Article · July 003 Source: oai citation reads 789 authors

concentrate at the two main operating system available,
namely Palm OS and Windows CE in this section, although
we are aware that there are several other operating systems
that the current handheld devices use such as Linux or Sym-
bian OS. In section 3, we provide the basic overview of a
firewall and illustrate the importance of a personal firewall
to protect such a system. In section 5, we describe the de-
sign and implementation of personal firewall in a Pocket PC
operating system. We note that this design can be easily
adopted to another operating system such as Symbian or
Linux. Section 6 concludes the paper.
Our Contributions
In this paper, firstly we examine the threats to handheld
devices. We also propose two feasible solutions to secure
handheld devices. One solution is to use a desktop com-
puter to act as a bastion host to protect handheld devices.
The second proposed solution is by building a personal fire-
wall for handheld devices. We will highlight the difficulty
of building such a system and proceed with our design and
implementation on a Pocket PC handheld device.
2
Handheld Devices
In this section, we briefly describe two main operating
systems used in handheld devices, namely Windows CE and
PalmOS, which currently dominate the market. A Windows
CE device (which is usually referred to a Pocket PC device)
uses a proprietary operating system from Microsoft which
is known as the Windows CE operating system. The latest
version of Windows CE operating system is Pocket PC 2002
which is designed to run in either a Strong ARM or an Intel
XScale processor. Palm operating system has evolved to its
current version Palm OS 5 which is used by several devices
such as Palm Tungsten or Sony Clie handheld. There are
several other operating systems that are available for hand-
held devices. Psion PDA uses EPOC and MSI’s Eznow
PDA uses Mine OS. Sharp Zaurus uses Embedded Linux
with Linux kernel 2.4.x as its operating system. Newton
OS is used by Apple’s MessagePad and eMate operating
system. The current smart phones, such as Ericsson P800
or Nokia 7650, uses the Symbian operating system.
In the following discussion, we concentrate on Pocket PC
devices. Any handheld devices will have similar features.
A Pocket PC can be connected to the Internet via sev-
eral methods: using a modem connection, using a wired
or wireless LAN connection, using a Bluetooth connection,
an Infrared connection with a cellular phone, or using a
TCP/IP connection with the assistance of a desktop PC.
As we mentioned in section 1, the Internet connections will
make the Pocket PCs vulnerable to attackers and viruses.
It is noted in [16] [15] that firewalls are important to ensure
security in wireless networks.
In the case of a Pocket PC device, a synchronization
software called ActiveSync is used to enable the commu-
nication between a Pocket PC PDA and a desktop com-
puter. This synchronization ensures that both the PDA
and desktop computer have the latest information. The
current version of ActiveSync is 3.7. Pocket PCs can be
connected with desktop PCs by using infrared, Ethernet
networking or modems [13]. ActiveSync offers a convenient
way to Synchronize data between a PDA and a desktop
PC, but it may also potentially deliver viruses between a
PDA and a PC. Viruses may spread via email, software in-
stallation or file transfer from a desktop PC to a PDA and
vice versa. McAfee’s VirusScan for Pocket PC can protect
desktop computers from infection by scanning the files on
handheld devices whenever they are connected to desktop
PCs [20]. PC-cillin for Wireless 2.0 [7] also offers virus pro-
tection to handhelds running Palm, Pocket PC and EPOC.
PC-cillin for Wireless works by scanning data and devices
for viruses during beaming, sychronization, or Internet ac-
cess.
3
Review of Firewalls
A Firewall is a component or set of components that re-
stricts access between a protected network and the Internet,
or between other sets of networks [24]. A typical scenario
of a firewall is illustrated in Figure 1. A bastion host is
a computer system between a protected network and the
Internet. The bastion host is designed to defend against
attacks on the protected network. The perimeter network,
also called De-Militarized Zone, is a network between an
internal network and an external network. The perimeter
network offers additional protection for the internal net-
work.
There has been much work contributed to firewalls and
their applications [2] [4]. In the next section, we will de-
Internet
Firewall
Perimeter
Network
Internal Network
Router
Interior
Bastion Host
Router
Exterior