In the case of firewalls, a data driven attack is a concern since it may get through the firewall in data form and launch an attack against a system behind the firewall. May be used in conjunction with firewalls. In firewall configurations, a dual homed gateway usually acts to block or filter some or all of the traffic trying to pass between the networks. Also, an application-level gateway, a machine or set of machines that relays services between the internal and external networks by means of proxy applications. Gateway to Gateway Protocol, the protocol used primarily between gateways to control routing and other gateway functions.
Host based security is operating system and version dependent.
Internet Control Message Protocol, implemented in the internet module, the ICMP is used from gateways to hosts and between hosts to report errors and make routing suggestions. An intranet may have a connection to the Internet, or it may exist on the Internet, achieving controlled access through passwords or other means. IP is referred to as an unreliable datagram service, meaning that upper-level protocols should not depend upon IP to deliver the packet every time. For example, someone might determine the IP address of a legitimate user inside the firewall, then forge packets from outside the firewall which the firewall allows to pass because they are from a legitimate user. IP Splicing attacks may occur after an authentication has been made, permitting the attacker to assume the role of an already authorized user.