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NAME | SYNOPSIS | MATCH EXTENSIONS | TARGET EXTENSIONS | COLOPHON |
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iptables-extensions(8) iptables 1.8.10 iptables-extensions(8)
iptables-extensions — list of extensions in the standard iptables
distribution
ip6tables [-m name [module-options...]] [-j target-name [target-
options...]
iptables [-m name [module-options...]] [-j target-name [target-
options...]
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 available,
depending on the specific module. You can specify multiple
extended match modules in one line, and you can use the -h or
--help options after the module has been specified to receive
help specific to that module. The 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 definition of that
group depends on the specific layer three protocol.
The following address types are possible:
UNSPEC an unspecified address (i.e. 0.0.0.0)
UNICAST
an unicast address
LOCAL a local address
BROADCAST
a broadcast address
ANYCAST
an anycast packet
MULTICAST
a multicast address
BLACKHOLE
a blackhole address
UNREACHABLE
an unreachable address
PROHIBIT
a prohibited address
THROW FIXME
NAT FIXME
XRESOLVE
[!] --src-type type
Matches if the source address is of given type
[!] --dst-type type
Matches if the destination address is of given type
--limit-iface-in
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 path to an eBPF object
or a cBPF program in decimal format.
--object-pinned path
Pass a path to a pinned eBPF object.
Applications load eBPF programs into the kernel with the bpf()
system call and BPF_PROG_LOAD command and can pin them in a
virtual filesystem with BPF_OBJ_PIN. To use a pinned object in
iptables, mount the bpf filesystem using
mount -t bpf bpf ${BPF_MOUNT}
then insert the filter in iptables by path:
iptables -A OUTPUT -m bpf --object-pinned
${BPF_MOUNT}/{PINNED_PATH} -j ACCEPT
--bytecode code
Pass the BPF byte code format as generated by the
nfbpf_compile utility.
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 supported.
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
Or use tcpdump -ddd. In that case, generate BPF targeting a
device with the same data link type as the xtables match.
Iptables passes packets from the network layer up, without mac
layer. Select a device with data link type RAW, such as a tun
device:
ip tuntap add tun0 mode tun
ip link set tun0 up
tcpdump -ddd -i tun0 ip proto 6
See tcpdump -L -i $dev for a list of known data link types for a
given device.
You may want to learn more about BPF from FreeBSD's bpf(4)
manpage.
cgroup
[!] --path path
Match cgroup2 membership.
Each socket is associated with the v2 cgroup of the
creating process. This matches packets coming from or
going to all sockets in the sub-hierarchy of the specified
path. The path should be relative to the root of the
cgroup2 hierarchy.
[!] --cgroup classid
Match cgroup net_cls classid.
classid is the marker set through the cgroup net_cls
controller. This option and --path can't be used
together.
Example:
iptables -A OUTPUT -p tcp --sport 80 -m cgroup ! --path
service/http-server -j DROP
iptables -A OUTPUT -p tcp --sport 80 -m cgroup ! --cgroup
1 -j DROP
IMPORTANT: when being used in the INPUT chain, the cgroup matcher
is currently only of limited functionality, meaning it will only
match on packets that are processed for local sockets through
early socket demuxing. Therefore, general usage on the INPUT
chain is not advised unless the implications are well understood.
Available since Linux 3.14.
cluster
Allows you to deploy gateway and back-end load-sharing clusters
without the need of load-balancers.
This match requires that all the nodes see the same packets.
Thus, the cluster match decides if this node has to handle a
packet given the following options:
--cluster-total-nodes num
Set number of total nodes in cluster.
[!] --cluster-local-node num
Set the local node number ID.
[!] --cluster-local-nodemask mask
Set the local node number ID mask. You can use this option
instead of --cluster-local-node.
--cluster-hash-seed value
Set seed value of the Jenkins hash.
Example:
iptables -A PREROUTING -t mangle -i eth1 -m cluster
--cluster-total-nodes 2 --cluster-local-node 1
--cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth2 -m cluster
--cluster-total-nodes 2 --cluster-local-node 1
--cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark
0xffff -j DROP
iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark
0xffff -j DROP
And the following commands to make all nodes see the same
packets:
ip maddr add 01:00:5e:00:01:01 dev eth1
ip maddr add 01:00:5e:00:01:02 dev eth2
arptables -A OUTPUT -o eth1 --h-length 6 -j mangle
--mangle-mac-s 01:00:5e:00:01:01
arptables -A INPUT -i eth1 --h-length 6 --destination-mac
01:00:5e:00:01:01 -j mangle --mangle-mac-d
00:zz:yy:xx:5a:27
arptables -A OUTPUT -o eth2 --h-length 6 -j mangle
--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 mangle --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. Therefore, 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 connections will be byte/packet counted. Existing connection
flows will not be gaining/losing a/the accounting structure when
be sysctl flag is flipped.
[!] --connbytes from[:to]
match packets from a connection whose
packets/bytes/average packet size is more than FROM and
less than TO bytes/packets. if TO is omitted only FROM
check is done. "!" is used to match packets not falling in
the range.
--connbytes-dir {original|reply|both}
which packets to consider
--connbytes-mode {packets|bytes|avgpkt}
whether to check the amount of packets, number of bytes
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 example HTTP), the average packet size will
be about half of the actual data packets.
Example:
iptables .. -m connbytes --connbytes 10000:100000
--connbytes-dir both --connbytes-mode bytes ...
connlimit
Allows you to restrict the number of parallel connections to a
server per client IP address (or client address block).
--connlimit-upto n
Match if the number of existing connections is below or
equal n.
--connlimit-above n
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 (inclusive)
--ctdir {ORIGINAL|REPLY}
Match packets that are flowing in the specified direction.
If this flag is not specified at all, matches packets in
both directions.
States for --ctstate:
INVALID
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 directions.
RELATED
The packet is starting a new connection, but is associated
with an existing connection, such as an FTP data transfer
or an ICMP error.
UNTRACKED
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 network
traffic on different queues.
Example:
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j
REDIRECT --to-ports 8080
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j
REDIRECT --to-ports 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 packet's 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 in the
range 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 specify 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 compares 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 FORWARD 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 Fragment 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 examples).
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 --hashlimit-mode option is given,
hashlimit acts like limit, but at the expensive of doing
the hash housekeeping.
--hashlimit-srcmask prefix
When --hashlimit-mode srcip is used, all source addresses
encountered will be grouped according to the given prefix
length and the so-created subnet will be subject to
hashlimit. prefix must be between (inclusive) 0 and 32.
Note that --hashlimit-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.
--hashlimit-rate-match
Classify the flow instead of rate-limiting it. This acts
like a true/false match on whether the rate is above/below
a certain number
--hashlimit-rate-interval sec
Can be used with --hashlimit-rate-match to specify the
interval at which the rate should be sampled
Examples:
matching on source host
"1000 packets per second for every host in 192.168.0.0/16"
=> -s 192.168.0.0/16 --hashlimit-mode srcip
--hashlimit-upto 1000/sec
matching on source port
"100 packets per second for every service of 192.168.1.1"
=> -s 192.168.1.1 --hashlimit-mode srcport
--hashlimit-upto 100/sec
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 an 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 encapsulated 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
prot which matches any upper layer protocol header. A protocol
name from /etc/protocols and numeric value also allowed.
The number 255 is equivalent to prot.
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 provides 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 specified. 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 convenient alias for this option.
Multiple ports or port ranges are separated using a comma,
and a port range is specified using a colon.
53,1024:65535 would therefore match ports 53 and all from
1024 through 65535.
[!] --destination-ports,--dports port[,port|,port:port]...
Match if the destination port is one of the given ports.
The flag --dports is a convenient alias for this option.
[!] --ports port[,port|,port:port]...
Match if either the source or destination ports are equal
to one of the given ports.
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 https://www.netfilter.org or,
alternatively, from the git.netfilter.org repository.
osf
The osf module does passive operating system fingerprinting. This
module 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 downloaded 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. Forwarded packets do
not have any socket associated with them. Packets from kernel
threads do have a socket, but usually no owner.
[!] --uid-owner username
[!] --uid-owner userid[-userid]
Matches if the packet socket's file structure (if it has
one) is owned by the given user. You may also specify a
numerical UID, or an UID range.
[!] --gid-owner groupname
[!] --gid-owner groupid[-groupid]
Matches if the packet socket's file structure is owned by
the given group. You may also specify a numerical GID, or
a GID range.
--suppl-groups
Causes group(s) specified with --gid-owner to be also
checked in the supplementary groups of a process.
[!] --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 bridged packets entering the FORWARD and
POSTROUTING chains). If the interface name ends in a "+",
then any interface which begins with this name will match.
[!] --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 module matches the policy used by IPsec for handling a
packet.
--dir {in|out}
Used to select whether to match the policy used for
decapsulation or the policy that will be used for
encapsulation. in is valid in the PREROUTING, INPUT and
FORWARD chains, out is valid in the POSTROUTING, OUTPUT
and FORWARD chains.
--pol {none|ipsec}
Matches if the packet is subject to IPsec processing.
--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:
Absolute:
• rateest operator rateest-bps
• rateest operator rateest-pps
Absolute + Delta:
• (rateest minus rateest-bps1) operator rateest-bps2
• (rateest minus rateest-pps1) operator rateest-pps2
Relative:
• rateest1 operator rateest2 rateest-bps(without rate!)
• rateest1 operator rateest2 rateest-pps(without rate!)
Relative + Delta:
• (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-interval 250ms --rateest-ewma 0.5s
iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST
--rateest-name ppp0 --rateest-interval 250ms --rateest-ewma 0.5s
# Mark based on available bandwidth
iptables -t mangle -A balance -m conntrack --ctstate NEW -m
helper --helper ftp -m rateest --rateest-delta --rateest1 eth0
--rateest-bps1 2.5mbit --rateest-gt --rateest2 ppp0
--rateest-bps2 2mbit -j CONNMARK --set-mark 1
iptables -t mangle -A balance -m conntrack --ctstate NEW -m
helper --helper ftp -m rateest --rateest-delta --rateest1 ppp0
--rateest-bps1 2mbit --rateest-gt --rateest2 eth0 --rateest-bps2
2.5mbit -j CONNMARK --set-mark 2
iptables -t mangle -A balance -j CONNMARK --restore-mark
realm (IPv4-specific)
This matches the routing realm. Routing realms are used in
complex routing setups involving dynamic routing protocols like
BGP.
[!] --realm value[/mask]
Matches a given realm number (and optionally mask). If not
a number, value can be a named realm from
/etc/iproute2/rt_realms (mask can not be used in that
case). Both value and mask are four byte unsigned
integers and may be specified in decimal, hex (by
prefixing with "0x") or octal (if a leading zero is
given).
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
module. 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 sending 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 by rounding it up to the next power of two
(with ip_list_tot defaulting to 100, ip_list_hash_size
will calculate to 128 by default).
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 specify 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.
Match types:
all Match if all given chunk types are present and flags
match.
any Match if any of the given chunk types is present with
given flags.
only Match if only the given chunk types are present with given
flags and none are missing.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT
HEARTBEAT_ACK ABORT SHUTDOWN SHUTDOWN_ACK ERROR
COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE
I_DATA RE_CONFIG PAD ASCONF ASCONF_ACK FORWARD_TSN
I_FORWARD_TSN
chunk type available flags
DATA I U B E i u b e
I_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 packets, 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 transparent proxies bound to non-local addresses it
is recommended to use the --transparent option instead.
Example (assuming packets with mark 1 are delivered locally):
-t mangle -A PREROUTING -m socket --transparent -j MARK
--set-mark 1
--restore-skmark
Set the packet mark to the matching socket's mark. Can be
combined with the --transparent and --nowildcard options
to restrict the sockets to be matched when restoring the
packet mark.
Example: An application opens 2 transparent (IP_TRANSPARENT)
sockets and sets a mark on them with SO_MARK socket option. We
can filter matching packets:
-t mangle -I PREROUTING -m socket --transparent
--restore-skmark -j action
-t mangle -A action -m mark --mark 10 -j action2
-t mangle -A action -m mark --mark 11 -j action3
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, ESTABLISHED, NEW,
RELATED or UNTRACKED. For their description, see the
"conntrack" heading 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 module 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 (counting from 0) is the last one that is
scanned and the maximum position of pattern's last
character. If not passed, default is the packet size.
[!] --string pattern
Matches the given pattern.
[!] --hex-string pattern
Matches the given pattern in hex notation.
--icase
Ignore case when searching.
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|'
Note: Since Boyer-Moore (BM) performs searches for matches from
right to left and the kernel may store a packet in multiple
discontiguous blocks, it's possible that a match could be spread
over multiple blocks, in which case this algorithm won't find it.
If you wish to ensure that such thing won't ever happen, use the
Knuth-Pratt-Morris (KMP) algorithm instead. In conclusion,
choose the proper string search algorithm depending on your use-
case.
For example, if you're using the module for filtering, NIDS or
any similar security-focused purpose, then choose KMP. On the
other hand, if you really care about performance — for example,
you're classifying packets to apply Quality of Service (QoS)
policies — and you don't mind about missing possible matches
spread over multiple fragments, then choose BM.
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. 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 outgoing TCP connections will be
unaffected. It is equivalent to --tcp-flags
SYN,RST,ACK,FIN SYN. If the "!" flag precedes the
"--syn", the sense of the option is inverted.
[!] --tcp-option number
Match if TCP option set.
tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP
header. You can only use this on TCP SYN or SYN/ACK packets,
since the MSS is only negotiated during the TCP handshake at
connection startup time.
[!] --mss value[:value]
Match a given TCP MSS value or range. If a range is given,
the second value must be greater than or equal to the
first value.
time
This matches if the packet arrival time/date is within a given
range. All options are optional, but are ANDed when specified.
All times are interpreted as UTC by default.
--datestart YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
--datestop YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
Only match during the given time, which must be in ISO
8601 "T" notation. The possible time range is
1970-01-01T00:00:00 to 2038-01-19T04:17:07.
If --datestart or --datestop are not specified, it will
default to 1970-01-01 and 2038-01-19, respectively.
--timestart hh:mm[:ss]
--timestop hh:mm[:ss]
Only match during the given daytime. The possible time
range is 00:00:00 to 23:59:59. Leading zeroes are allowed
(e.g. "06:03") and correctly interpreted as base-10.
[!] --monthdays day[,day...]
Only match on the given days of the month. Possible values
are 1 to 31. Note that specifying 31 will of course not
match on months which do not have a 31st day; the same
goes for 28- or 29-day February.
[!] --weekdays day[,day...]
Only match on the given weekdays. Possible values are Mon,
Tue, Wed, Thu, Fri, Sat, Sun, or values from 1 to 7,
respectively. You may also use two-character variants (Mo,
Tu, etc.).
--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 kernel timezone, and instead only set the
system time. Even if a particular distribution does set the
timezone at boot, it is usually does not keep the kernel timezone
offset — which is what changes on DST — up to date. ntpd will
not touch the kernel timezone, so running it will not resolve the
issue. As such, one may encounter a timezone that is always
+0000, or one that is wrong half of the time of the year. As
such, using --kerneltz is highly discouraged.
EXAMPLES. To match on weekends, use:
-m time --weekdays Sa,Su
Or, to match (once) on a national holiday block:
-m time --datestart 2007-12-24 --datestop 2007-12-27
Since the stop time is actually inclusive, you would need the
following stop time to not match the first second of the new day:
-m time --datestart 2007-01-01T17:00 --datestop
2007-01-01T23:59:59
During lunch hour:
-m time --timestart 12:30 --timestop 13:30
The fourth Friday in the month:
-m time --weekdays Fr --monthdays 22,23,24,25,26,27,28
(Note that this exploits a certain mathematical property. It is
not possible to say "fourth Thursday OR fourth Friday" in one
rule. It is possible with multiple rules, though.)
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 example 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 logically 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 recognized 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 values. The specification of what to
extract is general enough to find data at given offsets from tcp
headers or payloads.
[!] --u32 tests
The argument amounts to a program in a small language
described below.
tests := location "=" value | tests "&&" location "="
value
value := range | value "," range
range := number | number ":" number
a single number, n, is interpreted the same as n:n. n:m is
interpreted as the range of numbers >=n and <=m.
location := number | location operator number
operator := "&" | "<<" | ">>" | "@"
The operators &, <<, >> and && mean the same as in C. The = is
really a set membership operator and the value syntax describes a
set. The @ operator is what allows moving to the next header and
is described 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 anything beyond the IP header. The last 6 bits of
byte 6 and all of byte 7 are 0 iff this is a complete
packet (not a fragment). 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 --destination-port option of the TCP
extension for details.
iptables can use extended target modules: the following are
included in the standard distribution.
AUDIT
This target creates audit records for packets hitting the target.
It can be used to record accepted, dropped, and rejected packets.
See auditd(8) for additional details.
--type {accept|drop|reject}
Set type of audit record. Starting with linux-4.12, this
option has no effect on generated audit messages anymore.
It is still accepted by iptables for compatibility
reasons, but ignored.
Example:
iptables -N AUDIT_DROP
iptables -A AUDIT_DROP -j AUDIT
iptables -A AUDIT_DROP -j DROP
CHECKSUM
This target selectively works around broken/old applications. It
can only be used in the mangle table.
--checksum-fill
Compute and fill in the checksum in a packet that lacks a
checksum. This is particularly useful, if you need to
work around old applications such as dhcp clients, that do
not work well with checksum offloads, but don't want to
disable checksum offload in your device.
CLASSIFY
This module allows you to set the skb->priority value (and thus
classify the packet into a specific CBQ class).
--set-class major:minor
Set the major and minor class value. The values are always
interpreted as hexadecimal even if no 0x prefix is given.
CLUSTERIP (IPv4-specific)
This module allows you to configure a simple cluster of nodes
that share a certain IP and MAC address without an explicit load
balancer in front of them. Connections are statically
distributed between the nodes in this cluster.
Please note that CLUSTERIP target is considered deprecated in
favour of cluster match which is more flexible and not limited to
IPv4.
--new Create a new ClusterIP. You always have to set this on
the first rule for a given ClusterIP.
--hashmode mode
Specify the hashing mode. Has to be one of sourceip,
sourceip-sourceport, sourceip-sourceport-destport.
--clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer
multicast address
--total-nodes num
Number of total nodes within this cluster.
--local-node num
Local node number within this cluster.
--hash-init rnd
Specify the random seed used for hash initialization.
CONNMARK
This module sets the netfilter mark value associated with a
connection. The mark is 32 bits wide.
--set-xmark value[/mask]
Zero out the bits given by mask and XOR value into the
ctmark.
--save-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the packet mark (nfmark) to the connection mark
(ctmark) using the given masks. The new nfmark value is
determined as follows:
ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask)
i.e. ctmask defines what bits to clear and nfmask what
bits of the nfmark to XOR into the ctmark. ctmask and
nfmask default to 0xFFFFFFFF.
--restore-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the connection mark (ctmark) to the packet mark
(nfmark) using the given masks. The new ctmark value is
determined as follows:
nfmark = (nfmark & ~nfmask) ^ (ctmark & ctmask);
i.e. nfmask defines what bits to clear and ctmask what
bits of the ctmark to XOR into the nfmark. ctmask and
nfmask default to 0xFFFFFFFF.
--restore-mark is only valid in the mangle table.
The following mnemonics are available for --set-xmark:
--and-mark bits
Binary AND the ctmark with bits. (Mnemonic for --set-xmark
0/invbits, where invbits is the binary negation of bits.)
--or-mark bits
Binary OR the ctmark with bits. (Mnemonic for --set-xmark
bits/bits.)
--xor-mark bits
Binary XOR the ctmark with bits. (Mnemonic for --set-xmark
bits/0.)
--set-mark value[/mask]
Set the connection mark. If a mask is specified then only
those bits set in the mask are modified.
--save-mark [--mask mask]
Copy the nfmark to the ctmark. If a mask is specified,
only those bits are copied.
--restore-mark [--mask mask]
Copy the ctmark to the nfmark. If a mask is specified,
only those bits are copied. This is only valid in the
mangle table.
CONNSECMARK
This module copies security markings from packets to connections
(if unlabeled), and from connections back to packets (also only
if unlabeled). Typically used in conjunction with SECMARK, it is
valid in the security table (for backwards compatibility with
older kernels, it is also valid in the mangle table).
--save If the packet has a security marking, copy it to the
connection if the connection is not marked.
--restore
If the packet does not have a security marking, and the
connection does, copy the security marking from the
connection to the packet.
CT
The CT target sets parameters for a packet or its associated
connection. The target attaches a "template" connection tracking
entry to the packet, which is then used by the conntrack core
when initializing a new ct entry. This target is thus only valid
in the "raw" table.
--notrack
Disables connection tracking for this packet.
--helper name
Use the helper identified by name for the connection. This
is more flexible than loading the conntrack helper modules
with preset ports.
--ctevents event[,...]
Only generate the specified conntrack events for this
connection. Possible event types are: new, related,
destroy, reply, assured, protoinfo, helper, mark (this
refers to the ctmark, not nfmark), natseqinfo, secmark
(ctsecmark).
--expevents event[,...]
Only generate the specified expectation events for this
connection. Possible event types are: new.
--zone-orig {id|mark}
For traffic coming from ORIGINAL direction, assign this
packet to zone id and only have lookups done in that zone.
If mark is used instead of id, the zone is derived from
the packet nfmark.
--zone-reply {id|mark}
For traffic coming from REPLY direction, assign this
packet to zone id and only have lookups done in that zone.
If mark is used instead of id, the zone is derived from
the packet nfmark.
--zone {id|mark}
Assign this packet to zone id and only have lookups done
in that zone. If mark is used instead of id, the zone is
derived from the packet nfmark. By default, packets have
zone 0. This option applies to both directions.
--timeout name
Use the timeout policy identified by name for the
connection. This is provides more flexible timeout policy
definition than global timeout values available at
/proc/sys/net/netfilter/nf_conntrack_*_timeout_*.
DNAT
This target is only valid in the nat table, in the PREROUTING and
OUTPUT chains, and user-defined chains which are only called from
those chains. It specifies that the destination address of the
packet should be modified (and all future packets in this
connection will also be mangled), and rules should cease being
examined. It takes the following options:
--to-destination [ipaddr[-ipaddr]][:port[-port[/baseport]]]
which can specify a single new destination IP address, an
inclusive range of IP addresses. Optionally a port range,
if the rule also specifies one of the following protocols:
tcp, udp, dccp or sctp. If no port range is specified,
then the destination port will never be modified. If no IP
address is specified then only the destination port will
be modified. If baseport is given, the difference of the
original destination port and its value is used as offset
into the mapping port range. This allows one to create
shifted portmap ranges and is available since kernel
version 4.18. For a single port or baseport, a service
name as listed in /etc/services may be used.
--random
Randomize source port mapping (kernel >= 2.6.22).
--persistent
Gives a client the same source-/destination-address for
each connection. This supersedes the SAME target. Support
for persistent mappings is available from 2.6.29-rc2.
IPv6 support available since Linux kernels >= 3.7.
DNPT (IPv6-specific)
Provides stateless destination IPv6-to-IPv6 Network Prefix
Translation (as described by RFC 6296).
You have to use this target in the mangle table, not in the nat
table. It takes the following options:
--src-pfx [prefix/length]
Set source prefix that you want to translate and length
--dst-pfx [prefix/length]
Set destination prefix that you want to use in the
translation and length
You have to use the SNPT target to undo the translation. Example:
ip6tables -t mangle -I POSTROUTING -s fd00::/64 ! -o
vboxnet0 -j SNPT --src-pfx fd00::/64 --dst-pfx
2001:e20:2000:40f::/64
ip6tables -t mangle -I PREROUTING -i wlan0 -d
2001:e20:2000:40f::/64 -j DNPT --src-pfx
2001:e20:2000:40f::/64 --dst-pfx fd00::/64
You may need to enable IPv6 neighbor proxy:
sysctl -w net.ipv6.conf.all.proxy_ndp=1
You also have to use the NOTRACK target to disable connection
tracking for translated flows.
DSCP
This target alters the value of the DSCP bits within the TOS
header of the IPv4 packet. As this manipulates a packet, it can
only be used in the mangle table.
--set-dscp value
Set the DSCP field to a numerical value (can be decimal or
hex)
--set-dscp-class class
Set the DSCP field to a DiffServ class.
ECN (IPv4-specific)
This target selectively works around known ECN blackholes. It
can only be used in the mangle table.
--ecn-tcp-remove
Remove all ECN bits from the TCP header. Of course, it
can only be used in conjunction with -p tcp.
HL (IPv6-specific)
This is used to modify the Hop Limit field in IPv6 header. The
Hop Limit field is similar to what is known as TTL value in IPv4.
Setting or incrementing the Hop Limit field can potentially be
very dangerous, so it should be avoided at any cost. This target
is only valid in mangle table.
Don't ever set or increment the value on packets that leave your
local network!
--hl-set value
Set the Hop Limit to `value'.
--hl-dec value
Decrement the Hop Limit `value' times.
--hl-inc value
Increment the Hop Limit `value' times.
HMARK
Like MARK, i.e. set the fwmark, but the mark is calculated from
hashing packet selector at choice. You have also to specify the
mark range and, optionally, the offset to start from. ICMP error
messages are inspected and used to calculate the hashing.
Existing options are:
--hmark-tuple tuple
Possible tuple members are: src meaning source address
(IPv4, IPv6 address), dst meaning destination address
(IPv4, IPv6 address), sport meaning source port (TCP, UDP,
UDPlite, SCTP, DCCP), dport meaning destination port (TCP,
UDP, UDPlite, SCTP, DCCP), spi meaning Security Parameter
Index (AH, ESP), and ct meaning the usage of the conntrack
tuple instead of the packet selectors.
--hmark-mod value (must be > 0)
Modulus for hash calculation (to limit the range of
possible marks)
--hmark-offset value
Offset to start marks from.
For advanced usage, instead of using --hmark-tuple, you can
specify custom
prefixes and masks:
--hmark-src-prefix cidr
The source address mask in CIDR notation.
--hmark-dst-prefix cidr
The destination address mask in CIDR notation.
--hmark-sport-mask value
A 16 bit source port mask in hexadecimal.
--hmark-dport-mask value
A 16 bit destination port mask in hexadecimal.
--hmark-spi-mask value
A 32 bit field with spi mask.
--hmark-proto-mask value
An 8 bit field with layer 4 protocol number.
--hmark-rnd value
A 32 bit random custom value to feed hash calculation.
Examples:
iptables -t mangle -A PREROUTING -m conntrack --ctstate NEW
-j HMARK --hmark-tuple ct,src,dst,proto --hmark-offset 10000
--hmark-mod 10 --hmark-rnd 0xfeedcafe
iptables -t mangle -A PREROUTING -j HMARK --hmark-offset 10000
--hmark-tuple src,dst,proto --hmark-mod 10 --hmark-rnd 0xdeafbeef
IDLETIMER
This target can be used to identify when interfaces have been
idle for a certain period of time. Timers are identified by
labels and are created when a rule is set with a new label. The
rules also take a timeout value (in seconds) as an option. If
more than one rule uses the same timer label, the timer will be
restarted whenever any of the rules get a hit. One entry for
each timer is created in sysfs. This attribute contains the
timer remaining for the timer to expire. The attributes are
located under the xt_idletimer class:
/sys/class/xt_idletimer/timers/<label>
When the timer expires, the target module sends a sysfs
notification to the userspace, which can then decide what to do
(eg. disconnect to save power).
--timeout amount
This is the time in seconds that will trigger the
notification.
--label string
This is a unique identifier for the timer. The maximum
length for the label string is 27 characters.
LED
This creates an LED-trigger that can then be attached to system
indicator lights, to blink or illuminate them when certain
packets pass through the system. One example might be to light up
an LED for a few minutes every time an SSH connection is made to
the local machine. The following options control the trigger
behavior:
--led-trigger-id name
This is the name given to the LED trigger. The actual name
of the trigger will be prefixed with "netfilter-".
--led-delay ms
This indicates how long (in milliseconds) the LED should
be left illuminated when a packet arrives before being
switched off again. The default is 0 (blink as fast as
possible.) The special value inf can be given to leave the
LED on permanently once activated. (In this case the
trigger will need to be manually detached and reattached
to the LED device to switch it off again.)
--led-always-blink
Always make the LED blink on packet arrival, even if the
LED is already on. This allows notification of new
packets even with long delay values (which otherwise would
result in a silent prolonging of the delay time.)
Example:
Create an LED trigger for incoming SSH traffic:
iptables -A INPUT -p tcp --dport 22 -j LED
--led-trigger-id ssh
Then attach the new trigger to an LED:
echo netfilter-ssh >/sys/class/leds/ledname/trigger
LOG
Turn on kernel logging of matching packets. When this option is
set for a rule, the Linux kernel will print some information on
all matching packets (like most IP/IPv6 header fields) via the
kernel log (where it can be read with dmesg(1) or read in the
syslog).
This is a "non-terminating target", i.e. rule traversal continues
at the next rule. So if you want to LOG the packets you refuse,
use two separate rules with the same matching criteria, first
using target LOG then DROP (or REJECT).
--log-level level
Level of logging, which can be (system-specific) numeric
or a mnemonic. Possible values are (in decreasing order
of priority): emerg, alert, crit, error, warning, notice,
info or debug.
--log-prefix prefix
Prefix log messages with the specified prefix; up to 29
letters long, and useful for distinguishing messages in
the logs.
--log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the
log is readable by users.
--log-tcp-options
Log options from the TCP packet header.
--log-ip-options
Log options from the IP/IPv6 packet header.
--log-uid
Log the userid of the process which generated the packet.
--log-macdecode
Log MAC addresses and protocol.
MARK
This target is used to set the Netfilter mark value associated
with the packet. It can, for example, be used in conjunction
with routing based on fwmark (needs iproute2). If you plan on
doing so, note that the mark needs to be set in either the
PREROUTING or the OUTPUT chain of the mangle table to affect
routing. The mark field is 32 bits wide.
--set-xmark value[/mask]
Zeroes out the bits given by mask and XORs value into the
packet mark ("nfmark"). If mask is omitted, 0xFFFFFFFF is
assumed.
--set-mark value[/mask]
Zeroes out the bits given by mask and ORs value into the
packet mark. If mask is omitted, 0xFFFFFFFF is assumed.
The following mnemonics are available:
--and-mark bits
Binary AND the nfmark with bits. (Mnemonic for --set-xmark
0/invbits, where invbits is the binary negation of bits.)
--or-mark bits
Binary OR the nfmark with bits. (Mnemonic for --set-xmark
bits/bits.)
--xor-mark bits
Binary XOR the nfmark with bits. (Mnemonic for --set-xmark
bits/0.)
MASQUERADE
This target is only valid in the nat table, in the POSTROUTING
chain. It should only be used with dynamically assigned IP
(dialup) connections: if you have a static IP address, you should
use the SNAT target. Masquerading is equivalent to specifying a
mapping to the IP address of the interface the packet is going
out, but also has the effect that connections are forgotten when
the interface goes down. This is the correct behavior when the
next dialup is unlikely to have the same interface address (and
hence any established connections are lost anyway).
--to-ports port[-port]
This specifies a range of source ports to use, overriding
the default SNAT source port selection heuristics (see
above). This is only valid if the rule also specifies one
of the following protocols: tcp, udp, dccp or sctp.
--random
Randomize source port mapping (kernel >= 2.6.21). Since
kernel 5.0, --random is identical to --random-fully.
--random-fully
Fully randomize source port mapping (kernel >= 3.13).
IPv6 support available since Linux kernels >= 3.7.
NETMAP
This target allows you to statically map a whole network of
addresses onto another network of addresses. It can only be used
from rules in the nat table.
--to address[/mask]
Network address to map to. The resulting address will be
constructed in the following way: All 'one' bits in the
mask are filled in from the new `address'. All bits that
are zero in the mask are filled in from the original
address.
IPv6 support available since Linux kernels >= 3.7.
NFLOG
This target provides logging of matching packets. When this
target is set for a rule, the Linux kernel will pass the packet
to the loaded logging backend to log the packet. This is usually
used in combination with nfnetlink_log as logging backend, which
will multicast the packet through a netlink socket to the
specified multicast group. One or more userspace processes may
subscribe to the group to receive the packets. Like LOG, this is
a non-terminating target, i.e. rule traversal continues at the
next rule.
--nflog-group nlgroup
The netlink group (0–2^16-1) to which packets are (only
applicable for nfnetlink_log). The default value is 0.
--nflog-prefix prefix
A prefix string to include in the log message, up to 64
characters long, useful for distinguishing messages in the
logs.
--nflog-range size
This option has never worked, use --nflog-size instead
--nflog-size size
The number of bytes to be copied to userspace (only
applicable for nfnetlink_log). nfnetlink_log instances may
specify their own range, this option overrides it.
--nflog-threshold size
Number of packets to queue inside the kernel before
sending them to userspace (only applicable for
nfnetlink_log). Higher values result in less overhead per
packet, but increase delay until the packets reach
userspace. The default value is 1.
NFQUEUE
This target passes the packet to userspace using the
nfnetlink_queue handler. The packet is put into the queue
identified by its 16-bit queue number. Userspace can inspect and
modify the packet if desired. Userspace must then drop or
reinject the packet into the kernel. Please see
libnetfilter_queue for details. nfnetlink_queue was added in
Linux 2.6.14. The queue-balance option was added in Linux 2.6.31,
queue-bypass in 2.6.39.
--queue-num value
This specifies the QUEUE number to use. Valid queue
numbers are 0 to 65535. The default value is 0.
--queue-balance value:value
This specifies a range of queues to use. Packets are then
balanced across the given queues. This is useful for
multicore systems: start multiple instances of the
userspace program on queues x, x+1, .. x+n and use
"--queue-balance x:x+n". Packets belonging to the same
connection are put into the same nfqueue. Due to
implementation details, a lower range value of 0 limits
the higher range value to 65534, i.e. one can only balance
between at most 65535 queues.
--queue-bypass
By default, if no userspace program is listening on an
NFQUEUE, then all packets that are to be queued are
dropped. When this option is used, the NFQUEUE rule
behaves like ACCEPT instead, and the packet will move on
to the next table.
--queue-cpu-fanout
Available starting Linux kernel 3.10. When used together
with --queue-balance this will use the CPU ID as an index
to map packets to the queues. The idea is that you can
improve performance if there's a queue per CPU. This
requires --queue-balance to be specified.
NOTRACK
This extension disables connection tracking for all packets
matching that rule. It is equivalent with -j CT --notrack. Like
CT, NOTRACK can only be used in the raw table.
RATEEST
The RATEEST target collects statistics, performs rate estimation
calculation and saves the results for later evaluation using the
rateest match.
--rateest-name name
Count matched packets into the pool referred to by name,
which is freely choosable.
--rateest-interval amount{s|ms|us}
Rate measurement interval, in seconds, milliseconds or
microseconds.
--rateest-ewmalog value
Rate measurement averaging time constant.
REDIRECT
This target is only valid in the nat table, in the PREROUTING and
OUTPUT chains, and user-defined chains which are only called from
those chains. It redirects the packet to the machine itself by
changing the destination IP to the primary address of the
incoming interface (locally-generated packets are mapped to the
localhost address, 127.0.0.1 for IPv4 and ::1 for IPv6, and
packets arriving on interfaces that don't have an IP address
configured are dropped).
--to-ports port[-port]
This specifies a destination port or range of ports to
use: without this, the destination port is never altered.
This is only valid if the rule also specifies one of the
following protocols: tcp, udp, dccp or sctp. For a single
port, a service name as listed in /etc/services may be
used.
--random
Randomize source port mapping (kernel >= 2.6.22).
IPv6 support available starting Linux kernels >= 3.7.
REJECT (IPv6-specific)
This is used to send back an error packet in response to the
matched packet: otherwise it is equivalent to DROP so it is a
terminating TARGET, ending rule traversal. This target is only
valid in the INPUT, FORWARD and OUTPUT chains, and user-defined
chains which are only called from those chains. The following
option controls the nature of the error packet returned:
--reject-with type
The type given can be icmp6-no-route, no-route,
icmp6-adm-prohibited, adm-prohibited,
icmp6-addr-unreachable, addr-unreach, or
icmp6-port-unreachable, which return the appropriate
ICMPv6 error message (icmp6-port-unreachable is the
default). Finally, the option tcp-reset can be used on
rules which only match the TCP protocol: this causes a TCP
RST packet to be sent back. This is mainly useful for
blocking ident (113/tcp) probes which frequently occur
when sending mail to broken mail hosts (which won't accept
your mail otherwise). tcp-reset can only be used with
kernel versions 2.6.14 or later.
Warning: You should not indiscriminately apply the REJECT target
to packets whose connection state is classified as INVALID;
instead, you should only DROP these.
Consider a source host transmitting a packet P, with P
experiencing so much delay along its path that the source host
issues a retransmission, P_2, with P_2 being successful in
reaching its destination and advancing the connection state
normally. It is conceivable that the late-arriving P may be
considered not to be associated with any connection tracking
entry. Generating a reject response for a packet so classed would
then terminate the healthy connection.
So, instead of:
-A INPUT ... -j REJECT
do consider using:
-A INPUT ... -m conntrack --ctstate INVALID -j DROP
-A INPUT ... -j REJECT
REJECT (IPv4-specific)
This is used to send back an error packet in response to the
matched packet: otherwise it is equivalent to DROP so it is a
terminating TARGET, ending rule traversal. This target is only
valid in the INPUT, FORWARD and OUTPUT chains, and user-defined
chains which are only called from those chains. The following
option controls the nature of the error packet returned:
--reject-with type
The type given can be icmp-net-unreachable,
icmp-host-unreachable, icmp-port-unreachable,
icmp-proto-unreachable, icmp-net-prohibited,
icmp-host-prohibited, or icmp-admin-prohibited (*), which
return the appropriate ICMP error message
(icmp-port-unreachable is the default). The option
tcp-reset can be used on rules which only match the TCP
protocol: this causes a TCP RST packet to be sent back.
This is mainly useful for blocking ident (113/tcp) probes
which frequently occur when sending mail to broken mail
hosts (which won't accept your mail otherwise).
(*) Using icmp-admin-prohibited with kernels that do not
support it will result in a plain DROP instead of REJECT
Warning: You should not indiscriminately apply the REJECT target
to packets whose connection state is classified as INVALID;
instead, you should only DROP these.
Consider a source host transmitting a packet P, with P
experiencing so much delay along its path that the source host
issues a retransmission, P_2, with P_2 being successful in
reaching its destination and advancing the connection state
normally. It is conceivable that the late-arriving P may be
considered not to be associated with any connection tracking
entry. Generating a reject response for a packet so classed would
then terminate the healthy connection.
So, instead of:
-A INPUT ... -j REJECT
do consider using:
-A INPUT ... -m conntrack --ctstate INVALID -j DROP
-A INPUT ... -j REJECT
SECMARK
This is used to set the security mark value associated with the
packet for use by security subsystems such as SELinux. It is
valid in the security table (for backwards compatibility with
older kernels, it is also valid in the mangle table). The mark is
32 bits wide.
--selctx security_context
SET
This module adds and/or deletes entries from IP sets which can be
defined by ipset(8).
--add-set setname flag[,flag...]
add the address(es)/port(s) of the packet to the set
--del-set setname flag[,flag...]
delete the address(es)/port(s) of the packet from the set
--map-set setname flag[,flag...]
[--map-mark] [--map-prio] [--map-queue] map packet
properties (firewall mark, tc priority, hardware queue)
where flag(s) are src and/or dst specifications and there
can be no more than six of them.
--timeout value
when adding an entry, the timeout value to use instead of
the default one from the set definition
--exist
when adding an entry if it already exists, reset the
timeout value to the specified one or to the default from
the set definition
--map-set set-name
the set-name should be created with --skbinfo option
--map-mark map firewall mark to packet by lookup of value
in the set --map-prio map traffic control priority to
packet by lookup of value in the set --map-queue map
hardware NIC queue to packet by lookup of value in the set
The --map-set option can be used from the mangle table
only. The --map-prio and --map-queue flags can be used in
the OUTPUT, FORWARD and POSTROUTING chains.
Use of -j SET requires that ipset kernel support is provided,
which, for standard kernels, is the case since Linux 2.6.39.
SNAT
This target is only valid in the nat table, in the POSTROUTING
and INPUT chains, and user-defined chains which are only called
from those chains. It specifies that the source address of the
packet should be modified (and all future packets in this
connection will also be mangled), and rules should cease being
examined. It takes the following options:
--to-source [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new source IP address, an
inclusive range of IP addresses. Optionally a port range,
if the rule also specifies one of the following protocols:
tcp, udp, dccp or sctp. If no port range is specified,
then source ports below 512 will be mapped to other ports
below 512: those between 512 and 1023 inclusive will be
mapped to ports below 1024, and other ports will be mapped
to 1024 or above. Where possible, no port alteration will
occur.
--random
Randomize source port mapping through a hash-based
algorithm (kernel >= 2.6.21).
--random-fully
Fully randomize source port mapping through a PRNG (kernel
>= 3.14).
--persistent
Gives a client the same source-/destination-address for
each connection. This supersedes the SAME target. Support
for persistent mappings is available from 2.6.29-rc2.
Kernels prior to 2.6.36-rc1 don't have the ability to SNAT in the
INPUT chain.
IPv6 support available since Linux kernels >= 3.7.
SNPT (IPv6-specific)
Provides stateless source IPv6-to-IPv6 Network Prefix Translation
(as described by RFC 6296).
You have to use this target in the mangle table, not in the nat
table. It takes the following options:
--src-pfx [prefix/length]
Set source prefix that you want to translate and length
--dst-pfx [prefix/length]
Set destination prefix that you want to use in the
translation and length
You have to use the DNPT target to undo the translation. Example:
ip6tables -t mangle -I POSTROUTING -s fd00::/64 ! -o
vboxnet0 -j SNPT --src-pfx fd00::/64 --dst-pfx
2001:e20:2000:40f::/64
ip6tables -t mangle -I PREROUTING -i wlan0 -d
2001:e20:2000:40f::/64 -j DNPT --src-pfx
2001:e20:2000:40f::/64 --dst-pfx fd00::/64
You may need to enable IPv6 neighbor proxy:
sysctl -w net.ipv6.conf.all.proxy_ndp=1
You also have to use the NOTRACK target to disable connection
tracking for translated flows.
SYNPROXY
This target will process TCP three-way-handshake parallel in
netfilter context to protect either local or backend system. This
target requires connection tracking because sequence numbers need
to be translated. The kernels ability to absorb SYNFLOOD was
greatly improved starting with Linux 4.4, so this target should
not be needed anymore to protect Linux servers.
--mss maximum segment size
Maximum segment size announced to clients. This must match
the backend.
--wscale window scale
Window scale announced to clients. This must match the
backend.
--sack-perm
Pass client selective acknowledgement option to backend
(will be disabled if not present).
--timestamps
Pass client timestamp option to backend (will be disabled
if not present, also needed for selective acknowledgement
and window scaling).
Example:
Determine tcp options used by backend, from an external system
tcpdump -pni eth0 -c 1 'tcp[tcpflags] ==
(tcp-syn|tcp-ack)'
port 80 &
telnet 192.0.2.42 80
18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
Flags [S.], seq 360414582, ack 788841994, win 14480,
options [mss 1460,sackOK,
TS val 1409056151 ecr 9690221,
nop,wscale 9],
length 0
Switch tcp_loose mode off, so conntrack will mark out-of-flow
packets as state INVALID.
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
Make SYN packets untracked
iptables -t raw -A PREROUTING -i eth0 -p tcp --dport 80
--syn -j CT --notrack
Catch UNTRACKED (SYN packets) and INVALID (3WHS ACK packets)
states and send them to SYNPROXY. This rule will respond to SYN
packets with SYN+ACK syncookies, create ESTABLISHED for valid
client response (3WHS ACK packets) and drop incorrect cookies.
Flags combinations not expected during 3WHS will not match and
continue (e.g. SYN+FIN, SYN+ACK).
iptables -A INPUT -i eth0 -p tcp --dport 80
-m state --state UNTRACKED,INVALID -j SYNPROXY
--sack-perm --timestamp --mss 1460 --wscale 9
Drop invalid packets, this will be out-of-flow packets that were
not matched by SYNPROXY.
iptables -A INPUT -i eth0 -p tcp --dport 80 -m state
--state INVALID -j DROP
TCPMSS
This target alters the MSS value of TCP SYN packets, to control
the maximum size for that connection (usually limiting it to your
outgoing interface's MTU minus 40 for IPv4 or 60 for IPv6,
respectively). Of course, it can only be used in conjunction
with -p tcp.
This target is used to overcome criminally braindead ISPs or
servers which block "ICMP Fragmentation Needed" or "ICMPv6 Packet
Too Big" packets. The symptoms of this problem are that
everything works fine from your Linux firewall/router, but
machines behind it can never exchange large packets:
1. Web browsers connect, then hang with no data received.
2. Small mail works fine, but large emails hang.
3. ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall
configuration like:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST
SYN
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value
Explicitly sets MSS option to specified value. If the MSS
of the packet is already lower than value, it will not be
increased (from Linux 2.6.25 onwards) to avoid more
problems with hosts relying on a proper MSS.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40 for IPv4;
-60 for IPv6). This may not function as desired where
asymmetric routes with differing path MTU exist — the
kernel uses the path MTU which it would use to send
packets from itself to the source and destination IP
addresses. Prior to Linux 2.6.25, only the path MTU to the
destination IP address was considered by this option;
subsequent kernels also consider the path MTU to the
source IP address.
These options are mutually exclusive.
TCPOPTSTRIP
This target will strip TCP options off a TCP packet. (It will
actually replace them by NO-OPs.) As such, you will need to add
the -p tcp parameters.
--strip-options option[,option...]
Strip the given option(s). The options may be specified by
TCP option number or by symbolic name. The list of
recognized options can be obtained by calling iptables
with -j TCPOPTSTRIP -h.
TEE
The TEE target will clone a packet and redirect this clone to
another machine on the local network segment. In other words, the
nexthop must be the target, or you will have to configure the
nexthop to forward it further if so desired.
--gateway ipaddr
Send the cloned packet to the host reachable at the given
IP address. Use of 0.0.0.0 (for IPv4 packets) or ::
(IPv6) is invalid.
To forward all incoming traffic on eth0 to an Network Layer
logging box:
-t mangle -A PREROUTING -i eth0 -j TEE --gateway 2001:db8::1
TOS
This module sets the Type of Service field in the IPv4 header
(including the "precedence" bits) or the Priority field in the
IPv6 header. Note that TOS shares the same bits as DSCP and ECN.
The TOS target is only valid in the mangle table.
--set-tos value[/mask]
Zeroes out the bits given by mask (see NOTE below) and
XORs value into the TOS/Priority field. If mask is
omitted, 0xFF is assumed.
--set-tos symbol
You can specify a symbolic name when using the TOS target
for IPv4. It implies a mask of 0xFF (see NOTE below). The
list of recognized TOS names can be obtained by calling
iptables with -j TOS -h.
The following mnemonics are available:
--and-tos bits
Binary AND the TOS value with bits. (Mnemonic for
--set-tos 0/invbits, where invbits is the binary negation
of bits. See NOTE below.)
--or-tos bits
Binary OR the TOS value with bits. (Mnemonic for --set-tos
bits/bits. See NOTE below.)
--xor-tos bits
Binary XOR the TOS value with bits. (Mnemonic for
--set-tos bits/0. See NOTE below.)
NOTE: In Linux kernels up to and including 2.6.38, with the
exception of longterm releases 2.6.32 (>=.42), 2.6.33 (>=.15),
and 2.6.35 (>=.14), there is a bug whereby IPv6 TOS mangling does
not behave as documented and differs from the IPv4 version. The
TOS mask indicates the bits one wants to zero out, so it needs to
be inverted before applying it to the original TOS field.
However, the aforementioned kernels forgo the inversion which
breaks --set-tos and its mnemonics.
TPROXY
This target is only valid in the mangle table, in the PREROUTING
chain and user-defined chains which are only called from this
chain. It redirects the packet to a local socket without changing
the packet header in any way. It can also change the mark value
which can then be used in advanced routing rules. It takes three
options:
--on-port port
This specifies a destination port to use. It is a required
option, 0 means the new destination port is the same as
the original. This is only valid if the rule also
specifies -p tcp or -p udp.
--on-ip address
This specifies a destination address to use. By default
the address is the IP address of the incoming interface.
This is only valid if the rule also specifies -p tcp or -p
udp.
--tproxy-mark value[/mask]
Marks packets with the given value/mask. The fwmark value
set here can be used by advanced routing. (Required for
transparent proxying to work: otherwise these packets will
get forwarded, which is probably not what you want.)
TRACE
This target marks packets so that the kernel will log every rule
which match the packets as those traverse the tables, chains,
rules. It can only be used in the raw table.
With iptables-legacy, a logging backend, such as ip(6)t_LOG or
nfnetlink_log, must be loaded for this to be visible. The
packets are logged with the string prefix: "TRACE:
tablename:chainname:type:rulenum " where type can be "rule" for
plain rule, "return" for implicit rule at the end of a user
defined chain and "policy" for the policy of the built in chains.
With iptables-nft, the target is translated into nftables' meta
nftrace expression. Hence the kernel sends trace events via
netlink to userspace where they may be displayed using
xtables-monitor --trace command. For details, refer to
xtables-monitor(8).
TTL (IPv4-specific)
This is used to modify the IPv4 TTL header field. The TTL field
determines how many hops (routers) a packet can traverse until
it's time to live is exceeded.
Setting or incrementing the TTL field can potentially be very
dangerous, so it should be avoided at any cost. This target is
only valid in mangle table.
Don't ever set or increment the value on packets that leave your
local network!
--ttl-set value
Set the TTL value to `value'.
--ttl-dec value
Decrement the TTL value `value' times.
--ttl-inc value
Increment the TTL value `value' times.
ULOG (IPv4-specific)
This is the deprecated IPv4-only predecessor of the NFLOG target.
It provides userspace logging of matching packets. When this
target is set for a rule, the Linux kernel will multicast this
packet through a netlink socket. One or more userspace processes
may then subscribe to various multicast groups and receive the
packets. Like LOG, this is a "non-terminating target", i.e. rule
traversal continues at the next rule.
--ulog-nlgroup nlgroup
This specifies the netlink group (1–32) to which the
packet is sent. Default value is 1.
--ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32
characters long, and useful for distinguishing messages in
the logs.
--ulog-cprange size
Number of bytes to be copied to userspace. A value of 0
always copies the entire packet, regardless of its size.
Default is 0.
--ulog-qthreshold size
Number of packet to queue inside kernel. Setting this
value to, e.g. 10 accumulates ten packets inside the
kernel and transmits them as one netlink multipart message
to userspace. Default is 1 (for backwards compatibility).
This page is part of the iptables (administer and maintain packet
filter rules) project. Information about the project can be
found at ⟨http://www.netfilter.org/⟩. If you have a bug report
for this manual page, see ⟨http://bugzilla.netfilter.org/⟩. This
page was obtained from the project's upstream Git repository
⟨git://git.netfilter.org/iptables⟩ on 2024-06-14. (At that time,
the date of the most recent commit that was found in the
repository was 2024-06-12.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
iptables 1.8.10 iptables-extensions(8)
Pages that refer to this page: proc_pid_net(5), iptables(8), tc-fw(8)
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