IPTABLES linux command manual
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 specifies what to do with a packet that matches. This is called
a 'target', which may be a jump to a user-defined chain in the same
table.
TARGETS
A firewall rule specifies criteria for a packet, and a target. If the
packet does not match, the next rule in the chain is the examined; if
it does match, then the next rule is specified by the value of the
target, which can be the name of a user-defined chain or one of the
special values ACCEPT, DROP, QUEUE, or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet
on the floor. QUEUE means to pass the packet to userspace (if sup-
ported by the kernel). 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 tar-
get RETURN is matched, the target specified by the chain policy deter-
mines 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
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 coming into
the box itself), 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
supported: INPUT (for packets coming into the box itself),
FORWARD (for altering packets being routed through the
box), and POSTROUTING (for altering packets as they are
about to go out).
OPTIONS
The options that are recognized by iptables can be divided into sev-
eral 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 differen-
tiate it from all other options.
-A, --append chain rule-specification
Append one or more rules to the end of the selected chain.
When the source and/or destination names resolve to more than
one address, a rule will be added for each possible address
combination.
-D, --delete chain rule-specification
-D, --delete chain rulenum
Delete one or more rules from the selected chain. There are
two versions of this command: the rule can be specified as a
number in the chain (starting at 1 for the first rule) or a
rule to match.
-I, --insert chain [rulenum] rule-specification
Insert one or more rules in the selected chain as the given
rule number. So, if the rule number is 1, the rule or rules
are inserted at the head of the chain. This is also the
default if no rule number is specified.
-R, --replace chain rulenum rule-specification
Replace a rule in the selected chain. If the source and/or
destination names resolve to multiple addresses, the command
will fail. Rules are numbered starting at 1.
-L, --list [chain]
List all rules in the selected chain. If no chain is selected,
all chains are listed. 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
counters immediately before they are cleared. (See above.)
-N, --new-chain chain
Create a new user-defined chain by the given name. There must
be no target of that name already.
-X, --delete-chain [chain]
Delete the optional user-defined chain specified. There must
be no references to the chain. If there are, you must delete
or replace the referring rules before the chain can be deleted.
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 ta-
ble.
-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
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 net-
work 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 immedi-
ately, or an extension (see EXTENSIONS below). If this option
is omitted in a rule, then matching the rule will have no
effect on the packet's fate, but the counters on the rule will
be incremented.
-i, --in-interface [!] name
Name of an interface via which a packet is going to be 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 opera-
tions).
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.
ah
This module matches the SPIs in AH header of IPSec packets.
--ahspi [!] spi[:spi]
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 sup-
porting 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 con-
nection 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, match-
ing if the original source address differs from the reply des-
tination. DNAT A virtual state, matching if the original des-
tination 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.
esp
This module matches the SPIs in ESP header of IPSec packets.
--espspi [!] spi[:spi]
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
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).
multiport
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.
owner
This module attempts to match various characteristics of the packet
creator, for locally-generated packets. It is only valid in the OUT-
PUT 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 supporting this feature)
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 use-
ful for kernel versions above version 2.5.44.
--physdev-in name
Name of a bridge port via which a packet is received (only for
packets entering the INPUT, FORWARD and PREROUTING chains). If
the interface name ends in a "+", then any interface which
begins with this name will match. If the packet didn't arrive
through a bridge device, this packet won't match this option,
unless '!' is used.
--physdev-out name
Name of a bridge port via which a packet is going to be sent
(for packets entering the FORWARD, OUTPUT and POSTROUTING
chains). If the interface name ends in a "+", then any inter-
face which begins with this name will match. Note that in the
nat and mangle OUTPUT chains one cannot match on the bridge
output port, however one can in the filter OUTPUT chain. If the
packet won't leave by a bridge device or it is yet unknown what
the output device will be, then the packet won't match this
option, unless
--physdev-is-in
Matches if the packet has entered through a bridge interface.
--physdev-is-out
Matches if the packet will leave through a bridge interface.
--physdev-is-bridged
Matches if the packet is being bridged and therefore is not
being routed. This is only useful in the FORWARD and POSTROUT-
ING chains.
pkttype
This module matches the link-layer packet type.
--pkt-type [unicast|broadcast|multicast]
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 asso-
ciated with a connection which has seen packets in both direc-
tions, NEW meaning that the packet has started a new connec-
tion, 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 and RST
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 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.
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 ttl
Matches the given TTL value.
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.
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
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.
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.
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
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.
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 tar-
get. 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 destination fields in the IP header and retransmits the packet.
It is only valid in the INPUT, FORWARD and PREROUTING chains, and
user-defined chains which are only called from those chains. Note
that the outgoing packets are NOT seen by any packet filtering chains,
connection tracking or NAT, to avoid loops and other problems.
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 alters the destination IP address to send the packet to
the machine itself (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
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 multi-
ple --to-source options, a simple round-robin (one after
another in cycle) takes place between these adresses.
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 fire-
wall/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.
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 charac-
ters long, and useful for distinguishing messages in the logs.
--ulog-cprange size
Number of bytes to be copied to userspace. A value of 0 always
copies the entire packet, regardless of its size. Default is
0.
--ulog-qthreshold size
Number of packet to queue inside kernel. Setting this value
to, e.g. 10 accumulates ten packets inside the kernel and
transmits them as one netlink multipart message to userspace.
Default is 1 (for backwards compatibility).
DIAGNOSTICS
Various error messages are printed to standard error. The exit code
is 0 for correct functioning. Errors which appear to be caused by
invalid or abused command line parameters cause an exit code of 2, and
other errors cause an exit code of 1.
BUGS
Bugs? What's this? ;-) Well... the counters are not reliable on
sparc64.
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' ta-
ble, 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 han-
dled 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).
The packet-filtering-HOWTO details iptables usage for packet filter-
ing, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO details
the extensions that are not in the standard distribution, and the net-
filter-hacking-HOWTO details the netfilter internals.
See https://www.netfilter.org/.
AUTHORS
Rusty Russell 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 target, TTL, DSCP, ECN matches and tar-
gets.
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef
Kadlecsik, James Morris, Harald Welte and Rusty Russell.
Man page written by Herve Eychenne .
Mar 09, 2002 IPTABLES(8)