Figure 6.
Filter Incoming and Outgoing Packets
5.3.2
Filter Outgoing Network Packets
When the intermediate driver receives an outgoing packet
from the protocol driver, it copies this packet to its pre-
allocated packet, and send this new copy down to the mini-
port driver. The miniport driver then send this packet out.
As mentioned in 5.3.1, each network packet includes its
5
Figure 7.
Filtering Incoming and Outgoing Net-
work Traffic
source and destination IP addresses in its header session,
the intermediate driver can extract these two IP addresses
from the packet header and perform the filtering based on
source and destination IP addresses. Thus, the interme-
diate driver can filter the outgoing network packets before
passing them down to the adapter (shown in Figure 6). Fig-
ure 7 illustrates an example of filtering outgoing network
traffic.
6
Conclusion
Handheld computers are important computing tools due
to their size and light weight. However, handheld devices
also become attackers’ targets as they become more popu-
lar. We reviewed some security threats to handheld com-
puters and proposed several possible solutions. The first
solution is to use a desktop computer as a bastion host.
The second one is to build a personal firewall in a hand-
held computer. We highlighted the difficulty of building a
personal firewall in a Pocket PC operating system. We de-
scribed an implementation in an iPAQ Pocket PC which is
equipped with a Strong ARM processor with 206 MHz.
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