Iptables

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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.

Installation

Note: Your kernel needs to be compiled with iptables support. All stock Arch Linux kernels have iptables support.

First, install the userland utilities:

# pacman -S iptables

Next, add iptables to the DAEMONS array in Template:Filename to have it load your settings on boot:

Template:File

Basic concepts

tables

iptables contains four tables: raw, filter, nat and mangle.

chains

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Chains are used to specify rulesets. A packet begins at the top of a chain and progresses downwards until it hits a rule. There are three built-in chains: INPUT, OUTPUT and FORWARD. All outbound traffic passes through the forward chain, and all inbound traffic passes through the FORWARD 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.

targets

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.

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

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.

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

The configuration file at Template:Filename points to the location of the configuration file. The ruleset is loaded when the daemon is started.

IPTABLES=/usr/sbin/iptables
IP6TABLES=/usr/sbin/ip6tables

IPTABLES_CONF=/etc/iptables/iptables.rules
IP6TABLES_CONF=/etc/iptables/ip6tables.rules
IPTABLES_FORWARD=0  # enable IP forwarding?

To save the current ruleset, use this command:

# /etc/rc.d/iptables save

To load the ruleset, use this command:

# /etc/rc.d/iptables restart

Saving counters

You can also, optionally, save byte and packet counters. To accomplish this, edit Template:Filename

In the save) section, change the line:

/usr/sbin/iptables-save > $IPTABLES_CONF

to

/usr/sbin/iptables-save -c > $IPTABLES_CONF

In the stop) section, add the following to save before stopping:

stop)
     $0 save
     sleep 2

In the start) section, change the line:

/usr/sbin/iptables-restore < $IPTABLES_CONF

to

/usr/sbin/iptables-restore -c < $IPTABLES_CONF

and save the file

Guides

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.

## /etc/iptables/iptables.rules

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]

... 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 Template:Filename 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.

syslog-ng

Assuming you are using syslog-ng which is the default in Archlinux, you can control where iptables' log output goes this way:

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 Template:Filename.

If you also want iptables to log to a different file than Template:Filename, you can simply change the file value of destination d_iptables here (still in Template:Filename)

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. An AUR package can be found here.

Further Reading