screen the user sees when they access the hotspot), and control which sites users can
access. The system is designed to support user IDs and passwords, as well, so you™ll have the
potential to expand to that in the future.
Chapter 10 ” Creating a Free Wireless Hotspot

You™ll need the following hardware:

Wireless access point (AP).
A PC (486 or better) with at least 10GB disk space, 256MB RAM and two Ethernet
network interface cards (NICs). This computer serves as a gateway between your local
wireless network and the Internet. It will run the Linux operating system.
At least one client computer (usually Windows or Mac) for testing. Each client
must have either a wireless NIC or a standard Ethernet card and cable to connect
to the AP.
A high-speed Internet connection such as DSL, with an appropriate wide area network
(WAN) device such as a DSL modem and an Ethernet cable to connect this device to
the Linux computer.

You™ll also need the following software:

Linux. Ideally, you should have Red Hat 9.x with kernel version 2.4.x, with IPtables and
gpgv. NoCatAuth, the free software that will provide the management and control capa-
bilities for your hotspot, uses IPtables to route traffic between the two NICs. We™ll give
instructions for Red Hat 9 in this chapter. You may have to adapt these instructions to
other flavors of Linux. Another software-related requirement is root access to the Linux
server. You should do everything in this chapter as the root user.
The nightly build of NoCatAuth. Download it from the NoCat network site
( It is standard procedure to warn you that a particular nightly build could
be buggy. Peruse the mailing list a bit before downloading, to see if people seem to be having
trouble with recent builds. If so, download the most recent stable build. That being said, when
I downloaded NoCatAuth in early 2004, it looked like the code hadn™t changed since mid-
2002. (I guess they were on to working on NoCatSplash, a software package being groomed
as the successor to NoCatAuth. Dates on NoCatSplash files were very recent.)
DHCP (Dynamic Host Control Protocol) server somewhere on your network. This
could be a DHCP daemon running on your Linux machine (this seems to work best in
most situations), or it could be on your AP or another server. In general, implementing a
DHCP server is not a huge stumbling block. For instance, the default configuration for
many access points includes DHCP service. If your network clients are configured to
obtain an IP address automatically, and they are able to access the network, then they
already have access to a DHCP server.
If you want to set per-user bandwidth limits using throttle.fw, you will need tc installed
on your server. (This is a standard component of Red Hat 9.x.)
Optionally, you can install a local caching DNS (domain name service) server. You get an
option to install a DNS when you install Red Hat 9.x from scratch.

Be aware that you are undertaking a challenging project. There are a lot of things that have to
go right for your hotspot to work, and each one of these things is capable of going wrong in
many different ways. Basically, there are five areas where you may have to do some tinkering:
the AP, the WAN hardware (such as a DSL “modem”), Ethernet and TCP/IP networking, the
240 Part III ” Playing with Access Points

Linux computer, and the client computer(s). If you™re not a guru in all these areas, here™s an
opportunity to learn, and achieve something cool in the process.
The centerpiece of the solution described in this chapter is “captive portal” software,
NoCatAuth, which you download free from the NoCat network site ( and
run on a Linux computer, which thus becomes a gateway. Just the potential complications asso-
ciated with this one piece of software can drive you batty.
It is not unusual for questions on the NoCat mailing list”especially newbie questions”to go
unanswered for days, or even forever, despite sometimes piteous pleas (“Nobody will help
me?”). There is no official support for the NoCat software, and if you can find somebody to
provide support for a price, the price could be fairly steep (say, $75 an hour).
NoCat isn™t the only software available for setting up a free hotspot. If you™re interested in
checking out alternatives, a good starting point is
index.cgi/PortalSoftware. But NoCat seems to be the most popular of the free

What Is NoCatAuth?
NoCatAuth is a “captive portal” software. A portal is a Web site or service that offers access to
an array of other resources and services. A captive portal is the one in which users are initially
“captured” and restricted in what they™re able to do. They may be restricted to just a login
screen, or a screen describing an acceptable use policy (AUP). In that case, they must log in or
accept the policy before they can do anything else. Alternatively, the captive portal may allow
users to access a restricted number of Web sites without logging in or agreeing to the AUP. It is
also possible to include a Skip button on the login screen, allowing the user to skip the login
In this chapter, you™ll set up two basic NoCatAuth configurations: One uses NoCatAuth™s
“Open” mode to create a portal that does not allow login using a user name and password, but
does redirect users to a splash screen. Users have to click a button to continue.
The other configuration uses NoCatAuth™s “Passive” mode to create a portal that allows, but
does not require, a login. The user can press the Skip button, not provide a user name or pass-
word, and be automatically logged in as “unknown.”
You can create a Passive mode NoCatAuth system in which the user has to log in”no
Skip button allowed. However, this requires that you install not only the gateway compo-
nent of NoCat, but also the authorization server (“auth server”) component. The gateway
component provides (or refuses to provide) access to the Internet. Every NoCat hotspot is
based on a gateway, which manages local connections, enforces locally configurable firewall
rules (and optionally bandwidth limitations), and times out idle logins. The auth server
displays the login and logout screens in Passive mode (though not the splash screen in
Open mode”that™s served locally at the gateway) and handles the “backend” processing
for the login. This chapter does not go into any detail about setting up your own auth
Chapter 10 ” Creating a Free Wireless Hotspot

There is an auth server at that everyone is free to use. This chapter assumes that you
will use that. There are some very significant limitations for “outsiders” using this server. In
particular, when it hands out permissions, the auth server assigns one of three classes of service:
Owner (sometimes called “Priority”), Co-op, or Public. Outsiders always get Public class
access. In other words, you have to create a one-size-fits-all security configuration. Setting up
your own auth server gives you much more flexibility.
Even if you do plan to eventually set up your own auth server, you probably want to start by
getting a gateway working with the auth server at first. That way, you™ll have a
“known good” gateway for testing, and will not be trying to debug both the gateway and the
auth server at the same time.
After getting past the splash or login screen (see Figure 10-2), the user can be redirected to the
site originally requested. In the Open mode configuration, it is also possible to edit the splash
screen to redirect all users to a site that you specify. (If you have your own auth server, you can
edit the login screen, too.)
In either Open or Passive mode, you can configure a set of allowed domains, and the user will
not be able to browse any domains other than those. Any attempt to access nonallowed
domains will just bring up the splash screen or login screen. This can be a bit tricky to config-
ure, depending on the particular allowed site. The basic configuration is easy, as explained in

FIGURE 10-2: Typical “splash” page with three options.
242 Part III ” Playing with Access Points

the “Configuring NoCat” section later in this chapter. However, you may encounter situations
where you put a site into the AllowedWebHosts list and still can™t get through to it consis-
tently. Instead, you encounter situations where you are eternally returned to the splash screen or
login screen. It can take some troubleshooting to determine what is wrong and how to correct
it, as you™ll see in the section on “Troubleshooting NoCat.” Unfortunately, you™ll also see why it
is not possible to guarantee a smooth experience for every user when only a limited number of
domains is allowed.
The NoCat log (/usr/local/nocat/nocat.log) records the user™s IP address, the hard-
ware (MAC) address of the user™s Ethernet card, the URL of the site that the user originally
requested, and what level of access (class of service) is granted. In Passive mode, the log also
shows the login name, if the user provides one. Otherwise, it shows “unknown.” All log entries
are time-stamped.
If you were ever required to demonstrate that it was someone else, not you, who did something
on your network (something illegal, for instance), the MAC address could be particularly use-
ful, since it is associated with a particular computer”or, more precisely, with a particular
Ethernet card. This contrasts sharply with IP addresses, which you will probably be assigning
dynamically (using a DHCP server, for example), so that any given IP address gets used over
and over again for different clients.
Both Open and Passive configurations enforce idle time-outs. After a configurable period of
inactivity, the user will be forced to go through the splash page or login page again in order to
continue accessing the Internet.
Neither configuration requires any special client software”just an ordinary browser. This is a
major strength of the captive portal approach.
Note that, in addition to the NoCat software described in this chapter, there is a NoCat com-
munity network operating in Sebastopol, California. This chapter assumes “outsider” status, as
far as this network goes. That is, I assume that, in the security database maintained by the
NoCat community network, users of your gateway will not be defined as members of the
NoCat community or of any other group defined in that database.
There is a newer piece of software, NoCatSplash, which currently did not support authoriza-
tion at the time I tested it. It simply displayed a splash screen, forcing the user to click a button
in order to continue. NoCatSplash is billed as the successor to NoCatAuth. However, when I
played with it in early 2004, NoCatSplash was alpha software, and not as stable as NoCatAuth.
Therefore, I decided to stick with NoCatAuth for this chapter. However, once you are familiar
with NoCatAuth, you will probably find it very easy to migrate to NoCatSplash, should you
decide to do so.

Risk Management
When you decide to provide wireless data services, you essentially become a wireless ISP. As
such, you have a responsibility to try to prevent your hotspot from being used irresponsibly or
for illegal purposes. That could mean anything from spam to child pornography. Although I
don™t know of any cases where a free hotspot operator has been prosecuted for traffic on his or
Chapter 10 ” Creating a Free Wireless Hotspot

her network (and I am not an attorney and do not mean to offer legal advice), it seems only
prudent to take some basic precautions. Anyway, you™re probably a basically good person and
don™t want your hotspot used for bad purposes.
There are three things you can do to control what happens on your hotspot, and perhaps cover
yourself if violations of the law or of Internet etiquette occur:

1. Make users agree to an Acceptable Use Policy (AUP). Your “splash” page (the page that all
users are initially redirected to) might say, for example, that the user will not use your net-
work to send spam, or access or upload child pornography, and so on. This doesn™t actually
stop anybody from doing anything, but it demonstrates your good intentions, and lets
“law-abiding” users know what is expected of them. Either NoCatSplash or NoCatAuth
can ensure that users click a button before being allowed to do anything else.
Check out the following Personal Telco sites for more ideas about what exactly you
might want to put on your splash page:
2. Set up a system that limits what users can do. Either NoCatSplash or NoCatAuth can
apply some blanket rules to all users. For instance:
As an anti-spam measure, both are configured by default to prevent outgoing SMTP
packets, which prevents most e-mail clients from sending mail. (Web mail services, such
as Hotmail and Yahoo! Mail, are not affected.)
Users can be restricted to particular Web sites.
In addition, the auth server can place users in one of the three classes of service (Owner,
Co-op, Public), based on user names and passwords. You can define different rights and
permissions depending on which class they belong to. For instance, some users can be
limited to browsing a few specific Web sites, while others may be free to browse the
whole Internet. However, you can modify the auth server database in order to enable the
different classes of service. This generally means setting up your own auth server.
There is also a (“highly experimental”) facility for throttling bandwidth based on mem-
bership in these same groups. (After you install the NoCatAuth gateway, check out the
throttle.fw file for more information on throttling bandwidth.)
3. Monitor use. This could allow you to detect network abuse and take steps to end it. Most
free wireless network operators seem to have done little, if any, monitoring in the past”a
policy that has been likened to walking around with your eyes closed. However, both his-
torical and real-time monitoring are possible.
Historical monitoring involves analyzing the NoCat log file. Ongoing monitoring of the log
file would have to be automated. Try a Google search on “nocat.log analyzer” to find out
about work that has been done in this area, which you may be able to take advantage of.
Real-time monitoring could be based on a tool like MRTG (Multi Router Traffic
Grapher), which produces graphical images at regular intervals (every 5 minutes by
244 Part III ” Playing with Access Points

default) representing traffic on network links. You could use this to detect a user flooding
your network with traffic, for example, either maliciously or unintentionally. (MRTG
may come free with your Linux system. To download it or just to find out more about it,
go to To get more detailed information on who is causing the problem
and what exactly they are doing, you could go to the NoCat log. You might also use a
packet capture utility such as Ethereal, a free network protocol analyzer for Unix and
Windows. (See This type of analyzer gives you the most detailed
information, though not always the easiest to interpret.

You may have reasons other than security for wanting to monitor your network. For
instance, perhaps you want to be able to limit each user™s free access to a particular length of
time, such as half an hour. Or perhaps you are starting with free access now, but want to
position yourself to charge in the future. Time limitations (other than time-outs after a
period of idleness) and billing are not standard parts of the current NoCat implementation.
However, a number of approaches have been discussed and tried. One approach starts by
analyzing the log file to determine usage. A more flexible and sophisticated approach is to
record information in a MySQL database. Perhaps the most natural approach, however, is
integrating with the RADIUS authorization and accounting server. To find information on
this, try, or do a Google search on “nocat

Pre-Install Setup
You have to install and configure your network hardware before installing NoCat. There are
three basic areas you need to work in: the AP, the Linux box (including configuring the
two Ethernet cards), and the WAN connection (which I will assume to be DSL, for this
The basic hardware setup looks like that shown in Figures 10-3 and 10-4. It involves three
boxes: the wireless AP, the Linux gateway and the DSL “modem.” One Ethernet card connects
to the DSL modem. The other Ethernet card connects to the AP. The names assigned to the
Ethernet cards in Linux (“eth0” and “eth1”) are typical. However, you can assign any names you
want (using System Settings, Network in Red Hat).
If you don™t already have your DSL working, it™s probably best to make sure that things are at
least minimally functional on the Linux side first. Then get the Linux box talking to the WAN.
Finally, add the AP into the mix.

The Linux Box
To set up your NICs on the Linux box, click on the red hat and go to System Settings ➪
Network. A minimal requirement here is to have two active NICs. If one or both are either not
on the list or inactive, things are not going to work. In addition, the status should be ok.
If one of the cards is inactive, try clicking Activate. It may activate, or you may get an error
message giving you a clue about what is wrong.
Chapter 10 ” Creating a Free Wireless Hotspot

FIGURE 10-3: A basic hardware setup, with a DSL modem, a Linux box with
two Ethernet cards, and an AP.

If one of the cards is not on the list, click the New button and use the installation wizard
to install it. If nothing else seems to get a card working, you may want to try deactivating,
deleting, and reinstalling the card. It only takes a few minutes, and it™s a low-risk operation, if
you make sure to write down all the information necessary for reinstalling. So, before you
delete a card, write down all the information in all the tabs on the Network Configuration
screen (Devices, Hardware, IPsec, DNS, Hosts), as well as the information available by clicking
the Edit button.
246 Part III ” Playing with Access Points


Ethernet cable

Phone Line


Standard Ethernet

FIGURE 10-4: Diagram of a hardware setup as shown in Figure 10-3.

Software Installation for a NIC
The procedure for installing a card is as follows. Click the New button. Select “Ethernet
connection” from the “Device Type” list, and click the Forward button in the lower right.
Hopefully, on the “Select Ethernet Adapter” screen, both your cards will be there in the list,
making it easy to select them. If not, click on “Other Ethernet Card” and choose from the
drop-down menu based on your knowledge of the manufacturer and model of the card. Do
not worry about the empty boxes in the “Resource” section, or the “Unknown IRQ.”
Although you can set these things manually, they usually auto-configure correctly. Click
The information you enter on the “Configure Network Settings” screen will be crucial to mak-
ing your NoCat server work correctly. One potentially easy way out is to accept the defaults,
“Automatically obtain IP address settings with dhcp,” and “Automatically obtain DNS infor-
mation from provider.” If this works, fine. If not, you can go to System Settings ➪ Network ➪
Edit and try something else.
If you end up having to set static IP addresses (because automatically obtaining them doesn™t
work, for whatever reason), there are many right ways, and probably more wrong ways, to
configure your network.
Chapter 10 ” Creating a Free Wireless Hotspot

Here are three general principles you should be aware of:

1. You can™t successfully configure your NICs in isolation from the other components of
your network. Thus, network setup may be an iterative process, in which you tinker with
one part of the network (such as AP configuration), and then go back and change other
parts of the network (such as NIC configuration) to make them compatible.
2. No two devices on your local network can have the same IP address.
3. Each NIC must be configured to talk directly to the network device to which it is con-
nected. That generally means being in the same Class C network. In terms of IP address-
ing, that means that, of the four sections of the IP address, the first three are the same. For
instance, if the IP address of your DSL modem is, the NIC that connects to
your DSL modem will have an address like 192.168.1.XXX, where XXX is a number
from 2 to 254. (It can™t be 1 in this case, because that™s already taken by the DSL modem.)
Similarly, if your wireless AP is, the NIC that connects to your DSL modem
will have an address like 192.168.2.XXX, where XXX is a number from 2 to 254.

You may find that it is easiest to set static IP addresses for both NICs. If you do this, I recom-
mend assigning them to different Class C networks, which means having different numbers in
the third section of the IP address (called the “network number”). For example,
First, however, make sure that the DSL modem and the AP use those same Class C network
numbers. Otherwise, you™ll find that you are unable to talk to the DSL modem and AP. You™ll
get a “connection refused” message in your browser, because you violated rule number 3 above.
There are at least three other items that you may have to set: the subnet mask, the default gate-
way, and one or more DNS addresses.
The subnet mask basically says which portions of the IP address define the subnet. Assuming
that you are using Class C networks, as we suggested, the subnet mask should be

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