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< h1 align = "center" > < font color = "#FFFFFF" > Basic Two-Interface Firewall< / font > < / h1 >
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< p align = "left" > Setting up a Linux system as a firewall for a small network is a
fairly straight-forward task if you understand the basics and follow the
documentation.< / p >
< p > This guide doesn't attempt to acquaint you with all of the features of
Shorewall. It rather focuses on what is required to configure Shorewall in its
most common configuration:< / p >
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< ul >
< li > Linux system used as a firewall/router for a small local network.< / li >
< li > Single external IP address.< / li >
< li > Internet connection through cable modem, DSL, ISDN, Frame Relay, dial-up
...< / li >
< / ul >
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< p align = "left" > Here is a schematic of a typical installation.< / p >
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< p align = "center" >
< img border = "0" src = "images/basics.png" width = "444" height = "635" > < / p >
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< 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 >
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< pre > [root@gateway root]# which ip
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/sbin/ip
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[root@gateway root]#< / pre > < p > I recommend that you first read through the
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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" >
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. 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 >
< li > < a href = "http://www.simtel.net/pub/pd/51438.html" > Windows Version of
dos2unix< / a > < / li >
< li > < a href = "http://www.megaloman.com/~hany/software/hd2u/" > Linux Version of
dos2unix< / a > < / li >
< / ul >
< h2 align = "left" > Shorewall Concepts< / h2 >
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< p > The configuration files for Shorewall are contained in the directory
/etc/shorewall -- for simple setups, you will only need to deal with a few of
these as described in this guide. After you have < a href = "Install.htm" > installed Shorewall< / a > ,
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download the < a href = "/pub/shorewall/LATEST.samples/two-interfaces.tgz" > two-interface sample< / a > , un-tar it (tar -zxvf two-interfaces.tgz) and and copy the files to /etc/shorewall
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(these files will replace files with the same name).< / 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 default entries.< / p >
< p > Shorewall views the network where it is running as being composed of a set of
< i > zones.< / i > In the two-interface sample configuration, the following zone names are used:< / p >
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< td > < u > < b > Name< / b > < / u > < / td >
< td > < u > < b > Description< / b > < / u > < / td >
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< / tr >
< tr >
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< td > < b > net< / b > < / td >
< td > < b > The Internet< / b > < / td >
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< / tr >
< tr >
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< td > < b > loc< / b > < / td >
< td > < b > Your Local Network< / b > < / td >
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< / tr >
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< / table >
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< 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 > < / 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 >
< 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 >
< / ul >
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< 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 the request is first
checked against the rules in /etc/shorewall/common (the samples provide that
file for you).< / p >
< p > The /etc/shorewall/policy file included with the two-interface sample has the
following policies:< / p >
< blockquote >
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< tr >
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< 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 >
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< / tr >
< tr >
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< td > loc< / td >
< td > net< / td >
< td > ACCEPT< / td >
< td > < / td >
< td > < / td >
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< / tr >
< tr >
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< td > net< / td >
< td > all< / td >
< td > DROP< / td >
< td > info< / td >
< td > < / td >
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< / tr >
< tr >
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< td > all< / td >
< td > all< / td >
< td > REJECT< / td >
< td > info< / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< blockquote >
< p > In the two-interface sample, the line below is included but commented out. If
you want your firewall system to have full access to servers on the internet,
uncomment that line.< / p >
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< tr >
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< 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 >
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< / tr >
< tr >
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< td > fw< / td >
< td > net< / td >
< td > ACCEPT< / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< p > The above policy will:< / p >
< ol >
< 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< / li >
< li > optionally accept all connection requests from the firewall to the
internet (if you uncomment the additional policy)< / li >
< li > reject all other connection requests.< / li >
< / ol >
< p > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" > At this point, edit your /etc/shorewall/policy and make any changes that you
wish.< / p >
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< h2 align = "left" > Network Interfaces< / h2 >
< p align = "center" >
< img border = "0" src = "images/basics.png" width = "444" height = "635" > < / p >
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< p align = "left" > The firewall has two 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 > )
< 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 via ISDN, your external
interface will be < b > ippp0.< / b > < / p >
< p align = "left" > < img border = "0" src = "images/BD21298_1.gif" width = "13" height = "13" > 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 > Internal Interface< / i > will be an ethernet adapter (eth1
or eth0) and will be connected to a hub or switch. Your other computers will be
connected to the same hub/switch (note: If you have only a single internal 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 the internal and external interface
to the same hub or switch (even for testing). It won't work the way that you think that it will and you will end up confused and
believing that Shorewall doesn't work at all.< / p >
< p align = "left" > < img border = "0" src = "images/BD21298_.gif" align = "left" width = "13" height = "13" > The Shorewall two-interface sample configuration assumes that
the external interface is < b > eth0< / b > and the internal interface is < b > eth1< / b > .
If your configuration is different, you will have to modify the sample
< a href = "Documentation.htm#Interfaces" > /etc/shorewall/interfaces< / a > file accordingly. While you are there, you may wish to
review the list of options that are specified for the interfaces. Some hints:< / p >
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< ul >
< li >
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< p align = "left" > If your external interface is < b > ppp0< / b > or < b > ippp0< / b > , you can replace the
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" detect" in the second column with " -" .< / li >
< li >
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< p align = "left" > If your external interface is < b > ppp0< / b > or < b > ippp0< / b > or if you have a static IP
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address, you can remove " dhcp" from the option list.< / li >
< / ul >
< h2 align = "left" > IP Addresses< / h2 >
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< p align = "left" > Before going further, we should say a few words about Internet
Protocol (IP) < i > addresses< / i > . Normally, your ISP will assign you a single < i >
Public< / i > IP address. This address may be assigned via the< i > Dynamic Host
Configuration Protocol< / i > (DHCP) or as part of establishing your connection
when you dial in (standard modem) or establish your PPP connection. In rare
cases, your ISP may assign you a< i > static< / i > IP address; that means that you
configure your firewall's external interface to use that address permanently.< i >
< / i > However your external address is assigned, it will be shared by all of your systems when you access the
Internet. You will have to assign your own addresses in your
internal network (the Internal Interface on your firewall plus your other
computers). RFC 1918 reserves several < i > Private < / i > IP address ranges for this
purpose:< / p >
< div align = "left" >
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< pre > 10.0.0.0 - 10.255.255.255
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172.16.0.0 - 172.31.255.255
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192.168.0.0 - 192.168.255.255< / pre >
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< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" >
Before starting Shorewall, you should look at the IP address of your external
interface and if it is one of the above ranges, you should remove the
'norfc1918' option from the external interface's entry in
/etc/shorewall/interfaces.< / div >
< div align = "left" >
< p align = "left" > You will want to assign your addresses from the same < i >
sub-network < / i > (< i > subnet)< / i > . For our purposes, we can consider a subnet
to consists of a range of addresses x.y.z.0 - x.y.z.255. Such a subnet will
have a < i > Subnet Mask < / i > of 255.255.255.0. The address x.y.z.0 is reserved as
the < i > Subnet Address< / i > and x.y.z.255 is reserved as the < i > Subnet Broadcast< / i >
< i > Address< / i > . In Shorewall, a subnet is described using
< a href = "subnet_masks.htm" > < i > Variable-Length
Subnet Mask < / i > (VLSM) notation< / a > with consists of the subnet address followed
by " /24" . The " 24" refers to the number of
consecutive leading " 1" bits from the left of the subnet mask.
< / div >
< div align = "left" >
< p align = "left" > Example sub-network:< / div >
< div align = "left" >
< blockquote >
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< tr >
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< td > < b > Range:< / b > < / td >
< td > 10.10.10.0 - 10.10.10.255< / td >
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< / tr >
< tr >
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< td > < b > Subnet Address:< / b > < / td >
< td > 10.10.10.0< / td >
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< / tr >
< tr >
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< td > < b > Broadcast Address:< / b > < / td >
< td > 10.10.10.255< / td >
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< / tr >
< tr >
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< td > < b > VLSM Notation:< / b > < / td >
< td > 10.10.10.0/24< / td >
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< / tr >
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< / table >
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< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > It is conventional to assign the internal interface either the
first usable address in the subnet (10.10.10.1 in the above example) or the
last usable address (10.10.10.254).< / div >
< div align = "left" >
< p align = "left" > One of the purposes of subnetting is to allow all computers in the
subnet to understand which other computers can be communicated with directly.
To communicate with systems outside of the subnetwork, systems send packets
through a< i > gateway< / i > (router).< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_1.gif" width = "13" height = "13" > Your local computers (computer
1 and computer 2 in the above diagram) should be configured with their< i >
default gateway< / i > to be the IP address of the firewall's internal
interface.< i > < / i >
< / div >
< p align = "left" > The foregoing short discussion barely scratches the surface
regarding subnetting and routing. If you are interested in learning more about
IP addressing and routing, 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 >
< p align = "left" > The remainder of this quide will assume that you have configured
your network as shown here:< / p >
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< p align = "center" >
< img border = "0" src = "images/basics1.png" width = "444" height = "635" > < / p >
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< p align = "left" > The default gateway for computer's 1 & 2 would be 10.10.10.254.< / p >
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< h2 align = "left" > IP Masquerading (SNAT)< / h2 >
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< 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. When one of your local systems
(let's assume computer 1) sends a connection request to an internet host, the
firewall must perform < i > Network Address Translation < / i > (NAT). The firewall
rewrites the source address in the packet to be the address of the firewall's
external interface; in other words, the firewall makes it look as if the firewall
itself is initiating the connection. This is necessary so that the
destination host will be able to route return packets back to the firewall
(remember that packets whose destination address is reserved by RFC 1918 can't
be routed across the internet so the remote host can't address its response to
computer 1). When the firewall receives a return packet, it
rewrites the destination address back to 10.10.10.1 and
forwards the packet on to computer 1. < / p >
< p align = "left" > On Linux systems, the above process is often referred to as< i >
IP Masquerading< / i > but you will also see the term < i > Source Network Address
Translation < / i > (SNAT) used. Shorewall follows the convention used with
Netfilter:< / p >
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< ul >
< li >
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< p align = "left" > < i > Masquerade< / i > describes the case where you let your
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firewall system automatically detect the external interface address.< / li >
< li >
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< p align = "left" > < i > SNAT< / i > refers to the case when you explicitly specify the
source address that you want outbound packets from your local network to use.
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< / li >
< / ul >
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< p align = "left" > In Shorewall, both Masquerading and SNAT are configured with
entries in the /etc/shorewall/masq file. You will normally use Masquerading if
your external IP is dynamic and SNAT if the IP is static.< / p >
< p align = "left" > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" > If your external firewall interface is < b > eth0< / b > , you do not
need to modify the file provided with the sample. Otherwise, edit
/etc/shorewall/masq and change the first column to the name of your external
interface and the second column to the name of your internal interface.< / p >
< p align = "left" > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" > If your external IP is
static, you can enter it in the third column in the /etc/shorewall/masq entry if
you like although your firewall will work fine if you leave that column empty.
Entering your static IP in column 3 makes processing outgoing packets a little
more efficient. < / p >
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< h2 align = "left" > Port Forwarding (DNAT)< / h2 >
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< p align = "left" > One of your goals may be to run one or more servers on your
local computers. Because these computers have RFC-1918 addresses, it is not
possible for clients on the internet to connect directly to them. It is rather
necessary for those clients to address their connection requests to the firewall
who rewrites the destination address to the address of your server and forwards
the packet to that server. When your server responds, the firewall automatically
performs SNAT to rewrite the source address in the response.< / p >
< p align = "left" > The above process is called< i > Port Forwarding< / i > or < i >
Destination Network Address Translation< / i > (DNAT). You configure port
forwarding using DNAT rules in the /etc/shorewall/rules file.< / p >
< p > The general form of a simple port forwarding rule in
/etc/shorewall/rules is:< / p >
< blockquote >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > DNAT< / td >
< td > net< / td >
< td > loc:< i > < server local ip address> < / i > [:< i > < server port> < / i > ]< / td >
< td > < i > < protocol> < / i > < / td >
< td > < i > < port> < / i > < / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< p > Example - you run a Web Server on computer 2 and you want to forward incoming
TCP port 80 to that system:< / p >
< blockquote >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > DNAT< / td >
< td > net< / td >
< td > loc:10.10.10.2< / td >
< td > tcp< / td >
< td > 80< / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< p > A couple of important points
to keep in mind:< / p >
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< ul >
< li > You must test the above rule from a client outside of your local network
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(i.e., don't test from a browser running on computers 1 or 2 or on the
firewall). If you want to be able to access your web server using the IP
address of your external interface, see < a href = "FAQ.htm#faq2" > Shorewall FAQ
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#2< / a > .< / li >
< li > Many ISPs block incoming connection requests to port 80. If you have
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problems connecting to your web server, try the following rule and try
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connecting to port 5000.< / li >
< / ul >
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< blockquote >
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< table border = "1" cellpadding = "2" style = "border-collapse: collapse" id = "AutoNumber4" >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > DNAT< / td >
< td > net< / td >
< td > loc:10.10.10.2:80< / td >
< td > tcp< / td >
< td > 5000< / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< p >
< img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" > At this point, modify
/etc/shorewall/rules to add any DNAT rules that you require.< / p >
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< h2 align = "left" > Domain Name Server (DNS)< / h2 >
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< p align = "left" > Normally, when you connect to your ISP, as part of getting an IP
address your firewall's < i > Domain Name Service < / i > (DNS) resolver will be
automatically configured (e.g., the /etc/resolv.conf file will be written).
Alternatively, your ISP may have given you the IP address of a pair of DNS < i >
name servers< / i > for you to manually configure as your primary and secondary
name servers. Regardless of how DNS gets configured on your firewall, it is < u > your< / u > responsibility to configure the resolver in your
internal systems. You can take one of two approaches:< / p >
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< ul >
< li >
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< p align = "left" > You can configure your internal systems to use your ISP's name
servers. If you ISP gave you the addresses of their servers or if those
addresses are available on their web site, you can configure your internal
systems to use those addresses. If that information isn't available, look in
/etc/resolv.conf on your firewall system -- the name servers are given in
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" nameserver" records in that file.< / li >
< li >
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< p align = "left" > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" > You can configure a< i > Caching Name Server < / i > on your
firewall.< i > < / i > Red Hat has an RPM for a caching name server (the RPM also
requires the 'bind' RPM) and for Bering users, there is dnscache.lrp. If you
take this approach, you configure your internal systems to use the firewall
itself as their primary (and only) name server. You use the internal IP
address of the firewall (10.10.10.254 in the example above) for the name
server address. To allow your local systems to talk to your caching name
server, you must open port 53 (both UDP and TCP) from the local network to the
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firewall; you do that by adding the following rules in /etc/shorewall/rules.< / li >
< / ul >
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< blockquote >
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< table border = "1" cellpadding = "2" style = "border-collapse: collapse" id = "AutoNumber4" >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > loc< / td >
< td > fw< / td >
< td > tcp< / td >
< td > 53< / td >
< td > < / td >
< td > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > loc< / td >
< td > fw< / td >
< td > udp< / td >
< td > 53< / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< div align = "left" >
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< h2 align = "left" > Other Connections< / h2 >
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< / div >
< div align = "left" >
< p align = "left" > The two-interface sample includes the following rules:< / div >
< div align = "left" >
< blockquote >
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< table border = "1" cellpadding = "2" style = "border-collapse: collapse" id = "AutoNumber4" >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > fw< / td >
< td > net< / td >
< td > tcp< / td >
< td > 53< / td >
< td > < / td >
< td > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > fw< / td >
< td > net< / td >
< td > udp< / td >
< td > 53< / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > Those rules allow DNS access from your firewall and may be
removed if you commented out the line in /etc/shorewall/policy allowing all
connections from the firewall to the internet.< / div >
< div align = "left" >
< p align = "left" > The sample also includes:< / div >
< div align = "left" >
< blockquote >
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< table border = "1" cellpadding = "2" style = "border-collapse: collapse" id = "AutoNumber4" >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > loc< / td >
< td > fw< / td >
< td > tcp< / td >
< td > 22< / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > That rule allows you to run an SSH server on your firewall and
connect to that server from your local systems.< / div >
< div align = "left" >
< p align = "left" > If you wish to enable other connections between your firewall
and other systems, the general format is:< / div >
< div align = "left" >
< blockquote >
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< table border = "1" cellpadding = "2" style = "border-collapse: collapse" id = "AutoNumber4" >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > < i > < source zone> < / i > < / td >
< td > < i > < destination zone> < / i > < / td >
< td > < i > < protocol> < / i > < / td >
< td > < i > < port> < / i > < / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > Example - You want to run a Web Server on your firewall
system:< / div >
< div align = "left" >
< blockquote >
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< table border = "1" cellpadding = "2" style = "border-collapse: collapse" id = "AutoNumber4" >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > net< / td >
< td > fw< / td >
< td > tcp< / td >
< td > 80< / td >
< td > #Allow web access< / td >
< td > from the internet< / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > loc< / td >
< td > fw< / td >
< td > tcp< / td >
< td > 80< / td >
< td > #Allow web access< / td >
< td > from the local network< / td >
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< / tr >
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< / table >
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< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > Those two rules would of course be in addition to the rules
listed above under " You can configure a Caching Name Server on your firewall" < / div >
< div align = "left" >
< p align = "left" > If you don't know what port and protocol a particular
application uses, look < a href = "ports.htm" > here< / a > .< / div >
< div align = "left" >
< p align = "left" > < b > Important: < / b > I don't recommend enabling telnet to/from
the internet because it uses clear text (even for login!). If you want shell
access to your firewall from the internet, use SSH:< / div >
< div align = "left" >
< blockquote >
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< table border = "1" cellpadding = "2" style = "border-collapse: collapse" id = "AutoNumber4" >
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< tr >
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< 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 ADDRESS< / b > < / u > < / td >
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< / tr >
< tr >
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< td > ACCEPT< / td >
< td > net< / td >
< td > fw< / td >
< td > tcp< / td >
< td > 22< / td >
< td > < / td >
< td > < / td >
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< / tr >
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< / table >
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< / blockquote >
< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" > Now edit your
/etc/shorewall/rules file to add or delete other connections as required.< / div >
< div align = "left" >
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< h2 align = "left" > Starting and Stopping Your Firewall< / h2 >
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< / div >
< div align = "left" >
< p align = "left" > The < a href = "Install.htm" > installation procedure < / a >
configures your system to start Shorewall at system boot.< / 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" .< / div >
< div align = "left" >
< p align = "left" > < img border = "0" src = "images/BD21298_.gif" width = "13" height = "13" > The two-interface sample assumes that you want to enable
routing to/from < b > eth1 < / b > (the local network) when Shorewall is stopped. If
your local network isn't connected to < b > eth1< / b > or if you wish to enable
access to/from other hosts, change /etc/shorewall/routestopped accordingly.< / 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 > .< / div >
< p align = "left" > < font size = "2" > Last updated
7/26/2002 - < 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 >
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< / body >
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< / html >
