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 addresstype. 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-typetype
Matches if the source address is of given type
[!] --dst-typetype
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.
[!] --ahspispi[:spi]
Matches SPI.
[!] --ahlenlength
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.
[!] --ahspispi[:spi]
bpf
Match using Linux Socket Filter. Expects a path to an eBPF object or a cBPF program in decimal format.
--object-pinnedpath
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
--bytecodecode
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
[!] --pathpath
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.
[!] --cgroupclassid
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-nodesnum
Set number of total nodes in cluster.
[!] --cluster-local-nodenum
Set the local node number ID.
[!] --cluster-local-nodemaskmask
Set the local node number ID mask. You can use this option instead of --cluster-local-node.
--cluster-hash-seedvalue
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_loosecomment
Allows you to add comments (up to 256 characters) to any rule.
--commentcomment
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.
[!] --connbytesfrom[: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 ...
connlabel
Module matches or adds connlabels to a connection. connlabels are similar to connmarks, except labels
are bit-based; i.e. all labels may be attached to a flow at the same time. Up to 128 unique labels are
currently supported.
[!] --labelname
matches if label name has been set on a connection. Instead of a name (which will be translated
to a number, see EXAMPLE below), a number may be used instead. Using a number always overrides
connlabel.conf.
--set if the label has not been set on the connection, set it. Note that setting a label can fail.
This is because the kernel allocates the conntrack label storage area when the connection is
created, and it only reserves the amount of memory required by the ruleset that exists at the time
the connection is created. In this case, the match will fail (or succeed, in case --label option
was negated).
This match depends on libnetfilter_conntrack 1.0.4 or later. Label translation is done via the
/etc/xtables/connlabel.conf configuration file.
Example:
0 eth0-in
1 eth0-out
2 ppp-in
3 ppp-out
4 bulk-traffic
5 interactive
connlimit
Allows you to restrict the number of parallel connections to a server per client IP address (or client
address block).
--connlimit-upton
Match if the number of existing connections is below or equal n.
--connlimit-aboven
Match if the number of existing connections is above n.
--connlimit-maskprefix_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).
[!] --markvalue[/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.
[!] --ctstatestateliststatelist is a comma separated list of the connection states to match. Possible states are listed
below.
[!] --ctprotol4proto
Layer-4 protocol to match (by number or name)
[!] --ctorigsrcaddress[/mask]
[!] --ctorigdstaddress[/mask]
[!] --ctreplsrcaddress[/mask]
[!] --ctrepldstaddress[/mask]
Match against original/reply source/destination address
[!] --ctorigsrcportport[:port]
[!] --ctorigdstportport[:port]
[!] --ctreplsrcportport[:port]
[!] --ctrepldstportport[: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.
[!] --ctstatusstateliststatuslist is a comma separated list of the connection statuses to match. Possible statuses are
listed below.
[!] --ctexpiretime[: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
[!] --cpunumber
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,--sportport[:port]
[!] --destination-port,--dportport[:port]
[!] --dccp-typesmask
Match when the DCCP packet type is one of 'mask'. 'mask' is a comma-separated list of packet
types. Packet types are: REQUESTRESPONSEDATAACKDATAACKCLOSEREQCLOSERESETSYNCSYNCACKINVALID.
[!] --dccp-optionnumber
Match if DCCP option set.
devgroup
Match device group of a packet's incoming/outgoing interface.
[!] --src-groupname
Match device group of incoming device
[!] --dst-groupname
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.
[!] --dscpvalue
Match against a numeric (decimal or hex) value in the range 0–63.
[!] --dscp-classclass
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-lenlength
Total length of this header in octets.
--dst-optstype[: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-ectnum
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.
[!] --espspispi[: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.
[!] --fragidid[:id]
Matches the given Identification or range of it.
[!] --fraglenlength
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.
hashlimithashlimit 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-uptoamount[/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-aboveamount[/second|/minute|/hour|/day]
Match if the rate is above amount/quantum.
--hashlimit-burstamount
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-srcmaskprefix
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-dstmaskprefix
Like --hashlimit-srcmask, but for destination addresses.
--hashlimit-namefoo
The name for the /proc/net/ipt_hashlimit/foo entry.
--hashlimit-htable-sizebuckets
The number of buckets of the hash table
--hashlimit-htable-maxentries
Maximum entries in the hash.
--hashlimit-htable-expiremsec
After how many milliseconds do hash entries expire.
--hashlimit-htable-gcintervalmsec
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-intervalsec
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-lenlength
Total length of this header in octets.
--hbh-optstype[: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.
[!] --helperstring
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-eqvalue
Matches if Hop Limit equals value.
--hl-ltvalue
Matches if Hop Limit is less than value.
--hl-gtvalue
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-typetype[/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-rangefrom[-to]
Match source IP in the specified range.
[!] --dst-rangefrom[-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.
[!] --headerheader[,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)
[!] --vprotoprotocol
VIP protocol to match; by number or name, e.g. "tcp"
[!] --vaddraddress[/mask]
VIP address to match
[!] --vportport
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
[!] --vportctlport
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.
[!] --lengthlength[: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.
--limitrate[/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-burstnumber
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-sourceaddress
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).
[!] --markvalue[/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-typetype[: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,--sportsport[,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,--dportsport[,port|,port:port]...
Match if the destination port is one of the given ports. The flag --dports is a convenient alias
for this option.
[!] --portsport[,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-namename
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.
[!] --genrestring
Match an operating system genre by using a passive fingerprinting.
--ttllevel
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.
--loglevel
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-ownerusername
[!] --uid-owneruserid[-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-ownergroupname
[!] --gid-ownergroupid[-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-inname
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-outname
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,INPUTandFORWARD chains, out is valid in the
POSTROUTING,OUTPUTandFORWARD chains.
--pol {none|ipsec}
Matches if the packet is subject to IPsec processing. --polnone 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.
[!] --reqidid
Matches the reqid of the policy rule. The reqid can be specified with setkey(8) using unique:id as
level.
[!] --spispi
Matches the SPI of the SA.
[!] --proto {ah|esp|ipcomp}
Matches the encapsulation protocol.
[!] --mode {tunnel|transport}
Matches the encapsulation mode.
[!] --tunnel-srcaddr[/mask]
Matches the source end-point address of a tunnel mode SA. Only valid with --modetunnel.
[!] --tunnel-dstaddr[/mask]
Matches the destination end-point address of a tunnel mode SA. Only valid with --modetunnel.
--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).
[!] --quotabytes
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:
• rateestoperatorrateest-bps
• rateestoperatorrateest-pps
Absolute + Delta:
• (rateest minus rateest-bps1) operatorrateest-bps2
• (rateest minus rateest-pps1) operatorrateest-pps2
Relative:
• rateest1operatorrateest2rateest-bps(without rate!)
• rateest1operatorrateest2rateest-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.
--rateestname
Name of the one rate estimator for absolute mode.
--rateest1name--rateest2name
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.
[!] --realmvalue[/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.
--namename
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.
--masknetmask
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.
--secondsseconds
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.
--hitcounthits
This option must be used in conjunction with one of --rcheck or --update. When used, this will
narrow the match to only happen when the address is in the list and packets had been received
greater than or equal to the given value. This option may be used along with --seconds to create
an even narrower match requiring a certain number of hits within a specific time frame.
--rttl This option may only be used in conjunction with one of --rcheck or --update. When used, this will
narrow the match to only happen when the address is in the list and the TTL of the current packet
matches that of the packet which hit the --set rule. This may be useful if you have problems with
people faking their source address in order to DoS you via this module by disallowing others
access to your site by 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=0
Number of packets per address remembered. This parameter is obsolete since kernel version 3.19
which started to calculate the table size based on given --hitcount parameter.
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-typetype
Match the type (numeric).
[!] --rt-segsleftnum[:num]
Match the `segments left' field (range).
[!] --rt-lenlength
Match the length of this header.
--rt-0-res
Match the reserved field, too (type=0)
--rt-0-addrsaddr[,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,--sportport[:port]
[!] --destination-port,--dportport[: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-setsetnameflag[,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-eqvalue
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-ltvalue
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-gtvalue
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-eqvalue
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-ltvalue
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-gtvalue
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.
[!] --statestate
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:
--modemode
Set the matching mode of the matching rule, supported modes are random and nth.
[!] --probabilityp
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.
[!] --everyn
Match one packet every nth packet. It works only with the nth mode (see also the --packet option).
--packetp
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)
--fromoffset
Set the offset from which it starts looking for any matching. If not passed, default is 0.
--tooffset
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.
[!] --stringpattern
Matches the given pattern.
[!] --hex-stringpattern
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,--sportport[: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,--dportport[:port]
Destination port or port range specification. The flag --dport is a convenient alias for this
option.
[!] --tcp-flagsmaskcomp
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: SYNACKFINRSTURGPSHALLNONE. 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-flagsSYN,RST,ACK,FINSYN. If the "!" flag precedes the "--syn", the
sense of the option is inverted.
[!] --tcp-optionnumber
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.
[!] --mssvalue[: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.
--datestartYYYY[-MM[-DD[Thh[:mm[:ss]]]]]
--datestopYYYY[-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.
--timestarthh:mm[:ss]
--timestophh: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.
[!] --monthdaysday[,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.
[!] --weekdaysday[,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--kerneltzishighlydiscouraged.
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.
[!] --tosvalue[/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.
[!] --tossymbol
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 -mtos-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-eqttl
Matches the given TTL value.
--ttl-gtttl
Matches if TTL is greater than the given TTL value.
--ttl-ltttl
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.
[!] --u32tests
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,--sportport[:port]
Source port or port range specification. See the description of the --source-port option of the
TCP extension for details.
[!] --destination-port,--dportport[:port]
Destination port or port range specification. See the description of the --destination-port
option of the TCP extension for details.
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