source: postlfs/security/iptables.xml@ 6d6c19d

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
  <!ENTITY % general-entities SYSTEM "../../general.ent">
  %general-entities;

  <!ENTITY iptables-download-http "http://www.netfilter.org/projects/iptables/files/iptables-&iptables-version;.tar.bz2">
  <!ENTITY iptables-download-ftp  "ftp://ftp.netfilter.org/pub/iptables/iptables-&iptables-version;.tar.bz2">
  <!ENTITY iptables-md5sum        "29de711d15c040c402cf3038c69ff513">
  <!ENTITY iptables-size          "699 KB">
  <!ENTITY iptables-buildsize     "17 MB">
  <!ENTITY iptables-time          "0.2 SBU">
]>

<sect1 id="iptables" xreflabel="iptables-&iptables-version;">
  <?dbhtml filename="iptables.html"?>

  <sect1info>
    <othername>$LastChangedBy$</othername>
    <date>$Date$</date>
  </sect1info>

  <title>iptables-&iptables-version;</title>

  <indexterm zone="iptables">
    <primary sortas="a-iptables">iptables</primary>
  </indexterm>

  <sect2 role="package">
    <title>Introduction to iptables</title>

    <para>
      <application>iptables</application> is a userspace command line program
      used to configure Linux 2.4 and later kernel packet filtering ruleset.
    </para>

    &lfs90_checked;

    <bridgehead renderas="sect3">Package Information</bridgehead>
    <itemizedlist spacing="compact">
      <listitem>
        <para>
          Download (HTTP): <ulink url="&iptables-download-http;"/>
        </para>
      </listitem>
      <listitem>
        <para>
          Download (FTP): <ulink url="&iptables-download-ftp;"/>
        </para>
      </listitem>
      <listitem>
        <para>
          Download MD5 sum: &iptables-md5sum;
        </para>
      </listitem>
      <listitem>
        <para>
          Download size: &iptables-size;
        </para>
      </listitem>
      <listitem>
        <para>
          Estimated disk space required: &iptables-buildsize;
        </para>
      </listitem>
      <listitem>
        <para>
          Estimated build time: &iptables-time;
        </para>
      </listitem>
    </itemizedlist>

    <bridgehead renderas="sect3">iptables Dependencies</bridgehead>

    <bridgehead renderas="sect4">Optional</bridgehead>
    <para role="optional">
      <xref linkend="nftables"/>,
      <xref linkend="libpcap"/> (required for nfsypproxy support),
      <ulink url="https://github.com/tadamdam/bpf-utils">bpf-utils</ulink>
      (required for Berkely Packet Filter support),
      <ulink url="https://netfilter.org/projects/libnfnetlink/">libnfnetlink</ulink>
      (required for connlable support), and 
      <ulink url="https://netfilter.org/projects/libnetfilter_conntrack/">libnetfilter_conntrack"</ulink>
      (required for connlabel support)
    </para>


    <para condition="html" role="usernotes">User Notes:
      <ulink url="&blfs-wiki;/iptables"/>
    </para>
  </sect2>

  <sect2 role="kernel" id="iptables-kernel">
    <title>Kernel Configuration</title>

    <para>
      A firewall in Linux is accomplished through the netfilter interface. To
      use <application>iptables</application> to configure netfilter, the
      following kernel configuration parameters are required:
    </para>

<screen><literal>[*] Networking support  ---&gt;                                    [CONFIG_NET]
      Networking Options  ---&gt;
        [*] Network packet filtering framework (Netfilter) ---&gt; [CONFIG_NETFILTER]
          Core Netfilter Configuration ---&gt;</literal></screen>

    <para>
      Include any connection tracking protocols that will be used, as well as
      any protocols that you wish to use for match suppport under the
      "Core Netfilter Configuration" section.
    </para>

    <indexterm zone="iptables iptables-kernel">
      <primary sortas="d-iptables">iptables</primary>
    </indexterm>

  </sect2>

    <sect2 role="installation">
      <title>Installation of iptables</title>

    <note>
      <para>
        The installation below does not include building some specialized
        extension libraries which require the raw headers in the
        <application>Linux</application> source code. If you wish to build the
        additional extensions (if you aren't sure, then you probably don't), you
        can look at the <filename>INSTALL</filename> file to see an example of
        how to change the <parameter>KERNEL_DIR=</parameter> parameter to point
        at the <application>Linux</application> source code. Note that if you
        upgrade the kernel version, you may also need to recompile
        <application>iptables</application> and that the BLFS team has not
        tested using the raw kernel headers.
      </para>
    </note>

    <para>
      Install <application>iptables</application> by running the following
      commands:
    </para>

<screen><userinput>./configure --prefix=/usr      \
            --sbindir=/sbin    \
            --disable-nftables \
            --enable-libipq    \
            --with-xtlibdir=/lib/xtables &amp;&amp;
make</userinput></screen>

    <para>
      This package does not come with a test suite.
    </para>

    <para>
      Now, as the <systemitem class="username">root</systemitem> user:
    </para>

<screen role="root"><userinput>make install &amp;&amp;
ln -sfv ../../sbin/xtables-legacy-multi /usr/bin/iptables-xml &amp;&amp;

for file in ip4tc ip6tc ipq iptc xtables
do
  mv -v /usr/lib/lib${file}.so.* /lib &amp;&amp;
  ln -sfv ../../lib/$(readlink /usr/lib/lib${file}.so) /usr/lib/lib${file}.so
done</userinput></screen>

  </sect2>

  <sect2 role="commands">
    <title>Command Explanations</title>

    <para>
      <parameter>--disable-nftables</parameter>: This switch disables building
      nftables compat. Omit this switch if you have installed
      <xref linkend="nftables"/>.
    </para>

    <para>
      <parameter>--enable-libipq</parameter>: This switch enables building
      of <filename class="libraryfile">libipq.so</filename> which
      can be used by some packages outside of BLFS.
    </para>

    <para>
      <parameter>--with-xtlibdir=/lib/xtables</parameter>: Ensure all
      <application>iptables</application> modules are installed in the
      <filename class="directory">/lib/xtables</filename> directory.
    </para>

    <para>
      <option>--enable-nfsynproxy</option>: This switch enables installation
      of <application>nfsynproxy</application> SYNPROXY configuration tool.
    </para>

    <para>
      <command>ln -sfv ../../sbin/xtables-legacy-multi /usr/bin/iptables-xml</command>:
      Ensure the symbolic link for <command>iptables-xml</command> is relative.
    </para>

  </sect2>

  <sect2 role="configuration">
    <title>Configuring iptables</title>

    <note>
      <para>
        If you intend to use <xref linkend="firewalld"/> to configure your
        firewall rules, you should not use the example configurations provided
        here, nor should you enable the
        <phrase revision="sysv">bootscript.</phrase>
        <phrase revision="systemd">systemd unit.</phrase>
      </para>
    </note>

    <note>
      <para>
        In the following example configurations, <emphasis
        role="strong">LAN1</emphasis> is used for the internal LAN interface,
        and <emphasis role="strong">WAN1</emphasis> is used for the external
        interace connected to the Internet. You will need to replace these
        values with appropriate interface names for your system.
      </para>
    </note>

    <sect3 id="fw-persFw-ipt"
    xreflabel="Creating a Personal Firewall With iptables">
      <title>Personal Firewall</title>

      <para>
        A Personal Firewall is designed to let you access all the
        services offered on the Internet, but keep your box secure and
        your data private.
      </para>

      <para>
        Below is a slightly modified version of Rusty Russell's
        recommendation from the <ulink
        url="http://www.netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html">
        Linux 2.4 Packet Filtering HOWTO</ulink>. It is still applicable
        to the Linux 3.x kernels.
      </para>

<screen role="root" revision="sysv"><?dbfo keep-together="auto"?><userinput>cat &gt; /etc/rc.d/rc.iptables &lt;&lt; "EOF"
<literal>#!/bin/sh

# Begin rc.iptables

# Insert connection-tracking modules
# (not needed if built into the kernel)
modprobe nf_conntrack
modprobe xt_LOG

# Enable broadcast echo Protection
echo 1 &gt; /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# Disable Source Routed Packets
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_source_route
echo 0 &gt; /proc/sys/net/ipv4/conf/default/accept_source_route

# Enable TCP SYN Cookie Protection
echo 1 &gt; /proc/sys/net/ipv4/tcp_syncookies

# Disable ICMP Redirect Acceptance
echo 0 &gt; /proc/sys/net/ipv4/conf/default/accept_redirects

# Do not send Redirect Messages
echo 0 &gt; /proc/sys/net/ipv4/conf/all/send_redirects
echo 0 &gt; /proc/sys/net/ipv4/conf/default/send_redirects

# Drop Spoofed Packets coming in on an interface, where responses
# would result in the reply going out a different interface.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/rp_filter
echo 1 &gt; /proc/sys/net/ipv4/conf/default/rp_filter

# Log packets with impossible addresses.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/log_martians
echo 1 &gt; /proc/sys/net/ipv4/conf/default/log_martians

# be verbose on dynamic ip-addresses  (not needed in case of static IP)
echo 2 &gt; /proc/sys/net/ipv4/ip_dynaddr

# disable Explicit Congestion Notification
# too many routers are still ignorant
echo 0 &gt; /proc/sys/net/ipv4/tcp_ecn

# Set a known state
iptables -P INPUT   DROP
iptables -P FORWARD DROP
iptables -P OUTPUT  DROP

# These lines are here in case rules are already in place and the
# script is ever rerun on the fly. We want to remove all rules and
# pre-existing user defined chains before we implement new rules.
iptables -F
iptables -X
iptables -Z

iptables -t nat -F

# Allow local-only connections
iptables -A INPUT  -i lo -j ACCEPT

# Free output on any interface to any ip for any service
# (equal to -P ACCEPT)
iptables -A OUTPUT -j ACCEPT

# Permit answers on already established connections
# and permit new connections related to established ones
# (e.g. port mode ftp)
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

# Log everything else. What's Windows' latest exploitable vulnerability?
iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "

# End $rc_base/rc.iptables</literal>
EOF
chmod 700 /etc/rc.d/rc.iptables</userinput></screen>

<screen role="root" revision="systemd"><?dbfo keep-together="auto"?><userinput>install -v -dm755 /etc/systemd/scripts

cat &gt; /etc/systemd/scripts/iptables &lt;&lt; "EOF"
<literal>#!/bin/sh

# Begin /etc/systemd/scripts/iptables

# Insert connection-tracking modules
# (not needed if built into the kernel)
modprobe nf_conntrack
modprobe xt_LOG

# Enable broadcast echo Protection
echo 1 &gt; /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# Disable Source Routed Packets
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_source_route
echo 0 &gt; /proc/sys/net/ipv4/conf/default/accept_source_route

# Enable TCP SYN Cookie Protection
echo 1 &gt; /proc/sys/net/ipv4/tcp_syncookies

# Disable ICMP Redirect Acceptance
echo 0 &gt; /proc/sys/net/ipv4/conf/default/accept_redirects

# Do not send Redirect Messages
echo 0 &gt; /proc/sys/net/ipv4/conf/all/send_redirects
echo 0 &gt; /proc/sys/net/ipv4/conf/default/send_redirects

# Drop Spoofed Packets coming in on an interface, where responses
# would result in the reply going out a different interface.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/rp_filter
echo 1 &gt; /proc/sys/net/ipv4/conf/default/rp_filter

# Log packets with impossible addresses.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/log_martians
echo 1 &gt; /proc/sys/net/ipv4/conf/default/log_martians

# be verbose on dynamic ip-addresses  (not needed in case of static IP)
echo 2 &gt; /proc/sys/net/ipv4/ip_dynaddr

# disable Explicit Congestion Notification
# too many routers are still ignorant
echo 0 &gt; /proc/sys/net/ipv4/tcp_ecn

# Set a known state
iptables -P INPUT   DROP
iptables -P FORWARD DROP
iptables -P OUTPUT  DROP

# These lines are here in case rules are already in place and the
# script is ever rerun on the fly. We want to remove all rules and
# pre-existing user defined chains before we implement new rules.
iptables -F
iptables -X
iptables -Z

iptables -t nat -F

# Allow local-only connections
iptables -A INPUT  -i lo -j ACCEPT

# Free output on any interface to any ip for any service
# (equal to -P ACCEPT)
iptables -A OUTPUT -j ACCEPT

# Permit answers on already established connections
# and permit new connections related to established ones
# (e.g. port mode ftp)
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

# Log everything else. What's Windows' latest exploitable vulnerability?
iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "

# End /etc/systemd/scripts/iptables</literal>
EOF
chmod 700 /etc/systemd/scripts/iptables</userinput></screen>

      <para>
        This script is quite simple, it drops all traffic coming
        into your computer that wasn't initiated from your computer, but
        as long as you are simply surfing the Internet you are unlikely
        to exceed its limits.
      </para>

      <para>
        If you frequently encounter certain delays at accessing
        FTP servers, take a look at <xref linkend="fw-BB-4-ipt"/>.
      </para>

      <para>
        Even if you have daemons or services running on your system,
        these will be inaccessible everywhere but from your computer itself.
        If you want to allow access to services on your machine, such as
        <command>ssh</command> or <command>ping</command>, take a look at
        <xref linkend="fw-busybox-ipt"/>.
      </para>

    </sect3>

    <sect3 id="fw-masqRouter-ipt"
     xreflabel="Creating a Masquerading Router With iptables">
      <title>Masquerading Router</title>

      <para>
        A network Firewall has two interfaces, one connected to an
        intranet, in this example <emphasis role="strong">LAN1</emphasis>,
        and one connected to the Internet, here <emphasis
        role="strong">WAN1</emphasis>. To provide the maximum security
        for the firewall itself, make sure that there are no unnecessary
        servers running on it such as <application>X11</application> et al.
        As a general principle, the firewall itself should not access
        any untrusted service (think of a remote server giving answers that
        makes a daemon on your system crash, or even worse, that implements
        a worm via a buffer-overflow).
      </para>

<screen role="root" revision="sysv"><?dbfo keep-together="auto"?><userinput>cat &gt; /etc/rc.d/rc.iptables &lt;&lt; "EOF"
<literal>#!/bin/sh

# Begin rc.iptables

echo
echo "You're using the example configuration for a setup of a firewall"
echo "from Beyond Linux From Scratch."
echo "This example is far from being complete, it is only meant"
echo "to be a reference."
echo "Firewall security is a complex issue, that exceeds the scope"
echo "of the configuration rules below."
echo "You can find additional information"
echo "about firewalls in Chapter 4 of the BLFS book."
echo "http://www.&lfs-domainname;/blfs"
echo

# Insert iptables modules (not needed if built into the kernel).

modprobe nf_conntrack
modprobe nf_conntrack_ftp
modprobe xt_conntrack
modprobe xt_LOG
modprobe xt_state

# Enable broadcast echo Protection
echo 1 &gt; /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# Disable Source Routed Packets
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_source_route

# Enable TCP SYN Cookie Protection
echo 1 &gt; /proc/sys/net/ipv4/tcp_syncookies

# Disable ICMP Redirect Acceptance
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_redirects

# Don't send Redirect Messages
echo 0 &gt; /proc/sys/net/ipv4/conf/default/send_redirects

# Drop Spoofed Packets coming in on an interface where responses
# would result in the reply going out a different interface.
echo 1 &gt; /proc/sys/net/ipv4/conf/default/rp_filter

# Log packets with impossible addresses.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/log_martians

# Be verbose on dynamic ip-addresses  (not needed in case of static IP)
echo 2 &gt; /proc/sys/net/ipv4/ip_dynaddr

# Disable Explicit Congestion Notification
# Too many routers are still ignorant
echo 0 &gt; /proc/sys/net/ipv4/tcp_ecn

# Set a known state
iptables -P INPUT   DROP
iptables -P FORWARD DROP
iptables -P OUTPUT  DROP

# These lines are here in case rules are already in place and the
# script is ever rerun on the fly. We want to remove all rules and
# pre-existing user defined chains before we implement new rules.
iptables -F
iptables -X
iptables -Z

iptables -t nat -F

# Allow local connections
iptables -A INPUT  -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

# Allow forwarding if the initiated on the intranet
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD ! -i WAN1 -m conntrack --ctstate NEW       -j ACCEPT

# Do masquerading
# (not needed if intranet is not using private ip-addresses)
iptables -t nat -A POSTROUTING -o WAN1 -j MASQUERADE

# Log everything for debugging
# (last of all rules, but before policy rules)
iptables -A INPUT   -j LOG --log-prefix "FIREWALL:INPUT "
iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD "
iptables -A OUTPUT  -j LOG --log-prefix "FIREWALL:OUTPUT "

# Enable IP Forwarding
echo 1 &gt; /proc/sys/net/ipv4/ip_forward</literal>
EOF
chmod 700 /etc/rc.d/rc.iptables</userinput></screen>

<screen role="root" revision="systemd"><?dbfo keep-together="auto"?><userinput>install -v -dm755 /etc/systemd/scripts

cat &gt; /etc/systemd/scripts/iptables &lt;&lt; "EOF"
<literal>#!/bin/sh

# Begin /etc/systemd/scripts/iptables

echo
echo "You're using the example configuration for a setup of a firewall"
echo "from Beyond Linux From Scratch."
echo "This example is far from being complete, it is only meant"
echo "to be a reference."
echo "Firewall security is a complex issue, that exceeds the scope"
echo "of the configuration rules below."

echo "You can find additional information"
echo "about firewalls in Chapter 4 of the BLFS book."
echo "http://www.&lfs-domainname;/blfs"
echo

# Insert iptables modules (not needed if built into the kernel).

modprobe nf_conntrack
modprobe nf_conntrack_ftp
modprobe xt_conntrack
modprobe xt_LOG
modprobe xt_state

# Enable broadcast echo Protection
echo 1 &gt; /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# Disable Source Routed Packets
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_source_route

# Enable TCP SYN Cookie Protection
echo 1 &gt; /proc/sys/net/ipv4/tcp_syncookies

# Disable ICMP Redirect Acceptance
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_redirects

# Don't send Redirect Messages
echo 0 &gt; /proc/sys/net/ipv4/conf/default/send_redirects

# Drop Spoofed Packets coming in on an interface where responses
# would result in the reply going out a different interface.
echo 1 &gt; /proc/sys/net/ipv4/conf/default/rp_filter

# Log packets with impossible addresses.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/log_martians

# Be verbose on dynamic ip-addresses  (not needed in case of static IP)
echo 2 &gt; /proc/sys/net/ipv4/ip_dynaddr

# Disable Explicit Congestion Notification
# Too many routers are still ignorant
echo 0 &gt; /proc/sys/net/ipv4/tcp_ecn

# Set a known state
iptables -P INPUT   DROP
iptables -P FORWARD DROP
iptables -P OUTPUT  DROP

# These lines are here in case rules are already in place and the
# script is ever rerun on the fly. We want to remove all rules and
# pre-existing user defined chains before we implement new rules.
iptables -F
iptables -X
iptables -Z

iptables -t nat -F

# Allow local connections
iptables -A INPUT  -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

# Allow forwarding if the initiated on the intranet
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD ! -i WAN1 -m conntrack --ctstate NEW       -j ACCEPT

# Do masquerading
# (not needed if intranet is not using private ip-addresses)
iptables -t nat -A POSTROUTING -o WAN1 -j MASQUERADE

# Log everything for debugging
# (last of all rules, but before policy rules)
iptables -A INPUT   -j LOG --log-prefix "FIREWALL:INPUT "
iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD "
iptables -A OUTPUT  -j LOG --log-prefix "FIREWALL:OUTPUT "

# Enable IP Forwarding
echo 1 &gt; /proc/sys/net/ipv4/ip_forward

# The following sections allow inbound packets for specific examples
# Uncomment the example lines and adjust as necessary

# Allow ping on the external interface
#iptables -A INPUT  -p icmp -m icmp --icmp-type echo-request -j ACCEPT
#iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply   -j ACCEPT

# Reject ident packets with TCP reset to avoid delays with FTP or IRC
#iptables -A INPUT  -p tcp --dport 113 -j REJECT --reject-with tcp-reset

# Allow HTTP and HTTPS to 192.168.0.2
#iptables -A PREROUTING -t nat -i WAN1 -p tcp --dport 80 -j DNAT --to 192.168.0.2
#iptables -A PREROUTING -t nat -i WAN1 -p tcp --dport 443 -j DNAT --to 192.168.0.2
#iptables -A FORWARD -p tcp -d 192.168.0.2 --dport 80 -j ACCEPT
#iptables -A FORWARD -p tcp -d 192.168.0.2 --dport 443 -j ACCEPT

# End /etc/systemd/scripts/iptables</literal>
EOF
chmod 700 /etc/systemd/scripts/iptables</userinput></screen>

      <para>
        With this script your intranet should be reasonably secure
        against external attacks. No one should be able to setup a new
        connection to any internal service and, if it's masqueraded,
        makes your intranet invisible to the Internet. Furthermore, your
        firewall should be relatively safe because there are no services
        running that a cracker could attack.
      </para>

    </sect3>
 
    <sect3 id="fw-busybox-ipt" xreflabel="Creating a BusyBox With iptables">
      <title>BusyBox</title>

      <para>
        This scenario isn't too different from the <xref
        linkend="fw-masqRouter-ipt"/>, but additionally offers some
        services to your intranet. Examples of this can be when
        you want to administer your firewall from another host on
        your intranet or use it as a proxy or a name server.
      </para>

      <note>
        <para>
          Outlining specifically how to protect a server that
          offers services on the Internet goes far beyond the scope of
          this document. See the references in <xref linkend="fw-extra-info"/>
          for more information.
        </para>
      </note>

      <para>
        Be cautious. Every service you have enabled makes your
        setup more complex and your firewall less secure. You are
        exposed to the risks of misconfigured services or running
        a service with an exploitable bug. A firewall should generally
        not run any extra services.  See the introduction to the
        <xref linkend="fw-masqRouter-ipt"/> for some more details.
      </para>

      <para>
        If you want to add services such as internal Samba or
        name servers that do not need to access the Internet themselves,
        the additional statements are quite simple and should still be
        acceptable from a security standpoint. Just add the following lines
        into the script <emphasis>before</emphasis> the logging rules.
      </para>

<screen><literal>iptables -A INPUT  -i ! WAN1  -j ACCEPT
iptables -A OUTPUT -o ! WAN1  -j ACCEPT</literal></screen>

      <para>
        If daemons, such as squid, have to access the Internet
        themselves, you could open OUTPUT generally and restrict
        INPUT.
      </para>

<screen><literal>iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -j ACCEPT</literal></screen>

      <para>
        However, it is generally not advisable to leave OUTPUT
        unrestricted. You lose any control over trojans who would like
        to "call home", and a bit of redundancy in case you've
        (mis-)configured a service so that it broadcasts its existence
        to the world.
      </para>

      <para>
        To accomplish this, you should restrict INPUT and OUTPUT
        on all ports except those that it's absolutely necessary to have
        open. Which ports you have to open depends on your needs: mostly
        you will find them by looking for failed accesses in your log
        files.
      </para>

      <itemizedlist spacing="compact" role='iptables'>
        <title>Have a Look at the Following Examples:</title>
        <listitem>
          <para>
            Squid is caching the web:
          </para>

<screen><literal>iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT  -p tcp --sport 80 -m conntrack --ctstate ESTABLISHED \
  -j ACCEPT</literal></screen>

        </listitem>
        <listitem>
          <para>
            Your caching name server (e.g., named) does its lookups via UDP:
          </para>

<screen><literal>iptables -A OUTPUT -p udp --dport 53 -j ACCEPT</literal></screen>

        </listitem>
        <listitem>
          <para>
            You want to be able to ping your computer to ensure it's still
            alive:
          </para>

<screen><literal>iptables -A INPUT  -p icmp -m icmp --icmp-type echo-request -j ACCEPT
iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply   -j ACCEPT</literal></screen>

        </listitem>
        <listitem>
          <para id='fw-BB-4-ipt' xreflabel="BusyBox with iptable example number 4">
            If you are frequently accessing FTP servers or enjoy chatting, you
            might notice delays because some implementations of these daemons
            query an identd daemon on your system to obtain usernames. Although
            there's really little harm in this, having an identd running is not
            recommended because many security experts feel the service gives
            out too much additional information.
          </para>

          <para>
            To avoid these delays you could reject the requests with a
            'tcp-reset' response:
          </para>

<screen><literal>iptables -A INPUT  -p tcp --dport 113 -j REJECT --reject-with tcp-reset</literal></screen>

        </listitem>
        <listitem>
          <para>
            To log and drop invalid packets (packets
            that came in after netfilter's timeout or some types of
            network scans) insert these rules at the top of the chain:
          </para>

<screen><literal>iptables -I INPUT 0 -p tcp -m conntrack --ctstate INVALID \
  -j LOG --log-prefix "FIREWALL:INVALID "
iptables -I INPUT 1 -p tcp -m conntrack --ctstate INVALID -j DROP</literal></screen>

        </listitem>
        <listitem>
          <para>
            Anything coming from the outside should not have a
            private address, this is a common attack called IP-spoofing:
          </para>

<screen><literal>iptables -A INPUT -i WAN1 -s 10.0.0.0/8     -j DROP
iptables -A INPUT -i WAN1 -s 172.16.0.0/12  -j DROP
iptables -A INPUT -i WAN1 -s 192.168.0.0/16 -j DROP</literal></screen>

          <para>
            There are other addresses that you may also want to drop:
            0.0.0.0/8, 127.0.0.0/8, 224.0.0.0/3 (multicast and
            experimental), 169.254.0.0/16 (Link Local Networks), and
            192.0.2.0/24 (IANA defined test network).
          </para>
        </listitem>
        <listitem>
          <para>
            If your firewall is a DHCP client, you need to allow those packets:
          </para>

<screen><literal>iptables -A INPUT  -i WAN1 -p udp -s 0.0.0.0 --sport 67 \
   -d 255.255.255.255 --dport 68 -j ACCEPT</literal></screen>

        </listitem>
        <listitem>
          <para>
            To simplify debugging and be fair to anyone who'd like
            to access a service you have disabled, purposely or by mistake,
            you could REJECT those packets that are dropped.
          </para>

          <para>
            Obviously this must be done directly after logging as the very
            last lines before the packets are dropped by policy:
          </para>

<screen><literal>iptables -A INPUT -j REJECT</literal></screen>

        </listitem>
      </itemizedlist>

      <para>
        These are only examples to show you some of the capabilities
        of the firewall code in Linux. Have a look at the man page of iptables.
        There you will find much more information. The port numbers needed for
        this can be found in <filename>/etc/services</filename>, in case you
        didn't find them by trial and error in your log file.
      </para>

    </sect3>

    <sect3  id="iptables-init">
      <title><phrase revision="sysv">Boot Script</phrase>
             <phrase revision="systemd">Systemd Unit</phrase></title>

      <para revision="sysv">
        To set up the iptables firewall at boot, install the
        <filename>/etc/rc.d/init.d/iptables</filename> init script included
        in the <xref linkend="bootscripts"/> package.
      </para>

      <para revision="systemd">
        To set up the iptables firewall at boot, install the
        <filename>iptables.service</filename> unit included in the
        <xref linkend="systemd-units"/> package.
      </para>

      <indexterm zone="iptables iptables-init">
        <primary sortas="f-iptables">iptables</primary>
      </indexterm>

<screen role="root"><userinput>make install-iptables</userinput></screen>

    </sect3>

  </sect2>

  <sect2 role="content">
    <title>Contents</title>

    <segmentedlist>
      <segtitle>Installed Programs</segtitle>
      <segtitle>Installed Libraries</segtitle>
      <segtitle>Installed Directories</segtitle>

      <seglistitem>
        <seg>
          ip6tables, ip6tables-restore, ip6tables-save, iptables, iptables-restore,
          iptables-save, iptables-xml, nfsynproxy (optional) and xtables-multi
        </seg>
        <seg>
          libip4tc.so, libip6tc.so, libipq.so, libiptc.so, and libxtables.so
        </seg>
        <seg>
          /lib/xtables and /usr/include/libiptc
        </seg>
      </seglistitem>
    </segmentedlist>

    <variablelist>
      <bridgehead renderas="sect3">Short Descriptions</bridgehead>
      <?dbfo list-presentation="list"?>
      <?dbhtml list-presentation="table"?>

      <varlistentry id="iptables-prog">
        <term><command>iptables</command></term>
        <listitem>
          <para>
            is used to set up, maintain, and inspect the tables of
            IP packet filter rules in the Linux kernel.
          </para>
          <indexterm zone="iptables iptables-prog">
            <primary sortas="b-iptables">iptables</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="iptables-restore">
        <term><command>iptables-restore</command></term>
        <listitem>
          <para>
            is used to restore IP Tables from data specified on
            STDIN. Use I/O redirection provided by your
            shell to read from a file.
          </para>
          <indexterm zone="iptables iptables-restore">
            <primary sortas="b-iptables-restore">iptables-restore</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="iptables-save">
        <term><command>iptables-save</command></term>
        <listitem>
          <para>
            is used to dump the contents of an IP Table in easily
            parseable format to STDOUT. Use I/O-redirection
            provided by your shell to write to a file.
          </para>
          <indexterm zone="iptables iptables-save">
            <primary sortas="b-iptables-save">iptables-save</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="iptables-xml">
        <term><command>iptables-xml</command></term>
        <listitem>
          <para>
            is used to convert the output of
            <command>iptables-save</command> to an XML format. Using the
            <filename>iptables.xslt</filename> stylesheet converts the XML
            back to the format of <command>iptables-restore</command>.
          </para>
          <indexterm zone="iptables iptables-xml">
            <primary sortas="b-iptables-xml">iptables-xml</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="ip6tables">
        <term><command>ip6tables*</command></term>
        <listitem>
          <para>
            are a set of commands for IPV6 that parallel the iptables
            commands above.
          </para>
          <indexterm zone="iptables ip6tables">
            <primary sortas="b-ip6tables">ip6tables</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="nfsynproxy">
        <term><command>nfsynproxy</command></term>
        <listitem>
          <para>
            (optional) configuration tool. SYNPROXY target makes handling of
            large SYN floods possible without the large performance penalties
            imposed by the connection tracking in such cases.
          </para>
          <indexterm zone="iptables nfsynproxy">
            <primary sortas="b-nfsynproxy">nfsynproxy</primary>
          </indexterm>
        </listitem>
      </varlistentry>

      <varlistentry id="xtables-multi">
        <term><command>xtables-multi</command></term>
        <listitem>
          <para>
            is a binary that behaves according to the name it is called by.
          </para>
          <indexterm zone="iptables xtables-multi">
            <primary sortas="b-xtables-multi">xtables-multi</primary>
          </indexterm>
        </listitem>
      </varlistentry>

    </variablelist>

  </sect2>

</sect1>