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Iptables – CheatSheet

Iptables CheatSheet

NAME

iptables – administration tool for IPv4 packet filtering and NAT

SYNOPSIS

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

DESCRIPTION

Iptables is used to set up, maintain, and inspect the tables of IP 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 the 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 or one of the special 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_queuequeue handler. Packets with a target of QUEUE will be sent to queue number ‘0’ in this case. Please also see the NFQUEUEtarget 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 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-generated packets before routing), 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 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_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 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 combination.
-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 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
-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 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).

-p, –protocol [!] protocol
The protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp, icmp, or all, or it can be a numeric value, representing one of these protocols or a different one. A protocol name from /etc/protocols is also allowed. A “!” argument before the protocol 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 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 andPREROUTING 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 andPOSTROUTING 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 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.
-c, –set-counters PKTS BYTES
This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACEoperations).

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.
-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 relevant for the -Lcommand.
–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 modules in one line, and you can use the -h or –help options after the module has been specified to receive help specific to that module.

The 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 addressUNREACHABLE an unreachable address PROHIBIT a prohibited address THROW FIXME NAT FIXME XRESOLVEFIXME
–src-type type
Matches if the source address is of given type
–dst-type type
Matches if the destination address is of given type

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 privatized IP block”

connbytes

Match by how many bytes or packets a connection (or one of the two flows constituting the connection) have transferred so far, or by average bytes per packet.

The counters are 64bit and are thus not expected to overflow ;)

The primary use is to detect long-lived downloads and mark them to be scheduled using a lower priority band in traffic control.

The 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 transferred or the average size (in bytes) of all packets received so far. Note that when “both” is used together with “avgpkt”, and data is going (mainly) only in one direction (for example HTTP), the average packet size will be about half of the actual data packets.
Example:
iptables .. -m connbytes –connbytes 10000:100000 –connbytes-dir both –connbytes-mode bytes …

connmark

This module matches the netfilter mark field associated with a connection (which can be set using the CONNMARK target below).

–mark value[/mask]
Matches packets in connections with the given mark value (if a mask is specified, this is logically ANDed with the mark before the comparison).

conntrack

This module, when combined with connection tracking, allows access to more connection tracking information than the “state” match. (this module is present only if iptables was compiled under a kernel supporting this feature)

–ctstate state
Where state is a comma separated list of the connection states to match. Possible states are INVALID meaning that the packet is associated with no known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in both directions, NEW meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions, and RELATED meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error. SNAT A virtual state, matching if the original source address differs from the reply destination. DNAT A virtual state, matching if the original destination differs from the reply source.
–ctproto proto
Protocol to match (by number or name)
–ctorigsrc [!] address[/mask]
Match against original source address
–ctorigdst [!] address[/mask]
Match against original destination address
–ctreplsrc [!] address[/mask]
Match against reply source address
–ctrepldst [!] address[/mask]
Match against reply destination address
–ctstatus [NONE|EXPECTED|SEEN_REPLY|ASSURED][,…]
Match against internal conntrack states
–ctexpire time[:time]
Match remaining lifetime in seconds against given value or range of values (inclusive)

dccp

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

dscp

This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.

–dscp value
Match against a numeric (decimal or hex) value [0-32].
–dscp-class DiffServ Class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be converted into its according 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

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

It gives you the ability to express

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

with a single iptables rule.

–hashlimit rate
A rate just like the limit match
–hashlimit-burst num
Burst value, just like limit match
–hashlimit-mode dstip,srcip,dstport,srcport
A comma-separated list of objects to take into consideration
–hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry
–hashlimit-htable-size num
The number of buckets of the hash table
–hashlimit-htable-max num
Maximum entries in the hash
–hashlimit-htable-expire num
After how many milliseconds do hash entries expire
–hashlimit-htable-gcinterval num
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 provides the following option:

–icmp-type [!] typename
This allows specification of the ICMP type, which can be a numeric ICMP type, or one of the ICMP type names shown by the command

 iptables -p icmp -h

iprange

This matches on a given arbitrary range of IPv4 addresses

(Please note: This match requires kernel support that might not be available in official Linux kernel sources or Debian’s packaged Linux kernel sources. And if support for this match is available for the specific Linux kernel source version, that support might not be enabled in the current Linux kernel binary.)

[!]–src-range ip-ip
Match source IP in the specified range.
[!]–dst-range ip-ip
Match destination IP in the specified range.

length

This module matches the length of a packet against a specific value or range of values.

–length [!] length[:length]

limit

This module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is reached (unless the `!’ flag is used). It can be used in combination with the LOG target to give limited logging, for example.

–limit rate
Maximum average matching rate: specified as a number, with an optional `/second’, `/minute’, `/hour’, or `/day’ suffix; the default is 3/hour.
–limit-burst number
Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not reached, up to this number; the default is 5.

mac

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

mark

This module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).

–mark value[/mask]
Matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the maskbefore the comparison).

multiport

This module matches a set of source or destination ports. Up to 15 ports can be specified. A port range (port:port) counts as two ports. It can only be used in conjunction with -p tcp or -p udp.

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

owner

This module attempts to match various characteristics of the packet creator, for locally-generated packets. It is only valid in theOUTPUT chain, and even this some packets (such as ICMP ping responses) may have no owner, and hence never match.

–uid-owner userid
Matches if the packet was created by a process with the given effective user id.
–gid-owner groupid
Matches if the packet was created by a process with the given effective group id.
–pid-owner processid
Matches if the packet was created by a process with the given process id. (Please note: This option requires kernel support that might not be available in official Linux kernel sources or Debian’s packaged Linux kernel sources. And if support for this option is available for the specific Linux kernel source version, that support might not be enabled in the current Linux kernel binary.)
–sid-owner sessionid
Matches if the packet was created by a process in the given session group. (Please note: This option requires kernel support that might not be available in official Linux kernel sources or Debian’s packaged Linux kernel sources. And if support for this option is available for the specific Linux kernel source version, that support might not be enabled in the current Linux kernel binary.)
–cmd-owner name
Matches if the packet was created by a process with the given command name. (Please note: This option requires kernel support that might not be available in official Linux kernel sources or Debian’s packaged Linux kernel sources. And if support for this option is available for the specific Linux kernel source version, that support might not be enabled in the current Linux kernel binary.)
NOTE: pid, sid and command matching are broken on SMP

physdev

This module matches on the bridge port input and output devices enslaved to a bridge device. This module is a part of the infrastructure that enables a transparent bridging IP firewall and is only useful for kernel versions above version 2.5.44.

–physdev-in [!] name
Name of a bridge port via which a packet is received (only for packets entering the INPUT, FORWARD andPREROUTING 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 andPOSTROUTING chains). If the interface name ends in a “+”, then any interface which begins with this name will match. Note that in the nat and mangle OUTPUT chains one cannot match on the bridge output port, however one can in thefilter 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 ‘!’ is used.
[!] –physdev-is-in
Matches if the packet has entered through a bridge interface.
[!] –physdev-is-out
Matches if the packet will leave through a bridge interface.
[!] –physdev-is-bridged
Matches if the packet is being bridged and therefore is not being routed. This is only useful in the FORWARD and POSTROUTING chains.

pkttype

This module matches the link-layer packet type.

–pkt-type [unicast|broadcast|multicast]

policy

This modules matches the policy used by IPsec for handling a packet.

–dir in|out
Used to select whether to match the policy used for decapsulation or the policy that will be used for encapsulation. inis 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 –setrule. This may be useful if you have problems with people faking their source address in order to DoS you via this module by disallowing others access to your site by sending bogus packets to you.
–name name
Name of the recent list to be used. DEFAULT used if none given.
–rsource
Match/Save the source address of each packet in the recent list table (default).
–rdest
Match/Save the destination address of each packet in the recent list table.

Examples:

# iptables -A FORWARD -m recent –name badguy –rcheck –seconds 60 -j DROP# iptables -A FORWARD -p tcp -i eth0 –dport 139 -m recent –name badguy –set -j DROP

Official website (http://snowman.net/projects/ipt_recent/) also has some examples of usage.

/proc/net/ipt_recent/* are the current lists of addresses and information about each entry of each list.

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

echo xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Add to the DEFAULT list
echo -xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Remove from the DEFAULT list
echo clear > /proc/net/ipt_recent/DEFAULT
to empty the DEFAULT list.

The module itself accepts parameters, defaults shown:

ip_list_tot=100
Number of addresses remembered per table
ip_pkt_list_tot=20
Number of packets per address remembered
ip_list_hash_size=0
Hash table size. 0 means to calculate it based on ip_list_tot, default: 512
ip_list_perms=0644
Permissions for /proc/net/ipt_recent/* files
debug=0
Set to 1 to get lots of debugging info

sctp

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

chunk type available flags
DATA U B E u b e
ABORT T t
SHUTDOWN_COMPLETE T t

(lowercase means flag should be “off”, uppercase means “on”)

Examples:

iptables -A INPUT -p sctp –dport 80 -j DROP

iptables -A INPUT -p sctp –chunk-types any DATA,INIT -j DROP

iptables -A INPUT -p sctp –chunk-types any DATA:Be -j ACCEPT

state

This module, when combined with connection tracking, allows access to the connection tracking state for this packet.

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

string

This modules matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.

–algo bm|kmp
Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
–from offset
Set the offset from which it starts looking for any matching. If not passed, default is 0.
–to offset
Set the offset from which it starts looking for any matching. If not passed, default is the packet size.
–string pattern
Matches the given pattern. –hex-string pattern Matches the given pattern in hex notation.

tcp

These extensions can be used if `–protocol tcp’ is specified. It provides the following options:

–source-port [!] port[:port]
Source port or port range specification. This can either be a service name or a port number. An inclusive range can also be specified, using the format port:port. If the first port is omitted, “0” is assumed; if the last is omitted, “65535” is assumed. If the second port greater then the first they will be swapped. The flag –sport is a convenient alias for this option.
–destination-port [!] port[:port]
Destination port or port range specification. The flag –dport is a convenient alias for this option.
–tcp-flags [!] mask comp
Match when the TCP flags are as specified. The first argument is the flags which we should examine, written as a comma-separated list, and the second argument is a comma-separated list of flags which must be set. Flags are: SYN ACK FIN RST URG PSH ALL NONE. Hence the command

 iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN

will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset.

[!] –syn
Only match TCP packets with the SYN bit set and the ACK,RST and FIN bits cleared. Such packets are used to request TCP connection initiation; for example, blocking such packets coming in an interface will prevent incoming TCP connections, but outgoing TCP connections will be unaffected. It is equivalent to –tcp-flags SYN,RST,ACK,FIN SYN. If the “!” flag precedes the “–syn”, the sense of the option is inverted.
–tcp-option [!] number
Match if TCP option set.

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.

tos

This module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence bits).

–tos tos
The argument is either a standard name, (use

iptables -m tos -h
to see the list), or a numeric value to match.

ttl

This module matches the time to live field in the IP header.

–ttl-eq ttl
Matches the given TTL value.
–ttl-gt ttl
Matches if TTL is greater than the given TTL value.
–ttl-lt ttl
Matches if TTL is less than the given TTL value.

udp

These extensions can be used if `–protocol udp’ is specified. It provides the following options:

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

unclean

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

TARGET EXTENSIONS

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

CLASSIFY

This module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).

–set-class MAJOR:MINOR
Set the major and minor class value.

CLUSTERIP

This module allows you to configure a simple cluster of nodes that share a certain IP and MAC address without an explicit load balancer in front of them. Connections are statically distributed between the nodes in this cluster.

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

CONNMARK

This module sets the netfilter mark value associated with a connection

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

CONNSECMARK

This module copies security markings from packets to connections (if unlabeled), and from connections back to packets (also only if unlabeled). Typically used in conjunction with SECMARK, it is 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 connection 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 OUTPUT chains, and user-defined chains which are only called from those chains. It specifies that the destination address of the packet should be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes one type of option:

–to-destination [ipaddr][-ipaddr][:portport]
which can specify a single new destination IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then the destination port will never be modified. If no IP address is specified then only the destination 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 destination address, either via an address range or multiple –to-destination options, a simple round-robin (one after another in cycle) load balancing takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don’t have the ability to NAT to multiple ranges anymore.

–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 matching packets (like most IP header fields) via the kernel log (where it can be read with dmesg or syslogd(8)). This is a “non-terminating target”, i.e. rule traversal continues at the next rule. So if you want to LOG the packets you refuse, use two separate rules with the same matching criteria, first using target LOG then DROP (or REJECT).

–log-level level
Level of logging (numeric or see syslog.conf(5)).
–log-prefix prefix
Prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs.
–log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is readable by users.
–log-tcp-options
Log options from the TCP packet header.
–log-ip-options
Log options from the IP packet header.
–log-uid
Log the userid of the process which generated the packet.

MARK

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

–set-mark value
Set nfmark value
–and-mark value
Binary AND the nfmark with value
–or-mark value
Binary OR the nfmark with value

MASQUERADE

This target is only valid in the nat table, in the POSTROUTING chain. It should only be used with dynamically assigned IP (dialup) connections: if you have a static IP address, you should use the SNAT target. Masquerading is equivalent to specifying a mapping to the IP address of the interface the packet is going out, but also has the effect that connections areforgotten when the interface goes down. This is the correct behavior when the next dialup is unlikely to have the same interface address (and hence any established connections are lost anyway). It takes one option:

–to-ports port[-port]
This specifies a range of source ports to use, overriding the default SNAT source port-selection heuristics (see above). This is only valid if the rule also specifies -p tcp or -p udp.
–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, connection tracking or NAT, to avoid loops and other problems.

NETMAP

This target allows you to statically map a whole network of addresses onto another network of addresses. It can only be used from rules in the nat table.

–to address[/mask]
Network address to map to. The resulting address will be constructed in the following way: All ‘one’ bits in the mask are filled in from the new `address’. All bits that are zero in the mask are filled in from the original address.

NFQUEUE

This target is an extension of the QUEUE target. As opposed to QUEUE, it allows you to put a packet into any specific queue, identified by its 16-bit queue number.

–queue-num value
This specifies the QUEUE number to use. 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.

REDIRECT

This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called from those chains. It redirects the packet to the machine itself by changing the destination IP to the primary address of the incoming interface (locally-generated packets are mapped to the 127.0.0.1 address). It takes one option:

–to-ports port[-port]
This specifies a destination port or range of ports to use: without this, the destination port is never altered. This is only valid if the rule also specifies -p tcp or -p udp.
–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 TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains which are only called from those chains. The following option controls the nature of the error packet returned:

–reject-with type
The type given can be

 icmp-net-unreachable
 icmp-host-unreachable
 icmp-port-unreachable
 icmp-proto-unreachable
 icmp-net-prohibited
 icmp-host-prohibited or
 icmp-admin-prohibited (*)

which return the appropriate ICMP error message (port-unreachable is the default). The option tcp-reset can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending mail to broken mail hosts (which won’t accept your mail otherwise).

(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT

SAME

Similar to SNAT/DNAT depending on chain: it takes a range of addresses (`–to 1.2.3.4-1.2.3.7′) and gives a client the same source-/destination-address for each connection.

–to <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

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][:portport]
which can specify a single new source IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then source ports below 512 will be mapped to other ports below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be mapped to 1024 or above. Where possible, no port alteration willIn 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 control the maximum size for that connection (usually limiting it to your outgoing interface’s MTU minus 40). Of course, it can only be used in conjunction with -p tcp. It is only valid in themangle table.
This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation Needed packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines behind it can never exchange large packets:

1)
Web browsers connect, then hang with no data received.
2)
Small mail works fine, but large emails hang.
3)
ssh works fine, but scp hangs after initial handshaking.

Workaround: activate this option and add a rule to your firewall configuration like:

 iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
             -j TCPMSS --clamp-mss-to-pmtu
–set-mss value
Explicitly set MSS option to specified value.
–clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU – 40).
These options are mutually exclusive.

TOS

This is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mangle table.

–set-tos tos
You can use a numeric TOS values, or use

 iptables -j TOS -h

to see the list of valid TOS names.

TTL

This is used to modify the IPv4 TTL header field. The TTL field determines how many hops (routers) a packet can traverse until it’s time to live is exceeded.

Setting or incrementing the TTL field can potentially be very dangerous,
so it should be avoided at any cost.
Don’t ever set or increment the value on packets that leave your local network!
mangle table.
–ttl-set value
Set the TTL value to `value’.
–ttl-dec value
Decrement the TTL value `value’ times.
–ttl-inc value
Increment the TTL value `value’ times.

ULOG

This target provides userspace logging of matching packets. When this target is set for a rule, the Linux kernel will multicast this packet through a netlink socket. One or more userspace processes may then subscribe to various multicast groups and receive the packets. Like LOG, this is a “non-terminating target”, i.e. rule traversal continues at the next rule.

–ulog-nlgroup nlgroup
This specifies the netlink group (1-32) to which the packet is sent. Default value is 1.
–ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32 characters long, and useful for distinguishing messages in the logs.
–ulog-cprange size
Number of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. Default is 0.
–ulog-qthreshold size
Number of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets inside the kernel and transmits them as one netlink multipart message to userspace. Default is 1 (for backwards compatibility).

DIAGNOSTICS

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

BUGS

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

COMPATIBILITY WITH IPCHAINS

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

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

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

 -j MASQ
 -M -S
 -M -L

There are several other changes in iptables.

SEE ALSO

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

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

AUTHORS

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

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

James Morris wrote the TOS target, and tos match.

Jozsef Kadlecsik wrote the REJECT target.

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

The Netfilter Core Team is: Marc Boucher, Martin Josefsson, 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>.

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