iptables (Magyar)

Related articles
nftables
Fail2ban
sshguard
Simple stateful firewall
Sysctl#TCP/IP stack hardening
Uncomplicated Firewall
iptables
is a command line utility for configuring Linux kernel
firewall
implemented within the
Netfilter
project. The term
iptables
is also commonly used to refer to this kernel-level firewall. It can be configured directly with iptables, or by using one of the many
console
and
graphical
front-ends.
iptables
is used for
IPv4
and
ip6tables
is used for
IPv6
. Both
iptables
and
ip6tables
have the same syntax, but some options are specific to either IPv4 or IPv6.
Note
iptables
is a legacy framework,
nftables
aims to provide a modern replacement including a compatibility layer.
Telepítés
The stock Arch Linux kernel is compiled with iptables support. You will only need to
install
the userland utilities, which are provided by the package
iptables
. The
iptables
package is an indirect dependency of the
base
meta package
, so it should be installed on your system by default.
Front-ends
Console
Arno's firewall
— Secure firewall for both single and multi-homed machines. Very easy to configure, handy to manage and highly customizable. Supports: NAT and SNAT, port forwarding, ADSL ethernet modems with both static and dynamically assigned IPs, MAC address filtering, stealth port scan detection, DMZ and DMZ-2-LAN forwarding, protection against SYN/ICMP flooding, extensive user definable logging with rate limiting to prevent log flooding, all IP protocols and VPNs such as IPsec, plugin support to add extra features.
https://rocky.eld.leidenuniv.nl/
||
arno-iptables-firewall
AUR
ferm
— Tool to maintain complex firewalls, without having the trouble to rewrite the complex rules over and over again. It allows the entire firewall rule set to be stored in a separate file, and to be loaded with one command. The firewall configuration resembles structured programming-like language, which can contain levels and lists.
http://ferm.foo-projects.org/
||
ferm
FireHOL
— Language to express firewalling rules, not just a script that produces some kind of a firewall. It makes building even sophisticated firewalls easy - the way you want it.
https://firehol.sourceforge.net/
||
firehol
AUR
Firetable
— Tool to maintain an IPtables firewall. Each interface can be configured separately via its own configuration file, which holds an easy and human readable syntax.
https://gitlab.com/hsleisink/firetable
||
firetable
AUR
firewalld
(firewall-cmd)
— Daemon and console interface for configuring network and firewall zones as well as setting up and configuring firewall rules.
https://firewalld.org/
||
firewalld
Shorewall
— High-level tool for configuring Netfilter. You describe your firewall/gateway requirements using entries in a set of configuration files.
http://www.shorewall.net/
||
shorewall
AUR
Uncomplicated Firewall
— Simple front-end for iptables.
https://launchpad.net/ufw
||
ufw
PeerGuardian
(pglcmd)
— Privacy oriented firewall application. It blocks connections to and from hosts specified in huge block lists (thousands or millions of IP ranges).
https://sourceforge.net/projects/peerguardian/
||
pgl
AUR
Vuurmuur
— Powerful firewall manager. It has a simple and easy to learn configuration that allows both simple and complex configurations. The configuration can be fully configured through an
ncurses
GUI, which allows secure remote administration through SSH or on the console. Vuurmuur supports traffic shaping, has powerful monitoring features, which allow the administrator to look at the logs, connections and bandwidth usage in realtime.
https://www.vuurmuur.org/
||
vuurmuur
AUR
Graphical
Firewall Builder
— GUI firewall configuration and management tool that supports iptables (netfilter), ipfilter, pf, ipfw, Cisco PIX (FWSM, ASA) and Cisco routers extended access lists. The program runs on Linux, FreeBSD, OpenBSD, Windows and macOS and can manage both local and remote firewalls.
https://fwbuilder.sourceforge.net/
||
fwbuilder
firewalld
(firewall-config)
— Daemon and graphical interface for configuring network and firewall zones as well as setting up and configuring firewall rules.
https://firewalld.org/
||
firewalld
Gufw
— GTK-based front-end to
ufw
which happens to be a CLI front-end to iptables (gufw->ufw->iptables). It is super easy and super simple to use.
https://gufw.org/
||
gufw
PeerGuardian
GUI (pglgui)
— Privacy oriented firewall application. It blocks connections to and from hosts specified in huge block lists (thousands or millions of IP ranges).
https://sourceforge.net/projects/peerguardian/
||
pgl
AUR
Portmaster
— Portmaster is a free and open-source application firewall with defaults to improve your privacy.
https://safing.io/
||
portmaster-bin
AUR
Basic concepts
iptables is used to inspect, modify, forward, redirect, and/or drop IP packets.  The code for filtering IP packets is already built into the kernel and is organized into a collection of
tables
, each with a specific purpose.  The tables are made up of a set of predefined
chains
, and the chains contain rules which are traversed in order. Each rule consists of a predicate of potential matches and a corresponding action (called a
target
) which is executed if the predicate is true; i.e. the conditions are matched.  If the IP packet reaches the end of a built-in chain, including an empty chain, then the chain's
policy
target determines the final destination of the IP packet.  iptables is the user utility which allows you to work with these chains/rules.  Most new users find the complexities of linux IP routing quite daunting, but, in practice, the most common use cases (NAT and/or basic Internet firewall) are considerably less complex.
The key to understanding how iptables works is
this chart
.  The lowercase word on top is the table and the uppercase word below is the chain.  Every IP packet that comes in
on any network interface
passes through this flow chart from top to bottom.  A common misconception is that packets entering from, say, an internal interface are handled differently than packets from an Internet-facing interface.  All interfaces are handled the same way; it is up to you to define rules that treat them differently. Of course, some packets are intended for local processes, hence they come in from the top of the chart and stop at <Local Process>, while other packets are generated by local processes; hence they start at <Local Process> and proceed downward through the flowchart.  A detailed explanation of how this flow chart works can be found
here
.
In the vast majority of use cases, you will not need to use the
raw
,
mangle
, or
security
tables at all. Consequently, the following chart depicts a simplified network packet flow through
iptables
:
XXXXXXXXXXXXXXXXXX
                             XXX     Network    XXX
                               XXXXXXXXXXXXXXXXXX
                                       +
                                       |
                                       v
 +-------------+              +------------------+
 |table: filter| <---+        | table: nat       |
 |chain: INPUT |     |        | chain: PREROUTING|
 +-----+-------+     |        +--------+---------+
       |             |                 |
       v             |                 v
 [local process]     |           ****************          +--------------+
       |             +---------+ Routing decision +------> |table: filter |
       v                         ****************          |chain: FORWARD|
****************                                           +------+-------+
Routing decision                                                  |
****************                                                  |
       |                                                          |
       v                        ****************                  |
+-------------+       +------>  Routing decision  <---------------+
|table: nat   |       |         ****************
|chain: OUTPUT|       |               +
+-----+-------+       |               |
      |               |               v
      v               |      +-------------------+
+--------------+      |      | table: nat        |
|table: filter | +----+      | chain: POSTROUTING|
|chain: OUTPUT |             +--------+----------+
+--------------+                      |
                                      v
                               XXXXXXXXXXXXXXXXXX
                             XXX    Network     XXX
                               XXXXXXXXXXXXXXXXXX
Tables
iptables contains five tables:
raw
is used only for configuring packets so that they are exempt from connection tracking.
filter
is the default table, and is where all the actions typically associated with a firewall take place.
nat
is used for
network address translation
(e.g. port forwarding).
mangle
is used for specialized packet alterations.
security
is used for
Mandatory Access Control
networking rules (e.g. SELinux -- see
this article
for more details).
In most common use cases, you will only use two of these:
filter
and
nat
.  The other tables are aimed at complex configurations involving multiple routers and routing decisions and are in any case beyond the scope of these introductory remarks.
Chains
Tables consist of
chains
, which are lists of rules which are followed in order. The default table,
filter
, contains three built-in chains:
INPUT
,
OUTPUT
and
FORWARD
which are activated at different points of the packet filtering process, as illustrated in the
flow chart
. The nat table includes
PREROUTING
,
POSTROUTING
, and
OUTPUT
chains.
See
iptables(8)
for a description of built-in chains in other tables.
By default, none of the chains contain any rules.  It is up to you to append rules to the chains that you want to use.  Chains
do
have a default policy, which is generally set to
ACCEPT
, but can be reset to
DROP
, if you want to be sure that nothing slips through your ruleset. The default policy always applies at the end of a chain only. Hence, the packet has to pass through all existing rules in the chain before the default policy is applied.
User-defined chains can be added to make rulesets more efficient or more easily modifiable.  See
Simple stateful firewall
for an example of how user-defined chains are used.
Rules
Packet filtering is based on
rules
, which are specified by multiple
matches
(conditions the packet must satisfy so that the rule can be applied), and one
target
(action taken when the packet matches all conditions).  The typical things a rule might match on are what interface the packet came in on (e.g eth0 or eth1), what type of packet it is (ICMP, TCP, or UDP), or the destination port of the packet.
Targets are specified using the
-j
or
--jump
option. Targets can be either user-defined chains (i.e. if these conditions are matched, jump to the following user-defined chain and continue processing there), one of the special built-in targets, or a target extension. Built-in targets are
ACCEPT
,
DROP
,
QUEUE
and
RETURN
, target extensions are, for example,
REJECT
and
LOG
. If the target is a built-in target, the fate of the packet is decided immediately and processing of the packet in current table is stopped. If the target is a user-defined chain and the fate of the packet is not decided by this second chain, it will be filtered against the remaining rules of the original chain. Target extensions can be either
terminating
(as built-in targets) or
non-terminating
(as user-defined chains), see
iptables-extensions(8)
for details.
Traversing Chains
A network packet received on any interface traverses the traffic control chains of tables in the order shown in the
flow chart
. The first routing decision involves deciding if the final destination of the packet is the local machine (in which case the packet traverses through the
INPUT
chains) or elsewhere (in which case the packet traverses through the
FORWARD
chains). Subsequent routing decisions involve deciding what interface to assign to an outgoing packet. At each chain in the path, every rule in that chain is evaluated in order and whenever a rule matches, the corresponding target/jump action is executed.  The 3 most commonly used targets are
ACCEPT
,
DROP
, and jump to a user-defined chain.  While built-in chains can have default policies, user-defined chains can not.  If every rule in a chain that you jumped fails to provide a complete match, the packet is dropped back into the calling chain; see the following
illustration
. If at any time a complete match is achieved for a rule with a
DROP
target, the packet is dropped and no further processing is done. If a packet reaches a jump to the ACCEPT target, it will not traverse any further rules of the chain and subsequent chains of the table. Its processing will jump to the first chain of the next table in order. See also
Traversing tables and chains
and
Accept Target
of the frozentux tutorial.
Modules
There are many modules which can be used to extend iptables such as connlimit, conntrack, limit and recent. These modules add extra functionality to allow complex filtering rules.
Configuration and usage
By default,
iptables
provides an empty set of rules in
/etc/iptables/iptables.rules
. To be able to load iptables rules automatically on boot, you can
enable
the
iptables.service
unit provided by iptables or
start
the
systemd
service unit to load iptables rules at once.
Similarly, iptables rules for IPv6 are, by default, stored in
/etc/iptables/ip6tables.rules
, which is read by
ip6tables.service
. You can start it the same way as above.
After adding rules via command-line as shown in the following sections, the configuration file is not changed automatically — you have to save it manually:
# iptables-save -f /etc/iptables/iptables.rules
If you edit the configuration file manually, you have to
reload
iptables.
Or you can load it directly through iptables:
# iptables-restore /etc/iptables/iptables.rules
From the command line
Showing the current rules
The basic command to list current rules is
--list-rules
(
-S
), which is similar in output format to the
iptables-save
utility. The main difference of the two is that the latter outputs the rules of all tables per default, while all
iptables
commands default to the
filter
table only.
When working with iptables on the command line, the
--list
(
-L
) command accepts more modifiers and shows more information. For example, you can check the current ruleset and the number of hits per rule by using the command:
# iptables -nvL
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
If the output looks like the above, then there are no rules (i.e. nothing is blocked) in the default
filter
table. Other tables can be specified with the
-t
option.
To show the line numbers when listing rules, append
--line-numbers
to that input. The line numbers are a useful shorthand when
#Editing rules
on the command line.
Resetting rules
You can flush and reset iptables to default using these commands:
# iptables -F
# iptables -X
# iptables -t nat -F
# iptables -t nat -X
# iptables -t mangle -F
# iptables -t mangle -X
# iptables -t raw -F
# iptables -t raw -X
# iptables -t security -F
# iptables -t security -X
# iptables -P INPUT ACCEPT
# iptables -P FORWARD ACCEPT
# iptables -P OUTPUT ACCEPT
The
-F
command with no arguments flushes all the chains in its current table. Similarly,
-X
deletes all empty non-default chains in a table.
Individual chains may be flushed or deleted by following
-F
and
-X
with a
[chain]
argument.
Editing rules
Rules can be edited by appending
-A
a rule to a chain, inserting
-I
it at a specific position on the chain, replacing
-R
an existing rule, or deleting
-D
it. The first three commands are exemplified in the following.
First of all, our computer is not a router (unless, of course, it
is a router
). We want to change the default policy on the
FORWARD
chain from
ACCEPT
to
DROP
.
# iptables -P FORWARD DROP
Warning
The rest of this section is meant to teach the syntax and concepts behind iptables rules. It is not intended as a means for securing servers. For improving the security of your system, see
Simple stateful firewall
for a minimally secure iptables configuration and
Security
for hardening Arch Linux in general.
The
Dropbox
LAN sync feature
broadcasts packets every 30 seconds
to all computers it can see. If we happen to be on a LAN with Dropbox clients and do not use this feature, then we might wish to reject those packets.
# iptables -A INPUT -p tcp --dport 17500 -j REJECT --reject-with icmp-port-unreachable
# iptables -nvL --line-numbers
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
1        0     0 REJECT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:17500 reject-with icmp-port-unreachable

Chain FORWARD (policy DROP 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
Note
We use
REJECT
rather than
DROP
here, because
RFC 1122
requires hosts return ICMP errors whenever possible, instead of dropping packets.
This page
explains why it is almost always better to REJECT rather than DROP packets.
Now, say we change our mind about Dropbox and decide to install it on our computer. We also want to LAN sync, but only with one particular IP on our network. So we should use
-R
to replace our old rule. Where
10.0.0.85
is our other IP:
# iptables -R INPUT 1 -p tcp --dport 17500 ! -s 10.0.0.85 -j REJECT --reject-with icmp-port-unreachable
# iptables -nvL --line-numbers
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
1        0     0 REJECT     tcp  --  *      *      !10.0.0.85            0.0.0.0/0            tcp dpt:17500 reject-with icmp-port-unreachable

Chain FORWARD (policy DROP 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
We have now replaced our original rule with one that allows
10.0.0.85
to access port
17500
on our computer. But now we realize that this is not scalable. If our friendly Dropbox user is attempting to access port
17500
on our device, we should allow them immediately, not test them against any firewall rules that might come afterwards!
So we write a new rule to allow our trusted user immediately. Using
-I
to insert the new rule before our old one:
# iptables -I INPUT -p tcp --dport 17500 -s 10.0.0.85 -j ACCEPT -m comment --comment "Friendly Dropbox"
# iptables -nvL --line-numbers
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
1        0     0 ACCEPT     tcp  --  *      *       10.0.0.85            0.0.0.0/0            tcp dpt:17500 /* Friendly Dropbox */
2        0     0 REJECT     tcp  --  *      *      !10.0.0.85            0.0.0.0/0            tcp dpt:17500 reject-with icmp-port-unreachable

Chain FORWARD (policy DROP 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
And replace our second rule with one that rejects everything on port
17500
:
# iptables -R INPUT 2 -p tcp --dport 17500 -j REJECT --reject-with icmp-port-unreachable
Our final rule list now looks like this:
# iptables -nvL --line-numbers
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
1        0     0 ACCEPT     tcp  --  *      *       10.0.0.85            0.0.0.0/0            tcp dpt:17500 /* Friendly Dropbox */
2        0     0 REJECT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:17500 reject-with icmp-port-unreachable

Chain FORWARD (policy DROP 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
Allowing multicast traffic
Protocols that use multicast identification (e.g.
SANE
searching for network scanners) will send traffic to the network's broadcast IP and responses will come back from a specific client's IP. Since these IP addresses are different, iptables will not recognize the response as
RELATED
or
ESTABLISHED
, and it will block the response. See
[1]
for how to accept multicast traffic without creating an overly-permissive firewall.
First, create an
ipset
hash container. The timeout is the window of time to accept client responses.
# ipset create upnp hash:ip,port timeout 3
Second, create a rule to add outgoing multicast traffic to the ipset hash.
# iptables -A OUTPUT -d 239.255.255.250/32 -p udp -m udp -j SET --add-set upnp src,src --exist
Third, create a rule to allow incoming traffic that matches against the ipset hash.
# iptables -A INPUT -p udp -m set --match-set upnp dst,dst -j ACCEPT
Finally, remember to save the new rules (see
#Configuration and usage
and
ipset#Making ipset persistent
), and ensure
iptables.service
and
ipset.service
are
enabled
so the rules load upon system start.
Guides
Simple stateful firewall
Router
Logging
The
LOG
target can be used to log packets that hit a rule. Unlike other targets like
ACCEPT
or
DROP
, the packet will continue moving through the chain after hitting a
LOG
target. This means that in order to enable logging for all dropped packets, you would have to add a duplicate
LOG
rule before each
DROP
rule. Since this reduces efficiency and makes things less simple, a
logdrop
chain can be created instead.
Create the chain with:
# iptables -N logdrop
And add the following rules to the newly created chain:
# iptables -A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG
# iptables -A logdrop -j DROP
Explanation for
limit
and
limit-burst
options is given
below
.
Now whenever we want to drop a packet and log this event, we just jump to the
logdrop
chain, for example:
# iptables -A INPUT -m conntrack --ctstate INVALID -j logdrop
Limiting log rate
The above
logdrop
chain uses the limit module to prevent the
iptables
log from growing too large or causing needless hard drive writes. Without limiting an erroneously configured service trying to connect, or an attacker, could fill the drive (or at least the
/var
partition) by causing writes to the iptables log.
The limit module is called with
-m limit
. You can then use
--limit
to set an average rate and
--limit-burst
to set an initial burst rate. In the
logdrop
example above:
iptables -A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG
appends a rule which will log all packets that pass through it. The first 10 consecutive packets will be logged, and from then on only 5 packets per minute will be logged. The "limit burst" count is reset every time the "limit rate" is not broken, i.e. logging activity returns to normal automatically.
Viewing logged packets
Logged packets are visible as kernel messages in the
systemd journal
.
To view all packets that were logged since the machine was last booted:
# journalctl -k --grep="IN=.*OUT=.*"
syslog-ng
Assuming you are using
syslog-ng
, you can control where iptables' log output goes in
syslog-ng.conf
. Replace:
filter f_everything { level(debug..emerg) and not facility(auth, authpriv); };
to
filter f_everything { level(debug..emerg) and not facility(auth, authpriv) and not filter(f_iptables); };
This will stop logging iptables output to
/var/log/everything.log
.
If you also want iptables to log to a different file than
/var/log/iptables.log
, you can simply change the file value of destination
d_iptables
here (still in
syslog-ng.conf
):
destination d_iptables { file("/var/log/iptables.log"); };
ulogd
ulogd
is a specialized userspace packet logging daemon for netfilter that can replace the default
LOG
target.
További olvasnivaló a témában
Wikipedia article
Port knocking
Official iptables web site
iptables Tutorial 1.2.2
by Oskar Andreasson
Debian Wiki - iptables
Secure use of connection tracking helpers