iptables
administration tool for IPv4 packet filtering and NAT
see also :
iptables-save - iptables-restore - ip6tables - ip6tables-save - ip6tables-restore
Synopsis
iptables
[-t table]
{-A|-C|-D} chain
rule-specification
iptables
[-t table] -I chain
[rulenum] rule-specification
iptables
[-t table] -R chain
rulenum rule-specification
iptables
[-t table] -D chain
rulenum
iptables
[-t table] -S [chain
[rulenum]]
iptables
[-t table]
{-F|-L|-Z}
[chain [rulenum]] [options...]
iptables
[-t table] -N
chain
iptables
[-t table] -X
[chain]
iptables
[-t table] -P chain
target
iptables
[-t table] -E
old-chain-name new-chain-name
rule-specification
= [matches...] [target]
match =
-m matchname
[per-match-options]
target =
-j targetname
[per-target-options]
add an example, a script, a trick and tips
examples
iptables -L -n -v
## What does it do ?
For status
## Output
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
example added by LeBerger
source
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -F
iptables -X
source
chkconfig iptables off
service iptables stop
source
iptables -F
iptables -P OUTPUT ACCEPT
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
service iptables save
service iptables restart
source
cat > /etc/conf.d/iptables <<
EOF
IPTABLES_CONF=/etc/iptables/iptables.rules
EOF
systemctl enable
iptables.service
source
With Linux iptables, is it possible to log the process/command name that initiates an outbound connection?
You want the owner match module, which only works on the OUTPUT
chain (and maybe PREROUTING...?). Read the docs, but it will work
something like this:
iptables --append OUTPUT -m owner --cmd-owner "$app" \
--jump LOG --log-level DEBUG --log-prefix "OUTPUT $app packet died: "
source
Open up firewall automatically to anybody who has successfully connected via SSH
You can put commands in ~/.bashrc
, anything in there
is executed each time a user logs in.
For your commands to only run when logging in via ssh (and not
when logging in physically), you can test for the presence of the
SSH_CONNECTION
environment variable.
source
IP forwarding on the same network
Ethernet bridging
What you are describing is basically that your RaspPi should be
transparent for the network that connect your XBox to your
router. Which means that when your XBox request an IP address (it
does not have one yet) it will broadcast a message on the network
which should reach the router. This bridging between the physical
link between your raspPi and XBox to the other link between your
RaspPi and router, should be done at ethernet level. So you
describes an ethernet bridge, and the bridge utils should be the
way to go. Perhaps you could give us more information to spot why
your bridge is regularly dropping the connection.
here is an how to do Ethernet bridging on Linux and here is another
article on Linux as an Ethernet Bridge.
Configuration
This configuration bridge the Wireless LAN (connected to your
internet router) to your XBox. On your RaspPi:
# The loopback network interface
auto lo
iface lo inet loopback
# This is a list of hotpluggable network interfaces.
# They will be activated automatically by the hotplug subsystem.
auto eth0
allow-hotplug wlan0
auto br0
# The internet network interface
iface eth0 inet static
address 192.168.2.1
netmask 255.255.255.0
# The wireless side of the bridge
iface wlan0 inet manual
wireless-essid MY_ESSID
wireless-key **********
wireless-mode master
# The local network bridge
iface br0 inet dhcp
bridge_ports wlan0 eth0
And on your XBox set the IP address to be manual and
192.168.2.2/255.255.255.0 with the default gateway 192.168.2.1.
More advanced configuration and information here: Bridging with a wireless NIC
IP routing/gateway
At IP level, this is called routing. This technique however is
meant to inter-connect to IP networks together, implying that
they are not in the same address space. This can be done at
iptables level using IP masquerading (aka NAT), and from your
question this is not the way you want to go.
This implies that the IP subnet which belongs to your internet
router network would be different than the one from your
RaspPI/XBox link. You could try to fix an IP that belong from the
internet router subnet by manually setting the IP address, and
then you would need to set a static route on your internet router
so that it is using your RaspPI to reach your XBox. But you need
to be able to add those configuration on the internet router and
XBox.
You can find a few articles:
source
How to deliberately introduce a delay for incoming UDP packets
tc qdisc add dev eth1 root netem delay 250ms
hack
allows to do it globally for the given interface. It includes UDP
packets.
source
Setup routing and iptables for new VPN connection to redirect **only** ports 80 and 443
routing per protocol is a tad complicated. Usually routing table
is used to check the gateway according to destination IP and use
either the openvpn or the 192.168.0.1 default gateway.
It would be easier to set up e.g. Squid http proxy on the other
end of the VPN and set browser to use the proxy.
You wouldn't use the iptables as it would change the destination
IP of the HTTP connection and it would not work.
You could create a new routing table (/etc/iproute2/rt_tables)
with default route set to the VPN endpoint, use iptables fwmark (
-j MARK ) to mark all the HTTP packets and then use ip
rule to create a custom rule for the marked packages to use
the new routing table.
source
iptables vs route
route
is a command that displays, adds and deletes
entries from the kernel's TCP/IP routing table (aka "Forwarding
Information Base").
iptables
is a command that displays, adds, and
deletes entries from Netfilter, the Linux kernel's packet
filtering and manipulating subsystem. It handles NAT.
Since IP forwarding, i.e. routing, is basically rewriting a
packet with a different source address and shipping it out of a
different network interface, I believe you could technically do
static routing with the proper iptables
rules in the
mangle
table, but I believe it's generally fastest
to let the routing part of the kernel do that.
There are many diagrams that are out there that illustrate
exactly how a TCP/IP packet traverses the kernel (including
Netfilter and the routing facility) - an example is this:
http://www.adminsehow.com/2011/09/iptables-packet-traverse-map/
source
Sharing transparently proxied Internet connection with PS3
You could set the PS3 to use a static DNS server. You could try
OpenDNS,
which has servers on 208.67.222.222
and
208.67.220.220
, or try Google's public DNS server on 8.8.8.8
and 8.8.4.4
.
source
Blocking access to a specific url
You can use iptables:
iptables -I OUTPUT -p tcp --sport 80 -m string --string "superuser.com" --algo kmp -j DROP
You may also be interested in DansGuardian.
source
How to use iptables to forward all data from an IP to a Virtual Machine
Since you already allocate public addresses to your VMs, maybe
you should consider bridged networking instead of NAT ? You'd get
a much cleaner setup.
If you are worried about your guests interfering with each other
in a bridged setup, you can still use ebtables
and/or static arp to harden things.
source
Why small TCP connections succeeds, but big ones fails?
This is the problem with MTU and filtered ICMP messages
somewhere.
Workaround is setting MTU on the client device or using TCP MSS
clamping on router:
iptables -t mangle -A FORWARD -o ppp4 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
source
Block a user from accessing internet on Linux
This command will only block the user from accessing the World
Wide Web, not the entire Internet.
Apart from that it should work, assuming it is run on the same
machine $USERNAME
is working on.
source
Seting up IPTables to forward multiple GoPro cameras
You're looking for something like:
iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 10.5.5.9:80
However as each of your wifi adapters has the same IP this won't
work because your routing table will be fubar.
To get this working you need to force each wifi adapter to have a
unique address on the 10.5.5.0 network and then also set up your
routing appropriately:
route add -host <CAMERA IP> gw <WLAN INTERFACE ADDRESS> dev <WLAN INTERFACE DEV>
You'd setup each interface something like so:
ifconfig wlan0 <WLAN IF ADDR> -pointopoint <CAMERA ADDR>
And that should setup the route for you, if not use the "route"
command above.
source
CentOS 6 - iptables preventing web access via port 80
I'm not a master of iptables but the way mine is setup I used a
separate custom chain for my port opening:
iptables -N TCP
iptables -A TCP -p tcp --dport 80 -j ACCEPT
Make sure you save
iptables-save > /etc/iptables/iptables.rules
To load
iptables-restore < /etc/iptables/iptables.rules
^^ if the daemon doesn't load it for you. I see that you said you
already ran the method described above (just for the INPUT chain
which I don't think matters) so that leaves me to believe that
you might not be saving and reloading correctly. Is your iptables
daemon running correctly? Make sure the after running the above
commands and using iptables -L you see the newly added chain and
rule.
In response to your further trouble. iptables-save >
location_of_rules I'm not sure where it is on CentOS. But check
to see if the new chain is there by using iptables -L
I also recommend checking to make sure that your website is not
accessable. To do this go to your webbrowser (on the computer)
and put in the url http: //127.0.0.1/ <-Without the sapce
between : and // If the website is accessible then the problem is
with your router. You need to port-forward to open the port in
your firewall. http://en.wikipedia.org/wiki/Port_forwarding
source
Load balancing with multiple gateways
Unless you can somehow split up the traffic based on their local
parameters (like LAN IP), using this solution you will end up
having a lot of random errors because a lot of sites don't allow
you to use the same session from radically different IP
addresses, not to mention other protocols like FTP, DNS, etc.
If you really want to do this, you will need to rent a (virtual)
server with at least 2 IP addresses and build a VPN on each of
your connections, then set up an OSPF load balancing to utilize
both connections equally. Your server will need to have at least
the double of your two connections' speed together.
In summary: without network support failover works well enough,
load balancing however is going to be a constant pain.
Update: What you need to do:
- Set up the server and make sure your local machine sees the
two server IP's through different connections.
- Start two OpenVPN servers on the server, listening on each of
the addresses.
- Start two OpenVPN clients on the client.
- Install Quagga on both the server and the client.
- Make sure the server and the client see each other through
the OpenVPN without advertising any routes yet.
- Set up routing advertisement on the server so the default
route (0.0.0.0/0) gets advertised over OSPF one link to the
client via redistributing static routes. (This is hard.)
- Add the second link as a backup link over OSPF.
- Change the configuration so the two links are used in load
balancing modes.
OSPF is a whole new world, it's an integral part of how the
internet out there works. If you really want to do this, I
recommend you give yourself time and read some books as it is way
out of scope for this description. I've done this once and I
might get back to writing a tutorial on this, but it will take
considerable time so I'll make no promises.
description
Iptables
is used to set up, maintain, and inspect the tables of IPv4
packet filter rules in the Linux kernel. Several different
tables may be defined. Each table contains a number of
built-in chains and may also contain user-defined
chains.
Each chain is a
list of rules which can match a set of packets. Each rule
specifies what to do with a packet that matches. This is
called a ’target’, which may be a jump to a
user-defined chain in the same table.
options
The options
that are recognized by iptables can be divided into
several different groups.
COMMANDS
These options specify the desired action to perform. Only
one of them can be specified on the command line unless
otherwise stated below. For long versions of the command and
option names, you need to use only enough letters to ensure
that iptables can differentiate it from all other
options.
-A, --append chain
rule-specification
Append one or more rules to the
end of the selected chain. When the source and/or
destination names resolve to more than one address, a rule
will be added for each possible address combination.
-C,
--check chain
rule-specification
Check whether a rule matching
the specification does exist in the selected chain. This
command uses the same logic as -D to find a
matching entry, but does not alter the existing iptables
configuration and uses its exit code to indicate success or
failure.
-D,
--delete chain rule-specification
-D, --delete chain
rulenum
Delete one or more rules from
the selected chain. There are two versions of this command:
the rule can be specified as a number in the chain (starting
at 1 for the first rule) or a rule to match.
-I,
--insert chain [rulenum]
rule-specification
Insert one or more rules in the
selected chain as the given rule number. So, if the rule
number is 1, the rule or rules are inserted at the head of
the chain. This is also the default if no rule number is
specified.
-R,
--replace chain rulenum
rule-specification
Replace a rule in the selected
chain. If the source and/or destination names resolve to
multiple addresses, the command will fail. Rules are
numbered starting at 1.
-L,
--list [chain]
List all rules in the selected
chain. If no chain is selected, all chains are listed. Like
every other iptables command, it applies to the specified
table (filter is the default), so NAT rules get listed by
iptables -t nat -n -L
Please note that it is often used with the -n
option, in order to avoid long reverse DNS lookups. It is
legal to specify the -Z (zero) option as well,
in which case the chain(s) will be atomically listed and
zeroed. The exact output is affected by the other arguments
given. The exact rules are suppressed until you use
iptables -L -v
-S,
--list-rules [chain]
Print all rules in the selected
chain. If no chain is selected, all chains are printed like
iptables-save. Like every other iptables command, it applies
to the specified table (filter is the default).
-F,
--flush [chain]
Flush the selected chain (all
the chains in the table if none is given). This is
equivalent to deleting all the rules one by one.
-Z,
--zero [chain
[rulenum]]
Zero the packet and byte
counters in all chains, or only the given chain, or only the
given rule in a chain. It is legal to specify the
-L, --list (list) option as
well, to see the counters immediately before they are
cleared. (See above.)
-N,
--new-chain chain
Create a new user-defined chain
by the given name. There must be no target of that name
already.
-X,
--delete-chain [chain]
Delete the optional
user-defined chain specified. There must be no references to
the chain. If there are, you must delete or replace the
referring rules before the chain can be deleted. The chain
must be empty, i.e. not contain any rules. If no argument is
given, it will attempt to delete every non-builtin chain in
the table.
-P,
--policy chain target
Set the policy for the chain to
the given target. See the section TARGETS for the
legal targets. Only built-in (non-user-defined) chains can
have policies, and neither built-in nor user-defined chains
can be policy targets.
-E,
--rename-chain old-chain
new-chain
Rename the user specified chain
to the user supplied name. This is cosmetic, and has no
effect on the structure of the table.
-h
Help. Give a (currently very brief) description of the
command syntax.
PARAMETERS
The following parameters make up a rule specification (as
used in the add, delete, insert, replace and append
commands).
[!] -p, --protocol
protocol
The protocol of the rule or of
the packet to check. The specified protocol can be one of
tcp, udp, udplite, icmp,
esp, ah, sctp or the special keyword
"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.
"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, a
network IP address (with /mask), or a plain IP
address. Hostnames will be resolved once only, before the
rule is submitted to the kernel. Please note that specifying
any name to be resolved with a remote query such as DNS is a
really bad idea. 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.
Multiple addresses can be specified, but this will expand
to multiple rules (when adding with -A), or will
cause multiple rules to be deleted (with -D).
[!] -d,
--destination
address[/mask][,...]
Destination specification. See
the description of the -s (source) flag for a
detailed description of the syntax. The flag
--dst is an alias for this option.
-j,
--jump target
This specifies the target of
the rule; i.e., what to do if the packet matches it. The
target can be a user-defined chain (other than the one this
rule is in), one of the special builtin targets which decide
the fate of the packet immediately, or an extension (see
EXTENSIONS below). If this option is omitted in a
rule (and -g is not used), then matching the
rule will have no effect on the packet’s fate, but the
counters on the rule will be incremented.
-g,
--goto chain
This specifies that the
processing should continue in a user specified chain. Unlike
the --jump option return will not continue
processing in this chain but instead in the chain that
called us via --jump.
[!] -i,
--in-interface name
Name of an interface via which
a packet was received (only for packets entering the
INPUT, FORWARD 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 packets
bytes
This enables the administrator
to initialize the packet and byte counters of a rule (during
INSERT, APPEND, REPLACE
operations).
OTHER
OPTIONS
The following additional options can be specified:
-v, --verbose
Verbose output. This option
makes the list command show the interface name, the rule
options (if any), and the TOS masks. The packet and byte
counters are also listed, with the suffix ’K’,
’M’ or ’G’ for 1000, 1,000,000 and
1,000,000,000 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.
-v may be specified multiple times to possibly
emit more detailed debug statements.
-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, 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).
compatibility with ipchains
This iptables is very similar to ipchains by Rusty
Russell. The main difference is that the chains INPUT and
OUTPUT are only traversed for packets coming into the
local host and originating from the local host respectively.
Hence every packet only passes through one of the three chains
(except loopback traffic, which involves both INPUT and OUTPUT
chains); previously a forwarded packet would pass through all
three.
The other main difference is that -i refers to the input
interface; -o refers to the output interface, and both are
available for packets entering the FORWARD chain.
The various forms of NAT have been separated out; iptables
is a pure packet filter when using the default ’filter’ table,
with optional extension modules. This should simplify much of the
previous confusion over the combination of IP masquerading and
packet filtering seen previously. So the following options are
handled differently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
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.
match extensions
iptables can use extended packet matching modules. These are
loaded in two ways: implicitly, when -p or
--protocol is specified, or with the -m or
--match options, followed by the matching module name;
after these, various extra command line options become available,
depending on the specific module. You can specify multiple
extended match modules in one line, and you can use the -h
or --help options after the module has been specified to
receive help specific to that module.
addrtype
This module matches packets based on their address type.
Address types are used within the kernel networking stack and
categorize addresses into various groups. The exact definition of
that group depends on the specific layer three protocol.
The following address types are possible:
UNSPEC
an unspecified address (i.e. 0.0.0.0)
UNICAST
an unicast address
LOCAL
a local address
BROADCAST
a broadcast address
ANYCAST
an anycast packet
MULTICAST
a multicast address
BLACKHOLE
a blackhole address
UNREACHABLE
an unreachable address
PROHIBIT
a prohibited address
THROW
FIXME
NAT
FIXME
XRESOLVE
[!] --src-type type
Matches if the source address is of given type
[!] --dst-type type
Matches if the destination address is of given type
--limit-iface-in
The address type checking can be limited to the interface the
packet is coming in. This option is only valid in the
PREROUTING, INPUT and FORWARD chains. It
cannot be specified with the --limit-iface-out option.
--limit-iface-out
The address type checking can be limited to the interface the
packet is going out. This option is only valid in the
POSTROUTING, OUTPUT and FORWARD chains. It
cannot be specified with the --limit-iface-in option.
ah
This module matches the SPIs in Authentication header of IPsec
packets.
[!] --ahspi spi[:spi]
cluster
Allows you to deploy gateway and back-end load-sharing clusters
without the need of load-balancers.
This match requires that all the nodes see the same packets.
Thus, the cluster match decides if this node has to handle a
packet given the following options:
--cluster-total-nodes num
Set number of total nodes in cluster.
[!] --cluster-local-node num
Set the local node number ID.
[!] --cluster-local-nodemask mask
Set the local node number ID mask. You can use this option
instead of --cluster-local-node.
--cluster-hash-seed value
Set seed value of the Jenkins hash.
Example:
iptables -A PREROUTING -t mangle -i eth1 -m cluster
--cluster-total-nodes 2 --cluster-local-node 1
--cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth2 -m cluster
--cluster-total-nodes 2 --cluster-local-node 1
--cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark 0xffff
-j DROP
iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark 0xffff
-j DROP
And the following commands to make all nodes see the same
packets:
ip maddr add 01:00:5e:00:01:01 dev eth1
ip maddr add 01:00:5e:00:01:02 dev eth2
arptables -A OUTPUT -o eth1 --h-length 6 -j mangle --mangle-mac-s
01:00:5e:00:01:01
arptables -A INPUT -i eth1 --h-length 6 --destination-mac
01:00:5e:00:01:01 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27
arptables -A OUTPUT -o eth2 --h-length 6 -j mangle --mangle-mac-s
01:00:5e:00:01:02
arptables -A INPUT -i eth2 --h-length 6 --destination-mac
01:00:5e:00:01:02 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27
In the case of TCP connections, pickup facility has to be
disabled to avoid marking TCP ACK packets coming in the reply
direction as valid.
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
comment
Allows you to add comments (up to 256 characters) to any rule.
--comment comment
Example:
iptables -A INPUT -i eth1 -m comment --comment "my local LAN"
connbytes
Match by how many bytes or packets a connection (or one of the
two flows constituting the connection) has transferred so far, or
by average bytes per packet.
The counters are 64-bit and are thus not expected to overflow ;)
The primary use is to detect long-lived downloads and mark them
to be scheduled using a lower priority band in traffic control.
The transferred bytes per connection can also be viewed through
’conntrack -L’ and accessed via ctnetlink.
NOTE that for connections which have no accounting information,
the match will always return false. The
"net.netfilter.nf_conntrack_acct" sysctl flag controls whether
new connections will be byte/packet counted. Existing
connection flows will not be gaining/losing a/the accounting
structure when be sysctl flag is flipped.
[!] --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 connections to a
server per client IP address (or client address block).
--connlimit-upto n
Match if the number of existing connections is below or equal
n.
--connlimit-above n
Match if the number of existing connections is above n.
--connlimit-mask prefix_length
Group hosts using the prefix length. For IPv4, this must be a
number between (including) 0 and 32. For IPv6, between 0 and 128.
If not specified, the maximum prefix length for the applicable
protocol is used.
--connlimit-saddr
Apply the limit onto the source group.
--connlimit-daddr
Apply the limit onto the destination group.
Examples:
# allow 2 telnet connections per client host
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit
--connlimit-above 2 -j REJECT
# you can also match the other way around:
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit
--connlimit-upto 2 -j ACCEPT
# limit the number of parallel HTTP requests to 16 per class C
sized
source network (24 bit netmask)
iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above
16 --connlimit-mask 24 -j REJECT
# limit the number of parallel HTTP requests to 16 for the link
local
network
(ipv6) ip6tables -p tcp --syn --dport 80 -s fe80::/64 -m
connlimit --connlimit-above 16 --connlimit-mask 64 -j REJECT
# Limit the number of connections to a particular host:
ip6tables -p tcp --syn --dport 49152:65535 -d 2001:db8::1 -m
connlimit --connlimit-above 100 -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 specified, this is logically ANDed with the mark before
the comparison).
conntrack
This module, when combined with connection tracking, allows
access to the connection tracking state for this
packet/connection.
[!] --ctstate statelist
statelist is a comma separated list of the connection
states to match. Possible states are listed below.
[!] --ctproto l4proto
Layer-4 protocol to match (by number or name)
[!] --ctorigsrc
address[/mask]
[!] --ctorigdst
address[/mask]
[!] --ctreplsrc
address[/mask]
[!] --ctrepldst address[/mask]
Match against original/reply source/destination address
[!] --ctorigsrcport
port[:port]
[!] --ctorigdstport
port[:port]
[!] --ctreplsrcport
port[:port]
[!] --ctrepldstport
port[:port]
Match against original/reply source/destination port
(TCP/UDP/etc.) or GRE key. Matching against port ranges is only
supported in kernel versions above 2.6.38.
[!] --ctstatus statelist
statuslist is a comma separated list of the connection
statuses to match. Possible statuses are listed below.
[!] --ctexpire time[:time]
Match remaining lifetime in seconds against given value or range
of values (inclusive)
--ctdir {ORIGINAL|REPLY}
Match packets that are flowing in the specified direction. If
this flag is not specified at all, matches packets in both
directions.
States for --ctstate:
INVALID
meaning that the packet is associated with no known connection
NEW
meaning that the packet has started a new connection, or
otherwise associated with a connection which has not seen packets
in both directions, and
ESTABLISHED
meaning that the packet is associated with a connection which has
seen packets in both directions,
RELATED
meaning that the packet is starting a new connection, but is
associated with an existing connection, such as an FTP data
transfer, or an ICMP error.
UNTRACKED
meaning that the packet is not tracked at all, which happens if
you use the NOTRACK target in raw table.
SNAT
A virtual state, matching if the original source address differs
from the reply destination.
DNAT
A virtual state, matching if the original destination differs
from the reply source.
Statuses for --ctstatus:
none
None of the below.
EXPECTED
This is an expected connection (i.e. a conntrack helper set it
up)
SEEN_REPLY
Conntrack has seen packets in both directions.
ASSURED
Conntrack entry should never be early-expired.
CONFIRMED
Connection is confirmed: originating packet has left box.
cpu
[!] --cpu number
Match cpu handling this packet. cpus are numbered from 0 to
NR_CPUS-1 Can be used in combination with RPS (Remote Packet
Steering) or multiqueue NICs to spread network traffic on
different queues.
Example:
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j
REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j
REDIRECT --to-port 8081
Available since Linux 2.6.36.
dccp
[!] --source-port,--sport
port[:port]
[!] --destination-port,--dport
port[:port]
[!] --dccp-types mask
Match when the DCCP packet type is one of ’mask’. ’mask’ is a
comma-separated list of packet types. Packet types are:
REQUEST RESPONSE DATA ACK DATAACK CLOSEREQ CLOSE RESET SYNC
SYNCACK INVALID.
[!] --dccp-option number
Match if DCP option set.
dscp
This module matches the 6 bit DSCP field within the TOS field in
the IP header. DSCP has superseded TOS within the IETF.
[!] --dscp value
Match against a numeric (decimal or hex) value [0-63].
[!] --dscp-class class
Match the DiffServ class. This value may be any of the BE, EF,
AFxx or CSx classes. It will then be converted into its according
numeric value.
ecn
This allows you to match the ECN bits of the IPv4 and TCP header.
ECN is the Explicit Congestion Notification mechanism as
specified in RFC3168
[!] --ecn-tcp-cwr
This matches if the TCP ECN CWR (Congestion Window Received) bit
is set.
[!] --ecn-tcp-ece
This matches if the TCP ECN ECE (ECN Echo) bit is set.
[!] --ecn-ip-ect num
This matches a particular IPv4 ECT (ECN-Capable Transport). You
have to specify a number between ’0’ and ’3’.
esp
This module matches the SPIs in ESP header of IPsec packets.
[!] --espspi spi[:spi]
hashlimit
hashlimit uses hash buckets to express a rate limiting match
(like the limit match) for a group of connections using a
single iptables rule. Grouping can be done per-hostgroup
(source and/or destination address) and/or per-port. It gives you
the ability to express "N packets per time quantum per
group" (see below for some examples).
A hash limit option (--hashlimit-upto,
--hashlimit-above) and --hashlimit-name are
required.
--hashlimit-upto
amount[/second|/minute|/hour|/day]
Match if the rate is below or equal to amount/quantum. It
is specified as a number, with an optional time quantum suffix;
the default is 3/hour.
--hashlimit-above
amount[/second|/minute|/hour|/day]
Match if the rate is above amount/quantum.
--hashlimit-burst amount
Maximum initial number of packets to match: this number gets
recharged by one every time the limit specified above is not
reached, up to this number; the default is 5.
--hashlimit-mode
{srcip|srcport|dstip|dstport},...
A comma-separated list of objects to take into consideration. If
no --hashlimit-mode option is given, hashlimit acts like limit,
but at the expensive of doing the hash housekeeping.
--hashlimit-srcmask prefix
When --hashlimit-mode srcip is used, all source addresses
encountered will be grouped according to the given prefix length
and the so-created subnet will be subject to hashlimit.
prefix must be between (inclusive) 0 and 32. Note that
--hashlimit-srcmask 0 is basically doing the same thing as not
specifying srcip for --hashlimit-mode, but is technically more
expensive.
--hashlimit-dstmask prefix
Like --hashlimit-srcmask, but for destination addresses.
--hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry.
--hashlimit-htable-size buckets
The number of buckets of the hash table
--hashlimit-htable-max entries
Maximum entries in the hash.
--hashlimit-htable-expire msec
After how many milliseconds do hash entries expire.
--hashlimit-htable-gcinterval msec
How many milliseconds between garbage collection intervals.
Examples:
matching on source host
"1000 packets per second for every host in 192.168.0.0/16" =>
-s 192.168.0.0/16 --hashlimit-mode srcip --hashlimit-upto
1000/sec
matching on source port
"100 packets per second for every service of 192.168.1.1" =>
-s 192.168.1.1 --hashlimit-mode srcport --hashlimit-upto 100/sec
matching on subnet
"10000 packets per minute for every /28 subnet (groups of 8
addresses) in 10.0.0.0/8" => -s 10.0.0.8 --hashlimit-mask 28
--hashlimit-upto 10000/min
helper
This module matches packets related to a specific
conntrack-helper.
[!] --helper string
Matches packets related to the specified conntrack-helper.
string can be "ftp" for packets related to a ftp-session on
default port. For other ports append -portnr to the value, ie.
"ftp-2121".
Same rules apply for other conntrack-helpers.
icmp
This extension can be used if ’--protocol icmp’ is specified. It
provides the following option:
[!] --icmp-type
{type[/code]|typename}
This allows specification of the ICMP type, which can be a
numeric ICMP type, type/code pair, or one of the ICMP type names
shown by the command
iptables -p icmp -h
iprange
This matches on a given arbitrary range of IP addresses.
[!] --src-range from[-to]
Match source IP in the specified range.
[!] --dst-range from[-to]
Match destination IP in the specified range.
ipvs
Match IPVS connection properties.
[!] --ipvs
packet belongs to an IPVS connection
Any of the following options implies --ipvs (even negated)
[!] --vproto protocol
VIP protocol to match; by number or name, e.g. "tcp"
[!] --vaddr address[/mask]
VIP address to match
[!] --vport port
VIP port to match; by number or name, e.g. "http"
--vdir {ORIGINAL|REPLY}
flow direction of packet
[!] --vmethod {GATE|IPIP|MASQ}
IPVS forwarding method used
[!] --vportctl port
VIP port of the controlling connection to match, e.g. 21 for FTP
length
This module matches the length of the layer-3 payload (e.g.
layer-4 packet) 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. It can be used in combination with the LOG
target to give limited logging, for example.
xt_limit has no negation support - you will have to use -m
hashlimit ! --hashlimit rate in this case whilst omitting
--hashlimit-mode.
--limit
rate[/second|/minute|/hour|/day]
Maximum average matching rate: specified as a number, with an
optional ’/second’, ’/minute’, ’/hour’, or ’/day’ suffix; the
default is 3/hour.
--limit-burst number
Maximum initial number of packets to match: this number gets
recharged by one every time the limit specified above is not
reached, up to this number; the default is 5.
mac
[!] --mac-source address
Match source MAC address. It must be of the form
XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets
coming from an Ethernet device and entering the
PREROUTING, 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. A port range (port:port) counts as two
ports. It can only be used in conjunction with -p tcp or
-p udp.
[!] --source-ports,--sports
port[,port|,port:port]...
Match if the source port is one of the given ports. The flag
--sports is a convenient alias for this option. Multiple
ports or port ranges are separated using a comma, and a port
range is specified using a colon. 53,1024:65535 would
therefore match ports 53 and all from 1024 through 65535.
[!] --destination-ports,--dports
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.
osf
The osf module does passive operating system fingerprinting. This
modules compares some data (Window Size, MSS, options and their
order, TTL, DF, and others) from packets with the SYN bit
set.
[!] --genre string
Match an operating system genre by using a passive
fingerprinting.
--ttl level
Do additional TTL checks on the packet to determine the operating
system. level can be one of the following values:
•
0 - True IP address and fingerprint TTL comparison. This
generally works for LANs.
•
1 - Check if the IP header’s TTL is less than the fingerprint
one. Works for globally-routable addresses.
•
2 - Do not compare the TTL at all.
--log level
Log determined genres into dmesg even if they do not match the
desired one. level can be one of the following values:
•
0 - Log all matched or unknown signatures
•
1 - Log only the first one
•
2 - Log all known matched signatures
You may find something like this in syslog:
Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024
-> 11.22.33.44:139 hops=3 Linux [2.5-2.6:] : 1.2.3.4:42624
-> 1.2.3.5:22 hops=4
OS fingerprints are loadable using the nfnl_osf program.
To load fingerprints from a file, use:
nfnl_osf -f /usr/share/xtables/pf.os
To remove them again,
nfnl_osf -f /usr/share/xtables/pf.os -d
The fingerprint database can be downlaoded 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. This match is only
valid in the OUTPUT and POSTROUTING chains. Forwarded packets do
not have any socket associated with them. Packets from kernel
threads do have a socket, but usually no owner.
[!] --uid-owner username
[!] --uid-owner
userid[-userid]
Matches if the packet socket’s file structure (if it has one) is
owned by the given user. You may also specify a numerical UID, or
an UID range.
[!] --gid-owner groupname
[!] --gid-owner
groupid[-groupid]
Matches if the packet socket’s file structure is owned by the
given group. You may also specify a numerical GID, or a GID
range.
[!] --socket-exists
Matches if the packet is associated with a socket.
physdev
This module matches on the bridge port input and output devices
enslaved to a bridge device. This module is a part of the
infrastructure that enables a transparent bridging IP firewall
and is only useful for kernel versions above version 2.5.44.
[!] --physdev-in name
Name of a bridge port via which a packet is received (only for
packets entering the INPUT, FORWARD 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
if it is yet unknown what the output device will be, then the
packet won’t match this option, unless ’!’ is used.
[!] --physdev-is-in
Matches if the packet has entered through a bridge interface.
[!] --physdev-is-out
Matches if the packet will leave through a bridge interface.
[!] --physdev-is-bridged
Matches if the packet is being bridged and therefore is not being
routed. This is only useful in the FORWARD and POSTROUTING
chains.
pkttype
This module matches the link-layer packet type.
[!] --pkt-type
{unicast|broadcast|multicast}
policy
This modules matches the policy used by IPsec for handling a
packet.
--dir {in|out}
Used to select whether to match the policy used for decapsulation
or the policy that will be used for encapsulation. in is
valid in the PREROUTING, INPUT and FORWARD chains,
out is valid in the POSTROUTING, OUTPUT and FORWARD
chains.
--pol {none|ipsec}
Matches if the packet is subject to IPsec processing. --pol
none cannot be combined with --strict.
--strict
Selects whether to match the exact policy or match if any rule of
the policy matches the given policy.
For each policy element that is to be described, one can use one
or more of the following options. When --strict is in
effect, at least one must be used per element.
[!] --reqid id
Matches the reqid of the policy rule. The reqid can be specified
with setkey(8) using unique:id as level.
[!] --spi spi
Matches the SPI of the SA.
[!] --proto {ah|esp|ipcomp}
Matches the encapsulation protocol.
[!] --mode {tunnel|transport}
Matches the encapsulation mode.
[!] --tunnel-src addr[/mask]
Matches the source end-point address of a tunnel mode SA. Only
valid with --mode tunnel.
[!] --tunnel-dst addr[/mask]
Matches the destination end-point address of a tunnel mode SA.
Only valid with --mode tunnel.
--next
Start the next element in the policy specification. Can only be
used with --strict.
quota
Implements network quotas by decrementing a byte counter with
each packet. The condition matches until the byte counter reaches
zero. Behavior is reversed with negation (i.e. the condition does
not match until the byte counter reaches zero).
[!] --quota bytes
The quota in bytes.
rateest
The rate estimator can match on estimated rates as collected by
the RATEEST target. It supports matching on absolute bps/pps
values, comparing two rate estimators and matching on the
difference between two rate estimators.
For a better understanding of the available options, these are
all possible combinations:
•
rateest operator rateest-bps
•
rateest operator rateest-pps
•
(rateest minus rateest-bps1) operator
rateest-bps2
•
(rateest minus rateest-pps1) operator
rateest-pps2
•
rateest1 operator rateest2
rateest-bps(without rate!)
•
rateest1 operator rateest2
rateest-pps(without rate!)
•
(rateest1 minus rateest-bps1) operator
(rateest2 minus rateest-bps2)
•
(rateest1 minus rateest-pps1) operator
(rateest2 minus rateest-pps2)
--rateest-delta
For each estimator (either absolute or relative mode), calculate
the difference between the estimator-determined flow rate and the
static value chosen with the BPS/PPS options. If the flow rate is
higher than the specified BPS/PPS, 0 will be used instead of a
negative value. In other words, "max(0, rateest#_rate -
rateest#_bps)" is used.
[!] --rateest-lt
Match if rate is less than given rate/estimator.
[!] --rateest-gt
Match if rate is greater than given rate/estimator.
[!] --rateest-eq
Match if rate is equal to given rate/estimator.
In the so-called "absolute mode", only one rate estimator is used
and compared against a static value, while in "relative mode",
two rate estimators are compared against another.
--rateest name
Name of the one rate estimator for absolute mode.
--rateest1 name
--rateest2 name
The names of the two rate estimators for relative mode.
--rateest-bps [value]
--rateest-pps [value]
--rateest-bps1 [value]
--rateest-bps2 [value]
--rateest-pps1 [value]
--rateest-pps2 [value]
Compare the estimator(s) by bytes or packets per second, and
compare against the chosen value. See the above bullet list for
which option is to be used in which case. A unit suffix may be
used - available ones are: bit, [kmgt]bit, [KMGT]ibit, Bps,
[KMGT]Bps, [KMGT]iBps.
Example: This is what can be used to route outgoing data
connections from an FTP server over two lines based on the
available bandwidth at the time the data connection was started:
# Estimate outgoing rates
iptables -t mangle -A POSTROUTING -o eth0 -j RATEEST
--rateest-name eth0 --rateest-interval 250ms --rateest-ewma 0.5s
iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST
--rateest-name ppp0 --rateest-interval 250ms --rateest-ewma 0.5s
# Mark based on available bandwidth
iptables -t mangle -A balance -m conntrack --ctstate NEW -m
helper --helper ftp -m rateest --rateest-delta --rateest1 eth0
--rateest-bps1 2.5mbit --rateest-gt --rateest2 ppp0
--rateest-bps2 2mbit -j CONNMARK --set-mark 1
iptables -t mangle -A balance -m conntrack --ctstate NEW -m
helper --helper ftp -m rateest --rateest-delta --rateest1 ppp0
--rateest-bps1 2mbit --rateest-gt --rateest2 eth0 --rateest-bps2
2.5mbit -j CONNMARK --set-mark 2
iptables -t mangle -A balance -j CONNMARK --restore-mark
realm
This matches the routing realm. Routing realms are used in
complex routing setups involving dynamic routing protocols like
BGP.
[!] --realm value[/mask]
Matches a given realm number (and optionally mask). If not a
number, value can be a named realm from /etc/iproute2/rt_realms
(mask can not be used in that case).
recent
Allows you to dynamically create a list of IP addresses and then
match against that list in a few different ways.
For example, you can create a "badguy" list out of people
attempting to connect to port 139 on your firewall and then DROP
all future packets from them without considering them.
--set, --rcheck, --update and
--remove are mutually exclusive.
--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).
--rsource
Match/save the source address of each packet in the recent list
table. This is the default.
--rdest
Match/save the destination address of each packet in the recent
list table.
[!] --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.
--reap reap
This option can only be used in conjunction with
--seconds. When used, this will cause entries older then
’seconds’ to be purged.
--hitcount hits
This option must be used in conjunction with one of
--rcheck or --update. When used, this will narrow
the match to only happen when the address is in the list and
packets had been received greater than or equal to the given
value. This option may be used along with --seconds to
create an even narrower match requiring a certain number of hits
within a specific time frame. The maximum value for the hitcount
parameter is given by the "ip_pkt_list_tot" parameter of the
xt_recent kernel module. Exceeding this value on the command line
will cause the rule to be rejected.
--rttl
This option may only be used in conjunction with one of
--rcheck or --update. When used, this will narrow
the match to only happen when the address is in the list and the
TTL of the current packet matches that of the packet which hit
the --set rule. This may be useful if you have problems
with people faking their source address in order to DoS you via
this module by 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
Steve’s ipt_recent website
(http://snowman.net/projects/ipt_recent/) also has some examples
of usage.
/proc/net/xt_recent/* are the current lists of addresses
and information about each entry of each list.
Each file in /proc/net/xt_recent/ can be read from to see
the current list or written two using the following commands to
modify the list:
echo +addr >/proc/net/xt_recent/DEFAULT
to add addr to the DEFAULT list
echo -addr >/proc/net/xt_recent/DEFAULT
to remove addr from the DEFAULT list
echo / >/proc/net/xt_recent/DEFAULT
to flush the DEFAULT list (remove all entries).
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/xt_recent/* files.
ip_list_uid=0
Numerical UID for ownership of /proc/net/xt_recent/* files.
ip_list_gid=0
Numerical GID for ownership of /proc/net/xt_recent/* files.
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 FORWARD_TSN
chunk type available flags
DATA I U B E i 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 module matches IP sets which can be defined by ipset(8).
[!] --match-set setname
flag[,flag]...
where flags are the comma separated list of src and/or
dst specifications and there can be no more than six of
them. Hence the command
iptables -A FORWARD -m set --match-set test src,dst
will match packets, for which (if the set type is ipportmap) the
source address and destination port pair can be found in the
specified set. If the set type of the specified set is single
dimension (for example ipmap), then the command will match
packets for which the source address can be found in the
specified set.
The option --match-set can be replaced by --set if
that does not clash with an option of other extensions.
Use of -m set requires that ipset kernel support is provided. As
standard kernels do not ship this currently, the ipset or
Xtables-addons package needs to be installed.
socket
This matches if an open socket can be found by doing a socket
lookup on the packet.
--transparent
Ignore non-transparent sockets.
state
This module, when combined with connection tracking, allows
access to the connection tracking state for this packet.
[!] --state state
Where state is a comma separated list of the connection states to
match. Possible states are INVALID meaning that the packet
could not be identified for some reason which includes running
out of memory and ICMP errors which don’t correspond to any known
connection, ESTABLISHED meaning that the packet is
associated with a connection which has seen packets in both
directions, NEW meaning that the packet has started a new
connection, or otherwise associated with a connection which has
not seen packets in both directions, and RELATED meaning
that the packet is starting a new connection, but is associated
with an existing connection, such as an FTP data transfer, or an
ICMP error. UNTRACKED meaning that the packet is not
tracked at all, which happens if you use the NOTRACK target in
raw table.
statistic
This module matches packets based on some statistic condition. It
supports two distinct modes settable with the --mode
option.
Supported options:
--mode mode
Set the matching mode of the matching rule, supported modes are
random and nth.
[!] --probability p
Set the probability for a packet to be randomly matched. It only
works with the random mode. p must be within 0.0
and 1.0. The supported granularity is in 1/2147483648th
increments.
[!] --every n
Match one packet every nth packet. It works only with the
nth mode (see also the --packet option).
--packet p
Set the initial counter value (0 <= p <= n-1, default 0)
for the nth mode.
string
This modules matches a given string by using some pattern
matching strategy. It requires a linux kernel >= 2.6.14.
--algo {bm|kmp}
Select the pattern matching strategy. (bm = Boyer-Moore, kmp =
Knuth-Pratt-Morris)
--from offset
Set the offset from which it starts looking for any matching. If
not passed, default is 0.
--to offset
Set the offset up to which should be scanned. That is, byte
offset-1 (counting from 0) is the last one that is
scanned. If not passed, default is the packet size.
[!] --string pattern
Matches the given pattern.
[!] --hex-string pattern
Matches the given pattern in hex notation.
tcp
These extensions can be used if ’--protocol tcp’ is specified. It
provides the following options:
[!] --source-port,--sport
port[:port]
Source port or port range specification. This can either be a
service name or a port number. An inclusive range can also be
specified, using the format first:last. If
the first port is omitted, "0" is assumed; if the last is
omitted, "65535" is assumed. If the first port is greater than
the second one they will be swapped. The flag --sport is a
convenient alias for this option.
[!] --destination-port,--dport
port[:port]
Destination port or port range specification. The flag
--dport is a convenient alias for this option.
[!] --tcp-flags mask comp
Match when the TCP flags are as specified. The first argument
mask is the flags which we should examine, written as a
comma-separated list, and the second argument comp is a
comma-separated list of flags which must be set. Flags are:
SYN ACK FIN RST URG PSH ALL NONE. Hence the command
iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and the ACK, FIN
and RST flags unset.
[!] --syn
Only match TCP packets with the SYN bit set and the ACK,RST and
FIN bits cleared. Such packets are used to request TCP connection
initiation; for example, blocking such packets coming in an
interface will prevent incoming TCP connections, but outgoing TCP
connections will be unaffected. It is equivalent to
--tcp-flags SYN,RST,ACK,FIN SYN. If the "!" flag precedes
the "--syn", the sense of the option is inverted.
[!] --tcp-option number
Match if TCP option set.
tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP
header. You can only use this on TCP SYN or SYN/ACK packets,
since the MSS is only negotiated during the TCP handshake at
connection startup time.
[!] --mss value[:value]
Match a given TCP MSS value or range.
time
This matches if the packet arrival time/date is within a given
range. All options are optional, but are ANDed when specified.
All times are interpreted as UTC by default.
--datestart
YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
--datestop
YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
Only match during the given time, which must be in ISO 8601 "T"
notation. The possible time range is 1970-01-01T00:00:00 to
2038-01-19T04:17:07.
If --datestart or --datestop are not specified, it will default
to 1970-01-01 and 2038-01-19, respectively.
--timestart hh:mm[:ss]
--timestop hh:mm[:ss]
Only match during the given daytime. The possible time range is
00:00:00 to 23:59:59. Leading zeroes are allowed (e.g. "06:03")
and correctly interpreted as base-10.
[!] --monthdays day[,day...]
Only match on the given days of the month. Possible values are
1 to 31. Note that specifying 31 will of
course not match on months which do not have a 31st day; the same
goes for 28- or 29-day February.
[!] --weekdays day[,day...]
Only match on the given weekdays. Possible values are Mon,
Tue, Wed, Thu, Fri, Sat,
Sun, or values from 1 to 7, respectively.
You may also use two-character variants (Mo, Tu,
etc.).
--kerneltz
Use the kernel timezone instead of UTC to determine whether a
packet meets the time regulations.
About kernel timezones: Linux keeps the system time in UTC, and
always does so. On boot, system time is initialized from a
referential time source. Where this time source has no timezone
information, such as the x86 CMOS RTC, UTC will be assumed. If
the time source is however not in UTC, userspace should provide
the correct system time and timezone to the kernel once it has
the information.
Local time is a feature on top of the (timezone independent)
system time. Each process has its own idea of local time,
specified via the TZ environment variable. The kernel also has
its own timezone offset variable. The TZ userspace environment
variable specifies how the UTC-based system time is displayed,
e.g. when you run date(1), or what you see on your desktop clock.
The TZ string may resolve to different offsets at different
dates, which is what enables the automatic time-jumping in
userspace. when DST changes. The kernel’s timezone offset
variable is used when it has to convert between non-UTC sources,
such as FAT filesystems, to UTC (since the latter is what the
rest of the system uses).
The caveat with the kernel timezone is that Linux distributions
may ignore to set the kernel timezone, and instead only set the
system time. Even if a particular distribution does set the
timezone at boot, it is usually does not keep the kernel timezone
offset - which is what changes on DST - up to date. ntpd will not
touch the kernel timezone, so running it will not resolve the
issue. As such, one may encounter a timezone that is always
+0000, or one that is wrong half of the time of the year. As
such, using --kerneltz is highly discouraged.
EXAMPLES. To match on weekends, use:
-m time --weekdays Sa,Su
Or, to match (once) on a national holiday block:
-m time --datestart 2007-12-24 --datestop 2007-12-27
Since the stop time is actually inclusive, you would need the
following stop time to not match the first second of the new day:
-m time --datestart 2007-01-01T17:00 --datestop
2007-01-01T23:59:59
During lunch hour:
-m time --timestart 12:30 --timestop 13:30
The fourth Friday in the month:
-m time --weekdays Fr --monthdays 22,23,24,25,26,27,28
(Note that this exploits a certain mathematical property. It is
not possible to say "fourth Thursday OR fourth Friday" in one
rule. It is possible with multiple rules, though.)
tos
This module matches the 8-bit Type of Service field in the IPv4
header (i.e. including the "Precedence" bits) or the (also 8-bit)
Priority field in the IPv6 header.
[!] --tos value[/mask]
Matches packets with the given TOS mark value. If a mask is
specified, it is logically ANDed with the TOS mark before the
comparison.
[!] --tos symbol
You can specify a symbolic name when using the tos match for
IPv4. The list of recognized TOS names can be obtained by calling
iptables with -m tos -h. Note that this implies a mask of
0x3F, i.e. all but the ECN bits.
ttl
This module matches the time to live field in the IP header.
--ttl-eq ttl
Matches the given TTL value.
--ttl-gt ttl
Matches if TTL is greater than the given TTL value.
--ttl-lt ttl
Matches if TTL is less than the given TTL value.
u32
U32 tests whether quantities of up to 4 bytes extracted from a
packet have specified values. The specification of what to
extract is general enough to find data at given offsets from tcp
headers or payloads.
[!] --u32 tests
The argument amounts to a program in a small language described
below.
tests := location "=" value | tests "&&" location "="
value
value := range | value "," range
range := number | number ":" number
a single number, n, is interpreted the same as n:n.
n:m is interpreted as the range of numbers >=n
and <=m.
location := number | location operator number
operator := "&" | "<<" | ">>" | "@"
The operators &, <<, >> and
&& mean the same as in C. The = is really a set
membership operator and the value syntax describes a set. The
@ operator is what allows moving to the next header and is
described further below.
There are currently some artificial implementation limits on the
size of the tests:
*
no more than 10 of "=" (and 9 "&&"s) in the u32
argument
*
no more than 10 ranges (and 9 commas) per value
*
no more than 10 numbers (and 9 operators) per location
To describe the meaning of location, imagine the following
machine that interprets it. There are three registers:
A is of type char *, initially the address of the IP
header
B and C are unsigned 32 bit integers, initially zero
The instructions are:
number B = number;
C = (*(A+B)<<24) + (*(A+B+1)<<16) +
(*(A+B+2)<<8) + *(A+B+3)
&number C = C & number
<< number C = C << number
>> number C = C >> number
@number A = A + C; then do the instruction number
Any access of memory outside [skb->data,skb->end] causes
the match to fail. Otherwise the result of the computation is the
final value of C.
Whitespace is allowed but not required in the tests. However, the
characters that do occur there are likely to require shell
quoting, so it is a good idea to enclose the arguments in quotes.
Example:
match IP packets with total length >= 256
The IP header contains a total length field in bytes 2-3.
--u32 "0 & 0xFFFF = 0x100:0xFFFF"
read bytes 0-3
AND that with 0xFFFF (giving bytes 2-3), and test whether that is
in the range [0x100:0xFFFF]
Example: (more realistic, hence more complicated)
match ICMP packets with icmp type 0
First test that it is an ICMP packet, true iff byte 9 (protocol)
= 1
--u32 "6 & 0xFF = 1 && ...
read bytes 6-9, use & to throw away bytes 6-8 and compare
the result to 1. Next test that it is not a fragment. (If so, it
might be part of such a packet but we cannot always tell.) N.B.:
This test is generally needed if you want to match anything
beyond the IP header. The last 6 bits of byte 6 and all of byte 7
are 0 iff this is a complete packet (not a fragment).
Alternatively, you can allow first fragments by only testing the
last 5 bits of byte 6.
... 4 & 0x3FFF = 0 && ...
Last test: the first byte past the IP header (the type) is 0.
This is where we have to use the @syntax. The length of the IP
header (IHL) in 32 bit words is stored in the right half of byte
0 of the IP header itself.
... 0 >> 22 & 0x3C @ 0 >> 24 = 0"
The first 0 means read bytes 0-3, >>22 means shift
that 22 bits to the right. Shifting 24 bits would give the first
byte, so only 22 bits is four times that plus a few more bits.
&3C then eliminates the two extra bits on the right
and the first four bits of the first byte. For instance, if
IHL=5, then the IP header is 20 (4 x 5) bytes long. In this case,
bytes 0-1 are (in binary) xxxx0101 yyzzzzzz, >>22
gives the 10 bit value xxxx0101yy and &3C gives
010100. @ means to use this number as a new offset into
the packet, and read four bytes starting from there. This is the
first 4 bytes of the ICMP payload, of which byte 0 is the ICMP
type. Therefore, we simply shift the value 24 to the right to
throw out all but the first byte and compare the result with 0.
Example:
TCP payload bytes 8-12 is any of 1, 2, 5 or 8
First we test that the packet is a tcp packet (similar to ICMP).
--u32 "6 & 0xFF = 6 && ...
Next, test that it is not a fragment (same as above).
... 0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 =
1,2,5,8"
0>>22&3C as above computes the number of bytes
in the IP header. @ makes this the new offset into the
packet, which is the start of the TCP header. The length of the
TCP header (again in 32 bit words) is the left half of byte 12 of
the TCP header. The 12>>26&3C computes this
length in bytes (similar to the IP header before). "@" makes this
the new offset, which is the start of the TCP payload. Finally, 8
reads bytes 8-12 of the payload and = checks whether the
result is any of 1, 2, 5 or 8.
udp
These extensions can be used if ’--protocol udp’ is specified. It
provides the following options:
[!] --source-port,--sport
port[:port]
Source port or port range specification. See the description of
the --source-port option of the TCP extension for details.
[!] --destination-port,--dport
port[:port]
Destination port or port range specification. See the 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.
tables
There are currently three independent tables (which tables are
present at any time depends on the kernel configuration options
and which modules are present).
-t, --table table
This option specifies the packet matching table which the command
should operate on. If the kernel is configured with automatic
module loading, an attempt will be made to load the appropriate
module for that table if it is not already there.
The tables are as follows:
filter:
This is the default table (if no -t option is passed). It
contains the built-in chains INPUT (for packets destined
to local sockets), FORWARD (for packets being routed
through the box), and OUTPUT (for locally-generated
packets).
nat:
This table is consulted when a packet that creates a new
connection is encountered. It consists of three built-ins:
PREROUTING (for altering packets as soon as they come in),
OUTPUT (for altering locally-generated packets before
routing), and POSTROUTING (for altering packets as they
are about to go out).
mangle:
This table is used for specialized packet alteration. Until
kernel 2.4.17 it had two built-in chains: PREROUTING (for
altering incoming packets before routing) and OUTPUT (for
altering locally-generated packets before routing). Since kernel
2.4.18, three other built-in chains are also supported:
INPUT (for packets coming into the box itself),
FORWARD (for altering packets being routed through the
box), and POSTROUTING (for altering packets as they are
about to go out).
raw:
This table is used mainly for configuring exemptions from
connection tracking in combination with the NOTRACK target. It
registers at the netfilter hooks with higher priority and is thus
called before ip_conntrack, or any other IP tables. It provides
the following built-in chains: PREROUTING (for packets
arriving via any network interface) OUTPUT (for packets
generated by local processes)
security:
This table is used for Mandatory Access Control (MAC) networking
rules, such as those enabled by the SECMARK and
CONNSECMARK targets. Mandatory Access Control is
implemented by Linux Security Modules such as SELinux. The
security table is called after the filter table, allowing any
Discretionary Access Control (DAC) rules in the filter table to
take effect before MAC rules. This table provides the following
built-in chains: INPUT (for packets coming into the box
itself), OUTPUT (for altering locally-generated packets
before routing), and FORWARD (for altering packets being
routed through the box).
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 target of
QUEUE will be sent to queue number ’0’ in this case. Please also
see the NFQUEUE target as described later in this man
page.) RETURN means stop traversing this chain and resume
at the next rule in the previous (calling) chain. If the end of a
built-in chain is reached or a rule in a built-in chain with
target RETURN is matched, the target specified by the
chain policy determines the fate of the packet.
target extensions
iptables can use extended target modules: the following are
included in the standard distribution.
AUDIT
This target allows to create audit records for packets hitting
the target. It can be used to record accepted, dropped, and
rejected packets. See auditd(8) for additional details.
--type {accept|drop|reject}
Set type of audit record.
Example:
iptables -N AUDIT_DROP
iptables -A AUDIT_DROP -j AUDIT --type drop
iptables -A AUDIT_DROP -j DROP
CHECKSUM
This target allows to selectively work around broken/old
applications. It can only be used in the mangle table.
--checksum-fill
Compute and fill in the checksum in a packet that lacks a
checksum. This is particularly useful, if you need to work around
old applications such as dhcp clients, that do not work well with
checksum offloads, but don’t want to disable checksum offload in
your device.
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. The values are always
interpreted as hexadecimal even if no 0x prefix is given.
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. The mark is 32 bits wide.
--set-xmark value[/mask]
Zero out the bits given by mask and XOR value into
the ctmark.
--save-mark [--nfmask nfmask]
[--ctmask ctmask]
Copy the packet mark (nfmark) to the connection mark (ctmark)
using the given masks. The new nfmark value is determined as
follows:
ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask)
i.e. ctmask defines what bits to clear and nfmask
what bits of the nfmark to XOR into the ctmark. ctmask and
nfmask default to 0xFFFFFFFF.
--restore-mark [--nfmask nfmask]
[--ctmask ctmask]
Copy the connection mark (ctmark) to the packet mark (nfmark)
using the given masks. The new ctmark value is determined as
follows:
nfmark = (nfmark & ~nfmask) ^ (ctmark & ctmask);
i.e. nfmask defines what bits to clear and ctmask
what bits of the ctmark to XOR into the nfmark. ctmask and
nfmask default to 0xFFFFFFFF.
--restore-mark is only valid in the mangle table.
The following mnemonics are available for --set-xmark:
--and-mark bits
Binary AND the ctmark with bits. (Mnemonic for
--set-xmark 0/invbits, where invbits is the
binary negation of bits.)
--or-mark bits
Binary OR the ctmark with bits. (Mnemonic for
--set-xmark bits/bits.)
--xor-mark bits
Binary XOR the ctmark with bits. (Mnemonic for
--set-xmark bits/0.)
--set-mark value[/mask]
Set the connection mark. If a mask is specified then only those
bits set in the mask are modified.
--save-mark [--mask mask]
Copy the nfmark to the ctmark. If a mask is specified, only those
bits are copied.
--restore-mark [--mask mask]
Copy the ctmark to the nfmark. If a mask is specified, only those
bits are copied. This is only valid in the mangle table.
CONNSECMARK
This module copies security markings from packets to connections
(if unlabeled), and from connections back to packets (also only
if unlabeled). Typically used in conjunction with SECMARK, it is
valid in the security table (for backwards compatibility
with older kernels, it is also valid in the mangle table).
--save
If the packet has a security marking, copy it to the connection
if the connection is not marked.
--restore
If the packet does not have a security marking, and the
connection does, copy the security marking from the connection to
the packet.
CT
The CT target allows to set parameters for a packet or its
associated connection. The target attaches a "template"
connection tracking entry to the packet, which is then used by
the conntrack core when initializing a new ct entry. This target
is thus only valid in the "raw" table.
--notrack
Disables connection tracking for this packet.
--helper name
Use the helper identified by name for the connection. This
is more flexible than loading the conntrack helper modules with
preset ports.
--ctevents event[,...]
Only generate the specified conntrack events for this connection.
Possible event types are: new, related,
destroy, reply, assured, protoinfo,
helper, mark (this refers to the ctmark, not
nfmark), natseqinfo, secmark (ctsecmark).
--expevents event[,...]
Only generate the specified expectation events for this
connection. Possible event types are: new.
--zone id
Assign this packet to zone id and only have lookups done
in that zone. By default, packets have zone 0.
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. If no IP address is
specified then only the destination port will be modified.
In Kernels up to 2.6.10 you can add several --to-destination
options. For those kernels, if you specify more than one
destination address, either via an address range or multiple
--to-destination options, a simple round-robin (one after another
in cycle) load balancing takes place between these addresses.
Later Kernels (>= 2.6.11-rc1) don’t have the ability to NAT to
multiple ranges anymore.
--random
If option --random is used then port mapping will be
randomized (kernel >= 2.6.22).
--persistent
Gives a client the same source-/destination-address for each
connection. This supersedes the SAME target. Support for
persistent mappings is available from 2.6.29-rc2.
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.
IDLETIMER
This target can be used to identify when interfaces have been
idle for a certain period of time. Timers are identified by
labels and are created when a rule is set with a new label. The
rules also take a timeout value (in seconds) as an option. If
more than one rule uses the same timer label, the timer will be
restarted whenever any of the rules get a hit. One entry for each
timer is created in sysfs. This attribute contains the timer
remaining for the timer to expire. The attributes are located
under the xt_idletimer class:
/sys/class/xt_idletimer/timers/<label>
When the timer expires, the target module sends a sysfs
notification to the userspace, which can then decide what to do
(eg. disconnect to save power).
--timeout amount
This is the time in seconds that will trigger the notification.
--label string
This is a unique identifier for the timer. The maximum length for
the label string is 27 characters.
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 target is used to set the Netfilter mark value associated
with the packet. It can, for example, be used in conjunction with
routing based on fwmark (needs iproute2). If you plan on doing
so, note that the mark needs to be set in the PREROUTING chain of
the mangle table to affect routing. The mark field is 32 bits
wide.
--set-xmark value[/mask]
Zeroes out the bits given by mask and XORs value
into the packet mark ("nfmark"). If mask is omitted,
0xFFFFFFFF is assumed.
--set-mark value[/mask]
Zeroes out the bits given by mask and ORs value
into the packet mark. If mask is omitted, 0xFFFFFFFF is
assumed.
The following mnemonics are available:
--and-mark bits
Binary AND the nfmark with bits. (Mnemonic for
--set-xmark 0/invbits, where invbits is the
binary negation of bits.)
--or-mark bits
Binary OR the nfmark with bits. (Mnemonic for
--set-xmark bits/bits.)
--xor-mark bits
Binary XOR the nfmark with bits. (Mnemonic for
--set-xmark bits/0.)
MASQUERADE
This target is only valid in the nat table, in the
POSTROUTING chain. It should only be used with dynamically
assigned IP (dialup) 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).
--to-ports port[-port]
This specifies a range of source ports to use, overriding the
default SNAT source port-selection heuristics (see above).
This is only valid if the rule also specifies -p tcp or
-p udp.
--random
Randomize source port mapping If option --random is used
then port mapping will be randomized (kernel >= 2.6.21).
MIRROR
This is an experimental demonstration target which inverts the
source and destination fields in the IP header and retransmits
the packet. It is only valid in the INPUT, FORWARD
and PREROUTING chains, and user-defined chains which are
only called from those chains. Note that the outgoing packets are
NOT seen by any packet filtering chains, connection
tracking or NAT, to avoid loops and other problems.
NETMAP
This target allows you to statically map a whole network of
addresses onto another network of addresses. It can only be used
from rules in the nat table.
--to address[/mask]
Network address to map to. The resulting address will be
constructed in the following way: All ’one’ bits in the mask are
filled in from the new ’address’. All bits that are zero in the
mask are filled in from the original address.
NFLOG
This target provides logging of matching packets. When this
target is set for a rule, the Linux kernel will pass the packet
to the loaded logging backend to log the packet. This is usually
used in combination with nfnetlink_log as logging backend, which
will multicast the packet through a netlink socket to the
specified multicast group. One or more userspace processes may
subscribe to the group to receive the packets. Like LOG, this is
a non-terminating target, i.e. rule traversal continues at the
next rule.
--nflog-group nlgroup
The netlink group (0 - 2^16-1) to which packets are (only
applicable for nfnetlink_log). The default value is 0.
--nflog-prefix prefix
A prefix string to include in the log message, up to 64
characters long, useful for distinguishing messages in the logs.
--nflog-range size
The number of bytes to be copied to userspace (only applicable
for nfnetlink_log). nfnetlink_log instances may specify their own
range, this option overrides it.
--nflog-threshold size
Number of packets to queue inside the kernel before sending them
to userspace (only applicable for nfnetlink_log). Higher values
result in less overhead per packet, but increase delay until the
packets reach userspace. The default value is 1.
NFQUEUE
This target is an extension of the QUEUE target. As opposed to
QUEUE, it allows you to put a packet into any specific queue,
identified by its 16-bit queue number. It can only be used with
Kernel versions 2.6.14 or later, since it requires the
nfnetlink_queue kernel support. The queue-balance
option was added in Linux 2.6.31, queue-bypass in 2.6.39.
--queue-num value
This specifies the QUEUE number to use. Valid queue numbers are 0
to 65535. The default value is 0.
--queue-balance value:value
This specifies a range of queues to use. Packets are then
balanced across the given queues. This is useful for multicore
systems: start multiple instances of the userspace program on
queues x, x+1, .. x+n and use "--queue-balance
x:x+n". Packets belonging to the same
connection are put into the same nfqueue.
--queue-bypass
By default, if no userspace program is listening on an NFQUEUE,
then all packets that are to be queued are dropped. When this
option is used, the NFQUEUE rule is silently bypassed instead.
The packet will move on to the next rule.
NOTRACK
This target disables connection tracking for all packets matching
that rule.
It can only be used in the raw table.
RATEEST
The RATEEST target collects statistics, performs rate estimation
calculation and saves the results for later evaluation using the
rateest match.
--rateest-name name
Count matched packets into the pool referred to by name,
which is freely choosable.
--rateest-interval
amount{s|ms|us}
Rate measurement interval, in seconds, milliseconds or
microseconds.
--rateest-ewmalog value
Rate measurement averaging time constant.
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).
--to-ports port[-port]
This specifies a destination port or range of ports to use:
without this, the destination port is never altered. This is only
valid if the rule also specifies -p tcp or -p udp.
--random
If option --random is used then port mapping will be
randomized (kernel >= 2.6.22).
REJECT
This is used to send back an error packet in response to the
matched packet: otherwise it is equivalent to DROP so it
is a terminating TARGET, ending rule traversal. This target is
only valid in the INPUT, FORWARD and OUTPUT
chains, and user-defined chains which are only called from those
chains. The following option controls the nature of the error
packet returned:
--reject-with type
The type given can be icmp-net-unreachable,
icmp-host-unreachable, icmp-port-unreachable,
icmp-proto-unreachable, icmp-net-prohibited,
icmp-host-prohibited or icmp-admin-prohibited (*)
which return the appropriate ICMP error message
(port-unreachable is the default). The option
tcp-reset can be used on rules which only match the TCP
protocol: this causes a TCP RST packet to be sent back. This is
mainly useful for blocking ident (113/tcp) probes which
frequently occur when sending mail to broken mail hosts (which
won’t accept your mail otherwise).
(*) Using icmp-admin-prohibited with kernels that do not support
it will result in a plain DROP instead of REJECT
SAME
Similar to SNAT/DNAT depending on chain: it takes a range of
addresses (’--to 1.2.3.4-1.2.3.7’) and gives a client the same
source-/destination-address for each connection.
N.B.: The DNAT target’s --persistent option replaced the
SAME target.
--to ipaddr[-ipaddr]
Addresses to map source to. May be specified more than once for
multiple ranges.
--nodst
Don’t use the destination-ip in the calculations when selecting
the new source-ip
--random
Port mapping will be forcibly randomized to avoid attacks based
on port prediction (kernel >= 2.6.21).
SECMARK
This is used to set the security mark value associated with the
packet for use by security subsystems such as SELinux. It is
valid in the security table (for backwards compatibility
with older kernels, it is also valid in the mangle table).
The mark is 32 bits wide.
--selctx security_context
SET
This modules adds and/or deletes entries from IP sets which can
be defined by ipset(8).
--add-set setname flag[,flag...]
add the address(es)/port(s) of the packet to the sets
--del-set setname flag[,flag...]
delete the address(es)/port(s) of the packet from the sets
where flags are src and/or dst specifications and
there can be no more than six of them.
--timeout value
when adding entry, the timeout value to use instead of the
default one from the set definition
--exist
when adding entry if it already exists, reset the timeout value
to the specified one or to the default from the set definition
Use of -j SET requires that ipset kernel support is provided. As
standard kernels do not ship this currently, the ipset or
Xtables-addons package needs to be installed.
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.
--random
If option --random is used then port mapping will be
randomized (kernel >= 2.6.21).
--persistent
Gives a client the same source-/destination-address for each
connection. This supersedes the SAME target. Support for
persistent mappings is available from 2.6.29-rc2.
TCPMSS
This target allows to alter the MSS value of TCP SYN packets, to
control the maximum size for that connection (usually limiting it
to your outgoing interface’s MTU minus 40 for IPv4 or 60 for
IPv6, respectively). 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" or "ICMPv6 Packet
Too Big" packets. The symptoms of this problem are that
everything works fine from your Linux firewall/router, but
machines behind it can never exchange large packets:
1.
Web browsers connect, then hang with no data received.
2.
Small mail works fine, but large emails hang.
3.
ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall
configuration like:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value
Explicitly sets MSS option to specified value. If the MSS of the
packet is already lower than value, it will not be
increased (from Linux 2.6.25 onwards) to avoid more problems with
hosts relying on a proper MSS.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40 for IPv4; -60 for
IPv6). This may not function as desired where asymmetric routes
with differing path MTU exist — the kernel uses the path MTU
which it would use to send packets from itself to the source and
destination IP addresses. Prior to Linux 2.6.25, only the path
MTU to the destination IP address was considered by this option;
subsequent kernels also consider the path MTU to the source IP
address.
These options are mutually exclusive.
TCPOPTSTRIP
This target will strip TCP options off a TCP packet. (It will
actually replace them by NO-OPs.) As such, you will need to add
the -p tcp parameters.
--strip-options option[,option...]
Strip the given option(s). The options may be specified by TCP
option number or by symbolic name. The list of recognized options
can be obtained by calling iptables with -j TCPOPTSTRIP
-h.
TEE
The TEE target will clone a packet and redirect this clone
to another machine on the local network segment. In other
words, the nexthop must be the target, or you will have to
configure the nexthop to forward it further if so desired.
--gateway ipaddr
Send the cloned packet to the host reachable at the given IP
address. Use of 0.0.0.0 (for IPv4 packets) or :: (IPv6) is
invalid.
To forward all incoming traffic on eth0 to an Network Layer
logging box:
-t mangle -A PREROUTING -i eth0 -j TEE --gateway 2001:db8::1
TOS
This module sets the Type of Service field in the IPv4 header
(including the "precedence" bits) or the Priority field in the
IPv6 header. Note that TOS shares the same bits as DSCP and ECN.
The TOS target is only valid in the mangle table.
--set-tos value[/mask]
Zeroes out the bits given by mask (see NOTE below) and
XORs value into the TOS/Priority field. If mask is
omitted, 0xFF is assumed.
--set-tos symbol
You can specify a symbolic name when using the TOS target for
IPv4. It implies a mask of 0xFF (see NOTE below). The list of
recognized TOS names can be obtained by calling iptables with
-j TOS -h.
The following mnemonics are available:
--and-tos bits
Binary AND the TOS value with bits. (Mnemonic for
--set-tos 0/invbits, where invbits is the
binary negation of bits. See NOTE below.)
--or-tos bits
Binary OR the TOS value with bits. (Mnemonic for
--set-tos bits/bits. See NOTE below.)
--xor-tos bits
Binary XOR the TOS value with bits. (Mnemonic for
--set-tos bits/0. See NOTE below.)
NOTE: In Linux kernels up to and including 2.6.38, with the
exception of longterm releases 2.6.32.42 (or later) and 2.6.33.15
(or later), there is a bug whereby IPv6 TOS mangling does not
behave as documented and differs from the IPv4 version. The TOS
mask indicates the bits one wants to zero out, so it needs to be
inverted before applying it to the original TOS field. However,
the aformentioned kernels forgo the inversion which breaks
--set-tos and its mnemonics.
TPROXY
This target is only valid in the mangle table, in the
PREROUTING chain and user-defined chains which are only
called from this chain. It redirects the packet to a local socket
without changing the packet header in any way. It can also change
the mark value which can then be used in advanced routing rules.
It takes three options:
--on-port port
This specifies a destination port to use. It is a required
option, 0 means the new destination port is the same as the
original. This is only valid if the rule also specifies -p
tcp or -p udp.
--on-ip address
This specifies a destination address to use. By default the
address is the IP address of the incoming interface. This is only
valid if the rule also specifies -p tcp or -p udp.
--tproxy-mark value[/mask]
Marks packets with the given value/mask. The fwmark value set
here can be used by advanced routing. (Required for transparent
proxying to work: otherwise these packets will get forwarded,
which is probably not what you want.)
TRACE
This target marks packes so that the kernel will log every rule
which match the packets as those traverse the tables, chains,
rules.
A logging backend, such as ip(6)t_LOG or nfnetlink_log, must be
loaded for this to be visible. The packets are logged with the
string prefix: "TRACE: tablename:chainname:type:rulenum " where
type can be "rule" for plain rule, "return" for implicit rule at
the end of a user defined chain and "policy" for the policy of
the built in chains.
It can only be used in the raw table.
TTL
This is used to modify the IPv4 TTL header field. The TTL field
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. This target is
only valid in mangle table.
Don’t ever set or increment the value on packets that leave
your local network!
--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).
version
This manual page applies to iptables @PACKAGE_VERSION@.
bugs
Bugs?
What’s this? ;-) Well, you might want to have a look
at http://bugzilla.netfilter.org/
see also
iptables-save ,
iptables-restore , ip6tables ,
ip6tables-save ,
ip6tables-restore , libipq.
The
packet-filtering-HOWTO details iptables usage for packet
filtering, the NAT-HOWTO details NAT, the
netfilter-extensions-HOWTO details the extensions that are
not in the standard distribution, and the
netfilter-hacking-HOWTO details the netfilter internals.
See http://www.netfilter.org/.
authors
Rusty Russell
originally wrote iptables, in early consultation with
Michael Neuling.
Marc Boucher
made Rusty abandon ipnatctl by lobbying for a generic packet
selection framework in iptables, then wrote the mangle
table, the owner match, the mark stuff, and ran around doing
cool stuff everywhere.
James Morris
wrote the TOS target, and tos match.
Jozsef
Kadlecsik wrote the REJECT target.
Harald Welte
wrote the ULOG and NFQUEUE target, the new libiptc, as well
as the TTL, DSCP, ECN matches and targets.
The Netfilter
Core Team is: Marc Boucher, Martin Josefsson, Yasuyuki
Kozakai, Jozsef Kadlecsik, Patrick McHardy, James Morris,
Pablo Neira Ayuso, Harald Welte and Rusty Russell.
Man page
originally written by Herve Eychenne
<rv[:at:]wallfire[:dot:]org>.