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Additional Chain Operations and Rules: General Considerations

Additional Chain Operations

Assign chain policy (P CHAIN TARGET)

ACCEPT (default a built in target)
DROP (a built in target)
REJECT (not permitted, an extension target)

Flush all rules of a chain (F)
Does not flush the policy

Zero byte and packet counters Z CHAIN
 Useful for monitoring chain statistics or troubleshooting

Manage custom chains N, X
N Your Cha1n Name (adds Chain)
X Your Chain Name (deletes Chain)

Use P to change the default TARGET, or policy of a chain. The default installed policy is ACCEPT. Only

built-in targets (DROP and ACCEPT) may be a chain policy due to efficiency. While it is desirable to inform a client process that its connection is refused (REJECT), a new packet must be generated and routed to

the client. F is used to flush, or remove all rules from a chain. This does not reset the chain policy.

Use 2 to "zero" or set the byte and packet counters for all rules of a chain to zero (0) This is useful in the systematic collection, and statistical analysis of a rule's effectiveness. Remember that all packets traversing an interface are inspected. A rule which matches no packets requires the same and important resources as a rule that matches frequently. Analysis of rule counters is also helpful as a metric of system

and service activity. To display these counters, use the v option to the ip tables command.

Use -N to create a new, empty custom chain. Custom chains do not have a chain policy (see above). Custom chains are useful in the apportioning of rules which more effectively test packets of a Specific network matching on the interface, the source, or destination address or service matching on a port PrOper implementation of custom chains reduces unnecessary inspection of all packets when only a few, well defined groups of packets are required. To remove a custom chain, use x. Built in chains cantrol be expunged.

Rules: General Considerations

Mostly closed is appropriate
ip tables P INPUT DROP or
ip tables A INPUT i DROP
ip tables A INPUT j REJECT

a default installed Red Hat Enterprise Linux system without any ip tables rules asserted, will have empty

buiIt in chains with a policy of ACCEPT. In this way the paoket filtering facility can be present without having any effect on system resources.

Approaches to rule design can be classified as either mostly open or mostly closed. A mostly open

approach allows all packets by default and only blocks known bad traffic. A mostly closed approach blocks

all packets by default and only allows known good. A mostly closed approach to rule design is more cautious and usually considered more appropriate.

There are two techniques for creating a mostly closed ruleset for a chain. One is to set the chain's policy to
ip tables P INPUT DROP
The other is to create a "catchall" DROP rule at the bottom of the chain:
ip tables A INPUT j DROP
Both techniques will cause all traffic that is not explicitly allowed to be blocked. Note that this means NO network services, including ones that listen on local host ( will work if there are no other rules in the chain. The difference between the two techniques has to do with ip tables' behavior when the chain is "flushed" and all the rules are deleted. Since a chain's policy is not affected by a flush, flushing a chain with a DROP policy will cause all network services to become unavailable very bad if you are administering

the system remotely. However, flushing a chain with a catchall DROP rule will remove the rule so that the chain reverts to its default ACCEPT policy. This will not interrupt access to any services but it will leave the System wide open, as if no firewall at all were running. Which technique you use should be dependent upon your level of access to the machine and the number of sensitive services being protected by the firewall.

packet filtering rules applied using the ip tables commands alone, are not automatically reapplied on reboot (net persistent). However they can be made persistent as described later In this unit.


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