IPTABLES

iptables − administration tool for IPv4 packet filtering and NAT

iptables [-t table] -[AD] chain rule-specification [options]
iptables [-t table] -I chain [rulenum] rule-specification [options]
iptables [-t table] -R chain rulenum rule-specification [options]
iptables [-t table] -D chain rulenum [options]
iptables [-t table] -[LFZ] [chain] [options]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target [options]
iptables [-t table] -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 currently 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 (if no -t option is passed). 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 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

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. The exact rules are suppressed until you use

 iptables -L -v

-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 non-builtin 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-user-defined) 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

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 was 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 administrator 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 name, 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.

addrtype

This module matches packets based on their address type. Address types are used within the kernel networking stack and categorize addresses into various groups. The exact definition of that group depends on the specific layer three protocol.

The following address types are possible:

UNSPEC

an unspecified address (i.e. 0.0.0.0) UNICAST an unicast address LOCAL a local address BROADCAST a broadcast address ANYCAST an anycast packet MULTICAST a multicast address BLACKHOLE a blackhole address UNREACHABLE an unreachable address PROHIBIT a prohibited address THROW FIXME NAT FIXME XRESOLVE FIXME

--src-type type

Matches if the source address is of given type

--dst-type type

Matches if the destination address is of given type

ah

This module matches the SPIs in AH header of IPSec packets.

--ahspi [!] spi[:spi]

childlevel

This is an experimental module. It matches on whether the packet is part of a master connection or one of its children (or grandchildren, etc). For instance, most packets are level 0. FTP data transfer is level 1.

--childlevel [!] level

condition

This matches if a specific /proc filename is ’0’ or ’1’.

--condition [!] filename

Match on boolean value stored in /proc/net/ipt_condition/filename file

connmark

This module matches the netfilter mark field associated with a connection (which can be set using the CONNMARK target below).

--mark value[/mask]

Matches packets in connections with the given mark value (if a mask is specified, this is logically ANDed with the mark before the comparison).

connrate

This module matches the current transfer rate in a connection.

--connrate [!] [from]:[to]

Match against the current connection transfer rate being within ’from’ and ’to’ bytes per second. When the "!" argument is used before the range, the sense of the match is inverted.

conntrack

This module, when combined with connection tracking, allows access to more connection tracking information than the "state" match. (this module is present only if iptables was compiled under a kernel supporting this feature)

--ctstate 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. SNAT A virtual state, matching if the original source address differs from the reply destination. DNAT A virtual state, matching if the original destination differs from the reply source.

--ctproto proto

Protocol to match (by number or name)

--ctorigsrc [!] address[/mask]

Match against original source address

--ctorigdst [!] address[/mask]

Match against original destination address

--ctreplsrc [!] address[/mask]

Match against reply source address

--ctrepldst [!] address[/mask]

Match against reply destination address

--ctstatus [NONE|EXPECTED|SEEN_REPLY|ASSURED][,...]

Match against internal conntrack states

--ctexpire time[:time]

Match remaining lifetime in seconds against given value or range of values (inclusive)

dscp

This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.

--dscp value

Match against a numeric (decimal or hex) value [0-32].

--dscp-class DiffServ Class

Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be converted into it’s according numeric value.

dstlimit

This module allows you to limit the packet per second (pps) rate on a per destination IP or per destination port base. As opposed to the ‘limit’ match, every destination ip / destination port has it’s own limit.

--dstlimit avg

Maximum average match rate (packets per second unless followed by /sec /minute /hour /day postfixes).

--dstlimit-mode mode

The limiting hashmode. Is the specified limit per dstip, dstip-dstport tuple, srcip-dstip tuple, or per srcipdstip-dstport tuple.

--dstlimit-name name

Name for /proc/net/ipt_dstlimit/* file entry

[--dstlimit-burst burst]

Number of packets to match in a burst. Default: 5

[--dstlimit-htable-size size]

Number of buckets in the hashtable

[--dstlimit-htable-max max]

Maximum number of entries in the hashtable

[--dstlimit-htable-gcinterval interval]

Interval between garbage collection runs of the hashtable (in miliseconds). Default is 1000 (1 second).

[--dstlimit-htable-expire time

After which time are idle entries expired from hashtable (in miliseconds)? Default is 10000 (10 seconds).

ecn

This allows you to match the ECN bits of the IPv4 and TCP header. ECN is the Explicit Congestion Notification mechanism as specified in RFC3168

--ecn-tcp-cwr

This matches if the TCP ECN CWR (Congestion Window Received) bit is set.

--ecn-tcp-ece

This matches if the TCP ECN ECE (ECN Echo) bit is set.

--ecn-ip-ect num

This matches a particular IPv4 ECT (ECN-Capable Transport). You have to specify a number between ‘0’ and ‘3’.

esp

This module matches the SPIs in ESP header of IPSec packets.

--espspi [!] spi[:spi]

fuzzy

This module matches a rate limit based on a fuzzy logic controller [FLC]

--lower-limit number"

Specifies the lower limit (in packets per second).

--upper-limit number

Specifies the upper limit (in packets per second).

helper

This module matches packets related to a specific conntrack-helper.

--helper string

Matches packets related to the specified conntrack-helper.

string can be "ftp" for packets related to a ftp-session on default port. For other ports append -portnr to the value, ie. "ftp-2121".

Same rules apply for other conntrack-helpers.

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

iprange

This matches on a given arbitrary range of IPv4 addresses

[!]--src-range ip-ip

Match source IP in the specified range.

[!]--dst-range ip-ip

Match destination IP in the specified range.

length

This module matches the length of a packet against a specific value or range of values.

--length length[:length]

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.

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, FORWARD or INPUT chains.

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).

mport

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.

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.

nth

This module matches every ‘n’th packet

--every value

Match every ‘value’ packet

[--counter num]

Use internal counter number ‘num’. Default is ‘0’.

[--start num]

Initialize the counter at the number ‘num’ insetad of ‘0’. Most between ‘0’ and ‘value’-1.

[--packet num]

Match on ‘num’ packet. Most be between ‘0’ and ‘value’-1.

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)

physdev

This module matches on the bridge port input and output devices enslaved to a bridge device. This module is a part of the infrastructure that enables a transparent bridging IP firewall and is only useful for kernel versions above version 2.5.44.

--physdev-in name

Name of a bridge port via which a packet is received (only for packets entering the INPUT, FORWARD and PREROUTING chains). If the interface name ends in a "+", then any interface which begins with this name will match. If the packet didn’t arrive through a bridge device, this packet won’t match this option, unless ’!’ is used.

--physdev-out name

Name of a bridge port via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING chains). If the interface name ends in a "+", then any interface which begins with this name will match. Note that in the nat and mangle OUTPUT chains one cannot match on the bridge output port, however one can in the filter OUTPUT chain. If the packet won’t leave by a bridge device or it is yet unknown what the output device will be, then the packet won’t match this option, unless

--physdev-is-in

Matches if the packet has entered through a bridge interface.

--physdev-is-out

Matches if the packet will leave through a bridge interface.

--physdev-is-bridged

Matches if the packet is being bridged and therefore is not being routed. This is only useful in the FORWARD and POSTROUTING chains.

pkttype

This module matches the link-layer packet type.

--pkt-type [unicast|broadcast|multicast]

policy

This modules matches the policy used by IPsec for handling a packet.

--dir in|out

Used to select whether to match the policy used for decapsulation or the policy that will be used for encapsulation. in is valid in the PREROUTING, INPUT and FORWARD chains, out is valid in the POSTROUTING, OUTPUT and FORWARD chains.

--pol none|ipsec

Matches if the packet is subject to IPsec processing.

--strict

Selects whether to match the exact policy or match if any rule of the policy matches the given policy.

--reqid id

Matches the reqid of the policy rule. The reqid can be specified with setkey(8) using unique:id as level.

--spi spi

Matches the SPI of the SA.

--proto ah|esp|ipcomp

Matches the encapsulation protocol.

--mode tunnel|transport

Matches the encapsulation mode.

--tunnel-src addr[/mask]

Matches the source address of a tunnel. Only valid with --mode tunnel.

--tunnel-dst addr[/mask]

Matches the destination address of a tunnel. Only valid with --mode tunnel.

--next

Start the next element in the policy specification. Can only be used with --strict

random

This module randomly matches a certain percentage of all packets.

--average percent

Matches the given percentage. If omitted, a probability of 50% is set.

realm

This matches the routing realm. Routing realms are used in complex routing setups involving dynamic routing protocols like BGP.

--realm [!]value[/mask]

Matches a given realm number (and optionally mask).

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 could not be identified for some reason which includes running out of memory and ICMP errors which don’t correspond to any 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.

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 RST 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.

tcpmss

This matches the TCP MSS (maximum segment size) field of the TCP header. You can only use this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated during the TCP handshake at connection startup time.

[!] --mss value[:value]"

Match a given TCP MSS value or range.

time

This matches if the packet arrival time/date is within a given range. All options are facultative.

--timestart value

Match only if it is after ‘value’ (Inclusive, format: HH:MM ; default 00:00).

--timestop value

Match only if it is before ‘value’ (Inclusive, format: HH:MM ; default 23:59).

--days listofdays

Match only if today is one of the given days. (format: Mon,Tue,Wed,Thu,Fri,Sat,Sun ; default everyday)

--datestart date

Match only if it is after ‘date’ (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 1970)

--datestop date

Match only if it is before ‘date’ (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 2037)

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.

ttl

This module matches the time to live field in the IP header.

--ttl-eq ttl

Matches the given TTL value.

--ttl-gt ttl

Matches if TTL is greater than the given TTL value.

--ttl-lt ttl

Matches if TTL is less than the given TTL value.

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.

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.

BALANCE

This allows you to DNAT connections in a round-robin way over a given range of destination addresses.

--to-destination ipaddr-ipaddr

Address range to round-robin over.

CLASSIFY

This module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).

--set-class MAJOR:MINOR

Set the major and minor class value.

CLUSTERIP

This module allows you to configure a simple cluster of nodes that share a certain IP and MAC address without an explicit load balancer in front of them. Connections are statically distributed between the nodes in this cluster.

--new

Create a new ClusterIP. You always have to set this on the first rule for a given ClusterIP.

--hashmode mode

Specify the hashing mode. Has to be one of sourceip, sourceip-sourceport, sourceip-sourceport-destport

--clustermac mac

Specify the ClusterIP MAC address. Has to be a link-layer multicast address

--total-nodes num

Number of total nodes within this cluster.

--local-node num

Local node number within this cluster.

--hash-init rnd

Specify the random seed used for hash initialization.

CONNMARK

This module sets the netfilter mark value associated with a connection

--set-mark mark[/mask]

Set connection mark. If a mask is specified then only those bits set in the mask is modified.

--save-mark [--mask mask]

Copy the netfilter packet mark value to the connection mark. If a mask is specified then only those bits are copied.

--restore-mark [--mask mask]

Copy the connection mark value to the packet. If a mask is specified then only those bits are copied. This is only valid in the mangle table.

DNAT

This target is only valid in the nat table, in the PREROUTING 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 mangled), and rules should cease being examined. It takes one type of 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.

You can add several --to-destination options. If you specify more than one destination address, either via an address range or multiple --to-destination options, a simple round-robin (one after another in cycle) load balancing takes place between these adresses.

DSCP

This target allows to alter the value of the DSCP bits within the TOS header of the IPv4 packet. As this manipulates a packet, it can only be used in the mangle table.

--set-dscp value

Set the DSCP field to a numerical value (can be decimal or hex)

--set-dscp-class class

Set the DSCP field to a DiffServ class.

ECN

This target allows to selectively work around known ECN blackholes. It can only be used in the mangle table.

--ecn-tcp-remove

Remove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p tcp.

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)). 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 criteria, first using target LOG then DROP (or REJECT).

--log-level level

Level of logging (numeric or see syslog.conf(5)). The default level is warning.

--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

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

MASQUERADE

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.

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 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.

NETMAP

This target allows you to statically map a whole network of addresses onto another network of addresses. It can only be used from rules in the nat table.

--to address[/mask]

Network address to map to. The resulting address will be constructed in the following way: All ’one’ bits in the mask are filled in from the new ‘address’. All bits that are zero in the mask are filled in from the original address.

NOTRACK

This target disables connection tracking for all packets matching that rule.

It can only be used in the

raw table.

REDIRECT

This target is only valid in the nat table, in the PREROUTING and OUTPUT 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.

REJECT

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-unreachable
 icmp-port-unreachable
 icmp-proto-unreachable
 icmp-net-prohibited
 icmp-host-prohibited or
 icmp-admin-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).

(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT

ROUTE

This is used to explicitly override the core network stack’s routing decision. mangle table.

--oif ifname

Route the packet through ‘ifname’ network interface

--iif ifname

Change the packet’s incoming interface to ‘ifname’

--gw IP_address

Route the packet via this gateway

--continue

Behave like a non-terminating target and continue traversing the rules. Not valid in combination with ‘--iif’

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 type of 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.

You can add several --to-source options. If you specify more than one source address, either via an address range or multiple --to-source options, a simple round-robin (one after another in cycle) takes place between these adresses.

TCPMSS

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.

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.

TRACE

This target has no options. It just turns on packet tracing for all packets that match this rule.

TTL

This is used to modify the IPv4 TTL header field. The TTL field determines how many hops (routers) a packet can traverse until it’s time to live is exceeded.

Setting or incrementing the TTL field can potentially be very dangerous,

so it should be avoided at any cost.

Don’t ever set or increment the value on packets that leave your local network!

mangle table.

--ttl-set value

Set the TTL value to ‘value’.

--ttl-dec value

Decrement the TTL value ‘value’ times.

--ttl-inc value

Increment the TTL value ‘value’ times.

ULOG

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. Like LOG, this is a "non-terminating target", i.e. rule traversal continues at the next rule.

--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 for 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).

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? What’s this? ;-) Well... the counters are not reliable on sparc64.

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 (except loopback traffic, which involves both INPUT and OUTPUT 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.

iptables-save(8), iptables-restore(8), ip6tables(8), ip6tables-save(8), ip6tables-restore(8).

The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO details the extensions that are not in the standard distribution, and the netfilter-hacking-HOWTO details the netfilter 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, DSCP, ECN matches and targets.

The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef Kadlecsik, Patrick McHardy, James Morris, Harald Welte and Rusty Russell.

Man page written by Herve Eychenne <rv@wallfire.org>.