Translate "man iptables" (updating)

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STRRL 5月 27, 2019

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强迫自己看 iptables 的文档.
翻的很烂, 自己明白就行.

Netfilter-packet-flow

Name

iptables - administration tool for IPv4 packet filtering and NAT
iptables - IPv4 包过滤和 NAT 的管理工具

Synopsis 简介

iptables [-t table] -[AD] chain rule-specification [options]
iptables [-t table] -I chain [rulenum] rule-specification [options]
iptables [-t table] -R chain rulenum rule-specification [options]
iptables [-t table] -D chain rulenum [options]
iptables [-t table] -[LFZ][chain] [options]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target [options]
iptables [-t table] -E old-chain-name new-chain-name

Description 描述

Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel.
在 linux 内核中 iptable 被用来配置, 维护和检查 IP 包过滤规则表.

Several different tables may be defined.
可能会定义多个不同的表.

Each table contains a number of built-in chains and may also contain user-defined chains.
每个表都有几个内置的 Chain, 也有一些用户定义的 Chain.

Each chain is a list of rules which can match a set of packets.
每个 Chain 都有一组规则用来匹配一组数据包.

Each rule specifies what to do with a packet that matches.
每个规则都指明了如何处理被匹配的数据包.

This is called a ‘target’, which may be a jump to a user-defined chain in the same table.
这被称为 target, 它可能是到同一张表下某个用户定义的 Chain 的跳转.

Targets

A firewall rule specifies criteria for a packet, and a target.
一条防火墙规则表明了数据包的规范和它的 target.

If the packet does not match, the next rule in the chain is the examined;
如果数据包没有匹配上, 那么这条这个 Chain 内的下一条规则将继续尝试匹配.

if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined chain or one of the special values ACCEPT, DROP, QUEUE, or RETURN.
如果数据包匹配上了, 则下一条规则由 target 的值来指定, 它可能是一个用户定义的 Chain, 或者是下列特殊值之一: ACCEPT, DROP, QUEUE 或者 RETURN.

ACCEPT means to let the packet through.
ACCEPT 表示允许包通过.

DROP means to drop the packet on the floor.
DROP 表示将包丢在防火墙外.

QUEUE means to pass the packet to userspace.
QUEUE 表示将包发往用户空间.

(How the packet can be received by a userspace process differs by the particular queue handler.
(user space 进程如何收到数据包会因为 queue handler 的不同而不同.

2.4.x and 2.6.x kernels up to 2.6.13 include the ip_queue queue handler.
2.4.x 和 2.6.x 直到 2.6.13 的内核会包含一个 ip_queue queue handler.

Kernels 2.6.14 and later additionally include the nfnetlink_queue queue handler.
2.6.14 以及更高版本的内核内核额外包含了一个 nfnetlink_queue queue handler.

Packets with a target of QUEUE will be sent to queue number ‘0’ in this case.
在这种情况下, 目标为 QUEUE 的数据包会发往序号为 0 的 queue. (没明白)

Please also see the NFQUEUE target as described later in this man page.)
另请参阅本手册页后面所述的 NFQUEUE 目标.)

RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain.
RETURN 表示停止遍历这条 chain 并接着从上个(调用) chain 的下一条 rule 开始.

If the end of a built-in chain is reached or a rule in a built-in chain with target RETURN is matched, the target specified by the chain policy determines the fate of the packet.
如果到了内置 chain 的末尾或者一条目标为 RETURN 的内置链规则匹配上, 则 target 被这条 chain 的 policy 所决定.

Tables

There are currently three independent tables (which tables are present at any time depends on the kernel configuration options and which modules are present).
当前有三张独立的表(当前有哪些表存在取决于当前的 kernel 配置以及有哪些模块启用).

-t, --table table
This option specifies the packet matching table which the command should operate on.
这个选项指定了命令操作的哪张表.

If the kernel is configured with automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already there.
如果内核配置了自动模块加载,则会尝试加载该表的相应模块(如果该表尚不存在).

The tables are as follows:
有以下表:

filter:
This is the default table (if no -t option is passed).
这是默认的表(如果-t选项没有被指定).

It contains the built-in chains INPUT (for packets destined to local sockets), FORWARD (for packets being routed through the box), and OUTPUT (for locally-generated packets).
它包括了内置的链: INPUT(匹配目标是本机 socket 的包), FORWARD(匹配从本机经过的包)和 OUTPUT(匹配从本地生成的包)

nat:
This table is consulted when a packet that creates a new connection is encountered.
遇到创建新连接的数据包时,会查询此表.

It consists of three built-ins: PREROUTING (for altering packets as soon as they come in), OUTPUT (for altering locally-generated packets before routing), and POSTROUTING (for altering packets as they are about to go out).
它包括三条内置的链: PREROUTING(在数据包进入时立即更改), OUTPUT(在本地生成的数据包在路由之前进行修改)和 POSTROUTING (用于在数据包即将发送时更改数据包).

mangle:
This table is used for specialized packet alteration.
这个表被用来做特殊包修改.

Until kernel 2.4.17 it had two built-in chains: PREROUTING (for altering incoming packets before routing) and OUTPUT (for altering locally-generated packets before routing).
直到内核版本 2.4.17, 它有两个内置 CHAIN: PREROUTING(在包路由之前修改) 和 OUTPUT(在从本地产生的包在路由之前修改).

Since kernel 2.4.18, three other built-in chains are also supported: INPUT (for packets coming into the box itself), FORWARD (for altering packets being routed through the box), and POSTROUTING (for altering packets as they are about to go out).
自内核 2.4.18 后, 其他三个内置的 CHAIN 也被支持了: INPUT(进入本机的包), FORWARD(修改通过本机的包) 和 POSTROUTING(在包即将发送时修改).

raw:
This table is used mainly for configuring exemptions from connection tracking in combination with the NOTRACK target.
这个表主要和 NOTRACK 目标一起使用, 用来配置避免 connection tracking.

It registers at the netfilter hooks with higher priority and is thus called before ip_conntrack, or any other IP tables.
它在 netfilter hooks 中有更高的优先级, 因此它会在 ip_conntrack 或者任何其他的 IP 表之前被调用.

It provides the following built-in chains: PREROUTING (for packets arriving via any network interface) OUTPUT (for packets generated by local processes)
它提供了下列内置的 CHAIN: PREROUTING(修改到达任意网络接口的包), OUTPUT(修改本地生成的包).

Options

The options that are recognized by iptables can be divided into several different groups.
能被 iptables 识别的选项可被分成几个不同的组.

COMMANDS

These options specify the specific action to perform.
这些选项声明了将要执行的特定行为.

Only one of them can be specified on the command line unless otherwise specified below.
除非另有说明, 否则只能在一行命令中指定一个命令.

For all the long versions of the command and option names, you need to use only enough letters to ensure that iptables can differentiate it from all other options.
对于所有的长的命令, 你可以只用足够的字母, 能够保证 iptables 将它从命令中区分开即可。

-A, --append chain rule-specification
Append one or more rules to the end of the selected chain. When the source and/or destination names resolve to more than one address, a rule will be added for each possible address combination.
在选择的 CHAIN 后追加一条或多条规则. 当规则中的 source 和/或 destination 名可以处理多余一个地址时, 每个可行的地址组合都会新增一条规则.

-D, --delete chain rule-specification
-D, --delete chain rulenum
Delete one or more rules from the selected chain. There are two versions of this command: the rule can be specified as a number in the chain (starting at 1 for the first rule) or a rule to match.
从选择的 CHAIN 中删除一条或多条规则. 这条命令有两种使用方式: 使用 CHAIN 中的数字(从第一条规则以数字 1 开始)或者匹配规则.

-I, --insert chain [rulenum] rule-specification
Insert one or more rules in the selected chain as the given rule number.
向选择的 CHAIN 中以给定的 number 插入一条或多条规则.

So, if the rule number is 1, the rule or rules are inserted at the head of the chain. This is also the default if no rule number is specified.
所以如果 number 为 1, 规则就会被插入到 CHAIN 的头部. 如果没有声明 number, 这也是默认行为.

-R, --replace chain rulenum rule-specification
Replace a rule in the selected chain. If the source and/or destination names resolve to multiple addresses, the command will fail.
在选择的 CHAIN 中替换一条规则. 如果规则中的 source 和/或 destination 名匹配了多个地址, 这条命令会失败.

Rules are numbered starting at 1.
规则的序号从 1 开始.

-L, --list [chain]
List all rules in the selected chain. If no chain is selected, all chains are listed.
列出所选 CHAIN 的所有规则. 如果没有 CHAIN 被选择, 所有的 CHAIN 都会被列出.

As every other iptables command, it applies to the specified table (filter is the default), so NAT rules get listed by
就像其他的 iptables 命令一样, 它需要一个指定的表(默认表是 filter), 所以 NAT 规则可以这样被列出:

iptables -t nat -n -L

Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups.
请注意-n经常被使用, 他表示避免长时间的反向 DNS 查询.

It is legal to specify the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed.
也可以使用 -Z(zero)选项, 将把 CHAIN 中的计数器归零.

The exact output is affected by the other arguments given.
输出也会收到其他给定参数的影响.

The exact rules are suppressed until you use
输出都是被精简过的, 直到你使用

iptables -L -v

-F, --flush [chain]
Flush the selected chain (all the chains in the table if none is given).
清空选定的 CHAIN. (如果没有指定, 则是所有的 CHAIN)

This is equivalent to deleting all the rules one by one.
这和一条条删除所有的 rules 是等价的.

-Z, --zero [chain]
Zero the packet and byte counters in all chains.
将所有 CHAIN 的 packet 和 byte 计数器重置为 0.

It is legal to specify the -L, –list (list) option as well, to see the counters immediately before they are cleared. (See above.)
他可以和-L一起使用, 在重置之前查看计数器的值.

-N, --new-chain chain
Create a new user-defined chain by the given name.
使用给定的名字新建一个用户定义的 CHAIN.

There must be no target of that name already.
名字不能已经成为 target.

-X, --delete-chain [chain]
Delete the optional user-defined chain specified.
删除给定的用户定义的 CHAIN.

There must be no references to the chain.
这条 CHAIN 不能被引用.

If there are, you must delete or replace the referring rules before the chain can be deleted.
如果 CHAIN 被引用了, 你必须删除或者替换相关的规则, 才能使之能够被删除.

The chain must be empty, i.e. not contain any rules.
CHAIN 必须是空的, 或者说不能有任何 rules.

If no argument is given, it will attempt to delete every non-builtin chain in the table.
如果没有给定参数, 它会尝试删除表中所有非内置的 CHAIN.

-P, --policy chain target
Set the policy for the chain to the given target.
设置 CHIAN 的策略为给定的 target.

See the section TARGETS for the legal targets.
查看 TARGETS 章节来了解合法的 targets.

Only built-in (non-user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy targets.
只有内置的 CHINA 才有 policy, 内置或者用户定的 CHAIN 都不能成为 policy 的 targets.

-E, --rename-chain old-chain new-chain
Rename the user specified chain to the user supplied name.
重命名一个用户指定的 CHAIN 为用户给定的名字.

This is cosmetic, and has no effect on the structure of the table.
这只是个装饰品, 对表结构没有任何影响.

-h
Help. Give a (currently very brief) description of the command syntax.
帮助. 提供一个(目前很简单的)命令语法说明.

PARAMETERS

The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).
下面的参数组成了一条 rule specification(可以用在 add, delete, insert, replaceappend命令里).

-p, --protocol [!] protocol
The protocol of the rule or of the packet to check.
规则或者被检查包的协议.

The specified protocol can be one of tcp, udp, icmp, or all, or it can be a numeric value, representing one of these protocols or a different one.
规定的协议可以是 tcp, udp, icmp, 或者 all 的其中一个, 或者是一个数字, 代表协议之一或者不同协议.

A protocol name from /etc/protocols is also allowed.
/etc/protocols中的协议名都是允许的.

A “!” argument before the protocol inverts the test.
在协议名前的!会反转测试.

The number zero is equivalent to all.
数字 0 等于all.

Protocol all will match with all protocols and is taken as default when this option is omitted.
all将匹配所有的协议, 这也是该选项被忽略时的默认行为.

-s, --source [!] address[/mask]
Source specification.
声明源地址.

Address can be either a network name, a hostname (please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea), a network IP address (with /mask), or a plain IP address.
地址可以是一个网络名, 主机名(请注意, 使用 DNS 来解析任何名称都是不好的), IP 地址(带有/掩码), 或者单独一个 IP 地址.

The mask can be either a network mask or a plain number, specifying the number of 1’s at the left side of the network mask.
掩码的形式可以是一个网络掩码或者是一个数字, 来指定掩码中从左边数, 1 的数量.

Thus, a mask of 24 is equivalent to 255.255.255.0.
因此掩码 24255.255.255.0 是等效的.

A “!” argument before the address specification inverts the sense of the address.
在地址前的!参数代表将地址反转.

The flag –src is an alias for this option.
标志--src是该选项的别名.

-d, --destination [!] address[/mask]
Destination specification.
声明目标地址.

See the description of the -s (source) flag for a detailed description of the syntax.
查看-s(source)以获取语法的详细描述.

The flag –dst is an alias for this option.
标志--dst是该选项的别名.

-j, --jump target
This specifies the target of the rule; i.e., what to do if the packet matches it.
它标识了规则的目标,即规则匹配成功后要做什么.

The target can be a user-defined chain (other than the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately, or an extension (see EXTENSIONS below).
目标可以是一个用户定义的 CHAIN(除了这条规则所在的 CHAIN), 内置 targets 之一(将立刻决定包的去向), 或者是一个 extension(查看下面的 EXTENSIONS 章节).

If this option is omitted in a rule (and -g is not used), then matching the rule will have no effect on the packet’s fate, but the counters on the rule will be incremented.
如果在规则中省略了这个选项(而且没有使用-g), 则这条规则匹配到之后对包的去向没有任何影响, 但是增加这个条规则的计数器.

-g, --goto chain
This specifies that the processing should continue in a user specified chain.
它指定流程应该跳转到一个用户定义的 CHAIN 里.

Unlike the –jump option return will not continue processing in this chain but instead in the chain that called us via –jump.
不像--jump选项, (goto)不会重新回到调用方的 CHAIN 继续处理.

-i, --in-interface [!] name
Name of an interface via which a packet was received (only for packets entering the INPUT, FORWARD and PREROUTING chains).
接收包时所使用的网络接口名(只能在 INPUT, FORWARDPREROUTING中使用).

When the “!” argument is used before the interface name, the sense is inverted.
在网络接口名前加入!会反转其含义.

If the interface name ends in a “+”, then any interface which begins with this name will match.
如果网络接口以+结尾, 则任意以这个名字开头的接口都会匹配.

If this option is omitted, any interface name will match.
如果这个选择被忽略, 任何接口名都会匹配.

-o, --out-interface [!] name
Name of an interface via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING chains).
发送包时所使用的网络接口名(在FORWARD, OUTPUTPOSTROUTING中使用).

When the “!” argument is used before the interface name, the sense is inverted.
在网络接口名前加入!会反转其含义.

If the interface name ends in a “+”, then any interface which begins with this name will match.
如果网络接口以+结尾, 则任意以这个名字开头的接口都会匹配.

If this option is omitted, any interface name will match.
如果这个选择被忽略, 任何接口名都会匹配.

[!] -f, --fragment
This means that the rule only refers to second and further fragments of fragmented packets.
这代表这条规则只是为了被分片的包中的第二个以及以后的包.

Since there is no way to tell the source or destination ports of such a packet (or ICMP type), such a packet will not match any rules which specify them.
由于这样的包里不含有 source 或 destination 信息(或者 ICMP 类型), 这样的包不会匹配任意条规则.

When the “!” argument precedes the “-f” flag, the rule will only match head fragments, or unfragmented packets.
!-f前时, 规则会只匹配第一个分片, 或未分片的包.

-c, --set-counters PKTS BYTES
This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE operations).
这可以使管理员重置一条规则的包和字节计数器(在INSERT, APPEND, REPLACE操作时)

OTHER OPTIONS

The following additional options can be specified:
可以指定以下附加选项:

-v, --verbose
Verbose output.
详细输出.

This option makes the list command show the interface name, the rule options (if any), and the TOS masks.
这个选项使list命令显示网络接口名, 规则的选项和 TOS mask.

The packet and byte counters are also listed, with the suffix ‘K’, ‘M’ or ‘G’ for 1000, 1,000,000 and 1,000,000,000 multipliers respectively (but see the -x flag to change this).
包和字节计数器也会被列出,带有后缀K, M或者G代表 1000, 1,000,000 和 1,000,000,000 倍数(可以使用-x改变这个行为).

For appending, insertion, deletion and replacement, this causes detailed information on the rule or rules to be printed.
在添加, 插入, 删除和替换时, 会打印出规则或规则的详细信息.

-n, --numeric
Numeric output.
数字输出.

IP addresses and port numbers will be printed in numeric format.
IP 地址和端口号会以数字的形式输出.

By default, the program will try to display them as host names, network names, or services (whenever applicable).
默认情况下, 程序会尝试将他们以主机名, 网络名或者服务明(当适用时).

-x, --exact
Expand numbers.
展开数字.

Display the exact value of the packet and byte counters, instead of only the rounded number in K’s (multiples of 1000) M’s (multiples of 1000K) or G’s (multiples of 1000M).
显示包和字节计数器的精确值, 来替代使用 K,M 或 G 的大体值.

This option is only relevant for the -L command.
这个选项智能和-L命令一起使用.

--line-numbers
When listing rules, add line numbers to the beginning of each rule, corresponding to that rule’s position in the chain.
当列出规则时, 为每个规则在开头加一个行号, 对应着规则在 CHAIN 中的位置.

--modprobe=command
When adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).
当向 CHAIN 中添加或插入一条规则时, 使用命令来加载任意需要的模块(比如说 targets, 匹配扩展等).

Match Extensions

iptables can use extended packet matching modules.
iptables 可以使用扩展的包匹配模块.

These are loaded in two ways:
有两种方法可以加载它们

implicitly, when -p or –protocol is specified, or with the -m or –match options, followed by the matching module name;
隐式加载, 当指定了-p/--protocol, 或者带有-m/--match选项, 后面跟匹配模块的名称.

after these, various extra command line options become available, depending on the specific module.
除此之外, 根据具体模块, 可以使用各种不同的命令.

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.
你可以在一行中配置多个扩展匹配模块, 你可以在模块后使用 -h 或者 -help选项来获取他们的帮助信息.

The following are included in the base package, and most of these can be preceded by a ! to invert the sense of the match.
下面这些选项都内置在基本包中, 他们中大多数都可以使用 !来反转匹配的含义.

account

没用过 不熟

Account traffic for all hosts in defined network/netmask.
统计在某个网络/子网中的所有主机.

这里的 account 类似与 statistics, 相关信息: Traffic accounting.

Features:
特性:

  • long (one counter per protocol TCP/UDP/IMCP/Other) and short statistics 详细(每个协议一个单独的 counter)或精简的统计

  • one iptables rule for all hosts in network/netmask 每个在子网中的所有主机的每条规则

  • loading/saving counters (by reading/writting to procfs entries) 加载/保存 计数器( 通过读/写 /procfs 来统计)

--aaddr network/netmask
defines network/netmask for which make statistics.
定义需要统计的子网

--aname name
defines name of list where statistics will be kept. If no is specified DEFAULT will be used.
定义统计列表的名字. 如果未设置, 则使用 DEFAULT.

--ashort
table will colect only short statistics (only total counters without splitting it into protocols.)
只会统计简单的信息(只有总数的计数器, 没有细分到每个协议一个计数器).

Example usage:
用例:

account traffic for/to 192.168.0.0/24 network into table mynetwork:
统计 mynetwork 表中, 从 192.168.0.0/24 的发送/接收的情况

# iptables -A FORWARD -m account --aname mynetwork --aaddr 192.168.0.0/24

account traffic for/to WWW serwer for 192.168.0.0/24 network into table mywwwserver:
统计 mynetwork 表中, 从 192.168.0.0/24 的发送/接收的情况, 而且是 WWW 服务器的情况

# iptables -A INPUT -p tcp --dport 80 -m account --aname mywwwserver --aaddr 192.168.0.0/24 --ashort
# iptables -A OUTPUT -p tcp --sport 80 -m account --aname mywwwserver --aaddr 192.168.0.0/24 --ashort

read counters:
读取计数器

# cat /proc/net/ipt_account/mynetwork # cat /proc/net/ipt_account/mywwwserver

set counters:
设置计数器

# echo "ip = 192.168.0.1 packets_src = 0" > /proc/net/ipt_account/mywwserver

Webpage: http://www.barbara.eu.org/~quaker/ipt_account/

ddrtype

This module matches packets based on their address type.
这个模块通过地址类型来匹配包.

Address types are used within the kernel networking stack and categorize addresses into various groups.
地址类型在内核网络栈中使用, 并且被分为不同的组.

The exact definition of that group depends on the specific layer three protocol.
组的确切定义依赖于特定的三层协议.

The following address types are possible:
下面的地址类型是有可能出现的

UNSPEC an unspecified address (i.e. 0.0.0.0)
UNSPEC 一个不明确的地址(比如 0.0.0.0)

UNICAST an unicast address
UNICAST 一个单播地址

LOCAL a local address
LOCAL 一个本地地址

BROADCAST a broadcast address
BOARDCAST 一个广播地址

ANYCAST an anycast packet
ANYCAST 一个任播地址

Anycast

MULTICAST a multicast address
MULTICAST 一个组播地址

BLACKHOLE a blackhole address
BLACKHOLE 一个黑洞地址

Black hole (networking)

UNREACHABLE an unreachable address
UNREACHABLE 一个无法到达的地址

PROHIBIT a prohibited address
PROHIBIT 一个禁止的地址

THROW FIXME
THROW 待补充

NAT FIXME
NAT 待补充

XRESOLVE

--src-type type
Matches if the source address is of given type
匹配源地址类型

--dst-type type
Matches if the destination address is of given type
匹配目标地址类型

ah

This module matches the SPIs in Authentication header of IPsec packets.
这个模块会匹配 IPSec 包中认证头的 SPIs.

IPsec 不熟悉, 不懂.

--ahspi [!] spi[:spi]

原文也木有其他解释了. SPI 是个什么东西, 不知道, 挖个坑以后填(咕咕咕).

childlevel

This is an experimental module.
这是一个实验性的模块.

It matches on whether the packet is part of a master connection or one of its children (or grandchildren, etc).
它匹配某个包是否是主连接的一部分或者是它的子连接(或者孙子连接, 之类的).

For instance, most packets are level 0. FTP data transfer is level 1.
举例, 大多数的包在 level 0. FTP 的 数据传输在 level 1.

FTP 的控制流用 20 端口, 数据流在 21 端口. 这里也挖个坑.
类似的 websocket 应该也可以用 childlevel 匹配.
这个选项好像没了? 或者是因为试验性的原因, 这个文档里没有这个参数.

--childlevel [!] level

comment

Allows you to add comments (up to 256 characters) to any rule.
为任意一条 rule 增加注释(最多 256 个字符).

--comment comment
Example:
iptables -A INPUT -s 192.168.0.0/16 -m comment –comment “A privatized IP block”

condition

This matches if a specific /proc filename is ‘0’ or ‘1’.
匹配在/proc下的文件是否是 0 或者 1.

--condition [!] filename
Match on boolean value stored in /proc/net/ipt_condition/filename file
匹配储存在/proc/net/ipt_condition/filename的布尔值

connbytes

Match by how many bytes or packets a connection (or one of the two flows constituting the connection) have tranferred so far, or by average bytes per packet.
The counters are 64bit 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 transfered bytes per connection can also be viewed through /proc/net/ip_conntrack and accessed via ctnetlink

[!] –connbytes from:[to]
match packets from a connection whose packets/bytes/average packet size is more than FROM and less than TO bytes/packets. if TO is omitted only FROM check is done. “!” is used to match packets not falling in the range.
–connbytes-dir [original|reply|both]
which packets to consider
–connbytes-mode [packets|bytes|avgpkt]
whether to check the amount of packets, number of bytes transferred or the average size (in bytes) of all packets received so far. Note that when “both” is used together with “avgpkt”, and data is going (mainly) only in one direction (for example HTTP), the average packet size will be about half of the actual data packets.
Example:
iptables .. -m connbytes –connbytes 10000:100000 –connbytes-dir both –connbytes-mode bytes …

connlimit

Allows you to restrict the number of parallel TCP connections to a server per client IP address (or address block).
[!] –connlimit-above n
match if the number of existing tcp connections is (not) above n
–connlimit-mask bits
group hosts using mask
Examples:

allow 2 telnet connections per client host

iptables -p tcp –syn –dport 23 -m connlimit –connlimit-above 2 -j REJECT

you can also match the other way around:

iptables -p tcp –syn –dport 23 -m connlimit ! –connlimit-above 2 -j ACCEPT

limit the nr of parallel http requests to 16 per class C sized network (24 bit netmask)

iptables -p tcp –syn –dport 80 -m connlimit –connlimit-above 16 –connlimit-mask 24 -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).
connrate

This module matches the current transfer rate in a connection.
–connrate [!][from]:[to]
Match against the current connection transfer rate being within ‘from’ and ‘to’ bytes per second. When the “!” argument is used before the range, the sense of the match is inverted.

conntrack

This module, when combined with connection tracking, allows access to more connection tracking information than the “state” match. (this module is present only if iptables was compiled under a kernel supporting this feature)
–ctstate state
Where state is a comma separated list of the connection states to match. Possible states are INVALID meaning that the packet is associated with no known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in both directions, NEW meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions, and RELATED meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error. 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.
–ctproto proto
Protocol to match (by number or name)
–ctorigsrc [!] address[/mask]
Match against original source address
–ctorigdst [!] address[/mask]
Match against original destination address
–ctreplsrc [!] address[/mask]
Match against reply source address
–ctrepldst [!] address[/mask]
Match against reply destination address
–ctstatus [NONE|EXPECTED|SEEN_REPLY|ASSURED][,…]
Match against internal conntrack states
–ctexpire time[:time]
Match remaining lifetime in seconds against given value or range of values (inclusive)

dccp

–source-port,–sport [!] port[:port]
–destination-port,–dport [!] port[:port]
–dccp-types [!] mask
Match when the DCCP packet type is one of ‘mask’. ‘mask’ is a comma-separated list of packet types. Packet types are: REQUEST RESPONSE DATA ACK DATAACK CLOSEREQ CLOSE RESET SYNC SYNCACK INVALID.
–dccp-option [!] number
Match if DCP option set.

dscp

This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.
–dscp value
Match against a numeric (decimal or hex) value [0-63].
–dscp-class DiffServ Class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be converted into it’s according numeric value.

dstlimit

This module allows you to limit the packet per second (pps) rate on a per destination IP or per destination port base. As opposed to the ‘limit’ match, every destination ip / destination port has it’s own limit.
THIS MODULE IS DEPRECATED AND HAS BEEN REPLACED BY ‘’hashlimit’’
–dstlimit avg
Maximum average match rate (packets per second unless followed by /sec /minute /hour /day postfixes).
–dstlimit-mode mode
The limiting hashmode. Is the specified limit per dstip, dstip-dstport tuple, srcip-dstip tuple, or per srcipdstip-dstport tuple.
–dstlimit-name name
Name for /proc/net/ipt_dstlimit/* file entry
[–dstlimit-burst burst]
Number of packets to match in a burst. Default: 5
[–dstlimit-htable-size size]
Number of buckets in the hashtable
[–dstlimit-htable-max max]
Maximum number of entries in the hashtable
[–dstlimit-htable-gcinterval interval]
Interval between garbage collection runs of the hashtable (in miliseconds). Default is 1000 (1 second).
[–dstlimit-htable-expire time
After which time are idle entries expired from hashtable (in miliseconds)? Default is 10000 (10 seconds).

ecn

This allows you to match the ECN bits of the IPv4 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 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]

fuzzy

This module matches a rate limit based on a fuzzy logic controller [FLC]
–lower-limit number
Specifies the lower limit (in packets per second).
–upper-limit number
Specifies the upper limit (in packets per second).

hashlimit

This patch adds a new match called ‘hashlimit’. The idea is to have something like ‘limit’, but either per destination-ip or per (destip,destport) tuple.
It gives you the ability to express

‘1000 packets per second for every host in 192.168.0.0/16’
‘100 packets per second for every service of 192.168.1.1’

with a single iptables rule.
–hashlimit rate
A rate just like the limit match
–hashlimit-burst num
Burst value, just like limit match
–hashlimit-mode destip | destip-destport
Limit per IP or per port
–hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry
–hashlimit-htable-size num
The number of buckets of the hash table
–hashlimit-htable-max num
Maximum entries in the hash
–hashlimit-htable-expire num
After how many miliseconds do hash entries expire
–hashlimit-htable-gcinterval num
How many miliseconds between garbage collection intervals

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.

icmp

This extension is loaded if ‘–protocol icmp’ is specified. It provides the following option:
–icmp-type [!] typename
This allows specification of the ICMP type, which can be a numeric ICMP type, or one of the ICMP type names shown by the command
iptables -p icmp -h

iprange

This matches on a given arbitrary range of IPv4 addresses
[!]–src-range ip-ip
Match source IP in the specified range.
[!]–dst-range ip-ip
Match destination IP in the specified range.

ipv4options

Match on IPv4 header options like source routing, record route, timestamp and router-alert.
–ssrr
To match packets with the flag strict source routing.

–lsrr

To match packets with the flag loose source routing.

–no-srr
To match packets with no flag for source routing.
[!] –rr
To match packets with the RR flag.
[!] –ts
To match packets with the TS flag.
[!] –ra
To match packets with the router-alert option.
[!] –any-opt
To match a packet with at least one IP option, or no IP option at all if ! is chosen.
Examples:
$ iptables -A input -m ipv4options –rr -j DROP
will drop packets with the record-route flag.
$ iptables -A input -m ipv4options –ts -j DROP
will drop packets with the timestamp flag.

length

This module matches the length of a packet against a specific value or range of values.
–length [!] length[:length]

limit

This module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is reached (unless the ‘!’ flag is used). It can be used in combination with the LOG target to give limited logging, for example.
–limit rate
Maximum average matching rate: specified as a number, with an optional ‘/second’, ‘/minute’, ‘/hour’, or ‘/day’ suffix; the default is 3/hour.
–limit-burst number
Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not reached, up to this number; the default is 5.

mac

–mac-source [!] address
Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets coming from an Ethernet device and entering the PREROUTING, FORWARD or INPUT chains.

mark

This module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).
–mark value[/mask]
Matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the mask before the comparison).

mport

This module matches a set of source or destination ports. Up to 15 ports can be specified. It can only be used in conjunction with -p tcp or -p udp.
–source-ports port[,port[,port…]]
Match if the source port is one of the given ports. The flag –sports is a convenient alias for this option.
–destination-ports port[,port[,port…]]
Match if the destination port is one of the given ports. The flag –dports is a convenient alias for this option.
–ports port[,port[,port…]]
Match if the both the source and destination ports are equal to each other and to one of the given ports.

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 -p tcp or -p udp.
–source-ports [!] 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.
–destination-ports [!] port[,port[,port:port…]]
Match if the destination port is one of the given ports. The flag –dports is a convenient alias for this option.
–ports [!] port[,port[,port:port…]]
Match if either the source or destination ports are equal to one of the given ports.

nth

This module matches every ‘n’th packet
–every value
Match every ‘value’ packet
[–counter num]
Use internal counter number ‘num’. Default is ‘0’.
[–start num]
Initialize the counter at the number ‘num’ insetad of ‘0’. Most between ‘0’ and ‘value’-1.
[–packet num]
Match on ‘num’ packet. Most be between ‘0’ and ‘value’-1.

osf

The idea of passive OS fingerprint matching exists for quite a long time, but was created as extension fo OpenBSD pf only some weeks ago. Original idea was lurked in some OpenBSD mailing list (thanks grange@open…) and than adopted for Linux netfilter in form of this code.
Original fingerprint table was created by Michal Zalewski lcamtuf@coredump.cx.

This module compares some data(WS, MSS, options and it’s order, ttl, df and others) from first SYN packet (actually from packets with SYN bit set) with dynamically loaded OS fingerprints.

–log 1/0
If present, OSF will log determined genres even if they don’t match desired one.

0 - log all determined entries, 1 - only first one.
In syslog you find something like this:
ipt_osf: Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 -> 11.22.33.44:139
ipt_osf: Unknown: 16384:106:1:48:020405B401010402 44.33.22.11:1239 -> 11.22.33.44:80

–smart
if present, OSF will use some smartness to determine remote OS. OSF will use initial TTL only if source of connection is in our local network.
–netlink
If present, OSF will log all events also through netlink NETLINK_NFLOG groupt 1.
–genre [!] string
Match a OS genre by passive fingerprinting
Example:
#iptables -I INPUT -j ACCEPT -p tcp -m osf --genre Linux --log 1 --smart

NOTE: -p tcp is obviously required as it is a TCP match.

Fingerprints can be loaded and read through /proc/sys/net/ipv4/osf file. One can flush all fingerprints with following command:

echo -en FLUSH > /proc/sys/net/ipv4/osf
Only one fingerprint per open/write/close.
Fingerprints 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. It is only valid in the OUTPUT chain, and even this some packets (such as ICMP ping responses) may have no owner, and hence never match.
–uid-owner userid
Matches if the packet was created by a process with the given effective user id.
–gid-owner groupid
Matches if the packet was created by a process with the given effective group id.
–pid-owner processid
Matches if the packet was created by a process with the given process id.
–sid-owner sessionid
Matches if the packet was created by a process in the given session group.
–cmd-owner name
Matches if the packet was created by a process with the given command name. (this option is present only if iptables was compiled under a kernel supporting this feature)
NOTE: pid, sid and command matching are broken on SMP

physdev

This module matches on the bridge port input and output devices enslaved to a bridge device. This module is a part of the infrastructure that enables a transparent bridging IP firewall and is only useful for kernel versions above version 2.5.44.
–physdev-in [!] name
Name of a bridge port via which a packet is received (only for packets entering the INPUT, FORWARD and PREROUTING chains). If the interface name ends in a “+”, then any interface which begins with this name will match. If the packet didn’t arrive through a bridge device, this packet won’t match this option, unless ‘!’ is used.
–physdev-out [!] name
Name of a bridge port via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING chains). If the interface name ends in a “+”, then any interface which begins with this name will match. Note that in the nat and mangle OUTPUT chains one cannot match on the bridge output port, however one can in the filter OUTPUT chain. If the packet won’t leave by a bridge device or it is yet unknown what the output device will be, then the packet won’t match this option, unless
[!] –physdev-is-in
Matches if the packet has entered through a bridge interface.
[!] –physdev-is-out
Matches if the packet will leave through a bridge interface.
[!] –physdev-is-bridged
Matches if the packet is being bridged and therefore is not being routed. This is only useful in the FORWARD and POSTROUTING chains.

pkttype

This module matches the link-layer packet type.
–pkt-type [unicast|broadcast|multicast]

policy

This modules matches the policy used by IPsec for handling a packet.
–dir in|out
Used to select whether to match the policy used for decapsulation or the policy that will be used for encapsulation. in is valid in the PREROUTING, INPUT and FORWARD chains, out is valid in the POSTROUTING, OUTPUT and FORWARD chains.
–pol none|ipsec
Matches if the packet is subject to IPsec processing.
–strict
Selects whether to match the exact policy or match if any rule of the policy matches the given policy.
–reqid id
Matches the reqid of the policy rule. The reqid can be specified with setkey(8) using unique:id as level.
–spi spi
Matches the SPI of the SA.
–proto ah|esp|ipcomp
Matches the encapsulation protocol.
–mode tunnel|transport
Matches the encapsulation mode.
–tunnel-src addr[/mask]
Matches the source end-point address of a tunnel mode SA. Only valid with –mode tunnel.
–tunnel-dst addr[/mask]
Matches the destination end-point address of a tunnel mode SA. Only valid with –mode tunnel.
–next
Start the next element in the policy specification. Can only be used with –strict

psd

Attempt to detect TCP and UDP port scans. This match was derived from Solar Designer’s scanlogd.
–psd-weight-threshold threshold
Total weight of the latest TCP/UDP packets with different destination ports coming from the same host to be treated as port scan sequence.
–psd-delay-threshold delay
Delay (in hundredths of second) for the packets with different destination ports coming from the same host to be treated as possible port scan subsequence.
–psd-lo-ports-weight weight
Weight of the packet with privileged (<=1024) destination port.
–psd-hi-ports-weight weight
Weight of the packet with non-priviliged destination port.

quota

Implements network quotas by decrementing a byte counter with each packet.
–quota bytes
The quota in bytes.
KNOWN BUGS: this does not work on SMP systems.

random

This module randomly matches a certain percentage of all packets.
–average percent
Matches the given percentage. If omitted, a probability of 50% is set.

realm

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).

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.

–name name
Specify the list to use for the commands. If no name is given then ‘DEFAULT’ will be used.
[!] –set
This will add the source address of the packet to the list. If the source address is already in the list, this will update the existing entry. This will always return success (or failure if ‘!’ is passed in).
[!] –rcheck
Check if the source address of the packet is currently in the list.
[!] –update
Like –rcheck, except it will update the “last seen” timestamp if it matches.
[!] –remove
Check if the source address of the packet is currently in the list and if so that address will be removed from the list and the rule will return true. If the address is not found, false is returned.
[!] –seconds seconds
This option must be used in conjunction with one of –rcheck or –update. When used, this will narrow the match to only happen when the address is in the list and was seen within the last given number of seconds.
[!] –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.
–rttl
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 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

Official website (http://snowman.net/projects/ipt_recent/) also has some examples of usage.
/proc/net/ipt_recent/* are the current lists of addresses and information about each entry of each list.

Each file in /proc/net/ipt_recent/ can be read from to see the current list or written two using the following commands to modify the list:

echo xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Add to the DEFAULT list
echo -xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Remove from the DEFAULT list
echo clear > /proc/net/ipt_recent/DEFAULT
to empty the DEFAULT list.
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/ipt_recent/* files
debug=0
Set to 1 to get lots of debugging info

sctp

–source-port,–sport [!] port[:port]
–destination-port,–dport [!] port[:port]
–chunk-types [!] all|any|only chunktype[:flags][…]
The flag letter in upper case indicates that the flag is to match if set, in the lower case indicates to match if unset.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUTDOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE ASCONF ASCONF_ACK

chunk type available flags
DATA U B E 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 modules macthes IP sets which can be defined by ipset(8).
–set setname flag[,flag…]
where flags are src and/or dst and there can be no more than six of them. Hence the command
iptables -A FORWARD -m set –set test src,dst
will match packets, for which (depending on the type of the set) the source address or port number of the packet can be found in the specified set. If there is a binding belonging to the mached set element or there is a default binding for the given set, then the rule will match the packet only if additionally (depending on the type of the set) the destination address or port number of the packet can be found in the set according to the binding.
state

This module, when combined with connection tracking, 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. Possible states are INVALID meaning that the packet could not be identified for some reason which includes running out of memory and ICMP errors which don’t correspond to any known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in both directions, NEW meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions, and RELATED meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error.

string

This modules matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.
–algo bm|kmp
Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
–from offset
Set the offset from which it starts looking for any matching. If not passed, default is 0.
–to offset
Set the offset from which it starts looking for any matching. If not passed, default is the packet size.
–string pattern
Matches the given pattern. –hex-string pattern Matches the given pattern in hex notation.

tcp

These extensions are loaded if ‘–protocol tcp’ is specified. It provides the following options:
–source-port [!] 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 port:port. If the first port is omitted, “0” is assumed; if the last is omitted, “65535” is assumed. If the second port greater then the first they will be swapped. The flag –sport is a convenient alias for this option.
–destination-port [!] 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 is the flags which we should examine, written as a comma-separated list, and the second argument 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.
–mss value[:value]
Match TCP SYN or SYN/ACK packets with the specified MSS value (or range), which control the maximum packet size for that connection.

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.

time

This matches if the packet arrival time/date is within a given range. All options are facultative.
–timestart value
Match only if it is after ‘value’ (Inclusive, format: HH:MM ; default 00:00).
–timestop value
Match only if it is before ‘value’ (Inclusive, format: HH:MM ; default 23:59).
–days listofdays
Match only if today is one of the given days. (format: Mon,Tue,Wed,Thu,Fri,Sat,Sun ; default everyday)
–datestart date
Match only if it is after ‘date’ (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 1970)
–datestop date
Match only if it is before ‘date’ (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 2037)

tos

This module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence bits).
–tos tos
The argument is either a standard name, (use
iptables -m tos -h
to see the list), or a numeric value to match.

ttl

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 allows you to extract quantities of up to 4 bytes from a packet, AND them with specified masks, shift them by specified amounts and test whether the results are in any of a set of specified ranges. The specification of what to extract is general enough to skip over headers with lengths stored in the packet, as in IP or TCP header lengths.
Details and examples are in the kernel module source.

udp

These extensions are loaded if ‘–protocol udp’ is specified. It provides the following options:
–source-port [!] port[:port]
Source port or port range specification. See the description of the –source-port option of the TCP extension for details.
–destination-port [!] port[:port]
Destination port or port range specification. See the description of the –destination-port option of the TCP extension for details.

unclean

This module takes no options, but attempts to match packets which seem malformed or unusual. This is regarded as experimental.

Target Extensions

iptables can use extended target modules: the following are included in the standard distribution.

BALANCE

This allows you to DNAT connections in a round-robin way over a given range of destination addresses.
–to-destination ipaddr-ipaddr
Address range to round-robin over.

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.

CLUSTERIP

This module allows you to configure a simple cluster of nodes that share a certain IP and MAC address without an explicit load balancer in front of them. Connections are statically distributed between the nodes in this cluster.
–new
Create a new ClusterIP. You always have to set this on the first rule for a given ClusterIP.

–hashmode mode
Specify the hashing mode. Has to be one of sourceip, sourceip-sourceport, sourceip-sourceport-destport
–clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer multicast address
–total-nodes num
Number of total nodes within this cluster.
–local-node num
Local node number within this cluster.
–hash-init rnd
Specify the random seed used for hash initialization.

CONNMARK

This module sets the netfilter mark value associated with a connection
–set-mark mark[/mask]
Set connection mark. If a mask is specified then only those bits set in the mask is modified.
–save-mark [–mask mask]
Copy the netfilter packet mark value to the connection mark. If a mask is specified then only those bits are copied.
–restore-mark [–mask mask]
Copy the connection mark value to the packet. If a mask is specified then only those bits are copied. This is only valid in the mangle table.

DNAT

This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called from those chains. It specifies that the destination address of the packet should be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes one type of option:
–to-destination ipaddr[-ipaddr][:port-port]
which can specify a single new destination IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then the destination port will never be modified.
In Kernels up to 2.6.10 you can add several –to-destination options. For those kernels, if you specify more than one destination address, either via an address range or multiple –to-destination options, a simple round-robin (one after another in cycle) load balancing takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don’t have the ability to NAT to multiple ranges anymore.

DSCP

This target allows to alter 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

This target allows to selectively work around known ECN blackholes. It can only be used in the mangle table.
–ecn-tcp-remove
Remove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p tcp.

IPMARK

Allows you to mark a received packet basing on its IP address. This can replace many mangle/mark entries with only one, if you use firewall based classifier.
This target is to be used inside the mangle table, in the PREROUTING, POSTROUTING or FORWARD hooks.

–addr src/dst
Use source or destination IP address.
–and-mask mask
Perform bitwise ‘and’ on the IP address and this mask.
–or-mask mask
Perform bitwise ‘or’ on the IP address and this mask.
The order of IP address bytes is reversed to meet “human order of bytes”: 192.168.0.1 is 0xc0a80001. At first the ‘and’ operation is performed, then ‘or’.
Examples:

We create a queue for each user, the queue number is adequate to the IP address of the user, e.g.: all packets going to/from 192.168.5.2 are directed to 1:0502 queue, 192.168.5.12 -> 1:050c etc.

We have one classifier rule:

tc filter add dev eth3 parent 1:0 protocol ip fw
Earlier we had many rules just like below:
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.2 -j MARK –set-mark 0x10502
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.3 -j MARK –set-mark 0x10503

Using IPMARK target we can replace all the mangle/mark rules with only one:
iptables -t mangle -A POSTROUTING -o eth3 -j IPMARK –addr=dst –and-mask=0xffff –or-mask=0x10000
On the routers with hundreds of users there should be significant load decrease (e.g. twice).
IPV4OPTSSTRIP

Strip all the IP options from a packet.
The target doesn’t take any option, and therefore is extremly easy to use :

# iptables -t mangle -A PREROUTING -j IPV4OPTSSTRIP

LOG

Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print some information on all matching packets (like most IP header fields) via the kernel log (where it can be read with dmesg or syslogd(8)). This is a “non-terminating target”, i.e. rule traversal continues at the next rule. So if you want to LOG the packets you refuse, use two separate rules with the same matching criteria, first using target LOG then DROP (or REJECT).
–log-level level
Level of logging (numeric or see syslog.conf(5)).
–log-prefix prefix
Prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs.
–log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is readable by users.
–log-tcp-options
Log options from the TCP packet header.
–log-ip-options
Log options from the IP packet header.
–log-uid
Log the userid of the process which generated the packet.

MARK

This is used to set the netfilter mark value associated with the packet. It is only valid in the mangle table. It can for example be used in conjunction with iproute2.
–set-mark mark

MASQUERADE

This target is only valid in the nat table, in the POSTROUTING chain. It should only be used with dynamically assigned IP (dialup) connections: if you have a static IP address, you should use the SNAT target. Masquerading is equivalent to specifying a mapping to the IP address of the interface the packet is going out, but also has the effect that connections are forgotten when the interface goes down. This is the correct behavior when the next dialup is unlikely to have the same interface address (and hence any established connections are lost anyway). It takes one option:
–to-ports port[-port]
This specifies a range of source ports to use, overriding the default SNAT source port-selection heuristics (see above). This is only valid if the rule also specifies -p tcp or -p udp.

MIRROR

This is an experimental demonstration target which inverts the source and destination fields in the IP header and retransmits the packet. It is only valid in the INPUT, FORWARD and PREROUTING chains, and user-defined chains which are only called from those chains. Note that the outgoing packets are NOT seen by any packet filtering chains, connection tracking or NAT, to avoid loops and other problems.

NETMAP

This target allows you to statically map a whole network of addresses onto another network of addresses. It can only be used from rules in the nat table.
–to address[/mask]
Network address to map to. The resulting address will be constructed in the following way: All ‘one’ bits in the mask are filled in from the new ‘address’. All bits that are zero in the mask are filled in from the original address.

NFQUEUE

This target is an extension of the QUEUE target. As opposed to QUEUE, it allows you to put a packet into any specific queue, identified by its 16-bit queue number.
–queue-num value
This specifies the QUEUE number to use. Valud queue numbers are 0 to 65535. The default value is 0.
It can only be used with Kernel versions 2.6.14 or later, since it requires
the nfnetlink_queue kernel support.

NOTRACK

This target disables connection tracking for all packets matching that rule.
It can only be used in the
raw table.

REDIRECT

This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called from those chains. It redirects the packet to the machine itself by changing the destination IP to the primary address of the incoming interface (locally-generated packets are mapped to the 127.0.0.1 address). It takes one option:
–to-ports port[-port]
This specifies a destination port or range of ports to use: without this, the destination port is never altered. This is only valid if the rule also specifies -p tcp or -p udp.

REJECT

This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains which are only called from those chains. The following option controls the nature of the error packet returned:
–reject-with type
The type given can be
icmp-net-unreachable
icmp-host-unreachable
icmp-port-unreachable
icmp-proto-unreachable
icmp-net-prohibited
icmp-host-prohibited or
icmp-admin-prohibited ()
which return the appropriate ICMP error message (port-unreachable is the default). The option tcp-reset can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending mail to broken mail hosts (which won’t accept your mail otherwise).
(
) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT

SAME

Similar to SNAT/DNAT depending on chain: it takes a range of addresses (‘–to 1.2.3.4-1.2.3.7’) and gives a client the same source-/destination-address for each connection.
–to -
Addresses to map source to. May be specified more than once for multiple ranges.
–nodst
Don’t use the destination-ip in the calculations when selecting the new source-ip

SET

This modules adds and/or deletes entries from IP sets which can be defined by ipset(8).
–add-set setname flag[,flag…]
add the address(es)/port(s) of the packet to the sets
–del-set setname flag[,flag…]
delete the address(es)/port(s) of the packet from the sets, where flags are src and/or dst and there can be no more than six of them.
The bindings to follow must previously be defined in order to use
multilevel adding/deleting by the SET target.

SNAT

This target is only valid in the nat table, in the POSTROUTING chain. It specifies that the source address of the packet should be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes one type of option:
–to-source ipaddr[-ipaddr][:port-port]
which can specify a single new source IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then source ports below 512 will be mapped to other ports below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be mapped to 1024 or above. Where possible, no port alteration will occur.
In Kernels up to 2.6.10, you can add several –to-source options. For those kernels, if you specify more than one source address, either via an address range or multiple –to-source options, a simple round-robin (one after another in cycle) takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don’t have the ability to NAT to multiple ranges anymore.

TARPIT

Captures and holds incoming TCP connections using no local per-connection resources. Connections are accepted, but immediately switched to the persist state (0 byte window), in which the remote side stops sending data and asks to continue every 60-240 seconds. Attempts to close the connection are ignored, forcing the remote side to time out the connection in 12-24 minutes.
This offers similar functionality to LaBrea http://www.hackbusters.net/LaBrea/ but doesn’t require dedicated hardware or IPs. Any TCP port that you would normally DROP or REJECT can instead become a tarpit.

To tarpit connections to TCP port 80 destined for the current machine:

iptables -A INPUT -p tcp -m tcp –dport 80 -j TARPIT
To significantly slow down Code Red/Nimda-style scans of unused address space, forward unused ip addresses to a Linux box not acting as a router (e.g. “ip route 10.0.0.0 255.0.0.0 ip.of.linux.box” on a Cisco), enable IP forwarding on the Linux box, and add:
iptables -A FORWARD -p tcp -j TARPIT
iptables -A FORWARD -j DROP

NOTE:
If you use the conntrack module while you are using TARPIT, you should also use the NOTRACK target, or the kernel will unnecessarily allocate resources for each TARPITted connection. To TARPIT incoming connections to the standard IRC port while using conntrack, you could:

iptables -t raw -A PREROUTING -p tcp –dport 6667 -j NOTRACK
iptables -A INPUT -p tcp –dport 6667 -j TARPIT

TCPMSS

This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting it to your outgoing interface’s MTU minus 40). Of course, it can only be used in conjunction with -p tcp. It is only valid in the mangle table.
This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation Needed packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines behind it can never exchange large packets: 1)
Web browsers connect, then hang with no data received.

2)

Small mail works fine, but large emails hang.

3)

ssh works fine, but scp hangs after initial handshaking.

Workaround: activate this option and add a rule to your firewall configuration like:
iptables -t mangle -A FORWARD -p tcp –tcp-flags SYN,RST SYN
-j TCPMSS –clamp-mss-to-pmtu
–set-mss value
Explicitly set MSS option to specified value.
–clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40).
These options are mutually exclusive.

TOS

This is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mangle table.
–set-tos tos
You can use a numeric TOS values, or use
iptables -j TOS -h
to see the list of valid TOS names.

TRACE

This target has no options. It just turns on packet tracing for all packets that match this rule.

TTL

This is used to modify the IPv4 TTL header field. The TTL field determines how many hops (routers) a packet can traverse until it’s time to live is exceeded.
Setting or incrementing the TTL field can potentially be very dangerous,
so it should be avoided at any cost.
Don’t ever set or increment the value on packets that leave your local network!
mangle table.
–ttl-set value
Set the TTL value to ‘value’.
–ttl-dec value
Decrement the TTL value ‘value’ times.
–ttl-inc value
Increment the TTL value ‘value’ times.

ULOG

This target provides userspace logging of matching packets. When this target is set for a rule, the Linux kernel will multicast this packet through a netlink socket. One or more userspace processes may then subscribe to various multicast groups and receive the packets. Like LOG, this is a “non-terminating target”, i.e. rule traversal continues at the next rule.
–ulog-nlgroup nlgroup
This specifies the netlink group (1-32) to which the packet is sent. Default value is 1.
–ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32 characters long, and useful for distinguishing messages in the logs.
–ulog-cprange size
Number of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. Default is 0.
–ulog-qthreshold size
Number of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets inside the kernel and transmits them as one netlink multipart message to userspace. Default is 1 (for backwards compatibility).

XOR

Encrypt TCP and UDP traffic using a simple XOR encryption
–key string
Set key to “string”
–block-size
Set block size

Diagnostics

Various error messages are printed to standard error. The exit code is 0 for correct functioning. Errors which appear to be caused by invalid or abused command line parameters cause an exit code of 2, and other errors cause an exit code of 1.

Bugs

Bugs? What’s this? ;-) Well, you might want to have a look at http://bugzilla.netfilter.org/

Compatibility With Ipchains

This iptables is very similar to ipchains by Rusty Russell. The main difference is that the chains INPUT and OUTPUT are only traversed for packets coming into the local host and originating from the local host respectively. Hence every packet only passes through one of the three chains (except loopback traffic, which involves both INPUT and OUTPUT chains); previously a forwarded packet would pass through all three.

The other main difference is that -i refers to the input interface; -o refers to the output interface, and both are available for packets entering the FORWARD chain.

iptables is a pure packet filter when using the default ‘filter’ table, with optional extension modules. This should simplify much of the previous confusion over the combination of IP masquerading and packet filtering seen previously. So the following options are handled differently:

-j MASQ
-M -S
-M -L
There are several other changes in iptables.

See Also

iptables-save(8), iptables-restore(8), ip6tables(8), ip6tables-save(8), ip6tables-restore(8), libipq(3).

The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO details the extensions that are not in the standard distribution, and the netfilter-hacking-HOWTO details the netfilter internals.
See http://www.netfilter.org/.

Authors

Rusty Russell originally wrote iptables, in early consultation with Michael Neuling.

Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection framework in iptables, then wrote the mangle table, the owner match, the mark stuff, and ran around doing cool stuff everywhere.

James Morris wrote the TOS target, and tos match.

Jozsef Kadlecsik wrote the REJECT target.

Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as well as the TTL, DSCP, ECN matches and targets.

The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef Kadlecsik, Patrick McHardy, James Morris, Harald Welte and Rusty Russell.

Man page originally written by Herve Eychenne rv@wallfire.org.
Referenced By
arptables(8), brctl(8), collectd.conf(5), ebtables(8), ferm(1), firehol(1), firehol.conf(5), fwsnort(8), ip6tables-1.4.7(8), ipq_destroy_handle(3), ipq_get_msgerr(3), ipq_perror(3), ipq_read(3), ipq_set_mode(3), ipq_set_verdict(3), iptables-restore-1.4.7(8), iptables-save-1.4.7(8), iptables-xml-1.4.7(8), iptables_selinux(8), iptstate(8), ipvsadm(8), mountd(8), rpc.statd(8), shorewall(8), shorewall-lite(8), shorewall.conf(5), shorewall6-lite(8)