A computer program or hardware device used to provide
additional security on networks by blocking access from the
public network to certain services in the private network. Firewalls contain
rule sets that either grant or deny data traffic flowing into or out of a
network. Simply put, firewalls are to the perimeter of a network what a moat
and wall are to a castle.
Because system administrators need to grant access from the
outside world to some services within the perimeter, such as email or a Web server, they need to
drill holes for these services in their firewalls. Unfortunately, these holes
can be exploited by perpetrators. For example, control of outgoing traffic is
an often neglected area; there is a real risk that users can introduce
malicious code into the network by opening an email attachment or by surfing to
a Website having malicious content that installs a back door program on an
internal system. These back doors
initiate connections to an attacker that, from the firewalls perspective, seem
to be coming from inside and are therefore allowed. The reality is that back
doors can allow attackers to take over control of an internal system and create
considerable damage.
See Also: Back
or Trap Door; Electronic Mail or Email; Network; Security.
Security software that can actively block unauthorized entities from gaining access to internal resources such as systems, servers, databases, and networks. A firewall may also act to prevent internal users from accessing unauthorized external resources. A firewall is installed in a communications router, server, or some other device that physically and/or logically is a first point of access into a networked system. A packet-filtering firewall examines all data packets, forwarding or dropping individual packets based on predefined rules that specify where a packet is permitted to go, in consideration of both the authenticated identification of the user and the originating address of the request. A proxy firewall acts as an intermediary for user access requests by setting up a second connection to the resource. The proxy then decides if the message or file is safe. A stateful inspection firewall examines packets, notes the port numbers that they use for each connection, and shuts down those ports once the connection is terminated. See also
authentication,
authorization,
proxy firewall, and
security.
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