Shorewall 1.3 Reference

This documentation is intended primarily for reference. Step-by-step instructions for configuring Shorewall in common setups may be found in the QuickStart Guides.

Components

Shorewall consists of the following components:

/etc/shorewall/params

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

Variables may be used anywhere in the other configuration files.

/etc/shorewall/zones

This file is used to define 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.

/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 GNU/Linux System. Your /etc/shorewall/interfaces file would be:

/etc/shorewall/hosts Configuration

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)

The interface name much match an entry in /etc/shorewall/interfaces.

routestopped - Beginning with Shorewall 1.3.4, this option is deprecated in favor of the /etc/shorewall/routestopped file. 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: The setting of the MERGE_HOSTS variable in /etc/shorewall/shorewall.conf has an important effect on how the host file is processed. Please read the description of that variable carefully.

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.

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.

Hosts that belong to more than one zone may be managed by the rules of all of those zones. This is done through use of the special CONTINUE policy described below.

/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. SOURCE - The name of a client zone (a zone defined in the /etc/shorewall/zones file , the name of the firewall zone or "all").
2. DEST - 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 SOURCE 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.
5. LIMIT:BURST - Optional. If left empty, TCP connection requests from the SOURCE zone to the DEST zone will not be rate-limited. Otherwise, this column specifies the maximum rate at which TCP connection requests will be accepted followed by a colon (":") followed by the maximum burst size that will be tolerated. Example: 10/sec:40 specifies that the maximum rate of TCP connection requests allowed will be 10 per second and a burst of 40 connections will be tolerated. Connection requests in excess of these limits will be dropped.

In the SOURCE and DEST 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.

Sometimes it is necessary to suppress port forwarding for a sub-zone. For example, suppose that all hosts can SSH to the firewall and be forwarded to 192.168.1.5 EXCEPT Sam. When Sam connects to the firewall's external IP, he should be connected to the firewall itself. Because of the way that Netfilter is constructed, this requires two rules as follows:

The first rule allows Sam SSH access to the firewall. The second rule says that any clients from the net zone with the exception of those in the 'sam' zone should have their connection port forwarded to 192.168.1.3. If you need to exclude more than one zone in this way, you can list the zones separated by commas (e.g., net!sam,joe,fred). This technique also may be used when the ACTION is REDIRECT.

/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 local www connection requests EXCEPT those to your own http server (206.124.146.177) to a Squid transparent proxy running on the firewall and listening on port 3128. Squid will of course require access to remote web servers. This example shows yet another use for the ORIGINAL DEST column; here, connection requests that were NOT (notice the "!") originally destined to 206.124.146.177 are redirected to local port 3128.

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 internet in addition to the local 192.168.1.0/24 and dmz 192.168.2.0/24 subnetworks. Note that since 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 (but see FAQ 2). Note that unless you have more than one external IP address, you can leave the ORIGINAL DEST column blank in the first rule. You cannot leave it blank in the second rule though because then all ftp connections originating in the local subnet 192.168.1.0/24 would be sent to 192.168.2.2 regardless of the site that the user was trying to connect to. That is clearly not what you want .

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.

/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.

/etc/shorewall/masq

The /etc/shorewall/masq file is used to define classical IP Masquerading and Source Network Address Translation  (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.

/etc/shorewall/proxyarp

If you want to use proxy ARP on an entire sub-network, I suggest that you look at http://www.tldp.org/HOWTO/mini/Proxy-ARP-Subnet/. If you decide to use the technique described in that HOWTO, you can set the proxy_arp flag for an interface (/proc/sys/net/ipv4/conf/<interface>/proxy_arp) by including the proxyarp option in the interface's record in /etc/shorewall/interfaces. When using Proxy ARP sub-netting, you do NOT include any entries in /etc/shorewall/proxyarp.

The /etc/shorewall/proxyarp file is used to define Proxy ARP. The file is typically used for enabling Proxy ARP on a small set of systems since you need one entry in this file for each system using proxy ARP. Columns are:

Note: After you have made a change to the /etc/shorewall/proxyarp file, you may need to flush the ARP cache of all routers on the LAN segment connected to the interface specified in the EXTERNAL column of the change/added entry(s). If you are having problems communicating between an individual host (A) on that segment and a system whose entry has changed, you may need to flush the ARP cache on host A as well.

ISPs typically have ARP configured with long TTL (hours!) so if your ISPs router has a stale cache entry (as seen using "tcpdump -nei <external interface> host <IP addr>"), it may take a long while to time out. I personally have had to contact my ISP and ask them to delete a stale entry in order to restore a system to working order after changing my proxy ARP settings.

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 (http://www.tldp.org/HOWTO/mini/Proxy-ARP-Subnet/) 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

/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.

/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.

/etc/shorewall/shorewall.conf

This file is used to set the following firewall parameters:

/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>

/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.

WARNING: Users have reported that odd routing problems result from adding the ESP and AH protocols to the /etc/shorewall/tos file.

/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. The black list is designed to prevent listed hosts/subnets from accessing services on your network.

Shorewall also has a dynamic blacklist capability.

IMPORTANT: The Shorewall blacklist file is NOT designed to police your users' web browsing -- to do that, I suggest that you install and configure Squid (http://www.squid-cache.org).

/etc/shorewall/rfc1918 (Added in Version 1.3.1)

This file lists the subnets affected by the norfc1918 interface option. Columns in the file are:

25. /etc/shorewall/routestopped (Added in Version 1.3.4)

This fine defines the hosts that are accessible from the firewall when the firewall is stopped.  Columns in the file are:

Example: When your firewall is stopped, you want firewall accessibility from local hosts 192.168.1.0/24 and from your DMZ. Your DMZ interfaces through eth1 and your local hosts through eth2.

Updated 8/6/2002 - Tom Eastep

Copyright © 2001, 2002 Thomas M. Eastep.