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iptables-extensions(8) - phpMan
iptables-extensions(8) iptables 1.4.21 iptables-extensions(8)
NAME
iptables-extensions — list of extensions in the standard iptables distribution
SYNOPSIS
ip6tables [-m name [module-options...]] [-j target-name [target-options...]
iptables [-m name [module-options...]] [-j target-name [target-options...]
MATCH EXTENSIONS
iptables can use extended packet matching modules with the -m or --match options, followed
by the matching module name; after these, various extra command line options become avail‐
able, depending on the specific module. You can specify multiple extended match modules
in one line, and you can use the -h or --help options after the module has been specified
to receive help specific to that module. The extended match modules are evaluated in the
order they are specified in the rule.
If the -p or --protocol was specified and if and only if an unknown option is encountered,
iptables will try load a match module of the same name as the protocol, to try making the
option available.
addrtype
This module matches packets based on their address type. Address types are used within
the kernel networking stack and categorize addresses into various groups. The exact defi‐
nition of that group depends on the specific layer three protocol.
The following address types are possible:
UNSPEC an unspecified address (i.e. 0.0.0.0)
UNICAST
an unicast address
LOCAL a local address
BROADCAST
a broadcast address
ANYCAST
an anycast packet
MULTICAST
a multicast address
BLACKHOLE
a blackhole address
UNREACHABLE
an unreachable address
PROHIBIT
a prohibited address
THROW FIXME
NAT FIXME
XRESOLVE
[!] --src-type type
Matches if the source address is of given type
[!] --dst-type type
Matches if the destination address is of given type
--limit-iface-in
The address type checking can be limited to the interface the packet is coming in.
This option is only valid in the PREROUTING, INPUT and FORWARD chains. It cannot be
specified with the --limit-iface-out option.
--limit-iface-out
The address type checking can be limited to the interface the packet is going out.
This option is only valid in the POSTROUTING, OUTPUT and FORWARD chains. It cannot
be specified with the --limit-iface-in option.
ah (IPv6-specific)
This module matches the parameters in Authentication header of IPsec packets.
[!] --ahspi spi[:spi]
Matches SPI.
[!] --ahlen length
Total length of this header in octets.
--ahres
Matches if the reserved field is filled with zero.
ah (IPv4-specific)
This module matches the SPIs in Authentication header of IPsec packets.
[!] --ahspi spi[:spi]
bpf
Match using Linux Socket Filter. Expects a BPF program in decimal format. This is the for‐
mat generated by the nfbpf_compile utility.
--bytecode code
Pass the BPF byte code format (described in the example below).
The code format is similar to the output of the tcpdump -ddd command: one line that stores
the number of instructions, followed by one line for each instruction. Instruction lines
follow the pattern 'u16 u8 u8 u32' in decimal notation. Fields encode the operation, jump
offset if true, jump offset if false and generic multiuse field 'K'. Comments are not sup‐
ported.
For example, to read only packets matching 'ip proto 6', insert the following, without the
comments or trailing whitespace:
4 # number of instructions
48 0 0 9 # load byte ip->proto
21 0 1 6 # jump equal IPPROTO_TCP
6 0 0 1 # return pass (non-zero)
6 0 0 0 # return fail (zero)
You can pass this filter to the bpf match with the following command:
iptables -A OUTPUT -m bpf --bytecode '4,48 0 0 9,21 0 1 6,6 0 0 1,6 0 0 0' -j
ACCEPT
Or instead, you can invoke the nfbpf_compile utility.
iptables -A OUTPUT -m bpf --bytecode "`nfbpf_compile RAW 'ip proto 6'`" -j ACCEPT
You may want to learn more about BPF from FreeBSD's bpf(4) manpage.
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 --clus‐
ter-local-node.
--cluster-hash-seed value
Set seed value of the Jenkins hash.
Example:
iptables -A PREROUTING -t mangle -i eth1 -m cluster --cluster-total-nodes 2 --clus‐
ter-local-node 1 --cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth2 -m cluster --cluster-total-nodes 2 --clus‐
ter-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 man‐
gle --mangle-mac-d 00:zz:yy:xx:5a:27
arptables -A OUTPUT -o eth2 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:02
arptables -A INPUT -i eth2 --h-length 6 --destination-mac 01:00:5e:00:01:02 -j man‐
gle --mangle-mac-d 00:zz:yy:xx:5a:27
NOTE: the arptables commands above use mainstream syntax. If you are using arptables-jf
included in some RedHat, CentOS and Fedora versions, you will hit syntax errors. There‐
fore, you'll have to adapt these to the arptables-jf syntax to get them working.
In the case of TCP connections, pickup facility has to be disabled to avoid marking TCP
ACK packets coming in the reply direction as valid.
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
comment
Allows you to add comments (up to 256 characters) to any rule.
--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 average 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 `conntrack -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 con‐
nections 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 transferred or the average
size (in bytes) of all packets received so far. Note that when "both" is used
together with "avgpkt", and data is going (mainly) only in one direction (for exam‐
ple HTTP), the average packet size will be about half of the actual data packets.
Example:
iptables .. -m connbytes --connbytes 10000:100000 --connbytes-dir both
--connbytes-mode bytes ...
connlabel
Module matches or adds connlabels to a connection. connlabels are similar to connmarks,
except labels are bit-based; i.e. all labels may be attached to a flow at the same time.
Up to 128 unique labels are currently supported.
[!] --label name
matches if label name has been set on a connection. Instead of a name (which will
be translated to a number, see EXAMPLE below), a number may be used instead. Using
a number always overrides connlabel.conf.
--set if the label has not been set on the connection, set it. Note that setting a label
can fail. This is because the kernel allocates the conntrack label storage area
when the connection is created, and it only reserves the amount of memory required
by the ruleset that exists at the time the connection is created. In this case,
the match will fail (or succeed, in case --label option was negated).
This match depends on libnetfilter_conntrack 1.0.4 or later. Label translation is done
via the /etc/xtables/connlabel.conf configuration file.
Example:
0 eth0-in
1 eth0-out
2 ppp-in
3 ppp-out
4 bulk-traffic
5 interactive
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
Match if the number of existing connections is above n.
--connlimit-mask prefix_length
Group hosts using the prefix length. For IPv4, this must be a number between
(including) 0 and 32. For IPv6, between 0 and 128. If not specified, the maximum
prefix length for the applicable protocol is used.
--connlimit-saddr
Apply the limit onto the source group. This is the default if --connlimit-daddr is
not specified.
--connlimit-daddr
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 connection (which can be
set using the CONNMARK target below).
[!] --mark value[/mask]
Matches packets in connections with the given mark value (if a mask is specified,
this is logically ANDed with the mark before the comparison).
conntrack
This module, when combined with connection tracking, allows access to the connection
tracking state for this packet/connection.
[!] --ctstate statelist
statelist is a comma separated list of the connection states to match. Possible
states are listed below.
[!] --ctproto l4proto
Layer-4 protocol to match (by number or name)
[!] --ctorigsrc address[/mask]
[!] --ctorigdst address[/mask]
[!] --ctreplsrc address[/mask]
[!] --ctrepldst address[/mask]
Match against original/reply source/destination address
[!] --ctorigsrcport port[:port]
[!] --ctorigdstport port[:port]
[!] --ctreplsrcport port[:port]
[!] --ctrepldstport port[:port]
Match against original/reply source/destination port (TCP/UDP/etc.) or GRE key.
Matching against port ranges is only supported in kernel versions above 2.6.38.
[!] --ctstatus statelist
statuslist is a comma separated list of the connection statuses to match. Possible
statuses are listed below.
[!] --ctexpire time[:time]
Match remaining lifetime in seconds against given value or range of values (inclu‐
sive)
--ctdir {ORIGINAL|REPLY}
Match packets that are flowing in the specified direction. If this flag is not
specified at all, matches packets in both directions.
States for --ctstate:
INVALID
The packet is associated with no known connection.
NEW The packet has started a new connection or otherwise associated with a connection
which has not seen packets in both directions.
ESTABLISHED
The packet is associated with a connection which has seen packets in both direc‐
tions.
RELATED
The packet is starting a new connection, but is associated with an existing connec‐
tion, such as an FTP data transfer or an ICMP error.
UNTRACKED
The packet is not tracked at all, which happens if you explicitly untrack it by
using -j CT --notrack in the 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.
CONFIRMED
Connection is confirmed: originating packet has left box.
cpu
[!] --cpu number
Match cpu handling this packet. cpus are numbered from 0 to NR_CPUS-1 Can be used
in combination with RPS (Remote Packet Steering) or multiqueue NICs to spread net‐
work traffic on different queues.
Example:
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j REDIRECT --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 DCCP option set.
devgroup
Match device group of a packets incoming/outgoing interface.
[!] --src-group name
Match device group of incoming device
[!] --dst-group name
Match device group of outgoing device
dscp
This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has
superseded TOS within the IETF.
[!] --dscp value
Match against a numeric (decimal or hex) value [0-63].
[!] --dscp-class class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes.
It will then be converted into its according numeric value.
dst (IPv6-specific)
This module matches the parameters in Destination Options header
[!] --dst-len length
Total length of this header in octets.
--dst-opts type[:length][,type[:length]...]
numeric type of option and the length of the option data in octets.
ecn
This allows you to match the ECN bits of the IPv4/IPv6 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/IPv6 ECT (ECN-Capable Transport). You have to spec‐
ify a number between `0' and `3'.
esp
This module matches the SPIs in ESP header of IPsec packets.
[!] --espspi spi[:spi]
eui64 (IPv6-specific)
This module matches the EUI-64 part of a stateless autoconfigured IPv6 address. It com‐
pares the EUI-64 derived from the source MAC address in Ethernet frame with the lower 64
bits of the IPv6 source address. But "Universal/Local" bit is not compared. This module
doesn't match other link layer frame, and is only valid in the PREROUTING, INPUT and FOR‐
WARD chains.
frag (IPv6-specific)
This module matches the parameters in Fragment header.
[!] --fragid id[:id]
Matches the given Identification or range of it.
[!] --fraglen length
This option cannot be used with kernel version 2.6.10 or later. The length of Frag‐
ment header is static and this option doesn't make sense.
--fragres
Matches if the reserved fields are filled with zero.
--fragfirst
Matches on the first fragment.
--fragmore
Matches if there are more fragments.
--fraglast
Matches if this is the last fragment.
hashlimit
hashlimit uses hash buckets to express a rate limiting match (like the limit match) for a
group of connections using a single iptables rule. Grouping can be done per-hostgroup
(source and/or destination address) and/or per-port. It gives you the ability to express
"N packets per time quantum per group" or "N bytes per seconds" (see below for some exam‐
ples).
A hash limit option (--hashlimit-upto, --hashlimit-above) and --hashlimit-name are
required.
--hashlimit-upto amount[/second|/minute|/hour|/day]
Match if the rate is below or equal to amount/quantum. It is specified either as a
number, with an optional time quantum suffix (the default is 3/hour), or as
amountb/second (number of bytes per second).
--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.
When byte-based rate matching is requested, this option specifies the amount of
bytes that can exceed the given rate. This option should be used with caution --
if the entry expires, the burst value is reset too.
--hashlimit-mode {srcip|srcport|dstip|dstport},...
A comma-separated list of objects to take into consideration. If no --hash‐
limit-mode option is given, hashlimit acts like limit, but at the expensive of
doing the hash housekeeping.
--hashlimit-srcmask prefix
When --hashlimit-mode srcip is used, all source addresses encountered will be
grouped according to the given prefix length and the so-created subnet will be sub‐
ject to hashlimit. prefix must be between (inclusive) 0 and 32. Note that --hash‐
limit-srcmask 0 is basically doing the same thing as not specifying srcip for
--hashlimit-mode, but is technically more expensive.
--hashlimit-dstmask prefix
Like --hashlimit-srcmask, but for destination addresses.
--hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry.
--hashlimit-htable-size buckets
The number of buckets of the hash table
--hashlimit-htable-max entries
Maximum entries in the hash.
--hashlimit-htable-expire msec
After how many milliseconds do hash entries expire.
--hashlimit-htable-gcinterval msec
How many milliseconds between garbage collection intervals.
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 --hash‐
limit-mode srcport --hashlimit-upto 100/sec
matching on subnet
"10000 packets per minute for every /28 subnet (groups of 8 addresses) in
10.0.0.0/8" => -s 10.0.0.0/8 --hashlimit-mask 28 --hashlimit-upto 10000/min
matching bytes per second
"flows exceeding 512kbyte/s" => --hashlimit-mode srcip,dstip,srcport,dstport
--hashlimit-above 512kb/s
matching bytes per second
"hosts that exceed 512kbyte/s, but permit up to 1Megabytes without matching"
--hashlimit-mode dstip --hashlimit-above 512kb/s --hashlimit-burst 1mb
hbh (IPv6-specific)
This module matches the parameters in Hop-by-Hop Options header
[!] --hbh-len length
Total length of this header in octets.
--hbh-opts type[:length][,type[:length]...]
numeric type of option and the length of the option data in octets.
helper
This module matches packets related to a specific conntrack-helper.
[!] --helper string
Matches packets related to the specified conntrack-helper.
string can be "ftp" for packets related to a ftp-session on default port. For
other ports append -portnr to the value, ie. "ftp-2121".
Same rules apply for other conntrack-helpers.
hl (IPv6-specific)
This module matches the Hop Limit field in the IPv6 header.
[!] --hl-eq value
Matches if Hop Limit equals value.
--hl-lt value
Matches if Hop Limit is less than value.
--hl-gt value
Matches if Hop Limit is greater than value.
icmp (IPv4-specific)
This extension can be used if `--protocol icmp' is specified. It provides 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
icmp6 (IPv6-specific)
This extension can be used if `--protocol ipv6-icmp' or `--protocol icmpv6' is specified.
It provides the following option:
[!] --icmpv6-type type[/code]|typename
This allows specification of the ICMPv6 type, which can be a numeric ICMPv6 type,
type and code, or one of the ICMPv6 type names shown by the command
ip6tables -p ipv6-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.
ipv6header (IPv6-specific)
This module matches IPv6 extension headers and/or upper layer header.
--soft Matches if the packet includes any of the headers specified with --header.
[!] --header header[,header...]
Matches the packet which EXACTLY includes all specified headers. The headers encap‐
sulated with ESP header are out of scope. Possible header types can be:
hop|hop-by-hop
Hop-by-Hop Options header
dst Destination Options header
route Routing header
frag Fragment header
auth Authentication header
esp Encapsulating Security Payload header
none No Next header which matches 59 in the 'Next Header field' of IPv6 header or any
IPv6 extension headers
proto which matches any upper layer protocol header. A protocol name from /etc/protocols
and numeric value also allowed. The number 255 is equivalent to proto.
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.
[!] --length length[:length]
limit
This module matches at a limited rate using a token bucket filter. A rule using this
extension will match until this limit is reached. It can be used in combination with the
LOG target to give limited logging, for example.
xt_limit has no negation support - you will have to use -m hashlimit ! --hashlimit rate
in this case whilst omitting --hashlimit-mode.
--limit rate[/second|/minute|/hour|/day]
Maximum average matching rate: specified as a number, with an optional `/second',
`/minute', `/hour', or `/day' suffix; the default is 3/hour.
--limit-burst number
Maximum initial number of packets to match: this number gets recharged by one every
time the limit specified above is not reached, up to this number; the default is 5.
mac
[!] --mac-source address
Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that
this only makes sense for packets coming from an Ethernet device and entering the
PREROUTING, FORWARD or INPUT chains.
mark
This module matches the netfilter mark field associated with a packet (which can be set
using the MARK target below).
[!] --mark value[/mask]
Matches packets with the given unsigned mark value (if a mask is specified, this is
logically ANDed with the mask before the comparison).
mh (IPv6-specific)
This extension is loaded if `--protocol ipv6-mh' or `--protocol mh' is specified. It pro‐
vides the following option:
[!] --mh-type type[:type]
This allows specification of the Mobility Header(MH) type, which can be a numeric
MH type, type or one of the MH type names shown by the command
ip6tables -p mh -h
multiport
This module matches a set of source or destination ports. Up to 15 ports can be speci‐
fied. A port range (port:port) counts as two ports. It can only be used in conjunction
with one of the following protocols: tcp, udp, udplite, dccp and sctp.
[!] --source-ports,--sports port[,port|,port:port]...
Match if the source port is one of the given ports. The flag --sports is a conve‐
nient alias for this option. Multiple ports or port ranges are separated using a
comma, and a port range is specified using a colon. 53,1024:65535 would therefore
match ports 53 and all from 1024 through 65535.
[!] --destination-ports,--dports port[,port|,port:port]...
Match if the destination port is one of the given ports. The flag --dports is a
convenient alias for this option.
[!] --ports port[,port|,port:port]...
Match if either the source or destination ports are equal to one of the given
ports.
nfacct
The nfacct match provides the extended accounting infrastructure for iptables. You have
to use this match together with the standalone user-space utility nfacct(8)
The only option available for this match is the following:
--nfacct-name name
This allows you to specify the existing object name that will be use for accounting
the traffic that this rule-set is matching.
To use this extension, you have to create an accounting object:
nfacct add http-traffic
Then, you have to attach it to the accounting object via iptables:
iptables -I INPUT -p tcp --sport 80 -m nfacct --nfacct-name http-traffic
iptables -I OUTPUT -p tcp --dport 80 -m nfacct --nfacct-name http-traffic
Then, you can check for the amount of traffic that the rules match:
nfacct get http-traffic
{ pkts = 00000000000000000156, bytes = 00000000000000151786 } = http-traffic;
You can obtain nfacct(8) from http://www.netfilter.org or, alternatively, from the
git.netfilter.org repository.
osf
The osf module does passive operating system fingerprinting. This modules 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 fingerprinting.
--ttl level
Do additional TTL checks on the packet to determine the operating system. level
can be one of the following values:
· 0 - True IP address and fingerprint TTL comparison. This generally works for LANs.
· 1 - Check if the IP header's TTL is less than the fingerprint one. Works for globally-
routable addresses.
· 2 - Do not compare the TTL at all.
--log level
Log determined genres into dmesg even if they do not match the desired one. level can
be one of the following values:
· 0 - Log all matched or unknown signatures
· 1 - Log only the first one
· 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 fingerprints 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.openbsd.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. For‐
warded packets do not have any socket associated with them. Packets from kernel threads do
have a socket, but usually no owner.
[!] --uid-owner username
[!] --uid-owner userid[-userid]
Matches if the packet socket's file structure (if it has one) is owned by the given
user. You may also specify a numerical UID, or an UID range.
[!] --gid-owner groupname
[!] --gid-owner groupid[-groupid]
Matches if the packet socket's file structure is owned by the given group. You may
also specify a numerical GID, or a GID range.
[!] --socket-exists
Matches if the packet is associated with a socket.
physdev
This module matches on the bridge port input and output devices enslaved to a bridge
device. This module is a part of the infrastructure that enables a transparent bridging IP
firewall and is only useful for kernel versions above version 2.5.44.
[!] --physdev-in name
Name of a bridge port via which a packet is received (only for packets entering the
INPUT, FORWARD and PREROUTING chains). If the interface name ends in a "+", then
any interface which begins with this name will match. If the packet didn't arrive
through a bridge device, this packet won't match this option, unless '!' is used.
[!] --physdev-out name
Name of a bridge port via which a packet is going to be sent (for packets entering
the FORWARD, OUTPUT and POSTROUTING chains). If the interface name ends in a "+",
then any interface which begins with this name will match. Note that in the nat and
mangle OUTPUT chains one cannot match on the bridge output port, however one can in
the filter OUTPUT chain. If the packet won't leave by a bridge device or 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 POSTROUTING chains.
pkttype
This module matches the link-layer packet type.
[!] --pkt-type {unicast|broadcast|multicast}
policy
This modules matches the policy used by IPsec for handling a packet.
--dir {in|out}
Used to select whether to match the policy used for decapsulation or the policy
that will be used for encapsulation. in is valid in the PREROUTING, INPUT and FOR‐
WARD chains, out is valid in the POSTROUTING, OUTPUT and FORWARD chains.
--pol {none|ipsec}
Matches if the packet is subject to IPsec processing. --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.
[!] --proto {ah|esp|ipcomp}
Matches the encapsulation protocol.
[!] --mode {tunnel|transport}
Matches the encapsulation mode.
[!] --tunnel-src addr[/mask]
Matches the source end-point address of a tunnel mode SA. Only valid with --mode
tunnel.
[!] --tunnel-dst addr[/mask]
Matches the destination end-point address of a tunnel mode SA. Only valid with
--mode tunnel.
--next Start the next element in the policy specification. Can only be used with --strict.
quota
Implements network quotas by decrementing a byte counter with each packet. 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, comparing two rate estimators and matching
on the difference between two rate estimators.
For a better understanding of the available options, these are all possible combinations:
· rateest operator rateest-bps
· rateest operator rateest-pps
· (rateest minus rateest-bps1) operator rateest-bps2
· (rateest minus rateest-pps1) operator rateest-pps2
· rateest1 operator rateest2 rateest-bps(without rate!)
· rateest1 operator rateest2 rateest-pps(without rate!)
· (rateest1 minus rateest-bps1) operator (rateest2 minus rateest-bps2)
· (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 negative 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
Match if rate is greater than given rate/estimator.
[!] --rateest-eq
Match if rate is equal to given rate/estimator.
In the so-called "absolute mode", only one rate estimator is used and compared against a
static value, while in "relative mode", two rate estimators are compared against another.
--rateest name
Name of the one rate estimator for absolute mode.
--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-inter‐
val 250ms --rateest-ewma 0.5s
iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST --rateest-name ppp0 --rateest-inter‐
val 250ms --rateest-ewma 0.5s
# Mark based on available bandwidth
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest
--rateest-delta --rateest1 eth0 --rateest-bps1 2.5mbit --rateest-gt --rateest2 ppp0
--rateest-bps2 2mbit -j CONNMARK --set-mark 1
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest
--rateest-delta --rateest1 ppp0 --rateest-bps1 2mbit --rateest-gt --rateest2 eth0
--rateest-bps2 2.5mbit -j CONNMARK --set-mark 2
iptables -t mangle -A balance -j CONNMARK --restore-mark
realm (IPv4-specific)
This matches the routing realm. Routing realms are used in complex routing setups involv‐
ing dynamic routing protocols like BGP.
[!] --realm value[/mask]
Matches a given realm number (and optionally mask). If not a number, value can be a
named realm from /etc/iproute2/rt_realms (mask can not be used in that case).
recent
Allows you to dynamically create a list of IP addresses and then match against that list
in a few different ways.
For example, you can create a "badguy" list out of people attempting to connect to port
139 on your firewall and then DROP all future packets from them without considering them.
--set, --rcheck, --update and --remove are mutually exclusive.
--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.
--mask netmask
Netmask that will be applied to this recent list.
[!] --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 This option can only be used in conjunction with --seconds. When used, this will
cause entries older than the last given number of 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 mod‐
ule. Exceeding this value on the command line will cause the rule to be rejected.
--rttl This option may only be used in conjunction with one of --rcheck or --update. When
used, this will narrow the match to only happen when the address is in the list and
the TTL of the current packet matches that of the packet which hit the --set rule.
This may be useful if you have problems with people faking their source address in
order to DoS you via this module by disallowing others access to your site by send‐
ing bogus packets to you.
Examples:
iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP
iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy --set -j
DROP
/proc/net/xt_recent/* are the current lists of addresses and information 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.
rpfilter
Performs a reverse path filter test on a packet. If a reply to the packet would be sent
via the same interface that the packet arrived on, the packet will match. Note that,
unlike the in-kernel rp_filter, packets protected by IPSec are not treated specially.
Combine this match with the policy match if you want this. Also, packets arriving via the
loopback interface are always permitted. This match can only be used in the PREROUTING
chain of the raw or mangle table.
--loose
Used to specifiy that the reverse path filter test should match even if the
selected output device is not the expected one.
--validmark
Also use the packets' nfmark value when performing the reverse path route lookup.
--accept-local
This will permit packets arriving from the network with a source address that is
also assigned to the local machine.
--invert
This will invert the sense of the match. Instead of matching packets that passed
the reverse path filter test, match those that have failed it.
Example to log and drop packets failing the reverse path filter test:
iptables -t raw -N RPFILTER
iptables -t raw -A RPFILTER -m rpfilter -j RETURN
iptables -t raw -A RPFILTER -m limit --limit 10/minute -j NFLOG --nflog-prefix "rpfilter
drop"
iptables -t raw -A RPFILTER -j DROP
iptables -t raw -A PREROUTING -j RPFILTER
Example to drop failed packets, without logging:
iptables -t raw -A RPFILTER -m rpfilter --invert -j DROP
rt (IPv6-specific)
Match on IPv6 routing header
[!] --rt-type type
Match the type (numeric).
[!] --rt-segsleft num[:num]
Match the `segments left' field (range).
[!] --rt-len length
Match the length of this header.
--rt-0-res
Match the reserved field, too (type=0)
--rt-0-addrs addr[,addr...]
Match type=0 addresses (list).
--rt-0-not-strict
List of type=0 addresses is not a strict list.
sctp
This module matches Stream Control Transmission Protocol headers.
[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --chunk-types {all|any|only} chunktype[:flags] [...]
The flag letter in upper case indicates that the flag is to match if set, in the
lower case indicates to match if unset.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUT‐
DOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE ASCONF
ASCONF_ACK FORWARD_TSN
chunk type available flags
DATA I U B E i u b e
ABORT T t
SHUTDOWN_COMPLETE T t
(lowercase means flag should be "off", uppercase means "on")
Examples:
iptables -A INPUT -p sctp --dport 80 -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA,INIT -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT
set
This 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 specifications 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
packets for which the source address can be found in the specified set.
--return-nomatch
If the --return-nomatch option is specified and the set type supports the nomatch
flag, then the matching is reversed: a match with an element flagged with nomatch
returns true, while a match with a plain element returns false.
! --update-counters
If the --update-counters flag is negated, then the packet and byte counters of the
matching element in the set won't be updated. Default the packet and byte counters
are updated.
! --update-subcounters
If the --update-subcounters flag is negated, then the packet and byte counters of
the matching element in the member set of a list type of set won't be updated.
Default the packet and byte counters are updated.
[!] --packets-eq value
If the packet is matched an element in the set, match only if the packet counter of
the element matches the given value too.
--packets-lt value
If the packet is matched an element in the set, match only if the packet counter of
the element is less than the given value as well.
--packets-gt value
If the packet is matched an element in the set, match only if the packet counter of
the element is greater than the given value as well.
[!] -bytes-eq value
If the packet is matched an element in the set, match only if the byte counter of
the element matches the given value too.
--bytes-lt value
If the packet is matched an element in the set, match only if the byte counter of
the element is less than the given value as well.
--bytes-gt value
If the packet is matched an element in the set, match only if the byte counter of
the element is greater than the given value as well.
The packet and byte counters related options and flags are ignored when the set was
defined without counter support.
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, which, for standard kernels,
is the case since Linux 2.6.39.
socket
This matches if an open TCP/UDP socket can be found by doing a socket lookup on the
packet. It matches if there is an established or non-zero bound listening socket (possibly
with a non-local address). The lookup is performed using the packet tuple of TCP/UDP pack‐
ets, or the original TCP/UDP header embedded in an ICMP/ICPMv6 error packet.
--transparent
Ignore non-transparent sockets.
--nowildcard
Do not ignore sockets bound to 'any' address. The socket match won't accept
zero-bound listeners by default, since then local services could intercept traffic
that would otherwise be forwarded. This option therefore has security implications
when used to match traffic being forwarded to redirect such packets to local
machine with policy routing. When using the socket match to implement fully trans‐
parent proxies bound to non-local addresses it is recommended to use the --trans‐
parent option instead.
Example (assuming packets with mark 1 are delivered locally):
-t mangle -A PREROUTING -m socket --transparent -j MARK --set-mark 1
state
The "state" extension is a subset of the "conntrack" module. "state" 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. Only a
subset of the states unterstood by "conntrack" are recognized: INVALID, ESTAB‐
LISHED, NEW, RELATED or UNTRACKED. For their description, see the "conntrack" head‐
ing in this manpage.
statistic
This module matches packets based on some statistic condition. It supports two distinct
modes settable with the --mode option.
Supported options:
--mode mode
Set the matching mode of the matching rule, supported modes are random and nth.
[!] --probability p
Set the probability 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 offset-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.
Examples:
# The string pattern can be used for simple text characters.
iptables -A INPUT -p tcp --dport 80 -m string --algo bm --string 'GET /index.html'
-j LOG
# The hex string pattern can be used for non-printable characters, like |0D 0A| or
|0D0A|.
iptables -p udp --dport 53 -m string --algo bm --from 40 --to 57 --hex-string
'|03|www|09|netfilter|03|org|00|'
tcp
These extensions can be used if `--protocol tcp' is specified. It provides the following
options:
[!] --source-port,--sport port[:port]
Source port or port range specification. This can either be a service name or a
port number. An inclusive range can also be specified, using the format 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.
[!] --tcp-flags mask comp
Match when the TCP flags are as specified. The first argument mask is the flags
which we should examine, written as a comma-separated list, and the second argument
comp is a comma-separated list of flags which must be set. Flags are: SYN ACK FIN
RST URG PSH ALL NONE. Hence the command
iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and the ACK, FIN and RST flags
unset.
[!] --syn
Only match TCP packets with the SYN bit set and the ACK,RST and FIN bits cleared.
Such packets are used to request TCP connection initiation; for example, blocking
such packets coming in an interface will prevent incoming TCP connections, but out‐
going TCP connections will be unaffected. It is equivalent to --tcp-flags
SYN,RST,ACK,FIN SYN. If the "!" flag precedes the "--syn", the sense of the option
is inverted.
[!] --tcp-option number
Match if TCP option set.
tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP header. You can only use
this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated during the TCP hand‐
shake at connection startup time.
[!] --mss value[:value]
Match a given TCP MSS value or range.
time
This matches if the packet arrival time/date is within a given range. All options are
optional, but are ANDed when specified. 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 pos‐
sible time range is 1970-01-01T00:00:00 to 2038-01-19T04:17:07.
If --datestart or --datestop are not specified, it will default to 1970-01-01 and
2038-01-19, respectively.
--timestart hh:mm[:ss]
--timestop hh:mm[:ss]
Only match during the given daytime. The possible time range is 00:00:00 to
23:59:59. Leading zeroes are allowed (e.g. "06:03") and correctly interpreted as
base-10.
[!] --monthdays day[,day...]
Only match on the given days of the month. Possible values are 1 to 31. Note that
specifying 31 will of course not match on months which do not have a 31st day; the
same goes for 28- or 29-day February.
[!] --weekdays day[,day...]
Only match on the given weekdays. Possible values are Mon, Tue, Wed, Thu, Fri, Sat,
Sun, or values from 1 to 7, respectively. You may also use two-character variants
(Mo, Tu, etc.).
--contiguous
When --timestop is smaller than --timestart value, match this as a single time
period instead distinct intervals. See EXAMPLES.
--kerneltz
Use the kernel timezone instead of UTC to determine whether a packet meets the time
regulations.
About kernel timezones: Linux keeps the system time in UTC, and always does so. On boot,
system time is initialized from a referential time source. Where this time source has no
timezone information, such as the x86 CMOS RTC, UTC will be assumed. If the time source is
however not in UTC, userspace should provide the correct system time and timezone to the
kernel once it has the information.
Local time is a feature on top of the (timezone independent) system time. Each process has
its own idea of local time, specified via the TZ environment variable. The kernel also has
its own timezone offset variable. 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 ker‐
nel 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.)
Matching across days might not do what is expected. For instance,
-m time --weekdays Mo --timestart 23:00 --timestop 01:00 Will match Monday, for
one hour from midnight to 1 a.m., and then again for another hour from 23:00
onwards. If this is unwanted, e.g. if you would like 'match for two hours from
Montay 23:00 onwards' you need to also specify the --contiguous option in the exam‐
ple above.
tos
This module matches the 8-bit Type of Service field in the IPv4 header (i.e. including
the "Precedence" bits) or the (also 8-bit) Priority field in the IPv6 header.
[!] --tos value[/mask]
Matches packets with the given TOS mark value. If a mask is specified, it is logi‐
cally ANDed with the TOS mark before the comparison.
[!] --tos symbol
You can specify a symbolic name when using the tos match for IPv4. The list of rec‐
ognized TOS names can be obtained by calling iptables with -m tos -h. Note that
this implies a mask of 0x3F, i.e. all but the ECN bits.
ttl (IPv4-specific)
This module matches the time to live field in the IP header.
[!] --ttl-eq ttl
Matches the given TTL value.
--ttl-gt ttl
Matches if TTL is greater than the given TTL value.
--ttl-lt ttl
Matches if TTL is less than the given TTL value.
u32
U32 tests whether quantities of up to 4 bytes extracted from a packet have specified val‐
ues. The specification of what to extract is general enough to find data at given offsets
from tcp headers or payloads.
[!] --u32 tests
The argument amounts to a program in a small language described below.
tests := location "=" value | tests "&&" location "=" value
value := range | value "," range
range := number | number ":" number
a single number, n, is interpreted the same as n:n. n:m is interpreted as the range of
numbers >=n and <=m.
location := number | location operator number
operator := "&" | "<<" | ">>" | "@"
The operators &, <<, >> and && mean the same as in C. The = is really a set membership
operator and the value syntax describes a set. The @ operator is what allows moving to the
next header and is described further below.
There are currently some artificial implementation limits on the size of the tests:
* no more than 10 of "=" (and 9 "&&"s) in the u32 argument
* no more than 10 ranges (and 9 commas) per value
* no more than 10 numbers (and 9 operators) per location
To describe the meaning of location, imagine the following machine that interprets it.
There are three registers:
A is of type char *, initially the address of the IP header
B and C are unsigned 32 bit integers, initially zero
The instructions are:
number B = number;
C = (*(A+B)<<24) + (*(A+B+1)<<16) + (*(A+B+2)<<8) + *(A+B+3)
&number C = C & number
<< number C = C << number
>> number C = C >> number
@number A = A + C; then do the instruction number
Any access of memory outside [skb->data,skb->end] causes the match to fail. Otherwise the
result of the computation is the final value of C.
Whitespace is allowed but not required in the tests. However, the characters that do occur
there are likely to require shell quoting, so it is a good idea to enclose the arguments
in quotes.
Example:
match IP packets with total length >= 256
The IP header contains a total length field in bytes 2-3.
--u32 "0 & 0xFFFF = 0x100:0xFFFF"
read bytes 0-3
AND that with 0xFFFF (giving bytes 2-3), and test whether that is in the range
[0x100:0xFFFF]
Example: (more realistic, hence more complicated)
match ICMP packets with icmp type 0
First test that it is an ICMP packet, true iff byte 9 (protocol) = 1
--u32 "6 & 0xFF = 1 && ...
read bytes 6-9, use & to throw away bytes 6-8 and compare the result to 1. Next
test that it is not a fragment. (If so, it might be part of such a packet but we
cannot always tell.) N.B.: This test is generally needed if you want to match any‐
thing beyond the IP header. The last 6 bits of byte 6 and all of byte 7 are 0 iff
this is a complete packet (not a fragment). Alternatively, you can allow first
fragments by only testing the last 5 bits of byte 6.
... 4 & 0x3FFF = 0 && ...
Last test: the first byte past the IP header (the type) is 0. This is where we have
to use the @syntax. The length of the IP header (IHL) in 32 bit words is stored in
the right half of byte 0 of the IP header itself.
... 0 >> 22 & 0x3C @ 0 >> 24 = 0"
The first 0 means read bytes 0-3, >>22 means shift that 22 bits to the right.
Shifting 24 bits would give the first byte, so only 22 bits is four times that plus
a few more bits. &3C then eliminates the two extra bits on the right and the first
four bits of the first byte. For instance, if IHL=5, then the IP header is 20 (4 x
5) bytes long. In this case, bytes 0-1 are (in binary) xxxx0101 yyzzzzzz, >>22
gives the 10 bit value xxxx0101yy and &3C gives 010100. @ means to use this number
as a new offset into the packet, and read four bytes starting from there. This is
the first 4 bytes of the ICMP payload, of which byte 0 is the ICMP type. Therefore,
we simply shift the value 24 to the right to throw out all but the first byte and
compare the result with 0.
Example:
TCP payload bytes 8-12 is any of 1, 2, 5 or 8
First we test that the packet is a tcp packet (similar to ICMP).
--u32 "6 & 0xFF = 6 && ...
Next, test that it is not a fragment (same as above).
... 0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 = 1,2,5,8"
0>>22&3C as above computes the number of bytes in the IP header. @ makes this the
new offset into the packet, which is the start of the TCP header. The length of the
TCP header (again in 32 bit words) is the left half of byte 12 of the TCP header.
The 12>>26&3C computes this length in bytes (similar to the IP header before). "@"
makes this the new offset, which is the start of the TCP payload. Finally, 8 reads
bytes 8-12 of the payload and = checks whether the result is any of 1, 2, 5 or 8.
udp
These extensions can be used if `--protocol udp' is specified. It provides the following
options:
[!] --source-port,--sport port[:port]
Source port or port range specification. See the description of the --source-port
option of the TCP extension for details.
[!] --destination-port,--dport port[:port]
Destination port or port range specification. See the description of the --desti‐
nation-port option of the TCP extension for details.
unclean (IPv4-specific)
This module takes no options, but attempts to match packets which seem malformed or
unusual. This is regarded as experimental.
TARGET EXTENSIONS
iptables can use extended target modules: the following are included in the standard dis‐
tribution.
AUDIT
This target allows 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.
Example:
iptables -N AUDIT_DROP
iptables -A AUDIT_DROP -j AUDIT --type drop
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 par‐
ticularly 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 hexadeci‐
mal 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 stati‐
cally distributed between the nodes in this cluster.
--new Create a new ClusterIP. You always have to set this on the first rule for a given
ClusterIP.
--hashmode mode
Specify the hashing mode. Has to be one of sourceip, sourceip-sourceport, sour‐
ceip-sourceport-destport.
--clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer multicast address
--total-nodes num
Number of total nodes within this cluster.
--local-node num
Local node number within this cluster.
--hash-init rnd
Specify the random seed used for hash initialization.
CONNMARK
This module sets the netfilter mark value associated with a connection. 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 ker‐
nels, 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 id
Assign this packet to zone id and only have lookups done in that zone. By default,
packets have zone 0.
--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 desti‐
nation 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 destina‐
tion 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 fol‐
lowing 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 con‐
junction 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 reat‐
tached 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 cri‐
teria, 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 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
Binary AND the nfmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits
is the binary negation of bits.)
--or-mark bits
Binary OR the nfmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits
Binary XOR the nfmark with bits. (Mnemonic for --set-xmark bits/0.)
MASQUERADE
This target is only valid in the nat table, in the POSTROUTING chain. It should only be
used with dynamically assigned IP (dialup) connections: if you have a static IP address,
you should use the SNAT target. Masquerading is equivalent to specifying a mapping to the
IP address of the interface the packet is going out, but also has the effect that connec‐
tions are forgotten when the interface goes down. This is the correct behavior when the
next dialup is unlikely to have the same interface address (and hence any established con‐
nections are lost anyway).
--to-ports port[-port]
This specifies a range of source ports to use, overriding the default SNAT source
port-selection heuristics (see above). This is only valid if the rule also speci‐
fies 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).
IPv6 support available since Linux kernels >= 3.7.
MIRROR (IPv4-specific)
This is an experimental demonstration target which inverts the source and destination
fields in the IP header and retransmits the packet. It is only valid in the INPUT, FOR‐
WARD and PREROUTING chains, and user-defined chains which are only called from those
chains. Note that the outgoing packets are NOT seen by any packet filtering chains, con‐
nection tracking or NAT, to avoid loops and other problems.
NETMAP
This target allows you to statically map a whole network of addresses onto another network
of addresses. It can only be used from rules in the nat table.
--to address[/mask]
Network address to map to. The resulting address will be constructed in the fol‐
lowing way: All 'one' bits in the mask are filled in from the new `address'. All
bits that are zero in the mask are filled in from the original address.
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
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". Pack‐
ets 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).
--to-ports port[-port]
This specifies a destination port or range of ports to use: without this, the des‐
tination port is never altered. This is only valid if the rule also specifies 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-prohib‐
ited, 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 proto‐
col: 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.
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-pro‐
hibited, or icmp-admin-prohibited (*), which return the appropriate ICMP error mes‐
sage (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 other‐
wise).
(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain
DROP instead of REJECT
SAME (IPv4-specific)
Similar to SNAT/DNAT depending on chain: it takes a range of addresses (`--to
1.2.3.4-1.2.3.7') and gives a client the same source-/destination-address for each connec‐
tion.
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 compatibil‐
ity 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
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
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 (kernel >= 2.6.21).
--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 fol‐
lowing 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.
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 con‐
figure 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/Pri‐
ority 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 man‐
gling does not behave as documented and differs from the IPv4 version. The TOS mask indi‐
cates 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 pack‐
ets 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.
A logging backend, such as ip(6)t_LOG or nfnetlink_log, must be loaded for this to be vis‐
ible. The packets are logged with the string prefix: "TRACE: tablename:chain‐
name:type:rulenum " where type can be "rule" for plain rule, "return" for implicit rule at
the end of a user defined chain and "policy" for the policy of the built in chains.
It can only be used in the raw table.
TTL (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 accumu‐
lates ten packets inside the kernel and transmits them as one netlink multipart
message to userspace. Default is 1 (for backwards compatibility).
iptables 1.4.21 iptables-extensions(8)
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