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268 lines
11 KiB
XML
268 lines
11 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article id="MAC_Validation">
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<!--$Id$-->
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<articleinfo>
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<title>MAC Verification</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
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<copyright>
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<year>2001-2005</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<para>All traffic from an interface or from a subnet on an interface can be
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verified to originate from a defined set of MAC addresses. Furthermore, each
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MAC address may be optionally associated with one or more IP
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addresses.</para>
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<important>
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<para><emphasis role="bold">MAC addresses are only visible within an
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ethernet segment so all MAC addresses used in verification must belong to
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devices physically connected to one of the LANs to which your firewall is
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connected.</emphasis></para>
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<para><emphasis role="bold">This means what it says! MAC addresses are
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only used within a LAN and never go outside of that LAN so please don't
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post on the mailing list asking how to use MAC addresses of computers
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connected to remote networks. The only MAC address that your firewall is
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going to see from these hosts is the MAC address of your upstream
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router.</emphasis></para>
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</important>
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<important>
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<para><emphasis role="bold">Your kernel must include MAC match support
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(CONFIG_IP_NF_MATCH_MAC - module name ipt_mac.o).</emphasis></para>
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</important>
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<important>
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<para><emphasis role="bold">MAC verification is only applied to new
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incoming connection requests. </emphasis></para>
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</important>
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<important>
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<para><emphasis role="bold">DO NOT use MAC verification as your only
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security measure . MAC addresses can be easily spoofed. You can use it in
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combination with either <ulink url="IPSEC-2.6.html">IPSEC</ulink> or
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<ulink url="OPENVPN.html">OpenVPN</ulink>.</emphasis></para>
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</important>
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<section>
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<title>Components</title>
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<para>There are six components to this facility.</para>
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<orderedlist>
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<listitem>
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<para>The <emphasis role="bold">maclist</emphasis> interface option in
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<ulink
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url="Documentation.htm#Interfaces">/etc/shorewall/interfaces</ulink>.
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When this option is specified, all new connection requests arriving on
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the interface are subject to MAC verification.</para>
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</listitem>
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<listitem>
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<para>The <emphasis role="bold">maclist</emphasis> option in <ulink
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url="Documentation.htm#Hosts">/etc/shorewall/hosts</ulink>. When this
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option is specified for a subnet, all new connection requests from
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that subnet are subject to MAC verification.</para>
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</listitem>
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<listitem>
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<para>The /etc/shorewall/maclist file. This file is used to associate
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MAC addresses with interfaces and to optionally associate IP addresses
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with MAC addresses.</para>
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</listitem>
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<listitem>
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<para>The <emphasis role="bold">MACLIST_DISPOSITION</emphasis> and
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<emphasis role="bold">MACLIST_LOG_LEVEL</emphasis> variables in <ulink
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url="Documentation.htm#Conf">/etc/shorewall/shorewall.conf</ulink>.
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The MACLIST_DISPOSITION variable has the value DROP, REJECT or ACCEPT
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and determines the disposition of connection requests that fail MAC
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verification. The MACLIST_LOG_LEVEL variable gives the syslogd level
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at which connection requests that fail verification are to be logged.
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If set the empty value (e.g., MACLIST_LOG_LEVEL="") then failing
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connection requests are not logged.</para>
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</listitem>
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<listitem>
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<para>Beginning with Shorewall 2.2.3, the <emphasis
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role="bold">MACLIST_TTL</emphasis> variable in <ulink
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url="???">/etc/shorewall/shorewall.conf</ulink>. The performance of
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configurations with a large numbers of entries in
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/etc/shorewall/maclist can be improved by setting the MACLIST_TTL
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variable.</para>
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<para>If your iptables and kernel support the "Recent Match" (see the
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output of "shorewall check" near the top), you can cache the results
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of a 'maclist' file lookup and thus reduce the overhead associated
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with MAC Verification.</para>
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<para>When a new connection arrives from a 'maclist' interface, the
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packet passes through the list of entries for that interface in
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/etc/shorewall/maclist. If there is a match then the source IP address
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is added to the 'Recent' set for that interface. Subsequent connection
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attempts from that IP address occurring within $MACLIST_TTL seconds
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will be accepted without having to scan all of the entries. After
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$MACLIST_TTL from the first accepted connection request from an IP
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address, the next connection request from that IP address will be
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checked against the entire list.</para>
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<para>If MACLIST_TTL is not specified or is specified as empty (e.g,
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MACLIST_TTL="" or is specified as zero then 'maclist' lookups will not
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be cached).</para>
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</listitem>
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<listitem>
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<para>Beginning with Shorewall 2.4.6, the <emphasis
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role="bold">MACLIST_TABLE</emphasis> variable in <ulink
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url="???">/etc/shorewall/shorewall.conf</ulink>. Normally, MAC
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verification occurs in the filter table (INPUT and FORWARD) chains.
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When forwarding a packet from an interface with MAC verification to a
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bridge interface, that doesn't work.</para>
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<para>This problem can be worked around by setting
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MACLIST_TABLE=mangle which will cause MAC verification to occur out of
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the PREROUTING chain. Because REJECT isn't available in that
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environment, you may not specify MACLIST_DISPOSITION=REJECT with
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MACLIST_TABLE=mangle.</para>
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</listitem>
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</orderedlist>
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</section>
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<section>
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<title>/etc/shorewall/maclist</title>
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<para>The columns in /etc/shorewall/maclist are:</para>
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<variablelist>
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<varlistentry>
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<term>DISPOSITION (Added in Shorewall version 3.1)</term>
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<listitem>
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<para>Must be ACCEPT, DROP or REJECT (REJECT may not be specified if
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<emphasis role="bold">MACLIST_TABLE</emphasis>=mangle). May be
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optionally followed by ":" and a log level to cause packets matching
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the rule to be logged.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>INTERFACE</term>
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<listitem>
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<para>The name of an ethernet interface on the Shorewall
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system.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>MAC</term>
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<listitem>
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<para>The MAC address of a device on the ethernet segment connected
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by INTERFACE. It is not necessary to use the Shorewall MAC format in
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this column although you may use that format if you so choose.
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Beginning with Shorewall 3.1, you may specify "-" here if you enter
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an IP address in the next column.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>IP Address</term>
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<listitem>
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<para>An optional comma-separated list of IP addresses for the
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device whose MAC is listed in the MAC column.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</section>
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<section>
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<title>Examples</title>
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<example>
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<title>Here are my files (look <ulink url="myfiles.htm">here</ulink> for
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details about my setup)</title>
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<para>/etc/shorewall/shorewall.conf:</para>
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<programlisting>MACLIST_DISPOSITION=REJECT
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MACLIST_LOG_LEVEL=info</programlisting>
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<para>/etc/shorewall/interfaces:</para>
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<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
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net $EXT_IF 206.124.146.255 dhcp,norfc1918,routefilter,logmartians,blacklist,tcpflags,nosmurfs
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loc $INT_IF 192.168.1.255 dhcp
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dmz $DMZ_IF -
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vpn tun+ -
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Wifi $WIFI_IF - maclist,dhcp
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#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE</programlisting>
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<para>/etc/shorewall/maclist:</para>
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<programlisting>#INTERFACE MAC IP ADDRESSES (Optional)
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$WIFI_IF 00:04:5e:3f:85:b9 #WAP11
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$WIFI_IF 00:06:25:95:33:3c #WET11
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$WIFI_IF 00:0b:4d:53:cc:97 192.168.3.8 #TIPPER
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$WIFI_IF 00:1f:79:cd:fe:2e 192.168.3.6 #Work Laptop
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#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
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</programlisting>
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<para>As shown above, I use MAC Verification on my wireless zone.</para>
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<para><note>
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<para>While marketed as a wireless bridge, the WET11 behaves like a
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wireless router with DHCP relay. When forwarding DHCP traffic, it
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uses the MAC address of the host (TIPPER) but for other forwarded
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traffic it uses it's own MAC address. Consequently, I list the IP
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addresses of both devices in /etc/shorewall/maclist.</para>
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</note></para>
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</example>
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<example>
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<title>Router in Wireless Zone</title>
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<para>Suppose now that I add a second wireless segment to my wireless
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zone and gateway that segment via a router with MAC address
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00:06:43:45:C6:15 and IP address 192.168.3.253. Hosts in the second
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segment have IP addresses in the subnet 192.168.4.0/24. I would add the
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following entry to my /etc/shorewall/maclist file:</para>
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<programlisting> $WIFI_IF 00:06:43:45:C6:15 192.168.3.253,192.168.4.0/24</programlisting>
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<para>This entry accommodates traffic from the router itself
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(192.168.3.253) and from the second wireless segment (192.168.4.0/24).
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Remember that all traffic being sent to my firewall from the
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192.168.4.0/24 segment will be forwarded by the router so that traffic's
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MAC address will be that of the router (00:06:43:45:C6:15) and not that
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of the host sending the traffic.</para>
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</example>
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</section>
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</article> |