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Included with Red Hat Enterprise Linux are advanced tools for network packet filtering — the process of controlling network packets as they enter, move through, and exit the network stack within the kernel. Kernel versions prior to 2.4 relied on ipchains for packet filtering and used lists of rules applied to packets at each step of the filtering process. The introduction of the 2.4 kernel brought with it iptables (also called netfilter), which is similar to ipchains but greatly expands the scope and control available for filtering network packets.
This chapter focuses on packet filtering basics, defines the differences between ipchains and iptables, explains various options available with iptables commands, and explains how filtering rules can be preserved between system reboots.
For instructions on constructing iptables rules or setting up a firewall based on these rules, refer to Section 18.7 Additional Resources.
The Linux kernel has the built-in ability to filter packets, allowing some of them to be received by or pass through the system while stopping others. The kernel's netfilter has three built-in tables or rules lists. They are as follows:
Each table has a group of built-in chains which correspond to the actions performed on the packet by the netfilter.
The built-in chains for the filter table are as follows:
The built-in chains for the nat table are as follows:
The built-in chains for the mangle table are as follows:
Every network packet received by or sent from a Linux system is subject to at least one table. However, a packet may be subjected to multiple rules within each table before emerging at the end of the chain. The structure and purpose of these rules may vary, but they usually seek to identify a packet coming from or going to a particular IP address, or set of addresses, when using a particular protocol and network service.
Regardless of their destination, when packets match a particular rule in one of the tables, a target or action is applied to them. If the rule specifies an ACCEPT target for a matching packet, the packet skips the rest of the rule checks and is allowed to continue to its destination. If a rule specifies a DROP target, that packet is refused access to the system and nothing is sent back to the host that sent the packet. If a rule specifies a QUEUE target, the packet is passed to user-space. If a rule specifies the optional REJECT target, the packet is dropped, but an error packet is sent to the packet's originator.
Every chain has a default policy to ACCEPT, DROP, REJECT, or QUEUE. If none of the rules in the chain apply to the packet, then the packet is dealt with in accordance with the default policy.
The iptables command configures these tables, as well as sets up new tables if necessary.
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