Difference between revisions of "Iptables"

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=== tables ===
=== tables ===
iptables contains four tables:  
iptables contains four ''tables'', which are areas where a chain of rules can apply:  
# {{ic|raw}} filters packets before any of the other tables.
# {{ic|raw}} filters packets before any of the other tables.

Revision as of 05:41, 20 July 2013

zh-CN:Iptables Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary end

Iptables is a powerful firewall built into the Linux kernel and is part of the netfilter project. It can be configured directly, or by using one of the many frontends and GUIs. iptables is used for IPv4 and ip6tables is used for IPv6.


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 in the official repositories.

Basic concepts


iptables contains four tables, which are areas where a chain of rules can apply:

  1. raw filters packets before any of the other tables.
  2. filter filters packets.
  3. nat is for network address translation (e.g. port forwarding). Because of limitations in iptables, filtering should not be done here.
  4. mangle mangles packets.


Tables contain chains, which are lists of rules for packets that are followed in order. There are three built-in chains: INPUT, OUTPUT and FORWARD.

  1. Inbound traffic addressed to the machine itself hits the INPUT chain.
  2. Outbound, locally-generated traffic hits the OUTPUT chain.
  3. Routed traffic which should not be delivered locally hits the FORWARD chain.

A rule in one of these chains can be to send a packet down another chain. If the packet passes successfully through this second chain, it will move to the next rule in the original chain. The three built-in chains have default targets which are used if no rules are hit. User-defined chains can be added to make rulesets more efficient.


A "target" is the result that occurs when a packet hits a rule. Targets are specified using "jump" (-j). The most common targets are ACCEPT, DROP, REJECT and LOG.


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.


From the command line

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 0K packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

If the output looks like the above, then there are no rules. Nothing is blocked.

You can flush and reset iptables to default using these commands:

# iptables -P INPUT ACCEPT
# iptables -P FORWARD ACCEPT
# iptables -P OUTPUT ACCEPT
# iptables -F
# iptables -X

Configuration file

Iptables rules are by default stored in /etc/iptables/iptables.rules, this file is read by iptables.service:

# systemctl enable iptables.service
# systemctl start iptables.service

Iptables rules for ipv6 are by default stored in /etc/iptables/ip6tables.rules, this file is read by ip6tables.service. You can start it the same way as above.

After adding rules via command-line, the configuration file is not changed automatically - you have to save it manually:

# iptables-save > /etc/iptables/iptables.rules

If you edit the configuration file manually, you have to reload it:

# systemctl reload iptables



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

Then define it:

## /etc/iptables/iptables.rules


... other user defined chains ..

## logdrop chain
:logdrop - [0:0]

-A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG
-A logdrop -j DROP

... rules ...

## log AND drop packets that hit this rule:
-A INPUT -m state --state INVALID -j logdrop

... more rules ...

Limiting log rate

The limit module should be used to prevent your iptables log from growing too large or causing needless hard drive writes. Without limiting, an attacker could fill your drive (or at least your /var partition) by causing writes to the iptables log.

-m limit is used to call on the limit module. You can then use --limit to set an average rate and --limit-burst to set an initial burst rate. Example:

-A logdrop -m limit --limit 5/m --limit-burst 10 -j LOG

This appends a rule to the logdrop chain which will log all packets that pass through it. The first 10 packets will the be logged, and from then on only 5 packets per minute will be logged. The "limit burst" is restored by one every time the "limit rate" is not broken.


Assuming you are using syslog-ng, you can control where iptables' log output goes this way:

filter f_everything { level(debug..emerg) and not facility(auth, authpriv); };


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 is a specialized userspace packet logging daemon for netfilter that can replace the default LOG target. The package ulogd is available in the [community] reopository.

See also