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IPTABLES(8) IPTABLES(8)
NAME
iptables - administration tool for IPv4 packet filtering and NAT
SYNOPSIS
iptables [-t table] {-A|-D} chain rule-specification [options...]
iptables [-t table] -I [rulenum] rule-specification [options...]
iptables [-t table] -R rulenum rule-specification [options...]
iptables [-t table] -D chain rulenum [options...]
iptables [-t table] -S [chain]
iptables [-t table] {-F|-L|-Z} [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
DESCRIPTION
Iptables is used to set up, maintain, and inspect the tables of IPv4 packet filter rules
in the Linux kernel. Several different tables may be defined. Each table contains a num-
ber 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. (How the packet can be received by a
userspace process differs by the particular queue handler. 2.4.x and 2.6.x kernels up to
2.6.13 include the ip_queue queue handler. Kernels 2.6.14 and later additionally include
the nfnetlink_queue queue handler. Packets with a target of QUEUE will be sent to queue
number '0' in this case. Please also see the NFQUEUE target as described later in this man
page.) RETURN means stop traversing this chain and resume at the next rule in the previ-
ous (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 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 destined to local sockets), 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 encoun-
tered. 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 rout-
ing) and OUTPUT (for altering locally-generated packets before routing). Since
kernel 2.4.18, three other built-in chains are also supported: INPUT (for pack-
ets 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).
raw:
This table is used mainly for configuring exemptions from connection tracking
in combination with the NOTRACK target. It registers at the netfilter hooks
with higher priority and is thus called before ip_conntrack, or any other IP
tables. It provides the following built-in chains: PREROUTING (for packets
arriving via any network interface) OUTPUT (for packets generated by local pro-
cesses)
OPTIONS
The options that are recognized by iptables can be divided into several different groups.
COMMANDS
These options specify the desired action to perform. Only one of them can be specified on
the command line unless otherwise stated below. For long versions of the command and
option names, you need to use only enough letters to ensure that iptables can differenti-
ate 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. Like every other iptables command, it applies to the specified table (fil-
ter 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
-S, --list-rules [chain]
Print all rules in the selected chain. If no chain is selected, all chains are
printed like iptables-save. Like every other iptables command, it applies to the
specified table (filter is the default).
-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. The chain must be empty, i.e. not contain any rules. 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 nei-
ther 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, udplite, icmp, esp, ah, sctp 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 pro-
tocols 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 (and -g is
not used), then matching the rule will have no effect on the packet's fate, but the
counters on the rule will be incremented.
-g, --goto chain
This specifies that the processing should continue in a user specified chain.
Unlike the --jump option return will not continue processing in this chain but
instead in the chain that called us via --jump.
[!] -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 omit-
ted, 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 frag-
ments, or unfragmented packets.
-c, --set-counters packets 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 mul-
tipliers respectively (but see the -x flag to change this). For appending, inser-
tion, 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 mod-
ules (targets, match extensions, etc).
MATCH EXTENSIONS
iptables can use extended packet matching modules. These are loaded in two ways: implic-
itly, 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 avail-
able, 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 defi-
nition 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
[!] --src-type type
Matches if the source address is of given type
[!] --dst-type type
Matches if the destination address is of given type
--limit-iface-in
The address type checking can be limited to the interface the packet is coming in.
This option is only valid in the PREROUTING, INPUT and FORWARD chains. It cannot be
specified with the --limit-iface-out option.
--limit-iface-out
The address type checiking can be limited to the interface the packet is going out.
This option is only valid in the POSTROUTING, OUTPUT and FORWARD chains. It cannot
be specified with the --limit-iface-in option.
ah
This module matches the SPIs in Authentication header of IPsec packets.
[!] --ahspi spi[:spi]
comment
Allows you to add comments (up to 256 characters) to any rule.
--comment comment
Example:
iptables -A INPUT -s 192.168.0.0/16 -m comment --comment "A privatized IP block"
connbytes
Match by how many bytes or packets a connection (or one of the two flows constituting the
connection) has transferred so far, or by average bytes per packet.
The counters are 64bit and are thus not expected to overflow ;)
The primary use is to detect long-lived downloads and mark them to be scheduled using a
lower priority band in traffic control.
The transferred bytes per connection can also be viewed through /proc/net/ip_conntrack and
accessed via ctnetlink
[!] --connbytes from[:to]
match packets from a connection whose packets/bytes/average packet size is more
than FROM and less than TO bytes/packets. if TO is omitted only FROM check is done.
"!" is used to match packets not falling in the range.
--connbytes-dir {original|reply|both}
which packets to consider
--connbytes-mode {packets|bytes|avgpkt}
whether to check the amount of packets, number of bytes transferred or the average
size (in bytes) of all packets received so far. Note that when "both" is used
together with "avgpkt", and data is going (mainly) only in one direction (for exam-
ple HTTP), the average packet size will be about half of the actual data packets.
Example:
iptables .. -m connbytes --connbytes 10000:100000 --connbytes-dir both --connbytes-
mode bytes ...
connlimit
Allows you to restrict the number of parallel connections to a server per client IP
address (or client address block).
[!] --connlimit-above n
Match if the number of existing connections is (not) above n.
--connlimit-mask prefix_length
Group hosts using the prefix length. For IPv4, this must be a number between
(including) 0 and 32. For IPv6, between 0 and 128.
Examples:
# allow 2 telnet connections per client host
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j
REJECT
# you can also match the other way around:
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit ! --connlimit-above 2 -j
ACCEPT
# limit the number of parallel HTTP requests to 16 per class C sized network (24 bit net-
mask)
iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16 --connlimit-mask
24 -j REJECT
# limit the number of parallel HTTP requests to 16 for the link local network (ipv6)
ip6tables -p tcp --syn --dport 80 -s fe80::/64 -m connlimit --connlimit-above 16
--connlimit-mask 64 -j REJECT
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).
conntrack
This module, when combined with connection tracking, allows access to the connection
tracking state for this packet/connection.
[!] --ctstate statelist
statelist is a comma separated list of the connection states to match. Possible
states are listed below.
[!] --ctproto l4proto
Layer-4 protocol to match (by number or name)
[!] --ctorigsrc address[/mask]
[!] --ctorigdst address[/mask]
[!] --ctreplsrc address[/mask]
[!] --ctrepldst address[/mask]
Match against original/reply source/destination address
[!] --ctorigsrcport port
[!] --ctorigdstport port
[!] --ctreplsrcport port
[!] --ctrepldstport port
Match against original/reply source/destination port (TCP/UDP/etc.) or GRE key.
[!] --ctstatus statelist
statuslist is a comma separated list of the connection statuses to match. Possible
statuses are listed below.
[!] --ctexpire time[:time]
Match remaining lifetime in seconds against given value or range of values (inclu-
sive)
--ctdir {ORIGINAL|REPLY}
Match packets that are flowing in the specified direction. If this flag is not
specified at all, matches packets in both directions.
States for --ctstate:
INVALID
meaning that the packet is associated with no known connection
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
ESTABLISHED
meaning that the packet is associated with a connection which has seen packets in
both directions,
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.
Statuses for --ctstatus:
NONE None of the below.
EXPECTED
This is an expected connection (i.e. a conntrack helper set it up)
SEEN_REPLY
Conntrack has seen packets in both directions.
ASSURED
Conntrack entry should never be early-expired.
CONFIRMED
Connection is confirmed: originating packet has left box.
dccp
[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --dccp-types mask
Match when the DCCP packet type is one of 'mask'. 'mask' is a comma-separated list
of packet types. Packet types are: REQUEST RESPONSE DATA ACK DATAACK CLOSEREQ
CLOSE RESET SYNC SYNCACK INVALID.
[!] --dccp-option number
Match if DCP option set.
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 [0-63].
[!] --dscp-class class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes.
It will then be converted into its according numeric value.
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]
hashlimit
hashlimit uses hash buckets to express a rate limiting match (like the limit match) for a
group of connections using a single iptables rule. Grouping can be done per-hostgroup
(source and/or destination address) and/or per-port. It gives you the ability to express
"N packets per time quantum per group":
matching on source host
"1000 packets per second for every host in 192.168.0.0/16"
matching on source prot
"100 packets per second for every service of 192.168.1.1"
matching on subnet
"10000 packets per minute for every /28 subnet in 10.0.0.0/8"
A hash limit option (--hashlimit-upto, --hashlimit-above) and --hashlimit-name are
required.
--hashlimit-upto amount[/second|/minute|/hour|/day]
Match if the rate is below or equal to amount/quantum. It is specified as a number,
with an optional time quantum suffix; the default is 3/hour.
--hashlimit-above amount[/second|/minute|/hour|/day]
Match if the rate is above amount/quantum.
--hashlimit-burst amount
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.
--hashlimit-mode {srcip|srcport|dstip|dstport},...
A comma-separated list of objects to take into consideration. If no --hashlimit-
mode option is given, hashlimit acts like limit, but at the expensive of doing the
hash housekeeping.
--hashlimit-srcmask prefix
When --hashlimit-mode srcip is used, all source addresses encountered will be
grouped according to the given prefix length and the so-created subnet will be sub-
ject to hashlimit. prefix must be between (inclusive) 0 and 32. Note that --hash-
limit-srcmask 0 is basically doing the same thing as not specifying srcip for
--hashlimit-mode, but is technically more expensive.
--hashlimit-dstmask prefix
Like --hashlimit-srcmask, but for destination addresses.
--hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry.
--hashlimit-htable-size buckets
The number of buckets of the hash table
--hashlimit-htable-max entries
Maximum entries in the hash.
--hashlimit-htable-expire msec
After how many milliseconds do hash entries expire.
--hashlimit-htable-gcinterval msec
How many milliseconds between garbage collection intervals.
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 can be used 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 IP addresses.
[!] --src-range from[-to]
Match source IP in the specified range.
[!] --dst-range from[-to]
Match destination IP in the specified range.
length
This module matches the length of the layer-3 payload (e.g. layer-4 packet) f 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[/second|/minute|/hour|/day]
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).
multiport
This module matches a set of source or destination ports. Up to 15 ports can be speci-
fied. A port range (port:port) counts as two ports. It can only be used in conjunction
with -p tcp or -p udp.
[!] --source-ports,--sports port[,port|,port:port]...
Match if the source port is one of the given ports. The flag --sports is a conve-
nient alias for this option. Multiple ports or port ranges are separated using a
comma, and a port range is specified using a colon. 53,1024:65535 would therefore
match ports 53 and all from 1024 through 65535.
[!] --destination-ports,--dports port[,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:port]...
Match if either the source or destination ports are equal to one of the given
ports.
owner
This module attempts to match various characteristics of the packet creator, for locally
generated packets. This match is only valid in the OUTPUT and POSTROUTING chains. For-
warded packets do not have any socket associated with them. Packets from kernel threads do
have a socket, but usually no owner.
[!] --uid-owner username
[!] --uid-owner userid[-userid]
Matches if the packet socket's file structure (if it has one) is owned by the given
user. You may also specify a numerical UID, or an UID range.
[!] --gid-owner groupname
[!] --gid-owner groupid[-groupid]
Matches if the packet socket's file structure is owned by the given group. You may
also specify a numerical GID, or a GID range.
[!] --socket-exists
Matches if the packet is associated with a socket.
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 '!' is used.
[!] --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 FOR-
WARD 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 end-point address of a tunnel mode SA. Only valid with --mode
tunnel.
[!] --tunnel-dst addr[/mask]
Matches the destination end-point address of a tunnel mode SA. Only valid with
--mode tunnel.
--next Start the next element in the policy specification. Can only be used with --strict.
quota
Implements network quotas by decrementing a byte counter with each packet.
--quota bytes
The quota in bytes.
rateest
The rate estimator can match on estimated rates as collected by the RATEEST target. It
supports matching on absolute bps/pps values, comparing two rate estimators and matching
on the difference between two rate estimators.
--rateest1 name
Name of the first rate estimator.
--rateest2 name
Name of the second rate estimator (if difference is to be calculated).
--rateest-delta
Compare difference(s) to given rate(s)
--rateest1-bps value
--rateest2-bps value
Compare bytes per second.
--rateest1-pps value
--rateest2-pps value
Compare packets per second.
[!] --rateest-lt
Match if rate is less than given rate/estimator.
[!] --rateest-gt
Match if rate is greater than given rate/estimator.
[!] --rateest-eq
Match if rate is equal to given rate/estimator.
Example: This is what can be used to route outgoing data connections from an FTP server
over two lines based on the available bandwidth at the time the data connection was
started:
# Estimate outgoing rates
iptables -t mangle -A POSTROUTING -o eth0 -j RATEEST --rateest-name eth0 --rateest-inter-
val 250ms --rateest-ewma 0.5s
iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST --rateest-name ppp0 --rateest-inter-
val 250ms --rateest-ewma 0.5s
# Mark based on available bandwidth
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest
--rateest-delta --rateest1 eth0 --rateest-bps1 2.5mbit --rateest-gt --rateest2 ppp0
--rateest-bps2 2mbit -j CONNMARK --set-mark 1
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest
--rateest-delta --rateest1 ppp0 --rateest-bps1 2mbit --rateest-gt --rateest2 eth0
--rateest-bps2 2.5mbit -j CONNMARK --set-mark 2
iptables -t mangle -A balance -j CONNMARK --restore-mark
realm
This matches the routing realm. Routing realms are used in complex routing setups involv-
ing dynamic routing protocols like BGP.
[!] --realm value[/mask]
Matches a given realm number (and optionally mask). If not a number, value can be a
named realm from /etc/iproute2/rt_realms (mask can not be used in that case).
recent
Allows you to dynamically create a list of IP addresses and then match against that list
in a few different ways.
For example, you can create a `badguy' list out of people attempting to connect to port
139 on your firewall and then DROP all future packets from them without considering them.
--name name
Specify the list to use for the commands. If no name is given then 'DEFAULT' will
be used.
[!] --set
This will add the source address of the packet to the list. If the source address
is already in the list, this will update the existing entry. This will always
return success (or failure if `!' is passed in).
--rsource
Match/save the source address of each packet in the recent list table. This is the
default.
--rdest
Match/save the destination address of each packet in the recent list table.
[!] --rcheck
Check if the source address of the packet is currently in the list.
[!] --update
Like --rcheck, except it will update the "last seen" timestamp if it matches.
[!] --remove
Check if the source address of the packet is currently in the list and if so that
address will be removed from the list and the rule will return true. If the address
is not found, false is returned.
[!] --seconds seconds
This option must be used in conjunction with one of --rcheck or --update. When
used, this will narrow the match to only happen when the address is in the list and
was seen within the last given number of seconds.
[!] --hitcount hits
This option must be used in conjunction with one of --rcheck or --update. When
used, this will narrow the match to only happen when the address is in the list and
packets had been received greater than or equal to the given value. This option may
be used along with --seconds to create an even narrower match requiring a certain
number of hits within a specific time frame.
--rttl This option may only be used in conjunction with one of --rcheck or --update. When
used, this will narrow the match to only happen when the address is in the list and
the TTL of the current packet matches that of the packet which hit the --set rule.
This may be useful if you have problems with people faking their source address in
order to DoS you via this module by disallowing others access to your site by send-
ing bogus packets to you.
--name name
Name of the recent list to be used. DEFAULT used if none given.
--rsource
Match/Save the source address of each packet in the recent list table (default).
--rdest
Match/Save the destination address of each packet in the recent list table.
Examples:
# iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP
# iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy --set -j
DROP
Official website (http://snowman.net/projects/ipt_recent/) also has some examples of
usage.
/proc/net/ipt_recent/* are the current lists of addresses and information about each entry
of each list.
Each file in /proc/net/ipt_recent/ can be read from to see the current list or written two
using the following commands to modify the list:
echo xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Add to the DEFAULT list
echo -xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Remove from the DEFAULT list
echo clear > /proc/net/ipt_recent/DEFAULT
to empty the DEFAULT list.
The module itself accepts parameters, defaults shown:
ip_list_tot=100
Number of addresses remembered per table
ip_pkt_list_tot=20
Number of packets per address remembered
ip_list_hash_size=0
Hash table size. 0 means to calculate it based on ip_list_tot, default: 512
ip_list_perms=0644
Permissions for /proc/net/ipt_recent/* files
debug=0
Set to 1 to get lots of debugging info
sctp
[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --chunk-types {all|any|only} chunktype[:flags] [...]
The flag letter in upper case indicates that the flag is to match if set, in the
lower case indicates to match if unset.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUT-
DOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE ASCONF
ASCONF_ACK
chunk type available flags
DATA U B E u b e
ABORT T t
SHUTDOWN_COMPLETE T t
(lowercase means flag should be "off", uppercase means "on")
Examples:
iptables -A INPUT -p sctp --dport 80 -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA,INIT -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT
set
This modules macthes IP sets which can be defined by ipset(8).
[!] --set setname flag[,flag]...
where flags are src and/or dst and there can be no more than six of them. Hence the
command
iptables -A FORWARD -m set --set test src,dst
will match packets, for which (depending on the type of the set) the source address
or port number of the packet can be found in the specified set. If there is a bind-
ing belonging to the mached set element or there is a default binding for the given
set, then the rule will match the packet only if additionally (depending on the
type of the set) the destination address or port number of the packet can be found
in the set according to the binding.
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 start-
ing a new connection, but is associated with an existing connection, such as an FTP
data transfer, or an ICMP error.
statistic
This module matches packets based on some statistic condition. It supports two distinct
modes settable with the --mode option.
Supported options:
--mode mode
Set the matching mode of the matching rule, supported modes are random and nth.
--probability p
Set the probability from 0 to 1 for a packet to be randomly matched. It works only
with the random mode.
--every n
Match one packet every nth packet. It works only with the nth mode (see also the
--packet option).
--packet p
Set the initial counter value (0 <= p <= n-1, default 0) for the nth mode.
string
This modules matches a given string by using some pattern matching strategy. It requires a
linux kernel >= 2.6.14.
--algo {bm|kmp}
Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
--from offset
Set the offset from which it starts looking for any matching. If not passed,
default is 0.
--to offset
Set the offset from which it starts looking for any matching. If not passed,
default is the packet size.
[!] --string pattern
Matches the given pattern.
[!] --hex-string pattern
Matches the given pattern in hex notation.
tcp
These extensions can be used if `--protocol tcp' is specified. It provides the following
options:
[!] --source-port,--sport 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 is greater than the first they will be swapped. The
flag --sport is a convenient alias for this option.
[!] --destination-port,--dport 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 mask is the flags
which we should examine, written as a comma-separated list, and the second argument
comp 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,RST 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 out-
going TCP connections will be unaffected. It is equivalent to --tcp-flags
SYN,RST,ACK,FIN SYN. If the "!" flag precedes the "--syn", the sense of the option
is inverted.
[!] --tcp-option number
Match if TCP option set.
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 hand-
shake 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
optional, but are ANDed when specified.
--datestart YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
--datestop YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
Only match during the given time, which must be in ISO 8601 "T" notation. The pos-
sible time range is 1970-01-01T00:00:00 to 2038-01-19T04:17:07.
If --datestart or --datestop are not specified, it will default to 1970-01-01 and
2038-01-19, respectively.
--timestart hh:mm[:ss]
--timestop hh:mm[:ss]
Only match during the given daytime. The possible time range is 00:00:00 to
23:59:59. Leading zeroes are allowed (e.g. "06:03") and correctly interpreted as
base-10.
[!] --monthdays day[,day...]
Only match on the given days of the month. Possible values are 1 to 31. Note that
specifying 31 will of course not match on months which do not have a 31st day; the
same goes for 28- or 29-day February.
[!] --weekdays day[,day...]
Only match on the given weekdays. Possible values are Mon, Tue, Wed, Thu, Fri, Sat,
Sun, or values from 1 to 7, respectively. You may also use two-character variants
(Mo, Tu, etc.).
--utc
Interpret the times given for --datestart, --datestop, --timestart and --timestop
to be UTC.
--localtz
Interpret the times given for --datestart, --datestop, --timestart and --timestop
to be local kernel time. (Default)
EXAMPLES. To match on weekends, use:
-m time --weekdays Sa,Su
Or, to match (once) on a national holiday block:
-m time --datestart 2007-12-24 --datestop 2007-12-27
Since the stop time is actually inclusive, you would need the following stop time to not
match the first second of the new day:
-m time --datestart 2007-01-01T17:00 --datestop 2007-01-01T23:59:59
During lunch hour:
-m time --timestart 12:30 --timestop 13:30
The fourth Friday in the month:
-m time --weekdays Fr --monthdays 22,23,24,25,26,27,28
(Note that this exploits a certain mathematical property. It is not possible to say
"fourth Thursday OR fourth Friday" in one rule. It is possible with multiple rules,
though.)
tos
This module matches the 8-bit Type of Service field in the IPv4 header (i.e. including
the "Precedence" bits) or the (also 8-bit) Priority field in the IPv6 header.
[!] --tos value[/mask]
Matches packets with the given TOS mark value. If a mask is specified, it is logi-
cally ANDed with the TOS mark before the comparison.
[!] --tos symbol
You can specify a symbolic name when using the tos match for IPv4. The list of rec-
ognized TOS names can be obtained by calling iptables with -m tos -h. Note that
this implies a mask of 0x3F, i.e. all but the ECN bits.
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.
u32
U32 tests whether quantities of up to 4 bytes extracted from a packet have specified val-
ues. The specification of what to extract is general enough to find data at given offsets
from tcp headers or payloads.
[!] --u32 tests
The argument amounts to a program in a small language described below.
tests := location "=" value | tests "&&" location "=" value
value := range | value "," range
range := number | number ":" number
a single number, n, is interpreted the same as n:n. n:m is interpreted as the range of
numbers >=n and <=m.
location := number | location operator number
operator := "&" | "<<" | ">>" | "@"
The operators &, <<, >> and && mean the same as in C. The = is really a set membership
operator and the value syntax describes a set. The @ operator is what allows moving to the
next header and is described further below.
There are currently some artificial implementation limits on the size of the tests:
* no more than 10 of "=" (and 9 "&&"s) in the u32 argument
* no more than 10 ranges (and 9 commas) per value
* no more than 10 numbers (and 9 operators) per location
To describe the meaning of location, imagine the following machine that interprets it.
There are three registers:
A is of type char *, initially the address of the IP header
B and C are unsigned 32 bit integers, initially zero
The instructions are:
number B = number;
C = (*(A+B)<<24) + (*(A+B+1)<<16) + (*(A+B+2)<<8) + *(A+B+3)
&number C = C & number
<< number C = C << number
>> number C = C >> number
@number A = A + C; then do the instruction number
Any access of memory outside [skb->data,skb->end] causes the match to fail. Otherwise the
result of the computation is the final value of C.
Whitespace is allowed but not required in the tests. However, the characters that do occur
there are likely to require shell quoting, so it is a good idea to enclose the arguments
in quotes.
Example:
match IP packets with total length >= 256
The IP header contains a total length field in bytes 2-3.
--u32 "0 & 0xFFFF = 0x100:0xFFFF"
read bytes 0-3
AND that with 0xFFFF (giving bytes 2-3), and test whether that is in the range
[0x100:0xFFFF]
Example: (more realistic, hence more complicated)
match ICMP packets with icmp type 0
First test that it is an ICMP packet, true iff byte 9 (protocol) = 1
--u32 "6 & 0xFF = 1 && ...
read bytes 6-9, use & to throw away bytes 6-8 and compare the result to 1. Next
test that it is not a fragment. (If so, it might be part of such a packet but we
cannot always tell.) N.B.: This test is generally needed if you want to match any-
thing beyond the IP header. The last 6 bits of byte 6 and all of byte 7 are 0 iff
this is a complete packet (not a fragment). Alternatively, you can allow first
fragments by only testing the last 5 bits of byte 6.
... 4 & 0x3FFF = 0 && ...
Last test: the first byte past the IP header (the type) is 0. This is where we have
to use the @syntax. The length of the IP header (IHL) in 32 bit words is stored in
the right half of byte 0 of the IP header itself.
... 0 >> 22 & 0x3C @ 0 >> 24 = 0"
The first 0 means read bytes 0-3, >>22 means shift that 22 bits to the right.
Shifting 24 bits would give the first byte, so only 22 bits is four times that plus
a few more bits. &3C then eliminates the two extra bits on the right and the first
four bits of the first byte. For instance, if IHL=5, then the IP header is 20 (4 x
5) bytes long. In this case, bytes 0-1 are (in binary) xxxx0101 yyzzzzzz, >>22
gives the 10 bit value xxxx0101yy and &3C gives 010100. @ means to use this number
as a new offset into the packet, and read four bytes starting from there. This is
the first 4 bytes of the ICMP payload, of which byte 0 is the ICMP type. Therefore,
we simply shift the value 24 to the right to throw out all but the first byte and
compare the result with 0.
Example:
TCP payload bytes 8-12 is any of 1, 2, 5 or 8
First we test that the packet is a tcp packet (similar to ICMP).
--u32 "6 & 0xFF = 6 && ...
Next, test that it is not a fragment (same as above).
... 0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 = 1,2,5,8"
0>>22&3C as above computes the number of bytes in the IP header. @ makes this the
new offset into the packet, which is the start of the TCP header. The length of the
TCP header (again in 32 bit words) is the left half of byte 12 of the TCP header.
The 12>>26&3C computes this length in bytes (similar to the IP header before). "@"
makes this the new offset, which is the start of the TCP payload. Finally, 8 reads
bytes 8-12 of the payload and = checks whether the result is any of 1, 2, 5 or 8.
udp
These extensions can be used if `--protocol udp' is specified. It provides the following
options:
[!] --source-port,--sport port[:port]
Source port or port range specification. See the description of the --source-port
option of the TCP extension for details.
[!] --destination-port,--dport port[:port]
Destination port or port range specification. See the description of the --desti-
nation-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.
TARGET EXTENSIONS
iptables can use extended target modules: the following are included in the standard dis-
tribution.
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 stati-
cally 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, sour-
ceip-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-xmark value[/mask]
Zero out the bits given by mask and XOR value into the ctmark.
--save-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the packet mark (nfmark) to the connection mark (ctmark) using the given
masks. The new nfmark value is determined as follows:
ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask)
i.e. ctmask defines what bits to clear and nfmask what bits of the nfmark to XOR
into the ctmark. ctmask and nfmask default to 0xFFFFFFFF.
--restore-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the connection mark (ctmark) to the packet mark (nfmark) using the given
masks. The new ctmark value is determined as follows:
nfmark = (nfmark & ~nfmask) ^ (ctmark & ctmask);
i.e. nfmask defines what bits to clear and ctmask what bits of the ctmark to XOR
into the nfmark. ctmask and nfmask default to 0xFFFFFFFF.
--restore-mark is only valid in the mangle table.
The following mnemonics are available for --set-xmark:
--and-mark bits
Binary AND the ctmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits
is the binary negation of bits.)
--or-mark bits
Binary OR the ctmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits
Binary XOR the ctmark with bits. (Mnemonic for --set-xmark bits/0.)
--set-mark value[/mask]
Set the connection mark. If a mask is specified then only those bits set in the
mask are modified.
--save-mark [--mask mask]
Copy the nfmark to the ctmark. If a mask is specified, only those bits are copied.
--restore-mark [--mask mask]
Copy the ctmark to the nfmark. If a mask is specified, only those bits are copied.
This is only valid in the mangle table.
CONNSECMARK
This module copies security markings from packets to connections (if unlabeled), and from
connections back to packets (also only if unlabeled). Typically used in conjunction with
SECMARK, it is only valid in the mangle table.
--save If the packet has a security marking, copy it to the connection if the connection
is not marked.
--restore
If the packet does not have a security marking, and the connection does, copy the
security marking from the connection to the packet.
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 spec-
ifies -p tcp or -p udp). If no port range is specified, then the destination port
will never be modified. If no IP address is specified then only the destination
port will be modified.
In Kernels up to 2.6.10 you can add several --to-destination options. For those
kernels, 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 addresses. Later Kernels (>=
2.6.11-rc1) don't have the ability to NAT to multiple ranges anymore.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.22).
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 con-
junction 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-termi-
nating 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)).
--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.
--log-uid
Log the userid of the process which generated the packet.
MARK
This target is used to set the Netfilter mark value associated with the packet. The tar-
get can only be used in the mangle table. It can, for example, be used in conjunction with
routing based on fwmark (needs iproute2).
--set-xmark value[/mask]
Zeroes out the bits given by mask and XORs value into the packet mark ("nfmark").
If mask is omitted, 0xFFFFFFFF is assumed.
--set-mark value[/mask]
Zeroes out the bits given by mask and ORs value into the packet mark. If mask is
omitted, 0xFFFFFFFF is assumed.
The following mnemonics are available:
--and-mark bits
Binary AND the nfmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits
is the binary negation of bits.)
--or-mark bits
Binary OR the nfmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits
Binary XOR the nfmark with bits. (Mnemonic for --set-xmark bits/0.)
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 connec-
tions 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 con-
nections 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 speci-
fies -p tcp or -p udp.
--random
Randomize source port mapping If option --random is used then port mapping will be
randomized (kernel >= 2.6.21).
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, FOR-
WARD 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, con-
nection 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 fol-
lowing 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.
NFLOG
This target provides logging of matching packets. When this target is set for a rule, the
Linux kernel will pass the packet to the loaded logging backend to log the packet. This is
usually used in combination with nfnetlink_log as logging backend, which will multicast
the packet through a netlink socket to the specified multicast group. One or more
userspace processes may subscribe to the group to receive the packets. Like LOG, this is a
non-terminating target, i.e. rule traversal continues at the next rule.
--nflog-group nlgroup
The netlink group (1 - 2^32-1) to which packets are (only applicable for
nfnetlink_log). The default value is 0.
--nflog-prefix prefix
A prefix string to include in the log message, up to 64 characters long, useful for
distinguishing messages in the logs.
--nflog-range size
The number of bytes to be copied to userspace (only applicable for nfnetlink_log).
nfnetlink_log instances may specify their own range, this option overrides it.
--nflog-threshold size
Number of packets to queue inside the kernel before sending them to userspace (only
applicable for nfnetlink_log). Higher values result in less overhead per packet,
but increase delay until the packets reach userspace. The default value is 1.
NFQUEUE
This target is an extension of the QUEUE target. As opposed to QUEUE, it allows you to put
a packet into any specific queue, identified by its 16-bit queue number.
--queue-num value
This specifies the QUEUE number to use. Valid queue numbers are 0 to 65535. The
default value is 0.
It can only be used with Kernel versions 2.6.14 or later, since it requires the
nfnetlink_queue kernel support.
NOTRACK
This target disables connection tracking for all packets matching that rule.
It can only be used in the raw table.
RATEEST
The RATEEST target collects statistics, performs rate estimation calculation and saves the
results for later evaluation using the rateest match.
--rateest-name name
Count matched packets into the pool referred to by name, which is freely choosable.
--rateest-interval amount{s|ms|us}
Rate measurement interval, in seconds, milliseconds or microseconds.
--rateest-ewmalog value
Rate measurement averaging time constant.
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 redirects the packet to the
machine itself by changing the destination IP to the primary address of the incoming
interface (locally-generated packets are mapped to the 127.0.0.1 address).
--to-ports port[-port]
This specifies a destination port or range of ports to use: without this, the des-
tination port is never altered. This is only valid if the rule also specifies -p
tcp or -p udp.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.22).
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
SAME
Similar to SNAT/DNAT depending on chain: it takes a range of addresses (`--to
1.2.3.4-1.2.3.7') and gives a client the same source-/destination-address for each connec-
tion.
--to ipaddr[-ipaddr]
Addresses to map source to. May be specified more than once for multiple ranges.
--nodst
Don't use the destination-ip in the calculations when selecting the new source-ip
--random
Port mapping will be forcibly randomized to avoid attacks based on port prediction
(kernel >= 2.6.21).
SECMARK
This is used to set the security mark value associated with the packet for use by security
subsystems such as SELinux. It is only valid in the mangle table.
--selctx security_context
SET
This modules adds and/or deletes entries from IP sets which can be defined by ipset(8).
--add-set setname flag[,flag...]
add the address(es)/port(s) of the packet to the sets
--del-set setname flag[,flag...]
delete the address(es)/port(s) of the packet from the sets, where flags are src
and/or dst and there can be no more than six of them.
The bindings to follow must previously be defined in order to use multilevel adding/delet-
ing by the SET target.
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 con-
nection 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 spec-
ifies -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
In Kernels up to 2.6.10, you can add several --to-source options. For those ker-
nels, 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 addresses. Later Kernels (>= 2.6.11-rc1) don't have the
ability to NAT to multiple ranges anymore.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.21).
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 for
IPv4 or 60 for IPv6, respectively). Of course, it can only be used in conjunction with -p
tcp. It is only valid in the mangle table.
This target is used to overcome criminally braindead ISPs or servers which block "ICMP
Fragmentation Needed" or "ICMPv6 Packet Too Big" 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 -t mangle -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 for IPv4; -60 for IPv6).
These options are mutually exclusive.
TCPOPTSTRIP
This target will strip TCP options off a TCP packet. (It will actually replace them by NO-
OPs.) As such, you will need to add the -p tcp parameters.
--strip-options option[,option...]
Strip the given option(s). The options may be specified by TCP option number or by
symbolic name. The list of recognized options can be obtained by calling iptables
with -j TCPOPTSTRIP -h.
TOS
This module sets the Type of Service field in the IPv4 header (including the 'precedence'
bits) or the Priority field in the IPv6 header. Note that TOS shares the same bits as DSCP
and ECN. The TOS target is only valid in the mangle table.
--set-tos value[/mask]
Zeroes out the bits given by mask and XORs value into the TOS/Priority field. If
mask is omitted, 0xFF is assumed.
--set-tos symbol
You can specify a symbolic name when using the TOS target for IPv4. It implies a
mask of 0xFF. The list of recognized TOS names can be obtained by calling iptables
with -j TOS -h.
The following mnemonics are available:
--and-tos bits
Binary AND the TOS value with bits. (Mnemonic for --set-tos 0/invbits, where
invbits is the binary negation of bits.)
--or-tos bits
Binary OR the TOS value with bits. (Mnemonic for --set-tos bits/bits.)
--xor-tos bits
Binary XOR the TOS value with bits. (Mnemonic for --set-tos bits/0.)
TRACE
This target marks packes so that the kernel will log every rule which match the packets as
those traverse the tables, chains, rules. (The ipt_LOG or ip6t_LOG module is required for
the logging.) The packets are logged with the string prefix: "TRACE: tablename:chain-
name:type:rulenum " where type can be "rule" for plain rule, "return" for implicit rule at
the end of a user defined chain and "policy" for the policy of the built in chains.
It can only be used in the raw table.
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 pack-
ets. 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 accumu-
lates 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 parame-
ters cause an exit code of 2, and other errors cause an exit code of 1.
BUGS
Bugs? What's this? ;-) Well, you might want to have a look at http://bugzilla.netfil-
ter.org/
COMPATIBILITY WITH IPCHAINS
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 out-
put interface, and both are available for packets entering the FORWARD chain.
The various forms of NAT have been separated out; 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 filter-
ing seen previously. So the following options are handled differently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
SEE ALSO
iptables-save(8), iptables-restore(8), ip6tables(8), ip6tables-save(8), ip6tables-
restore(8), libipq(3).
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/.
AUTHORS
Rusty Russell originally wrote iptables, in early consultation with Michael Neuling.
Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection frame-
work 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 and NFQUEUE target, the new libiptc, as well as the TTL, DSCP,
ECN matches and targets.
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Yasuyuki Kozakai, Jozsef
Kadlecsik, Patrick McHardy, James Morris, Pablo Neira Ayuso, Harald Welte and Rusty Rus-
sell.
Man page originally written by Herve Eychenne <rv AT wallfire.org>.
Jul 03, 2008 IPTABLES(8)
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