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< h1 align = "center" > Shorewall Setup Guide< / h1 >
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< p > < a href = "#Introduction" > 1.0 Introduction< / a > < br >
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< a href = "#Concepts" > 2.0 Shorewall Concepts< / a > < br >
< a href = "#Interfaces" > 3.0 Network Interfaces< / a > < br >
< a href = "#Addressing" > 4.0 Addressing, Subnets and Routing< / a > < / p >
< blockquote >
< p > < a href = "#Addresses" > 4.1 IP Addresses< / a > < br >
< a href = "#Subnets" > 4.2 Subnets< / a > < br >
< a href = "#Routing" > 4.3 Routing< / a > < br >
< a href = "#ARP" > 4.4 Address Resolution Protocol< / a > < br >
< a href = "#RFC1918" > 4.5 RFC 1918< / a > < / p >
< / blockquote >
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< p > < a href = "#Options" > 5.0 Setting up your Network< / a > < / p >
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< blockquote >
< p > < a href = "#Routed" > 5.1 Routed< / a > < br >
< a href = "#NonRouted" > 5.2 Non-routed< / a > < / p >
< blockquote >
< p > < a href = "#SNAT" > 5.2.1 SNAT< / a > < br >
< a href = "#DNAT" > 5.2.2 DNAT< / a > < br >
< a href = "#ProxyARP" > 5.2.3 Proxy ARP< / a > < br >
< a href = "#NAT" > 5.2.4 Static NAT< / a > < / p >
< / blockquote >
< p > < a href = "#Rules" > 5.3 Rules< / a > < br >
< a href = "#OddsAndEnds" > 5.4 Odds and Ends< / a > < / p >
< / blockquote >
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< p > < a href = "#DNS" > 6.0 DNS< / a > < br >
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< a href = "#StartingAndStopping" > 7.0 Starting and Stopping the Firewall< / a > < / p >
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< h2 > < a name = "Introduction" > < / a > 1.0 Introduction< / h2 >
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< p > This guide is intended for users who are setting up Shorewall in an environment
where a set of public IP addresses must be managed or who want to know more
about Shorewall than is contained in the < a
href="shorewall_quickstart_guide.htm">single-address guides< / a > . Because
the range of possible applications is so broad, the Guide will give you
general guidelines and will point you to other resources as necessary.< / p >
< p > < img border = "0" src = "images/j0213519.gif" width = "60" height = "60" >
<EFBFBD> <EFBFBD> <EFBFBD> If you run LEAF Bering, your Shorewall configuration is NOT what I release
-- I suggest that you consider installing a stock Shorewall lrp from the
shorewall.net site before you proceed.< / p >
< p > This guide assumes that you have the iproute/iproute2 package installed
(on RedHat, the package is called < i > iproute< / i > )< i > . < / i > You can tell if
this package is installed by the presence of an < b > ip< / b > program on your
firewall system. As root, you can use the 'which' command to check for this
program:< / p >
< pre > [root@gateway root]# which ip< br > /sbin/ip< br > [root@gateway root]#< / pre >
< p > I recommend that you first read through the guide to familiarize yourself
with what's involved then go back through it again making your configuration
changes. Points at which configuration changes are recommended are flagged
with < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" >
.< / p >
< p > < img border = "0" src = "images/j0213519.gif" width = "60" height = "60" >
<EFBFBD> <EFBFBD> <EFBFBD> If you edit your configuration files on a Windows system, you must save
them as Unix files if your editor supports that option or you must run them
through dos2unix before trying to use them with Shorewall. Similarly, if
you copy a configuration file from your Windows hard drive to a floppy disk,
you must run dos2unix against the copy before using it with Shorewall.< / p >
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< ul >
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< li > < a href = "http://www.simtel.net/pub/pd/51438.html" > Windows Version
of dos2unix< / a > < / li >
< li > < a href = "http://www.megaloman.com/%7Ehany/software/hd2u/" > Linux Version
of dos2unix< / a > < / li >
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< / ul >
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< h2 align = "left" > < a name = "Concepts" > < / a > 2.0 Shorewall Concepts< / h2 >
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< p > The configuration files for Shorewall are contained in the directory /etc/shorewall
-- for most setups, you will only need to deal with a few of these as described
in this guide. Skeleton files are created during the < a
href="Install.htm">Shorewall Installation Process< / a > .< / p >
< p > As each file is introduced, I suggest that you look through the actual
file on your system -- each file contains detailed configuration instructions
and some contain default entries.< / p >
< p > Shorewall views the network where it is running as being composed of a
set of < i > zones.< / i > In the default installation, the following zone names
are used:< / p >
< table border = "0" style = "border-collapse: collapse;" cellpadding = "3"
cellspacing="0" id="AutoNumber2">
< tbody >
< tr >
< td > < u > < b > Name< / b > < / u > < / td >
< td > < u > < b > Description< / b > < / u > < / td >
< / tr >
< tr >
< td > < b > net< / b > < / td >
< td > < b > The Internet< / b > < / td >
< / tr >
< tr >
< td > < b > loc< / b > < / td >
< td > < b > Your Local Network< / b > < / td >
< / tr >
< tr >
< td > < b > dmz< / b > < / td >
< td > < b > Demilitarized Zone< / b > < / td >
< / tr >
< / tbody >
< / table >
< p > Zones are defined in the < a href = "Documentation.htm#Zones" > /etc/shorewall/zones< / a >
file.< / p >
< p > Shorewall also recognizes the firewall system as its own zone - by default,
the firewall itself is known as < b > fw< / b > but that may be changed in the
< a href = "Documentation.htm#Configs" > /etc/shorewall/shorewall.conf< / a > file.
In this guide, the default name (< b > fw< / b > ) will be used.< / p >
< p > With the exception of < b > fw< / b > , Shorewall attaches absolutely no meaning
to zone names. Zones are entirely what YOU make of them. That means that
you should not expect Shorewall to do something special "because this is
the internet zone" or "because that is the DMZ".< / p >
< p > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" >
<EFBFBD> <EFBFBD> <EFBFBD> Edit the /etc/shorewall/zones file and make any changes necessary.< / p >
< p > Rules about what traffic to allow and what traffic to deny are expressed
in terms of zones.< / p >
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< ul >
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< li > You express your default policy for connections from one zone to another
zone in the< a href = "Documentation.htm#Policy" > /etc/shorewall/policy < / a > file.< / li >
< li > You define exceptions to those default policies in the < a
href="Documentation.htm#Rules">/etc/shorewall/rules < / a > file.< / li >
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< / ul >
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< p > Shorewall is built on top of the < a href = "http://www.netfilter.org" > Netfilter< / a >
kernel facility. Netfilter implements a < a
href="http://www.cs.princeton.edu/%7Ejns/security/iptables/iptables_conntrack.html">connection
tracking function< / a > that allows what is often referred to as < i > stateful
inspection< / i > of packets. This stateful property allows firewall rules
to be defined in terms of < i > connections< / i > rather than in terms of packets.
With Shorewall, you:< / p >
< ol >
< li > Identify the source zone.< / li >
< li > Identify the destination 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 from zone
A to the firewall and are also allowed from the firewall to zone B < font
color="#ff6633">< b > < u > DOES NOT mean that these connections are allowed
from zone A to 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 > For each connection request entering the firewall, the request is first
checked against the /etc/shorewall/rules file. If no rule in that file matches
the connection request then the first policy in /etc/shorewall/policy that
matches the request is applied. If that policy is REJECT or DROP<4F> the request
is first checked against the rules in /etc/shorewall/common.def.< / p >
< p > The default /etc/shorewall/policy file has the following policies:< / p >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber3">
< tbody >
< tr >
< td > < u > < b > Source Zone< / b > < / u > < / td >
< td > < u > < b > Destination Zone< / b > < / u > < / td >
< td > < u > < b > Policy< / b > < / u > < / td >
< td > < u > < b > Log Level< / b > < / u > < / td >
< td > < u > < b > Limit:Burst< / b > < / u > < / td >
< / tr >
< tr >
< td > loc< / td >
< td > net< / td >
< td > ACCEPT< / td >
< td > <EFBFBD> < / td >
< td > <EFBFBD> < / td >
< / tr >
< tr >
< td > net< / td >
< td > all< / td >
< td > DROP< / td >
< td > info< / td >
< td > <EFBFBD> < / td >
< / tr >
< tr >
< td > all< / td >
< td > all< / td >
< td > REJECT< / td >
< td > info< / td >
< td > <EFBFBD> < / td >
< / tr >
< / tbody >
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< / table >
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< / blockquote >
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< p > The above policy will:< / p >
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< ol >
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< li > allow all connection requests from your local network to the internet< / li >
< li > drop (ignore) all connection requests from the internet to your firewall
or local network and log a message at the < i > info< / i > level (see "man
syslog").< / li >
< li > reject all other connection requests and log a message at the < i > info< / i >
level. When a request is rejected, the firewall will return an RST (if
the protocol is TCP) or an ICMP port-unreachable packet for other protocols.< / li >
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< / ol >
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< p > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" >
<EFBFBD> <EFBFBD> <EFBFBD> At this point, edit your /etc/shorewall/policy and make any changes that
you wish.< / p >
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< h2 align = "left" > < a name = "Interfaces" > < / a > 3.0 Network Interfaces< / h2 >
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< p align = "left" > For the remainder of this guide, we'll refer to the following
diagram. While it may not look like your own network, it can be used to
illustrate the important aspects of Shorewall configuration.< / p >
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< p align = "left" > In this diagram:< / p >
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< ul >
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< li > The DMZ Zone consists of systems DMZ 1 and DMZ 2. A DMZ is used to
isolate your internet-accessible servers from your local systems so that
if one of those servers is compromised, you still have the firewall between
the compromised system and your local systems. < / li >
< li > The Local Zone consists of systems Local 1, Local 2 and Local 3. < / li >
< li > All systems from the ISP outward comprise the Internet Zone. < / li >
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< / ul >
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< p align = "center" > < img border = "0" src = "images/dmz3.png" width = "692"
height="635">
< / p >
< p align = "left" > The simplest way to define zones is to simply associate the
zone name (previously defined in /etc/shorewall/zones) with a network interface.
This is done in the < a href = "Documentation.htm#Interfaces" > /etc/shorewall/interfaces< / a >
file.< / p >
< p align = "left" > The firewall illustrated above has three network interfaces.
Where Internet connectivity is through a cable or DSL "Modem", the < i > External
Interface< / i > will be the Ethernet adapter that is connected to that "Modem"
(e.g., < b > eth0< / b > )<29> < u > unless< / u > you connect via < i > < u > P< / u > oint-to-< u > P< / u > oint
< u > P< / u > rotocol over < u > E< / u > thernet< / i > (PPPoE) or < i > < u > P< / u > oint-to-< u > P< / u > oint
< u > T< / u > unneling < u > P< / u > rotocol < / i > (PPTP) in which case the External Interface
will be a ppp interface (e.g., < b > ppp0< / b > ). If you connect via a regular
modem, your External Interface will also be < b > ppp0< / b > . If you connect
using ISDN, you external interface will be < b > ippp0.< / b > < / p >
< p align = "left" > < img border = "0" src = "images/BD21298_1.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> If your external interface is < b > ppp0< / b > or < b > ippp0 < / b > then you will
want to set CLAMPMSS=yes in < a href = "Documentation.htm#Conf" > /etc/shorewall/shorewall.conf.< / a > < / p >
< p align = "left" > Your < i > Local Interface< / i > will be an Ethernet adapter (eth0,
eth1 or eth2) and will be connected to a hub or switch. Your local computers
will be connected to the same switch (note: If you have only a single local
system, you can connect the firewall directly to the computer using a < i > cross-over
< / i > cable).< / p >
< p align = "left" > Your < i > DMZ Interface< / i > will also be an Ethernet adapter
(eth0, eth1 or eth2) and will be connected to a hub or switch. Your DMZ
computers will be connected to the same switch (note: If you have only a
single DMZ system, you can connect the firewall directly to the computer
using a < i > cross-over < / i > cable).< / p >
< p align = "left" > < u > < b > < img border = "0" src = "images/j0213519.gif"
width="60" height="60">
< / b > < / u > Do not connect more than one interface to the same hub or switch
(even for testing). It won't work the way that you expect it to and you
will end up confused and believing that Linux networking doesn't work at
all.< / p >
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< p align = "left" > For the remainder of this Guide, we will assume that:< / p >
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< ul >
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< li >
< p align = "left" > The external interface is < b > eth0< / b > .< / p >
< / li >
< li >
< p align = "left" > The Local interface is < b > eth1< / b > .< / p >
< / li >
< li >
< p align = "left" > The DMZ interface is < b > eth2< / b > .< / p >
< / li >
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< / ul >
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< p align = "left" > The Shorewall default configuration does not define the contents
of any zone. To define the above configuration using the /etc/shorewall/interfaces
file, that file would might contain:< / p >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber4">
< tbody >
< tr >
< td > < u > < b > Zone< / b > < / u > < / td >
< td > < u > < b > Interface< / b > < / u > < / td >
< td > < u > < b > Broadcast< / b > < / u > < / td >
< td > < u > < b > Options< / b > < / u > < / td >
< / tr >
< tr >
< td > net< / td >
< td > eth0< / td >
< td > detect< / td >
< td > norfc1918< / td >
< / tr >
< tr >
< td > loc< / td >
< td > eth1< / td >
< td > detect< / td >
< td > <EFBFBD> < / td >
< / tr >
< tr >
< td > dmz< / td >
< td > eth2< / td >
< td > detect< / td >
< td > <EFBFBD> < / td >
< / tr >
< / tbody >
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< / table >
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< / blockquote >
< p align = "left" > < img border = "0" src = "images/BD21298_2.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> Edit the /etc/shorewall/interfaces file and define the network interfaces
on your firewall and associate each interface with a zone. If you have a
zone that is interfaced through more than one interface, simply include
one entry for each interface and repeat the zone name as many times as necessary.< / p >
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< p align = "left" > Example:< / p >
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< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber4">
< tbody >
< tr >
< td > < u > < b > Zone< / b > < / u > < / td >
< td > < u > < b > Interface< / b > < / u > < / td >
< td > < u > < b > Broadcast< / b > < / u > < / td >
< td > < u > < b > Options< / b > < / u > < / td >
< / tr >
< tr >
< td > net< / td >
< td > eth0< / td >
< td > detect< / td >
< td > norfc1918< / td >
< / tr >
< tr >
< td > loc< / td >
< td > eth1< / td >
< td > detect< / td >
< td > <EFBFBD> < / td >
< / tr >
< tr >
< td > loc< / td >
< td > eth2< / td >
< td > detect< / td >
< td > dhcp< / td >
< / tr >
< / tbody >
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< / table >
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< / blockquote >
< div align = "left" >
< p align = "left" > When you have more than one interface to a zone, you will
usually want a policy that permits intra-zone traffic:< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber3">
< tbody >
< tr >
< td > < u > < b > Source Zone< / b > < / u > < / td >
< td > < u > < b > Destination Zone< / b > < / u > < / td >
< td > < u > < b > Policy< / b > < / u > < / td >
< td > < u > < b > Log Level< / b > < / u > < / td >
< td > < u > < b > Limit:Burst< / b > < / u > < / td >
< / tr >
< tr >
< td > loc< / td >
< td > loc< / td >
< td > ACCEPT< / td >
< td > <EFBFBD> < / td >
< td > <EFBFBD> < / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< p align = "left" > < img border = "0" src = "images/BD21298_2.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> You may define more complicated zones using the < a
href="Documentation.htm#Hosts">/etc/shorewall/hosts< / a > file but in most
cases, that isn't necessary.< / p >
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< h2 align = "left" > < a name = "Addressing" > < / a > 4.0 Addressing, Subnets and Routing< / h2 >
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< p align = "left" > Normally, your ISP will assign you a set of < i > Public< / i >
IP addresses. You will configure your firewall's external interface to use
one of those addresses permanently and you will then have to decide how
you are going to use the rest of your addresses. Before we tackle that question
though, some background is in order.< / p >
< p align = "left" > If you are thoroughly familiar with IP addressing and routing,
you may < a href = "#Options" > go to the next section< / a > .< / p >
< p align = "left" > The following discussion barely scratches the surface of
addressing and routing. If you are interested in learning more about this
subject, I highly recommend < i > "IP Fundamentals: What Everyone Needs to
Know about Addressing & Routing",< / i > Thomas A. Maufer, Prentice-Hall,
1999, ISBN 0-13-975483-0.< / p >
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< h3 align = "left" > < a name = "Addresses" > < / a > 4.1 IP Addresses< / h3 >
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< p align = "left" > IP version 4 (< i > IPv4) < / i > addresses are 32-bit numbers.
The notation w.x.y.z refers to an address where the high-order byte has value
"w", the next byte has value "x", etc. If we take the address 192.0.2.14
and express it in hexadecimal, we get:< / p >
< blockquote >
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< p align = "left" > C0.00.02.0E< / p >
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< / blockquote >
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< p align = "left" > or looking at it as a 32-bit integer< / p >
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< blockquote >
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< p align = "left" > C000020E< / p >
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< / blockquote >
< h3 align = "left" > < a name = "Subnets" > < / a > 4.2 Subnets< / h3 >
< p align = "left" > You will still hear the terms "Class A network", "Class B
network" and "Class C network". In the early days of IP, networks only
came in three sizes (there were also Class D networks but they were used
differently):< / p >
< blockquote >
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< p align = "left" > Class A - netmask 255.0.0.0, size = 2 ** 24< / p >
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< p align = "left" > Class B - netmask 255.255.0.0, size = 2 ** 16< / p >
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< p align = "left" > Class C - netmask 255.255.255.0, size = 256< / p >
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< / blockquote >
< p align = "left" > The class of a network was uniquely determined by the value
of the high order byte of its address so you could look at an IP address
and immediately determine the associated < i > netmask< / i > . The netmask is
a number that when logically ANDed with an address isolates the < i > network
number< / i > ; the remainder of the address is the < i > host number< / i > . For
example, in the Class C address 192.0.2.14, the network number is hex C00002
and the host number is hex 0E.< / p >
< p align = "left" > As the internet grew, it became clear that such a gross partitioning
of the 32-bit address space was going to be very limiting (early on, large
corporations and universities were assigned their own class A network!).
After some false starts, the current technique of < i > subnetting< / i > these
networks into smaller < i > subnetworks< / i > evolved; that technique is referred
to as<61> < i > Classless InterDomain Routing< / i > (CIDR). Today, any system that
you are likely to work with will understand CIDR and Class-based networking
is largely a thing of the past.< / p >
< p align = "left" > A < i > subnetwork< / i > (often referred to as a < i > subnet) < / i > is
a contiguous set of IP addresses such that:< / p >
2002-08-07 16:28:04 +02:00
< ol >
2002-09-30 20:11:25 +02:00
< li >
< p align = "left" > The number of addresses in the set is a power of 2; and< / p >
< / li >
< li >
< p align = "left" > The first address in the set is a multiple of the set
size.< / p >
< / li >
< li >
< p align = "left" > The first address in the subnet is reserved and is referred
to as the < i > subnet address.< / i > < / p >
< / li >
< li >
< p align = "left" > The last address in the subnet is reserved as the < i > subnet's
broadcast address.< / i > < / p >
< / li >
2002-08-07 16:28:04 +02:00
< / ol >
2002-09-30 20:11:25 +02:00
< p align = "left" > As you can see by this definition, in each subnet of size
< b > n< / b > there are (< b > n< / b > - 2) usable addresses (addresses that can
be assigned to hosts). The first and last address in the subnet are used
for the subnet address and subnet broadcast address respectively. Consequently,
small subnetworks are more wasteful of IP addresses than are large ones.
< / p >
< p align = "left" > Since < b > n< / b > is a power of two, we can easily calculate
the < i > Natural Logarithm< / i > (< b > log2< / b > ) of < b > n< / b > . For the more common
subnet sizes, the size and its natural logarithm are given in the following
table:< / p >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber5">
< tbody >
< tr >
< td > < u > < b > n< / b > < / u > < / td >
< td > < u > < b > log2 n< / b > < / u > < / td >
< td > < u > < b > (32 - log2 n)< / b > < / u > < / td >
< / tr >
< tr >
< td > 8< / td >
< td > 3< / td >
< td > 29< / td >
< / tr >
< tr >
< td > 16< / td >
< td > 4< / td >
< td > 28< / td >
< / tr >
< tr >
< td > 32< / td >
< td > 5< / td >
< td > 27< / td >
< / tr >
< tr >
< td > 64< / td >
< td > 6< / td >
< td > 26< / td >
< / tr >
< tr >
< td > 128< / td >
< td > 7< / td >
< td > 25< / td >
< / tr >
< tr >
< td > 256< / td >
< td > 8< / td >
< td > 24< / td >
< / tr >
< tr >
< td > 512< / td >
< td > 9< / td >
< td > 23< / td >
< / tr >
< tr >
< td > 1024< / td >
< td > 10< / td >
< td > 22< / td >
< / tr >
< tr >
< td > 2048< / td >
< td > 11< / td >
< td > 21< / td >
< / tr >
< tr >
< td > 4096< / td >
< td > 12< / td >
< td > 20< / td >
< / tr >
< tr >
< td > 8192< / td >
< td > 13< / td >
< td > 19< / td >
< / tr >
< tr >
< td > 16384< / td >
< td > 14< / td >
< td > 18< / td >
< / tr >
< tr >
< td > 32768< / td >
< td > 15< / td >
< td > 17< / td >
< / tr >
< tr >
< td > 65536< / td >
< td > 16< / td >
< td > 16< / td >
< / tr >
< / tbody >
2002-08-22 23:33:54 +02:00
< / table >
2002-09-30 20:11:25 +02:00
< / blockquote >
< p align = "left" > You will notice that the above table also contains a column
for (32 - log2 < b > n< / b > ). That number is the < i > Variable Length Subnet
Mask< / i > for a network of size < b > n< / b > . From the above table, we can
derive the following one which is a little easier to use.< / p >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber5">
< tbody >
< tr >
< td > < u > < b > Size of Subnet< / b > < / u > < / td >
< td > < u > < b > VLSM< / b > < / u > < / td >
< td > < u > < b > Subnet Mask< / b > < / u > < / td >
< / tr >
< tr >
< td > 8< / td >
< td > /29< / td >
< td > 255.255.255.248< / td >
< / tr >
< tr >
< td > 16< / td >
< td > /28< / td >
< td > 255.255.255.240< / td >
< / tr >
< tr >
< td > 32< / td >
< td > /27< / td >
< td > 255.255.255.224< / td >
< / tr >
< tr >
< td > 64< / td >
< td > /26< / td >
< td > 255.255.255.192< / td >
< / tr >
< tr >
< td > 128< / td >
< td > /25< / td >
< td > 255.255.255.128< / td >
< / tr >
< tr >
< td > 256< / td >
< td > /24< / td >
< td > 255.255.255.0< / td >
< / tr >
< tr >
< td > 512< / td >
< td > /23< / td >
< td > 255.255.254.0< / td >
< / tr >
< tr >
< td > 1024< / td >
< td > /22< / td >
< td > 255.255.252.0< / td >
< / tr >
< tr >
< td > 2048< / td >
< td > /21< / td >
< td > 255.255.248.0< / td >
< / tr >
< tr >
< td > 4096< / td >
< td > /20< / td >
< td > 255.255.240.0< / td >
< / tr >
< tr >
< td > 8192< / td >
< td > /19< / td >
< td > 255.255.224.0< / td >
< / tr >
< tr >
< td > 16384< / td >
< td > /18< / td >
< td > 255.255.192.0< / td >
< / tr >
< tr >
< td > 32768< / td >
< td > /17< / td >
< td > 255.255.128.0< / td >
< / tr >
< tr >
< td > 65536< / td >
< td > /16< / td >
< td > 255.255.0.0< / td >
< / tr >
< tr >
< td > 2 ** 24< / td >
< td > /8< / td >
< td > 255.0.0.0< / td >
< / tr >
< / tbody >
2002-08-22 23:33:54 +02:00
< / table >
2002-09-30 20:11:25 +02:00
< / blockquote >
< p align = "left" > Notice that the VLSM is written with a slash ("/") -- you
will often hear a subnet of size 64 referred to as a "slash 26" subnet
and one of size 8 referred to as a "slash 29".< / p >
< p align = "left" > The subnet's mask (also referred to as its < i > netmask) < / i > is
simply a 32-bit number with the first "VLSM" bits set to one and the remaining
bits set to zero. For example, for a subnet of size 64, the subnet mask
has 26 leading one bits:< / p >
< blockquote >
< p align = "left" > 11111111111111111111111111000000 = FFFFFFC0 = FF.FF.FF.C0
= 255.255.255.192< / p >
< / blockquote >
< p align = "left" > The subnet mask has the property that if you logically AND
the subnet mask with an address in the subnet, the result is the subnet
address. Just as important, if you logically AND the subnet mask with
an address outside the subnet, the result is NOT the subnet address. As
we will see below, this property of subnet masks is very useful in routing.< / p >
< p align = "left" > For a subnetwork whose address is < b > a.b.c.d< / b > and whose
Variable Length Subnet Mask is < b > /v< / b > , we denote the subnetwork as
"< b > a.b.c.d/v< / b > " using < i > CIDR< / i > < i > Notation< / i > .<2E> < / p >
< p align = "left" > Example:< / p >
< blockquote >
< table border = "2" style = "border-collapse: collapse;" id = "AutoNumber1"
cellpadding="2">
< tbody >
< tr >
< td > < b > Subnet:< / b > < / td >
< td > 10.10.10.0 - 10.10.10.127< / td >
< / tr >
< tr >
< td > < b > Subnet Size:< / b > < / td >
< td > 128< / td >
< / tr >
< tr >
< td > < b > Subnet Address:< / b > < / td >
< td > 10.10.10.0< / td >
< / tr >
< tr >
< td > < b > Broadcast Address:< / b > < / td >
< td > 10.10.10.127< / td >
< / tr >
< tr >
< td > < b > CIDR<EFBFBD> Notation:< / b > < / td >
< td > 10.10.10.0/25< / td >
< / tr >
< / tbody >
2002-08-22 23:33:54 +02:00
< / table >
2002-09-30 20:11:25 +02:00
< / blockquote >
< p align = "left" > There are two degenerate subnets that need mentioning; namely,
the subnet with one member and the subnet with 2 ** 32 members.< / p >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber5">
< tbody >
< tr >
< td > < u > < b > Size of Subnetwork< / b > < / u > < / td >
< td > < u > < b > VLSM Length< / b > < / u > < / td >
< td > < u > < b > Subnet Mask< / b > < / u > < / td >
< td > < u > < b > CIDR<EFBFBD> Notation< / b > < / u > < / td >
< / tr >
< tr >
< td > 1< / td >
< td > 32< / td >
< td > 255.255.255.255< / td >
< td > a.b.c.d/32< / td >
< / tr >
< tr >
< td > 2 ** 32< / td >
< td > 0< / td >
< td > 0.0.0.0< / td >
< td > 0.0.0.0/0< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< p align = "left" > So any address < b > a.b.c.d < / b > may also be written < b > a.b.c.d/32< / b >
and the set of all possible IP addresses is written < b > 0.0.0.0/0< / b > .< / p >
< p align = "left" > Later in this guide, you will see the notation < b > a.b.c.d/v< / b >
used to describe the ip configuration of a network interface (the 'ip' utility
also uses this syntax). This simply means that the interface is configured
with ip address < b > a.b.c.d< / b > and with the netmask that corresponds to
VLSM < b > /v< / b > .< / p >
2002-08-07 16:28:04 +02:00
< p align = "left" > Example: 192.0.2.65/29< / p >
2002-09-30 20:11:25 +02:00
< p align = "left" > <EFBFBD> <EFBFBD> <EFBFBD> The interface is configured with IP address 192.0.2.65
and netmask 255.255.255.248.< / p >
2002-08-22 23:33:54 +02:00
< h3 align = "left" > < a name = "Routing" > < / a > 4.3 Routing< / h3 >
2002-09-30 20:11:25 +02:00
< p align = "left" > One of the purposes of subnetting is that it forms the basis
for routing. Here's the routing table on < a href = "myfiles.htm" > my firewall< / a > :< / p >
< div align = "left" >
< blockquote >
< pre > [root@gateway root]# netstat -nr< br > Kernel IP routing table< br > Destination Gateway Genmask Flags MSS Window irtt Iface< br > 192.168.9.1 0.0.0.0 255.255.255.255 UH 40 0 0 texas< br > 206.124.146.177 0.0.0.0 255.255.255.255 UH 40 0 0 eth1< br > 206.124.146.180 0.0.0.0 255.255.255.255 UH 40 0 0 eth3< br > 192.168.3.0 0.0.0.0 255.255.255.0 U 40 0 0 eth3< br > 192.168.2.0 0.0.0.0 255.255.255.0 U 40 0 0 eth1< br > 192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2< br > 206.124.146.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0< br > 192.168.9.0 192.0.2.223 255.255.255.0 UG 40 0 0 texas< br > 127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo< br > 0.0.0.0 206.124.146.254 0.0.0.0 UG 40 0 0 eth0< br > [root@gateway root]#< / pre >
< / blockquote >
< / div >
< p align = "left" > The device < i > texas< / i > is a GRE tunnel to a peer site in
the Dallas, Texas area.< br >
< br >
The first three routes are < i > host routes< / i > since they indicate how to
get to a single host. In the 'netstat' output this can be seen by the "Genmask"
(Subnet Mask) of 255.255.255.255 and the "H" in the Flags column. The remainder
are 'net' routes since they tell the kernel how to route packets to a subnetwork.
The last route is the < i > default route< / i > and the gateway mentioned in
that route is called the < i > default gateway< / i > .< / p >
< p align = "left" > When the kernel is trying to send a packet to IP address
< b > A< / b > , it starts at the top of the routing table and:< / p >
2002-08-22 23:33:54 +02:00
< ul >
2002-09-30 20:11:25 +02:00
< li >
< p align = "left" > < b > A< / b > is logically ANDed with the 'Genmask' value
in the table entry.< / p >
< / li >
< li >
< p align = "left" > The result is compared with the 'Destination' value in
the table entry.< / p >
< / li >
< li >
< p align = "left" > If the result and the 'Destination' value are the same,
then:< / p >
< ul >
< li >
< p align = "left" > If the 'Gateway' column is non-zero, the packet is
sent to the gateway over the interface named in the 'Iface' column.< / p >
< / li >
< li >
< p align = "left" > Otherwise, the packet is sent directly to < b > A < / b > over
the interface named in the 'iface' column.< / p >
< / li >
< / ul >
< / li >
< li >
< p align = "left" > Otherwise, the above steps are repeated on the next entry
in the table.< / p >
< / li >
< / ul >
< p align = "left" > Since the default route matches any IP address (< b > A< / b >
land 0.0.0.0 = 0.0.0.0), packets that don't match any of the other routing
table entries are sent to the < i > default gateway< / i > which is usually a
router at your ISP.< / p >
< p align = "left" > Lets take an example. Suppose that we want to route a packet
to 192.168.1.5. That address clearly doesn't match any of the host routes
in the table but if we logically and that address with 255.255.255.0, the
result is 192.168.1.0 which matches this routing table entry:< / p >
< div align = "left" >
< blockquote >
< pre > 192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2< / pre >
< / blockquote >
< p > So to route a packet to 192.168.1.5, the packet is sent directly over
eth2.< / p >
< / div >
< p align = "left" > One more thing needs to be emphasized -- all outgoing packet
are sent using the routing table and reply packets are not a special case.
There seems to be a common mis-conception whereby people think that request
packets are like salmon and contain a genetic code that is magically transferred
to reply packets so that the replies follow the reverse route taken by the
request. That isn't the case; the replies may take a totally different route
back to the client than was taken by the requests -- they are totally independent.< / p >
< h3 align = "left" > < a name = "ARP" > < / a > 4.4 Address Resolution Protocol< / h3 >
< p align = "left" > When sending packets over Ethernet, IP addresses aren't used.
Rather Ethernet addressing is based on < i > Media Access Control< / i > (MAC)
addresses. Each Ethernet device has it's own unique<75> MAC address which is
burned into a PROM on the device during manufacture. You can obtain the
MAC of an Ethernet device using the 'ip' utility:< / p >
< blockquote >
< div align = "left" >
< pre > [root@gateway root]# ip addr show eth0< br > 2: eth0: < BROADCAST,MULTICAST,UP> mtu 1500 qdisc htb qlen 100< br > link/ether < u > 02:00:08:e3:fa:55< / u > brd ff:ff:ff:ff:ff:ff< br > inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0< br > inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0< br > inet 206.124.146.179/24 brd 206.124.146.255 scope global secondary eth0< br > [root@gateway root]#< / pre >
< / div >
< / blockquote >
< div align = "left" >
< p align = "left" > As you can see from the above output, the MAC is 6 bytes
(48 bits) wide. A card's MAC is usually also printed on a label attached
to the card itself. < / p >
< / div >
< div align = "left" >
< p align = "left" > Because IP uses IP addresses and Ethernet uses MAC addresses,
a mechanism is required to translate an IP address into a MAC address;
that is the purpose of the < i > Address Resolution Protocol< / i > (ARP). Here
is ARP in action:< / p >
< / div >
< div align = "left" >
< blockquote >
< div align = "left" >
< pre > [root@gateway root]# tcpdump -nei eth2 arp< br > tcpdump: listening on eth2< br > 09:56:49.766757 2:0:8:e3:4c:48 0:6:25:aa:8a:f0 arp 42: arp who-has 192.168.1.19 tell 192.168.1.254< br > 09:56:49.769372 0:6:25:aa:8a:f0 2:0:8:e3:4c:48 arp 60: arp reply 192.168.1.19 is-at 0:6:25:aa:8a:f0< br > < br > 2 packets received by filter< br > 0 packets dropped by kernel< br > [root@gateway root]#< br > < / pre >
< / div >
< / blockquote >
< / div >
< p align = "left" > In this exchange, 192.168.1.254 (MAC 2:0:8:e3:4c:48) wants
to know the MAC of the device with IP address 192.168.1.19. The system having
that IP address is responding that the MAC address of the device with IP
address 192.168.1.19 is 0:6:25:aa:8a:f0.< / p >
< p align = "left" > In order to avoid having to exchange ARP information each
time that an IP packet is to be sent, systems maintain an < i > ARP cache< / i >
of IP< -> MAC correspondences. You can see the ARP cache on your system
(including your Windows system) using the 'arp' command:< / p >
< blockquote >
< div align = "left" >
< pre > [root@gateway root]# arp -na< br > ? (206.124.146.177) at 00:A0:C9:15:39:78 [ether] on eth1< br > ? (192.168.1.3) at 00:A0:CC:63:66:89 [ether] on eth2< br > ? (192.168.1.5) at 00:A0:CC:DB:31:C4 [ether] on eth2< br > ? (206.124.146.254) at 00:03:6C:8A:18:38 [ether] on eth0< br > ? (192.168.1.19) at 00:06:25:AA:8A:F0 [ether] on eth2< / pre >
< / div >
< / blockquote >
< p align = "left" > The leading question marks are a result of my having specified
the 'n' option (Windows 'arp' doesn't allow that option) which causes the
'arp' program to forego IP-> DNS name translation. Had I not given that
option, the question marks would have been replaced with the FQDN corresponding
to each IP address. Notice that the last entry in the table records the
information we saw using tcpdump above.< / p >
< h3 align = "left" > < a name = "RFC1918" > < / a > 4.5 RFC 1918< / h3 >
< p align = "left" > IP addresses are allocated by the < i > < a
href="http://www.iana.org">Internet Assigned Number Authority< / a > < / i > (IANA)
who delegates allocations on a geographic basis to < i > Regional Internet
Registries< / i > (RIRs). For example, allocation for the Americas and for
sub-Sahara Africa is delegated to the < i > < a href = "http://www.arin.net" > American
Registry for Internet Numbers< / a > < / i > (ARIN). These RIRs may in turn delegate
to national registries. Most of us don't deal with these registrars but
rather get our IP addresses from our ISP.< / p >
< p align = "left" > It's a fact of life that most of us can't afford as many
Public IP addresses as we have devices to assign them to so we end up making
use of < i > Private < / i > IP addresses. RFC 1918 reserves several IP address
ranges for this purpose:< / p >
< div align = "left" >
< pre > 10.0.0.0 - 10.255.255.255< br > 172.16.0.0 - 172.31.255.255< br > 192.168.0.0 - 192.168.255.255< / pre >
< / div >
< div align = "left" >
< p align = "left" > The addresses reserved by RFC 1918 are sometimes referred
to as < i > non-routable< / i > because the Internet backbone routers don't
forward packets which have an RFC-1918 destination address. This is understandable
given that anyone can select any of these addresses for their private
use.< / p >
< / div >
< div align = "left" >
< p align = "left" > When selecting addresses from these ranges, there's a couple
of things to keep in mind:< / p >
< / div >
< div align = "left" >
< ul >
< li >
< p align = "left" > As the IPv4 address space becomes depleted, more and
more organizations (including ISPs) are beginning to use RFC 1918 addresses
in their infrastructure. < / p >
2002-08-22 23:33:54 +02:00
< / li >
< li >
2002-09-30 20:11:25 +02:00
< p align = "left" > You don't want to use addresses that are being used by
your ISP or by another organization with whom you want to establish
a VPN relationship. < / p >
2002-08-22 23:33:54 +02:00
< / li >
2002-09-30 20:11:25 +02:00
< / ul >
< / div >
< div align = "left" >
< p align = "left" > So it's a good idea to check with your ISP to see if they
are using (or are planning to use) private addresses before you decide
the addresses that you are going to use.< / p >
< / div >
< div align = "left" >
< h2 align = "left" > < a name = "Options" > < / a > 5.0 Setting up your Network< / h2 >
< / div >
< div align = "left" >
< p align = "left" > The choice of how to set up your network depends primarily
on how many Public IP addresses you have vs. how many addressable entities
you have in your network. Regardless of how many addresses you have, your
ISP will handle that set of addresses in one of two ways:< / p >
< / div >
< div align = "left" >
< ol >
< li >
< p align = "left" > < b > Routed - < / b > Traffic to any of your addresses will
be routed through a single < i > gateway address< / i > . This will generally
only be done if your ISP has assigned you a complete subnet (/29 or larger).
In this case, you will assign the gateway address as the IP address of
your firewall/router's external interface. < / p >
2002-08-22 23:33:54 +02:00
< / li >
2002-09-30 20:11:25 +02:00
< li >
< p align = "left" > < b > Non-routed - < / b > Your ISP will send traffic to each
of your addresses directly. < / p >
< / li >
< / ol >
< / div >
< div align = "left" >
< p align = "left" > In the subsections that follow, we'll look at each of these
separately.< / p >
< / div >
< div align = "left" >
< h3 align = "left" > < a name = "Routed" > < / a > 5.1 Routed< / h3 >
< / div >
< div align = "left" >
< p align = "left" > Let's assume that your ISP has assigned you the subnet
192.0.2.64/28 routed through 192.0.2.65. That means that you have IP addresses
192.0.2.64 - 192.0.2.79 and that your firewall's external IP address is
192.0.2.65. Your ISP has also told you that you should use a netmask of
255.255.255.0 (so your /28 is part of a larger /24). With this many IP
addresses, you are able to subnet your /28 into two /29's and set up your
network as shown in the following diagram.< / p >
< / div >
< div align = "left" >
< p align = "center" > < img border = "0" src = "images/dmz4.png" width = "726"
height="635">
< / p >
< / div >
< div align = "left" >
< p align = "left" > Here, the DMZ comprises the subnet 192.0.2.64/29 and the
Local network is 192.0.2.72/29. The default gateway for hosts in the DMZ
would be configured to 192.0.2.66 and the default gateway for hosts in
the local network would be 192.0.2.73.< / p >
< / div >
< div align = "left" >
< p align = "left" > Notice that this arrangement is rather wasteful of public
IP addresses since it is using 192.0.2.64 and 192.0.2.72 for subnet addresses,
192.0.2.71 and 192.0.2.79 for subnet broadcast addresses and 192.0.2.66
and 168.0.2.73 for internal addresses on the firewall/router. Nevertheless,
it shows how subnetting can work and if we were dealing with a /24 rather
than a /28 network, the use of 6 IP addresses out of 256 would be justified
because of the simplicity of the setup.< / p >
< / div >
< div align = "left" >
< p align = "left" > The astute reader may have noticed that the Firewall/Router's
external interface is actually part of the DMZ subnet (192.0.2.64/29).
What if DMZ 1 (192.0.2.67) tries to communicate with 192.0.2.65? The routing
table on DMZ 1 will look like this:< / p >
< / div >
< div align = "left" >
< blockquote >
< pre > Kernel IP routing table< br > Destination Gateway Genmask Flags MSS Window irtt Iface< br > 192.0.2.64 0.0.0.0 255.255.255.248 U 40 0 0 eth0< br > 0.0.0.0 192.0.2.66 0.0.0.0 UG 40 0 0 eth0< / pre >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > This means that DMZ 1 will send an ARP "who-has 192.0.2.65"
request and no device on the DMZ Ethernet segment has that IP address.
Oddly enough, the firewall will respond to the request with the MAC address
of its < u > DMZ Interface!!< / u > DMZ 1 can then send Ethernet frames addressed
to that MAC address and the frames will be received (correctly) by the
firewall/router.< / p >
< / div >
< div align = "left" >
< p align = "left" > It is this rather unexpected ARP behavior on the part of
the Linux Kernel that prompts the warning earlier in this guide regarding
the connecting of multiple firewall/router interfaces to the same hub
or switch. When an ARP request for one of the firewall/router's IP addresses
is sent by another system connected to the hub/switch, all of the firewall's
interfaces that connect to the hub/switch can respond! It is then a race
as to which "here-is" response reaches the sender first.< / p >
< / div >
< div align = "left" >
< h3 align = "left" > < a name = "NonRouted" > < / a > 5.2 Non-routed< / h3 >
< / div >
< div align = "left" >
< p align = "left" > If you have the above situation but it is non-routed,
you can configure your network exactly as described above with one additional
twist; simply specify the "proxyarp" option on all three firewall interfaces
in the /etc/shorewall/interfaces file.< / p >
< / div >
< div align = "left" >
< p align = "left" > Most of us don't have the luxury of having enough public
IP addresses to set up our networks as shown in the preceding example
(even if the setup is routed). < / p >
< / div >
< div align = "left" >
< p align = "left" > < b > For the remainder of this section, assume that your ISP
has assigned you IP addresses 192.0.2.176-180 and has told you to use
netmask 255.255.255.0 and default gateway 192.0.2.254.< / b > < / p >
< / div >
< div align = "left" >
< p align = "left" > Clearly, that set of addresses doesn't comprise a subnetwork
and there aren't enough addresses for all of the network interfaces. There
are four different techniques that can be used to work around this problem.< / p >
< / div >
< div align = "left" >
< ul >
< li >
< p align = "left" > < i > Source Network Address Translation < / i > (SNAT).
< / p >
< / li >
< li >
< p align = "left" > < i > Destination Network Address Translation < / i > (DNAT)
also known as < i > Port Forwarding.< / i > < / p >
< / li >
< li >
< p align = "left" > < i > Proxy ARP.< / i > < / p >
< / li >
< li >
< p align = "left" > < i > Network Address Translation< / i > (NAT) also referred
to as < i > Static NAT< / i > . < / p >
2002-08-22 23:33:54 +02:00
< / li >
< / ul >
2002-09-30 20:11:25 +02:00
< / div >
< div align = "left" >
< p align = "left" > Often a combination of these techniques is used. Each of
these will be discussed in the sections that follow.< / p >
< / div >
< div align = "left" >
< h4 align = "left" > < a name = "SNAT" > < / a > <EFBFBD> 5.2.1 SNAT< / h4 >
< / div >
< div align = "left" >
< p align = "left" > With SNAT, an internal LAN segment is configured using RFC
1918 addresses. When a host < b > A < / b > on this internal segment initiates
a connection to host < b > B< / b > on the internet, the firewall/router rewrites
the IP header in the request to use one of your public IP addresses as
the source address. When < b > B< / b > responds and the response is received
by the firewall, the firewall changes the destination address back to
the RFC 1918 address of < b > A< / b > and forwards the response back to < b > A.< / b > < / p >
< / div >
< div align = "left" >
< p align = "left" > Let's suppose that you decide to use SNAT on your local zone
and use public address 192.0.2.176 as both your firewall's external IP
address and the source IP address of internet requests sent from that
zone.< / p >
< / div >
< div align = "left" >
< h2 align = "center" > < img border = "0" src = "images/dmz5.png" width = "745"
height="635">
< / h2 >
< / div >
< div align = "left" > The local zone has been subnetted as 192.168.201.0/29
(netmask 255.255.255.248).< / div >
< div align = "left" > <20> < / div >
< div align = "left" > < img border = "0" src = "images/BD21298_2.gif"
width="13" height="13">
<EFBFBD> <EFBFBD> <EFBFBD> The systems in the local zone would be configured with a default gateway
of 192.168.201.1 (the IP address of the firewall's local interface).< / div >
< div align = "left" > <20> < / div >
< div align = "left" > < img border = "0" src = "images/BD21298_2.gif"
width="13" height="13">
<EFBFBD> <EFBFBD> <EFBFBD> SNAT is configured in Shorewall using the < a
href="Documentation.htm#Masq"> /etc/shorewall/masq file< / a > .< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber6">
< tbody >
< tr >
< td width = "33%" > < u > < b > INTERFACE< / b > < / u > < / td >
< td width = "33%" > < u > < b > SUBNET< / b > < / u > < / td >
< td width = "34%" > < u > < b > ADDRESS< / b > < / u > < / td >
< / tr >
< tr >
< td width = "33%" > eth0< / td >
< td width = "33%" > 192.168.201.0/29< / td >
< td width = "34%" > 192.0.2.176< / td >
< / tr >
< / tbody >
2002-08-22 23:33:54 +02:00
< / table >
2002-09-30 20:11:25 +02:00
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > This example used the normal technique of assigning the same
public IP address for the firewall external interface and for SNAT. If
you wanted to use a different IP address, you would either have to use
your distributions network configuration tools to add that IP address
to the external interface or you could set ADD_SNAT_ALIASES=Yes in
/etc/shorewall/shorewall.conf and Shorewall will add the address for you.< / p >
< / div >
< div align = "left" >
< h4 align = "left" > < a name = "DNAT" > < / a > 5.2.2 DNAT< / h4 >
< / div >
< div align = "left" >
< p align = "left" > When SNAT is used, it is impossible for hosts on the internet
to initiate a connection to one of the internal systems since those systems
do not have a public IP address. DNAT provides a way to allow selected
connections from the internet.< / p >
< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_2.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> <EFBFBD> Suppose that your daughter wants to run a web server on her system
"Local 3". You could allow connections to the internet to her server by
adding the following entry in < a href = "Documentation.htm#Rules" > /etc/shorewall/rules< / a > :< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber7">
< tbody >
< tr >
< td > < u > < b > ACTION< / b > < / u > < / td >
< td > < u > < b > SOURCE< / b > < / u > < / td >
< td > < u > < b > DESTINATION< / b > < / u > < / td >
< td > < u > < b > PROTOCOL< / b > < / u > < / td >
< td > < u > < b > PORT< / b > < / u > < / td >
< td > < u > < b > SOURCE PORT< / b > < / u > < / td >
< td > < u > < b > ORIGINAL DESTINATION< / b > < / u > < / td >
< / tr >
< tr >
< td > DNAT< / td >
< td > net< / td >
< td > loc:192.168.201.4< / td >
< td > tcp< / td >
< td > www< / td >
< td > -< / td >
< td > 192.0.2.176< / td >
< / tr >
< / tbody >
2002-08-22 23:33:54 +02:00
< / table >
2002-09-30 20:11:25 +02:00
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > If one of your daughter's friends at address < b > A< / b > wants
to access your daughter's server, she can connect to < a
href="http://192.0.2.176"> http://192.0.2.176< / a > (the firewall's external
IP address) and the firewall will rewrite the destination IP address to
192.168.201.4 (your daughter's system) and forward the request. When your
daughter's server responds, the firewall will rewrite the source address
back to 192.0.2.176 and send the response back to < b > A.< / b > < / p >
2002-08-07 16:28:04 +02:00
< / div >
2002-09-30 20:11:25 +02:00
< div align = "left" >
< p align = "left" > This example used the firewall's external IP address for
DNAT. You can use another of your public IP addresses but Shorewall will
not add that address to the firewall's external interface for you.< / p >
< / div >
< div align = "left" >
< h4 align = "left" > < a name = "ProxyARP" > < / a > 5.2.3 Proxy ARP< / h4 >
< / div >
< div align = "left" >
< p align = "left" > The idea behind proxy ARP is that:< / p >
< / div >
< div align = "left" >
< ul >
< li >
< p align = "left" > A host < b > H < / b > behind your firewall is assigned one
of your public IP addresses (< b > A)< / b > and is assigned the same netmask
< b > (M) < / b > as the firewall's external interface. < / p >
< / li >
< li >
< p align = "left" > The firewall responds to ARP "who has" requests for < b > A.< / b >
< / p >
< / li >
< li >
< p align = "left" > When < b > H< / b > issues an ARP "who has" request for an
address in the subnetwork defined by < b > A< / b > and < b > M< / b > , the firewall
will respond (with the MAC if the firewall interface to < b > H< / b > ). < / p >
< / li >
< / ul >
< / div >
< div align = "left" >
< p align = "left" > Let suppose that we decide to use Proxy ARP on the DMZ in
our example network.< / p >
< / div >
< div align = "left" >
< p align = "center" > < img border = "0" src = "images/dmz6.png" width = "745"
height="635">
< / p >
< / div >
< div align = "left" > Here, we've assigned the IP addresses 192.0.2.177 to
system DMZ 1 and 192.0.2.178 to DMZ 2. Notice that we've just assigned
an arbitrary RFC 1918 IP address and subnet mask to the DMZ interface
on the firewall. That address and netmask isn't relevant - just be sure
it doesn't overlap another subnet that you've defined.< / div >
< div align = "left" > <20> < / div >
< div align = "left" > < img border = "0" src = "images/BD21298_2.gif"
width="13" height="13">
<EFBFBD> <EFBFBD> <EFBFBD> The Shorewall configuration of Proxy ARP is done using the < a
href="Documentation.htm#ProxyArp">/etc/shorewall/proxyarp< / a > file.< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" id = "AutoNumber8"
style="border-collapse: collapse;">
< tbody >
< tr >
< td > < u > < b > ADDRESS< / b > < / u > < / td >
< td > < u > < b > INTERFACE< / b > < / u > < / td >
< td > < u > < b > EXTERNAL< / b > < / u > < / td >
< td > < u > < b > HAVE ROUTE< / b > < / u > < / td >
< / tr >
< tr >
< td > 192.0.2.177< / td >
< td > eth2< / td >
< td > eth0< / td >
< td > No< / td >
< / tr >
< tr >
< td > 192.0.2.178< / td >
< td > eth2< / td >
< td > eth0< / td >
< td > No< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > Because the HAVE ROUTE column contains No, Shorewall will
add host routes thru eth2 to 192.0.2.177 and 192.0.2.178.< / p >
< / div >
< div align = "left" >
< p align = "left" > A word of warning is in order here. ISPs typically configure
their routers with a long ARP cache timeout. If you move a system from
parallel to your firewall to behind your firewall with Proxy ARP, it will
probably be HOURS before that system can communicate with the internet.
You can call your ISP and ask them to purge the stale ARP cache entry
but many either can't or won't purge individual entries. You can determine
if your ISP's gateway ARP cache is stale using ping and tcpdump. Suppose
that we suspect that the gateway router has a stale ARP cache entry for
192.0.2.177. On the firewall, run tcpdump as follows:< / p >
< / div >
< div align = "left" >
< pre > tcpdump -nei eth0 icmp< / pre >
< / div >
< div align = "left" >
< p align = "left" > Now from 192.0.2.177, ping the default gateway (which we
are assuming is 192.0.2.254):< / p >
< / div >
< div align = "left" >
< pre > ping 192.0.2.254< / pre >
< / div >
< div align = "left" >
< p align = "left" > We can now observe the tcpdump output:< / p >
< / div >
< div align = "left" >
< pre > 13:35:12.159321 < u > 0:4:e2:20:20:33< / u > 0:0:77:95:dd:19 ip 98: 192.0.2.177 > 192.0.2.254: icmp: echo request (DF)< br > 13:35:12.207615 0:0:77:95:dd:19 < u > 0:c0:a8:50:b2:57< / u > ip 98: 192.0.2.254 > 192.0.2.177 : icmp: echo reply< / pre >
< / div >
< div align = "left" >
< p align = "left" > Notice that the source MAC address in the echo request is
different from the destination MAC address in the echo reply!! In this
case 0:4:e2:20:20:33 was the MAC of the firewall's eth0 NIC while 0:c0:a8:50:b2:57
was the MAC address of DMZ 1. In other words, the gateway's ARP cache
still associates 192.0.2.177 with the NIC in DMZ 1 rather than with the
firewall's eth0.< / p >
< / div >
< div align = "left" >
< h4 align = "left" > < a name = "NAT" > < / a > 5.2.4 Static NAT< / h4 >
< / div >
< div align = "left" >
< p align = "left" > With static NAT, you assign local systems RFC 1918 addresses
then establish a one-to-one mapping between those addresses and public
IP addresses. For outgoing connections SNAT occurs and on incoming connections
DNAT occurs. Let's go back to our earlier example involving your daughter's
web server running on system Local 3.< / p >
< / div >
< div align = "left" >
< p align = "center" > < img border = "0" src = "images/dmz5.png" width = "745"
height="635">
< / p >
< / div >
< div align = "left" >
< p align = "left" > Recall that in this setup, the local network is using SNAT
and is sharing the firewall external IP (192.0.2.176) for outbound connections.
This is done with the following entry in /etc/shorewall/masq:< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber6">
< tbody >
< tr >
< td width = "33%" > < u > < b > INTERFACE< / b > < / u > < / td >
< td width = "33%" > < u > < b > SUBNET< / b > < / u > < / td >
< td width = "34%" > < u > < b > ADDRESS< / b > < / u > < / td >
< / tr >
< tr >
< td width = "33%" > eth0< / td >
< td width = "33%" > 192.168.201.0/29< / td >
< td width = "34%" > 192.0.2.176< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_1.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> Suppose now that you have decided to give your daughter her own IP
address (192.0.2.179) for both inbound and outbound connections. You would
do that by adding an entry in < a href = "Documentation.htm#NAT" > /etc/shorewall/nat< / a > .< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellspacing = "0" cellpadding = "2" id = "AutoNumber9"
style="border-collapse: collapse;">
< tbody >
< tr >
< td > EXTERNAL< / td >
< td > INTERFACE< / td >
< td > INTERNAL< / td >
< td > ALL INTERFACES < / td >
< td > LOCAL< / td >
< / tr >
< tr >
< td > 192.0.2.179< / td >
< td > eth0< / td >
< td > 192.168.201.4< / td >
< td > No< / td >
< td > No< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > With this entry in place, you daughter has her own IP address
and the other two local systems share the firewall's IP address.< / p >
< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_1.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> Once the relationship between 192.0.2.179 and 192.168.201.4 is established
by the nat file entry above, it is no longer appropriate to use a DNAT
rule for you daughter's web server -- you would rather just use an ACCEPT
rule:< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber7">
< tbody >
< tr >
< td > < u > < b > ACTION< / b > < / u > < / td >
< td > < u > < b > SOURCE< / b > < / u > < / td >
< td > < u > < b > DESTINATION< / b > < / u > < / td >
< td > < u > < b > PROTOCOL< / b > < / u > < / td >
< td > < u > < b > PORT< / b > < / u > < / td >
< td > < u > < b > SOURCE PORT< / b > < / u > < / td >
< td > < u > < b > ORIGINAL DESTINATION< / b > < / u > < / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > loc:192.168.201.4< / td >
< td > tcp< / td >
< td > www< / td >
< td > <EFBFBD> < / td >
< td > <EFBFBD> < / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< h3 align = "left" > < a name = "Rules" > < / a > 5.3 Rules< / h3 >
< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_1.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> With the default policies, your local systems (Local 1-3) can access
any servers on the internet and the DMZ can't access any other host (including
the firewall). With the exception of < a href = "#DNAT" > DNAT rules< / a > which
cause address translation and allow the translated connection request
to pass through the firewall, the way to allow connection requests through
your firewall is to use ACCEPT rules.< / p >
< / div >
< div align = "left" >
< p align = "left" > < b > NOTE: Since the SOURCE PORT and ORIG. DEST. Columns aren't
used in this section, they won't be shown< / b > < / p >
< / div >
< div align = "left" >
< p align = "left" > You probably want to allow ping between your zones:< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber7">
< tbody >
< tr >
< td > < u > < b > ACTION< / b > < / u > < / td >
< td > < u > < b > SOURCE< / b > < / u > < / td >
< td > < u > < b > DESTINATION< / b > < / u > < / td >
< td > < u > < b > PROTOCOL< / b > < / u > < / td >
< td > < u > < b > PORT< / b > < / u > < / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz< / td >
< td > icmp< / td >
< td > echo-request< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > loc< / td >
< td > icmp< / td >
< td > echo-request< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz< / td >
< td > loc< / td >
< td > icmp< / td >
< td > echo-request< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz< / td >
< td > icmp< / td >
< td > echo-request< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > Let's suppose that you run mail and pop3 servers on DMZ 2
and a Web Server on DMZ 1. The rules that you would need are:< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber7">
< tbody >
< tr >
< td > < u > < b > ACTION< / b > < / u > < / td >
< td > < u > < b > SOURCE< / b > < / u > < / td >
< td > < u > < b > DESTINATION< / b > < / u > < / td >
< td > < u > < b > PROTOCOL< / b > < / u > < / td >
< td > < u > < b > PORT< / b > < / u > < / td >
< td > < u > < b > COMMENTS< / b > < / u > < / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail from the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > pop3< / td >
< td > # Pop3 from the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail from the Local Network< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > pop3< / td >
< td > # Pop3 from the Local Network< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > fw< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail from the Firewall< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz:192.0.2.178< / td >
< td > net< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail to the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > http< / td >
< td > # WWW from the Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > https< / td >
< td > # Secure HTTP from the Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > https< / td >
< td > # Secure HTTP from the Local Net< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > If you run a public DNS server on 192.0.2.177, you would
need to add the following rules:< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber7">
< tbody >
< tr >
< td > < u > < b > ACTION< / b > < / u > < / td >
< td > < u > < b > SOURCE< / b > < / u > < / td >
< td > < u > < b > DESTINATION< / b > < / u > < / td >
< td > < u > < b > PROTOCOL< / b > < / u > < / td >
< td > < u > < b > PORT< / b > < / u > < / td >
< td > < u > < b > COMMENTS< / b > < / u > < / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.177< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS from the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS from the internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > fw< / td >
< td > dmz:192.0.2.177< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS from firewall< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > fw< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS from firewall< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.177< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS from the local Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS from the local Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz:192.0.2.177< / td >
< td > net< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS to the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz:192.0.2.177< / td >
< td > net< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS to the Internet< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > You probably want some way to communicate with your firewall
and DMZ systems from the local network -- I recommend SSH which through
its scp utility can also do publishing and software update distribution.< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber7">
< tbody >
< tr >
< td > < u > < b > ACTION< / b > < / u > < / td >
< td > < u > < b > SOURCE< / b > < / u > < / td >
< td > < u > < b > DESTINATION< / b > < / u > < / td >
< td > < u > < b > PROTOCOL< / b > < / u > < / td >
< td > < u > < b > PORT< / b > < / u > < / td >
< td > < u > < b > COMMENTS< / b > < / u > < / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz< / td >
< td > tcp< / td >
< td > ssh< / td >
< td > # SSH to the DMZ< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > fw< / td >
< td > tcp< / td >
< td > ssh< / td >
< td > # SSH to the Firewall< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< h3 align = "left" > < a name = "OddsAndEnds" > < / a > 5.4 Odds and Ends< / h3 >
< / div >
< div align = "left" >
< p align = "left" > The above discussion reflects my personal preference for
using Proxy ARP for my servers in my DMZ and SNAT/NAT for my local systems.
I prefer to use NAT only in cases where a system that is part of an RFC
1918 subnet needs to have it's own public IP.<2E> < / p >
< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_2.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> If you haven't already, it would be a good idea to browse through
< a href = "Documentation.htm#Conf" > /etc/shorewall/shorewall.conf< / a > just
to see if there is anything there that might be of interest. You might
also want to look at the other configuration files that you haven't touched
yet just to get a feel for the other things that Shorewall can do.< / p >
< / div >
< div align = "left" >
< p align = "left" > In case you haven't been keeping score, here's the final
set of configuration files for our sample network. Only those that were
modified from the original installation are shown.< / p >
< / div >
< div align = "left" >
< p align = "left" > /etc/shorewall/interfaces (The "options" will be very
site-specific).< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "0" cellspacing = "0"
style="border-collapse: collapse;" id="AutoNumber4">
< tbody >
< tr >
< td > < u > < b > Zone< / b > < / u > < / td >
< td > < u > < b > Interface< / b > < / u > < / td >
< td > < u > < b > Broadcast< / b > < / u > < / td >
< td > < u > < b > Options< / b > < / u > < / td >
< / tr >
< tr >
< td > net< / td >
< td > eth0< / td >
< td > detect< / td >
< td > norfc1918,routefilter < / td >
< / tr >
< tr >
< td > loc< / td >
< td > eth1< / td >
< td > detect< / td >
< td > <EFBFBD> < / td >
< / tr >
< tr >
< td > dmz< / td >
< td > eth2< / td >
< td > detect< / td >
< td > <EFBFBD> < / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > The setup described here requires that your network interfaces
be brought up before Shorewall can start. This opens a short window during
which you have no firewall protection. If you replace 'detect' with the
actual broadcast addresses in the entries above, you can bring up Shorewall
before you bring up your network interfaces.< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "0" cellspacing = "0"
style="border-collapse: collapse;" id="AutoNumber4">
< tbody >
< tr >
< td > < u > < b > Zone< / b > < / u > < / td >
< td > < u > < b > Interface< / b > < / u > < / td >
< td > < u > < b > Broadcast< / b > < / u > < / td >
< td > < u > < b > Options< / b > < / u > < / td >
< / tr >
< tr >
< td > net< / td >
< td > eth0< / td >
< td > 192.0.2.255< / td >
< td > norfc1918,routefilter < / td >
< / tr >
< tr >
< td > loc< / td >
< td > eth1< / td >
< td > 192.168.201.7< / td >
< td > <EFBFBD> < / td >
< / tr >
< tr >
< td > dmz< / td >
< td > eth2< / td >
< td > 192.168.202.7< / td >
< td > <EFBFBD> < / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > /etc/shorewall/masq - Local subnet< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber6">
< tbody >
< tr >
< td width = "33%" > < u > < b > INTERFACE< / b > < / u > < / td >
< td width = "33%" > < u > < b > SUBNET< / b > < / u > < / td >
< td width = "34%" > < u > < b > ADDRESS< / b > < / u > < / td >
< / tr >
< tr >
< td width = "33%" > eth0< / td >
< td width = "33%" > 192.168.201.0/29< / td >
< td width = "34%" > 192.0.2.176< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > /etc/shorewall/proxyarp - DMZ< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" id = "AutoNumber8"
style="border-collapse: collapse;">
< tbody >
< tr >
< td > < u > < b > ADDRESS< / b > < / u > < / td >
< td > < u > < b > INTERFACE< / b > < / u > < / td >
< td > < u > < b > EXTERNAL< / b > < / u > < / td >
< td > < u > < b > HAVE ROUTE< / b > < / u > < / td >
< / tr >
< tr >
< td > 192.0.2.177< / td >
< td > eth2< / td >
< td > eth0< / td >
< td > No< / td >
< / tr >
< tr >
< td > 192.0.2.178< / td >
< td > eth2< / td >
< td > eth0< / td >
< td > No< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > /etc/shorewall/nat- Daughter's System< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" id = "AutoNumber9"
style="border-collapse: collapse;">
< tbody >
< tr >
< td > EXTERNAL< / td >
< td > INTERFACE< / td >
< td > INTERNAL< / td >
< td > ALL INTERFACES < / td >
< td > LOCAL< / td >
< / tr >
< tr >
< td > 192.0.2.179< / td >
< td > eth0< / td >
< td > 192.168.201.4< / td >
< td > No< / td >
< td > No< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > /etc/shorewall/rules< / p >
< / div >
< div align = "left" >
< blockquote >
< table border = "1" cellpadding = "2" style = "border-collapse: collapse;"
id="AutoNumber7">
< tbody >
< tr >
< td > < u > < b > ACTION< / b > < / u > < / td >
< td > < u > < b > SOURCE< / b > < / u > < / td >
< td > < u > < b > DESTINATION< / b > < / u > < / td >
< td > < u > < b > PROTOCOL< / b > < / u > < / td >
< td > < u > < b > PORT< / b > < / u > < / td >
< td > < u > < b > COMMENTS< / b > < / u > < / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail from the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > pop3< / td >
< td > # Pop3 from the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail from the Local Network< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > pop3< / td >
< td > # Pop3 from the Local Network< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > fw< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail from the Firewall< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz:192.0.2.178< / td >
< td > net< / td >
< td > tcp< / td >
< td > smtp< / td >
< td > # Mail to the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > http< / td >
< td > # WWW from the Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > https< / td >
< td > # Secure HTTP from the Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.178< / td >
< td > tcp< / td >
< td > https< / td >
< td > # Secure HTTP from the Local Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.177< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS from the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS from the internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > fw< / td >
< td > dmz:192.0.2.177< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS from firewall< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > fw< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS from firewall< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.177< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS from the local Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz:192.0.2.177< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS from the local Net< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz:192.0.2.177< / td >
< td > net< / td >
< td > udp< / td >
< td > domain< / td >
< td > # UDP DNS to the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz:192.0.2.177< / td >
< td > net< / td >
< td > tcp< / td >
< td > domain< / td >
< td > # TCP DNS to the Internet< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > dmz< / td >
< td > icmp< / td >
< td > echo-request< / td >
< td > # Ping< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > net< / td >
< td > loc< / td >
< td > icmp< / td >
< td > echo-request< / td >
< td > #<23> "< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > dmz< / td >
< td > loc< / td >
< td > icmp< / td >
< td > echo-request< / td >
< td > # "< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz< / td >
< td > icmp< / td >
< td > echo-request< / td >
< td > # "< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > dmz< / td >
< td > tcp< / td >
< td > ssh< / td >
< td > # SSH to the DMZ< / td >
< / tr >
< tr >
< td > ACCEPT< / td >
< td > loc< / td >
< td > fw< / td >
< td > tcp< / td >
< td > ssh< / td >
< td > # SSH to the Firewall< / td >
< / tr >
< / tbody >
< / table >
< / blockquote >
< / div >
< div align = "left" >
< h2 align = "left" > < a name = "DNS" > < / a > 6.0 DNS< / h2 >
< / div >
< div align = "left" >
< p align = "left" > Given the collection of RFC 1918 and public addresses in
this setup, it only makes sense to have separate internal and external
DNS servers. You can combine the two into a single BIND 9 server using
< i > Views. < / i > If you are not interested in Bind 9 views, you can < a
href="#StartingAndStopping">go to the next section< / a > .< / p >
< / div >
< div align = "left" >
< p align = "left" > Suppose that your domain is foobar.net and you want the two
DMZ systems named www.foobar.net and mail.foobar.net and you want the
three local systems named "winken.foobar.net, blinken.foobar.net and nod.foobar.net.
You want your firewall to be known as firewall.foobar.net externally and
it's interface to the local network to be know as gateway.foobar.net and
its interface to the dmz as dmz.foobar.net. Let's have the DNS server
on 192.0.2.177 which will also be known by the name ns1.foobar.net.< / p >
< / div >
< div align = "left" >
< p align = "left" > The /etc/named.conf file would look like this:< / p >
< / div >
< div align = "left" >
< blockquote >
< div align = "left" >
< pre > options {< br > directory "/var/named";< br > listen-on { 127.0.0.1 ; 192.0.2.177; };< br > };< br > < br > logging {< br > channel xfer-log {< br > file "/var/log/named/bind-xfer.log";< br > print-category yes;< br > print-severity yes;< br > print-time yes;< br > severity info;< br > };< br > category xfer-in { xfer-log; };< br > category xfer-out { xfer-log; };< br > category notify { xfer-log; };< br > };< / pre >
< / div >
< div align = "left" >
< pre > #< br > # This is the view presented to our internal systems< br > #< br > < br > view "internal" {< br > #< br > # These are the clients that see this view< br > #< br > match-clients { 192.168.201.0/29;< br > 192.168.202.0/29;< br > 127.0.0/24;< br > 192.0.2.176/32; < br > 192.0.2.178/32;< br > 192.0.2.179/32;< br > 192.0.2.180/32; };< br > #< br > # If this server can't complete the request, it should use outside< br > # servers to do so< br > #< br > recursion yes;< br > < br > zone "." in {< br > type hint;< br > file "int/root.cache";< br > };< br > < br > zone "foobar.net" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "int/db.foobar";< br > };< br > < br > zone "0.0.127.in-addr.arpa" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "int/db.127.0.0"; < br > };< br > < br > zone "201.168.192.in-addr.arpa" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "int/db.192.168.201";< br > };< br > < br > zone "202.168.192.in-addr.arpa" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "int/db.192.168.202";< br > };< br > < br > zone "176.2.0.192.in-addr.arpa" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "db.192.0.2.176";< br > };< br > < br > zone "177.2.0.192.in-addr.arpa" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "db.192.0.2.177";< br > };< br > < br > zone "178.2.0.192.in-addr.arpa" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "db.192.0.2.178";< br > };< br > < br > zone "179.2.0.192.in-addr.arpa" in {< br > type master;< br > notify no;< br > allow-update { none; };< br > file "db.206.124.146.179";< br > };< br > < br > };< br > #< br > # This is the view that we present to the outside world< br > #< br > view "external" {< br > match-clients { any; };< br > #< br > # If we can't answer the query, we tell the client so< br > #< br > recursion no;< br > < br > zone "foobar.net" in {< br > type master;< br > notify yes;< br > allow-update {none; };< br > allow-transfer { < i > < secondary NS IP> < / i > ; };< br > file "ext/db.foobar";< br > };< br > < br > zone "176.2.0.192.in-addr.arpa" in {< br > type master;< br > notify yes;< br > allow-update { none; };< br > allow-transfer { < i > < secondary NS IP> < / i > ; };< br > file "db.192.0.2.176";< br > };< br > < br > zone "177.2.0.192.in-addr.arpa" in {< br > type master;< br > notify yes;< br > allow-update { none; };< br > allow-transfer { < i > < secondary NS IP> < / i > ; };< br > file "db.192.0.2.177";< br > };< br > < br > zone "178.2.0.192.in-addr.arpa" in {< br > type master;< br > notify yes;< br > allow-update { none; };< br > allow-transfer { < i > < secondary NS IP> < / i > ; };< br > file "db.192.0.2.178";< br > };< br > < br > zone "179.2.0.192.in-addr.arpa" in {< br > type master;< br > notify yes;< br > allow-update { none; };< br > allow-transfer { < i > < secondary NS IP> < / i > ; };< br > file "db.192.0.2.179";< br > };< br > };< / pre >
< / div >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > Here are the files in /var/named (those not shown are usually
included in your bind disbribution).< / p >
< p align = "left" > db.192.0.2.176 - This is the reverse zone for the firewall's
external interface< / p >
< blockquote >
< pre > ; ############################################################< br > ; Start of Authority (Inverse Address Arpa) for 192.0.2.176/32< br > ; Filename: db.192.0.2.176< br > ; ############################################################< br > @ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (< br > 2001102303 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ) ; minimum (1 day)< br > ;< br > ; ############################################################< br > ; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)< br > ; ############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > @ 604800 IN NS < i > < name of secondary ns> < / i > .< br > ;< br > ; ############################################################< br > ; Iverse Address Arpa Records (PTR's) < br > ; ############################################################< br > 176.2.0.192.in-addr.arpa. 86400 IN PTR firewall.foobar.net.< br > < / pre >
< / blockquote >
< / div >
< div align = "left" >
< div align = "left" > db.192.0.2.177 - This is the reverse zone for the www/DNS
server
< blockquote >
< pre > ; ############################################################< br > ; Start of Authority (Inverse Address Arpa) for 192.0.2.177/32< br > ; Filename: db.192.0.2.177< br > ; ############################################################< br > @ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (< br > 2001102303 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ) ; minimum (1 day)< br > ;< br > ; ############################################################< br > ; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)< br > ; ############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > @ 604800 IN NS < i > < name of secondary ns> < / i > .< br > ;< br > ; ############################################################< br > ; Iverse Address Arpa Records (PTR's) < br > ; ############################################################< br > 177.2.0.192.in-addr.arpa. 86400 IN PTR www.foobar.net.< br > < / pre >
< / blockquote >
< / div >
< / div >
< div align = "left" >
< div align = "left" > db.192.0.2.178 - This is the reverse zone for the mail
server
< blockquote >
< pre > ; ############################################################< br > ; Start of Authority (Inverse Address Arpa) for 192.0.2.178/32< br > ; Filename: db.192.0.2.178< br > ; ############################################################< br > @ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (< br > 2001102303 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ) ; minimum (1 day)< br > ;< br > ; ############################################################< br > ; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)< br > ; ############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > @ 604800 IN NS < i > < name of secondary ns> < / i > .< br > ;< br > ; ############################################################< br > ; Iverse Address Arpa Records (PTR's) < br > ; ############################################################< br > 178.2.0.192.in-addr.arpa. 86400 IN PTR mail.foobar.net.< br > < / pre >
< / blockquote >
< / div >
< / div >
< div align = "left" >
< div align = "left" > db.192.0.2.179 - This is the reverse zone for daughter's
web server's public IP
< blockquote >
< pre > ; ############################################################< br > ; Start of Authority (Inverse Address Arpa) for 192.0.2.179/32< br > ; Filename: db.192.0.2.179< br > ; ############################################################< br > @ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (< br > 2001102303 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ) ; minimum (1 day)< br > ;< br > ; ############################################################< br > ; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)< br > ; ############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > @ 604800 IN NS < i > < name of secondary ns> < / i > .< br > ;< br > ; ############################################################< br > ; Iverse Address Arpa Records (PTR's) < br > ; ############################################################< br > 179.2.0.192.in-addr.arpa. 86400 IN PTR nod.foobar.net.< br > < / pre >
< / blockquote >
< / div >
< / div >
< div align = "left" >
< p align = "left" > int/db.127.0.0 - The reverse zone for localhost< / p >
< / div >
< div align = "left" >
< blockquote >
< pre > ; ############################################################< br > ; Start of Authority (Inverse Address Arpa) for 127.0.0.0/8< br > ; Filename: db.127.0.0< br > ; ############################################################< br > @ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (< br > 2001092901 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ) ; minimum (1 day)< br > ; ############################################################< br > ; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)< br > ; ############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > < br > ; ############################################################< br > ; Iverse Address Arpa Records (PTR's)< br > ; ############################################################< br > 1 86400 IN PTR localhost.foobar.net.< / pre >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > int/db.192.168.201 - Reverse zone for the local net. This
is only shown to internal clients< / p >
< / div >
< div align = "left" >
< blockquote >
< pre > ; ############################################################< br > ; Start of Authority (Inverse Address Arpa) for 192.168.201.0/29< br > ; Filename: db.192.168.201< br > ; ############################################################< br > @ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (< br > 2002032501 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ) ; minimum (1 day)< br > < br > ; ############################################################< br > ; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)< br > ; ############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > < br > ; ############################################################< br > ; Iverse Address Arpa Records (PTR's)< br > ; ############################################################< br > 1 86400 IN PTR gateway.foobar.net.< br > 2 86400 IN PTR winken.foobar.net.< br > 3 86400 IN PTR blinken.foobar.net.< br > 4 86400 IN PTR nod.foobar.net.< / pre >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > int/db.192.168.202 - Reverse zone for the firewall's DMZ
interface< / p >
< / div >
< div align = "left" >
< blockquote >
< div align = "left" >
< pre > ; ############################################################< br > ; Start of Authority (Inverse Address Arpa) for 192.168.202.0/29< br > ; Filename: db.192.168.202< br > ; ############################################################< br > @ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (< br > 2002032501 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ) ; minimum (1 day)< br > < br > ; ############################################################< br > ; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)< br > ; ############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > < br > ; ############################################################< br > ; Iverse Address Arpa Records (PTR's)< br > ; ############################################################< br > 1 86400 IN PTR dmz.foobar.net.< / pre >
< / div >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > int/db.foobar - Forward zone for use by internal clients.< / p >
< / div >
< div align = "left" >
< blockquote >
< pre > ;##############################################################< br > ; Start of Authority for foobar.net.< br > ; Filename: db.foobar< br > ;##############################################################< br > @ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (< br > 2002071501 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ); minimum (1 day)< br > ;############################################################< br > ; foobar.net Nameserver Records (NS)< br > ;############################################################< br > @ 604800 IN NS ns1.foobar.net.< br > < br > ;############################################################< br > ; Foobar.net Office Records (ADDRESS)< br > ;############################################################< br > localhost 86400 IN A 127.0.0.1< br > < br > firewall 86400 IN A 192.0.2.176< br > www 86400 IN A 192.0.2.177< br > ns1 86400 IN A 192.0.2.177< br > www 86400 IN A 192.0.2.177< br > < br > gateway 86400 IN A 192.168.201.1< br > winken 86400 IN A 192.168.201.2< br > blinken 86400 IN A 192.168.201.3< br > nod 86400 IN A 192.168.201.4< / pre >
< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > ext/db.foobar - Forward zone for external clients< / p >
< / div >
< div align = "left" >
< blockquote >
< div align = "left" >
< pre > ;##############################################################< br > ; Start of Authority for foobar.net.< br > ; Filename: db.foobar< br > ;##############################################################< br > @ 86400 IN SOA ns1.foobar.net. netadmin.foobar.net. (< br > 2002052901 ; serial< br > 10800 ; refresh (3 hour)< br > 3600 ; retry (1 hour)< br > 604800 ; expire (7 days)< br > 86400 ); minimum (1 day)< br > ;############################################################< br > ; Foobar.net Nameserver Records (NS)< br > ;############################################################< br > @ 86400 IN NS ns1.foobar.net.< br > @ 86400 IN NS < i > < secondary NS> < / i > .< br > ;############################################################< br > ; Foobar.net Foobar Wa Office Records (ADDRESS)< br > ;############################################################< br > localhost 86400 IN A 127.0.0.1< br > ;< br > ; The firewall itself< br > ;< br > firewall 86400 IN A 192.0.2.176< br > ;< br > ; The DMZ< br > ;< br > ns1 86400 IN A 192.0.2.177< br > www 86400 IN A 192.0.2.177< br > mail 86400 IN A 192.0.2.178< br > ;< br > ; The Local Network< br > ;< br > nod 86400 IN A 192.0.2.179< br > < br > ;############################################################< br > ; Current Aliases for foobar.net (CNAME)< br > ;############################################################< br > < br > ;############################################################< br > ; foobar.net MX Records (MAIL EXCHANGER)< br > ;############################################################< br > foobar.net. 86400 IN A 192.0.2.177< br > 86400 IN MX 0 mail.foobar.net.< br > 86400 IN MX 1 < i > < backup MX> < / i > .< / pre >
< / div >
< / blockquote >
< / div >
< div align = "left" >
< h2 align = "left" > < a name = "StartingAndStopping" > < / a > 7.0 Starting and Stopping
Your Firewall< / h2 >
< / div >
< div align = "left" >
< p align = "left" > The < a href = "Install.htm" > installation procedure < / a > configures
your system to start Shorewall at system boot.< / p >
< / div >
< div align = "left" >
< p align = "left" > The firewall is started using the "shorewall start" command
and stopped using "shorewall stop". When the firewall is stopped, routing
is enabled on those hosts that have an entry in < a
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped< / a > . A
running firewall may be restarted using the "shorewall restart" command.
If you want to totally remove any trace of Shorewall from your Netfilter
configuration, use "shorewall clear".< / p >
< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_2.gif" width = "13"
height="13">
<EFBFBD> <EFBFBD> <EFBFBD> Edit the /etc/shorewall/routestopped file and configure those systems
that you want to be able to access the firewall when it is stopped.< / p >
< / div >
< div align = "left" >
< p align = "left" > < b > WARNING: < / b > If you are connected to your firewall from
the internet, do not issue a "shorewall stop" command unless you have
added an entry for the IP address that you are connected from to < a
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped< / a > .
Also, I don't recommend using "shorewall restart"; it is better to create
an < i > < a href = "Documentation.htm#Configs" > alternate configuration< / a > < / i >
and test it using the < a href = "Documentation.htm#Starting" > "shorewall
try" command< / a > .< / p >
< / div >
< p align = "left" > < font size = "2" > Last updated 9/19/2002 - < a
href="support.htm">Tom Eastep< / a > < / font > < / p >
< p align = "left" > < a href = "copyright.htm" > < font size = "2" > Copyright 2002 Thomas
M. Eastep< / font > < / a > < / p >
< br >
< / body >
< / html >
