.1.0 Introduction
2.0 Shorewall Concepts
3.0 Network Interfaces
4.0 Addressing, Subnets and Routing
4.1 IP Addresses
4.2 Subnets
4.3 Routing
4.4 Address Resolution Protocol
4.5 RFC 1918
6.0 DNS
7.0 Starting and Stopping the Firewall
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.
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 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.
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.
As each file is introduced, I suggest that you look through the actual file on your system -- each file contains detailed configuration instructions and some contain default entries.
Shorewall views the network where it is running as being composed of a set of zones. In the default installation, the following zone names are used:
| Name | Description |
| net | The Internet |
| loc | Your Local Network |
| dmz | Demilitarized Zone |
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 but that may be changed in the /etc/shorewall/shorewall.conf file. In this guide, the default name (fw) will be used.
With the exception of fw, 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.
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:
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. 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. If that policy is REJECT or DROP the request is first checked against the rules in /etc/shorewall/common.def.
The default /etc/shorewall/policy file has the following policies:
Source Zone Destination Zone Policy Log Level Limit:Burst loc net ACCEPT net all DROP info all all REJECT info
The above policy will:
At this point, edit your /etc/shorewall/policy and make any changes that you
wish.
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 simplest way to define zones is to simply associate the zone name (previously defined in /etc/shorewall/zones) with a network interface. This is done in the /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 more than one interface
to the same hub or switch (even for testing). It won't work the way that you
expect it to and you will end up confused and believing that Linux networking doesn't work at all.
For the remainder of this Guide, we will assume that:
The external interface is eth0.
The Local interface is eth1.
The DMZ interface is 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:
Zone Interface Broadcast Options net eth0 detect norfc1918 loc eth1 detect dmz eth2 detect
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.
Example:
Zone Interface Broadcast Options net eth0 detect norfc1918 loc eth1 detect loc eth2 detect dhcp
When you have more than one interface to a zone, you will usually want a policy that permits intra-zone traffic:
Source Zone Destination Zone Policy Log Level Limit:Burst loc loc ACCEPT
You may define more complicated zones using the
/etc/shorewall/hosts file but in most
cases, that isn't necessary.
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 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:
C0.00.02.0E
or looking at it as a 32-bit integer
C000020E
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 -- today, any system that you are likely to work with will understand subnetting 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 Natural Logarithm (log2) of n. For the more common subnet sizes, the size and its natural logarithm are given in the following table:
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 for a network of size n. From the above table, we can derive the following one which is a little easier to use.
Size of Subnet 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 = 255.255.255.192
The subnet mask has the property that if you logically AND the subnet mask with an address in the subnet, the result is the subnet address. Just as important, if you logically AND the subnet mask with an address outside the subnet, the result is NOT the subnet address. As we will see below, this property of subnet masks is very useful in routing.
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 VLSM Notation.
Example:
Subnet: 10.10.10.0 - 10.10.10.127 Subnet Size: 128 Subnet Address: 10.10.10.0 Broadcast Address: 10.10.10.127 VLSM 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.
Size of Subnetwork VLSM Length Subnet Mask VLSM Notation 1 32 255.255.255.255 a.b.c.d/32 2 ** 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.
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.
Example: 192.0.2.65/29
The interface is configured with IP address 192.0.2.65 and netmask 255.255.255.248.
One of the purposes of subnetting is that it forms the basis for routing. Here's the routing table on my firewall:
[root@gateway root]# netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window 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.
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 it's 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 forego IP->DNS name translation. Had I not given that option, the question marks would have been replaced with the FQDN corresponding to each IP address. Notice that the last entry in the table records the information we saw using tcpdump above.
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.
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.
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.
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.
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 Static NAT.
Often a combination of these techniques is used. Each of these will be discussed in the sections that follow.
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 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.
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 DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL DESTINATION DNAT net loc:192.168.201.4 tcp www - 192.0.2.176
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 but Shorewall will not add that address to the firewall's external interface for you.
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.
When H issues an ARP "who has" request for an address in the subnetwork defined by A and M, the firewall will respond (with the MAC if the firewall interface to H).
Let suppose that we decide to use Proxy ARP on the DMZ in our example network.
The Shorewall
configuration of Proxy ARP is done using the
/etc/shorewall/proxyarp file.
ADDRESS INTERFACE EXTERNAL HAVE ROUTE 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.
A word of warning is in order here. ISPs typically configure there routers with a long ARP cache timeout. If you move a system from parallel to your firewall to behind your firewall with Proxy ARP, it will probably be HOURS before that system can communicate with the internet. You can call your ISP and ask them to purge the stale ARP cache entry but many either can't or won't purge individual entries. You can determine if your ISP's gateway ARP cache is stale using ping and tcpdump. Suppose that we suspect that the gateway router has a stale ARP cache entry for 192.0.2.177. On the firewall, run tcpdump as follows:
tcpdump -nei eth0 icmp
Now from 192.0.2.177, ping the default gateway (which we are assuming 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 reply
Notice that the source MAC address in the echo request is different from the destination MAC address in the echo reply!! In this case 0:4:e2:20:20:33 was the MAC of the firewall's eth0 NIC while 0:c0:a8:50:b2:57 was the MAC address of DMZ 1. In other words, the gateway's ARP cache still associates 192.0.2.177 with the NIC in DMZ 1 rather than with the firewall's eth0.
With static NAT, you assign local systems RFC 1918 addresses then establish a one-to-one mapping between those addresses and public IP addresses. For outgoing connections SNAT occurs and on incoming connections DNAT occurs. Let's go back to our earlier example involving your daughter's web server running on system Local 3.
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 ALL INTERFACES 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 DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL DESTINATION ACCEPT net loc:192.168.201.4 tcp www
With the default policies, your local systems (Local 1-3) can access any
servers on the internet and the DMZ can't access any other host (including the
firewall). With the exception of 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.
NOTE: Since the SOURCE PORT and ORIG. DEST. Columns aren't used in this section, they won't be shown
You probably want to allow ping between your zones:
ACTION SOURCE DESTINATION PROTOCOL PORT 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 DESTINATION PROTOCOL PORT COMMENTS ACCEPT net dmz:192.0.2.178 tcp smtp # Mail from the Internet ACCEPT net dmz:192.0.2.178 tcp pop3 # Pop3 from the Internet ACCEPT loc dmz:192.0.2.178 tcp smtp # Mail from the Local Network ACCEPT loc dmz:192.0.2.178 tcp pop3 # Pop3 from the 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 the Net ACCEPT net dmz:192.0.2.177 tcp https # Secure HTTP from the Net ACCEPT loc dmz:192.0.2.177 tcp https # Secure HTTP from the Local Net
If you run a public DNS server on 192.0.2.177, you would need to add the following rules:
ACTION SOURCE DESTINATION PROTOCOL PORT COMMENTS ACCEPT net dmz:192.0.2.177 udp domain # UDP DNS from the Internet ACCEPT net dmz:192.0.2.177 tcp domain # TCP DNS from the internet ACCEPT fw dmz:192.0.2.177 udp domain # UDP DNS from firewall ACCEPT fw dmz:192.0.2.177 tcp domain # TCP DNS from firewall ACCEPT loc dmz:192.0.2.177 udp domain # UDP DNS from the local Net ACCEPT loc dmz:192.0.2.177 tcp domain # TCP DNS from the local Net ACCEPT dmz:192.0.2.177 net udp domain # UDP DNS to the Internet ACCEPT dmz:192.0.2.177 net tcp domain # TCP 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 DESTINATION PROTOCOL PORT COMMENTS ACCEPT loc dmz tcp ssh # SSH to the DMZ ACCEPT loc fw tcp ssh # SSH to the Firewall
The above discussion reflects my personal preference for using Proxy ARP for my servers in my DMZ and SNAT/NAT for my local systems. I prefer to use NAT only in cases where a system that is part of an RFC 1918 subnet needs to have it's own public IP.
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).
Zone Interface Broadcast Options net eth0 detect norfc1918,routefilter loc eth1 detect dmz eth2 detect
The setup described here requires that your network interfaces be brought up before Shorewall can start. This opens a short window during which you have no firewall protection. If you replace 'detect' with the actual broadcast addresses in the entries above, you can bring up Shorewall before you bring up your network interfaces.
Zone Interface Broadcast Options net eth0 192.0.2.255 norfc1918,routefilter loc eth1 192.168.201.7 dmz eth2 192.168.202.7
/etc/shorewall/masq - Local subnet
INTERFACE SUBNET ADDRESS eth0 192.168.201.0/29 192.0.2.176
/etc/shorewall/proxyarp - DMZ
ADDRESS INTERFACE EXTERNAL 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 DESTINATION PROTOCOL PORT COMMENTS ACCEPT net dmz:192.0.2.178 tcp smtp # Mail from the Internet ACCEPT net dmz:192.0.2.178 tcp pop3 # Pop3 from the Internet ACCEPT loc dmz:192.0.2.178 tcp smtp # Mail from the Local Network ACCEPT loc dmz:192.0.2.178 tcp pop3 # Pop3 from the 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:192.0.2.178 tcp http # WWW from the Net ACCEPT net dmz:192.0.2.178 tcp https # Secure HTTP from the Net ACCEPT loc dmz:192.0.2.178 tcp https # Secure HTTP from the Local Net ACCEPT net dmz:192.0.2.177 udp domain # UDP DNS from the Internet ACCEPT net dmz:192.0.2.177 tcp domain # TCP DNS from the internet ACCEPT fw dmz:192.0.2.177 udp domain # UDP DNS from firewall ACCEPT fw dmz:192.0.2.177 tcp domain # TCP DNS from firewall ACCEPT loc dmz:192.0.2.177 udp domain # UDP DNS from the local Net ACCEPT loc dmz:192.0.2.177 tcp domain # TCP DNS from the local Net ACCEPT dmz:192.0.2.177 net udp domain # UDP DNS to the Internet ACCEPT dmz:192.0.2.177 net tcp domain # TCP DNS to the Internet ACCEPT net dmz icmp echo-request # Ping ACCEPT net loc icmp echo-request # " ACCEPT dmz loc icmp echo-request # " ACCEPT loc dmz icmp echo-request # " ACCEPT loc dmz tcp ssh # SSH to the DMZ ACCEPT loc fw tcp ssh # SSH to the Firewall
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 it's interface to the local network to be know as gateway.foobar.net and its interface to the dmz as dmz.foobar.net. Let's have the DNS server on 192.0.2.177 which will also be known by the name ns1.foobar.net.
The /etc/named.conf file would look like this:
options { directory "/var/named"; listen-on { 127.0.0.1 ; 192.0.2.177; }; }; 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; 127.0.0/24; 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; }; allow-transfer { <secondary NS IP>; }; file "ext/db.foobar"; }; zone "176.2.0.192.in-addr.arpa" in { type master; notify yes; allow-update { none; }; allow-transfer { <secondary NS IP>; }; file "db.192.0.2.176"; }; zone "177.2.0.192.in-addr.arpa" in { type master; notify yes; allow-update { none; }; allow-transfer { <secondary NS IP>; }; file "db.192.0.2.177"; }; zone "178.2.0.192.in-addr.arpa" in { type master; notify yes; allow-update { none; }; allow-transfer { <secondary NS IP>; }; file "db.192.0.2.178"; }; zone "179.2.0.192.in-addr.arpa" in { type master; notify yes; allow-update { none; }; allow-transfer { <secondary NS IP>; }; file "db.192.0.2.179"; }; };
Here are the files in /var/named (those not shown are usually included in your bind disbribution).
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>. ; ; ############################################################ ; Iverse Address Arpa Records (PTR's) ; ############################################################ 176.2.0.192.in-addr.arpa. 86400 IN PTR firewall.foobar.net.
; ############################################################ ; 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>. ; ; ############################################################ ; Iverse Address Arpa Records (PTR's) ; ############################################################ 177.2.0.192.in-addr.arpa. 86400 IN PTR www.foobar.net.
; ############################################################ ; 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>. ; ; ############################################################ ; Iverse Address Arpa Records (PTR's) ; ############################################################ 178.2.0.192.in-addr.arpa. 86400 IN PTR mail.foobar.net.
; ############################################################ ; 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>. ; ; ############################################################ ; Iverse Address Arpa Records (PTR's) ; ############################################################ 179.2.0.192.in-addr.arpa. 86400 IN PTR nod.foobar.net.
int/db.127.0.0 - The 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. ; ############################################################ ; Iverse Address Arpa Records (PTR's) ; ############################################################ 1 86400 IN PTR localhost.foobar.net.
int/db.192.168.201 - Reverse zone for the local net. 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. ; ############################################################ ; Iverse 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. ; ############################################################ ; Iverse Address Arpa Records (PTR's) ; ############################################################ 1 86400 IN PTR dmz.foobar.net.
int/db.foobar - Forward zone for use by 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 www 86400 IN A 192.0.2.177 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
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>.
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 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".
Edit the /etc/shorewall/routestopped file and configure those systems that you
want to be able to access the firewall when it is stopped.
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 8/10/2002 - Tom Eastep