Using Iptables on Linux and Setting the filtering rules

Using Iptables on Linux and Setting the filtering rules

iptables is a user-space application program that allows a system administrator to configure the tables provided by the Linux kernel firewall (implemented as different Netfilter modules) and the chains and rules it stores. Different kernel modules and programs are currently used for different protocols; iptables applies to IPv4, ip6tables to IPv6, arptables to ARP, and ebtables to Ethernet frames.

iptables requires elevated privileges to operate and must be executed by user root, otherwise it fails to function. On most Linux systems, iptables is installed as /usr/sbin/iptables and documented in its man pages, which can be opened using man iptables when installed. It may also be found in /sbin/iptables, but since iptables is more like a service rather than an "essential binary", the preferred location remains /usr/sbin.

The term iptables is also commonly used to inclusively refer to the kernel-level components. x_tables is the name of the kernel module carrying the shared code portion used by all four modules that also provides the API used for extensions; subsequently,Xtables is more or less used to refer to the entire firewall (v4, v6, arp, and eb) architecture.

There are many different types of cryptographic algorithms, though most of them fit into one of two classifications — symmetric and asymmetric. Some systems, however, use a hybrid of both classifications. Symmetric algorithms, also known as symmetric-key or shared-key algorithms, work by the use of a key known only to the two authorized parties. While these can be implemented in the form of block ciphers or stream ciphers, the same key is used for both encrypting and decrypting the message. The Data Encryption Standard (DES) and Advanced Encryption Standard (AES) are the most popular examples of symmetric cryptography algorithms.

Asymmetric cryptography algorithms rely on a pair of keys — a public key and a private key. The public key can be revealed, but, to protect the data, the private key must be concealed. Additionally, encryption and decryption of the data must be done by the associated private and public keys. For example, data encrypted by the private key must be decrypted by the public key, and vice versa. RSA is one of the most common examples of this algorithm.

Symmetric algorithms are usually much faster than asymmetric algorithms. This is largely related to the fact that only one key is required.