iptables manpage

IPTABLES(8)                                                                                                 iptables 1.4.21                                                                                                IPTABLES(8)

NAME
       iptables/ip6tables — administration tool for IPv4/IPv6 packet filtering and NAT

SYNOPSIS
       iptables [-t table] {-A|-C|-D} chain rule-specification

       ip6tables [-t table] {-A|-C|-D} chain rule-specification

       iptables [-t table] -I chain [rulenum] rule-specification

       iptables [-t table] -R chain rulenum rule-specification

       iptables [-t table] -D chain rulenum

       iptables [-t table] -S [chain [rulenum]]

       iptables [-t table] {-F|-L|-Z} [chain [rulenum]] [options...]

       iptables [-t table] -N chain

       iptables [-t table] -X [chain]

       iptables [-t table] -P chain target

       iptables [-t table] -E old-chain-name new-chain-name

       rule-specification = [matches...] [target]

       match = -m matchname [per-match-options]

       target = -j targetname [per-target-options]

DESCRIPTION
       Iptables  and  ip6tables  are  used  to  set up, maintain, and inspect the tables of IPv4 and IPv6 packet filter rules in the Linux kernel.  Several different tables may be defined.  Each table contains a number of built-in
       chains and may also contain user-defined chains.

       Each chain is a list of rules which can match a set of packets.  Each rule specifies what to do with a packet that matches.  This is called a `target', which may be a jump to a user-defined chain in the same table.

TARGETS
       A firewall rule specifies criteria for a packet and a target.  If the packet does not match, the next rule in the chain is examined; 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, one of the targets described in iptables-extensions(8), or one of the special values ACCEPT, DROP or RETURN.

       ACCEPT  means  to let the packet through.  DROP means to drop the packet on the floor.  RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain.  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.

TABLES
       There are currently five independent tables (which tables are present at any time depends on the kernel configuration options and which modules are present).

       -t, --table table
              This option specifies the packet matching table which the command should operate on.  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.

              The tables are as follows:

              filter:
                  This  is  the default table (if no -t option is passed). 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).

              nat:
                  This table is consulted when a packet that creates a new connection is encountered.  It consists of three built-ins: PREROUTING (for altering packets as soon as they come in), OUTPUT (for altering  locally-gener‐
                  ated packets before routing), and POSTROUTING (for altering packets as they are about to go out).  IPv6 NAT support is available since kernel 3.7.

              mangle:
                  This  table is used for specialized packet alteration.  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).  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).

              raw:
                  This  table  is  used mainly for configuring exemptions from connection tracking in combination with the NOTRACK target.  It registers at the netfilter hooks with higher priority and is thus called before ip_con‐
                  ntrack, or any other IP tables.  It provides the following built-in chains: PREROUTING (for packets arriving via any network interface) OUTPUT (for packets generated by local processes)

              security:
                  This table is used for Mandatory Access Control (MAC) networking rules, such as those enabled by the SECMARK and CONNSECMARK targets.  Mandatory Access Control is implemented by Linux  Security  Modules  such  as
                  SELinux.   The security table is called after the filter table, allowing any Discretionary Access Control (DAC) rules in the filter table to take effect before MAC rules.  This table provides the following built-
                  in chains: INPUT (for packets coming into the box itself), OUTPUT (for altering locally-generated packets before routing), and FORWARD (for altering packets being routed through the box).

OPTIONS
       The options that are recognized by iptables and ip6tables can be divided into several different groups.

   COMMANDS
       These options specify the desired action to perform. Only one of them can be specified on the command line unless otherwise stated below. For long versions of the command and option names, you need to use only  enough  let‐
       ters to ensure that iptables can differentiate it from all other options.

       -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.

       -C, --check chain rule-specification
              Check  whether  a rule matching the specification does exist in the selected chain. This command uses the same logic as -D to find a matching entry, but does not alter the existing iptables configuration and uses its
              exit code to indicate success or failure.

       -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.

       -I, --insert chain [rulenum] rule-specification
              Insert one or more rules in the selected chain as the given rule number.  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  speci‐
              fied.

       -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.  Rules are numbered starting at 1.

       -L, --list [chain]
              List all rules in the selected chain.  If no chain is selected, all chains are listed. Like every other iptables command, it applies to the specified table (filter is the default), so NAT rules get listed by
               iptables -t nat -n -L
              Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups.  It is legal to specify the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed.
              The exact output is affected by the other arguments given. The exact rules are suppressed until you use
               iptables -L -v

       -S, --list-rules [chain]
              Print all rules in the selected chain.  If no chain is selected, all chains are printed like iptables-save. Like every other iptables command, it applies to the specified table (filter is the default).

       -F, --flush [chain]
              Flush the selected chain (all the chains in the table if none is given).  This is equivalent to deleting all the rules one by one.

       -Z, --zero [chain [rulenum]]
              Zero the packet and byte counters in all chains, or only the given chain, or only the given rule in a chain. It is legal to specify the -L, --list (list) option as well, to see the counters  immediately  before  they
              are cleared. (See above.)

       -N, --new-chain chain
              Create a new user-defined chain by the given name.  There must be no target of that name already.

       -X, --delete-chain [chain]
              Delete  the  optional  user-defined chain specified.  There must be no references to the chain.  If there are, you must delete or replace the referring rules before the chain can be deleted.  The chain must be empty,
              i.e. not contain any rules.  If no argument is given, it will attempt to delete every non-builtin chain in the table.

       -P, --policy chain target
              Set the policy for the chain to the given target.  See the section TARGETS for the legal targets.  Only built-in (non-user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy
              targets.

       -E, --rename-chain old-chain new-chain
              Rename the user specified chain to the user supplied name.  This is cosmetic, and has no effect on the structure of the table.

       -h     Help.  Give a (currently very brief) description of the command syntax.

   PARAMETERS
       The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).

       -4, --ipv4
              This  option  has  no  effect in iptables and iptables-restore.  If a rule using the -4 option is inserted with (and only with) ip6tables-restore, it will be silently ignored. Any other uses will throw an error. This
              option allows to put both IPv4 and IPv6 rules in a single rule file for use with both iptables-restore and ip6tables-restore.

       -6, --ipv6
              If a rule using the -6 option is inserted with (and only with) iptables-restore, it will be silently ignored. Any other uses will throw an error. This option allows to put both IPv4 and IPv6 rules in  a  single  rule
              file for use with both iptables-restore and ip6tables-restore.  This option has no effect in ip6tables and ip6tables-restore.

       [!] -p, --protocol protocol
              The protocol of the rule or of the packet to check.  The specified protocol can be one of tcp, udp, udplite, icmp, icmpv6,esp, ah, sctp, mh or the special keyword "all", or it can be a numeric value, representing one
              of these protocols or a different one.  A protocol name from /etc/protocols is also allowed.  A "!" argument before the protocol inverts the test.  The number zero is equivalent to all. "all" will match with all pro‐
              tocols and is taken as default when this option is omitted.  Note that, in ip6tables, IPv6 extension headers except esp are not allowed.  esp and ipv6-nonext can be used with Kernel version 2.6.11 or later.  The num‐
              ber zero is equivalent to all, which means that you cannot test the protocol field for the value 0 directly. To match on a HBH header, even if it were the last, you cannot use -p 0, but always need -m hbh.

       [!] -s, --source address[/mask][,...]
              Source specification. Address can be either a network name, a hostname, a network IP address (with /mask), or a plain IP address. Hostnames will be resolved once only, before the rule  is  submitted  to  the  kernel.
              Please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea.  The mask can be either an ipv4 network mask (for iptables) or a plain number, specifying the number of 1's at
              the left side of the network mask.  Thus, an iptables mask of 24 is equivalent to 255.255.255.0.  A "!" argument before the address specification inverts the sense of the address. The flag --src is an alias for  this
              option.  Multiple addresses can be specified, but this will expand to multiple rules (when adding with -A), or will cause multiple rules to be deleted (with -D).

       [!] -d, --destination address[/mask][,...]
              Destination specification.  See the description of the -s (source) flag for a detailed description of the syntax.  The flag --dst is an alias for this option.

       -m, --match match
              Specifies a match to use, that is, an extension module that tests for a specific property. The set of matches make up the condition under which a target is invoked. Matches are evaluated first to last as specified on
              the command line and work in short-circuit fashion, i.e. if one extension yields false, evaluation will stop.

       -j, --jump target
              This specifies the target of the rule; i.e., what to do if the packet matches it.  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).  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.

       -g, --goto chain
              This specifies that the processing should continue in a user specified chain. Unlike the --jump option return will not continue processing in this chain but instead in the chain that called us via --jump.

       [!] -i, --in-interface name
              Name of an interface via which a packet was received (only for packets entering the INPUT, FORWARD and PREROUTING chains).  When the "!" argument is used before the interface name, the  sense  is  inverted.   If  the
              interface name ends in a "+", then any interface which begins with this name will match.  If this option is omitted, any interface name will match.

       [!] -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).  When the "!" argument is used before the interface name, the sense is inverted.  If the
              interface name ends in a "+", then any interface which begins with this name will match.  If this option is omitted, any interface name will match.

       [!] -f, --fragment
              This means that the rule only refers to second and further IPv4 fragments of fragmented packets.  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.  When the "!" argument precedes the "-f" flag, the rule will only match head fragments, or unfragmented packets. This option is IPv4 specific, it is not available in ip6tables.

       -c, --set-counters packets bytes
              This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE operations).

   OTHER OPTIONS
       The following additional options can be specified:

       -v, --verbose
              Verbose  output.   This  option  makes  the list command show the interface name, the rule options (if any), and the TOS masks.  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).  For appending, insertion, deletion and replacement, this causes detailed information on the rule or rules to be printed.  -v
              may be specified multiple times to possibly emit more detailed debug statements.

       -w, --wait [seconds]
              Wait for the xtables lock.  To prevent multiple instances of the program from running concurrently, an attempt will be made to obtain an exclusive lock at launch.  By default, the program will exit if the lock cannot
              be obtained.  This option will make the program wait (indefinitely or for optional seconds) until the exclusive lock can be obtained.

       -n, --numeric
              Numeric output.  IP addresses and port numbers will be printed in numeric format.  By default, the program will try to display them as host names, network names, or services (whenever applicable).

       -x, --exact
              Expand numbers.  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).  This option is only rele‐
              vant for the -L command.

       --line-numbers
              When listing rules, add line numbers to the beginning of each rule, corresponding to that rule's position in the chain.

       --modprobe=command
              When adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).

MATCH AND TARGET EXTENSIONS
       iptables can use extended packet matching and target modules.  A list of these is available in the iptables-extensions(8) manpage.

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  respec‐
       tively.  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.

       The various forms of NAT have been separated out; 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 combi‐
       nation 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-apply(8), iptables-save(8), iptables-restore(8), iptables-extensions(8),

       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, Yasuyuki Kozakai, Jozsef Kadlecsik, Patrick McHardy, James Morris, Pablo Neira Ayuso, Harald Welte and Rusty Russell.

       Man page originally written by Herve Eychenne <rv@wallfire.org>.

VERSION
       This manual page applies to iptables/ip6tables @PACKAGE_AND_VERSION@.

iptables 1.4.21   

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iptables 简介

iptables是linux防火墙管理工具，位于/sbin/iptables. 真正实现防火墙功能的事netfilter， 它是linux 内核中实现包过滤的内部结构。

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iptables的规则表和链

• 表（tables)

提供特定的功能，iptables内置了4个表：filter表（包过滤）， nat表（网络地址转换）， mangle表（包重构/修改）和raw表（数据跟踪处理）。

• 链（chains）

是数据包传播的路径，每一条链实际上就是众多规则中的一个检查清单，每一条链可以有一条或数条规则。当一个数据包到达一个链时， iptables就会从链中的第一条规则开始检查，看数据包是否满足规则所定义的条件。如果满足，系统就会根据该条规则所定义的方法处理数据包；否则iptbales将继续检查下一条规则，如果该数据包不符合链中的任意一条规则，iptables就会根据预先定义的策略来处理数据包。

iptabels采用链和表的分层结构，共有四张表和五个链。

raw表	mangle表	nat表	filter表
PREROUTING链	PREROUTING链	PREROUTING链	INPUT链
OUTPUT链	POSTROUTING链	POSTROUTING链	FORWARD链
	INPUT链	OUTPUT链	OUTPUT链
	OUTPUT链
	FORWARD链

1. 规则表

• filter表，过滤数据包，内核模块iptables_fillter
• nat表，用于网路地址转换，内核模块iptables_nat
• mangle表，修改数据包的服务类型、TTL、并且可以配置实现QOS，内核模块iptables_mangle
• raw表，决定数据包是否被状态跟踪机制处理，内核模块iptables_raw

2. 规则链

1. INPUT -- 应用于进来的数据包
2. OUTPUT -- 应用于外出的数据包
3. FORWARD -- 应用于转发数据包
4. PREROUTING -- 应用于路由选择前的数据包，进来的数据包优先由这个链处理
5. POSTROUTING -- 应用于路由选择后的数据包，出去的数据包优先由这个链处理
6. 规则表之间的优先顺序

image.png image.png

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常用iptables配置

1. 删除已有规则

iptables -F
iptables --flush

2. 设置chain策略

对于filter table， 默认的chain策略为ACCEPT，我们可以通过下面命令修改chain策略：

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP

3. 屏蔽指定的ip

block_ip ="x.x.x.x"
iptables -A INPUT -i eth0 -p tcp -s "$block_ip" -j DROP

4. 配置服务项

设置只能指定网段、由指定网口ssh连接本机

iptables -A  INPUT -i eth0 -p tcp -s 192.168.100.0/24 --dport 22 -m state --state NEW,ESTABLESHED -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp -sport 22 -m state --state ESTABLISHED -j ACCEPT

设置本机通过SSH连接其他机器

iptables -A INPUT -i eth0 -p tcp -s 192.168.100.0/24 --dport 22 -m state --state ESTABLESHED -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 22 -m state --state NEW,ESTABLISHED -j ACCEPT

基于udp的dns服务

iptables -A OUTPUT -p udp -o eth0 --dport 53 -j ACCEPT
iptables -A INPUT -p udp -i eth0 --sport 53 -j ACCEPT

5. 网口转发配置

eth0内网，eth1外网

iptables -A FORWARD -i eth0 -o eth1 -j ACCPET

6. 端口转发配置

将422端口的包转发到22端口

iptables -t nat -A PERROUTING -p tcp -d 192.168.102.37 --dport 422 -j DNAT --to 192.168.102.37:22

7. Dos攻击防范

利用limit模块,每分钟最大连接数为25，总连接最大为100

iptables -A  INPUT -p tcp --dport 80 -m limit --limit 25/minute --limit-burst 100 -j ACCEPT

8. 配置负载均衡

利用nth扩展模块

iptables -A PERROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every  3 --packet 0 -j DNAT --to-destination 192.168.1.101:80
iptables -A PERROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every  3 --packet 0 -j DNAT --to-destination 192.168.1.103:80
iptables -A PERROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every  3 --packet 0 -j DNAT --to-destination 192.168.1.105:80