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IPTABLES(8) IPTABLES(8)
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 speci-
fies 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_queue queue handler. Packets with a tar-
get 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 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-gen-
erated 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 pack-
ets before routing). Since kernel 2.4.18, three other built-in chains
are also supported: INPUT (for packets coming into the box itself), FOR-
WARD (for altering packets being routed through the box), and POSTROUT-
ING (for altering packets as they are about to go out).
raw:
This table is used mainly for configuring exemptions from connection
tracking in combination with the NOTRACK target. It registers at the
netfilter hooks with higher priority and is thus called before ip_con-
ntrack, or any other IP tables. It provides the following built-in
chains: PREROUTING (for packets arriving via any network interface) OUT-
PUT (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 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 num-
bered 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 sup-
pressed 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 refer-
ences 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 tar-
gets.
-E, --rename-chain old-chain new-chain
Rename the user specified chain to the user supplied name. This is cos-
metic, 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, repre-
senting 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 incre-
mented.
-g, --goto chain
This specifies that the processing should continue in a user specified
chain. Unlike the --jump option return will not continue processing in this
chain but instead in the chain that called us via --jump.
-i, --in-interface [!] name
Name of an interface via which a packet was received (only for packets
entering the INPUT, FORWARD and PREROUTING chains). When the "!" argument
is used before the interface name, the sense is inverted. If the interface
name ends in a "+", then any interface which begins with this name will
match. If this option is omitted, any interface name will match.
-o, --out-interface [!] name
Name of an interface via which a packet is going to be sent (for packets
entering the FORWARD, OUTPUT and POSTROUTING chains). When the "!" argument
is used before the interface name, the sense is inverted. If the interface
name ends in a "+", then any interface which begins with this name will
match. If this option is omitted, any interface name will match.
[!] -f, --fragment
This means that the rule only refers to second and further 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, REPLACE operations).
OTHER OPTIONS
The following additional options can be specified:
-v, --verbose
Verbose output. This option makes the list command show the interface name,
the rule options (if any), and the TOS masks. The packet and byte counters
are also listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and
1,000,000,000 multipliers respectively (but see the -x flag to change this).
For appending, insertion, deletion and replacement, this causes detailed
information on the rule or rules to be printed.
-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
-L command.
--line-numbers
When listing rules, add line numbers to the beginning of each rule, corre-
sponding to that rule's position in the chain.
--modprobe=command
When adding or inserting rules into a chain, use command to load any neces-
sary 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 multi-
ple 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.
account
Account traffic for all hosts in defined network/netmask.
Features:
- long (one counter per protocol TCP/UDP/IMCP/Other) and short statistics
- 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 split-
ting 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 MULTI-
CAST a multicast address BLACKHOLE a blackhole address UNREACHABLE an
unreachable address PROHIBIT a prohibited address THROW FIXME NAT FIXME XRE-
SOLVE 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 Authentication header of IPsec packets.
--ahspi [!] spi[:spi]
childlevel
This is an experimental module. It matches on whether the packet is part of a mas-
ter 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 privatized 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/filename file
connbytes
Match by how many bytes or packets a connection (or one of the two flows constitut-
ing the connection) have tranferred so far, or by average bytes per packet.
The counters are 64bit and are thus not expected to overflow ;)
The primary use is to detect long-lived downloads and mark them to be scheduled
using a lower priority band in traffic control.
The transfered bytes per connection can also be viewed through /proc/net/ip_con-
ntrack and accessed via ctnetlink
[!] --connbytes from:[to]
match packets from a connection whose packets/bytes/average packet size is
more than FROM and less than TO bytes/packets. if TO is omitted only FROM
check is done. "!" is used to match packets not falling in the range.
--connbytes-dir [original|reply|both]
which packets to consider
--connbytes-mode [packets|bytes|avgpkt]
whether to check the amount of packets, number of bytes transferred or the
average size (in bytes) of all packets received so far. Note that when
"both" is used together with "avgpkt", and data is going (mainly) only in
one direction (for example HTTP), the average packet size will be about half
of the actual data packets.
Example:
iptables .. -m connbytes --connbytes 10000:100000 --connbytes-dir both
--connbytes-mode bytes ...
connlimit
Allows you to restrict the number of parallel TCP connections to a server per
client IP address (or address block).
[!] --connlimit-above n
match if the number of existing tcp connections is (not) above n
--connlimit-mask bits
group hosts using mask
Examples:
# allow 2 telnet connections per client host
iptables -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT
# you can also match the other way around:
iptables -p tcp --syn --dport 23 -m connlimit ! --connlimit-above 2 -j
ACCEPT
# limit the nr of parallel http requests to 16 per class C sized network (24 bit
netmask)
iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16
--connlimit-mask 24 -j REJECT
connmark
This module matches the netfilter mark field associated with a connection (which
can be set using the CONNMARK target below).
--mark value[/mask]
Matches packets in connections with the given mark value (if a mask is spec-
ified, 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 connec-
tion 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 connec-
tion 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 desti-
nation. 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-sepa-
rated list of packet types. Packet types are: REQUEST RESPONSE DATA ACK
DATAACK CLOSEREQ CLOSE RESET SYNC SYNCACK INVALID.
--dccp-option [!] number
Match if DCP option set.
dscp
This module matches the 6 bit DSCP field within the TOS field in the IP header.
DSCP has superseded TOS within the IETF.
--dscp value
Match against a numeric (decimal or hex) value [0-63].
--dscp-class DiffServ Class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx
classes. It will then be converted into it's according numeric value.
dstlimit
This module allows you to limit the packet per second (pps) rate on a per destina-
tion IP or per destination port base. As opposed to the 'limit' match, every des-
tination ip / destination port has it's own limit.
THIS MODULE IS DEPRECATED AND HAS BEEN REPLACED BY ''hashlimit''
--dstlimit avg
Maximum average match rate (packets per second unless followed by /sec
/minute /hour /day postfixes).
--dstlimit-mode mode
The limiting hashmode. Is the specified limit per dstip, dstip-dstport
tuple, srcip-dstip tuple, or per srcipdstip-dstport tuple.
--dstlimit-name name
Name for /proc/net/ipt_dstlimit/* file entry
[--dstlimit-burst burst]
Number of packets to match in a burst. Default: 5
[--dstlimit-htable-size size]
Number of buckets in the hashtable
[--dstlimit-htable-max max]
Maximum number of entries in the hashtable
[--dstlimit-htable-gcinterval interval]
Interval between garbage collection runs of the hashtable (in miliseconds).
Default is 1000 (1 second).
[--dstlimit-htable-expire time
After which time are idle entries expired from hashtable (in miliseconds)?
Default is 10000 (10 seconds).
ecn
This allows you to match the ECN bits of the IPv4 and TCP header. ECN is the
Explicit Congestion Notification mechanism as specified in RFC3168
--ecn-tcp-cwr
This matches if the TCP ECN CWR (Congestion Window Received) bit is set.
--ecn-tcp-ece
This matches if the TCP ECN ECE (ECN Echo) bit is set.
--ecn-ip-ect num
This matches a particular IPv4 ECT (ECN-Capable Transport). You have to
specify a number between '0' and '3'.
esp
This module matches the SPIs in ESP header of IPsec packets.
--espspi [!] spi[:spi]
fuzzy
This module matches a rate limit based on a fuzzy logic controller [FLC]
--lower-limit number
Specifies the lower limit (in packets per second).
--upper-limit number
Specifies the upper limit (in packets per second).
hashlimit
This patch adds a new match called 'hashlimit'. The idea is to have something like
'limit', but either per destination-ip or per (destip,destport) tuple.
It gives you the ability to express
'1000 packets per second for every host in 192.168.0.0/16'
'100 packets per second for every service of 192.168.1.1'
with a single iptables rule.
--hashlimit rate
A rate just like the limit match
--hashlimit-burst num
Burst value, just like limit match
--hashlimit-mode destip | destip-destport
Limit per IP or per port
--hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry
--hashlimit-htable-size num
The number of buckets of the hash table
--hashlimit-htable-max num
Maximum entries in the hash
--hashlimit-htable-expire num
After how many miliseconds do hash entries expire
--hashlimit-htable-gcinterval num
How many miliseconds between garbage collection intervals
helper
This module matches packets related to a specific conntrack-helper.
--helper string
Matches packets related to the specified conntrack-helper.
string can be "ftp" for packets related to a ftp-session on default port.
For other ports append -portnr to the value, ie. "ftp-2121".
Same rules apply for other conntrack-helpers.
icmp
This extension is loaded if '--protocol icmp' is specified. It provides the fol-
lowing option:
--icmp-type [!] typename
This allows specification of the ICMP type, which can be a numeric ICMP
type, or one of the ICMP type names shown by the command
iptables -p icmp -h
iprange
This matches on a given arbitrary range of IPv4 addresses
[!]--src-range ip-ip
Match source IP in the specified range.
[!]--dst-range ip-ip
Match destination IP in the specified range.
ipv4options
Match on IPv4 header options like source routing, record route, timestamp and
router-alert.
--ssrr To match packets with the flag strict source routing.
--lsrr To match packets with the flag loose source routing.
--no-srr
To match packets with no flag for source routing.
[!] --rr
To match packets with the RR flag.
[!] --ts
To match packets with the TS flag.
[!] --ra
To match packets with the router-alert option.
[!] --any-opt
To match a packet with at least one IP option, or no IP option at all if !
is chosen.
Examples:
$ iptables -A input -m ipv4options --rr -j DROP
will drop packets with the record-route flag.
$ iptables -A input -m ipv4options --ts -j DROP
will drop packets with the timestamp flag.
length
This module matches the length of a packet against a specific value or range of
values.
--length [!] length[:length]
limit
This module matches at a limited rate using a token bucket filter. A rule using
this extension will match until this limit is reached (unless the '!' flag is
used). It can be used in combination with the LOG target to give limited logging,
for example.
--limit rate
Maximum average matching rate: specified as a number, with an optional
'/second', '/minute', '/hour', or '/day' suffix; the default is 3/hour.
--limit-burst number
Maximum initial number of packets to match: this number gets recharged by
one every time the limit specified above is not reached, up to this number;
the default is 5.
mac
--mac-source [!] address
Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note
that this only makes sense for packets coming from an Ethernet device and
entering the PREROUTING, FORWARD or INPUT chains.
mark
This module matches the netfilter mark field associated with a packet (which can be
set using the MARK target below).
--mark value[/mask]
Matches packets with the given unsigned mark value (if a mask is specified,
this is logically ANDed with the mask before the comparison).
mport
This module matches a set of source or destination ports. Up to 15 ports can be
specified. It can only be used in conjunction with -p tcp or -p udp.
--source-ports port[,port[,port...]]
Match if the source port is one of the given ports. The flag --sports is a
convenient alias for this option.
--destination-ports port[,port[,port...]]
Match if the destination port is one of the given ports. The flag --dports
is a convenient alias for this option.
--ports port[,port[,port...]]
Match if the both the source and destination ports are equal to each other
and to one of the given ports.
multiport
This module matches a set of source or destination ports. Up to 15 ports can be
specified. A port range (port:port) counts as two ports. It can only be used in
conjunction with -p tcp or -p udp.
--source-ports [!] port[,port[,port:port...]]
Match if the source port is one of the given ports. The flag --sports is a
convenient alias for this option.
--destination-ports [!] port[,port[,port:port...]]
Match if the destination port is one of the given ports. The flag --dports
is a convenient alias for this option.
--ports [!] port[,port[,port:port...]]
Match if either the source or destination ports are equal to one of the
given ports.
nth
This module matches every 'n'th packet
--every value
Match every 'value' packet
[--counter num]
Use internal counter number 'num'. Default is '0'.
[--start num]
Initialize the counter at the number 'num' insetad of '0'. Most between '0'
and 'value'-1.
[--packet num]
Match on 'num' packet. Most be between '0' and 'value'-1.
osf
The idea of passive OS fingerprint matching exists for quite a long time, but was
created as extension fo OpenBSD pf only some weeks ago. Original idea was lurked
in some OpenBSD mailing list (thanks grange@open...) and than adopted for Linux
netfilter in form of this code.
Original fingerprint table was created by Michal Zalewski <lcamtuf AT coredump.cx>.
This module compares some data(WS, MSS, options and it's order, ttl, df and others)
from first SYN packet (actually from packets with SYN bit set) with dynamically
loaded OS fingerprints.
--log 1/0
If present, OSF will log determined genres even if they don't match desired
one. 0 - log all determined entries, 1 - only first one.
In syslog you find something like this:
ipt_osf: Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024
-> 11.22.33.44:139
ipt_osf: Unknown: 16384:106:1:48:020405B401010402 44.33.22.11:1239 ->
11.22.33.44:80
--smart
if present, OSF will use some smartness to determine remote OS. OSF will
use initial TTL only if source of connection is in our local network.
--netlink
If present, OSF will log all events also through netlink NETLINK_NFLOG
groupt 1.
--genre [!] string
Match a OS genre by passive fingerprinting
Example:
#iptables -I INPUT -j ACCEPT -p tcp -m osf --genre Linux --log 1 --smart
NOTE: -p tcp is obviously required as it is a TCP match.
Fingerprints can be loaded and read through /proc/sys/net/ipv4/osf file. One can
flush all fingerprints with following command:
echo -en FLUSH > /proc/sys/net/ipv4/osf
Only one fingerprint per open/write/close.
Fingerprints can be downloaded from http://www.openbsd.org/cgi-
bin/cvsweb/src/etc/pf.os
owner
This module attempts to match various characteristics of the packet creator, for
locally-generated packets. It is only valid in the OUTPUT chain, and even this
some packets (such as ICMP ping responses) may have no owner, and hence never
match.
--uid-owner userid
Matches if the packet was created by a process with the given effective user
id.
--gid-owner groupid
Matches if the packet was created by a process with the given effective
group id.
--pid-owner processid
Matches if the packet was created by a process with the given process id.
--sid-owner sessionid
Matches if the packet was created by a process in the given session group.
--cmd-owner name
Matches if the packet was created by a process with the given command name.
(this option is present only if iptables was compiled under a kernel sup-
porting this feature)
NOTE: pid, sid and command matching are broken on SMP
physdev
This module matches on the bridge port input and output devices enslaved to a
bridge device. This module is a part of the infrastructure that enables a transpar-
ent bridging IP firewall and is only useful for kernel versions above version
2.5.44.
--physdev-in [!] name
Name of a bridge port via which a packet is received (only for packets
entering the INPUT, FORWARD and PREROUTING chains). If the interface name
ends in a "+", then any interface which begins with this name will match. If
the packet didn't arrive through a bridge device, this packet won't match
this option, unless '!' is used.
--physdev-out [!] name
Name of a bridge port via which a packet is going to be sent (for packets
entering the FORWARD, OUTPUT and POSTROUTING chains). If the interface name
ends in a "+", then any interface which begins with this name will match.
Note that in the nat and mangle OUTPUT chains one cannot match on the bridge
output port, however one can in the filter OUTPUT chain. If the packet won't
leave by a bridge device or it is yet unknown what the output device will
be, then the packet won't match this option, unless
[!] --physdev-is-in
Matches if the packet has entered through a bridge interface.
[!] --physdev-is-out
Matches if the packet will leave through a bridge interface.
[!] --physdev-is-bridged
Matches if the packet is being bridged and therefore is not being routed.
This is only useful in the FORWARD and POSTROUTING chains.
pkttype
This module matches the link-layer packet type.
--pkt-type [unicast|broadcast|multicast]
policy
This modules matches the policy used by IPsec for handling a packet.
--dir in|out
Used to select whether to match the policy used for decapsulation or the
policy that will be used for encapsulation. in is valid in the PREROUTING,
INPUT and FORWARD chains, out is valid in the POSTROUTING, OUTPUT and FOR-
WARD chains.
--pol none|ipsec
Matches if the packet is subject to IPsec processing.
--strict
Selects whether to match the exact policy or match if any rule of the policy
matches the given policy.
--reqid id
Matches the reqid of the policy rule. The reqid can be specified with
setkey(8) using unique:id as level.
--spi spi
Matches the SPI of the SA.
--proto ah|esp|ipcomp
Matches the encapsulation protocol.
--mode tunnel|transport
Matches the encapsulation mode.
--tunnel-src addr[/mask]
Matches the source end-point address of a tunnel mode SA. Only valid with
--mode tunnel.
--tunnel-dst addr[/mask]
Matches the destination end-point address of a tunnel mode SA. Only valid
with --mode tunnel.
--next Start the next element in the policy specification. Can only be used with
--strict
psd
Attempt to detect TCP and UDP port scans. This match was derived from Solar
Designer's scanlogd.
--psd-weight-threshold threshold
Total weight of the latest TCP/UDP packets with different destination ports
coming from the same host to be treated as port scan sequence.
--psd-delay-threshold delay
Delay (in hundredths of second) for the packets with different destination
ports coming from the same host to be treated as possible port scan subse-
quence.
--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 con-
sidering 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 nar-
rower 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 fak-
ing their source address in order to DoS you via this module by disallowing
others access to your site by sending bogus packets to you.
Examples:
# iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP
# iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy
--set -j DROP
Official website (http://snowman.net/projects/ipt_recent/) also has some examples
of usage.
/proc/net/ipt_recent/* are the current lists of addresses and information about
each entry of each list.
Each file in /proc/net/ipt_recent/ can be read from to see the current list or
written two using the following commands to modify the list:
echo xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Add to the DEFAULT list
echo -xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Remove from the DEFAULT list
echo clear > /proc/net/ipt_recent/DEFAULT
to empty the DEFAULT list.
The module itself accepts parameters, defaults shown:
ip_list_tot=100
Number of addresses remembered per table
ip_pkt_list_tot=20
Number of packets per address remembered
ip_list_hash_size=0
Hash table size. 0 means to calculate it based on ip_list_tot, default: 512
ip_list_perms=0644
Permissions for /proc/net/ipt_recent/* files
debug=0
Set to 1 to get lots of debugging info
sctp
--source-port,--sport [!] port[:port]
--destination-port,--dport [!] port[:port]
--chunk-types [!] all|any|only chunktype[:flags] [...]
The flag letter in upper case indicates that the flag is to match if set, in
the lower case indicates to match if unset.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN
SHUTDOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE
ASCONF ASCONF_ACK
chunk type available flags
DATA U B E u b e
ABORT T t
SHUTDOWN_COMPLETE T t
(lowercase means flag should be "off", uppercase means "on")
Examples:
iptables -A INPUT -p sctp --dport 80 -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA,INIT -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT
set
This modules macthes IP sets which can be defined by ipset(8).
--set setname flag[,flag...]
where flags are src and/or dst and there can be no more than six of them.
Hence the command
iptables -A FORWARD -m set --set test src,dst
will match packets, for which (depending on the type of the set) the source
address or port number of the packet can be found in the specified set. If
there is a binding belonging to the mached set element or there is a default
binding for the given set, then the rule will match the packet only if addi-
tionally (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 connec-
tion 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 direc-
tions, NEW meaning that the packet has started a new connection, or other-
wise associated with a connection which has not seen packets in both direc-
tions, and RELATED meaning that the packet is starting a new connection, but
is associated with an existing connection, such as an FTP data transfer, or
an ICMP error.
string
This modules matches a given string by using some pattern matching strategy. It
requires a linux kernel >= 2.6.14.
--algo bm|kmp
Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-
Morris)
--from offset
Set the offset from which it starts looking for any matching. If not passed,
default is 0.
--to offset
Set the offset from which it starts looking for any matching. If not passed,
default is the packet size.
--string pattern
Matches the given pattern. --hex-string pattern Matches the given pattern
in hex notation.
tcp
These extensions are loaded if '--protocol tcp' is specified. It provides the fol-
lowing 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 conve-
nient alias for this option.
--tcp-flags [!] mask comp
Match when the TCP flags are as specified. The first argument is the flags
which we should examine, written as a comma-separated list, and the second
argument is a comma-separated list of flags which must be set. Flags are:
SYN ACK FIN RST URG PSH ALL NONE. Hence the command
iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and the ACK, FIN and RST
flags unset.
[!] --syn
Only match TCP packets with the SYN bit set and the ACK,RST and FIN bits
cleared. Such packets are used to request TCP connection initiation; for
example, blocking such packets coming in an interface will prevent incoming
TCP connections, but outgoing TCP connections will be unaffected. It is
equivalent to --tcp-flags SYN,RST,ACK,FIN SYN. If the "!" flag precedes the
"--syn", the sense of the option is inverted.
--tcp-option [!] number
Match if TCP option set.
--mss value[:value]
Match TCP SYN or SYN/ACK packets with the specified MSS value (or range),
which control the maximum packet size for that connection.
tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP header. You can
only use this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated dur-
ing the TCP handshake at connection startup time.
[!] --mss value[:value]"
Match a given TCP MSS value or range.
time
This matches if the packet arrival time/date is within a given range. All options
are facultative.
--timestart value
Match only if it is after 'value' (Inclusive, format: HH:MM ; default
00:00).
--timestop value
Match only if it is before 'value' (Inclusive, format: HH:MM ; default
23:59).
--days listofdays
Match only if today is one of the given days. (format:
Mon,Tue,Wed,Thu,Fri,Sat,Sun ; default everyday)
--datestart date
Match only if it is after 'date' (Inclusive, format:
YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 1970)
--datestop date
Match only if it is before 'date' (Inclusive, format:
YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 2037)
tos
This module matches the 8 bits of Type of Service field in the IP header (ie.
including the precedence bits).
--tos tos
The argument is either a standard name, (use
iptables -m tos -h
to see the list), or a numeric value to match.
ttl
This module matches the time to live field in the IP header.
--ttl-eq ttl
Matches the given TTL value.
--ttl-gt ttl
Matches if TTL is greater than the given TTL value.
--ttl-lt ttl
Matches if TTL is less than the given TTL value.
u32
U32 allows you to extract quantities of up to 4 bytes from a packet, AND them with
specified masks, shift them by specified amounts and test whether the results are
in any of a set of specified ranges. The specification of what to extract is gen-
eral enough to skip over headers with lengths stored in the packet, as in IP or TCP
header lengths.
Details and examples are in the kernel module source.
udp
These extensions are loaded if '--protocol udp' is specified. It provides the fol-
lowing 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 stan-
dard distribution.
BALANCE
This allows you to DNAT connections in a round-robin way over a given range of des-
tination addresses.
--to-destination ipaddr-ipaddr
Address range to round-robin over.
CLASSIFY
This module allows you to set the skb->priority value (and thus classify the packet
into a specific CBQ class).
--set-class MAJOR:MINOR
Set the major and minor class value.
CLUSTERIP
This module allows you to configure a simple cluster of nodes that share a certain
IP and MAC address without an explicit load balancer in front of them. Connections
are statically distributed between the nodes in this cluster.
--new Create a new ClusterIP. You always have to set this on the first rule for a
given ClusterIP.
--hashmode mode
Specify the hashing mode. Has to be one of sourceip, sourceip-sourceport,
sourceip-sourceport-destport
--clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer multicast address
--total-nodes num
Number of total nodes within this cluster.
--local-node num
Local node number within this cluster.
--hash-init rnd
Specify the random seed used for hash initialization.
CONNMARK
This module sets the netfilter mark value associated with a connection
--set-mark mark[/mask]
Set connection mark. If a mask is specified then only those bits set in the
mask is modified.
--save-mark [--mask mask]
Copy the netfilter packet mark value to the connection mark. If a mask is
specified then only those bits are copied.
--restore-mark [--mask mask]
Copy the connection mark value to the packet. If a mask is specified then
only those bits are copied. This is only valid in the mangle table.
DNAT
This target is only valid in the nat table, in the PREROUTING and OUTPUT chains,
and user-defined chains which are only called from those chains. It specifies that
the destination address of the packet should be modified (and all future packets in
this connection will also be mangled), and rules should cease being examined. It
takes one type of option:
--to-destination ipaddr[-ipaddr][:port-port]
which can specify a single new destination IP address, an inclusive range of
IP addresses, and optionally, a port range (which is only valid if the rule
also specifies -p tcp or -p udp). If no port range is specified, then the
destination port will never be modified.
In Kernels up to 2.6.10 you can add several --to-destination options. For
those kernels, if you specify more than one destination address, either via
an address range or multiple --to-destination options, a simple round-robin
(one after another in cycle) load balancing takes place between these
addresses. Later Kernels (>= 2.6.11-rc1) don't have the ability to NAT to
multiple ranges anymore.
DSCP
This target allows to alter the value of the DSCP bits within the TOS header of the
IPv4 packet. As this manipulates a packet, it can only be used in the mangle ta-
ble.
--set-dscp value
Set the DSCP field to a numerical value (can be decimal or hex)
--set-dscp-class class
Set the DSCP field to a DiffServ class.
ECN
This target allows to selectively work around known ECN blackholes. It can only be
used in the mangle table.
--ecn-tcp-remove
Remove all ECN bits from the TCP header. Of course, it can only be used in
conjunction with -p tcp.
IPMARK
Allows you to mark a received packet basing on its IP address. This can replace
many mangle/mark entries with only one, if you use firewall based classifier.
This target is to be used inside the mangle table, in the PREROUTING, POSTROUTING
or FORWARD hooks.
--addr src/dst
Use source or destination IP address.
--and-mask mask
Perform bitwise 'and' on the IP address and this mask.
--or-mask mask
Perform bitwise 'or' on the IP address and this mask.
The order of IP address bytes is reversed to meet "human order of bytes":
192.168.0.1 is 0xc0a80001. At first the 'and' operation is performed, then 'or'.
Examples:
We create a queue for each user, the queue number is adequate to the IP address of
the user, e.g.: all packets going to/from 192.168.5.2 are directed to 1:0502 queue,
192.168.5.12 -> 1:050c etc.
We have one classifier rule:
tc filter add dev eth3 parent 1:0 protocol ip fw
Earlier we had many rules just like below:
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.2 -j MARK --set-mark
0x10502
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.3 -j MARK --set-mark
0x10503
Using IPMARK target we can replace all the mangle/mark rules with only one:
iptables -t mangle -A POSTROUTING -o eth3 -j IPMARK --addr=dst --and-
mask=0xffff --or-mask=0x10000
On the routers with hundreds of users there should be significant load decrease
(e.g. twice).
IPV4OPTSSTRIP
Strip all the IP options from a packet.
The target doesn't take any option, and therefore is extremly easy to use :
# iptables -t mangle -A PREROUTING -j IPV4OPTSSTRIP
LOG
Turn on kernel logging of matching packets. When this option is set for a rule,
the Linux kernel will print some information on all matching packets (like most IP
header fields) via the kernel log (where it can be read with dmesg or syslogd(8)).
This is a "non-terminating target", i.e. rule traversal continues at the next rule.
So if you want to LOG the packets you refuse, use two separate rules with the same
matching criteria, first using target LOG then DROP (or REJECT).
--log-level level
Level of logging (numeric or see syslog.conf(5)).
--log-prefix prefix
Prefix log messages with the specified prefix; up to 29 letters long, and
useful for distinguishing messages in the logs.
--log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is readable by
users.
--log-tcp-options
Log options from the TCP packet header.
--log-ip-options
Log options from the IP packet header.
--log-uid
Log the userid of the process which generated the packet.
MARK
This is used to set the netfilter mark value associated with the packet. It is
only valid in the mangle table. It can for example be used in conjunction with
iproute2.
--set-mark mark
MASQUERADE
This target is only valid in the nat table, in the POSTROUTING chain. It should
only be used with dynamically assigned IP (dialup) connections: if you have a
static IP address, you should use the SNAT target. Masquerading is equivalent to
specifying a mapping to the IP address of the interface the packet is going out,
but also has the effect that connections are forgotten when the interface goes
down. This is the correct behavior when the next dialup is unlikely to have the
same interface address (and hence any established connections are lost anyway). It
takes one option:
--to-ports port[-port]
This specifies a range of source ports to use, overriding the default SNAT
source port-selection heuristics (see above). This is only valid if the
rule also specifies -p tcp or -p udp.
MIRROR
This is an experimental demonstration target which inverts the source and destina-
tion 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 fil-
tering 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 origi-
nal address.
NFQUEUE
This target is an extension of the QUEUE target. As opposed to QUEUE, it allows you
to put a packet into any specific queue, identified by its 16-bit queue number.
--queue-num value
This specifies the QUEUE number to use. Valud queue numbers are 0 to 65535.
The default value is 0.
It can only be used with Kernel versions 2.6.14 or later, since it requires
the nfnetlink_queue kernel support.
NOTRACK
This target disables connection tracking for all packets matching that rule.
It can only be used in the
raw table.
REDIRECT
This target is only valid in the nat table, in the PREROUTING and OUTPUT chains,
and user-defined chains which are only called from those chains. It redirects the
packet to the machine itself by changing the destination IP to the primary address
of the incoming interface (locally-generated packets are mapped to the 127.0.0.1
address). It takes one option:
--to-ports port[-port]
This specifies a destination port or range of ports to use: without this,
the destination port is never altered. This is only valid if the rule also
specifies -p tcp or -p udp.
REJECT
This is used to send back an error packet in response to the matched packet: other-
wise 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 send-
ing 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
SET
This modules adds and/or deletes entries from IP sets which can be defined by
ipset(8).
--add-set setname flag[,flag...]
add the address(es)/port(s) of the packet to the sets
--del-set setname flag[,flag...]
delete the address(es)/port(s) of the packet from the sets, where flags are
src and/or dst and there can be no more than six of them.
The bindings to follow must previously be defined in order to use
multilevel adding/deleting by the SET target.
SNAT
This target is only valid in the nat table, in the POSTROUTING chain. It specifies
that the source address of the packet should be modified (and all future packets in
this connection will also be mangled), and rules should cease being examined. It
takes one type of option:
--to-source ipaddr[-ipaddr][:port-port]
which can specify a single new source IP address, an inclusive range of IP
addresses, and optionally, a port range (which is only valid if the rule
also specifies -p tcp or -p udp). If no port range is specified, then
source ports below 512 will be mapped to other ports below 512: those
between 512 and 1023 inclusive will be mapped to ports below 1024, and other
ports will be mapped to 1024 or above. Where possible, no port alteration
will occur.
In Kernels up to 2.6.10, you can add several --to-source options. For those
kernels, if you specify more than one source address, either via an address
range or multiple --to-source options, a simple round-robin (one after
another in cycle) takes place between these addresses. Later Kernels (>=
2.6.11-rc1) don't have the ability to NAT to multiple ranges anymore.
TARPIT
Captures and holds incoming TCP connections using no local per-connection
resources. Connections are accepted, but immediately switched to the persist state
(0 byte window), in which the remote side stops sending data and asks to continue
every 60-240 seconds. Attempts to close the connection are ignored, forcing the
remote side to time out the connection in 12-24 minutes.
This offers similar functionality to LaBrea <http://www.hackbusters.net/LaBrea/>
but doesn't require dedicated hardware or IPs. Any TCP port that you would normally
DROP or REJECT can instead become a tarpit.
To tarpit connections to TCP port 80 destined for the current machine:
iptables -A INPUT -p tcp -m tcp --dport 80 -j TARPIT
To significantly slow down Code Red/Nimda-style scans of unused address space, for-
ward unused ip addresses to a Linux box not acting as a router (e.g. "ip route
10.0.0.0 255.0.0.0 ip.of.linux.box" on a Cisco), enable IP forwarding on the Linux
box, and add:
iptables -A FORWARD -p tcp -j TARPIT
iptables -A FORWARD -j DROP
NOTE: If you use the conntrack module while you are using TARPIT, you should also
use the NOTRACK target, or the kernel will unnecessarily allocate resources
for each TARPITted connection. To TARPIT incoming connections to the stan-
dard IRC port while using conntrack, you could:
iptables -t raw -A PREROUTING -p tcp --dport 6667 -j NOTRACK
iptables -A INPUT -p tcp --dport 6667 -j TARPIT
TCPMSS
This target allows to alter the MSS value of TCP SYN packets, to control the maxi-
mum size for that connection (usually limiting it to your outgoing interface's MTU
minus 40). Of course, it can only be used in conjunction with -p tcp. It is only
valid in the mangle table.
This target is used to overcome criminally braindead ISPs or servers which block
ICMP Fragmentation Needed packets. The symptoms of this problem are that every-
thing works fine from your Linux firewall/router, but machines behind it can never
exchange large packets:
1) Web browsers connect, then hang with no data received.
2) Small mail works fine, but large emails hang.
3) ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall configuration
like:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value
Explicitly set MSS option to specified value.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40).
These options are mutually exclusive.
TOS
This is used to set the 8-bit Type of Service field in the IP header. It is only
valid in the mangle table.
--set-tos tos
You can use a numeric TOS values, or use
iptables -j TOS -h
to see the list of valid TOS names.
TRACE
This target has no options. It just turns on packet tracing for all packets that
match this rule.
TTL
This is used to modify the IPv4 TTL header field. The TTL field determines how
many hops (routers) a packet can traverse until it's time to live is exceeded.
Setting or incrementing the TTL field can potentially be very dangerous,
so it should be avoided at any cost.
Don't ever set or increment the value on packets that leave your local network!
mangle table.
--ttl-set value
Set the TTL value to 'value'.
--ttl-dec value
Decrement the TTL value 'value' times.
--ttl-inc value
Increment the TTL value 'value' times.
ULOG
This target provides userspace logging of matching packets. When this target is
set for a rule, the Linux kernel will multicast this packet through a netlink
socket. One or more userspace processes may then subscribe to various multicast
groups and receive the packets. Like LOG, this is a "non-terminating target", i.e.
rule traversal continues at the next rule.
--ulog-nlgroup nlgroup
This specifies the netlink group (1-32) to which the packet is sent.
Default value is 1.
--ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32 characters long, and
useful for distinguishing messages in the logs.
--ulog-cprange size
Number of bytes to be copied to userspace. A value of 0 always copies the
entire packet, regardless of its size. Default is 0.
--ulog-qthreshold size
Number of packet to queue inside kernel. Setting this value to, e.g. 10
accumulates ten packets inside the kernel and transmits them as one netlink
multipart message to userspace. Default is 1 (for backwards compatibility).
XOR
Encrypt TCP and UDP traffic using a simple XOR encryption
--key string
Set key to "string"
--block-size
Set block size
DIAGNOSTICS
Various error messages are printed to standard error. The exit code is 0 for cor-
rect 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.net-
filter.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 net-
filter internals.
See http://www.netfilter.org/.
AUTHORS
Rusty Russell originally wrote iptables, in early consultation with Michael Neul-
ing.
Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection
framework in iptables, then wrote the mangle table, the owner match, the mark
stuff, and ran around doing cool stuff everywhere.
James Morris wrote the TOS target, and tos match.
Jozsef Kadlecsik wrote the REJECT target.
Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as well as the
TTL, DSCP, ECN matches and targets.
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef Kadlecsik,
Patrick McHardy, James Morris, Harald Welte and Rusty Russell.
Man page originally written by Herve Eychenne <rv AT wallfire.org>.
Mar 09, 2002 IPTABLES(8)
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