iptables - IP packet filter administration
SYNOPSIS
iptables -[ADC] chain rule-specification [options]
iptables -[RI] chain rulenum rule-specification [options]
iptables -D chain rulenum [options]
iptables -[LFZ] [chain] [options]
iptables -[NX] chain
iptables -P chain target [options]
iptables -E old-chain-name new-chain-name
DESCRIPTION
Iptables is used to set up, maintain, and inspect the
tables of IP packet filter rules in the Linux kernel.
Several different tables may be defined. Each table con
tains a number of built-in chains and may also contain
user-defined chains.
Each chain is a list of rules which can match a set of
packets. Each rule specifies what to do with a packet
that matches. This is called a `target', which may be a
jump to a user-defined chain in the same table.
TARGETS
A firewall rule specifies criteria for a packet, and a
target. If the packet does not match, the next rule in
the chain is the examined; if it does match, then the next
rule is specified by the value of the target, which can be
the name of a user-defined chain or one of the special
values ACCEPT, DROP, QUEUE, or RETURN.
ACCEPT means to let the packet through. DROP means to
drop the packet on the floor. QUEUE means to pass the
packet to userspace (if supported by the kernel). RETURN
means stop traversing this chain and resume at the next
rule in the previous (calling) chain. If the end of a
built-in chain is reached or a rule in a built-in chain
with target RETURN is matched, the target specified by the
chain policy determines the fate of the packet.
TABLES
There are current three independent tables (which tables
are present at any time depends on the kernel configura
tion options and which modules are present).
-t, --table
This option specifies the packet matching table
which the command should operate on. If the kernel
is configured with automatic module loading, an
attempt will be made to load the appropriate module
for that table if it is not already there.
filter This is the default table. It contains the built-
in chains INPUT (for packets coming into the box
itself), FORWARD (for packets being routed through
the box), and OUTPUT (for locally-generated pack
ets).
nat This table is consulted when a packet that creates
a new connection is encountered. It consists of
three built-ins: PREROUTING (for altering packets
as soon as they come in), OUTPUT (for altering
locally-generated packets before routing), and
POSTROUTING (for altering packets as they are about
to go out).
mangle This table is used for specialized packet alter
ation. It has two built-in chains: PREROUTING (for
altering incoming packets before routing) and OUT
PUT (for altering locally-generated packets before
routing).
OPTIONS
The options that are recognized by iptables can be divided
into several different groups.
COMMANDS
These options specify the specific action to perform.
Only one of them can be specified on the command line
unless otherwise specified below. For all the long ver
sions of the command and option names, you need to use
only enough letters to ensure that iptables can differen
tiate it from all other options.
-A, --append
Append one or more rules to the end of the selected
chain. When the source and/or destination names
resolve to more than one address, a rule will be
added for each possible address combination.
-D, --delete
Delete one or more rules from the selected chain.
There are two versions of this command: the rule
can be specified as a number in the chain (starting
at 1 for the first rule) or a rule to match.
-R, --replace
Replace a rule in the selected chain. If the
source and/or destination names resolve to multiple
addresses, the command will fail. Rules are num
bered starting at 1.
-I, --insert
Insert one or more rules in the selected chain as
1, the rule or rules are inserted at the head of
the chain. This is also the default if no rule
number is specified.
-L, --list
List all rules in the selected chain. If no chain
is selected, all chains are listed. It is legal to
specify the -Z (zero) option as well, in which case
the chain(s) will be atomically listed and zeroed.
The exact output is affected by the other arguments
given.
-F, --flush
Flush the selected chain. This is equivalent to
deleting all the rules one by one.
-Z, --zero
Zero the packet and byte counters in all chains.
It is legal to specify the -L, --list (list) option
as well, to see the counters immediately before
they are cleared. (See above.)
-N, --new-chain
Create a new user-defined chain by the given name.
There must be no target of that name already.
-X, --delete-chain
Delete the specified user-defined chain. There
must be no references to the chain. If there are,
you must delete or replace the referring rules
before the chain can be deleted. If no argument is
given, it will attempt to delete every non-builtin
chain in the table.
-P, --policy
Set the policy for the chain to the given target.
See the section TARGETS for the legal targets.
Only non-user-defined chains can have policies, and
neither built-in nor user-defined chains can be
policy targets.
-E, --rename-chain
Rename the user specified chain to the user sup
plied name. This is cosmetic, and has no effect on
the structure of the table.
-h Help. Give a (currently very brief) description of
the command syntax.
PARAMETERS
The following parameters make up a rule specification (as
used in the add, delete, insert, replace and append com
mands).
The protocol of the rule or of the packet to check.
The specified protocol can be one of tcp, udp,
icmp, or all, or it can be a numeric value, repre
senting one of these protocols or a different one.
A protocol name from /etc/protocols is also
allowed. A "!" argument before the protocol
inverts the test. The number zero is equivalent to
all. Protocol all will match with all protocols
and is taken as default when this option is omit
ted.
-s, --source [!] address[/mask]
Source specification. Address can be either a
hostname, a network name, or a plain IP address.
The mask can be either a network mask or a plain
number, specifying the number of 1's at the left
side of the network mask. Thus, a mask of 24 is
equivalent to 255.255.255.0. A "!" argument before
the address specification inverts the sense of the
address. The flag --src is a convenient alias for
this option.
-d, --destination [!] address[/mask]
Destination specification. See the description of
the -s (source) flag for a detailed description of
the syntax. The flag --dst is an alias for this
option.
-j, --jump target
This specifies the target of the rule; i.e., what
to do if the packet matches it. The target can be
a user-defined chain (other than the one this rule
is in), one of the special builtin targets which
decide the fate of the packet immediately, or an
extension (see EXTENSIONS below). If this option
is omitted in a rule, then matching the rule will
have no effect on the packet's fate, but the coun
ters on the rule will be incremented.
-i, --in-interface [!] [name]
Optional name of an interface via which a packet is
received (for packets entering the INPUT, FORWARD
and PREROUTING chains). When the "!" argument is
used before the interface name, the sense is
inverted. If the interface name ends in a "+",
then any interface which begins with this name will
match. If this option is omitted, the string "+"
is assumed, which will match with any interface
name.
-o, --out-interface [!] [name]
Optional name of an interface via which a packet is
going to be sent (for packets entering the FORWARD,
ment is used before the interface name, the sense
is inverted. If the interface name ends in a "+",
then any interface which begins with this name will
match. If this option is omitted, the string "+"
is assumed, which will match with any interface
name.
[!] -f, --fragment
This means that the rule only refers to second and
further fragments of fragmented packets. Since
there is no way to tell the source or destination
ports of such a packet (or ICMP type), such a
packet will not match any rules which specify them.
When the "!" argument precedes the "-f" flag, the
rule will only match head fragments, or unfrag
mented packets.
-c, --set-counters PKTS BYTES
This enables the administrater to initialize the
packet and byte counters of a rule (during INSERT,
APPEND, REPLACE operations)
OTHER OPTIONS
The following additional options can be specified:
-v, --verbose
Verbose output. This option makes the list command
show the interface address, the rule options (if
any), and the TOS masks. The packet and byte coun
ters are also listed, with the suffix 'K', 'M' or
'G' for 1000, 1,000,000 and 1,000,000,000 multipli
ers respectively (but see the -x flag to change
this). For appending, insertion, deletion and
replacement, this causes detailed information on
the rule or rules to be printed.
-n, --numeric
Numeric output. IP addresses and port numbers will
be printed in numeric format. By default, the pro
gram will try to display them as host names, net
work names, or services (whenever applicable).
-x, --exact
Expand numbers. Display the exact value of the
packet and byte counters, instead of only the
rounded number in K's (multiples of 1000) M's (mul
tiples of 1000K) or G's (multiples of 1000M). This
option is only relevant for the -L command.
--line-numbers
When listing rules, add line numbers to the begin
ning of each rule, corresponding to that rule's
position in the chain.
When adding or inserting rules into a chain, use
command to load any necessary modules (targets,
match extensions, etc).
MATCH EXTENSIONS
iptables can use extended packet matching modules. These
are loaded in two ways: implicitly, when -p or --protocol
is specified, or with the -m or --match options, followed
by the matching module name; after these, various extra
command line options become available, depending on the
specific module. You can specify multiple extended match
modules in one line, and you can use the -h or --help
options after the module has been specified to receive
help specific to that module.
The following are included in the base package, and most
of these can be preceded by a ! to invert the sense of
the match.
tcp
These extensions are loaded if `--protocol tcp' is speci
fied. It provides the following options:
--source-port [!] [port[:port]]
Source port or port range specification. This can
either be a service name or a port number. An
inclusive range can also be specified, using the
format port:port. If the first port is omitted,
"0" is assumed; if the last is omitted, "65535" is
assumed. If the second port greater then the first
they will be swapped. The flag --sport is an alias
for this option.
--destination-port [!] [port[:port]]
Destination port or port range specification. The
flag --dport is an alias for this option.
--tcp-flags [!] mask comp
Match when the TCP flags are as specified. The
first argument is the flags which we should exam
ine, written as a comma-separated list, and the
second argument is a comma-separated list of flags
which must be set. Flags are: SYN ACK FIN RST URG
PSH ALL NONE. Hence the command
iptables -A FORWARD -p tcp --tcp-flags
SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and
the ACK, FIN and RST flags unset.
[!] --syn
Only match TCP packets with the SYN bit set and the
ACK and FIN bits cleared. Such packets are used to
request TCP connection initiation; for example,
prevent incoming TCP connections, but outgoing TCP
connections will be unaffected. It is equivalent
to --tcp-flags SYN,RST,ACK SYN. If the "!" flag
precedes the "--syn", the sense of the option is
inverted.
--tcp-option [!] number
Match if TCP option set.
udp
These extensions are loaded if `--protocol udp' is speci
fied. It provides the following options:
--source-port [!] [port[:port]]
Source port or port range specification. See the
description of the --source-port option of the TCP
extension for details.
--destination-port [!] [port[:port]]
Destination port or port range specification. See
the description of the --destination-port option of
the TCP extension for details.
icmp
This extension is loaded if `--protocol icmp' is speci
fied. It provides the following option:
--icmp-type [!] typename
This allows specification of the ICMP type, which
can be a numeric ICMP type, or one of the ICMP type
names shown by the command
iptables -p icmp -h
mac
--mac-source [!] address
Match source MAC address. It must be of the form
XX:XX:XX:XX:XX:XX. Note that this only makes sense
for packets entering the PREROUTING, FORWARD or
INPUT chains for packets coming from an ethernet
device.
limit
This module matches at a limited rate using a token bucket
filter: it can be used in combination with the LOG target
to give limited logging. A rule using this extension will
match until this limit is reached (unless the `!' flag is
used).
--limit rate
Maximum average matching rate: specified as a num
ber, with an optional `/second', `/minute',
`/hour', or `/day' suffix; the default is 3/hour.
The maximum initial number of packets to match:
this number gets recharged by one every time the
limit specified above is not reached, up to this
number; the default is 5.
multiport
This module matches a set of source or destination ports.
Up to 15 ports can be specified. It can only be used in
conjunction with -p tcp or -p udp.
--source-port [port[,port]]
Match if the source port is one of the given ports.
--destination-port [port[,port]]
Match if the destination port is one of the given
ports.
--port [port[,port]]
Match if the both the source and destination ports
are equal to each other and to one of the given
ports.
mark
This module matches the netfilter mark field associated
with a packet (which can be set using the MARK target
below).
--mark value[/mask]
Matches packets with the given unsigned mark value
(if a mask is specified, this is logically ANDed
with the mask before the comparison).
owner
This module attempts to match various characteristics of
the packet creator, for locally-generated packets. It is
only valid in the OUTPUT chain, and even this some packets
(such as ICMP ping responses) may have no owner, and hence
never match.
--uid-owner userid
Matches if the packet was created by a process with
the given effective user id.
--gid-owner groupid
Matches if the packet was created by a process with
the given effective group id.
--pid-owner processid
Matches if the packet was created by a process with
the given process id.
--sid-owner sessionid
Matches if the packet was created by a process in
state
This module, when combined with connection tracking,
allows access to the connection tracking state for this
packet.
--state state
Where state is a comma separated list of the con
nection states to match. Possible states are
INVALID meaning that the packet is associated with
no known connection, ESTABLISHED meaning that the
packet is associated with a connection which has
seen packets in both directions, NEW meaning that
the packet has started a new connection, or other
wise associated with a connection which has not
seen packets in both directions, and RELATED mean
ing that the packet is starting a new connection,
but is associated with an existing connection, such
as an FTP data transfer, or an ICMP error.
unclean
This module takes no options, but attempts to match pack
ets which seem malformed or unusual. This is regarded as
experimental.
tos
This module matches the 8 bits of Type of Service field in
the IP header (ie. including the precedence bits).
--tos tos
The argument is either a standard name, (use
iptables -m tos -h
to see the list), or a numeric value to match.
TARGET EXTENSIONS
iptables can use extended target modules: the following
are included in the standard distribution.
LOG
Turn on kernel logging of matching packets. When this
option is set for a rule, the Linux kernel will print some
information on all matching packets (like most IP header
fields) via the kernel log (where it can be read with
dmesg or syslogd(8)).
--log-level level
Level of logging (numeric or see syslog.conf(5)).
--log-prefix prefix
Prefix log messages with the specified prefix; up
to 29 letters long, and useful for distinguishing
messages in the logs.
Log TCP sequence numbers. This is a security risk
if the log is readable by users.
--log-tcp-options
Log options from the TCP packet header.
--log-ip-options
Log options from the IP packet header.
MARK
This is used to set the netfilter mark value associated
with the packet. It is only valid in the mangle table.
--set-mark mark
REJECT
This is used to send back an error packet in response to
the matched packet: otherwise it is equivalent to DROP.
This target is only valid in the INPUT, FORWARD and OUTPUT
chains, and user-defined chains which are only called from
those chains. Several options control the nature of the
error packet returned:
--reject-with type
The type given can be icmp-net-unreachable, icmp-
host-unreachable, icmp-port-unreachable, icmp-
proto-unreachable, icmp-net-prohibitedor icmp-host-
prohibited, which return the appropriate ICMP error
message (port-unreachable is the default). The
option echo-reply is also allowed; it can only be
used for rules which specify an ICMP ping packet,
and generates a ping reply. Finally, the option
tcp-reset can be used on rules which only match the
TCP protocol: this causes a TCP RST packet to be
sent back. This is mainly useful for blocking
ident probes which frequently occur when sending
mail to broken mail hosts (which won't accept your
mail otherwise).
TOS
This is used to set the 8-bit Type of Service field in the
IP header. It is only valid in the mangle table.
--set-tos tos
You can use a numeric TOS values, or use
iptables -j TOS -h
to see the list of valid TOS names.
MIRROR
This is an experimental demonstration target which inverts
the source and destination fields in the IP header and
retransmits the packet. It is only valid in the INPUT,
FORWARD and PREROUTING chains, and user-defined chains
outgoing packets are NOT seen by any packet filtering
chains, connection tracking or NAT, to avoid loops and
other problems.
SNAT
This target is only valid in the nat table, in the
POSTROUTING chain. It specifies that the source address
of the packet should be modified (and all future packets
in this connection will also be mangled), and rules should
cease being examined. It takes one option:
--to-source <ipaddr>[-<ipaddr>][:port-port]
which can specify a single new source IP address,
an inclusive range of IP addresses, and optionally,
a port range (which is only valid if the rule also
specifies -p tcp or -p udp). If no port range is
specified, then source ports below 512 will be
mapped to other ports below 512: those between 512
and 1023 inclusive will be mapped to ports below
1024, and other ports will be mapped to 1024 or
above. Where possible, no port alteration will
occur.
DNAT
This target is only valid in the nat table, in the PRE
ROUTING and OUTPUT chains, and user-defined chains which
are only called from those chains. It specifies that the
destination address of the packet should be modified (and
all future packets in this connection will also be man
gled), and rules should cease being examined. It takes
one option:
--to-destination <ipaddr>[-<ipaddr>][:port-port]
which can specify a single new destination IP
address, an inclusive range of IP addresses, and
optionally, a port range (which is only valid if
the rule also specifies -p tcp or -p udp). If no
port range is specified, then the destination port
will never be modified.
MASQUERADE
This target is only valid in the nat table, in the
POSTROUTING chain. It should only be used with dynami
cally assigned IP (dialup) connections: if you have a
static IP address, you should use the SNAT target. Mas
querading is equivalent to specifying a mapping to the IP
address of the interface the packet is going out, but also
has the effect that connections are forgotten when the
interface goes down. This is the correct behavior when
the next dialup is unlikely to have the same interface
address (and hence any established connections are lost
anyway). It takes one option:
This specifies a range of source ports to use,
overriding the default SNAT source port-selection
heuristics (see above). This is only valid with if
the rule also specifies -p tcp or -p udp).
REDIRECT
This target is only valid in the nat table, in the PRE
ROUTING and OUTPUT chains, and user-defined chains which
are only called from those chains. It alters the destina
tion IP address to send the packet to the machine itself
(locally-generated packets are mapped to the 127.0.0.1
address). It takes one option:
--to-ports <port>[-<port>]
This specifies a destination port or range or ports
to use: without this, the destination port is never
altered. This is only valid with if the rule also
specifies -p tcp or -p udp).
EXTRA EXTENSIONS
The following extensions are not included by default in
the standard distribution.
ttl
This module matches the time to live field in the IP
header.
--ttl ttl
Matches the given TTL value.
TTL
This target is used to modify the time to live field in
the IP header. It is only valid in the mangle table.
--ttl-set ttl
Set the TTL to the given value.
--ttl-dec ttl
Decrement the TTL by the given value.
--ttl-inc ttl
Increment the TTL by the given value.
ULOG
This target provides userspace logging of matching pack
ets. When this target is set for a rule, the Linux kernel
will multicast this packet through a netlink socket. One
or more userspace processes may then subscribe to various
multicast groups and receive the packets.
--ulog-nlgroup <nlgroup>
This specifies the netlink group (1-32) to which
the packet is sent. Default value is 1.
Prefix log messages with the specified prefix; up
to 32 characters long, and useful fro distinguish
ing messages in the logs.
--ulog-cprange <size>
Number of bytes to be copied to userspace. A value
of 0 always copies the entire packet, regardless of
its size. Default is 0
--ulog-qthreshold <size>
Number of packet to queue inside kernel. Setting
this value to, e.g. 10 accumulates ten packets
inside the kernel and transmits them as one netlink
multipart message to userspace. Default is 1 (for
backwards compatibility)
DIAGNOSTICS
Various error messages are printed to standard error. The
exit code is 0 for correct functioning. Errors which
appear to be caused by invalid or abused command line
parameters cause an exit code of 2, and other errors cause
an exit code of 1.
BUGS
Check is not implemented (yet).
COMPATIBILITY WITH IPCHAINS
This iptables is very similar to ipchains by Rusty Rus
sell. The main difference is that the chains INPUT and
OUTPUT are only traversed for packets coming into the
local host and originating from the local host respec
tively. Hence every packet only passes through one of the
three chains; previously a forwarded packet would pass
through all three.
The other main difference is that -i refers to the input
interface; -o refers to the output interface, and both are
available for packets entering the FORWARD chain.
iptables is a pure packet filter when using the default
`filter' table, with optional extension modules. This
should simplify much of the previous confusion over the
combination of IP masquerading and packet filtering seen
previously. So the following options are handled differ
ently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
SEE ALSO
The packet-filtering-HOWTO, which details more iptables
usage for packet filtering, the NAT-HOWTO, which details
internals.
AUTHORS
Rusty Russell wrote iptables, in early consultation with
Michael Neuling.
Marc Boucher made Rusty abandon ipnatctl by lobbying for a
generic packet selection framework in iptables, then wrote
the mangle table, the owner match, the mark stuff, and ran
around doing cool stuff everywhere.
James Morris wrote the TOS target, and tos match.
Jozsef Kadlecsik wrote the REJECT target.
Harald Welte wrote the ULOG target, TTL match+target and
libipulog.
The Netfilter Core Team is: Marc Boucher, James Morris,
Harald Welte and Rusty Russell.
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