iptables can use extended target modules: the following are
included in the standard distribution.
AUDIT
This target creates audit records for packets hitting the target.
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. Starting with linux-4.12, this
option has no effect on generated audit messages anymore.
It is still accepted by iptables for compatibility
reasons, but ignored.
Example:
iptables -N AUDIT_DROP
iptables -A AUDIT_DROP -j AUDIT
iptables -A AUDIT_DROP -j DROP
CHECKSUM
This target selectively works around broken/old applications. It
can only be used in the mangle table.
--checksum-fill
Compute and fill in the checksum in a packet that lacks a
checksum. This is particularly useful, if you need to
work around old applications such as dhcp clients, that do
not work well with checksum offloads, but don't want to
disable checksum offload in your device.
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. The values are always
interpreted as hexadecimal even if no 0x prefix is given.
CLUSTERIP (IPv4-specific)
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.
Please note that CLUSTERIP target is considered deprecated in
favour of cluster match which is more flexible and not limited to
IPv4.
--new Create a new ClusterIP. You always have to set this on
the first rule for a given ClusterIP.
--hashmode mode
Specify the hashing mode. Has to be one of sourceip,
sourceip-sourceport, sourceip-sourceport-destport.
--clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer
multicast address
--total-nodes num
Number of total nodes within this cluster.
--local-node num
Local node number within this cluster.
--hash-init rnd
Specify the random seed used for hash initialization.
CONNMARK
This module sets the netfilter mark value associated with a
connection. 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);
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
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
connection does, copy the security marking from the
connection to the packet.
CT
The CT target sets 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 initializing 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
connection. Possible event types are: new, related,
destroy, reply, assured, protoinfo, helper, mark (this
refers to the ctmark, not nfmark), natseqinfo, secmark
(ctsecmark).
--expevents event[,...]
Only generate the specified expectation events for this
connection. Possible event types are: new.
--zone-orig {id|mark}
For traffic coming from ORIGINAL direction, assign this
packet to zone id and only have lookups done in that zone.
If mark is used instead of id, the zone is derived from
the packet nfmark.
--zone-reply {id|mark}
For traffic coming from REPLY direction, assign this
packet to zone id and only have lookups done in that zone.
If mark is used instead of id, the zone is derived from
the packet nfmark.
--zone {id|mark}
Assign this packet to zone id and only have lookups done
in that zone. If mark is used instead of id, the zone is
derived from the packet nfmark. By default, packets have
zone 0. This option applies to both directions.
--timeout name
Use the timeout policy identified by name for the
connection. This is provides more flexible timeout policy
definition than global timeout values available at
/proc/sys/net/netfilter/nf_conntrack_*_timeout_*.
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 the following options:
--to-destination [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new destination IP address, an
inclusive range of IP addresses. Optionally a port range,
if the rule also specifies one of the following protocols:
tcp, udp, dccp or sctp. 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).
--persistent
Gives a client the same source-/destination-address for
each connection. This supersedes the SAME target. Support
for persistent mappings is available from 2.6.29-rc2.
IPv6 support available since Linux kernels >= 3.7.
DNPT (IPv6-specific)
Provides stateless destination IPv6-to-IPv6 Network Prefix
Translation (as described by RFC 6296).
You have to use this target in the mangle table, not in the nat
table. It takes the following options:
--src-pfx [prefix/length]
Set source prefix that you want to translate and length
--dst-pfx [prefix/length]
Set destination prefix that you want to use in the
translation and length
You have to use the SNPT target to undo the translation. Example:
ip6tables -t mangle -I POSTROUTING -s fd00::/64 -o
vboxnet0 -j SNPT --src-pfx fd00::/64 --dst-pfx
2001:e20:2000:40f::/64
ip6tables -t mangle -I PREROUTING -i wlan0 -d
2001:e20:2000:40f::/64 -j DNPT --src-pfx
2001:e20:2000:40f::/64 --dst-pfx fd00::/64
You may need to enable IPv6 neighbor proxy:
sysctl -w net.ipv6.conf.all.proxy_ndp=1
You also have to use the NOTRACK target to disable connection
tracking for translated flows.
DSCP
This target alters 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 (IPv4-specific)
This target selectively works 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.
HL (IPv6-specific)
This is used to modify the Hop Limit field in IPv6 header. The
Hop Limit field is similar to what is known as TTL value in IPv4.
Setting or incrementing the Hop Limit field can potentially be
very dangerous, so it should be avoided at any cost. This target
is only valid in mangle table.
Don't ever set or increment the value on packets that leave your
local network!
--hl-set value
Set the Hop Limit to `value'.
--hl-dec value
Decrement the Hop Limit `value' times.
--hl-inc value
Increment the Hop Limit `value' times.
HMARK
Like MARK, i.e. set the fwmark, but the mark is calculated from
hashing packet selector at choice. You have also to specify the
mark range and, optionally, the offset to start from. ICMP error
messages are inspected and used to calculate the hashing.
Existing options are:
--hmark-tuple tuple
Possible tuple members are: src meaning source address
(IPv4, IPv6 address), dst meaning destination address
(IPv4, IPv6 address), sport meaning source port (TCP, UDP,
UDPlite, SCTP, DCCP), dport meaning destination port (TCP,
UDP, UDPlite, SCTP, DCCP), spi meaning Security Parameter
Index (AH, ESP), and ct meaning the usage of the conntrack
tuple instead of the packet selectors.
--hmark-mod value (must be > 0)
Modulus for hash calculation (to limit the range of
possible marks)
--hmark-offset value
Offset to start marks from.
For advanced usage, instead of using --hmark-tuple, you can
specify custom
prefixes and masks:
--hmark-src-prefix cidr
The source address mask in CIDR notation.
--hmark-dst-prefix cidr
The destination address mask in CIDR notation.
--hmark-sport-mask value
A 16 bit source port mask in hexadecimal.
--hmark-dport-mask value
A 16 bit destination port mask in hexadecimal.
--hmark-spi-mask value
A 32 bit field with spi mask.
--hmark-proto-mask value
An 8 bit field with layer 4 protocol number.
--hmark-rnd value
A 32 bit random custom value to feed hash calculation.
Examples:
iptables -t mangle -A PREROUTING -m conntrack --ctstate NEW
-j HMARK --hmark-tuple ct,src,dst,proto --hmark-offset 10000
--hmark-mod 10 --hmark-rnd 0xfeedcafe
iptables -t mangle -A PREROUTING -j HMARK --hmark-offset 10000
--hmark-tuple src,dst,proto --hmark-mod 10 --hmark-rnd 0xdeafbeef
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 created when a rule is set with a new label. The
rules also take a timeout value (in seconds) as an option. If
more than one rule uses the same timer label, the timer will be
restarted whenever any of the rules get a hit. One entry for
each timer is created in sysfs. This attribute contains the
timer remaining for the timer to expire. The attributes are
located under the xt_idletimer class:
/sys/class/xt_idletimer/timers/<label>
When the timer expires, the target module sends a sysfs
notification to the userspace, which can then decide what to do
(eg. disconnect to save power).
--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.
LED
This creates an LED-trigger that can then be attached to system
indicator lights, to blink or illuminate them when certain
packets pass through the system. One example might be to light up
an LED for a few minutes every time an SSH connection is made to
the local machine. The following options control the trigger
behavior:
--led-trigger-id name
This is the name given to the LED trigger. The actual name
of the trigger will be prefixed with "netfilter-".
--led-delay ms
This indicates how long (in milliseconds) the LED should
be left illuminated when a packet arrives before being
switched off again. The default is 0 (blink as fast as
possible.) The special value inf can be given to leave the
LED on permanently once activated. (In this case the
trigger will need to be manually detached and reattached
to the LED device to switch it off again.)
--led-always-blink
Always make the LED blink on packet arrival, even if the
LED is already on. This allows notification of new
packets even with long delay values (which otherwise would
result in a silent prolonging of the delay time.)
Example:
Create an LED trigger for incoming SSH traffic:
iptables -A INPUT -p tcp --dport 22 -j LED
--led-trigger-id ssh
Then attach the new trigger to an LED:
echo netfilter-ssh >/sys/class/leds/ledname/trigger
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/IPv6 header fields) via the
kernel log (where it can be read with dmesg(1) or read in the
syslog).
This is a "non-terminating target", i.e. rule traversal continues
at the next rule. So if you want to LOG the packets you refuse,
use two separate rules with the same matching criteria, first
using target LOG then DROP (or REJECT).
--log-level level
Level of logging, which can be (system-specific) numeric
or a mnemonic. Possible values are (in decreasing order
of priority): emerg, alert, crit, error, warning, notice,
info or debug.
--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/IPv6 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 either the
PREROUTING or the OUTPUT 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
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 connections are forgotten when
the interface goes down. This is the correct behavior when the
next dialup is unlikely to have the same interface address (and
hence any established connections are lost anyway).
--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 one
of the following protocols: tcp, udp, dccp or sctp.
--random
Randomize source port mapping If option --random is used
then port mapping will be randomized (kernel >= 2.6.21).
Since kernel 5.0, --random is identical to --random-fully.
--random-fully
Full randomize source port mapping If option --random-
fully is used then port mapping will be fully randomized
(kernel >= 3.13).
IPv6 support available since Linux kernels >= 3.7.
NETMAP
This target allows you to statically map a whole network of
addresses onto another network of addresses. It can only be used
from rules in the nat table.
--to address[/mask]
Network address to map to. The resulting address will be
constructed in the following way: All 'one' bits in the
mask are filled in from the new `address'. All bits that
are zero in the mask are filled in from the original
address.
IPv6 support available since Linux kernels >= 3.7.
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 (0 - 2^16-1) 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
This option has never worked, use --nflog-size instead
--nflog-size 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 passes the packet to userspace using the
nfnetlink_queue handler. The packet is put into the queue
identified by its 16-bit queue number. Userspace can inspect and
modify the packet if desired. Userspace must then drop or
reinject the packet into the kernel. Please see
libnetfilter_queue for details. nfnetlink_queue was added in
Linux 2.6.14. 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
balanced 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
behaves like ACCEPT instead, and the packet will move on
to the next table.
--queue-cpu-fanout
Available starting Linux kernel 3.10. When used together
with --queue-balance this will use the CPU ID as an index
to map packets to the queues. The idea is that you can
improve performance if there's a queue per CPU. This
requires --queue-balance to be specified.
NOTRACK
This extension disables connection tracking for all packets
matching that rule. It is equivalent with -j CT --notrack. Like
CT, NOTRACK 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
localhost address, 127.0.0.1 for IPv4 and ::1 for IPv6, and
packets arriving on interfaces that don't have an IP address
configured are dropped).
--to-ports port[-port]
This specifies a destination port or range of ports to
use: without this, the destination port is never altered.
This is only valid if the rule also specifies one of the
following protocols: tcp, udp, dccp or sctp.
--random
If option --random is used then port mapping will be
randomized (kernel >= 2.6.22).
IPv6 support available starting Linux kernels >= 3.7.
REJECT (IPv6-specific)
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 icmp6-no-route, no-route,
icmp6-adm-prohibited, adm-prohibited,
icmp6-addr-unreachable, addr-unreach, or
icmp6-port-unreachable, which return the appropriate
ICMPv6 error message (icmp6-port-unreachable is the
default). Finally, the option tcp-reset can be used on
rules which only match the TCP protocol: this causes a TCP
RST packet to be sent back. This is mainly useful for
blocking ident (113/tcp) probes which frequently occur
when sending mail to broken mail hosts (which won't accept
your mail otherwise). tcp-reset can only be used with
kernel versions 2.6.14 or later.
Warning: You should not indiscriminately apply the REJECT target
to packets whose connection state is classified as INVALID;
instead, you should only DROP these.
Consider a source host transmitting a packet P, with P
experiencing so much delay along its path that the source host
issues a retransmission, P_2, with P_2 being successful in
reaching its destination and advancing the connection state
normally. It is conceivable that the late-arriving P may be
considered not to be associated with any connection tracking
entry. Generating a reject response for a packet so classed would
then terminate the healthy connection.
So, instead of:
-A INPUT ... -j REJECT
do consider using:
-A INPUT ... -m conntrack --ctstate INVALID -j DROP -A INPUT ...
-j REJECT
REJECT (IPv4-specific)
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
(icmp-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
Warning: You should not indiscriminately apply the REJECT target
to packets whose connection state is classified as INVALID;
instead, you should only DROP these.
Consider a source host transmitting a packet P, with P
experiencing so much delay along its path that the source host
issues a retransmission, P_2, with P_2 being successful in
reaching its destination and advancing the connection state
normally. It is conceivable that the late-arriving P may be
considered not to be associated with any connection tracking
entry. Generating a reject response for a packet so classed would
then terminate the healthy connection.
So, instead of:
-A INPUT ... -j REJECT
do consider using:
-A INPUT ... -m conntrack --ctstate INVALID -j DROP -A INPUT ...
-j REJECT
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 module 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 set
--del-set setname flag[,flag...]
delete the address(es)/port(s) of the packet from the set
--map-set setname flag[,flag...]
[--map-mark] [--map-prio] [--map-queue] map packet
properties (firewall mark, tc priority, hardware queue)
where flag(s) are src and/or dst specifications and there
can be no more than six of them.
--timeout value
when adding an entry, the timeout value to use instead of
the default one from the set definition
--exist
when adding an entry if it already exists, reset the
timeout value to the specified one or to the default from
the set definition
--map-set set-name
the set-name should be created with --skbinfo option
--map-mark map firewall mark to packet by lookup of value
in the set --map-prio map traffic control priority to
packet by lookup of value in the set --map-queue map
hardware NIC queue to packet by lookup of value in the set
The --map-set option can be used from the mangle table
only. The --map-prio and --map-queue flags can be used in
the OUTPUT, FORWARD and POSTROUTING chains.
Use of -j SET requires that ipset kernel support is provided,
which, for standard kernels, is the case since Linux 2.6.39.
SNAT
This target is only valid in the nat table, in the POSTROUTING
and INPUT chains, and user-defined chains which are only called
from those chains. 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 the following options:
--to-source [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new source IP address, an
inclusive range of IP addresses. Optionally a port range,
if the rule also specifies one of the following protocols:
tcp, udp, dccp or sctp. If no port range is specified,
then source ports below 512 will be mapped to other ports
below 512: those between 512 and 1023 inclusive will be
mapped to ports below 1024, and other ports will be mapped
to 1024 or above. Where possible, no port alteration will
occur. 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 through a hash-based algorithm (kernel >=
2.6.21).
--random-fully
If option --random-fully is used then port mapping will be
fully randomized through a PRNG (kernel >= 3.14).
--persistent
Gives a client the same source-/destination-address for
each connection. This supersedes the SAME target. Support
for persistent mappings is available from 2.6.29-rc2.
Kernels prior to 2.6.36-rc1 don't have the ability to SNAT in the
INPUT chain.
IPv6 support available since Linux kernels >= 3.7.
SNPT (IPv6-specific)
Provides stateless source IPv6-to-IPv6 Network Prefix Translation
(as described by RFC 6296).
You have to use this target in the mangle table, not in the nat
table. It takes the following options:
--src-pfx [prefix/length]
Set source prefix that you want to translate and length
--dst-pfx [prefix/length]
Set destination prefix that you want to use in the
translation and length
You have to use the DNPT target to undo the translation. Example:
ip6tables -t mangle -I POSTROUTING -s fd00::/64 -o
vboxnet0 -j SNPT --src-pfx fd00::/64 --dst-pfx
2001:e20:2000:40f::/64
ip6tables -t mangle -I PREROUTING -i wlan0 -d
2001:e20:2000:40f::/64 -j DNPT --src-pfx
2001:e20:2000:40f::/64 --dst-pfx fd00::/64
You may need to enable IPv6 neighbor proxy:
sysctl -w net.ipv6.conf.all.proxy_ndp=1
You also have to use the NOTRACK target to disable connection
tracking for translated flows.
SYNPROXY
This target will process TCP three-way-handshake parallel in
netfilter context to protect either local or backend system. This
target requires connection tracking because sequence numbers need
to be translated. The kernels ability to absorb SYNFLOOD was
greatly improved starting with Linux 4.4, so this target should
not be needed anymore to protect Linux servers.
--mss maximum segment size
Maximum segment size announced to clients. This must match
the backend.
--wscale window scale
Window scale announced to clients. This must match the
backend.
--sack-perm
Pass client selective acknowledgement option to backend
(will be disabled if not present).
--timestamps
Pass client timestamp option to backend (will be disabled
if not present, also needed for selective acknowledgement
and window scaling).
Example:
Determine tcp options used by backend, from an external system
tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-
ack)'
port 80 &
telnet 192.0.2.42 80
18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
Flags [S.], seq 360414582, ack 788841994, win 14480,
options [mss 1460,sackOK,
TS val 1409056151 ecr 9690221,
nop,wscale 9],
length 0
Switch tcp_loose mode off, so conntrack will mark out-of-flow
packets as state INVALID.
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
Make SYN packets untracked
iptables -t raw -A PREROUTING -i eth0 -p tcp --dport 80
--syn -j CT --notrack
Catch UNTRACKED (SYN packets) and INVALID (3WHS ACK packets)
states and send them to SYNPROXY. This rule will respond to SYN
packets with SYN+ACK syncookies, create ESTABLISHED for valid
client response (3WHS ACK packets) and drop incorrect cookies.
Flags combinations not expected during 3WHS will not match and
continue (e.g. SYN+FIN, SYN+ACK).
iptables -A INPUT -i eth0 -p tcp --dport 80
-m state --state UNTRACKED,INVALID -j SYNPROXY
--sack-perm --timestamp --mss 1460 --wscale 9
Drop invalid packets, this will be out-of-flow packets that were
not matched by SYNPROXY.
iptables -A INPUT -i eth0 -p tcp --dport 80 -m state
--state INVALID -j DROP
TCPMSS
This target alters 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.
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 sets MSS option to specified value. If the MSS
of the packet is already lower than value, it will not be
increased (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
(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 (see NOTE below) 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 (see NOTE below). 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. See NOTE below.)
--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), 2.6.33 (>=.15),
and 2.6.35 (>=.14), 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 inversion 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 redirects 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 packets so that the kernel will log every rule
which match the packets as those traverse the tables, chains,
rules. It can only be used in the raw table.
With iptables-legacy, 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.
With iptables-nft, the target is translated into nftables' meta
nftrace expression. Hence the kernel sends trace events via
netlink to userspace where they may be displayed using xtables-
monitor --trace command. For details, refer to
xtables-monitor(8).
TTL (IPv4-specific)
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. This target is
only valid in mangle table.
Don't ever set or increment the value on packets that leave your
local network!
--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 (IPv4-specific)
This is the deprecated ipv4-only predecessor of the NFLOG target.
It provides userspace logging of matching packets. When this
target is set for a rule, the Linux kernel will multicast this
packet through a netlink socket. One or more userspace processes
may then subscribe to various multicast groups and receive the
packets. Like LOG, this is a "non-terminating target", i.e. rule
traversal continues at the next rule.
--ulog-nlgroup nlgroup
This specifies the netlink group (1-32) to which the
packet is sent. Default value is 1.
--ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32
characters long, and useful for distinguishing messages in
the logs.
--ulog-cprange size
Number of bytes to be copied to userspace. A value of 0
always copies the entire packet, regardless of its size.
Default is 0.
--ulog-qthreshold size
Number of packet to queue inside kernel. Setting this
value to, e.g. 10 accumulates ten packets inside the
kernel and transmits them as one netlink multipart message
to userspace. Default is 1 (for backwards compatibility).
