What are Netfilter and Iptables?

[Bottlenecked]

I have recently started studying about the Netfilter and Iptables. We were given small notes for the same, but that wasn't enough to explain the basics also. So I thought that it would be better for me, to take some help from you guys. Please provide information in details about the netfilter and iptables. Any other information related to the topic would also be helpful.

[SreeKanth]

Netfilter is the kernel implementation of the firewall in Linux. A talented coder named Rusty Russell found it necessary to have a replacement for the famous ipfilter and can compete to see far exceed commercial firewalls. Currently, according to my knowledge, Netfilter is one of the most powerful firewall market. It is flexible, easy to configure and widely supported by the community. Iptables is the command to set the filter Netfilter kernel and thus configure its firewall.

[Baser-X]

Netfilter is a framework available in the core Linux that allows you to intercept and manipulate network packets . This framework enables package management in different stages of processing. Netfilter is also the name given to the project that is responsible for providing free tools for firewall based on Linux. The most popular component built on Netfilter is iptables, a tool firewall that allows not only filter packets, but also perform network address translation (NAT) for IPv4 or maintain records log. The Netfilter project not only provides components available as kernel modules, but also offers user-space tools and libraries.

[Luther]

iptables is the name of the tool user space through which the administrator can define policies for filtering traffic passing through the network. The name of iptables is often used incorrectly to refer to all the facilities offered by the Netfilter project. However, the project offers other independent subsystems such as iptables connection tracking system or system of connection tracking , or that allows packages to be glued treated from user space. iptables is a software available in virtually all distributions Linux today.

[Benedict]

Before iptables, most programs used to generate firewalls on Linux were ipchains in Linux kernel 2.2, furthermore ipfwadm on Linux 2.0 kernel, which in turn was based on ipfw from BSD . Both ipchains and ipfwadm revise the code to manipulate network packets, and that there was no framework for handling general packet until the manifestation of netfilter. iptables retains the basic idea introduced in Linux with ipfwadm: lists of regulations that specify that MATCH inside a package and what to do with this package. ipchains adds the perception of rule chains (chains) and iptables extended this to the idea of tables: one table was consulted to decide whether to NAT-ear package, and another was consulted to decide how a packet filter. Additionally, amendments were three points where the filtering is done on the trip of a package, so that a packet passes a point just for filtering.

[Allan.d]

Each rule specifies which packages the meet (match) and a destination that indicates what to do with the package if it satisfies the rule. Every network packet that arrives to a computer or sent from a computer runs at least a string and each rule in the chain is found with the package. If the rule complies with the datagram, the tour stops and the fate of the rule dictates what to do with the package. If the package reaches the end of a predefined string without being matched by any rule of the chain, the policy chain target dictates what to do with the package. If the packet reaches the end of a user-defined chain without complying with any rule of the chain or if the user-defined chain is empty, the tour continues in the calling chain (so-called implicit target RETURN RETURN target or implied). Only built-in chains have policies.

[Steinbach]

The fate of a rule may be the name of a user-defined chain or one of the destinations already incorporated ACCEPT, DROP, QUEUE, or RETURN (accept, discard, or return solvent, respectively). When a destination is the name of a user-defined chain, the package directs you to the chain for processing (such as with a call to a subroutine in a programming language ). If the package gets through the string defined by the user without any of the rules of the chain acts on it, the packet processing continues where it left off in the current string. These calls between networks can be nested to any level desired.

[Jackson2]

Until recently working with iptables, or its predecessor ipchains, required since the start of enough knowledge, should be used to recompile the kernel to add support, should be the same with certain modules if we wanted to load certain rules. Today is no longer true, support for iptables is already the default in the 2.6 kernel and the iptables utility is automatically installed with the system, and run just because a new rule stating iptables triggers the dynamic load iptables module so it is not even necessary to manually load the module using modprobe but if necessary for some specific modules.

[johnson22]

You can easily tell when a rule iptables is using an extra module, the module ip_tables addition, since the rule include explicitly using the-m, that module is. For example, a rule which contains-m state-state NEW explicitly used the form xt_state. However, there are modules that belong to iptables, for which not supported by default in the kernel and the collection will be necessary in this case, the reason is not having the support modules for tasks that are too specific and whose use is not is common, so adding the support is in the vast majority of cases unnecessary. A brief introduction to the modules and their utility can be obtained from iptables [man iptables] or the official website of the netfilter project. Among them are very useful modules related to bandwidth control, QoS and DoS prevention. Among them the most important are connbytes, connlimit, quota (according to the man, bad idea if we have symmetric) and realm.