# man > iptables-extensions(8) [_iptables-extensions_(8)](https://www.chedong.com/phpMan.php/man/iptables-extensions/8/markdown) iptables 1.8.10 [_iptables-extensions_(8)](https://www.chedong.com/phpMan.php/man/iptables-extensions/8/markdown) ## NAME iptables-extensions — list of extensions in the standard iptables distribution ## SYNOPSIS **ip6tables **[**-m _**name_ [_module-options_...]] [**-j _**target-name_ [_target-options_...] **iptables **[**-m _**name_ [_module-options_...]] [**-j _**target-name_ [_target-options_...] ## MATCH EXTENSIONS iptables can use extended packet matching modules with the **-m **or **--match **options, followed by the matching module name; after these, various extra command line options become 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 op‐ tion 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 deci‐ mal 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 fol‐ low 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)](https://www.chedong.com/phpMan.php/man/bpf/4/markdown) 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 lim‐ ited functionality, meaning it will only match on packets that are processed for local sock‐ ets 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-bal‐ ancers. 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-lo‐** **cal-node**. **--cluster-hash-seed _**value_ Set seed value of the Jenkins hash. Example: iptables -A PREROUTING -t mangle -i eth1 -m cluster --cluster-total-nodes 2 --clus‐ ter-local-node 1 --cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff iptables -A PREROUTING -t mangle -i eth2 -m cluster --cluster-total-nodes 2 --clus‐ ter-local-node 1 --cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark 0xffff -j DROP iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark 0xffff -j DROP And the following commands to make all nodes see the same packets: ip maddr add 01:00:5e:00:01:01 dev eth1 ip maddr add 01:00:5e:00:01:02 dev eth2 arptables -A OUTPUT -o eth1 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:01 arptables -A INPUT -i eth1 --h-length 6 --destination-mac 01:00:5e:00:01:01 -j 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 in‐ cluded 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 ac‐ counting 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 ... ### connlabel Module matches or adds connlabels to a connection. connlabels are similar to connmarks, ex‐ cept labels are bit-based; i.e. all labels may be attached to a flow at the same time. Up to 128 unique labels are currently supported. [**!**] **--label name** matches if label **name **has been set on a connection. Instead of a name (which will be translated to a number, see EXAMPLE below), a number may be used instead. Using a number always overrides connlabel.conf. **--set **if the label has not been set on the connection, set it. Note that setting a label can fail. This is because the kernel allocates the conntrack label storage area when the connection is created, and it only reserves the amount of memory required by the ruleset that exists at the time the connection is created. In this case, the match will fail (or succeed, in case **--label **option was negated). This match depends on libnetfilter_conntrack 1.0.4 or later. Label translation is done via the **/etc/xtables/connlabel.conf **configuration file. Example: 0 eth0-in 1 eth0-out 2 ppp-in 3 ppp-out 4 bulk-traffic 5 interactive ### connlimit Allows you to restrict the number of parallel connections to a server per client IP address (or client address block). **--connlimit-upto _**n_ Match if the number of existing connections is below or equal _n_. **--connlimit-above _**n_ Match if the number of existing connections is above _n_. **--connlimit-mask _**prefix_length_ Group hosts using the prefix length. For IPv4, this must be a number between (includ‐ ing) 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](https://www.chedong.com/phpMan.php/perldoc/db8%3A%3A1/markdown) -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 speci‐ fied 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 connec‐ tion, such as an FTP data transfer or an ICMP error. **UNTRACKED** The packet is not tracked at all, which happens if you explicitly untrack it by using -j CT --notrack in the raw table. **SNAT **A virtual state, matching if the original source address differs from the reply desti‐ nation. **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 RE‐** **SET 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 su‐ perseded TOS within the IETF. [**!**] **--dscp _**value_ Match against a numeric (decimal or hex) value [0-63]. [**!**] **--dscp-class _**class_ Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be converted into its according numeric value. ### dst (IPv6-specific) This module matches the parameters in Destination Options header [**!**] **--dst-len _**length_ Total length of this header in octets. **--dst-opts _**type_[**:_**length_][**,_**type_[**:_**length_]...] numeric type of option and the length of the option data in octets. ### ecn This allows you to match the ECN bits of the IPv4/IPv6 and TCP header. ECN is the Explicit Congestion Notification mechanism as specified in RFC3168 [**!**] **--ecn-tcp-cwr** This matches if the TCP ECN CWR (Congestion Window Received) bit is set. [**!**] **--ecn-tcp-ece** This matches if the TCP ECN ECE (ECN Echo) bit is set. [**!**] **--ecn-ip-ect _**num_ This matches a particular IPv4/IPv6 ECT (ECN-Capable Transport). You have to 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 _amount_b/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 en‐ try 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 --hash‐ limit-mode srcport --hashlimit-upto 100/sec matching on subnet "10000 packets per minute for every /28 subnet (groups of 8 addresses) in 10.0.0.0/8" => -s 10.0.0.0/8 --hashlimit-mask 28 --hashlimit-upto 10000/min matching bytes per second "flows exceeding 512kbyte/s" => --hashlimit-mode srcip,dstip,srcport,dstport --hash‐ limit-above 512kb/s matching bytes per second "hosts that exceed 512kbyte/s, but permit up to 1Megabytes without matching" --hash‐ limit-mode dstip --hashlimit-above 512kb/s --hashlimit-burst 1mb ### hbh (IPv6-specific) This module matches the parameters in Hop-by-Hop Options header [**!**] **--hbh-len _**length_ Total length of this header in octets. **--hbh-opts _**type_[**:_**length_][**,_**type_[**:_**length_]...] numeric type of option and the length of the option data in octets. ### helper This module matches packets related to a specific conntrack-helper. [**!**] **--helper _**string_ Matches packets related to the specified conntrack-helper. string can be "ftp" for packets related to a ftp-session on default port. For other ports append -portnr to the value, ie. "ftp-2121". Same rules apply for other conntrack-helpers. ### hl (IPv6-specific) This module matches the Hop Limit field in the IPv6 header. [**!**] **--hl-eq _**value_ Matches if Hop Limit equals _value_. **--hl-lt _**value_ Matches if Hop Limit is less than _value_. **--hl-gt _**value_ Matches if Hop Limit is greater than _value_. ### icmp (IPv4-specific) This extension can be used if `--protocol icmp' is specified. It provides the following op‐ tion: [**!**] **--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 encapsu‐ lated 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 exten‐ sion will match until this limit is reached. It can be used in combination with the **LOG **tar‐ get 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 **PREROUT‐** **ING**, **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 con‐ venient 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)](https://www.chedong.com/phpMan.php/man/nfacct/8/markdown)** 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)](https://www.chedong.com/phpMan.php/man/nfacct/8/markdown) **from or, alternatively, from the git.net‐ filter.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 - bin/cvsweb/src/etc/pf.os . ### owner This module attempts to match various characteristics of the packet creator, for locally gen‐ erated 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 en‐ tering 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 op‐ tions. 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)](https://www.chedong.com/phpMan.php/man/setkey/8/markdown) **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 tun‐** **nel**. [**!**] **--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 sup‐ ports matching on absolute bps/pps values, comparing two rate estimators and matching on the difference between two rate estimators. For a better understanding of the available options, these are all possible combinations: • **rateest _**operator_ **rateest-bps** • **rateest _**operator_ **rateest-pps** • (**rateest **minus **rateest-bps1**) _operator_ **rateest-bps2** • (**rateest **minus **rateest-pps1**) _operator_ **rateest-pps2** • **rateest1 _**operator_ **rateest2 rateest-bps**(without rate!) • **rateest1 _**operator_ **rateest2 rateest-pps**(without rate!) • (**rateest1 **minus **rateest-bps1**) _operator_ (**rateest2 **minus **rateest-bps2**) • (**rateest1 **minus **rateest-pps1**) _operator_ (**rateest2 **minus **rateest-pps2**) ### --rateest-delta For each estimator (either absolute or relative mode), calculate the difference between the estimator-determined flow rate and the static value chosen with the BPS/PPS options. If the flow rate is higher than the specified BPS/PPS, 0 will be used instead of a nega‐ tive 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 sta‐ tic 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 cho‐ sen 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 --ra‐ teest-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 --ra‐ teest-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 suc‐ cess (or failure if **! **is passed in). ### --rsource Match/save the source address of each packet in the recent list table. This is the de‐ fault. ### --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 ad‐ dress 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 or‐ der 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 us‐ ing the following commands to modify the list: **echo +_**addr_ **>/proc/net/xt_recent/DEFAULT** to add _addr_ to the DEFAULT list **echo -_**addr_ **>/proc/net/xt_recent/DEFAULT** to remove _addr_ from the DEFAULT list ### echo / >/proc/net/xt_recent/DEFAULT to flush the DEFAULT list (remove all entries). The module itself accepts parameters, defaults shown: **ip_list_tot**=_100_ Number of addresses remembered per table. **ip_pkt_list_tot**=_20_ Number of packets per address remembered. **ip_list_hash_size**=_0_ Hash table size. 0 means to calculate it based on ip_list_tot, default: 512. **ip_list_perms**=_0644_ Permissions for /proc/net/xt_recent/* files. **ip_list_uid**=_0_ Numerical UID for ownership of /proc/net/xt_recent/* files. **ip_list_gid**=_0_ Numerical GID for ownership of /proc/net/xt_recent/* files. ### rpfilter Performs a reverse path filter test on a packet. If a reply to the packet would be sent via the same interface that the packet arrived on, the packet will match. Note that, unlike the in-kernel rp_filter, packets protected by IPSec are not treated specially. Combine this match with the policy match if you want this. Also, packets arriving via the loopback inter‐ face 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 SHUT‐ DOWN_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)](https://www.chedong.com/phpMan.php/man/ipset/8/markdown). [**!**] **--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 speci‐ fied 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 **re‐ turns **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 prox‐ ies bound to non-local addresses it is recommended to use the --transparent option in‐ stead. 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 **--trans‐** **parent **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**, **RE‐** **LATED **or **UNTRACKED**. For their description, see the "conntrack" heading in this man‐ page. ### 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 in‐ crements. [**!**] **--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_-1 (counting from 0) is the last one that is scanned. If not passed, default is the packet size. [**!**] **--string _**pattern_ Matches the given pattern. [**!**] **--hex-string _**pattern_ Matches the given pattern in hex notation. ### --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) al‐ gorithm 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 exam‐ ple, 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 op‐ tions: [**!**] **--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 op‐ tional, but are ANDed when specified. All times are interpreted as UTC by default. **--datestart _**YYYY_[**-_**MM_[**-_**DD_[**T_**hh_[**:_**mm_[**:_**ss_]]]]] **--datestop _**YYYY_[**-_**MM_[**-_**DD_[**T_**hh_[**:_**mm_[**:_**ss_]]]]] Only match during the given time, which must be in ISO 8601 "T" notation. The possi‐ ble 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 ker‐ nel 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)](https://www.chedong.com/phpMan.php/man/date/1/markdown), or what you see on your desk‐ top 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 off‐ set 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 on‐ wards' 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 recog‐ nized 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 num‐ bers **>=n **and **<=m**. location := number | location operator number operator := "&" | "<<" | ">>" | "@" The operators **&**, **<<**, **>> **and **&& **mean the same as in C. The **= **is really a set membership oper‐ ator 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 al‐ ways 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 com‐ plete 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 op‐ tions: [**!**] **--source-port**,**--sport _**port_[**:_**port_] Source port or port range specification. See the description of the **--source-port **op‐ tion of the TCP extension for details. [**!**] **--destination-port**,**--dport _**port_[**:_**port_] Destination port or port range specification. See the description of the **--destina‐** **tion-port **option of the TCP extension for details. ## TARGET EXTENSIONS iptables can use extended target modules: the following are included in the standard distrib‐ ution. **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)](https://www.chedong.com/phpMan.php/man/auditd/8/markdown) for additional details. **--type **{**accept**|**drop**|**reject**} Set type of audit record. Starting with linux-4.12, this option has no effect on gen‐ erated audit messages anymore. It is still accepted by iptables for compatibility rea‐ sons, 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 particu‐ larly 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 dis‐ tributed 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**, **sour‐** **ceip-sourceport-destport**. **--clustermac _**mac_ Specify the ClusterIP MAC address. Has to be a link-layer multicast address **--total-nodes _**num_ Number of total nodes within this cluster. **--local-node _**num_ Local node number within this cluster. **--hash-init _**rnd_ Specify the random seed used for hash initialization. **CONNMARK** This module sets the netfilter mark value associated with a connection. The mark is 32 bits wide. **--set-xmark _**value_[**/_**mask_] Zero out the bits given by _mask_ and XOR _value_ into the ctmark. **--save-mark **[**--nfmask _**nfmask_] [**--ctmask _**ctmask_] Copy the packet mark (nfmark) to the connection mark (ctmark) using the given masks. The new nfmark value is determined as follows: ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask) i.e. _ctmask_ defines what bits to clear and _nfmask_ what bits of the nfmark to XOR into the ctmark. _ctmask_ and _nfmask_ default to 0xFFFFFFFF. **--restore-mark **[**--nfmask _**nfmask_] [**--ctmask _**ctmask_] Copy the connection mark (ctmark) to the packet mark (nfmark) using the given masks. The new ctmark value is determined as follows: nfmark = (nfmark & ~_nfmask_) ^ (ctmark & _ctmask_); i.e. _nfmask_ defines what bits to clear and _ctmask_ what bits of the ctmark to XOR into the nfmark. _ctmask_ and _nfmask_ default to 0xFFFFFFFF. **--restore-mark **is only valid in the **mangle **table. The following mnemonics are available for **--set-xmark**: **--and-mark _**bits_ Binary AND the ctmark with _bits_. (Mnemonic for **--set-xmark 0/_**invbits_, where _invbits_ is the binary negation of _bits_.) **--or-mark _**bits_ Binary OR the ctmark with _bits_. (Mnemonic for **--set-xmark _**bits_**/_**bits_.) **--xor-mark _**bits_ Binary XOR the ctmark with _bits_. (Mnemonic for **--set-xmark _**bits_**/0**.) **--set-mark _**value_[**/_**mask_] Set the connection mark. If a mask is specified then only those bits set in the mask are modified. **--save-mark **[**--mask _**mask_] Copy the nfmark to the ctmark. If a mask is specified, only those bits are copied. **--restore-mark **[**--mask _**mask_] Copy the ctmark to the nfmark. If a mask is specified, only those bits are copied. This is only valid in the **mangle **table. **CONNSECMARK** This module copies security markings from packets to connections (if unlabeled), and from connections back to packets (also only if unlabeled). Typically used in conjunction with SECMARK, it is valid in the **security **table (for backwards compatibility with older 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 se‐ curity 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 load‐ ing 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-de‐ fined chains which are only called from those chains. It specifies that the destination ad‐ dress 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 ad‐ dresses. Optionally a port range, if the rule also specifies one of the following pro‐ tocols: **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 destina‐ tion 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 super‐ sedes 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 conjunc‐ tion 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 poten‐ tially 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 **mean‐ ing destination address (IPv4, IPv6 address), **sport **meaning source port (TCP, UDP, UD‐ Plite, 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 con‐ ntrack 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/