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170 Chapter 14. Firewalling with iptables

The iptables commands are:

• -A — Appends the iptables rule to the end of the speciﬁed chain. This is the command used to

simply add a rule when rule order in the chain does not matter.

• -C — Checks a particular rule before adding it to the user-speciﬁed chain. This command can

help you construct complicated iptables rules by prompting you for additional parameters and

options.

• -D — Deletes a rule in a particular chain by number (such as 5 for the ﬁfth rule in a chain). You

can also type the entire rule, and iptables will delete the rule in the chain that matches it.

• -E — Renames a user-deﬁned chain. This does not affect the structure of the table at all. Rather, it

just saves you the trouble of deleting the chain, creating it under the new name, and reconﬁguring

all of your rules for that chain.

• -F — Flushes the selected chain, which effectively deletes every rule in the the chain. If no chain

is speciﬁed, this command ﬂushes every rule from every chain.

• -h — Provides a list of helpful command structures, as well as a quick summary of command

parameters and options.

• -I — Inserts a rule in a chain at a particular point. Assign a number to the rule to be inserted and

iptables will put it there. If no number is speciﬁed, iptables will place your command at the

top of the rule list.

Caution

Be aware of which option (-A or -I) you are using when adding a rule. The order of the rules can

be very important when determining if a particular packet applies to one rule or another. Make

sure when adding a rule to the beginning or end of the chain that it does not affect other rules in

that chain.

• -L — Lists all of the rules in the chain speciﬁed after the command. To list all rules in all chains in

the default filter table, do not specify a chain or table. Otherwise, the following syntax should

be used to list the rules in a speciﬁc chain in a particular table:

iptables -L chain-name -t table-name

Powerful options for the -L command that provide rule numbers and allow more verbose rule

descriptions, among others, are described in Section 14.3.7.

• -N — Creates a new chain with a user-speciﬁed name.

• -P — Sets the default policy for a particular chain, so that when packets traverse an entire chain

without matching a rule, they will be sent on to a particular target, such as ACCEPT or DROP.

• -R — Replaces a rule in a particular chain. You must use a rule’s number after the chain’s name to

replace that rule. The ﬁrst rule in a chain relates to rule number 1.

• -X — Deletes a user-speciﬁed chain. Deleting a built-in chain for any table is not allowed.

• -Z — Zeros the byte and packet counters in all chains for a particular table.

14.3.4. Parameters

Once certain iptables commands are speciﬁed, including those used to add, append, delete, insert,

or replace rules within a particular chain, parameters are required to begin the construction of the

packet ﬁltering rule.

• -c — Resets the counters for a particular rule. This parameter accepts the PKTS and BYTES options

to specify what counter to reset.

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 170

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