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:
Here is a schematic of a typical installation.
This guide assumes 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 
.
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.
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.
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).
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:
At this point, edit your /etc/shorewall/policy and make any changes
that you wish.
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 (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.
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.
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.
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.
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:<server local ip address> [:<server port>] <protocol> <port>
Example - 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
A couple of important points to keep in mind:
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.
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
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 <source zone> <destination zone> <protocol> <port>
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
Now edit your /etc/shorewall/rules file to add or delete other
connections as required.
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.
Last updated 11/21/2002 - Tom Eastep
Copyright 2002 Thomas M. Eastep