Xtables allows the system administrator to define tables containing chains of rules for the treatment of packets. Each table is associated with a different kind of packet processing. Packets are processed by sequentially traversing the rules in chains. A rule in a chain can cause a goto or jump to another chain, and this can be repeated to whatever level of nesting is desired. (A jump is like a “call”, i.e. the point that was jumped from is remembered.) Every network packet arriving at or leaving from the computer traverses at least one chain.
Packets start at a given box and will flow along a certain path, depending on the circumstances.
The origin of the packet determines which chain it traverses initially. There are five predefined chains (mapping to the five available Netfilter hooks), though a table may not have all chains. Predefined chains have a policy, for example DROP, which is applied to the packet if it reaches the end of the chain. The system administrator can create as many other chains as desired. These chains have no policy; if a packet reaches the end of the chain it is returned to the chain which called it. A chain may be empty.
“PREROUTING”: Packets will enter this chain before a routing decision is made.
“INPUT”: Packet is going to be locally delivered. (N.B.: It does not have anything to do with processes having a socket open. Local delivery is controlled by the “local-delivery” routing table: `ip route show table local`.)
“FORWARD”: All packets that have been routed and were not for local delivery will traverse this chain.
“OUTPUT”: Packets sent from the machine itself will be visiting this chain.
“POSTROUTING”: Routing decision has been made. Packets enter this chain just before handing them off to the hardware.
Each rule in a chain contains the specification of which packets it matches. It may also contain a target (used for extensions) or verdict (one of the built-in decisions). As a packet traverses a chain, each rule in turn is examined. If a rule does not match the packet, the packet is passed to the next rule. If a rule does match the packet, the rule takes the action indicated by the target/verdict, which may result in the packet being allowed to continue along the chain or it may not.
Matches make up the large part of rulesets, as they contain the conditions packets are tested for. These can happen for about any layer in the OSI model, as with e.g. the --mac-source and -p tcp --dport parameters, and there are also protocol-independent matches, such as -m time.
The packet continues to traverse the chain until either
Targets also return a verdict like ACCEPT (NAT modules will do this) or DROP (e.g. the “REJECT” module), but may also imply CONTINUE (e.g. the “LOG” module; CONTINUE is an internal name) to continue with the next rule as if no target/verdict was specified at all.
The firewall chain diagram is summarized in below picture (valid for version 2.15 core 88 with addon Guardian 2.0 being installed)