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175 lines
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<title>Shorewall Firewall Structure</title>
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<h1 align="center"><font color="#FFFFFF">Firewall Structure</font></h1>
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<p>
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Shorewall views the network in which it is running as a set of
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<i> zones. </i>Shorewall itself defines exactly one zone called "fw"
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which refers to the firewall system itself . The /etc/shorewall/zones file
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is used to define additional zones and the example file provided with Shorewall
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defines the zones:</p>
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<ol>
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<li>
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net -- the (untrusted) internet.</li>
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<li>
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dmz - systems that must be accessible from the internet and from the
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local network. These systems cannot be trusted completely since their servers
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may have been compromised through a security exploit.</li>
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<li>
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loc - systems in your local network(s). These systems must be protected
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from the internet and from the DMZ and in some cases, from each other.</li>
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</ol>
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<p><b>Note: </b><a href="#Conf">You can specify the name of the firewall zone</a>.
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For ease of description in this documentation, it is assumed
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that the firewall zone is named "fw".</p>
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<p>It can't be stressed enough that
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with the exception of the firewall zone, Shorewall itself attaches no meaning to
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zone names. Zone names are simply labels used to refer to a collection of
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network hosts.</p>
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<p>While zones are normally disjoint (no two zones have a host in common),
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there are cases where nested or overlapping zone definitions are appropriate.</p>
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<p>For a general picture of how packets traverse a Netfilter firewall, see
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<a href="http://www.netfilter.org/documentation/tutorials/blueflux/iptables-tutorial.html#TRAVERSINGOFTABLES">
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http://www.netfilter.org/documentation/tutorials/blueflux/iptables-tutorial.html#TRAVERSINGOFTABLES.</a><br>
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<br>
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Packets entering the firewall first pass through the <i>mangle </i>table's
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PREROUTING chain (you can see the mangle table by typing "shorewall show
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mangle"). If the packet entered through an interface that has the <b>norfc1918</b>
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option, then the packet is sent down the <b>man1918</b> which will drop
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the packet if its destination IP address is reserved (as specified in the
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/etc/shorewall/rfc1918 file). Next the packet passes through the<b> pretos</b>
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chain to set its TOS field as specified in the /etc/shorewall/tos file.
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Finally, if traffic control/shaping is being used, the packet is sent through
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the<b> tcpre</b> chain to be marked for later use in policy routing or traffic
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control.</p>
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<p>Next, if the packet isn't part of an established connection, it passes
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through the<i> nat</i> table's PREROUTING chain (you can see the nat table by
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typing "shorewall show nat"). If you are doing both static nat and
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port forwarding, the order in which chains are traversed is dependent on the
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setting of NAT_BEFORE_RULES in shorewall.conf. If NAT_BEFORE_RULES is on then
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packets will ender a chain called <i>interface_</i>in where <i>interface</i> is
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the name of the interface on which the packet entered. Here it's destination IP
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is compared to each of the <i>EXTERNAL</i> IP addresses from /etc/shorewall/nat
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that correspond to this interface; if there is a match, DNAT is applied and the
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packet header is modified to the IP in the <i>INTERNAL</i> column of the nat
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file record. If the destination address doesn't match any of the rules in the
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<i>interface_</i>in chain then the packet enters a chain called <i>sourcezone</i>_dnat
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where <i>sourcezone</i> is the source zone of the packet. There it is compared
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for a match against each of the DNAT records in the rules file that specify <i>
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sourcezone </i>as the source zone. If a match is found, the destination IP
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address (and possibly the destination port) is modified based on the rule
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matched. If NAT_BEFORE_RULES is off, then the order of traversal of the <i>
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interface_</i>in and <i>sourcezone</i>_dnat is reversed.</p>
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<p>
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Traffic is next sent to an<i> input </i>chain in the mail Netfilter table
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(called 'filter'). If the traffic is destined for the
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firewall itself, the name of the input chain is formed by appending "_in" to
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the interface name. So traffic on eth0 destined for the firewall will enter a
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chain called <i>eth0_in</i>. The input chain for traffic that will be routed to
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another system is formed by appending "_fwd" to the interface name. So traffic
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from eth1 that is going to be forwarded enters a chain called<i> eth1_fwd</i>.
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Interfaces described with the wild-card character ("+") in
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/etc/shorewall/interfaces, share input chains. if <i>ppp+ </i>appears in
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/etc/shorewall/interfaces then all PPP interfaces (ppp0, ppp1, ...) will share
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the input chains <i>ppp_in</i> and <i>ppp_fwd</i>. In other words, "+" is
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deleted from the name before forming the input chain names.</p>
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<p>
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While the use of input chains may seem wasteful in simple environments, in
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complex setups it substantially reduces the number of rules that each packet
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must traverse. </p>
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<p>
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Traffic directed from a zone to the firewall itself is sent through a
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chain named <<i>zone name></i>2fw. For example, traffic inbound from
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the internet and addressed to the firewall is sent through a chain named
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net2fw. Similarly, traffic originating in the firewall and being sent to
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a host in a given zone is sent through a chain named fw2<i><zone name>.
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</i>For example, traffic originating in the firewall and destined
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for a host in the local network is sent through a chain named <i>fw2loc.</i>
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<font face="Century Gothic, Arial, Helvetica">
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</font></p>
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<p>
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Traffic being forwarded between two zones (or from one interface to a
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zone to another interface to that zone) is sent through a chain named <i>
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<source zone></i>2<i> <destination zone></i>. So for example,
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traffic originating in a local system and destined for a remote web server
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is sent through chain <i>loc2net. </i>This chain is referred to
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as the <i>canonical</i> chain from <source zone> to <destination
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zone>. Any destination NAT will have occurred <u>before</u> the packet
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traverses one of these chains so rules in /etc/shorewall/rules should be
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expressed in terms of the destination system's real IP address as opposed
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to its apparent external address. Similarly, source NAT will occur <u>after</u>
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the packet has traversed the appropriate forwarding chain so the rules
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again will be expressed using the source system's real IP address.</p>
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<p>
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For each record in the /etc/shorewall/policy file, a chain is created. Policies
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in that file are expressed in terms of a source zone and destination zone
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where these zones may be a zone defined in /etc/shorewall/zones, "fw" or
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"all". Policies specifying the pseudo-zone "all" matches all defined zones
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and "fw". These chains are referred to as <i>Policy Chains.</i> Notice that
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for an ordered pair of zones (za,zb), the canonical chain (za2zb) may also
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be the policy chain for the pair or the policy chain may be a different
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chain (za2all, for example). Packets from one zone to another will traverse
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chains as follows:</p>
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<ol>
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<li>
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If the canonical chain exists, packets first traverse that chain.</li>
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<li>
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If the canonical chain and policy chain are different and the packet
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does not match a rule in the canonical chain, it then is sent to the
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policy chain.</li>
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<li>
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If the canonical chain does not exist, packets are sent immediately
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to the policy chain.</li>
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</ol>
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<p>
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The canonical chain from zone za to zone zb will be created only if there
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are exception rules defined in /etc/shorewall/rules for packets going from
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za to zb.</p>
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<p>
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Shorewall is built on top of the Netfilter kernel facility. Netfilter
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implements connection tracking function that allow what is often referred
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to as "statefull inspection" of packets. This statefull property allows
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firewall rules to be defined in terms of "connections" rather than in
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terms of "packets". With Shorewall, you:</p>
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<ol>
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<li>
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Identify the client's zone.</li>
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<li>
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Identify the server's zone.</li>
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<li>
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If the POLICY from the client's zone to the server's zone is what you
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want for this client/server pair, you need do nothing further.</li>
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<li>
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If the POLICY is not what you want, then you must add a rule. That rule
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is expressed in terms of the client's zone and the server's zone.</li>
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</ol>
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<p>
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Just because connections of a particular type are allowed between zone A
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and the firewall and are also allowed between the firewall and zone B <font color="#ff6633"><b><u>
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DOES NOT mean that these connections are allowed between zone A and zone
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B</u></b></font>. It rather means that you can have a proxy running on
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the firewall that accepts a connection from zone A and then establishes
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its own separate connection from the firewall to zone B.</p>
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<p>
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If you adopt the default policy of ACCEPT from the local zone to the internet
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zone and you are having problems connecting from a local client to an internet
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server, <font color="#ff6633"><b><u> adding a rule won't help</u></b></font>
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(see point 3 above).</p>
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<p><font size="2">Last modified 8/22/2002 - <a href="support.htm">Tom
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Eastep</a></font><p><font face="Trebuchet MS"><a href="copyright.htm">
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<font size="2">Copyright</font> © <font size="2">2001, 2002 Thomas M. Eastep.</font></a></font></body></html> |