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 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 specific action to perform. Only one of them
can be specified on the command line unless otherwise specified below.
For all the long versions of the command and option names, you need to
use only enough letters to ensure that iptables can differentiate it
from all other options.
-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. As 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 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. 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, icmp, or all, or it can be
a numeric value, representing one of these protocols or a dif-
ferent 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 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 PKTS 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.
account
Account traffic for all hosts in defined network/netmask.
Features:
- long (one counter per protocol TCP/UDP/IMCP/Other) and short statis-
tics
- one iptables rule for all hosts in network/netmask
- loading/saving counters (by reading/writting to procfs entries)
–aaddr network/netmask
defines network/netmask for which make statistics.
–aname name
defines name of list where statistics will be kept. If no is
specified DEFAULT will be used.
–ashort
table will colect only short statistics (only total counters
without splitting it into protocols.
Example usage:
account traffic for/to 192.168.0.0/24 network into table mynetwork:
# iptables -A FORWARD -m account –aname mynetwork –aaddr
192.168.0.0/24
account traffic for/to WWW serwer for 192.168.0.0/24 network into table
mywwwserver:
# iptables -A INPUT -p tcp –dport 80
-m account –aname mywwwserver –aaddr 192.168.0.0/24 –ashort
# iptables -A OUTPUT -p tcp –sport 80
-m account –aname mywwwserver –aaddr 192.168.0.0/24 –ashort
read counters:
# cat /proc/net/ipt_account/mynetwork # cat
/proc/net/ipt_account/mywwwserver
set counters:
# echo “ip = 192.168.0.1 packets_src = 0″ >
/proc/net/ipt_account/mywwserver
Webpage:
http://www.barbara.eu.org/~quaker/ipt_account/
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 black-
hole address UNREACHABLE an unreachable address PROHIBIT a pro-
hibited address THROW FIXME NAT FIXME XRESOLVE FIXME
–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 AH header of IPSec packets.
–ahspi [!] spi[:spi]
childlevel
This is an experimental module. It matches on whether the packet is
part of a master connection or one of its children (or grandchildren,
etc). For instance, most packets are level 0. FTP data transfer is
level 1.
–childlevel [!] level
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”
condition
This matches if a specific /proc filename is ‘0′ or ‘1′.
–condition [!] filename
Match on boolean value stored in /proc/net/ipt_condition/file-
name file
connbytes
Match by how many bytes or packets a connection (or one of the two
flows constituting the connection) have tranferred so far, or by 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 transfered bytes per connection can also be viewed through
/proc/net/ip_conntrack and accessed via ctnetlink
[!] –connbytes from:[to]
match packets from a connection whose packets/bytes/average
packet size is more than FROM and less than TO bytes/packets. if
TO is omitted only FROM check is done. “!” is used to match
packets not falling in the range.
–connbytes-dir [original|reply|both]
which packets to consider
–connbytes-mode [packets|bytes|avgpkt]
whether to check the amount of packets, number of bytes 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 TCP connections to a
server per client IP address (or address block).
[!] –connlimit-above n
match if the number of existing tcp connections is (not) above n
–connlimit-mask bits
group hosts using mask
Examples:
# allow 2 telnet connections per client host
iptables -p tcp –syn –dport 23 -m connlimit –connlimit-above
2 -j REJECT
# you can also match the other way around:
iptables -p tcp –syn –dport 23 -m connlimit ! –connlimit-
above 2 -j ACCEPT
# limit the nr of parallel http requests to 16 per class C sized net-
work (24 bit netmask)
iptables -p tcp –syn –dport 80 -m connlimit –connlimit-above
16 –connlimit-mask 24 -j REJECT
connmark
This module matches the netfilter mark field associated with a 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).
connrate
This module matches the current transfer rate in a connection.
–connrate [!] [from]:[to]
Match against the current connection transfer rate being within
‘from’ and ‘to’ bytes per second. When the “!” argument is used
before the range, the sense of the match is inverted.
conntrack
This module, when combined with connection tracking, allows access to
more connection tracking information than the “state” match. (this
module is present only if iptables was compiled under a kernel support-
ing 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 pack-
ets 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 mean-
ing that the packet is starting a new connection, but is associ-
ated with an existing connection, such as an FTP data transfer,
or an ICMP error. SNAT A virtual state, matching if the origi-
nal 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)
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 it’s
according numeric value.
dstlimit
This module allows you to limit the packet per second (pps) rate on a
per destination IP or per destination port base. As opposed to the
`limit’ match, every destination ip / destination port has it’s own
limit.
–dstlimit avg
Maximum average match rate (packets per second unless followed
by /sec /minute /hour /day postfixes).
–dstlimit-mode mode
The limiting hashmode. Is the specified limit per dstip, dstip-
dstport tuple, srcip-dstip tuple, or per srcipdstip-dstport
tuple.
–dstlimit-name name
Name for /proc/net/ipt_dstlimit/* file entry
[--dstlimit-burst burst]
Number of packets to match in a burst. Default: 5
[--dstlimit-htable-size size]
Number of buckets in the hashtable
[--dstlimit-htable-max max]
Maximum number of entries in the hashtable
[--dstlimit-htable-gcinterval interval]
Interval between garbage collection runs of the hashtable (in
miliseconds). Default is 1000 (1 second).
[--dstlimit-htable-expire time
After which time are idle entries expired from hashtable (in
miliseconds)? Default is 10000 (10 seconds).
ecn
This allows you to match the ECN bits of the IPv4 and TCP header. ECN
is the Explicit Congestion Notification mechanism as specified in
RFC3168
--ecn-tcp-cwr
This matches if the TCP ECN CWR (Congestion Window Received) bit
is set.
--ecn-tcp-ece
This matches if the TCP ECN ECE (ECN Echo) bit is set.
--ecn-ip-ect num
This matches a particular IPv4 ECT (ECN-Capable Transport). You
have to specify a number between `0' and `3'.
esp
This module matches the SPIs in ESP header of IPSec packets.
--espspi [!] spi[:spi]
fuzzy
This module matches a rate limit based on a fuzzy logic controller
[FLC]
–lower-limit number”
Specifies the lower limit (in packets per second).
–upper-limit number
Specifies the upper limit (in packets per second).
geoip
Match a packet by its source or destination country.
[!] –src-cc, –source-country country[,country,country,...]
Match packet coming from (one of) the specified country(ies)
[!] –dst-cc, –destination-country country[,country,country,...]
Match packet going to (one of) the specified country(ies)
NOTE: The country is inputed by its ISO3166 code.
The only extra files you need is a binary db (geoipdb.bin) & its index
file (geoipdb.idx). Both files are generated from a countries & sub-
nets database with the csv2bin tool, available at www.cook-
inglinux.org/geoip/. Both files MUST also be moved in /var/geoip/ as
the shared library is statically looking for that pathname (ex.:
/var/geoip/geoipdb.bin).
hashlimit
This patch adds a new match called ‘hashlimit’. The idea is to have
something like ‘limit’, but either per destination-ip or per (des-
tip,destport) tuple.
It gives you the ability to express
‘1000 packets per second for every host in 192.168.0.0/16′
‘100 packets per second for every service of 192.168.1.1′
with a single iptables rule.
–hashlimit rate
A rate just like the limit match
–hashlimit-burst num
Burst value, just like limit match
–hashlimit-mode destip | destip-destport
Limit per IP or per port
–hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry
–hashlimit-htable-size num
The number of buckets of the hash table
–hashlimit-htable-max num
Maximum entries in the hash
–hashlimit-htable-expire num
After how many miliseconds do hash entries expire
–hashlimit-htable-gcinterval num
How many miliseconds between garbage collection intervals
helper
This module matches packets related to a specific conntrack-helper.
–helper string
Matches packets related to the specified conntrack-helper.
string can be “ftp” for packets related to a ftp-session on
default port. For other ports append -portnr to the value, ie.
“ftp-2121″.
Same rules apply for other conntrack-helpers.
icmp
This extension is loaded if `–protocol icmp’ is specified. It 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
ipp2p
This module matches certain packets in P2P flows. It is not designed to
match all packets belonging to a P2P connection - use IPP2P together
with CONNMARK for this purpose. Also visit http://www.ipp2p.org for
detailed information.
Use it together with -p tcp or -p udp to search these protocols only or
without -p switch to search packets of both protocols.
IPP2P provides the following options:
–edk Matches as many eDonkey/eMule packets as possible.
–kazaa
Matches as many KaZaA packets as possible.
–gnu Matches as many Gnutella packets as possible.
–dc Matches as many Direct Connect packets as possible.
–bit Matches BitTorrent packets.
–apple
Matches AppleJuice packets.
–soul Matches some SoulSeek packets. Considered as beta, use careful!
–winmx
Matches some WinMX packets. Considered as beta, use careful!
–ares Matches Ares and AresLite packets. Use together with -j DROP
only.
–ipp2p
Short hand for: –edk –kazaa –gnu –dc
–debug
Prints some information about each hit into kernel logfile. May
produce huge logfiles so beware!
iprange
This matches on a given arbitrary range of IPv4 addresses
[!]–src-range ip-ip
Match source IP in the specified range.
[!]–dst-range ip-ip
Match destination IP in the specified range.
ipv4options
Match on IPv4 header options like source routing, record route, times-
tamp and router-alert.
–ssrr To match packets with the flag strict source routing.
–lsrr To match packets with the flag loose source routing.
–no-srr
To match packets with no flag for source routing.
[!] –rr
To match packets with the RR flag.
[!] –ts
To match packets with the TS flag.
[!] –ra
To match packets with the router-alert option.
[!] –any-opt
To match a packet with at least one IP option, or no IP option
at all if ! is chosen.
Examples:
$ iptables -A input -m ipv4options –rr -j DROP
will drop packets with the record-route flag.
$ iptables -A input -m ipv4options –ts -j DROP
will drop packets with the timestamp flag.
length
This module matches the length of a packet against a specific value or
range of values.
–length length[:length]
limit
This module matches at a limited rate using a token bucket filter. A
rule using this extension will match until this limit is reached
(unless the `!’ flag is used). It can be used in combination with the
LOG target to give limited logging, for example.
–limit rate
Maximum average matching rate: specified as a number, with an
optional `/second’, `/minute’, `/hour’, or `/day’ suffix; the
default is 3/hour.
–limit-burst number
Maximum initial number of packets to match: this number gets
recharged by one every time the limit specified above is not
reached, up to this number; the default is 5.
mac
–mac-source [!] address
Match source MAC address. It must be of the form
XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets
coming from an Ethernet device and entering the PREROUTING, 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).
mport
This module matches a set of source or destination ports. Up to 15
ports can be specified. It can only be used in conjunction with -p tcp
or -p udp.
–source-ports port[,port[,port...]]
Match if the source port is one of the given ports. The flag
–sports is a convenient alias for this option.
–destination-ports port[,port[,port...]]
Match if the destination port is one of the given ports. The
flag –dports is a convenient alias for this option.
–ports port[,port[,port...]]
Match if the both the source and destination ports are equal to
each other and to one of the given ports.
multiport
This module matches a set of source or destination ports. Up to 15
ports can be specified. A port range (port:port) counts as two ports.
It can only be used in conjunction with -p tcp or -p udp.
–source-ports [!] port[,port[,port:port...]]
Match if the source port is one of the given ports. The flag
–sports is a convenient alias for this option.
–destination-ports [!] port[,port[,port:port...]]
Match if the destination port is one of the given ports. The
flag –dports is a convenient alias for this option.
–ports [!] port[,port[,port:port...]]
Match if either the source or destination ports are equal to one
of the given ports.
nth
This module matches every `n’th packet
–every value
Match every `value’ packet
[--counter num]
Use internal counter number `num’. Default is `0′.
[--start num]
Initialize the counter at the number `num’ insetad of `0′. Most
between `0′ and `value’-1.
[--packet num]
Match on `num’ packet. Most be between `0′ and `value’-1.
osf
The idea of passive OS fingerprint matching exists for quite a long
time, but was created as extension fo OpenBSD pf only some weeks ago.
Original idea was lurked in some OpenBSD mailing list (thanks
grange@open…) and than adopted for Linux netfilter in form of this
code.
Original fingerprint table was created by Michal Zalewski
<lcamtuf@coredump.cx>.
This module compares some data(WS, MSS, options and it’s order, ttl, df
and others) from first SYN packet (actually from packets with SYN bit
set) with dynamically loaded OS fingerprints.
–log 1/0
If present, OSF will log determined genres even if they don’t
match desired one. 0 – log all determined entries, 1 – only
first one.
In syslog you find something like this:
ipt_osf: Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]:
11.22.33.55:4024 -> 11.22.33.44:139
ipt_osf: Unknown: 16384:106:1:48:020405B401010402
44.33.22.11:1239 -> 11.22.33.44:80
–smart
if present, OSF will use some smartness to determine remote OS.
OSF will use initial TTL only if source of connection is in our
local network.
–netlink
If present, OSF will log all events also through netlink
NETLINK_NFLOG groupt 1.
–genre [!] string
Match a OS genre by passive fingerprinting
Example:
#iptables -I INPUT -j ACCEPT -p tcp -m osf –genre Linux –log 1
–smart
NOTE: -p tcp is obviously required as it is a TCP match.
Fingerprints can be loaded and read through /proc/sys/net/ipv4/osf
file. One can flush all fingerprints with following command:
echo -en FLUSH > /proc/sys/net/ipv4/osf
Only one fingerprint per open/write/close.
Fingerprints can be downloaded from http://www.openbsd.org/cgi-
bin/cvsweb/src/etc/pf.os
owner
This module attempts to match various characteristics of the packet
creator, for locally-generated packets. It is valid in the INPUT, OUT-
PUT and POSTROUTING chains, however in the INPUT chain only TCP and UDP
packets can be matched. Also note that some packets (such as ICMP ping
responses) may have no owner, and hence never match.
–uid-owner userid
Matches if the packet was created by a process with the given
effective user id.
–gid-owner groupid
Matches if the packet was created by a process with the given
effective group id.
–pid-owner processid
Matches if the packet was created by a process with the given
process id.
–sid-owner sessionid
Matches if the packet was created by a process in the given ses-
sion group.
–cmd-owner name
Matches if the packet was created by a process with the given
command name. (this option is present only if iptables was com-
piled under a kernel supporting this feature)
NOTE: pid, sid and command matching are broken on SMP
physdev
This module matches on the bridge port input and output devices
enslaved to a bridge device. This module is a part of the 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 out-
put 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 address of a tunnel. Only valid with –mode
tunnel.
–tunnel-dst addr[/mask]
Matches the destination address of a tunnel. Only valid with
–mode tunnel.
–next Start the next element in the policy specification. Can only be
used with –strict
psd
Attempt to detect TCP and UDP port scans. This match was derived from
Solar Designer’s scanlogd.
–psd-weight-threshold threshold
Total weight of the latest TCP/UDP packets with different desti-
nation ports coming from the same host to be treated as port
scan sequence.
–psd-delay-threshold delay
Delay (in hundredths of second) for the packets with different
destination ports coming from the same host to be treated as
possible port scan subsequence.
–psd-lo-ports-weight weight
Weight of the packet with privileged (<=1024) destination port.
–psd-hi-ports-weight weight
Weight of the packet with non-priviliged destination port.
quota
Implements network quotas by decrementing a byte counter with each
packet.
–quota bytes
The quota in bytes.
KNOWN BUGS: this does not work on SMP systems.
random
This module randomly matches a certain percentage of all packets.
–average percent
Matches the given percentage. If omitted, a probability of 50%
is set.
realm
This matches the routing realm. Routing realms are used in complex
routing setups involving dynamic routing protocols like BGP.
–realm [!]value[/mask]
Matches a given realm number (and optionally mask).
recent
Allows you to dynamically create a list of IP addresses and then match
against that list in a few different ways.
For example, you can create a `badguy’ list out of people attempting to
connect to port 139 on your firewall and then DROP all future packets
from them without considering them.
–name name
Specify the list to use for the commands. If no name is given
then ‘DEFAULT’ will be used.
[!] –set
This will add the source address of the packet to the list. If
the source address is already in the list, this will update the
existing entry. This will always return success (or failure if
`!’ is passed in).
[!] –rcheck
Check if the source address of the packet is currently in the
list.
[!] –update
Like –rcheck, except it will update the “last seen” timestamp
if it matches.
[!] –remove
Check if the source address of the packet is currently in the
list and if so that address will be removed from the list and
the rule will return true. If the address is not found, false is
returned.
[!] –seconds seconds
This option must be used in conjunction with one of –rcheck or
–update. When used, this will narrow the match to only happen
when the address is in the list and was seen within the last
given number of seconds.
[!] –hitcount hits
This option must be used in conjunction with one of –rcheck or
–update. When used, this will narrow the match to only happen
when the address is in the list and packets had been received
greater than or equal to the given value. This option may be
used along with –seconds to create an even narrower match
requiring a certain number of hits within a specific time frame.
–rttl This option must be used in conjunction with one of –rcheck or
–update. When used, this will narrow the match to only happen
when the address is in the list and the TTL of the current
packet matches that of the packet which hit the –set rule. This
may be useful if you have problems with people faking their
source address in order to DoS you via this module by 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.
tcp
These extensions are loaded if `–protocol tcp’ is specified. It pro-
vides 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-sepa-
rated 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 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.
–mss value[:value]
Match TCP SYN or SYN/ACK packets with the specified MSS value
(or range), which control the maximum packet size for that con-
nection.
tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP
header. You can only use this on TCP SYN or SYN/ACK packets, since the
MSS is only negotiated during the TCP handshake at connection startup
time.
[!] –mss value[:value]”
Match a given TCP MSS value or range.
time
This matches if the packet arrival time/date is within a given range.
All options are facultative.
–timestart value
Match only if it is after `value’ (Inclusive, format: HH:MM ;
default 00:00).
–timestop value
Match only if it is before `value’ (Inclusive, format: HH:MM ;
default 23:59).
–days listofdays
Match only if today is one of the given days. (format:
Mon,Tue,Wed,Thu,Fri,Sat,Sun ; default everyday)
–datestart date
Match only if it is after `date’ (Inclusive, format:
YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to
1970)
–datestop date
Match only if it is before `date’ (Inclusive, format:
YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to
2037)
tos
This module matches the 8 bits of Type of Service field in the IP
header (ie. including the precedence bits).
–tos tos
The argument is either a standard name, (use
iptables -m tos -h
to see the list), or a numeric value to match.
ttl
This module matches the time to live field in the IP header.
–ttl-eq ttl
Matches the given TTL value.
–ttl-gt ttl
Matches if TTL is greater than the given TTL value.
–ttl-lt ttl
Matches if TTL is less than the given TTL value.
u32
U32 allows you to extract quantities of up to 4 bytes from a packet,
AND them with specified masks, shift them by specified amounts and test
whether the results are in any of a set of specified ranges. The spec-
ification of what to extract is general enough to skip over headers
with lengths stored in the packet, as in IP or TCP header lengths.
Details and examples are in the kernel module source.
udp
These extensions are loaded if `–protocol udp’ is specified. It pro-
vides 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 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.
BALANCE
This allows you to DNAT connections in a round-robin way over a given
range of destination addresses.
–to-destination ipaddr-ipaddr
Address range to round-robin over.
CLASSIFY
This module allows you to set the skb->priority value (and thus 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-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 spec-
ified then only those bits are copied. This is only valid in the
mangle table.
DNAT
This target is only valid in the nat table, in the PREROUTING and 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.
You can add several –to-destination options. 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
adresses.
DSCP
This target allows to alter the value of the DSCP bits within the TOS
header of the IPv4 packet. As this manipulates a packet, it can only
be used in the mangle table.
–set-dscp value
Set the DSCP field to a numerical value (can be decimal or hex)
–set-dscp-class class
Set the DSCP field to a DiffServ class.
ECN
This target allows to selectively work around known ECN blackholes. It
can only be used in the mangle table.
–ecn-tcp-remove
Remove all ECN bits from the TCP header. Of course, it can only
be used in conjunction with -p tcp.
IPMARK
Allows you to mark a received packet basing on its IP address. This can
replace many mangle/mark entries with only one, if you use firewall
based classifier.
This target is to be used inside the mangle table, in the PREROUTING,
POSTROUTING or FORWARD hooks.
–addr src/dst
Use source or destination IP address.
–and-mask mask
Perform bitwise `and’ on the IP address and this mask.
–or-mask mask
Perform bitwise `or’ on the IP address and this mask.
The order of IP address bytes is reversed to meet “human order of
bytes”: 192.168.0.1 is 0xc0a80001. At first the `and’ operation is per-
formed, then `or’.
Examples:
We create a queue for each user, the queue number is adequate to the IP
address of the user, e.g.: all packets going to/from 192.168.5.2 are
directed to 1:0502 queue, 192.168.5.12 -> 1:050c etc.
We have one classifier rule:
tc filter add dev eth3 parent 1:0 protocol ip fw
Earlier we had many rules just like below:
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.2 -j MARK
–set-mark 0×10502
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.3 -j MARK
–set-mark 0×10503
Using IPMARK target we can replace all the mangle/mark rules with only
one:
iptables -t mangle -A POSTROUTING -o eth3 -j IPMARK –addr=dst
–and-mask=0xffff –or-mask=0×10000
On the routers with hundreds of users there should be significant load
decrease (e.g. twice).
IPV4OPTSSTRIP
Strip all the IP options from a packet.
The target doesn’t take any option, and therefore is extremly easy to
use :
# iptables -t mangle -A PREROUTING -j IPV4OPTSSTRIP
LOG
Turn on kernel logging of matching packets. When this option is set
for a rule, the Linux kernel will print some information on all 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 is used to set the netfilter mark value associated with the
packet. It is only valid in the mangle table. It can for example be
used in conjunction with iproute2.
–set-mark mark
MASQUERADE
This target is only valid in the nat table, in the POSTROUTING chain.
It should only be used with dynamically assigned IP (dialup) 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.
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.
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.
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 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). It
takes one option:
–to-ports port[-port]
This specifies a destination port or range of ports to use:
without this, the destination port is never altered. This is
only valid if the rule also specifies -p tcp or -p udp.
REJECT
This is used to send back an error packet in response to the matched
packet: otherwise it is equivalent to DROP so it is a terminating 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
ROUTE
This is used to explicitly override the core network stack’s routing
decision. mangle table.
–oif ifname
Route the packet through `ifname’ network interface
–iif ifname
Change the packet’s incoming interface to `ifname’
–gw IP_address
Route the packet via this gateway
–continue
Behave like a non-terminating target and continue traversing the
rules. Not valid in combination with `–iif’ or `–tee’
–tee Make a copy of the packet, and route that copy to the given des-
tination. For the original, uncopied packet, behave like a non-
terminating target and continue traversing the rules. Not valid
in combination with `–iif’ or `–continue’
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
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 occur.
You can add several –to-source options. 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 adresses.
TARPIT
Captures and holds incoming TCP connections using no local per-connec-
tion resources. Connections are accepted, but immediately switched to
the persist state (0 byte window), in which the remote side stops send-
ing data and asks to continue every 60-240 seconds. Attempts to close
the connection are ignored, forcing the remote side to time out the
connection in 12-24 minutes.
This offers similar functionality to LaBrea <http://www.hack-
busters.net/LaBrea/> but doesn’t require dedicated hardware or IPs. Any
TCP port that you would normally DROP or REJECT can instead become a
tarpit.
To tarpit connections to TCP port 80 destined for the current machine:
iptables -A INPUT -p tcp -m tcp –dport 80 -j TARPIT
To significantly slow down Code Red/Nimda-style scans of unused address
space, forward unused ip addresses to a Linux box not acting as a
router (e.g. “ip route 10.0.0.0 255.0.0.0 ip.of.linux.box” on a Cisco),
enable IP forwarding on the Linux box, and add:
iptables -A FORWARD -p tcp -j TARPIT
iptables -A FORWARD -j DROP
NOTE: If you use the conntrack module while you are using TARPIT, you
should also use the NOTRACK target, or the kernel will unneces-
sarily allocate resources for each TARPITted connection. To
TARPIT incoming connections to the standard IRC port while using
conntrack, you could:
iptables -t raw -A PREROUTING -p tcp –dport 6667 -j NOTRACK
iptables -A INPUT -p tcp –dport 6667 -j TARPIT
TCPMSS
This target allows to alter the MSS value of TCP SYN packets, to con-
trol 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.
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 con-
figuration like:
iptables -A FORWARD -p tcp –tcp-flags SYN,RST SYN \
-j TCPMSS –clamp-mss-to-pmtu
–set-mss value
Explicitly set MSS option to specified value.
–clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU – 40).
These options are mutually exclusive.
TOS
This is used to set the 8-bit Type of Service field in the IP header.
It is only valid in the mangle table.
–set-tos tos
You can use a numeric TOS values, or use
iptables -j TOS -h
to see the list of valid TOS names.
TRACE
This target has no options. It just turns on packet tracing for all
packets that match this rule.
TTL
This is used to modify the IPv4 TTL header field. The TTL field 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).
XOR
Encrypt TCP and UDP traffic using a simple XOR encryption
–key string
Set key to “string”
–block-size
Set block size
DIAGNOSTICS
Various error messages are printed to standard error. The exit code is
0 for correct functioning. Errors which appear to be caused by invalid
or abused command line parameters cause an exit code of 2, and other
errors cause an exit code of 1.
BUGS
Bugs? What’s this? Well, you might want to have a look at
http://bugzilla.netfilter.org/
COMPATIBILITY WITH IPCHAINS
This iptables is very similar to ipchains by Rusty Russell. The main
difference is that the chains INPUT and OUTPUT are only traversed for
packets coming into the local host and originating from the local host
respectively. Hence every packet only passes through one of the three
chains (except loopback traffic, which involves both INPUT and OUTPUT
chains); previously a forwarded packet would pass through all three.
The other main difference is that -i refers to the input interface; -o
refers to the output interface, and both are available for packets
entering the FORWARD chain.
iptables is a pure packet filter when using the default `filter’ table,
with optional extension modules. This should simplify much of the pre-
vious confusion over the combination of IP masquerading and packet fil-
tering seen previously. So the following options are handled differ-
ently:
-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
everywhere.
James Morris wrote the TOS target, and tos match.
Jozsef Kadlecsik wrote the REJECT target.
Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as
well as the TTL, DSCP, ECN matches and targets.
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef
Kadlecsik, Patrick McHardy, James Morris, Harald Welte and Rusty Rus-
sell.
Man page originally written by Herve Eychenne <rv@wallfire.org>.
