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Articles taggués ‘network iptables’

Communication Networks/IP Tables

30/11/2015 Comments off

Operational summary

The netfilter framework, of which iptables is a part of, allows the system administrator to define rules for how to deal with network packets. Rules are grouped into chains—each chain is an ordered list of rules. Chains are grouped into tables—each table is associated with a different kind of packet processing.

Each rule contains a specification of which packets match it and a target that specifies what to do with the packet if it is matched by that rule. Every network packet arriving at or leaving from the computer traverses at least one chain, and each rule on that chain attempts to match the packet. If the rule matches the packet, the traversal stops, and the rule’s target dictates what to do with the packet. If a packet reaches the end of a predefined chain without being matched by any rule on the chain, the chain’s policy target dictates what to do with the packet. If a packet reaches the end of a user-defined chain without being matched by any rule on the chain or the user-defined chain is empty, traversal continues on the calling chain (implicit target RETURN). Only predefined chains have policies.

Rules in iptables are grouped into chains. A chain is a set of rules for IP packets, determining what to do with them. Each rule can possibly dump the packet out of the chain (short-circuit), and further chains are not considered. A chain may contain a link to another chain – if either the packet passes through that entire chain or matches a RETURN target rule it will continue in the first chain. There is no limit to how many nested chains there can be. There are three basic chains (INPUT, OUTPUT, and FORWARD), and the user can create as many as desired. A rule can merely be a pointer to a chain.

Tables

There are three built-in tables, each of which contains some predefined chains. It is possible for extension modules to create new tables. The administrator can create and delete user-defined chains within any table. Initially, all chains are empty and have a policy target that allows all packets to pass without being blocked or altered in any fashion.

  • filter table — This table is responsible for filtering (blocking or permitting a packet to proceed). Every packet passes through the filter table. It contains the following predefined chains, and any packet will pass through one of them:
    • INPUT chain — All packets destined for this system go through this chain (hence sometimes referred to as LOCAL_INPUT)
    • OUTPUT chain — All packets created by this system go through this chain (aka. LOCAL_OUTPUT)
    • FORWARD chain — All packets merely passing through the system (being routed) go through this chain.
  • nat table — This table is responsible for setting up the rules for rewriting packet addresses or ports. The first packet in any connection passes through this table: any verdicts here determine how all packets in that connection will be rewritten. It contains the following predefined chains:
    • PREROUTING chain — Incoming packets pass through this chain before the local routing table is consulted, primarily for DNAT (destination-NAT).
    • POSTROUTING chain — Outgoing packets pass through this chain after the routing decision has been made, primarily for SNAT (source-NAT).
    • OUTPUT chain — Allows limited DNAT on locally-generated packets
  • mangle table — This table is responsible for adjusting packet options, such as quality of service. All packets pass through this table. Because it is designed for advanced effects, it contains all the possible predefined chains:
    • PREROUTING chain — All packets entering the system in any way, before routing decides whether the packet is to be forwarded (FORWARD chain) or is destined locally (INPUT chain).
    • INPUT chain — All packets destined for this system go through this chain
    • FORWARD chain — All packets merely passing through the system go through this chain.
    • OUTPUT chain — All packets created by this system go through this chain
    • POSTROUTING chain — All packets leaving the system go through this chain.

In addition to the built-in chains, the user can create any number of user-defined chains within each table, which allows them to group rules logically.

Each chain contains a list of rules. When a packet is sent to a chain, it is compared against each rule in the chain in order. The rule specifies what properties the packet must have for the rule to match, such as the port number or IP address. If the rule does not match then processing continues with the next rule. If, however, the rule does match the packet, then the rule’s target instructions are followed (and further processing of the chain is usually aborted). Some packet properties can only be examined in certain chains (for example, the outgoing network interface is not valid in the INPUT chain). Some targets can only be used in certain chains, and/or certain tables (for example, the SNAT target can only be used in the POSTROUTING chain of the nat table).

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