Shorewall Setup Guide
Tom
Eastep
2001-2005
Thomas M. Eastep
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
GNU Free Documentation
License
.
This article applies to Shorewall 3.0 and
later. If you are running a version of Shorewall earlier than Shorewall
3.0.0 then please see the documentation for that
release.
Introduction
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 single-address guides.
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.
Shorewall requires that the iproute/iproute2 package be 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 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.
Windows
Version of dos2unix
Linux Version
of dos2unix
Shorewall Concepts
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 Shorewall Installation Process.
Note to Debian Users
If you install using the .deb, you will find that your /etc/shorewall directory is almost empty.
This is intentional. The released configuration file skeletons may be
found on your system in the directory /usr/share/doc/shorewall/default-config.
Simply copy the files you need from that directory to /etc/shorewall and modify the
copies.
As each file is introduced, I suggest that you look through the
actual file on your system -- each file contains detailed configuration
instructions.
Shorewall views the network where it is running as being composed of
a set of zones. A zone is one or more hosts, which can be defined as
individual hosts or networks in /etc/shorewall/hosts, or as an entire
interface in /etc/shorewall/interfaces. In this guide, we
will use the following zones:
fw
The firewall system itself.
net
The public Internet.
loc
A private local network using private IP addresses.
dmz
A Demilitarized Zone holding publicly-accessible
servers.
Zones are defined in the file /etc/shorewall/zones.
The /etc/shorewall/zones file included in the
release is empty. You can create the standard set of zones described
above by copying and pasting the following into the file:
#ZONE TYPE OPTIONS
fw firewall
net ipv4
loc ipv4
dmz ipv4
Note that Shorewall recognizes the firewall system as its own zone -
The above example follows the usual convention of naming the Firewall zone
fw. The name specified for the firewall
zone (fw in the above example) is stored
in the shell variable $FW when the
/etc/shorewall/zones file is processed. With the exception of the name
assigned to the firewall zone, 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
.
Edit the /etc/shorewall/zones file and make any changes
necessary.
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.
Shorewall is built on top of the Netfilter kernel facility.
Netfilter implements a connection
tracking function that allows what is often referred to as
stateful inspection of packets. This stateful property allows firewall
rules to be defined in terms of connections rather than in terms of
packets. With Shorewall, you:
Identify the source (client) zone.
Identify destination (server) zone.
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.
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.
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
DOES NOT mean that these connections are allowed
from zone A to zone B (in other words, policies and rules
involving the firewall zone are not transitive). 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.
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 after the request is passed to the appropriate default action (if any).
Prior to Shorewall 2.2.0, the default
/etc/shorewall/policy file had the following
policies:
#SOURCE DEST POLICY LOGLEVEL LIMIT
loc net ACCEPT
net all DROP info
all all REJECT info
The currently released policy file is empty. You can copy and
paste the above entries to create a starting point from which to
customize your policies.
The above policies 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 and log a message at the info level (here is a description of log
levels).
reject all other connection requests and log a message at the
info 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.
At this point, edit your /etc/shorewall/policy
and make any changes that you wish.
Network Interfaces
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.
In this diagram:
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.
The Local Zone consists of systems Local 1, Local 2 and Local
3.
All systems from the ISP outward comprise the Internet
Zone.
![]()
The simplest way to define zones is to associate the zone name
(previously defined in /etc/shorewall/zones) with a network interface.
This is done in the /etc/shorewall/interfaces
file. The firewall illustrated above has three 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 using ISDN, you
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 Local Interface 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
cross-over cable).
Your DMZ Interface 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 cross-over cable).
Do not connect the internal and external interface to the same hub
or switch except for testing. You can test using this kind of
configuration if you specify the arp_filter option or the arp_ignore 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.
For the remainder of this Guide, we will assume that:
The External Interface is eth0.
The Local Interface eth1.
The DMZ Interface eth2.
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:
?FORMAT 2
#ZONE INTERFACE OPTIONS
net eth0
loc eth1
dmz eth2
Note that the $FW zone has no entry
in the /etc/shorewall/interfaces file.
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.
Multiple Interfaces to a Zone
?FORMAT 2
#ZONE INTERFACE BROADCAST OPTIONS
net eth0
loc eth1
loc eth2
You may define more complicated zones using the /etc/shorewall/hosts
file but in most cases, that isn't necessary. See Shorewall_and_Aliased_Interfaces.html
and Multiple_Zones.html for
examples.
Addressing, Subnets and Routing
Normally, your ISP will assign you a set of Public 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.
If you are thoroughly familiar with IP addressing and routing, you
may go to the next section.
The following discussion barely scratches the surface of addressing
and routing. If you are interested in learning more about this subject, I
highly recommend IP Fundamentals: What Everyone Needs to
Know about Addressing & Routing
, Thomas A. Maufer,
Prentice-Hall, 1999, ISBN 0-13-975483-0.
IP Addresses
IP version 4 (IPv4) 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:
C0.00.02.0Eor looking at it as a
32-bit integer
C000020E
Subnets
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):
Class A - netmask 255.0.0.0, size = 2 ** 24
Class B - netmask 255.255.0.0, size = 2 ** 16
Class C - netmask 255.255.255.0, size = 256
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 netmask. The netmask is a number
that when logically ANDed with an address isolates the network number;
the remainder of the address is the host number. For example, in the
Class C address 192.0.2.14, the network number is hex C00002 and the
host number is hex 0E.
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
subnetting these networks into smaller subnetworks evolved; that
technique is referred to as Classless InterDomain
Routing (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.
A subnetwork (often referred to as a
subnet) is a contiguous set of IP addresses such
that:
The number of addresses in the set is a power of 2; and
The first address in the set is a multiple of the set
size.
The first address in the subnet is reserved and is referred to
as the subnet address.
The last address in the subnet is reserved as the subnet's
broadcast address.
As you can see by this definition, in each subnet of size n there
are (n - 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.
Since n is a power of two, we can easily calculate the
Base-2 Logarithm (log2) of n. For the more common
subnet sizes, the size and its base-2 logarithm are given in the
following table:
Base-2 Logarithms
n
log2 n
(32 - log2 n)
8
3
29
16
4
28
32
5
27
64
6
26
128
7
25
256
8
24
512
9
23
1024
10
22
2048
11
21
4096
12
20
8192
13
19
16384
14
18
32768
15
17
65536
16
16
You will notice that the above table also contains a column for
(32 - log2 n). That number is the
Variable Length Subnet Mask (VLSM) for a network of
size n. From the above table, we can derive the following one which is a
little easier to use.
VLSM
Subnet Size
VLSM
Subnet Mask
8
/29
255.255.255.248
16
/28
255.255.255.240
32
/27
255.255.255.224
64
/26
255.255.255.192
128
/25
255.255.255.128
256
/24
255.255.255.0
512
/23
255.255.254.0
1024
/22
255.255.252.0
2048
/21
255.255.248.0
4096
/20
255.255.240.0
8192
/19
255.255.224.0
16384
/18
255.255.192.0
32768
/17
255.255.128.0
65536
/16
255.255.0.0
2 ** 24
/8
255.0.0.0
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
.
The subnet's mask (also referred to as its
netmask) 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:
11111111111111111111111111000000 = FFFFFFC0 = FF.FF.FF.C0 = 255.255.255.192The
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.
For a subnetwork whose address is a.b.c.d and whose Variable Length Subnet Mask is
/v, we denote the subnetwork as
a.b.c.d/v
using
CIDR Notation. Example:
Subnet
Subnet:
10.10.10.0 - 10.10.10.127
Subnet Size:
128
Subnet Address:
10.10.10.0
Broadcast
Address:
10.10.10.127
CIDR Notation:
10.10.10.0/25
There are two degenerate subnets that need mentioning; namely, the
subnet with one member and the subnet with 2 ** 32 members.
/32 and /0
Subnet Size
VLSM Length
Subnet Mask
CIDR Notation
1
32
255.255.255.255
a.b.c.d/32
32
0
0.0.0.0
0.0.0.0/0
So any address a.b.c.d may also
be written a.b.c.d/32 and the set of
all possible IP addresses is written 0.0.0.0/0.
A Shorewall user has contributed a useful
graphical summary of the above information.
Later in this guide, you will see the notation a.b.c.d/v 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 a.b.c.d and with the netmask
that corresponds to VLSM /v.
192.0.2.65/29
The interface is configured with IP address 192.0.2.65 and
netmask 255.255.255.248.
/sbin/shorewall supports an ipcalc command that automatically
calculates information about a [sub]network.
Using the <command>ipcalc </command>command
shorewall ipcalc 10.10.10.0/25
CIDR=10.10.10.0/25
NETMASK=255.255.255.128
NETWORK=10.10.10.0
BROADCAST=10.10.10.127
Using the <command>ipcalc</command> command
shorewall ipcalc 10.10.10.0 255.255.255.128
CIDR=10.10.10.0/25
NETMASK=255.255.255.128
NETWORK=10.10.10.0
BROADCAST=10.10.10.127
Routing
One of the purposes of subnetting is that it forms the basis for
routing. Here's the routing table on my firewall (compressed for
PDF):
[root@gateway root]# netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flgs MSS Win irtt Iface
192.168.9.1 0.0.0.0 255.255.255.255 UH 40 0 0 texas
206.124.146.177 0.0.0.0 255.255.255.255 UH 40 0 0 eth1
206.124.146.180 0.0.0.0 255.255.255.255 UH 40 0 0 eth3
192.168.3.0 0.0.0.0 255.255.255.0 U 40 0 0 eth3
192.168.2.0 0.0.0.0 255.255.255.0 U 40 0 0 eth1
192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2
206.124.146.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
192.168.9.0 192.0.2.223 255.255.255.0 UG 40 0 0 texas
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 206.124.146.254 0.0.0.0 UG 40 0 0 eth0
[root@gateway root]#
The device texas is a GRE tunnel to a peer
site in the Dallas, Texas area.
The first three routes are host routes 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 default route and the gateway mentioned in that
route is called the default gateway.
When the kernel is trying to send a packet to IP address A, it starts at the top of the routing table
and:
A is logically ANDed with the
Genmask
value in the table entry.
The result is compared with the Destination
value in the table entry.
If the result and the Destination
value are the
same, then:
If the Gateway
column is non-zero, the
packet is sent to the gateway over the interface named in the
Iface
column.
Otherwise, the packet is sent directly to A over the interface named in the
iface
column.
Otherwise, the above steps are repeated on the next entry in
the table.
Since the default route matches any IP address (A 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 default
gateway which is usually a router at your ISP. 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:
192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth2
So to route a packet to 192.168.1.5, the packet is sent directly
over eth2.
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 misconception 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.
Address Resolution Protocol (ARP)
When sending packets over Ethernet, IP addresses aren't used.
Rather Ethernet addressing is based on Media Access
Control (MAC) addresses. Each Ethernet device has its own
unique 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:
[root@gateway root]# ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc htb qlen 100
link/ether 02:00:08:e3:fa:55 brd ff:ff:ff:ff:ff:ff
inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0
inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0
inet 206.124.146.179/24 brd 206.124.146.255 scope global secondary eth0
[root@gateway root]#
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. 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 Address Resolution
Protocol (ARP). Here is ARP in action:
[root@gateway root]# tcpdump -nei eth2 arp
tcpdump: listening on eth2
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
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
2 packets received by filter
0 packets dropped by kernel
[root@gateway root]#
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.
In order to avoid having to exchange ARP information each time
that an IP packet is to be sent, systems maintain an ARP
cache of IP<->MAC correspondences. You can see the ARP
cache on your system (including your Windows system) using the
arp
command:
[root@gateway root]# arp -na
? (206.124.146.177) at 00:A0:C9:15:39:78 [ether] on eth1
? (192.168.1.3) at 00:A0:CC:63:66:89 [ether] on eth2
? (192.168.1.5) at 00:A0:CC:DB:31:C4 [ether] on eth2
? (206.124.146.254) at 00:03:6C:8A:18:38 [ether] on eth0
? (192.168.1.19) at 00:06:25:AA:8A:F0 [ether] on eth2
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 forgo 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.
RFC 1918
IP addresses are allocated by the Internet Assigned Number Authority
(IANA) who delegates allocations on a geographic basis to Regional
Internet Registries (RIRs). For example, allocation for the Americas and
for sub-Sahara Africa is delegated to the American Registry for Internet
Numbers (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. 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 Private IP addresses. RFC 1918 reserves
several 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
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. This is understandable given that anyone can select any of
these addresses for their private use but the term non-routable is
somewhat unfortunate because it leads people to the erroneous conclusion
that traffic destined for one of these addresses can't be sent through a
router. This is definitely not true; private routers (including your
Shorewall-based firewall) can forward RFC 1918 addressed traffic just
fine.
When selecting addresses from these ranges, there's a couple of
things to keep in mind:
As the IPv4 address space becomes depleted, more and more
organizations (including ISPs) are beginning to use RFC 1918
addresses in their infrastructure.
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.
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.
In this document, external
real
IP addresses are of the form 192.0.2.x.
192.0.2.0/24 is reserved by RFC 3330 for use as public IP addresses in
printed examples and test networks. These "real" addresses are not to
be confused with addresses in 192.168.0.0/16; as described above,
those addresses are reserved by RFC 1918 for private
use.
Setting Up Your Network
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:
Routed - Traffic to any of your
addresses will be routed through a single gateway address. 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.
Non-routed - Your ISP will send
traffic to each of your addresses directly.
In the subsections that follow, we'll look at each of these
separately.
Before we begin, there is one thing for you to check:
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:
IP_FORWARDING=On
Routed
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.
![]()
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.
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.
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:
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.0.2.64 0.0.0.0 255.255.255.248 U 40 0 0 eth0
0.0.0.0 192.0.2.66 0.0.0.0 UG 40 0 0 eth0
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 DMZ
Interface!! DMZ 1 can then send Ethernet frames addressed to
that MAC address and the frames will be received (correctly) by the
firewall/router.
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.
Non-routed
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.
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).
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.
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.
Source Network Address Translation
(SNAT).
Destination Network Address Translation
(DNAT) also known as Port Forwarding.
Proxy ARP.
Network Address Translation (NAT) also
referred to as One-to-one NAT.
Often a combination of these techniques is used. Each of these
will be discussed in the sections that follow.
SNAT
With SNAT, an internal LAN segment is configured using RFC 1918
addresses. When a host A on this
internal segment initiates a connection to host 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 responds
and the response is received by the firewall, the firewall changes the
destination address back to the RFC 1918 address of A and forwards the response back to A.
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.
![]()
The local zone has been subnetted as 192.168.201.0/29 (netmask
255.255.255.248).
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).
SNAT is configured in Shorewall using the /etc/shorewall/masq
file.
#INTERFACE SUBNET ADDRESS
eth0 192.168.201.0/29 192.0.2.176
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.
DNAT
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.
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
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
DNAT net loc:192.168.201.4 tcp www
If one of your daughter's friends at address A wants to access your daughter's server, she
can connect to http://192.0.2.176 (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 A.
This example used the firewall's external IP address for DNAT.
You can use another of your public IP addresses (place it in the
ORIGDEST column in the rule above) but Shorewall will not add that
address to the firewall's external interface for you.
When testing DNAT rules like those shown above, you must test
from a client OUTSIDE YOUR FIREWALL (in the 'net' zone). You cannot
test these rules from inside the firewall!
For DNAT troubleshooting tips, see
FAQs 1a and 1b.
Proxy ARP
The idea behind Proxy ARP is that:
A host H behind your
firewall is assigned one of your public IP addresses (A), and is assigned the same netmask
(M) as the firewall's external
interface.
The firewall responds to ARP who has
requests
for A from machines outside of
the firewall.
When H issues an ARP
who has
request for a machine with an address in
the network defined by M where
the target machine is outside of the firewall, the firewall will
respond to H (with the MAC of the
firewall interface that H is
connected to).
For a more complete description of how Proxy ARP works, please
see the Shorewall Proxy
Documentation.
Let us suppose that we decide to use Proxy ARP on the DMZ in our
example network.
![]()
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.
The Shorewall configuration of Proxy ARP is done using the/etc/shorewall/proxyarp
file.
#ADDRESS INTERFACE EXTERNAL HAVEROUTE PERSISTENT
192.0.2.177 eth2 eth0 No
192.0.2.178 eth2 eth0 No
Because the HAVE ROUTE column contains No, Shorewall will add
host routes thru eth2 to 192.0.2.177 and 192.0.2.178. The Ethernet
interfaces on DMZ 1 and DMZ 2 should be configured to have the IP
addresses shown but should have the same default gateway as the
firewall itself -- namely 192.0.2.254. In other words, they should be
configured just like they would be if they were parallel to the
firewall rather than behind it.
Do not add the Proxy ARP'ed address(es)
(192.0.2.177 and 192.0.2.178 in the above example) to the external
interface (eth0 in this example) of the firewall.
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. There are a couple of things that you can try:
(Courtesy of Bradey Honsinger) A reading of Stevens' TCP/IP
Illustrated, Vol 1 reveals that a
gratuitous
ARP packet should cause the
ISP's router to refresh their ARP cache (section 4.7). A
gratuitous ARP is simply a host requesting the MAC address for
its own IP; in addition to ensuring that the IP address isn't a
duplicate,...
if the host sending the gratuitous ARP has just
changed its hardware address..., this packet causes any other
host...that has an entry in its cache for the old hardware
address to update its ARP cache entry
accordingly.
Which is, of course, exactly what you want to do when you
switch a host from being exposed to the Internet to behind
Shorewall using proxy ARP (or one-to-one NAT for that matter).
Happily enough, recent versions of Redhat's iputils package
include arping
, whose -U
flag does
just that:
arping -U -I <net if> <newly proxied IP>
arping -U -I eth0 66.58.99.83 # for exampleStevens
goes on to mention that not all systems respond correctly to
gratuitous ARPs, but googling for arping -U
seems
to support the idea that it works most of the time.
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:
tcpdump -nei eth0 icmp
Now from 192.0.2.177, ping the ISP's gateway (which we will
assume is 192.0.2.254):
ping 192.0.2.254
We can now observe the tcpdump output:
13:35:12.159321 0:4:e2:20:20:33 0:0:77:95:dd:19 ip 98:
192.0.2.177 > 192.0.2.254: icmp: echo request (DF)
13:35:12.207615 0:0:77:95:dd:19 0:c0:a8:50:b2:57 ip 98:
192.0.2.254 > 192.0.2.177 : icmp: echo replyNotice
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.
One-to-one NAT
With one-to-one 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 (Source Network Address
Translation) occurs and on incoming connections DNAT (Destination
Network Address Translation) occurs. Let's go back to our earlier
example involving your daughter's web server running on system Local
3.
![]()
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:
#INTERFACE SUBNET ADDRESS
eth0 192.168.201.0/29 192.0.2.176
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 /etc/shorewall/nat.
#EXTERNAL INTERFACE INTERNAL ALLINTS LOCAL
192.0.2.179 eth0 192.168.201.4 No No
With this entry in place, you daughter has her own IP address
and the other two local systems share the firewall's IP
address.
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:
#ACTION SOURCE DEST PROTO DEST SPORT ORIGDEST
ACCEPT net loc:192.168.201.4 tcp www
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 one-to-one NAT,
it will probably be HOURS before that system can communicate with the
Internet. There are a couple of things that you can try:
(Courtesy of Bradey Honsinger) A reading of Stevens' TCP/IP
Illustrated, Vol 1 reveals that a
gratuitous
ARP packet should cause the
ISP's router to refresh their ARP cache (section 4.7). A
gratuitous ARP is simply a host requesting the MAC address for
its own IP; in addition to ensuring that the IP address isn't a
duplicate,...
if the host sending the gratuitous ARP has just
changed its hardware address..., this packet causes any other
host...that has an entry in its cache for the old hardware
address to update its ARP cache entry
accordingly.
Which is, of course, exactly what you want to do when you
switch a host from being exposed to the Internet to behind
Shorewall using one-to-one NAT. Happily enough, recent versions of
Redhat's iputils package include arping
, whose
-U
flag does just that:
arping -U -I <net if> <newly proxied IP>
arping -U -I eth0 66.58.99.83 # for exampleStevens
goes on to mention that not all systems respond correctly to
gratuitous ARPs, but googling for arping -U
seems
to support the idea that it works most of the time.
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:
tcpdump -nei eth0 icmp
Now from 192.0.2.177, ping the ISP's gateway (which we will
assume is 192.0.2.254):
ping 192.0.2.254
We can now observe the tcpdump output:
13:35:12.159321 0:4:e2:20:20:33 0:0:77:95:dd:19 ip 98:
192.0.2.177 > 192.0.2.254: icmp: echo request (DF)
13:35:12.207615 0:0:77:95:dd:19 0:c0:a8:50:b2:57 ip 98:
192.0.2.254 > 192.0.2.177 : icmp: echo replyNotice
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.
Rules
Shorewall has a macro facility
that includes macros for many standard applications. This section does
not use those macros but rather defines the rules directly.
With the default policies described earlier in this document, your
local systems (Local 1-3) can access any server on the Internet and the
DMZ can't access any other host (including the firewall). With the
exception of DNAT rules 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.
Since the SPORT and ORIGDEST. Columns aren't used in this
section, they won't be shown
You probably want to allow ping between your zones:
#ACTION SOURCE DEST PROTO DPORT
ACCEPT net dmz icmp echo-request
ACCEPT net loc icmp echo-request
ACCEPT dmz loc icmp echo-request
ACCEPT loc dmz icmp echo-request
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:
#ACTION SOURCE DEST PROTO DPORT
ACCEPT net dmz:192.0.2.178 tcp smtp #Mail from
#Internet
ACCEPT net dmz:192.0.2.178 tcp pop3 #Pop3 from
#Internet
ACCEPT loc dmz:192.0.2.178 tcp smtp #Mail from local
#Network
ACCEPT loc dmz:192.0.2.178 tcp pop3 #Pop3 from local
#Network
ACCEPT $FW dmz:192.0.2.178 tcp smtp #Mail from the
#Firewall
ACCEPT dmz:192.0.2.178 net tcp smtp #Mail to the
#Internet
ACCEPT net dmz:192.0.2.177 tcp http #WWW from
#Internet
ACCEPT net dmz:192.0.2.177 tcp https #Secure WWW
#from Internet
ACCEPT loc dmz:192.0.2.177 tcp https #Secure WWW
#from local
#Network
If you run a public DNS server on 192.0.2.177, you would need to
add the following rules:
#ACTION SOURCE DEST PROTO DPORT
ACCEPT net dmz:192.0.2.177 udp domain #UDP DNS from
#Internet
ACCEPT net dmz:192.0.2.177 tcp domain #TCP DNS from
#Internet
ACCEPT loc dmz:192.0.2.177 udp domain #UDP DNS from
#Local Network
ACCEPT loc dmz:192.0.2.177 tcp domain #TCP DNS from
#Local Network
ACCEPT $FW dmz:192.0.2.177 udp domain #UDP DNS from
#the Firewall
ACCEPT $FW dmz:192.0.2.177 tcp domain #TCP DNS from
#the Firewall
ACCEPT dmz:192.0.2.177 net udp domain #UDP DNS to
#the Internet
ACCEPT dmz:192.0.2.177 net tcp domain #TCPP DNS to
#the Internet
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.
#ACTION SOURCE DEST PROTO DPORT
ACCEPT loc dmz tcp ssh #SSH to the DMZ
ACCEPT net $FW tcp ssh #SSH to the
#Firewall
Odds and Ends
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 its own public IP.
If you haven't already, it would be a good idea to browse through
/etc/shorewall/shorewall.conf
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.
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.
/etc/shorewall/interfaces (The
options
will be very site-specific).
?FORMAT 2
#ZONE INTERFACE OPTIONS
net eth0 routefilter
loc eth1
dmz eth2
/etc/shorewall/masq - Local Subnet
#INTERFACE SUBNET ADDRESS
eth0 192.168.201.0/29 192.0.2.176
/etc/shorewall/proxyarp - DMZ
#ADDRESS EXTERNAL INTERFACE HAVE ROUTE
192.0.2.177 eth2 eth0 No
192.0.2.178 eth2 eth0 No
/etc/shorewall/nat- Daughter's System
#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL
192.0.2.179 eth0 192.168.201.4 No No
/etc/shorewall/rules
#ACTION SOURCE DEST PROTO DPORT
ACCEPT net dmz icmp echo-request
ACCEPT net loc icmp echo-request
ACCEPT dmz loc icmp echo-request
ACCEPT loc dmz icmp echo-request
ACCEPT net loc:192.168.201.4 tcp www #Daughter's
#Server
ACCEPT net dmz:192.0.2.178 tcp smtp #Mail from
#Internet
ACCEPT net dmz:192.0.2.178 tcp pop3 #Pop3 from
#Internet
ACCEPT loc dmz:192.0.2.178 tcp smtp #Mail from local
#Network
ACCEPT loc dmz:192.0.2.178 tcp pop3 #Pop3 from local
#Network
ACCEPT $FW dmz:192.0.2.178 tcp smtp #Mail from the
#Firewall
ACCEPT dmz:192.0.2.178 net tcp smtp #Mail to the
#Internet
ACCEPT net dmz:192.0.2.177 tcp http #WWW from
#Internet
ACCEPT net dmz:192.0.2.177 tcp https #Secure WWW
#from Internet
ACCEPT loc dmz:192.0.2.177 tcp https #Secure WWW
#from local
#Network
ACCEPT net dmz:192.0.2.177 udp domain #UDP DNS from
#Internet
ACCEPT net dmz:192.0.2.177 tcp domain #TCP DNS from
#Internet
ACCEPT loc dmz:192.0.2.177 udp domain #UDP DNS from
#Local Network
ACCEPT loc dmz:192.0.2.177 tcp domain #TCP DNS from
#Local Network
ACCEPT $FW dmz:192.0.2.177 udp domain #UDP DNS from
#the Firewall
ACCEPT $FW dmz:192.0.2.177 tcp domain #TCP DNS from
#the Firewall
ACCEPT dmz:192.0.2.177 net udp domain #UDP DNS to
#the Internet
ACCEPT dmz:192.0.2.177 net tcp domain #TCPP DNS to
#the Internet
ACCEPT loc dmz tcp ssh #SSH to the DMZ
ACCEPT net $FW tcp ssh #SSH to the
#Firewall
DNS
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 Views. If you
are not interested in Bind 9 views, you can go to the next section.
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 its 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.
The /etc/named.conf file would look like
this:
options {
directory "/var/named";
listen-on { 127.0.0.1 ; 192.0.2.177; };
transfer-format many-answers;
max-transfer-time-in 60;
allow-transfer {
// Servers allowed to request zone transfers
<secondary NS IP>; };
};
logging {
channel xfer-log {
file "/var/log/named/bind-xfer.log";
print-category yes;
print-severity yes;
print-time yes;
severity info;
};
category xfer-in { xfer-log; };
category xfer-out { xfer-log; };
category notify { xfer-log; };
};
#
# This is the view presented to our internal systems
#
view "internal" {
#
# These are the clients that see this view
#
match-clients { 192.168.201.0/29;
192.168.202.0/29;
127.0.0.0/8;
192.0.2.176/32;
192.0.2.178/32;
192.0.2.179/32;
192.0.2.180/32; };
#
# If this server can't complete the request, it should use
# outside servers to do so
#
recursion yes;
zone "." in {
type hint;
file "int/root.cache";
};
zone "foobar.net" in {
type master;
notify no;
allow-update { none; };
file "int/db.foobar";
};
zone "0.0.127.in-addr.arpa" in {
type master;
notify no;
allow-update { none; };
file "int/db.127.0.0";
};
zone "201.168.192.in-addr.arpa" in {
type master;
notify no;
allow-update { none; };
file "int/db.192.168.201";
};
zone "202.168.192.in-addr.arpa" in {
type master;
notify no;
allow-update { none; };
file "int/db.192.168.202";
};
zone "176.2.0.192.in-addr.arpa" in {
type master;
notify no;
allow-update { none; };
file "db.192.0.2.176";
};
zone "177.2.0.192.in-addr.arpa" in {
type master;
notify no;
allow-update { none; };
file "db.192.0.2.177";
};
zone "178.2.0.192.in-addr.arpa" in {
type master;
notify no;
allow-update { none; };
file "db.192.0.2.178";
};
zone "179.2.0.192.in-addr.arpa" in {
type master;
notify no;
allow-update { none; };
file "db.206.124.146.179";
};
};
#
# This is the view that we present to the outside world
#
view "external" {
match-clients { any; };
#
# If we can't answer the query, we tell the client so
#
recursion no;
zone "foobar.net" in {
type master;
notify yes;
allow-update {none; };
file "ext/db.foobar";
};
zone "176.2.0.192.in-addr.arpa" in {
type master;
notify yes;
allow-update { none; };
file "db.192.0.2.176";
};
zone "177.2.0.192.in-addr.arpa" in {
type master;
notify yes;
allow-update { none; };
file "db.192.0.2.177";
};
zone "178.2.0.192.in-addr.arpa" in {
type master;
notify yes;
allow-update { none; };
file "db.192.0.2.178";
};
zone "179.2.0.192.in-addr.arpa" in {
type master;
notify yes;
allow-update { none; };
file "db.192.0.2.179";
};
};
Here are the files in /var/named (those not shown are usually
included in your bind distribution).
db.192.0.2.176 - This is the reverse zone for
the firewall's external interface
; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.176/32
; Filename: db.192.0.2.176
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS <name of secondary ns>.
;
; ############################################################
; Inverse Address Arpa Records (PTR's)
; ############################################################
176.2.0.192.in-addr.arpa. 86400 IN PTR firewall.foobar.net.
db.192.0.2.177 - Reverse zone www server
; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.177/32
; Filename: db.192.0.2.177
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS <name of secondary ns>.
;
; ############################################################
; Inverse Address Arpa Records (PTR's)
; ############################################################
177.2.0.192.in-addr.arpa. 86400 IN PTR www.foobar.net.
db.192.0.2.178 - Reverse zone for the mail
server
; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.178/32
; Filename: db.192.0.2.178
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS <name of secondary ns>.
;
; ############################################################
; Inverse Address Arpa Records (PTR's)
; ############################################################
178.2.0.192.in-addr.arpa. 86400 IN PTR mail.foobar.net.
db.192.0.2.179 - Reverse zone for Daughter's
public web server
; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.0.2.179/32
; Filename: db.192.0.2.179
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001102303 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
;
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
@ 604800 IN NS <name of secondary ns>.
;
; ############################################################
; Inverse Address Arpa Records (PTR's)
; ############################################################
179.2.0.192.in-addr.arpa. 86400 IN PTR nod.foobar.net.
int/db.127.0.0 - Reverse zone for
localhost
; ############################################################
; Start of Authority (Inverse Address Arpa) for 127.0.0.0/8
; Filename: db.127.0.0
; ############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2001092901 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
; ############################################################
; Inverse Address Arpa Records (PTR's)
; ############################################################
1 86400 IN PTR localhost.foobar.net.
int/db.192.168.201 - Reverse zone for the local
network. This is only shown to internal clients.
; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.168.201.0/29
; Filename: db.192.168.201
; ############################################################
@ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (
2002032501 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
; ############################################################
; Inverse Address Arpa Records (PTR's)
; ############################################################
1 86400 IN PTR gateway.foobar.net.
2 86400 IN PTR winken.foobar.net.
3 86400 IN PTR blinken.foobar.net.
4 86400 IN PTR nod.foobar.net.
int/db.192.168.202 - Reverse zone for the
firewall's DMZ Interface
; ############################################################
; Start of Authority (Inverse Address Arpa) for 192.168.202.0/29
; Filename: db.192.168.202
; ############################################################
@ 604800 IN SOA ns1.foobar.net netadmin.foobar.net. (
2002032501 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ) ; minimum (1 day)
; ############################################################
; Specify Name Servers for all Reverse Lookups (IN-ADDR.ARPA)
; ############################################################
@ 604800 IN NS ns1.foobar.net.
; ############################################################
; Inverse Address Arpa Records (PTR's)
; ############################################################
1 86400 IN PTR dmz.foobar.net.
int/db.foobar - Forward zone for internal
clients.
;##############################################################
; Start of Authority for foobar.net.
; Filename: db.foobar
;##############################################################
@ 604800 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2002071501 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ); minimum (1 day)
;############################################################
; foobar.net Nameserver Records (NS)
;############################################################
@ 604800 IN NS ns1.foobar.net.
;############################################################
; Foobar.net Office Records (ADDRESS)
;############################################################
localhost 86400 IN A 127.0.0.1
firewall 86400 IN A 192.0.2.176
www 86400 IN A 192.0.2.177
ns1 86400 IN A 192.0.2.177
mail 86400 IN A 192.0.2.178
gateway 86400 IN A 192.168.201.1
winken 86400 IN A 192.168.201.2
blinken 86400 IN A 192.168.201.3
nod 86400 IN A 192.168.201.4
dmz 86400 IN A 192.168.202.1
ext/db.foobar - Forward zone for external
clients.
;##############################################################
; Start of Authority for foobar.net.
; Filename: db.foobar
;##############################################################
@ 86400 IN SOA ns1.foobar.net. netadmin.foobar.net. (
2002052901 ; serial
10800 ; refresh (3 hour)
3600 ; retry (1 hour)
604800 ; expire (7 days)
86400 ); minimum (1 day)
;############################################################
; Foobar.net Nameserver Records (NS)
;############################################################
@ 86400 IN NS ns1.foobar.net.
@ 86400 IN NS <secondary NS>.
;############################################################
; Foobar.net Foobar Wa Office Records (ADDRESS)
;############################################################
localhost 86400 IN A 127.0.0.1
;
; The firewall itself
;
firewall 86400 IN A 192.0.2.176
;
; The DMZ
;
ns1 86400 IN A 192.0.2.177
www 86400 IN A 192.0.2.177
mail 86400 IN A 192.0.2.178
;
; The Local Network
;
nod 86400 IN A 192.0.2.179
;############################################################
; Current Aliases for foobar.net (CNAME)
;############################################################
;############################################################
; foobar.net MX Records (MAIL EXCHANGER)
;############################################################
foobar.net. 86400 IN A 192.0.2.177
86400 IN MX 0 mail.foobar.net.
86400 IN MX 1 <backup MX>.
Some Things to Keep in Mind
You cannot test your firewall from the
inside. Just because you send requests to your firewall
external IP address does not mean that the request will be associated
with the external interface or the net
zone. Any
traffic that you generate from the local network will be associated
with your local interface and will be treated as loc->$FW
traffic.
IP addresses are properties of systems,
not of interfaces. It is a mistake to believe that your
firewall is able to forward packets just because you can ping the IP
address of all of the firewall's interfaces from the local network.
The only conclusion you can draw from such pinging success is that the
link between the local system and the firewall works and that you
probably have the local system's default gateway set correctly.
All IP addresses configured on firewall
interfaces are in the $FW (fw) zone. If 192.168.1.254 is
the IP address of your internal interface then you can write
$FW:192.168.1.254
in a
rule but you may not write loc:192.168.1.254
. Similarly, it is
nonsensical to add 192.168.1.254 to the loc zone using an entry in
/etc/shorewall/hosts.
Reply packets do NOT automatically follow
the reverse path of the one taken by the original request.
All packets are routed according to the routing table of the host at
each step of the way. This issue commonly comes up when people install
a Shorewall firewall parallel to an existing gateway and try to use
DNAT through Shorewall without changing the default gateway of the
system receiving the forwarded requests. Requests come in through the
Shorewall firewall where the destination IP address gets rewritten but
replies go out unmodified through the old gateway.
Shorewall itself has no notion of inside
or outside. These concepts are embodied in how Shorewall is
configured.
Starting and Stopping the Firewall
The Installation procedure
configures your system to start Shorewall at system boot.
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 ACCEPT entry in
/etc/shorewall/stoppedrules.
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
.
Edit the /etc/shorewall/stoppedrules
file and add ACCEPT rules for those systems that you want to be able to
access the firewall when it is stopped.
If you are connected to your firewall from the Internet, do not
issue a shorewall stop
command unless you have added an
ACCEPT entry for the IP address that you are connected from to
/etc/shorewall/stoppedrules.
Also, I don't recommend using shorewall restart
; it is
better to create an an alternate
configuration and test it using the shorewall
try
command.