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서버를 사용하다보면 DDOS 공격을 하는 사람들이 종종있다.

다름 사람들에게 피해를 주면서.. 돈을 요구하는 XX들이 있다.

이렇게 남에게 피해를 주는 네트웍 트래픽에 시간과 돈을 투자하지 마시고,

남에게 도움이 되는것, 생산적인 것에 투자 하시기 바랍니다.

===

이와 같은 경우, iptables와 fail2ban을 사용하면 간단히 막을 수 있을것.

iptables에 대한 공부는 상단의 링크 메뉴얼을 번역하여 공부한다.

번역가능하신분은 번역해서 올려주시면 좋겠다.

CentOS 5.2 를 기준으로..

일단 ssh 서버는 fail2ban을 사용하고, 웹 공격에 대해서는 iptables 옵션을 사용하자.

fail2ban에 보면 ssh 접속 시도에 대하여 접근 차단을 하도록 설정할 수 있다.

1. fail2ban은 소스로 설치하고, init 시작 스크립트도 제공된다.

http://www.fail2ban.org

단, init 스크립트에서 fail2ban-client 명령은 /usr/local/bin 디렉토리에 설치되므로,
시작 스크립트를 수정한다.

ssh 서비스에 대한 로그파일을 기준으로 판단하게 되는데, CentOS 5.2에서는 /var/log/secure 파일에 기본적인 ssh 로그파일이 기록되므로, fail2ban의 jail.conf에서 디렉토리 경로를 /var/log/secure 로 수정한다.

2. iptables 명령을 사용하여 100초 동안 150번 접속을 한 연결은 DROP한다.

# vim /etc/sysconfig/iptables
#####################DDOS PROTECT
-N syn-flood
-A syn-flood -m limit --limit 100/second --limit-burst 150 -j RETURN
-A syn-flood -j LOG --log-prefix "SYN flood: "
-A syn-flood -j DROP

######################

[root@localhost ~]# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
fail2ban-ssh  tcp  --  anywhere             anywhere            tcp dpt:ssh
RH-Firewall-1-INPUT  all  --  anywhere             anywhere

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
RH-Firewall-1-INPUT  all  --  anywhere             anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Chain RH-Firewall-1-INPUT (2 references)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     icmp --  anywhere             anywhere            icmp any
ACCEPT     esp  --  anywhere             anywhere
ACCEPT     ah   --  anywhere             anywhere
ACCEPT     udp  --  anywhere             224.0.0.251         udp dpt:mdns
ACCEPT     udp  --  anywhere             anywhere            udp dpt:ipp
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ipp
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:telnet
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:http
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ftp
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:https
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Chain fail2ban-ssh (1 references)
target     prot opt source               destination
RETURN     all  --  anywhere             anywhere

Chain syn-flood (0 references)
target     prot opt source               destination
RETURN     all  --  anywhere             anywhere            limit: avg 100/sec burst 150
LOG        all  --  anywhere             anywhere            LOG level warning prefix `SYN flood: '
DROP       all  --  anywhere             anywhere
[root@localhost ~]#

connlimit 옵션을 사용하여 10개의 syn 접속(80포트 웹접속)이 있으면 다음 접속부터는 접속 시도를 DROP할 수 있다.
# iptables -A INPUT -p tcp --dport 80 --syn -m connlimit --connlimit-above 10 -j DROP

3. 프로세스를 트리 형태로 보기

[root@localhost ~]# ps aux --forest

4. 네트웍 접속상황 보기

[root@localhost ~]# netstat -plan

5. 현재 네트웍 세션 보기(요약정보 보기)

[root@localhost ~]# netstat -anp |grep 'tcp\|udp' | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n
      1 192.168.1.200
      5
     20 0.0.0.0
[root@localhost ~]#

awk 쉘스크립트 프로그래밍 공부 필수.


- iptables 맨페이지 보기 -

[root@localhost ~]# man iptables | col -b | cat
IPTABLES(8)                                                        IPTABLES(8)

 

NAME
       iptables - administration tool for IPv4 packet filtering and NAT

SYNOPSIS
       iptables [-t table] {-A|-D} chain rule-specification [options...]
       iptables [-t table] -I [rulenum] rule-specification [options...]
       iptables [-t table] -R rulenum rule-specification [options...]
       iptables [-t table] -D chain rulenum [options...]
       iptables [-t table] -S [chain]
       iptables [-t table] {-F|-L|-Z} [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
       Iptables  is  used  to set up, maintain, and inspect the tables of IPv4
       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  ta-
       ble.


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 the  examined;  if
       it does match, then the next rule is specified by the value of the tar-
       get, which can be the name of a user-defined chain or one of  the  spe-
       cial values ACCEPT, DROP, QUEUE or RETURN.

       ACCEPT  means to let the packet through.  DROP means to drop the packet
       on the floor.  QUEUE means to pass the packet to userspace.   (How  the
       packet can be received by a userspace process differs by the particular
       queue handler.  2.4.x and  2.6.x  kernels  up  to  2.6.13  include  the
       ip_queue  queue handler.  Kernels 2.6.14 and later additionally include
       the nfnetlink_queue queue handler.  Packets with a target of QUEUE will
       be  sent  to queue number ’0’ in this case. Please also see the NFQUEUE
       target as described  later  in  this  man  page.)   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 three independent tables (which tables are  present
       at  any time depends on the kernel configuration options and which mod-
       ules are present).

       -t, --table table
              This option specifies the packet matching table which  the  com-
              mand  should operate on.  If the kernel is configured with auto-
              matic module loading, an attempt will be made to load the appro-
              priate 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-generated packets before rout-
                  ing), and POSTROUTING (for  altering  packets  as  they  are
                  about to go out).

              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 sup-
                  ported: INPUT (for packets coming into the box itself), FOR-
                  WARD  (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_conntrack, 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)

OPTIONS
       The options that are recognized by iptables 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  letters  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 combina-
              tion.

       -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 specified.

       -R, --replace chain rulenum rule-specification
              Replace a rule in the selected chain.  If the source and/or des-
              tination 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 ipt-
              ables  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]
              Zero the packet and byte counters in all chains.  It is legal to
              specify  the  -L, --list (list) option as well, to see the coun-
              ters 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  sec-
              tion  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).

       [!] -p, --protocol protocol
              The protocol of the rule or of the packet to check.  The  speci-
              fied  protocol  can  be one of tcp, udp, udplite, icmp, esp, ah,
              sctp or 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  pro-
              tocol  inverts  the test.  The number zero is equivalent to all.
              Protocol all will match with  all  protocols  and  is  taken  as
              default when this option is omitted.

       [!] -s, --source address[/mask]
              Source  specification.  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.  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.  Thus, a 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.

       [!] -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.

       -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 con-
              tinue 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 frag-
              ments 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.

       -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 multipli-
              ers 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.

       -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  appli-
              cable).

       -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 relevant 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 EXTENSIONS
       iptables can use extended packet matching modules.  These are loaded in
       two  ways:  implicitly, when -p or --protocol is specified, or 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  mod-
       ules  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 following are included in the base package, and most of  these  can
       be preceded by a "!" to invert the sense of the match.

   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 PREROUT-
              ING, INPUT and FORWARD chains. It cannot be specified  with  the
              --limit-iface-out option.

       --limit-iface-out
              The  address  type checiking can be limited to the interface the
              packet is going out. This option is only valid in the  POSTROUT-
              ING,  OUTPUT and FORWARD chains. It cannot be specified with the
              --limit-iface-in option.

   ah
       This module matches the SPIs in Authentication header of IPsec packets.

       [!] --ahspi spi[:spi]

   comment
       Allows you to add comments (up to 256 characters) to any rule.

       --comment comment

       Example:
              iptables -A INPUT -s 192.168.0.0/16 -m comment --comment "A pri-
              vatized IP block"

   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 aver-
       age 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  transferred  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 trans-
              ferred or the average size (in bytes) of all packets received so
              far.  Note  that when "both" is used together with "avgpkt", and
              data is going (mainly) only in one direction (for example HTTP),
              the  average  packet  size will be about half of the actual data
              packets.

       Example:
              iptables .. -m connbytes --connbytes  10000:100000  --connbytes-
              dir both --connbytes-mode bytes ...

   connlimit
       Allows  you  to restrict the number of parallel connections to a server
       per client IP address (or client address block).

       [!] --connlimit-above n
              Match if the number of existing connections is (not) above n.

       --connlimit-mask prefix_length
              Group hosts using the prefix length. For IPv4, this  must  be  a
              number  between  (including)  0  and 32. For IPv6, between 0 and
              128.

       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-above 2 -j ACCEPT

       # limit the number 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

       #  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

   connmark
       This module matches the netfilter mark field associated with a  connec-
       tion (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

       [!] --ctorigdstport port

       [!] --ctreplsrcport port

       [!] --ctrepldstport port
              Match    against    original/reply    source/destination    port
              (TCP/UDP/etc.) or GRE key.

       [!] --ctstatus statelist
              statuslist is a comma separated list of the connection  statuses
              to match.  Possible statuses are listed below.

       [!] --ctexpire time[:time]
              Match remaining lifetime in seconds against given value or range
              of values (inclusive)

       --ctdir {ORIGINAL|REPLY}
              Match packets that are flowing in the  specified  direction.  If
              this  flag  is  not  specified  at  all, matches packets in both
              directions.

       States for --ctstate:

       INVALID
              meaning that the packet is associated with no known connection

       NEW    meaning that the packet has started a new connection, or  other-
              wise  associated with a connection which has not seen packets in
              both directions, and

       ESTABLISHED
              meaning that the packet is associated with  a  connection  which
              has seen packets in both directions,

       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.

       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.

   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 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 accord-
              ing numeric value.

   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]

   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":

       matching on source host
              "1000 packets per second for every host in 192.168.0.0/16"

       matching on source prot
              "100 packets per second for every service of 192.168.1.1"

       matching on subnet
              "10000 packets per minute for every /28 subnet in 10.0.0.0/8"

       A hash limit option (--hashlimit-upto, --hashlimit-above)  and  --hash-
       limit-name are required.

       --hashlimit-upto amount[/second|/minute|/hour|/day]
              Match  if  the  rate  is below or equal to amount/quantum. It is
              specified as a number, with an optional time quantum suffix; the
              default is 3/hour.

       --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.

       --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-src-
              mask 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.

   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 can be used if ‘--protocol icmp’ is specified.  It  pro-
       vides 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 IP addresses.

       [!] --src-range from[-to]
              Match source IP in the specified range.

       [!] --dst-range from[-to]
              Match destination IP in the specified range.

   length
       This  module  matches  the  length of the layer-3 payload (e.g. layer-4
       packet) f 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[/second|/minute|/hour|/day]
              Maximum  average  matching  rate: specified as a number, with an
              optional ‘/second’, ‘/minute’, ‘/hour’, or  ‘/day’  suffix;  the
              default is 3/hour.

       --limit-burst number
              Maximum  initial  number  of  packets to match: this number gets
              recharged by one every time the limit  specified  above  is  not
              reached, up to this number; the default is 5.

   mac
       [!] --mac-source address
              Match   source   MAC   address.    It   must   be  of  the  form
              XX:XX:XX:XX:XX:XX.  Note that this only makes sense for  packets
              coming from an Ethernet device and entering the PREROUTING, FOR-
              WARD 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 com-
              parison).

   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,--sport 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,--dport 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.

   owner
       This  module  attempts  to  match various characteristics of the packet
       creator, for locally generated packets. This match is only valid in the
       OUTPUT and POSTROUTING chains. Forwarded packets do not have any socket
       associated with them. Packets from kernel threads do have a socket, but
       usually no owner.

       [!] --uid-owner username

       [!] --uid-owner userid[-userid]
              Matches if the packet socket’s file structure (if it has one) is
              owned by the given user. You may also specify a  numerical  UID,
              or an UID range.

       [!] --gid-owner groupname

       [!] --gid-owner groupid[-groupid]
              Matches  if  the  packet socket’s file structure is owned by the
              given group.  You may also specify a numerical  GID,  or  a  GID
              range.

       [!] --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  infrastruc-
       ture 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 inter-
              face 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  POSTROUT-
              ING 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 decapsula-
              tion 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.

   quota
       Implements  network  quotas  by  decrementing  a byte counter with each
       packet.

       --quota bytes
              The quota in bytes.


   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). If not a
              number, value can be a named realm from  /etc/iproute2/rt_realms
              (mask can not be used in that case).

   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 disallow-
              ing 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  informa-
       tion 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 associ-
              ated with a connection which has seen  packets  in  both  direc-
              tions, 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.

   statistic
       This module matches packets based on some statistic condition.  It sup-
       ports 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  from  0  to 1 for a packet to be randomly
              matched. It works only with the random mode.

       --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 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 can be used if ‘--protocol tcp’ is specified. It  pro-
       vides the following options:

       [!] --source-port,--sport port[:port]
              Source  port  or  port range specification. This can either be a
              service name or a port number. An inclusive range  can  also  be
              specified,  using  the  format  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,--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-sepa-
              rated  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  connec-
              tion 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.

   time
       This matches if the packet arrival time/date is within a  given  range.
       All options are optional, but are ANDed when specified.

       --datestart YYYY[-MM[-DD[Thh[:mm[:ss]]]]]

       --datestop YYYY[-MM[-DD[Thh[:mm[:ss]]]]]

              Only  match during the given time, which must be in ISO 8601 "T"
              notation.  The possible time  range  is  1970-01-01T00:00:00  to
              2038-01-19T04:17:07.

              If  --datestart or --datestop are not specified, it will default
              to 1970-01-01 and 2038-01-19, respectively.

       --timestart hh:mm[:ss]

       --timestop hh:mm[:ss]

              Only match during the given daytime. The possible time range  is
              00:00:00  to 23:59:59. Leading zeroes are allowed (e.g. "06:03")
              and correctly interpreted as base-10.

       [!] --monthdays day[,day...]

              Only match on the given days of the month. Possible values are 1
              to  31.  Note  that  specifying  31  will of course not match on
              months which do not have a 31st day; the same goes  for  28-  or
              29-day February.

       [!] --weekdays day[,day...]

              Only  match on the given weekdays. Possible values are Mon, Tue,
              Wed, Thu, Fri, Sat, Sun, or values from 1  to  7,  respectively.
              You may also use two-character variants (Mo, Tu, etc.).

       --utc

              Interpret  the times given for --datestart, --datestop, --times-
              tart and --timestop to be UTC.

       --localtz

              Interpret the times given for --datestart, --datestop,  --times-
              tart and --timestop to be local kernel time. (Default)

       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.)

   tos
       This  module matches the 8-bit Type of Service field in the IPv4 header
       (i.e.  including the "Precedence" bits) or the  (also  8-bit)  Priority
       field in the IPv6 header.

       [!] --tos value[/mask]
              Matches  packets  with  the  given  TOS mark value. If a mask is
              specified, it is logically ANDed with the TOS  mark  before  the
              comparison.

       [!] --tos symbol
              You  can  specify  a  symbolic name when using the tos match for
              IPv4. The list of recognized TOS names can be obtained by  call-
              ing  iptables  with -m tos -h.  Note that this implies a mask of
              0x3F, i.e. all but the ECN bits.

   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 tests whether quantities of up to 4 bytes extracted from  a  packet
       have  specified values. The specification of what to extract is general
       enough to find data at given offsets from tcp headers or payloads.

       [!] --u32 tests
              The argument amounts to a program in a small language  described
              below.

              tests := location "=" value | tests "&&" location "=" value

              value := range | value "," range

              range := number | number ":" number

       a  single number, n, is interpreted the same as n:n. n:m is interpreted
       as the range of numbers >=n and <=m.

           location := number | location operator number

           operator := "&" | "<<" | ">>" | "@"

       The operators &, <<, >> and && mean the same as in C.  The = is  really
       a  set  membership operator and the value syntax describes a set. The @
       operator is what allows moving to the next header and is described fur-
       ther 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 char-
       acters that do occur there are likely to require shell quoting,  so  it
       is a good idea to enclose the arguments in quotes.

       Example:

              match IP packets with total length >= 256

              The IP header contains a total length field in bytes 2-3.

              --u32 "0 & 0xFFFF = 0x100:0xFFFF"

              read bytes 0-3

              AND  that  with 0xFFFF (giving bytes 2-3), and test whether that
              is in the range [0x100:0xFFFF]

       Example: (more realistic, hence more complicated)

              match ICMP packets with icmp type 0

              First test that it is an ICMP packet, true iff byte 9 (protocol)
              = 1

              --u32 "6 & 0xFF = 1 && ...

              read  bytes  6-9,  use & to throw away bytes 6-8 and compare the
              result to 1. Next test that it is not a  fragment.  (If  so,  it
              might be part of such a packet but we cannot always tell.) N.B.:
              This test is generally needed if  you  want  to  match  anything
              beyond  the IP header. The last 6 bits of byte 6 and all of byte
              7 are 0 iff this is a complete packet (not a fragment). Alterna-
              tively, 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 pro-
       vides the following options:

       [!] --source-port,--sport port[:port]
              Source port or port range specification.  See the description of
              the --source-port option of the TCP extension for details.

       [!] --destination-port,--dport port[:port]
              Destination port or port range specification.  See the  descrip-
              tion  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.

   CLASSIFY
       This module allows you to set the skb->priority value (and  thus  clas-
       sify 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  mul-
              ticast 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-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 unla-
       beled).  Typically used in conjunction with SECMARK, it is  only  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  connec-
              tion  does, copy the security marking from the connection to the
              packet.


   DNAT
       This target is only valid in the nat table, in the PREROUTING and  OUT-
       PUT  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 inclu-
              sive 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. If no IP address is specified then only the desti-
              nation port will 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 desti-
              nation 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.

       --random
              If option --random is used then port mapping will be  randomized
              (kernel >= 2.6.22).


   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.

   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  match-
       ing  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 tar-
       get",  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 target is used to set the Netfilter mark value associated with the
       packet.  The target can only be used in the mangle table. It  can,  for
       example,  be  used  in  conjunction with routing based on fwmark (needs
       iproute2).

       --set-xmark value[/mask]
              Zeroes out the bits given by mask and XORs value into the packet
              mark ("nfmark"). If mask is omitted, 0xFFFFFFFF is assumed.

       --set-mark value[/mask]
              Zeroes  out the bits given by mask and ORs value into the packet
              mark. If mask is omitted, 0xFFFFFFFF is assumed.

       The following mnemonics are available:

       --and-mark bits
              Binary AND the  nfmark  with  bits.  (Mnemonic  for  --set-xmark
              0/invbits, where invbits is the binary negation of bits.)

       --or-mark bits
              Binary  OR  the  nfmark  with  bits.  (Mnemonic  for --set-xmark
              bits/bits.)

       --xor-mark bits
              Binary XOR the  nfmark  with  bits.  (Mnemonic  for  --set-xmark
              bits/0.)

   MASQUERADE
       This  target  is only valid in the nat table, in the POSTROUTING chain.
       It should only be used with dynamically assigned  IP  (dialup)  connec-
       tions: 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  any-
       way).  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.

       --random
              Randomize  source  port  mapping If option --random is used then
              port mapping will be randomized (kernel >= 2.6.21).


   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,  connec-
       tion 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 con-
              structed 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.

   NFLOG
       This target provides logging of matching packets. When this  target  is
       set  for  a  rule,  the Linux kernel will pass the packet to the loaded
       logging backend to log the packet. This is usually used in  combination
       with  nfnetlink_log as logging backend, which will multicast the packet
       through a netlink socket to the specified multicast group. One or  more
       userspace  processes may subscribe to the group to receive the packets.
       Like LOG, this is a non-terminating target, i.e. rule traversal contin-
       ues at the next rule.

       --nflog-group nlgroup
              The netlink group (1 - 2^32-1) to which packets are (only appli-
              cable for nfnetlink_log). The default value is 0.

       --nflog-prefix prefix
              A prefix string to include in the log message, up to 64  charac-
              ters long, useful for distinguishing messages in the logs.

       --nflog-range size
              The  number  of bytes to be copied to userspace (only applicable
              for nfnetlink_log). nfnetlink_log instances  may  specify  their
              own range, this option overrides it.

       --nflog-threshold size
              Number of packets to queue inside the kernel before sending them
              to userspace (only applicable for nfnetlink_log). Higher  values
              result in less overhead per packet, but increase delay until the
              packets reach userspace. The default value is 1.

   NFQUEUE
       This target 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. Valid 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.

   RATEEST
       The RATEEST target collects statistics, performs rate estimation calcu-
       lation  and  saves  the  results for later evaluation using the rateest
       match.

       --rateest-name name
              Count matched packets into the pool referred to by  name,  which
              is freely choosable.

       --rateest-interval amount{s|ms|us}
              Rate measurement interval, in seconds, milliseconds or microsec-
              onds.

       --rateest-ewmalog value

   REDIRECT
       This target is only valid in the nat table, in the PREROUTING and  OUT-
       PUT  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).

       --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.

       --random
              If option --random is used then port mapping will be  randomized
              (kernel >= 2.6.22).


   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  TAR-
       GET,  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-unreach-
              able 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-/destina-
       tion-address for each connection.

       --to ipaddr[-ipaddr]
              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

       --random
              Port  mapping will be forcibly randomized to avoid attacks based
              on port prediction (kernel >= 2.6.21).

   SECMARK
       This is used to set the security mark value associated with the  packet
       for  use  by  security subsystems such as SELinux.  It is only valid in
       the mangle table.

       --selctx security_context

   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  alter-
              ation will

              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.

       --random
              If option --random is used then port mapping will be  randomized
              (kernel >= 2.6.21).


   TCPMSS
       This  target  allows to alter the MSS value of TCP SYN packets, to con-
       trol the maximum size for that connection (usually limiting it to  your
       outgoing  interface’s  MTU  minus  40  for IPv4 or 60 for IPv6, respec-
       tively).  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"  or  "ICMPv6  Packet  Too  Big"
       packets.   The  symptoms of this problem are that everything works fine
       from your Linux firewall/router,  but  machines  behind  it  can  never
       exchange large packets:
        1) Web browsers connect, then hang with no data received.
        2) Small mail works fine, but large emails hang.
        3) ssh works fine, but scp hangs after initial handshaking.
       Workaround:  activate  this option and add a rule to your firewall con-
       figuration 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  for  IPv4;  -60
              for IPv6).

       These options are mutually exclusive.

   TCPOPTSTRIP
       This  target will strip TCP options off a TCP packet. (It will actually
       replace them by NO-OPs.) As such, you will  need  to  add  the  -p  tcp
       parameters.

       --strip-options option[,option...]
              Strip  the  given option(s). The options may be specified by TCP
              option number or  by  symbolic  name.  The  list  of  recognized
              options  can be obtained by calling iptables with -j TCPOPTSTRIP
              -h.

   TOS
       This module sets the Type of Service field in the IPv4 header  (includ-
       ing  the  shares  the same bits as DSCP and ECN. The TOS target is only
       valid in the mangle table.

       --set-tos value[/mask]
              Zeroes out the bits given  by  mask  and  XORs  value  into  the
              TOS/Priority field. If mask is omitted, 0xFF is assumed.

       --set-tos symbol
              You  can  specify  a symbolic name when using the TOS target for
              IPv4. It implies a mask of 0xFF.  The  list  of  recognized  TOS
              names can be obtained by calling iptables with -j TOS -h.

       The following mnemonics are available:

       --and-tos bits
              Binary  AND  the  TOS  value  with bits. (Mnemonic for --set-tos
              0/invbits, where invbits is the binary negation of bits.)

       --or-tos bits
              Binary OR the TOS  value  with  bits.  (Mnemonic  for  --set-tos
              bits/bits.)

       --xor-tos bits
              Binary  XOR  the  TOS  value  with bits. (Mnemonic for --set-tos
              bits/0.)

   TRACE
       This target marks packes so that the kernel will log every  rule  which
       match  the  packets  as  those traverse the tables, chains, rules. (The
       ipt_LOG or ip6t_LOG module is required for the  logging.)  The  packets
       are   logged   with   the   string   prefix:  "TRACE:  tablename:chain-
       name:type:rulenum " where type can be "rule" for plain  rule,  "return"
       for  implicit  rule at the end of a user defined chain and "policy" for
       the policy of the built in chains.
       It can only be used in the raw table.

   TTL
       This is used to modify the IPv4 TTL header field.  The TTL field deter-
       mines  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  danger-
       ous,
              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 sub-
       scribe  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  charac-
              ters 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).

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.

       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 combination of IP masquerading and packet filtering seen  pre-
       viously.  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  netfil-
       ter-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 every-
       where.

       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>.

 

                                 Jul 03, 2008                      IPTABLES(8)
[root@localhost ~]#


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