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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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interface (most firewalls have a mini-Web server built in to allow this). In the sim-
plest kind of firewall, the
stateless firewall
, the header of each packet passing
through is inspected and a decision is made to pass or discard the packet based
solely on the information in the header and the firewall’s rules. The information in
the packet header includes the source and destination IP addresses, source and
destination ports, type of service and protocol. Other fields are available, but rarely
occur in the rules.
In the example of Fig. 9-32 we see three servers, each with a unique IP address
of the form 207.68.160.x, where
is 190, 191, and 192, respectively. These are the
addresses to which packets must be sent to get to these servers. Incoming packets
also contain a 16-bit
port number
, which specifies which process on the machine
gets the packet (a process can listen on a port for incoming traffic). Some ports
have standard services associated with them.
In particular, port 80 is used for the
Web, port 25 is used for email, and port 21 is used for FTP (file transfer) service,
but most of the others are available for user-defined services. Under these condi-
tions, the firewall might be configured as follows:
IP address
These rules allow packets to go to machine, but only if they are ad-
dressed to port 80; all other ports on this machine are disallowed and packets sent
to them will be silently discarded by the firewall. Similarly, packets can go to the
other two servers if addressed to ports 25 and 21, respectively. All other traffic is
discarded. This ruleset makes it hard for an attacker to get any access to the LAN
except for the three public services being offered.
Despite the firewall, it is still possible to attack the LAN.
For example, if the
Web server is
and the cracker has discovered a bug in
that can be
exploited, he might be able to send a very long URL to on port 80
and force a buffer overflow, thus taking over one of the machines inside the fire-
wall, which could then be used to launch an attack on other machines on the LAN.
Another potential attack is to write and publish a multiplayer game and get it
widely accepted. The game software needs some port to connect to other players,
so the game designer may select one, say, 9876, and tell the players to change their
firewall settings to allow incoming and outgoing traffic on this port. People who
have opened this port are now subject to attacks on it, which may be easy especial-
ly if the game contains a Trojan horse that accepts certain commands from afar and
just runs them blindly. But even if the game is legitimate, it might contain poten-
ially exploitable bugs. The more ports are open, the greater the chance of an attack
succeeding. Every hole increases the odds of an attack getting through.

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