Basic Two-Interface Firewall
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.
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:
Linux system used as a firewall/router for a small local network.
Single public IP address. If you have more than one public IP address,
this is not the guide you want -- see the Shorewall Setup Guide instead.
Internet connection through cable modem, DSL, ISDN, Frame Relay, dial-up
...
Here is a schematic of a typical installation.
If you are running Shorewall under Mandrake 9.0 or later, you can easily
configure the above setup using the Mandrake "Internet Connection Sharing"
applet. From the Mandrake Control Center, select "Network & Internet" then
"Connection Sharing".
Note however, that the Shorewall configuration produced by Mandrake
Internet Connection Sharing is strange and is apt to confuse you if you
use the rest of this documentation (it has two local zones; "loc" and
"masq" where "loc" is empty; this conflicts with this documentation which
assumes a single local zone "loc"). We therefore recommend that once you
have set up this sharing that you uninstall the Mandrake Shorewall RPM and
install the one from the download page then follow the instructions in
this Guide.
Shorewall requires that you have the iproute/iproute2 package installed
(on RedHat, the package is called iproute). You can tell if this package
is installed by the presence of an ip program on your firewall system. As
root, you can use the 'which' command to check for this program:
[root@gateway root]# which ip /sbin/ip [root@gateway root]#
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 . Configuration notes that are unique to
LEAF/Bering are marked with
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.
Windows Version of dos2unix
Linux Version of dos2unix
PPTP/ADSL
If you have an ADSL Modem and you use PPTP to communicate with a
server in that modem, you must make the changes recommended here in
addition to those detailed below. ADSL with PPTP is most commonly found in
Europe, notably in Austria.
Shorewall Concepts
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 installed Shorewall,
download the two-interface sample, un-tar it (tar -zxvf
two-interfaces.tgz) and and copy the files to /etc/shorewall (these files
will replace files with the same name).
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.
Shorewall views the network where it is running as being composed of a set
of zones. In the two-interface sample configuration, the following zone
names are used:
Name Description
net The Internet
loc Your Local Network
Zones are defined in the /etc/shorewall/zones file.
Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as fw.
Rules about what traffic to allow and what traffic to deny are expressed
in terms of zones.
You express your default policy for connections from one zone to another
zone in the /etc/shorewall/policy file.
You define exceptions to those default policies in the
/etc/shorewall/rules file.
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 if that file exists; otherwise the rules in
/etc/shorewall/common.def are checked.
The /etc/shorewall/policy file included with the two-interface sample has
the following policies:
Source Zone Destination Zone Policy Log Level Limit:Burst
loc net ACCEPT
net all DROP info
all all REJECT info
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.
Source Zone Destination Zone Policy Log Level Limit:Burst
fw net ACCEPT
The above policy will:
allow all connection requests from your local network to the internet
drop (ignore) all connection requests from the internet to your firewall
or local network
optionally accept all connection requests from the firewall to the
internet (if you uncomment the additional policy)
reject all other connection requests.
At this point, edit your /etc/shorewall/policy and make any changes
that you wish.
Network Interfaces
The firewall has two network interfaces. Where Internet connectivity is
through a cable or DSL "Modem", the External Interface will be the
ethernet adapter that is connected to that "Modem" (e.g., eth0) unless
you connect via Point-to-Point Protocol over Ethernet (PPPoE) or
Point-to-Point Tunneling Protocol (PPTP) in which case the External
Interface will be a ppp interface (e.g., ppp0). If you connect via a
regular modem, your External Interface will also be ppp0. If you connect
via ISDN, your external interface will be ippp0.
If your external interface is ppp0 or ippp0 then you will want to
set CLAMPMSS=yes in /etc/shorewall/shorewall.conf.
Your Internal Interface 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
cross-over cable).
Do not connect the internal and external interface to the same hub or
switch except for testing AND you are running Shorewall version 1.4.7 or
later. When using these recent versions, you can test using this kind of
configuration if you specify the arp_filter option in
/etc/shorewall/interfaces for all interfaces connected to the common
hub/switch. Using such a setup with a production firewall is strongly
recommended against.
The Shorewall two-interface sample configuration assumes that the
external interface is eth0 and the internal interface is eth1. If your
configuration is different, you will have to modify the sample
/etc/shorewall/interfaces file accordingly. While you are there, you may
wish to review the list of options that are specified for the interfaces.
Some hints:
If your external interface is ppp0 or ippp0, you can replace the "detect"
in the second column with "-".
If your external interface is ppp0 or ippp0 or if you have a static IP
address, you can remove "dhcp" from the option list.
IP Addresses
Before going further, we should say a few words about Internet Protocol
(IP) addresses. Normally, your ISP will assign you a single Public IP
address. This address may be assigned via the Dynamic Host Configuration
Protocol (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 static IP address; that means that you configure your
firewall's external interface to use that address permanently. 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 Private IP
address ranges for this purpose:
10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
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.
You will want to assign your addresses from the same sub-network (subnet).
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 Subnet Mask of
255.255.255.0. The address x.y.z.0 is reserved as the Subnet Address and
x.y.z.255 is reserved as the Subnet Broadcast Address. In Shorewall, a
subnet is described using Classless InterDomain Routing (CIDR) notation
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.
Example sub-network:
Range: 10.10.10.0 - 10.10.10.255
Subnet Address: 10.10.10.0
Broadcast Address: 10.10.10.255
CIDR Notation: 10.10.10.0/24
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).
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 gateway (router).
Your local computers (computer 1 and computer 2 in the above diagram)
should be configured with their default gateway to be the IP address of
the firewall's internal interface.
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 "IP Fundamentals: What Everyone
Needs to Know about Addressing & Routing", Thomas A. Maufer,
Prentice-Hall, 1999, ISBN 0-13-975483-0.
The remainder of this quide will assume that you have configured your
network as shown here:
The default gateway for computer's 1 & 2 would be 10.10.10.254.
WARNING: Your ISP might assign your external interface an RFC 1918
address. If that address is in the 10.10.10.0/24 subnet then you will need
to select a DIFFERENT RFC 1918 subnet for your local network.
IP Masquerading (SNAT)
The addresses reserved by RFC 1918 are sometimes referred to as
non-routable 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 Network Address Translation (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.
On Linux systems, the above process is often referred to as IP
Masquerading but you will also see the term Source Network Address
Translation (SNAT) used. Shorewall follows the convention used with
Netfilter:
Masquerade describes the case where you let your firewall system
automatically detect the external interface address.
SNAT refers to the case when you explicitly specify the source address
that you want outbound packets from your local network to use.
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.
If your external firewall interface is eth0, 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.
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.
If you are using the Debian package, please check your shorewall.conf
file to ensure that the following are set correctly; if they are not,
change them appropriately:
NAT_ENABLED=Yes (Shorewall versions earlier than 1.4.6)
IP_FORWARDING=On
Port Forwarding (DNAT)
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.
The above process is called Port Forwarding or Destination Network Address
Translation (DNAT). You configure port forwarding using DNAT rules in the
/etc/shorewall/rules file.
The general form of a simple port forwarding rule in /etc/shorewall/rules
is:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net loc: [:]
Example 1 - you run a Web Server on computer 2 and you want to forward
incoming TCP port 80 to that system:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net loc:10.10.10.2 tcp 80
Example 2 - you run an FTP Server on computer 1 so you want to forward
incoming TCP port 21 to that system:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net loc:10.10.10.1 tcp 21
For FTP, you will also need to have FTP connection tracking and NAT
support in your kernel. For vendor-supplied kernels, this means that the
ip_conntrack_ftp and ip_nat_ftp modules must be loaded. Shorewall will
automatically load these modules if they are available and located in the
standard place under /lib/modules//kernel/net/ipv4/netfilter.
A couple of important points to keep in mind:
You must test the above rule from a client outside of your local network
(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 and/or FTP
server from inside your firewall using the IP address of your external
interface, see Shorewall FAQ #2.
Many ISPs block incoming connection requests to port 80. If you have
problems connecting to your web server, try the following rule and try
connecting to port 5000.
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
DNAT net loc:10.10.10.2:80 tcp 5000
At this point, modify /etc/shorewall/rules to add any DNAT rules that
you require.
Domain Name Server (DNS)
Normally, when you connect to your ISP, as part of getting an IP address
your firewall's Domain Name Service (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
name servers for you to manually configure as your primary and secondary
name servers. Regardless of how DNS gets configured on your firewall, it
is your responsibility to configure the resolver in your internal systems.
You can take one of two approaches:
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
"nameserver" records in that file.
You can configure a Caching Name Server on your firewall. 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 firewall;
you do that by adding the following rules in /etc/shorewall/rules.
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT loc fw tcp 53
ACCEPT loc fw udp 53
Other Connections
The two-interface sample includes the following rules:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT fw net tcp 53
ACCEPT fw net udp 53
Those rules allow DNS access from your firewall and may be removed if you
uncommented the line in /etc/shorewall/policy allowing all connections
from the firewall to the internet.
The sample also includes:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT loc fw tcp 22
That rule allows you to run an SSH server on your firewall and connect to
that server from your local systems.
If you wish to enable other connections between your firewall and other
systems, the general format is:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT
Example - You want to run a Web Server on your firewall system:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT net fw tcp 80 #Allow web access from the internet
ACCEPT loc fw tcp 80 #Allow web access from the local network
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"
If you don't know what port and protocol a particular application uses,
look here.
Important: 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:
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT net fw tcp 22
Bering users will want to add the following two rules to be
compatible with Jacques's Shorewall configuration.
ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT loc
fw udp
53
#Allow DNS Cache to work
ACCEPT loc fw tcp 80 #Allow weblet to work
Now edit your /etc/shorewall/rules file to add or delete other
connections as required.
Starting and Stopping Your Firewall
The installation procedure configures your system to start Shorewall
at system boot but beginning with Shorewall version 1.3.9 startup is
disabled so that your system won't try to start Shorewall before
configuration is complete. Once you have completed configuration of your
firewall, you can enable Shorewall startup by removing the file
/etc/shorewall/startup_disabled.
IMPORTANT: Users of the .deb package must edit /etc/default/shorewall and
set 'startup=1'.
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 /etc/shorewall/routestopped. 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".
The two-interface sample assumes that you want to enable routing
to/from eth1 (the local network) when Shorewall is stopped. If your local
network isn't connected to eth1 or if you wish to enable access to/from
other hosts, change /etc/shorewall/routestopped accordingly.
WARNING: 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 /etc/shorewall/routestopped. Also,
I don't recommend using "shorewall restart"; it is better to create an
alternate configuration and test it using the "shorewall try" command.
Additional Recommended Reading
I highly recommend that you review the Common Configuration File Features
page -- it contains helpful tips about Shorewall features than make
administering your firewall easier.
Last updated 11/15/2003 - Tom Eastep
Copyright 2002, 2003 Thomas M. Eastep
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