The options used with given iptables rules must be
grouped logically, based on the purpose and conditions of the overall
rule, for the rule to be valid. The remainder of this section explains
commonly-used options for the iptables command.
The <table-name> option allows the
user to select a table other than the default filter
table to use with the command. The
<command> option dictates a specific
action to perform, such as appending or deleting the rule specified by
the <chain-name> option. Following
the <chain-name> are pairs of
parameters and options that define what happens when a packet
matches the rule.
When looking at the structure of an iptables
command, it is important to remember that, unlike most other commands,
the length and complexity of an iptables command
can change based on its purpose. A command to remove a rule from a
chain can be very short, while a command designed to filter packets
from a particular subnet using a variety of specific parameters and
options can be rather lengthy. When creating
iptables commands, it is helpful to recognize that
some parameters and options may create the need for other parameters
and options to further specify the previous option's request. To
construct a valid rule, this must continue until every parameter and
option that requires another set of options is satisfied.
Type iptables -h to view a comprehensive list of
iptables command structures.
Command options instruct iptables to perform a
specific action. Only one command option is allowed per
iptables command. With the exception of the help
command, all commands are written in upper-case characters.
The iptables commands are as follows:
-A — Appends the iptables
rule to the end of the specified chain. This is the command used
to 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 deletes the rule in the chain that
matches it.
-E — Renames a user-defined chain. This
does not affect the structure of the table.
-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 command
structures, as well as a quick summary of command parameters and
options.
-I — Inserts a rule in a chain at a
point specified by a user-defined integer value. If no number is
specified, iptables places the command at
the top of the chain.
Caution
Be aware when using the -A or
-I option that the order of the rules within
a chain are important for determining which rules apply to
which packets.
-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>
Additional options for the -L command option,
which provide rule numbers and allow more verbose rule
descriptions, are described in Section 18.3.6 Listing Options.
-N — Creates a new chain with a
user-specified name.
-P — Sets the default policy for the
specified chain, so that when packets traverse an entire chain
without matching a rule, they are sent on to the specified
target, such as ACCEPT or DROP.
-R — Replaces a rule in the specified
chain. The rule's number must be specified after the chain's
name. The first rule in a chain corresponds to rule number one.
-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 table.
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 construct a
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.
-d — Sets the destination hostname, IP
address, or network of a packet that matches the rule. When
matching a network, the following IP address/netmask formats are
supported:
N.N.N.N/M.M.M.M
— Where N.N.N.N is the IP
address range and M.M.M.M is the
netmask.
N.N.N.N/M
— Where N.N.N.N is the IP
address range and M is the bitmask.
-f — Applies this rule only to
fragmented packets.
By using the exclamation point character (!)
option after this parameter, only unfragmented packets are
matched.
-i — Sets the incoming network
interface, such as eth0 or
ppp0. With iptables, this
optional parameter may only be used with the INPUT and FORWARD
chains when used with the filter table and the
PREROUTING chain with the nat and
mangle tables.
This parameter also supports the following special options:
Exclamation point character (!) —
Reverses the directive, meaning any specified interfaces are
excluded from this rule.
Plus character (+) — A wildcard
character used to match all interfaces that match the
specified string. For example, the parameter -i
eth+ would apply this rule to any Ethernet interfaces
but exclude any other interfaces, such as
ppp0.
If the -i parameter is used but no interface
is specified, then every interface is affected by the rule.
-j — Jumps to the specified target when
a packet matches a particular rule. Valid targets to use after
the -j option include standard options
(ACCEPT, DROP,
QUEUE, and RETURN) as well as
extended options that are available through modules loaded by
default with the Red Hat Enterprise Linux iptables RPM package,
such as LOG, MARK, and
REJECT, among others. Refer to the
iptables man page for more information about
these and other targets.
It is also possible to direct a packet matching this rule to a
user-defined chain outside of the current chain so that other
rules can be applied to the packet.
If no target is specified, the packet moves past the rule with
no action taken. However, the counter for this rule increases by
one.
-o — Sets the outgoing network
interface for a rule and may only be used with OUTPUT and FORWARD
chains in the filter table, and the POSTROUTING
chain in the nat and mangle
tables. This parameter's options are the same as those of the
incoming network interface parameter (-i).
-p — Sets the IP protocol for the rule,
which can be either icmp, tcp,
udp, or all, to match every
supported protocol. In addition, any protocols listed in
/etc/protocols may also be used. If this
option is omitted when creating a rule, the all
option is the default.
-s — Sets the source for a particular
packet using the same syntax as the destination
(-d) parameter.
Different network protocols provide specialized matching options which
can be configured to match a particular packet using that
protocol. However, the protocol must first be specified in the
iptables command. For example -p tcp
<protocol-name> (where
<protocol-name> is the target
protocol), makes options for the specified protocol available.
These match options are available for the TCP protocol (-p
tcp):
--dport — Sets the destination port for
the packet. Use either a network service name (such as
www or smtp), port number, or
range of port numbers to configure this option. To browse the
names and aliases of network services and the port numbers they
use, view the /etc/services file. The
--destination-port match option is synonymous
with --dport.
To specify a range of port numbers, separate the two
numbers with a colon (:), such as
-p tcp --dport 3000:3200. The largest
acceptable valid
range is 0:65535.
Use an exclamation point character (!) after
the --dport option to match all packets which
do not use that network service or port.
--sport — Sets the source port of the
packet using the same options as --dport. The
--source-port match option is synonymous with
--sport.
--syn — Applies to all TCP packets
designed to initiate communication, commonly called
SYN packets. Any packets that carry a
data payload are not touched. Placing an exclamation point
character (!) as a flag after the
--syn option causes all non-SYN packets to be
matched.
--tcp-flags — Allows TCP packets with
specific set bits, or flags, to match a rule. The
--tcp-flags match option accepts two
parameters. The first parameter is the mask, which sets the flags
to be examined in the packet. The second parameter refers to the
flag that must be set to match.
The possible flags are:
ACK
FIN
PSH
RST
SYN
URG
ALL
NONE
For example, an iptables rule which contains
-p tcp --tcp-flags ACK,FIN,SYN SYN only matches
TCP packets that have the SYN flag set and the ACK and FIN flags
unset.
Using the exclamation point character (!) after
--tcp-flags reverses the effect of the match
option.
--tcp-option — Attempts to match with
TCP-specific options that can be set within a particular
packet. This match option can also be reversed with the
exclamation point character (!).
These match options are available for the UDP protocol (-p
udp):
--dport — Specifies the destination port
of the UDP packet, using the service name, port number, or range
of port numbers. The --destination-port match
option is synonymous with --dport.
--sport — Specifies the source port of
the UDP packet, using the service name, port number, or range of
port numbers. The --source-port match option is
synonymous with --sport.
The following match options are available for the Internet
Control Message Protocol (ICMP) (-p icmp):
--icmp-type — Sets the name or number of
the ICMP type to match with the rule. A list of valid ICMP names
can be retrieved by typing the iptables -p icmp
-h command.
Additional match options are also available through modules
loaded by the iptables command. To use a match
option module, load the module by name using the -m
option, such as -m
<module-name> (replacing
<module-name> with the name of the
module).
A large number of modules are available by default. It is even
possible to create modules that provide additional functionality.
The following is a partial list of the most commonly used modules:
limit module — Places limits on how
many packets are matched to a particular rule. This is
especially beneficial when used in conjunction with the
LOG target as it can prevent a flood of
matching packets from filling up the system log with repetitive
messages or using up system resources. Refer to Section 18.3.5 Target Options for more information about
the LOG target.
The limit module enables the following options:
--limit — Sets the number of matches
for a particular range of time, specified with a number and
time modifier arranged in a
<number>/<time>
format. For example, using --limit 5/hour
only lets a rule match 5 times in a
single hour.
If a number and time modifier are not used, the default value of
3/hour is assumed.
--limit-burst — Sets a limit on the
number of packets able to match a rule at one time. This option
should be used in conjunction with the --limit
option, and it accepts a number to set the burst threshold.
If no number is specified, only five packets are initially able
to match the rule.
state module — Enables state
matching.
The state module enables the following options:
--state — match a packet with the following
connection states:
ESTABLISHED — The matching packet is
associated with other packets in an established connection.
INVALID — The matching packet cannot be
tied to a known connection.
NEW — The matching packet is either
creating a new connection or is part of a two-way connection not
previously seen.
RELATED — The matching packet is starting
a new connection related in some way to an existing connection.
These connection states can be used in combination with
one another by separating them with commas, such as
-m state --state INVALID,NEW.
mac module — Enables hardware MAC
address matching.
The mac module enables the following option:
--mac-source — Matches a MAC address
of the network interface card that sent the packet. To
exclude a MAC address from a rule, place an exclamation
point character (!) after the
--mac-source match option.
To view other match options available through modules, refer to
the iptables man page.
Once a packet has matched a particular rule, the rule can direct the
packet to a number of different targets that decide its fate and,
possibly, take additional actions. Each chain has a default target,
which is used if none of the rules on that chain match a packet or if
none of the rules which match the packet specify a target.
The following are the standard targets:
<user-defined-chain>
— Replace
<user-defined-chain> with the
name of a user-defined chain within the table. This target passes
the packet to the target chain.
ACCEPT — Allows the packet to
successfully move on to its destination or another chain.
DROP — Drops the packet without
responding to the requester. The system that sent the packet is
not notified of the failure.
QUEUE — The packet is queued for
handling by a user-space application.
RETURN — Stops checking the packet
against rules in the current chain. If the packet with a
RETURN target matches a rule in a chain called
from another chain, the packet is returned to the first chain to
resume rule checking where it left off. If the
RETURN rule is used on a built-in chain and the
packet cannot move up to its previous chain, the default target
for the current chain decides what action to take.
In addition to these standard targets, various other targets may be
used with extensions called target modules. For
more information about match option modules, refer to Section 18.3.4.4 Additional Match Option Modules.
There are many extended target modules, most of which only apply to
specific tables or situations. A couple of the most popular target
modules included by default in Red Hat Enterprise Linux are:
LOG — Logs all packets that match this
rule. Since the packets are logged by the kernel, the
/etc/syslog.conf file determines where these
log entries are written. By default, they are placed in the
/var/log/messages file.
Additional options can be used after the LOG
target to specify the way in which logging occurs:
--log-level — Sets the priority level
of a logging event. A list of priority levels can be found
within the syslog.conf man page.
--log-ip-options — Logs any options set
in the header of a IP packet.
--log-prefix — Places a string of up to
29 characters before the log line when it is written. This is
useful for writing syslog filters for use in conjunction with
packet logging.
--log-tcp-options — Logs any options
set in the header of a TCP packet.
--log-tcp-sequence — Writes the TCP
sequence number for the packet in the log.
REJECT — Sends an error packet back to
the remote system and drops the packet.
The REJECT target accepts
--reject-with
<type> (where
<type> is the rejection type)
allowing more detailed information to be sent back with the error
packet. The message
port-unreachable is the default
<type> error
given if no other option is used. For a full list of
<type> options,
refer to the iptables man page.
Other target extensions, including several that are useful for IP
masquerading using the nat table or with packet
alteration using the mangle table, can be found in
the iptables man page.
The default list command, iptables -L, provides a
very basic overview of the default filter table's current chains.
Additional options provide more information:
-v — Displays verbose output, such as
the number of packets and bytes each chain has seen, the number of
packets and bytes each rule has matched, and which interfaces
apply to a particular rule.
-x — Expands numbers into their exact
values. On a busy system, the number of packets and bytes seen by
a particular chain or rule may be abbreviated using
K (thousands),
M (millions), and
G (billions) at the end of the
number. This option forces the full number to be displayed.
-n — Displays IP addresses and port
numbers in numeric format, rather than the default hostname and
network service format.
--line-numbers — Lists rules in each
chain next to their numeric order in the chain. This option is
useful when attempting to delete the specific rule in a chain or
to locate where to insert a rule within a chain.
