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shorewall_code/Shorewall-docs/shorewall_firewall_structure.htm
teastep cbc3ac56b1 Add Corporate Network Example 
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@664 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
2003-07-16 18:59:33 +00:00

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content="text/html; charset=windows-1252">
<title>Shorewall Firewall Structure</title>
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<table border="0" cellpadding="0" cellspacing="0"
style="border-collapse: collapse;" bordercolor="#111111" width="100%"
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<h1 align="center"><font color="#ffffff">Firewall Structure (Under
Construction)</font></h1>
</td>
</tr>
</tbody>
</table>
<p> Shorewall views the network in which it is running as a set of
<i> zones. </i>Shorewall itself defines exactly one zone called "fw" which
refers to the firewall system itself . The /etc/shorewall/zones file
is used to define additional zones and the example file provided with
Shorewall defines the zones:</p>
<ol>
<li> net -- the (untrusted) internet.</li>
<li> dmz - systems that must be accessible from the internet
and from the local network.  These systems cannot be trusted completely
since their servers may have been compromised through a security exploit.</li>
<li> loc - systems in your local network(s). These systems
must be protected from the internet and from the DMZ and in some
cases, from each other.</li>
</ol>
<p><b>Note: </b><a href="#Conf">You can specify the name of the firewall
zone</a>. For ease of description in this documentation, it is assumed
that the firewall zone is named "fw".</p>
<p>It can't be stressed enough that with the exception of the firewall zone,
Shorewall itself attaches no meaning to zone names. Zone names are simply
labels used to refer to a collection of network hosts.</p>
<p>While zones are normally disjoint (no two zones have a host in common),
there are cases where nested or overlapping zone definitions are appropriate.</p>
<p>Netfilter has the concept of <i>tables</i> and <i>chains. </i>For the purpose
of this document, we will consider Netfilter to have three tables:</p>
<ol>
<li>Filter table -- this is the main table for packet filtering and
can be displayed with the command "shorewall show".</li>
<li>Nat table -- used for all forms of Network Address Translation (NAT);
SNAT, DNAT and MASQUERADE.</li>
<li>Mangle table -- used to modify fields in the packet header.<br>
</li>
</ol>
<p>Netfilter defines a number of inbuilt chains: PREROUTING, INPUT, OUTPUT,
FORWARD and POSTROUTING. Not all inbuilt chains are present in all tables
as shown in this table.<br>
</p>
<div align="center">
<table cellpadding="2" cellspacing="2" border="1">
<tbody>
<tr>
<td valign="top">CHAIN<br>
</td>
<td valign="top">Filter<br>
</td>
<td valign="top">Nat<br>
</td>
<td valign="top">Mangle<br>
</td>
</tr>
<tr>
<td valign="top">PREROUTING<br>
</td>
<td valign="top"><br>
</td>
<td valign="top">X<br>
</td>
<td valign="top">X<br>
</td>
</tr>
<tr>
<td valign="top">INPUT<br>
</td>
<td valign="top">X<br>
</td>
<td valign="top"><br>
</td>
<td valign="top">X<br>
</td>
</tr>
<tr>
<td valign="top">OUTPUT<br>
</td>
<td valign="top">X<br>
</td>
<td valign="top">X<br>
</td>
<td valign="top">X<br>
</td>
</tr>
<tr>
<td valign="top">FORWARD<br>
</td>
<td valign="top">X<br>
</td>
<td valign="top"><br>
</td>
<td valign="top">X<br>
</td>
</tr>
<tr>
<td valign="top">POSTROUTING<br>
</td>
<td valign="top"><br>
</td>
<td valign="top">X<br>
</td>
<td valign="top">X<br>
</td>
</tr>
</tbody>
</table>
</div>
<p>Shorewall doesn't create rules in all of the builtin chains. In the large
diagram below are boxes such as  shown below.  This box represents in INPUT
chain and shows that packets first flow through the INPUT chain in the Mangle
table followed by the INPUT chain in the Filter table. The parentheses around
"Mangle" indicate that while the packets will flow through the INPUT chain
in the Mangle table, Shorewall does not create any rules in that chain.<br>
</p>
<div align="center"><img src="images/Legend.png" alt="(Box Legend)"
width="145" height="97" align="middle">
<br>
</div>
<p></p>
<p>Here is a picture of how packets traverse the various chains and tables
in Netfilter. In that diagram, "Local Process" refers to a process running
on the Firewall itself (in the 'fw' zone).</p>
<div align="center"><img src="images/Netfilter.png"
alt="Netfilter Flow Diagram" width="541" height="767">
</div>
<p><br>
<br>
In the text that follows, the paragraph numbers correspond to the box
number in the diagram above.<br>
</p>
<ol>
<li>Packets entering the firewall first pass through the <i>mangle </i>table's
PREROUTING chain (you can see the mangle table by typing "shorewall show
mangle"). If the packet entered through an interface that has the <b>norfc1918</b>
option and if iptables/netfilter doesn't support the connection tracking
match extension, then the packet is sent down the <b>man1918</b> chain which
will drop the packet if its destination IP address is reserved (as specified
in the /etc/shorewall/rfc1918 file). Next the packet passes through the<b>
pretos</b> chain to set its TOS field as specified in the /etc/shorewall/tos
file. Finally, if traffic control/shaping is being used, the packet is
sent through the<b> tcpre</b> chain to be marked for later use in policy
routing or traffic control.<br>
<br>
Next, if the packet isn't part of an established connection, it passes
through the<i> nat</i> table's PREROUTING chain (you can see the nat table
by typing "shorewall show nat"). If you are doing both static nat and
port forwarding, the order in which chains are traversed is dependent on
the setting of NAT_BEFORE_RULES in shorewall.conf. If NAT_BEFORE_RULES is
on then packets will ender a chain called<b> <i>interface_</i>in</b> where
<i>interface</i> is the name of the interface on which the packet entered.
Here it's destination IP is compared to each of the <i>EXTERNAL</i> IP
addresses from /etc/shorewall/nat that correspond to this interface; if
there is a match, DNAT is applied and the packet header is modified to
the IP in the <i>INTERNAL</i> column of the nat file record. If the destination
address doesn't match any of the rules in the <b><i>interface_</i>in</b>
chain then the packet enters a chain called <b><i>sourcezone</i>_dnat</b>
where <i>sourcezone</i> is the source zone of the packet. There it is compared
for a match against each of the DNAT records in the rules file that specify
<i> sourcezone </i>as the source zone. If a match is found, the
destination IP address (and possibly the destination port) is modified based
on the rule matched. If NAT_BEFORE_RULES is off, then the order of traversal
of the <b><i> interface_</i>in</b> and <b><i>sourcezone</i>_dnat</b> is
reversed.<br>
<br>
</li>
<li>Depending on whether the packet is destined for the firewall itself
or for another system, it follows either the left or the right path. Traffic
going to the firewall goes through chain called INPUT in the mangle table.
Shorewall doesn't add any rules to that chain.<br>
<br>
</li>
<li>Traffic that is to be forwarded to another host goes through the chains
called FORWARD in the mangle table. If MARK_IN_FORWARD=Yes in shorewall.conf,
all rules in /etc/shorewall/tcrules that do not specify Prerouting (:P) are
processed in a chain called <br>
<br>
</li>
<ol>
</ol>
<li>Traffic is next sent to an<i> interface </i>chain in the main Netfilter
table (called 'filter'). If the traffic is destined for the firewall
itself, the name of the interface chain is formed by appending "_in" to
the interface name. So traffic on eth0 destined for the firewall will
enter a chain called <i>eth0_in</i>. The interface chain for traffic
that will be routed to another system is formed by appending "_fwd" to
the interface name. So traffic from eth1 that is going to be forwarded
enters a chain called<i> eth1_fwd</i>. Interfaces described with the wild-card
character ("+") in /etc/shorewall/interfaces, share input chains. if <i>ppp+
</i>appears in /etc/shorewall/interfaces then all PPP interfaces (ppp0,
ppp1, ...) will share the interface chains <i>ppp_in</i> and <i>ppp_fwd</i>.
In other words, "+" is deleted from the name before forming the input chain
names.<br>
<br>
While the use of interfacechains may seem wasteful in simple environments,
in complex setups it substantially reduces the number of rules that each
packet must traverse.  </li>
</ol>
<p> Traffic directed from a zone to the firewall itself is sent through
a chain named &lt;<i>zone name&gt;</i>2fw. For example, traffic inbound from
the internet and addressed to the firewall is sent through a chain named
net2fw. Similarly, traffic originating in the firewall and being sent
to a host in a given zone is sent through a chain named fw2<i>&lt;zone
name&gt;. </i>For example, traffic originating in the firewall and
destined for a host in the local network is sent through a chain named
<i>fw2loc.</i> <font face="Century Gothic, Arial, Helvetica">  </font></p>
<p> Traffic being forwarded between two zones (or from one interface to
a zone to another interface to that zone) is sent through a chain named <i>
&lt;source zone&gt;</i>2<i> &lt;destination zone&gt;</i>. So for example,
traffic originating in a local system and destined for a remote web server
is sent through chain <i>loc2net. </i>This chain is referred to
as the <i>canonical</i> chain from &lt;source zone&gt; to &lt;destination
zone&gt;. Any destination NAT will have occurred <u>before</u> the packet
traverses one of these chains so rules in /etc/shorewall/rules should
be expressed in terms of the destination system's real IP address as opposed
to its apparent external address. Similarly, source NAT will occur <u>after</u>
the packet has traversed the appropriate forwarding chain so the rules
again will be expressed using the source system's real IP address.</p>
<p> For each record in the /etc/shorewall/policy file, a chain is created.
Policies in that file are expressed in terms of a source zone and destination
zone where these zones may be a zone defined in /etc/shorewall/zones,
"fw" or "all". Policies specifying the pseudo-zone "all" matches all defined
zones and "fw". These chains are referred to as <i>Policy Chains.</i> Notice
that for an ordered pair of zones (za,zb), the canonical chain (za2zb)
may also be the policy chain for the pair or the policy chain may be
a different chain (za2all, for example). Packets from one zone to another
will traverse chains as follows:</p>
<ol>
<li> If the canonical chain exists, packets first traverse
that chain.</li>
<li> If the canonical chain and policy chain are different
and the packet does not match a rule in the canonical chain, it then
is sent to the policy chain.</li>
<li> If the canonical chain does not exist, packets are sent
immediately to the policy chain.</li>
</ol>
<p> The canonical chain from zone za to zone zb will be created only if
there are exception rules defined in /etc/shorewall/rules for packets going
from za to zb.</p>
<p> Shorewall is built on top of the Netfilter kernel facility. Netfilter
implements connection tracking function that allow what is often referred
to as "statefull inspection" of packets. This statefull property allows
firewall rules to be defined in terms of "connections" rather than in
terms of "packets". With Shorewall, you:</p>
<ol>
<li> Identify the client's zone.</li>
<li> Identify the server's zone.</li>
<li> If the POLICY from the client's zone to the server's zone
is what you want for this client/server pair, you need do nothing further.</li>
<li> If the POLICY is not what you want, then you must add
a rule. That rule is expressed in terms of the client's zone and
the server's zone.</li>
</ol>
<p> Just because connections of a particular type are allowed between zone
A and the firewall and are also allowed between the firewall and zone
B <font color="#ff6633"><b><u> DOES NOT mean that these connections
are allowed between zone A and zone B</u></b></font>. It rather means
that you can have a proxy running on the firewall that accepts a connection
from zone A and then establishes its own separate connection from the
firewall to zone B.</p>
<p> If you adopt the default policy of ACCEPT from the local zone to the
internet zone and you are having problems connecting from a local client
to an internet server, <font color="#ff6633"><b><u> adding a rule won't
help</u></b></font> (see point 3 above).</p>
<p><font size="2">Last modified 5/22/2003 - <a href="support.htm">Tom Eastep</a></font></p>
<p><font face="Trebuchet MS"><a href="copyright.htm"> <font size="2">Copyright</font>
© <font size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a></font></p>
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