iptables
SYNOPSIS
iptables [-t table] {-A|-C|-D} chain rule-specification
iptables [-t table] -I chain [rulenum] rule-specification
iptables [-t table] -R chain rulenum rule-specification
iptables [-t table] -D chain rulenum
iptables [-t table] -S [chain [rulenum]]
iptables [-t table] {-F|-L|-Z} [chain [rulenum]] [options...]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target
iptables [-t table] -E old-chain-name new-chain-name
rule-specification = [matches...] [target]
match = -m matchname [per-match-options]
target = -j targetname [per-target-options]
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 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 ta-
ble.
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 tar-
get, which can be the name of a user-defined chain or one of the spe-
cial 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
mand should operate on. If the kernel is configured with auto-
matic module loading, an attempt will be made to load the appro-
priate 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 encountered. It consists of three built-ins:
PREROUTING (for altering packets as soon as they come in),
OUTPUT (for altering locally-generated packets before rout-
ing), 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 sup-
ported: INPUT (for packets coming into the box itself), FOR-
WARD (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 processes)
security:
This table is used for Mandatory Access Control (MAC) net-
working rules, such as those enabled by the SECMARK and
CONNSECMARK targets. Mandatory Access Control is imple-
mented by Linux Security Modules such as SELinux. The secu-
rity table is called after the filter table, allowing any
Discretionary Access Control (DAC) rules in the filter table
to take effect before MAC rules. This table provides the
following built-in chains: INPUT (for packets coming into
the box itself), OUTPUT (for altering locally-generated
packets before routing), and FORWARD (for altering packets
being routed through the box).
OPTIONS
the source and/or destination names resolve to more than one
address, a rule will be added for each possible address combina-
tion.
-C, --check chain rule-specification
Check whether a rule matching the specification does exist in
the selected chain. This command uses the same logic as -D to
find a matching entry, but does not alter the existing iptables
configuration and uses its exit code to indicate success or
failure.
-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 des-
tination 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 (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
-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 ipt-
ables 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.
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 sec-
tion 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 speci-
fied protocol can be one of tcp, udp, udplite, icmp, esp, ah,
sctp or the special keyword "all", or it can be a numeric value,
representing one of these protocols or a different one. A pro-
tocol name from /etc/protocols is also allowed. A "!" argument
before the protocol inverts the test. The number zero is equiv-
alent to all. "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, a network IP address (with /mask), or a plain IP
address. Hostnames will be resolved once only, before the rule
is submitted to the kernel. Please note that specifying any
name to be resolved with a remote query such as DNS is a really
bad idea. The mask can be either a network mask or a plain num-
ber, specifying the number of 1's at the left side of the net-
work 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. Multiple addresses can be specified, but this will
expand to multiple rules (when adding with -A), or will cause
multiple rules to be deleted (with -D).
[!] -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
tinue 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 omitted, any interface name will match.
[!] -f, --fragment
This means that the rule only refers to second and further frag-
ments 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 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 multipli-
ers respectively (but see the -x flag to change this). For
appending, insertion, deletion and replacement, this causes
detailed information on the rule or rules to be printed. -v may
be specified multiple times to possibly emit more detailed debug
statements.
-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 appli-
cable).
-x, --exact
Expand numbers. Display the exact value of the packet and byte
counters, instead of only the rounded number in K's (multiples
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 mod-
ules 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.
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
[!] --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
This module matches the SPIs in Authentication header of IPsec packets.
[!] --ahspi spi[:spi]
cluster
Allows you to deploy gateway and back-end load-sharing clusters without
the need of load-balancers.
This match requires that all the nodes see the same packets. Thus, the
cluster match decides if this node has to handle a packet given the
following options:
--cluster-total-nodes num
Set number of total nodes in cluster.
[!] --cluster-local-node num
Set the local node number ID.
[!] --cluster-local-nodemask mask
Set the local node number ID mask. You can use this option
instead of --cluster-local-node.
--cluster-hash-seed value
Set seed value of the Jenkins hash.
Example:
iptables -A PREROUTING -t mangle -i eth1 -m cluster --clus-
ter-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed
0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth2 -m cluster --clus-
ter-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed
0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark 0xffff
-j DROP
iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark 0xffff
-j DROP
And the following commands to make all nodes see the same packets:
ip maddr add 01:00:5e:00:01:01 dev eth1
ip maddr add 01:00:5e:00:01:02 dev eth2
arptables -A OUTPUT -o eth1 --h-length 6 -j mangle --mangle-mac-
s 01:00:5e:00:01:01
arptables -A INPUT -i eth1 --h-length 6 --destination-mac
01:00:5e:00:01:01 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27
arptables -A OUTPUT -o eth2 --h-length 6 -j mangle --man-
--comment comment
Example:
iptables -A INPUT -i eth1 -m comment --comment "my local LAN"
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 aver-
age bytes per packet.
The counters are 64-bit 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 `con-
ntrack -L` and accessed via ctnetlink.
NOTE that for connections which have no accounting information, the
match will always return false. The "net.netfilter.nf_conntrack_acct"
sysctl flag controls whether new connections will be byte/packet
counted. Existing connection flows will not be gaining/losing a/the
accounting structure when be sysctl flag is flipped.
[!] --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 trans-
ferred 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 example 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-upto n
Match if the number of existing connections is below or equal n.
--connlimit-above n
Apply the limit onto the destination group.
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-upto 2 -j ACCEPT
# limit the number of parallel HTTP requests to 16 per class C sized
source network (24 bit netmask)
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
# Limit the number of connections to a particular host:
ip6tables -p tcp --syn --dport 49152:65535 -d 2001:db8::1 -m
connlimit --connlimit-above 100 -j REJECT
connmark
This module matches the netfilter mark field associated with a connec-
tion (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
[!] --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 (inclusive)
--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 other-
wise 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.
UNTRACKED
meaning that the packet is not tracked at all, which happens if
you use the NOTRACK target in raw table.
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.
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j REDI-
RECT --to-port 8080
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j REDI-
RECT --to-port 8081
Available since Linux 2.6.36.
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 accord-
ing 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.
--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 subject to hashlimit. prefix
must be between (inclusive) 0 and 32. Note that --hashlimit-src-
mask 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.
Examples:
matching on source host
"1000 packets per second for every host in 192.168.0.0/16" => -s
192.168.0.0/16 --hashlimit-mode srcip --hashlimit-upto 1000/sec
matching on source port
"100 packets per second for every service of 192.168.1.1" => -s
192.168.1.1 --hashlimit-mode srcport --hashlimit-upto 100/sec
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 pro-
vides the following option:
[!] --icmp-type {type[/code]|typename}
This allows specification of the ICMP type, which can be a
numeric ICMP type, type/code pair, 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.
ipvs
Match IPVS connection properties.
[!] --ipvs
packet belongs to an IPVS connection
Any of the following options implies --ipvs (even negated)
[!] --vproto protocol
VIP protocol to match; by number or name, e.g. "tcp"
[!] --vaddr address[/mask]
VIP address to match
[!] --vport port
VIP port to match; by number or name, e.g. "http"
--vdir {ORIGINAL|REPLY}
flow direction of packet
[!] --vmethod {GATE|IPIP|MASQ}
IPVS forwarding method used
[!] --vportctl port
VIP port of the controlling connection to match, e.g. 21 for FTP
length
This module matches the length of the layer-3 payload (e.g. layer-4
packet) of a packet against a specific value or range of values.
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, FOR-
WARD 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 com-
parison).
multiport
This module matches a set of source or destination ports. Up to 15
ports can be specified. 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 convenient 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.
osf
The osf module does passive operating system fingerprinting. This mod-
ules compares some data (Window Size, MSS, options and their order,
TTL, DF, and others) from packets with the SYN bit set.
[!] --genre string
Match an operating system genre by using a passive fingerprint-
ing.
--log level
Log determined genres into dmesg even if they do not match the
desired one. level can be one of the following values:
o 0 - Log all matched or unknown signatures
o 1 - Log only the first one
o 2 - Log all known matched signatures
You may find something like this in syslog:
Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 ->
11.22.33.44:139 hops=3 Linux [2.5-2.6:] : 1.2.3.4:42624 -> 1.2.3.5:22
hops=4
OS fingerprints are loadable using the nfnl_osf program. To load fin-
gerprints from a file, use:
nfnl_osf -f /usr/share/xtables/pf.os
To remove them again,
nfnl_osf -f /usr/share/xtables/pf.os -d
The fingerprint database can be downlaoded from http://www.open-
bsd.org/cgi-bin/cvsweb/src/etc/pf.os .
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. Forwarded 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
[!] --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 inter-
face which begins with this name will match. Note that in the
nat and mangle OUTPUT chains one cannot match on the bridge out-
put port, however one can in the filter OUTPUT chain. If the
packet won't leave by a bridge device or if 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 POSTROUT-
ING 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 decapsula-
tion 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. --pol none
cannot be combined with --strict.
--strict
Selects whether to match the exact policy or match if any rule
of the policy matches the given policy.
For each policy element that is to be described, one can use one or
more of the following options. When --strict is in effect, at least one
must be used per element.
[!] --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.
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. The condition matches until the byte counter reaches zero.
Behavior is reversed with negation (i.e. the condition does not match
until the byte counter reaches zero).
[!] --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, com-
paring two rate estimators and matching on the difference between two
rate estimators.
For a better understanding of the available options, these are all pos-
sible combinations:
o rateest operator rateest-bps
o rateest operator rateest-pps
o (rateest minus rateest-bps1) operator rateest-bps2
o (rateest minus rateest-pps1) operator rateest-pps2
o rateest1 operator rateest2 rateest-bps(without rate!)
o rateest1 operator rateest2 rateest-pps(without rate!)
o (rateest1 minus rateest-bps1) operator (rateest2 minus rateest-
bps2)
o (rateest1 minus rateest-pps1) operator (rateest2 minus rateest-
pps2)
--rateest-delta
For each estimator (either absolute or relative mode), calculate
the difference between the estimator-determined flow rate and the
static value chosen with the BPS/PPS options. If the flow rate is
higher than the specified BPS/PPS, 0 will be used instead of a neg-
ative value. In other words, "max(0, rateest#_rate - rateest#_bps)"
is used.
[!] --rateest-lt
Match if rate is less than given rate/estimator.
[!] --rateest-gt
--rateest1 name
--rateest2 name
The names of the two rate estimators for relative mode.
--rateest-bps [value]
--rateest-pps [value]
--rateest-bps1 [value]
--rateest-bps2 [value]
--rateest-pps1 [value]
--rateest-pps2 [value]
Compare the estimator(s) by bytes or packets per second, and
compare against the chosen value. See the above bullet list for
which option is to be used in which case. A unit suffix may be
used - available ones are: bit, [kmgt]bit, [KMGT]ibit, Bps,
[KMGT]Bps, [KMGT]iBps.
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-interval 250ms --rateest-ewma 0.5s
iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST --rateest-name
ppp0 --rateest-interval 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 involving dynamic routing protocols like BGP.
[!] --realm value[/mask]
Matches a given realm number (and optionally mask). If not a
--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.
--reap reap
This option can only be used in conjunction with --seconds.
When used, this will cause entries older then 'seconds' to be
purged.
--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.
The maximum value for the hitcount parameter is given by the
"ip_pkt_list_tot" parameter of the xt_recent kernel module.
Exceeding this value on the command line will cause the rule to
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
Steve's ipt_recent website (http://snowman.net/projects/ipt_recent/)
also has some examples of usage.
/proc/net/xt_recent/* are the current lists of addresses and informa-
tion about each entry of each list.
Each file in /proc/net/xt_recent/ can be read from to see the current
list or written two using the following commands to modify the list:
echo +addr >/proc/net/xt_recent/DEFAULT
to add addr to the DEFAULT list
echo -addr >/proc/net/xt_recent/DEFAULT
to remove addr from the DEFAULT list
echo / >/proc/net/xt_recent/DEFAULT
to flush the DEFAULT list (remove all entries).
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/xt_recent/* files.
ip_list_uid=0
Numerical UID for ownership of /proc/net/xt_recent/* files.
ip_list_gid=0
Numerical GID for ownership of /proc/net/xt_recent/* files.
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.
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 module matches IP sets which can be defined by ipset(8).
[!] --match-set setname flag[,flag]...
where flags are the comma separated list of src and/or dst spec-
ifications and there can be no more than six of them. Hence the
command
iptables -A FORWARD -m set --match-set test src,dst
will match packets, for which (if the set type is ipportmap) the
source address and destination port pair can be found in the
specified set. If the set type of the specified set is single
dimension (for example ipmap), then the command will match pack-
ets for which the source address can be found in the specified
set.
The option --match-set can be replaced by --set if that does not clash
with an option of other extensions.
Use of -m set requires that ipset kernel support is provided. As stan-
dard kernels do not ship this currently, the ipset or Xtables-addons
package needs to be installed.
socket
This matches if an open socket can be found by doing a socket lookup on
the packet.
--transparent
Ignore non-transparent sockets.
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 associ-
ated with a connection which has seen packets in both direc-
tions, 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.
[!] --probability p
Set the probability for a packet to be randomly matched. It only
works with the random mode. p must be within 0.0 and 1.0. The
supported granularity is in 1/2147483648th increments.
[!] --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 up to which should be scanned. That is, byte off-
set-1 (counting from 0) is the last one that is scanned. 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 pro-
vides 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 first:last. If the first port is
omitted, "0" is assumed; if the last is omitted, "65535" is
assumed. If the first port is greater than the second one 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.
FIN bits cleared. Such packets are used to request TCP connec-
tion 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,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 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 optional, but are ANDed when specified. All times are
interpreted as UTC by default.
--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 possible 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.).
--kerneltz
Use the kernel timezone instead of UTC to determine whether a
able. The TZ userspace environment variable specifies how the UTC-based
system time is displayed, e.g. when you run date(1), or what you see on
your desktop clock. The TZ string may resolve to different offsets at
different dates, which is what enables the automatic time-jumping in
userspace. when DST changes. The kernel's timezone offset variable is
used when it has to convert between non-UTC sources, such as FAT
filesystems, to UTC (since the latter is what the rest of the system
uses).
The caveat with the kernel timezone is that Linux distributions may
ignore to set the kernel timezone, and instead only set the system
time. Even if a particular distribution does set the timezone at boot,
it is usually does not keep the kernel timezone offset - which is what
changes on DST - up to date. ntpd will not touch the kernel timezone,
so running it will not resolve the issue. As such, one may encounter a
timezone that is always +0000, or one that is wrong half of the time of
the year. As such, using --kerneltz is highly discouraged.
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 logically ANDed with the TOS mark before the
comparison.
--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 values. 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 fur-
ther 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)
Whitespace is allowed but not required in the tests. However, the char-
acters 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 anything
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). Alterna-
tively, 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
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 pro-
vides 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 descrip-
tion 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.
TARGET EXTENSIONS
iptables can use extended target modules: the following are included in
the standard distribution.
AUDIT
This target allows to create audit records for packets hitting the tar-
get. It can be used to record accepted, dropped, and rejected packets.
See auditd(8) for additional details.
--type {accept|drop|reject}
Set type of audit record.
Example:
iptables -N AUDIT_DROP
iptables -A AUDIT_DROP -j AUDIT --type drop
iptables -A AUDIT_DROP -j DROP
CHECKSUM
This target allows to selectively work around broken/old applications.
It can only be used in the mangle table.
Set the major and minor class value. The values are always
interpreted as hexadecimal even if no 0x prefix is given.
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, sour-
ceip-sourceport, sourceip-sourceport-destport.
--clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer multi-
cast 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.
The mark is 32 bits wide.
--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);
--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 unla-
beled). Typically used in conjunction with SECMARK, it is valid in the
security table (for backwards compatibility with older kernels, it is
also 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 connec-
tion does, copy the security marking from the connection to the
packet.
CT
The CT target allows to set parameters for a packet or its associated
connection. The target attaches a "template" connection tracking entry
to the packet, which is then used by the conntrack core when initializ-
ing a new ct entry. This target is thus only valid in the "raw" table.
--notrack
Disables connection tracking for this packet.
--helper name
Use the helper identified by name for the connection. This is
more flexible than loading the conntrack helper modules with
preset ports.
--ctevents event[,...]
Only generate the specified conntrack events for this connec-
tion. Possible event types are: new, related, destroy, reply,
assured, protoinfo, helper, mark (this refers to the ctmark, not
PUT chains, and user-defined chains which are only called from those
chains. It specifies that the destination address of the packet should
be modified (and all future packets in this connection will also be
mangled), and rules should cease being examined. It takes one type of
option:
--to-destination [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new destination IP address, an inclu-
sive 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. If no IP address is specified then only the desti-
nation 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 desti-
nation 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).
--persistent
Gives a client the same source-/destination-address for each
connection. This supersedes the SAME target. Support for per-
sistent mappings is available from 2.6.29-rc2.
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.
IDLETIMER
This target can be used to identify when interfaces have been idle for
a certain period of time. Timers are identified by labels and are cre-
ated when a rule is set with a new label. The rules also take a time-
--timeout amount
This is the time in seconds that will trigger the notification.
--label string
This is a unique identifier for the timer. The maximum length
for the label string is 27 characters.
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 match-
ing 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 tar-
get", 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. It can, for example, be used in conjunction with routing based
on fwmark (needs iproute2). If you plan on doing so, note that the mark
needs to be set in the PREROUTING chain of the mangle table to affect
routing. The mark field is 32 bits wide.
--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
This target is only valid in the nat table, in the POSTROUTING chain.
It should only be used with dynamically assigned IP (dialup) connec-
tions: 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 any-
way).
--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.
--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, 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, connec-
tion 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 con-
structed 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.
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 contin-
ues at the next rule.
--nflog-group nlgroup
The netlink group (0 - 2^16-1) to which packets are (only appli-
cable for nfnetlink_log). The default value is 0.
--nflog-prefix prefix
A prefix string to include in the log message, up to 64 charac-
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. It can only be used with Kernel versions
2.6.14 or later, since it requires the nfnetlink_queue kernel support.
The queue-balance option was added in Linux 2.6.31, queue-bypass in
2.6.39.
--queue-num value
This specifies the QUEUE number to use. Valid queue numbers are
0 to 65535. The default value is 0.
--queue-balance value:value
This specifies a range of queues to use. Packets are then bal-
anced across the given queues. This is useful for multicore
systems: start multiple instances of the userspace program on
queues x, x+1, .. x+n and use "--queue-balance x:x+n". Packets
belonging to the same connection are put into the same nfqueue.
--queue-bypass
By default, if no userspace program is listening on an NFQUEUE,
then all packets that are to be queued are dropped. When this
option is used, the NFQUEUE rule is silently bypassed instead.
The packet will move on to the next rule.
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 calcu-
lation 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 microsec-
onds.
--rateest-ewmalog value
Rate measurement averaging time constant.
REDIRECT
This target is only valid in the nat table, in the PREROUTING and OUT-
PUT chains, and user-defined chains which are only called from those
chains. It 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).
packet: otherwise it is equivalent to DROP so it is a terminating TAR-
GET, ending rule traversal. This target is only valid in the INPUT,
FORWARD and OUTPUT chains, and user-defined chains which are only
called from those chains. The following option controls the nature of
the error packet returned:
--reject-with type
The type given can be icmp-net-unreachable, icmp-host-unreach-
able, icmp-port-unreachable, icmp-proto-unreachable,
icmp-net-prohibited, icmp-host-prohibited or icmp-admin-prohib-
ited (*) 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-/destina-
tion-address for each connection.
N.B.: The DNAT target's --persistent option replaced the SAME target.
--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 valid in the
security table (for backwards compatibility with older kernels, it is
also valid in the mangle table). The mark is 32 bits wide.
--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
when adding entry if it already exists, reset the timeout value
to the specified one or to the default from the set definition
Use of -j SET requires that ipset kernel support is provided. As stan-
dard kernels do not ship this currently, the ipset or Xtables-addons
package needs to be installed.
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 alter-
ation will occur.
In Kernels up to 2.6.10, you can add several --to-source
options. For those kernels, 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).
--persistent
Gives a client the same source-/destination-address for each
connection. This supersedes the SAME target. Support for per-
sistent mappings is available from 2.6.29-rc2.
TCPMSS
This target allows to alter the MSS value of TCP SYN packets, to con-
trol the maximum size for that connection (usually limiting it to your
outgoing interface's MTU minus 40 for IPv4 or 60 for IPv6, respec-
tively). 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" 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.
(from Linux 2.6.25 onwards) to avoid more problems with hosts
relying on a proper MSS.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40 for IPv4; -60
for IPv6). This may not function as desired where asymmetric
routes with differing path MTU exist -- the kernel uses the path
MTU which it would use to send packets from itself to the source
and destination IP addresses. Prior to Linux 2.6.25, only the
path MTU to the destination IP address was considered by this
option; subsequent kernels also consider the path MTU to the
source IP address.
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.
TEE
The TEE target will clone a packet and redirect this clone to another
machine on the local network segment. In other words, the nexthop must
be the target, or you will have to configure the nexthop to forward it
further if so desired.
--gateway ipaddr
Send the cloned packet to the host reachable at the given IP
address. Use of 0.0.0.0 (for IPv4 packets) or :: (IPv6) is
invalid.
To forward all incoming traffic on eth0 to an Network Layer logging
box:
-t mangle -A PREROUTING -i eth0 -j TEE --gateway 2001:db8::1
TOS
This module sets the Type of Service field in the IPv4 header (includ-
ing 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 (see NOTE below) and XORs
value into the TOS/Priority field. If mask is omitted, 0xFF is
assumed.
--set-tos symbol
--or-tos bits
Binary OR the TOS value with bits. (Mnemonic for --set-tos
bits/bits. See NOTE below.)
--xor-tos bits
Binary XOR the TOS value with bits. (Mnemonic for --set-tos
bits/0. See NOTE below.)
NOTE: In Linux kernels up to and including 2.6.38, with the exception
of longterm releases 2.6.32.42 (or later) and 2.6.33.15 (or later),
there is a bug whereby IPv6 TOS mangling does not behave as documented
and differs from the IPv4 version. The TOS mask indicates the bits one
wants to zero out, so it needs to be inverted before applying it to the
original TOS field. However, the aformentioned kernels forgo the inver-
sion which breaks --set-tos and its mnemonics.
TPROXY
This target is only valid in the mangle table, in the PREROUTING chain
and user-defined chains which are only called from this chain. It redi-
rects the packet to a local socket without changing the packet header
in any way. It can also change the mark value which can then be used in
advanced routing rules. It takes three options:
--on-port port
This specifies a destination port to use. It is a required
option, 0 means the new destination port is the same as the
original. This is only valid if the rule also specifies -p tcp
or -p udp.
--on-ip address
This specifies a destination address to use. By default the
address is the IP address of the incoming interface. This is
only valid if the rule also specifies -p tcp or -p udp.
--tproxy-mark value[/mask]
Marks packets with the given value/mask. The fwmark value set
here can be used by advanced routing. (Required for transparent
proxying to work: otherwise these packets will get forwarded,
which is probably not what you want.)
TRACE
This target marks packes so that the kernel will log every rule which
match the packets as those traverse the tables, chains, rules.
A logging backend, such as ip(6)t_LOG or nfnetlink_log, must be loaded
for this to be visible. The packets are logged with the string prefix:
"TRACE: tablename:chainname: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 deter-
mines how many hops (routers) a packet can traverse until it's time to
--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 sub-
scribe 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 charac-
ters 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).
DIAGNOSTICS
Various error messages are printed to standard error. The exit code is
0 for correct functioning. Errors which appear to be caused by invalid
or abused command line parameters cause an exit code of 2, and other
errors cause an exit code of 1.
BUGS
Bugs? What's this? ;-) Well, you might want to have a look at
http://bugzilla.netfilter.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 output interface, and both are available for packets
entering the FORWARD chain. 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 netfil-
ter-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 framework in iptables, then wrote the mangle table,
the owner match, the mark stuff, and ran around doing cool stuff every-
where.
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 Russell.
Man page originally written by Herve Eychenne <rv@wallfire.org>.
VERSION
This manual page applies to iptables @PACKAGE_VERSION@.
iptables 1.4.12 IPTABLES(8)
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