- External access to any part of a distributed system controlled with a special kind of reference monitor called a Firewalls.
- This monitor isolates the distributed system from the external world.
- All outgoing and incoming packets routed through a special computer and inspected before they passed.
- Thus, a Firewalls set of related programs located at a network gateway server that protects the resources of a private network from the users of other networks.
- The firewall itself should thoroughly protect against any security threat and should never fail.
- Basically, a firewall works closely with a router program. And examines each network packet to determine whether or not to forward it towards its destination.
A firewall also includes or works with a proxy server that makes network requests on behalf of workstation users.
- A firewall often installed in a specially designated computer separate from the rest of the network so that no incoming request can get directly from private network resources.
- It makes the decision of whether or not to pass a network packet based on the source and destination addresses in the packet.
- The other type of Firewalls are the application-level gateway.
- This type of firewall inspects the contents of the incoming and outgoing packet.
- For example, a mail gateway can discard incoming and outgoing messages exceeding a certain size.