iptables - IP packet filter administration
iptables -[ADC] chain rule-specification [options]
iptables -I chain [rulenum] rule-specification [options]
iptables -R chain rulenum rule-specification [options]
iptables -D chain rulenum [options]
iptables -[LFZ] [chain] [options]
iptables -N chain
iptables -X [chain]
iptables -P chain target [options]
iptables -E old-chain-name new-chain-name
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 contains 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.
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.
There are current three independent tables (which tables are present at
any time depends on the kernel configuration options and which modules
are present).
-t, --table 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.
The tables are as follows:
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 locallygenerated
packets).
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 alteration. Until
kernel 2.4.17 it had two built-in chains: PREROUTING (for altering
incoming packets before routing) and OUTPUT (for altering
locally-generated packets before routing). Since kernel 2.4.18,
three other built-in chains are also supported : INPUT (for
packets coming into the box itself), FORWARD (for altering packets
being routed through the box), and POSTROUTING (for altering
packets as they are about to go out).
The options that are recognized by iptables can be divided into several
different groups.
COMMANDS [Toc] [Back]
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 versions of the command and option names, you need to
use only enough letters to ensure that iptables can differentiate it
from all other options.
-A, --append chain rule-specification
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 chain rule-specification
-D, --delete chain rulenum
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.
-I, --insert chain [rulenum] rule-specification
Insert one or more rules in the selected chain as the given rule
number. So, if the rule number is 1, the rule or rules are
inserted at the head of the chain. This is also the default if
no rule number is specified.
-R, --replace chain rulenum rule-specification
Replace a rule in the selected chain. If the source and/or destination
names resolve to multiple addresses, the command will
fail. Rules are numbered starting at 1.
-L, --list [chain]
List all rules in the selected chain. If no chain is selected,
all chains are listed. As every other iptables command, it
applies to the specified table (filter is the default), so NAT
rules get listed by
iptables -t nat -n -L
Please note that it is often used with the -n option, in order
to avoid long reverse DNS lookups. 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 [chain]
Flush the selected chain (all the chains in the table if none is
given). This is equivalent to deleting all the rules one by
one.
-Z, --zero [chain]
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 chain
Create a new user-defined chain by the given name. There must
be no target of that name already.
-X, --delete-chain [chain]
Delete the optional user-defined chain specified. 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 nonbuiltin
chain in the table.
-P, --policy chain target
Set the policy for the chain to the given target. See the section
TARGETS for the legal targets. Only built-in (non-userdefined)
chains can have policies, and neither built-in nor
user-defined chains can be policy targets.
-E, --rename-chain old-chain new-chain
Rename the user specified chain to the user supplied 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 [Toc] [Back]
The following parameters make up a rule specification (as used in the
add, delete, insert, replace and append commands).
-p, --protocol [!] protocol
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, representing 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
omitted.
-s, --source [!] address[/mask]
Source specification. Address can be either a network name, a
hostname (please note that specifying any name to be resolved
with a remote query such as DNS is a really bad idea), a network
IP address (with /mask), 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 an 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 counters on the rule will be incremented.
-i, --in-interface [!] name
Name of an interface via which a packet is going to be received
(only 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, any interface name will match.
-o, --out-interface [!] name
Name of an interface via which a packet is going to be sent (for
packets entering the FORWARD, OUTPUT and POSTROUTING 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, any interface name will match.
[!] -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 unfragmented 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 [Toc] [Back]
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 counters are also listed, with the suffix
'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multipliers
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 program will try to display
them as host names, network 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 (multiples 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 beginning of each
rule, corresponding to that rule's position in the chain.
--modprobe=command
When adding or inserting rules into a chain, use command to load
any necessary modules (targets, match extensions, etc).
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 specified. 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 a convenient alias for this
option.
--destination-port [!] port[:port]
Destination port or port range specification. The flag --dport
is a convenient 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 examine, 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, blocking such packets coming in an
interface will 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.
--mss value[:value]
Match TCP SYN or SYN/ACK packets with the specified MSS value
(or range), which control the maximum packet size for that connection.
udp
These extensions are loaded if `--protocol udp' is specified. 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 specified. 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
coming from an Ethernet device and entering the PREROUTING, FOR-
WARD or INPUT chains.
limit
This module matches at a limited rate using a token bucket filter. A
rule using this extension will match until this limit is reached
(unless the `!' flag is used). It can be used in combination with the
LOG target to give limited logging, for example.
--limit rate
Maximum average matching rate: specified as a number, with an
optional `/second', `/minute', `/hour', or `/day' suffix; the
default is 3/hour.
--limit-burst number
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-ports port[,port[,port...]]
Match if the source port is one of the given ports. The flag
--sports is a convenient alias for this option.
--destination-ports port[,port[,port...]]
Match if the destination port is one of the given ports. The
flag --dports is a convenient alias for this option.
--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 the given session
group.
--cmd-owner name
Matches if the packet was created by a process with the given
command name. (this option is present only if iptables was compiled
under a kernel supporting this feature)
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 connection 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 otherwise associated with a connection
which has not seen packets in both directions, and RELATED meaning
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.
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.
ah
This module matches the SPIs in AH header of IPSec packets.
--ahspi [!] spi[:spi]
esp
This module matches the SPIs in ESP header of IPSec packets.
--espspi [!] spi[:spi]
length
This module matches the length of a packet against a specific value or
range of values.
--length length[:length]
ttl
This module matches the time to live field in the IP header.
--ttl ttl
Matches the given TTL value.
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. This is regarded as experimental.
--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 the given session
group.
unclean
This module takes no options, but attempts to match packets which seem
malformed or unusual. This is regarded as experimental.
iptables can use extended target modules: the following are included in
the standard distribution.
LOG [Toc] [Back]
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)). This is a "non-terminating target",
i.e. rule traversal continues at the next rule. So if you want
to LOG the packets you refuse, use two separate rules with the same
matching criterias, first using target LOG then DROP (or REJECT).
--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
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 [Toc] [Back]
This is used to set the netfilter mark value associated with the
packet. It is only valid in the mangle table. It can for example be
used in conjunction with iproute2.
--set-mark mark
REJECT [Toc] [Back]
This is used to send back an error packet in response to the matched
packet: otherwise it is equivalent to DROP so it is a terminating TARGET,
ending rule traversal. This target is only valid in the INPUT,
FORWARD and OUTPUT chains, and user-defined chains which are only
called from those chains. The following option controls the nature of
the error packet returned:
--reject-with type
The type given can be icmp-net-unreachable, icmp-host-unreach-
able, icmp-port-unreachable, icmp-proto-unreachable, icmp-net-
prohibited or icmp-host-prohibited, which return the appropriate
ICMP error message (port-unreachable is the default). 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 (113/tcp) probes which frequently
occur when sending mail to broken mail hosts (which
won't accept your mail otherwise).
TOS [Toc] [Back]
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 [Toc] [Back]
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 userdefined
chains which are only called from those chains. Note that the
outgoing packets are NOT seen by any packet filtering chains, connection
tracking or NAT, to avoid loops and other problems.
SNAT [Toc] [Back]
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 [Toc] [Back]
This target is only valid in the nat table, in the PREROUTING and OUT-
PUT 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
mangled), 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 [Toc] [Back]
This target is only valid in the nat table, in the POSTROUTING chain.
It should only be used with dynamically assigned IP (dialup) connections:
if you have a static IP address, you should use the SNAT target.
Masquerading 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:
--to-ports port[-port]
This specifies a range of source ports to use, overriding the
default SNAT source port-selection heuristics (see above). This
is only valid if the rule also specifies -p tcp or -p udp.
REDIRECT [Toc] [Back]
This target is only valid in the nat table, in the PREROUTING and OUT-
PUT chains, and user-defined chains which are only called from those
chains. It alters the destination 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 of ports to use:
without this, the destination port is never altered. This is
only valid if the rule also specifies -p tcp or -p udp.
ULOG [Toc] [Back]
This target provides userspace logging of matching packets. 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.
--ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32 characters
long, and useful fro distinguishing 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).
TCPMSS [Toc] [Back]
This target allows to alter the MSS value of TCP SYN packets, to control
the maximum size for that connection (usually limiting it to your
outgoing interface's MTU minus 40). Of course, it can only be used in
conjunction with -p tcp.
This target is used to overcome criminally braindead ISPs or servers
which block ICMP Fragmentation Needed packets. The symptoms of this
problem are that everything works fine from your Linux firewall/router,
but machines behind it can never exchange large packets:
1) Web browsers connect, then hang with no data received.
2) Small mail works fine, but large emails hang.
3) ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall configuration
like:
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value
Explicitly set MSS option to specified value.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40).
These options are mutually exclusive.
The following extensions are not included by default in the standard
distribution.
TTL [Toc] [Back]
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.
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.
Check is not implemented (yet).
COMPATIBILITY WITH IPCHAINS [Toc] [Back] This iptables is very similar to ipchains by Rusty Russell. 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
respectively. 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 differently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
The packet-filtering-HOWTO, which details more iptables usage for
packet filtering, the NAT-HOWTO, which details NAT, and the netfilterhacking-HOWTO
which details the internals.
See http://www.netfilter.org/.
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, Jozsef Kadlecsik, James Morris,
Harald Welte and Rusty Russell.
Man page written by Herve Eychenne <[email protected]>.
Mar 09, 2002 IPTABLES(8)
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