Shorewall Documentation

1. Philosophy

Shorewall is designed to allow flexible firewall configuration.

Shorewall:

2. Components

Shorewall consists of the following components:

3. Shorewall Configurations

Beginning with Shorewall version 1.1.14, Shorewall supports configuration directories other than /etc/shorewall. The shorewall start and restart commands allow you to specify an alternate configuration directory and Shorewall will use the files in the alternate directory rather than the corresponding files in /etc/shorewall. The alternate directory need not contain a complete configuration; those files not in the alternate directory will be read from /etc/shorewall.

This facility permits you to easily create a test or temporary configuration by:

1. copying the files that need modification from /etc/shorewall to a separate directory;
2. modify those files in the separate directory; and
3. specifying the separate directory in a shorewall start or shorewall restart command (e.g., shorewall -c /etc/testconfig restart ).

4. Firewall Structure

Shorewall views the network in which it is running as a set of disjoint zones. Shorewall itself defines exactly one zone called "fw" which refers to the firewall system itself . The /etc/shorewall/zones file is used to define additional zones and the example file provided with Shorewall defines the zones:

1. net -- the (untrusted) internet.
2. dmz - systems that must be accessible from the internet and from the local network.  These systems cannot be trusted completely since their servers may have been compromised through a security exploit.
3. loc - systems in your local network(s). These systems must be protected from the internet and from the DMZ and in some cases, from each other.

Note: Beginning with version 1.2.4, you can specify the name of the firewall zone. For ease of description in the remainder of this section, I am going to assume that the firewall zone is named "fw".

I can't stress enough that with the exception of the firewall zone, Shorewall itself attaches no meaning to zone names. Zone names are simply labels used to refer to a collection of network hosts.

Beginning with Shorewall 1.1.13, the firewall reserves the zone name "multi". This zone name is used to describe those network interface that are associated with multiple zones although it is not treated as a real zone.

Traffic directed from a zone to the firewall itself is sent through a chain named <zone name>2fw. For example, traffic inbound from the internet and addressed to the firewall is sent through a chain named net2fw. Similarly, traffic originating in the firewall and being sent to a host in a given zone is sent through a chain named fw2<zone name>. For example, traffic originating in the firewall and destined for a host in the local network is sent through a chain named fw2loc.  

Traffic being forwarded between two zones (or from one interface to a zone to another interface to that zone) is sent through a chain named <source zone>2 <destination zone>. So for example, traffic originating in a local system and destined for a remote web server is sent through chain loc2net. This chain is referred to as the canonical chain from <source zone> to <destination zone>. Any destination NAT will have occurred before the packet traverses one of these chains so rules in /etc/shorewall/rules should be expressed in terms of the destination system's real IP address as opposed to its apparent external address. Similarly, source NAT will occur after the packet has traversed the appropriate forwarding chain so the rules again will be expressed using the source system's real IP address.

Beginning with Shorewall 1.1, the firewall structure is slightly modified. For each record in the /etc/shorewall/policy file, a chain is created. Policies in that file are expressed in terms of a source zone and destination zone where these zones may be a zone defined in /etc/shorewall/zones, "fw" or "all". Policies specifying the pseudo-zone "all" matches all defined zones and "fw". These chains are referred to as Policy Chains. Notice that for an ordered pair of zones (za,zb), the canonical chain (za2zb) may also be the policy chain for the pair or the policy chain may be a different chain (za2all, for example). Packets from one zone to another will traverse chains as follows:

1. If the canonical chain exists, packets first traverse that chain.
2. If the canonical chain and policy chain are different and the packet does not match a rule in the canonical chain, it then is sent to the policy chain.
3. If the canonical chain does not exist, packets are sent immediately to the policy chain.

The canonical chain from zone za to zone zb will be created only if there are exception rules defined in /etc/shorewall/rules for packets going from za to zb.

Shorewall is built on top of the Netfilter kernel facility. Netfilter implements connection tracking function that allow what is often referred to as "statefull inspection" of packets. This statefull property allows firewall rules to be defined in terms of "connections" rather than in terms of "packets". With Shorewall, you:

1. Identify the client's zone.
2. Identify the server's zone.
3. 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.
4. 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 between zone A and the firewall and are also allowed between the firewall and zone B DOES NOT mean that these connections are allowed between zone A and 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.

If you adopt the default policy of ACCEPT from the local zone to the internet zone and you are having problems connecting from a local client to an internet server, adding a rule won't help (see point 3 above).

5. Installation

See the installation instructions . Also, Scott Merrill has written a set of step-by-step instructions for Installing a "Belt and Suspenders" Firewall Configuration under RedHat 7.2.

6. Getting Started

The steps involved in configuring Shorewall are as follows:

1. Define your Zones (/etc/shorewall/zones ). Zones allow you to define sets of systems that are to be treated similarly by the firewall. A zone may be all of the hosts accessible through a particular network interface (see /etc/shorewall/interfaces ) or may be a more specific set of hosts (see /etc/shorewall/hosts ).
2. Define your Policies (/etc/shorewall/policy ). Policies are like default rules for connections between zones.
3. Define your Rules (/etc/shorewall/rules ). Rules are exceptions to the policies defined in the previous step. 
4. Define NAT and/or Proxy ARP (/etc/shorewall/masq , /etc/shorewall/nat and /etc/shorewall/proxyarp ).
5. Review the other files in /etc/shorewall to see if you wish to make any changes.

A couple of things to note:

7. Using MAC Addresses

Beginning with Shorewall version 1.2.9, MAC (Media Access Control) addresses can be used to specify packet source in several of the configuration files. To use this feature, your kernel must have MAC Address Match support (CONFIG_IP_NF_MATCH_MAC) included.

MAC addresses are 48 bits wide and each Ethernet Controller has a unique MAC address.

In Linux, MAC addresses are usually written as a series of 6 hex numbers separated by colons. Example:

     [root@gateway root]# ifconfig eth0
     eth0 Link encap:Ethernet HWaddr 02:00:08:E3:FA:55
     inet addr:206.124.146.176 Bcast:206.124.146.255 Mask:255.255.255.0
     UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
     RX packets:2398102 errors:0 dropped:0 overruns:0 frame:0
     TX packets:3044698 errors:0 dropped:0 overruns:0 carrier:0
     collisions:30394 txqueuelen:100
     RX bytes:419871805 (400.4 Mb) TX bytes:1659782221 (1582.8 Mb)
     Interrupt:11 Base address:0x1800

Because Shorewall uses colons as a separator for address fields, Shorewall requires MAC addresses to be written in another way. In Shorewall, MAC addresses begin with a tilde ("~") and consist of 6 hex numbers separated by hyphens. In Shorewall, the MAC address in the example above would be written "~02-00-08-E3-FA-55".

8. Configuration Files

You may place comments in configuration files by making the first character a pound sign ("#"). Beginning with Shorewall version 1.2.4, you may also place comments at the end of any line, again by delimiting the comment from the rest of the line with a pound sign.

Warning: If you edit your configuration files on a system running Microsoft Windows, you must run them through dos2unix before you use them with Shorewall.

9. /etc/shorewall/params

Beginning with Shorewall 1.1.13, you may use the file /etc/shorewall/params file to set shell variables that you can then use in some of the other configuration files.

It is suggested that variable names begin with an upper case letter to distinguish them from variables used internally within the Shorewall programs

Example:

NET_IF=eth0
NET_BCAST=130.252.100.255
NET_OPTIONS=noping,norfc1918

Example (/etc/shorewall/interfaces record):

The result will be the same as if the record had been written

Beginning in version 1.2.6, variables may be used anywhere in the other configuration files.

10. /etc/shorewall/zones

This file was introduced in version 1.0.4 and is used to define the network zones. Prior to version 1.0.4, the firewall script itself defined the network zones. There is one entry in /etc/shorewall/zones for each zone; Columns in an entry are:

The /etc/shorewall/zones file released with Shorewall is as follows:

You may add, delete and modify entries in the /etc/shorewall/zones file as desired so long as you have at least one zone defined.

Warning 1: If you rename or delete a zone, you should perform "shorewall stop; shorewall start" to install the change rather than "shorewall restart".

Warning 2: The order of entries in the /etc/shorewall/zones file is significant in some cases.

11. /etc/shorewall/interfaces

This file is used to tell the firewall which of your firewall's network interfaces are connected to which zone. There will be one entry in /etc/shorewall/interfaces for each of your interfaces. Columns in an entry are:

Example 1: You have a conventional firewall setup in which eth0 connects to a Cable or DSL modem and eth1 connects to your local network and eth0 gets its IP address via DHCP. You want to ignore ping requests from the internet and you want to check all packets entering from the internet against the black list. Your /etc/shorewall/interfaces file would be as follows:

Example 2: You have a standalone dialup Linux System. Your /etc/shorewall/interfaces file would be:

12. /etc/shorewall/hosts Configuration (version 1.1 and later)

For most applications, specifying zones entirely in terms of network interfaces is sufficient. There may be times though where you need to define a zone to be a more general collection of hosts. This is the purpose of the /etc/shorewall/hosts file.

WARNING: 90% of Shorewall users don't need to put entries in this file and 80% of those who try to add such entries do it wrong. Unless you are ABSOLUTELY SURE that you need entries in this file, don't touch it.

Columns in this file are:

1. An IP address (example - eth1:192.168.1.3)
2. A subnet in the form <subnet address>/<width> (example - eth2:192.168.2.0/2)

If you want to specify an address or subnet without a network interface, use "+" as the interface (example - +:155.186.235.151). Note: Use of "+" weakens the firewall slightly and increases packet latency slightly.

routestopped - When the firewall is stopped, traffic to and from this host (these hosts) will be accepted and routing will occur between this host and other routestopped interfaces and hosts.

If you don't define any hosts for a zone, the hosts in the zone default to i0:0.0.0.0/0 , i1:0.0.0.0/0, ... where i0, i1, ... are the interfaces to the zone.

Note 1: You probably DON'T want to specify any hosts for your internet zone since the hosts that you specify will be the only ones that you will be able to access without adding additional rules.

Note 2: If you have any entry for a zone in /etc/shorewall/hosts then the zone must be entirely defined in /etc/shorewall/hosts. For example, if a zone has two interfaces but only one interface has an entry in /etc/shorewall/hosts then hosts attached to the other interface will not be considered part of the zone.

Example:

Your local interface is eth1 and you have two groups of local hosts that you want to make into separate zones:

Your /etc/shorewall/interfaces file might look like:

The '-' in the ZONE column for eth1 tells Shorewall that eth1 interfaces to multiple zones.

The multi option allows the firewall to route between the two zones (although normally, the subnet mask for all local systems would be 255.255.255.0 which would make such an entry unnecessary).

Your /etc/shorewall/hosts file might look like:

Hosts in 'loc2' can communicate with the firewall while Shorewall is stopped -- those in 'loc1' cannot.

Nested and Overlapping Zones

The /etc/shorewall/interfaces and /etc/shorewall/hosts file allow you to define nested or overlapping zones. Such overlapping/nested zones are allowed and Shorewall processes zones in the order that they appear in the /etc/shorewall/zones file. So if you have nested zones, you want the sub-zone to appear before the super-zone and in the case of overlapping zones, the rules that will apply to hosts that belong to both zones is determined by which zone appears first in /etc/shorewall/zones.

Beginning with Shorewall version 1.1.15, hosts that belong to more than one zone may be managed by the rules of all of the zones. This is done through use of the special CONTINUE policy described below.

13. /etc/shorewall/policy Configuration.

This file is used to describe the firewall policy regarding establishment of connections. Connection establishment is described in terms of clients who initiate connections and servers who receive those connection requests. Policies defined in /etc/shorewall/policy describe which zones are allowed to establish connections with other zones.

Policies established in /etc/shorewall/policy can be viewed as default policies. If no rule in /etc/shorewall/rules applies to a particular connection request then the policy from /etc/shorewall/policy is applied.

Four policies are defined:

For each policy specified in /etc/shorewall/policy, you can indicate that you want a message sent to your system log each time that the policy is applied.

Entries in /etc/shorewall/policy have four columns as follows:

1. CLIENT - The name of a client zone (a zone defined in the /etc/shorewall/zones file , the name of the firewall zone or "all").
2. SERVER - The name of a destination zone (a zone defined in the /etc/shorewall/zones file , the name of the firewall zone or "all").
3. POLICY - The default policy for connection requests from the CLIENT zone to the DESTINATION zone.
4. LOG LEVEL - Optional. If left empty, no log message is generated when the policy is applied. Otherwise, this column should contain an integer or name indicating a syslog level. See the syslog.conf man page for a description of each log level.

In the CLIENT and SERVER columns, you can enter "all" to indicate all zones. 

The policy file installed by default is as follows:

This table may be interpreted as follows:

WARNING:

The firewall script processes  the /etc/shorewall/policy file from top to bottom and uses the first applicable policy that it finds. For example, in the following policy file, the policy for (loc, loc) connections would be ACCEPT as specified in the first entry even though the third entry in the file specifies REJECT.

The CONTINUE policy

Where zones are nested or overlapping , the CONTINUE policy allows hosts that are within multiple zones to be managed under the rules of all of these zones. Let's look at an example:

/etc/shorewall/zones:

/etc/shorewall/interfaces:

/etc/shorewall/hosts:

Note that Sam's home system is a member of both the sam zone and the net zone and as described above , that means that sam must be listed before net  in /etc/shorewall/zones.

/etc/shorewall/policy:

The second entry above says that when Sam is the client, connection requests should first be process under rules where the source zone is sam and if there is no match then the connection request should be treated under rules where the source zone is net. It is important that this policy be listed BEFORE the next policy (net to all).

Partial /etc/shorewall/rules:

Given these two rules, Sam can connect to the firewall's internet interface with ssh and the connection request will be forwarded to 192.168.1.3. Like all hosts in the net zone, Sam can connect to the firewall's internet interface on TCP port 80 and the connection request will be forwarded to 192.168.1.5. The order of the rules is not significant.

14. /etc/shorewall/rules

The /etc/shorewall/rules file defines exceptions to the policies established in the /etc/shorewall/policy file. There is one entry in /etc/shorewall/rules for each of these rules. 

Entries in the file have the following columns:

Example 1. You wish to forward all ssh connection requests from the internet to local system 192.168.1.3. 

Example 2. You want to redirect all www requests from the local network to a Squid server running on the firewall and listening on port 8080 and the firewall zone is named "fw". Squid will require access to remote web servers.

Example 3. You want to run a web server at 155.186.235.222 in your DMZ and have it accessible remotely and locally. the DMZ is managed by Proxy ARP or by classical sub-netting.

Example 4. You want to run wu-ftpd on 192.168.2.2 in your masqueraded DMZ. Your internet interface address is 155.186.235.151 and you want the FTP server to be accessible from the local 192.168.1.0/24 and dmz 192.168.2.0/24 subnetworks. Note that size the server is in the 192.168.2.0/24 subnetwork, we can assume that access to the server from that subnet will not involve the firewall.

If you are running wu-ftpd, you should restrict the range of passive in your /etc/ftpaccess file. I only need a few simultaneous FTP sessions so I use port range 65500-65535. In /etc/ftpaccess, this entry is appropriate:

passive ports  0.0.0.0/0 65500 65534

If you are running pure-ftpd, you would include "-p 65500:65534" on the pure-ftpd runline.

The important point here is to ensure that the port range used for FTP passive connections is unique and will not overlap with any usage on the firewall system.

Example 5. You wish to allow unlimited DMZ access to the host with MAC address 02:00:08:E3:FA:55.

Look here for information on other services.

15. /etc/shorewall/common

Shorewall allows definition of rules that apply between all zones. By default, these rules are defined in the file /etc/shorewall/common.def but may be modified to suit individual requirements. Rather than modify /etc/shorewall/common.def, you should copy that file to /etc/shorewall/common and modify that file.

The /etc/shorewall/common file is expected to contain iptables commands; rather than running iptables directly, you should run it indirectly using the Shorewall function 'run_iptables'. That way, if iptables encounters an error, the firewall will be safely stopped.

16. /etc/shorewall/masq

The /etc/shorewall/masq file is used to define classical IP Masquerading. Beginning with Shorewall version 1.2.5, it can also be used to define Source NAT (SNAT). There is one entry in the file for each subnet that you want to masquerade. In order to make use of this feature, you must have NAT enabled .

Columns are:

Example 1: You have eth0 connected to a cable modem and eth1 connected to your local subnetwork 192.168.9.0/24. Your /etc/shorewall/masq file would look like:    

Example 2: You have a number of IPSEC tunnels through ipsec0 and you want to masquerade traffic from your 192.168.9.0/24 subnet to the remote subnet 10.1.0.0/16 only.

Example 3: You have a DSL line connected on eth0 and a local network (192.168.10.0/24) connected to eth1. You want all local->net connections to use source address 206.124.146.176.

Example 4: Same as example 3 except that you wish to exclude 192.168.10.44 and 192.168.10.45 from the SNAT rule (Version 1.2.11 and later).

17. /etc/shorewall/proxyarp

The /etc/shorewall/proxyarp file is used to define Proxy ARP. You need one entry in this file for each system to be proxy arp'd. Columns are:

Example: You have public IP addresses 155.182.235.0/28. You configure your firewall as follows:

In your DMZ, you want to install a Web/FTP server with public address 155.186.235.4. On the Web server, you subnet just like the firewall's eth0 and you configure 155.186.235.1 as the default gateway. In your /etc/shorewall/proxyarp file, you will have:

Note: You may want to configure the servers in your DMZ with a subnet that is smaller than the subnet of your internet interface. See the Proxy ARP Subnet Mini Howto for details. In this case you will want to place "Yes" in the HAVEROUTE column.

To learn how I use Proxy ARP in my DMZ, see my configuration files.

Warning: Do not use Proxy ARP and FreeS/Wan on the same system unless you are prepared to suffer the consequences. If you start or restart Shorewall with an IPSEC tunnel active, the proxied IP addresses are mistakenly assigned to the IPSEC tunnel device (ipsecX) rather than to the interface that you specify in the INTERFACE column of /etc/shorewall/proxyarp. I haven't had the time to debug this problem so I can't say if it is a bug in the Kernel or in FreeS/Wan. 

You might be able to work around this problem using the following (I haven't tried it):

In /etc/shorewall/init, include:

     qt service ipsec stop

In /etc/shorewall/start, include:

    qt service ipsec start

18. /etc/shorewall/nat

The /etc/shorewall/nat file is used to define static NAT. There is one entry in the file for each static NAT relationship that you wish to define. In order to make use of this feature, you must have NAT enabled .

IMPORTANT: If all you want to do is forward ports to servers behind your firewall, you do NOT want to use static NAT. Port forwarding can be accomplished with simple entries in the rules file. Also, in most cases Proxy ARP provides a superior solution to static NAT because the internal systems are accessed using the same IP address internally and externally.

Columns in an entry are:

Look here for additional information and an example.

19. /etc/shorewall/tunnels

The /etc/shorewall/tunnels file allows you to define IPSec, GRE and IPIP tunnels with end-points on your firewall. To use ipsec, you must install version 1.9, 1.91 or the current FreeS/WAN development snapshot. 

Note: For kernels 2.4.4 and above, you will need to use version 1.91 or a development snapshot as patching with version 1.9 results in kernel compilation errors.

Instructions for setting up IPSEC tunnels may be found here and instructions for IPIP tunnels are here . Look here for information about setting up PPTP tunnels under Shorewall.

20. /etc/shorewall/shorewall.conf

This file is used to set the following firewall parameters:

21. /etc/shorewall/modules Configuration

The file /etc/shorewall/modules contains commands for loading the kernel modules required by Shorewall-defined firewall rules. Shorewall will source this file during start/restart provided that it exists and that the directory specified by the MODULESDIR parameter exists (see /etc/shorewall/shorewall.conf above).

The file that is released with Shorewall calls the Shorewall function "loadmodule" for the set of modules that I load.

The loadmodule function is called as follows:

loadmodule <modulename> [ <module parameters> ]

where

<modulename>                

is the name of the modules without the trailing ".o" (example ip_conntrack).

<module parameters>

Optional parameters to the insmod utility.

The function determines if the module named by <modulename> is already loaded and if not then the function determines if the ".o" file corresponding to the module exists in the moduledirectory; if so, then the following command is executed:

insmod moduledirectory/<modulename>.o <module parameters>

If the file doesn't exist, the function determines of the ".o.gz" file corresponding to the module exists in the moduledirectory. If it does, the function assumes that the running configuration supports compressed modules and execute the following command:

insmod moduledirectory/<modulename>.o.gz <module parameters>

22. /etc/shorewall/tos Configuration

The /etc/shorewall/tos file allows you to set the Type of Service field in packet headers based on packet source, packet destination, protocol, source port and destination port. In order for this file to be processed by Shorewall, you must have mangle support enabled .

Entries in the file have the following columns:

Minimize-Delay (16)
Maximize-Throughput (8)
Maximize-Reliability (4)
Minimize-Cost (2)
Normal-Service (0)

The /etc/shorewall/tos file that is included with Shorewall contains the following entries.

23. /etc/shorewall/blacklist

Each line in /etc/shorewall/blacklist contains an IP address, a MAC address in Shorewall Format or subnet address. Example:

Packets from hosts listed in the blacklist file will be disposed of according to the value assigned to the BLACKLIST_DISPOSITION and BLACKLIST_LOGLEVEL variables in /etc/shorewall/shorewall.conf. Only packets arriving on interfaces that have the 'blacklist' option in /etc/shorewall/interfaces are checked against the blacklist.

24. Starting/Stopping and Monitoring the Firewall

If you have a permanent internet connection such as DSL or Cable, I recommend that you start the firewall automatically at boot. Once you have installed "firewall" in your init.d directory, simply type "chkconfig --add firewall". This will start the firewall in run levels 2-5 and stop it in run levels 1 and 6. If you want to configure your firewall differently from this default, you can use the "--level" option in chkconfig (see "man chkconfig") or using your favorite graphical run-level editor.

Important Note:

If you use dialup, you may want to start the firewall in your /etc/ppp/ip-up.local script. I recommend just placing "shorewall restart" in that script.

You can manually start and stop Shoreline Firewall using the "shorewall" shell program:

The "shorewall" program may also be used to monitor the firewall.

Beginning with Shorewall version 1.1.14, the shorewall start and shorewall restart commands allow you to specify which Shorewall configuration to use:

shorewall [ -c configuration-directory ] {start|restart}

If a configuration-directory is specified, each time that Shorewall is going to use a file in /etc/shorewall it will first look in the configuration-directory . If the file is present in the configuration-directory, that file will be used; otherwise, the file in /etc/shorewall will be used.

25. Extension Scripts

Beginning with version 1.1, Shorewall includes provisions for extending Shorewall through Extension Scripts. Extension scripts are user-provided scripts that are invoked at various points during firewall start, restart, stop and clear. The scripts are placed in /etc/shorewall and are processed using the Bourne shell "source" mechanism. The following scripts can be supplied:

You can also supply a script with the same name as any of the filter chains in the firewall and the script will be invoked after the /etc/shorewall/rules file has been processed but before the /etc/shorewall/policy file has been processed.

Beginning with version 1.1.2, the following two files receive special treatment:

Rather than running iptables directly, you should run it using the function run_iptables. Similarly, rather than running "ip" directly, you should use run_ip. These functions accept the same arguments as the underlying command but cause the firewall to be stopped if an error occurs during processing of the command.

26. LEAF

Jacques Nilo and Eric Wolzak have a LEAF offering that features Shorewall-1.2.9 and Kernel-2.4.18. You can find their work at:

http://leaf.sourceforge.net/devel/jnilo

Updated 4/19/2002 - Tom Eastep

Copyright © 2001, 2002 Thomas M. Eastep.