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Everything About Linux Iptables/Linux Firewall

Introduction
Firewalls are used to monitor and control the inbound and outbound traffic on the protected network. They have an ability to block and allow the internal as well as external services within the network. Before allowing access to the service, a firewall may also force the client / user to pass through an authentication. Sometimes a firewall can be also used in IPSEC tunnels as a platform. It monitors security-related events.

Packet Filtering
The packet filtering mechanism mainly contains inspection on TCP/IP and UDP packets. It also includes all ports in its inspection. In this process, certain rules are written for allowing and rejecting the packets passing through the network. The rules written in the firewall may contain TCP and UDP port numbers, source and destination addresses. One can implement firewall rules which may work in both inbound and outbound directions.

Types of Firewalls
There are basically four types of firewalls:
  1. Packet Filter Firewall
  2. Stateful Packet Filter Firewall
  3. Circuit Level Gateway
  4. Application Level Gateway
Packet Filter Firewall
This firewall comes into play when an administrator wants only certain packets to enter into the protected network. In this case, each packet will be monitored and inspected before passing through the network, and after monitoring and inspecting, the firewall will decide whether to let it pass or not.



There are two types of packet filter mechanisms:
  1. Stateful Packet Filtering
    These types of firewalls are known as a smart / clever firewall. If the firewall remembers the packets it allowed and blocked in the network, then it is known as Stateful packet filtering. Sometimes it is also called a dynamic packet process.
  2. Stateless Packet Filtering
    In this case, information about all those previous packets passed through the networks is not being remembered by a firewall. This type of firewall can be bypassed and easily fooled by attackers, and is especially dangerous for UDP data packets. This firewall will never come to know whether the given packet is a part of existing connection or any rough useless packet, because it isolates each and every packet.
  3. Stateful Filter Firewall
    I have already mentioned about the Stateful packet filtering process in the above section. Additionally, this type of firewall keeps a track record of TCP streams to inspect each and every packet passing through and in and out of the network. Generally this type of firewall is only constructed to inspect packets which are coming in only one direction, from client to server. There is an automatic process which handles counter requests (replies) going from server to client. It has an ability to support a wider range of protocols such as IRC, FTP, etc…


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