Compiled Firewall Programs and Shorewall Lite

Tom Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2009/06/05


Table of Contents

Overview
Restrictions
The "shorewall compile" command
Shorewall Lite
Converting a system from Shorewall to Shorewall Lite
The /etc/shorewall/capabilities file and the shorecap program
Running compiled programs directly

Caution

This article applies to Shorewall 4.3 and later. If you are running a version of Shorewall earlier than Shorewall 4.3.5 then please see the documentation appropriate for your version.

Overview

Shorewall has the capability to compile a Shorewall configuration and produce a runnable firewall program script. The script is a complete program which can be placed on a system with Shorewall Lite installed and can serve as the firewall creation script for that system.

Restrictions

While compiled Shorewall programs are useful in many cases, there are some important restrictions that you should be aware of before attempting to use them.

  1. All extension scripts used are copied into the program (with the exception of those executed at compile-time by the compiler). The ramifications of this are:

    • If you update an extension script, the compiled program will not use the updated script.

    • The params file is only processed at compile time if you set EXPORTPARAMS=No in shorewall.conf. For run-time setting of shell variables, use the init extension script. Although the default setting is EXPORTPARAMS=Yes for compatibility, the recommended setting is EXPORTPARAMS=No.

      If the params file needs to set shell variables based on the configuration of the firewall system, you can use this trick:

      EXT_IP=$(ssh root@firewall "/sbin/shorewall-lite call find_first_interface_address eth0")

      The shorewall-lite call command allows you to to call interactively any Shorewall function that you can call in an extension script.

  2. You must install Shorewall Lite on the system where you want to run the script. You then install the compiled program in /usr/share/shorewall-lite/firewall and use the /sbin/shorewall-lite program included with Shorewall Lite to control the firewall just as if the full Shorewall distribution was installed.

The "shorewall compile" command

A compiled script is produced using the compile command:

shorewall compile [ -e ] [ <directory name> ] [ <path name> ]

where

-e

Indicates that the program is to be "exported" to another system. When this flag is set, neither the "detectnets" interface option nor DYNAMIC_ZONES=Yes in shorewall.conf are allowed. The created program may be run on a system that has only Shorewall Lite installed

When this flag is given, Shorewall does not probe the current system to determine the kernel/iptables features that it supports. It rather reads those capabilities from /etc/shorewall/capabilities. See below for details.

<directory name>

specifies a directory to be searched for configuration files before those directories listed in the CONFIG_PATH variable in shorewall.conf.

When -e <directory-name> is included, only the SHOREWALL_SHELL and VERBOSITY settings from /etc/shorewall/shorewall.conf are used and these apply only to the compiler itself. The settings used by the compiled firewall script are determined by the contents of <directory name>/shorewall.conf.

<path name>

specifies the name of the script to be created. If not given, ${VARDIR}/firewall is assumed (by default, ${VARDIR} is /var/lib/shorewall/)

Shorewall Lite

Shorewall Lite is a companion product to Shorewall and is designed to allow you to maintain all Shorewall configuration information on a single system within your network.

  1. You install the full Shorewall release on one system within your network. You need not configure Shorewall there and you may totally disable startup of Shorewall in your init scripts. For ease of reference, we call this system the 'administrative system'.

  2. On each system where you wish to run a Shorewall-generated firewall, you install Shorewall Lite. For ease of reference, we will call these systems the 'firewall systems'.

    Note

    The firewall systems do NOT need to have the full Shorewall product installed but rather only the Shorewall Lite product. Shorewall and Shorewall Lite may be installed on the same system but that isn't encouraged.

  3. On the administrative system you create a separate 'export directory' for each firewall system. You copy the contents of /usr/share/shorewall/configfiles into each export directory.

  4. The /etc/shorewall/shorewall.conf file is used to determine the VERBOSITY setting which determines how much output the compiler generates. All other settings are taken from the shorewall.conf file in the remote systems export directory.

    Caution

    If you want to be able to allow non-root users to manage remote firewall systems, then the files /etc/shorewall/params and /etc/shorewall/shorewall.conf must be readable by all users on the administrative system. Not all packages secure the files that way and you may have to change the file permissions yourself.

  5. On each firewall system, If you are running Debian or one of its derivatives like Ubuntu then edit /etc/default/shorewall-lite and set startup=1.

  6. On the administrative system, for each firewall system you do the following (this may be done by a non-root user who has root ssh access to the firewall system):

    1. modify the files in the corresponding export directory appropriately. It's a good idea to include the IP address of the administrative system in the routestopped file.

      It is important to understand that with Shorewall Lite, the firewall's export directory on the administrative system acts as /etc/shorewall for that firewall. So when the Shorewall documentation gives instructions for placing entries in files in the firewall's /etc/shorewall, when using Shorewall Lite you make those changes in the firewall's export directory on the administrative system.

      The CONFIG_PATH variable is treated as follows:

      • The value of CONFIG_PATH in /etc/shorewall/shorewall.conf is ignored when compiling for export (the -e option in given) and when the load or reload command is being executed (see below).

      • The value of CONFIG_PATH in the shorewall.conf file in the export directory is used to search for configuration files during compilation of that configuration.

      • The value of CONFIG_PATH used when the script is run on the firewall system is "/etc/shorewall-lite:/usr/share/shorewall-lite".

    2. cd <export directory>
      /sbin/shorewall load -c firewall

      The load command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and starts Shorewall Lite on the remote system via ssh. The -c option causes the capabilities of the remote system to be generated and copied to a file named capabilities in the export directory. See below.

      Example (firewall's DNS name is 'gateway'):

      /sbin/shorewall load -c gateway

      Note

      Although scp and ssh are used by default, you can use other utilities by setting RSH_COMMAND and RCP_COMMAND in /etc/shorewall/shorewall.conf.

  7. If you later need to change the firewall's configuration, change the appropriate files in the firewall's export directory then:

    cd <export directory>
    /sbin/shorewall reload firewall

    The reload command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and restarts Shorewall Lite on the remote system via ssh. The reload command also supports the '-c' option.

    I personally place a Makefile in each export directory as follows:

    #     Shorewall Packet Filtering Firewall Export Directory Makefile - V3.3
    #
    #     This program is under GPL [http://www.gnu.org/licenses/old-licenses/gpl-2.0.txt]
    #
    #     (c) 2006 - Tom Eastep (teastep@shorewall.net)
    #
    #       Shorewall documentation is available at http://www.shorewall.net
    #
    #       This program is free software; you can redistribute it and/or modify
    #       it under the terms of Version 2 of the GNU General Public License
    #       as published by the Free Software Foundation.
    #
    #       This program is distributed in the hope that it will be useful,
    #       but WITHOUT ANY WARRANTY; without even the implied warranty of
    #       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    #       GNU General Public License for more details.
    #
    #       You should have received a copy of the GNU General Public License
    #       along with this program; if not, write to the Free Software
    #       Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
    ################################################################################
    # Place this file in each export directory. Modify each copy to set HOST
    # to the name of the remote firewall corresponding to the directory.
    #
    #       To make the 'firewall' script, type "make".
    # 
    #       Once the script is compiling correctly, you can install it by
    #       typing "make install".
    #  
    ################################################################################
    #                             V A R I A B L E S
    #
    # Files in the export directory on which the firewall script does not depend
    #
    IGNOREFILES =  firewall% Makefile% trace% %~
    #
    # Remote Firewall system
    #
    HOST = gateway
    #
    # Save some typing
    #
    LITEDIR = /var/lib/shorewall-lite
    #
    # Set this if the remote system has a non-standard modules directory
    #
    MODULESDIR=
    #
    # Default target is the firewall script
    #
    ################################################################################
    #                                T A R G E T S
    #
    all: firewall
    #
    # Only generate the capabilities file if it doesn't already exist
    #
    capabilities: 
            ssh root@$(HOST) "MODULESDIR=$(MODULESDIR) /usr/share/shorewall-lite/shorecap > $(LITEDIR)/capabilities"
            scp root@$(HOST):$(LITEDIR)/capabilities .
    #
    # Compile the firewall script. Using the 'wildcard' function causes "*" to be expanded so that
    # 'filter-out' will be presented with the list of files in this directory rather than "*"
    #
    firewall: $(filter-out $(IGNOREFILES) capabilities , $(wildcard *) ) capabilities
            shorewall compile -e . firewall
    #
    # Only reload on demand.
    #
    install: firewall
            scp firewall firewall.conf root@$(HOST):$(LITEDIR)
            ssh root@$(HOST) "/sbin/shorewall-lite restart"
    #
    # Save running configuration
    #
    save:
            ssh root@$(HOST) "/sbin/shorewall-lite save"
    #
    # Remove generated files
    #
    clean: 
            rm -f capabilities firewall firewall.conf reload
    

    That way, after I've changed the configuration, I can simply type make or make install.

    Note

    I omit trace% because I often trace compiler execution while I'm debugging new versions of Shorewall.

There is a shorewall-lite.conf file installed as part of Shorewall Lite (/etc/shorewall-lite/shorewall-lite.conf). You can use that file on the firewall system to override some of the settings from the shorewall.conf file in the export directory.

Settings that you can override are:

VERBOSITY
LOGFILE
LOGFORMAT
IPTABLES
PATH
SHOREWALL_SHELL
SUBSYSLOCK
RESTOREFILE

You will normally not need to touch /etc/shorewall-lite/shorewall-lite.conf unless you run Debian or one of its derivatives (see above).

The /sbin/shorewall-lite program included with Shorewall Lite supports the same set of commands as the /sbin/shorewall program in a full Shorewall installation with the following exceptions:

add
compile
delete
refresh
reload
try
safe-start
safe-restart
show actions
show macros

On systems with only Shorewall Lite installed, I recommend that you create a symbolic link /sbin/shorewall and point it at /sbin/shorewall-lite. That way, you can use shorewall as the command regardless of which product is installed.

ln -sf shorewall-lite /sbin/shorewall

Converting a system from Shorewall to Shorewall Lite

Converting a firewall system that is currently running Shorewall to run Shorewall Lite instead is straight-forward.

  1. On the administrative system, create an export directory for the firewall system.

  2. Copy the contents of /etc/shorewall/ from the firewall system to the export directory on the administrative system.

  3. On the firewall system:

    Be sure that the IP address of the administrative system is included in the firewall's export directory routestopped file.

    shorewall stop

    We recommend that you uninstall Shorewall at this point.

  4. Install Shorewall Lite on the firewall system.

    If you are running Debian or one of its derivatives like Ubuntu then edit /etc/default/shorewall-lite and set startup=1.

  5. On the administrative system:

    It's a good idea to include the IP address of the administrative system in the firewall system's routestopped file.

    Also, edit the shorewall.conf file in the firewall's export directory and change the CONFIG_PATH setting to remove /etc/shorewall. You can replace it with /usr/share/shorewall/configfiles if you like.

    Example:

    Before editing:

    CONFIG_PATH=/etc/shorewall:/usr/share/shorewall

    After editing:

    CONFIG_PATH=/usr/share/shorewall/configfiles:/usr/share/shorewall

    Changing CONFIG_PATH will ensure that subsequent compilations using the export directory will not include any files from /etc/shorewall other than shorewall.conf and params.

    If you set variables in the params file, there are a couple of issues:

    The params file is not processed at run time if you set EXPORTPARAMS=No in shorewall.conf. For run-time setting of shell variables, use the init extension script.

    If the params file needs to set shell variables based on the configuration of the firewall system, you can use this trick:

    EXT_IP=$(ssh root@firewall "/sbin/shorewall-lite call find_first_interface_address eth0")

    The shorewall-lite call command allows you to to call interactively any Shorewall function that you can call in an extension script.

    After having made the above changes to the firewall's export directory, execute the following commands.

    cd <export directory>
    /sbin/shorewall load -c <firewall system>
    

    Example (firewall's DNS name is 'gateway'):

    /sbin/shorewall load -c gateway

    The load command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and starts Shorewall Lite on the remote system via ssh.

  6. If you later need to change the firewall's configuration, change the appropriate files in the firewall's export directory then:

    cd <export directory>
    /sbin/shorewall reload firewall

    The reload command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and restarts Shorewall Lite on the remote system via ssh.

  7. If the kernel/iptables configuration on the firewall later changes and you need to create a new capabilities file, do the following on the firewall system:

    /usr/share/shorewall-lite/shorecap > capabilities
    scp capabilities <admin system>:<this system's config dir>

    Or simply use the -c option the next time that you use the reload command.

The /etc/shorewall/capabilities file and the shorecap program

As mentioned above, the /etc/shorewall/capabilities file specifies that kernel/iptables capabilities of the target system. Here is a sample file:

#
# Shorewall detected the following iptables/netfilter capabilities - Tue Jul 15 07:28:12 PDT 2008
#
NAT_ENABLED=Yes
MANGLE_ENABLED=Yes
MULTIPORT=Yes
XMULTIPORT=Yes
CONNTRACK_MATCH=Yes
USEPKTTYPE=Yes
POLICY_MATCH=Yes
PHYSDEV_MATCH=Yes
PHYSDEV_BRIDGE=Yes
LENGTH_MATCH=Yes
IPRANGE_MATCH=Yes
RECENT_MATCH=Yes
OWNER_MATCH=Yes
IPSET_MATCH=Yes
CONNMARK=Yes
XCONNMARK=Yes
CONNMARK_MATCH=Yes
XCONNMARK_MATCH=Yes
RAW_TABLE=Yes
IPP2P_MATCH=
CLASSIFY_TARGET=Yes
ENHANCED_REJECT=Yes
KLUDGEFREE=Yes
MARK=Yes
XMARK=Yes
MANGLE_FORWARD=Yes
COMMENTS=Yes
ADDRTYPE=Yes
TCPMSS_MATCH=Yes
HASHLIMIT_MATCH=Yes
NFQUEUE_TARGET=Yes
REALM_MATCH=Yes
CAPVERSION=40190

As you can see, the file contains a simple list of shell variable assignments — the variables correspond to the capabilities listed by the shorewall show capabilities command and they appear in the same order as the output of that command.

To aid in creating this file, Shorewall Lite includes a shorecap program. The program is installed in the /usr/share/shorewall-lite/ directory and may be run as follows:

[ IPTABLES=<iptables binary> ] [ MODULESDIR=<kernel modules directory> ] /usr/share/shorewall-lite/shorecap > capabilities

The IPTABLES and MODULESDIR options have their usual Shorewall default values.

The capabilities file may then be copied to a system with Shorewall installed and used when compiling firewall programs to run on the remote system.

The capabilities file may also be creating using /sbin/shorewall-lite:

shorewall-lite show -f capabilities > capabilities

Note that unlike the shorecap program, the show capabilities command shows the kernel's current capabilities; it does not attempt to load additional kernel modules.

Running compiled programs directly

Compiled firewall programs are complete shell programs that support the following command line forms:

<program> [ -q ] [ -v ] [ -n ] start
<program> [ -q ] [ -v ] [ -n ] stop
<program> [ -q ] [ -v ] [ -n ] clear
<program> [ -q ] [ -v ] [ -n ] refresh
<program> [ -q ] [ -v ] [ -n ] reset
<program> [ -q ] [ -v ] [ -n ] restart
<program> [ -q ] [ -v ] [ -n ] status
<program> [ -q ] [ -v ] [ -n ] version

The options have the same meanings as when they are passed to /sbin/shorewall itself. The default VERBOSITY level is the level specified in the shorewall.conf file used when the program was compiled.