Introduction of shorewall

Introduction

Tom Eastep

Copyright © 2003-2012 Thomas M. Eastep

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

2012/02/20

Introduction

The information in this document applies only to 4.3 and later releases of Shorewall.

Glossary

• Netfilter - the packet filter facility built into the 2.4 and later Linux kernels.
• ipchains - the packet filter facility built into the 2.2 Linux kernels. Also the name of the utility program used to configure and control that facility. Netfilter can be used in ipchains compatibility mode.
• iptables - the utility program used to configure and control Netfilter. The term “iptables” is often used to refer to the combination of iptables+Netfilter (with Netfilter not in ipchains compatibility mode).
• iptables-restore - a program included with iptables that allows for atomic installation of a set of Netfilter rules. This is a much more efficient way to install a rule set than running the iptables utility once for each rule in the rule set.
• ifconfig - An obsolete program included in the net-utils package. ifconfig was used to configure network interfaces.
• route - An obsolete program included in the net-utils package. route was used to configure routing.
• ip - A program included in the iproute2 package. ip replaces ifconfig and route in modern Linux systems.
• tc - A program included in the iproute2 package. tc is used to configure QOS/Traffic Shaping on Linux systems.

What is Shorewall?

The Shoreline Firewall, more commonly known as “Shorewall”, is high-level tool for configuring Netfilter. You describe your firewall/gateway requirements using entries in a set of configuration files. Shorewall reads those configuration files and with the help of the iptables, iptables-restore, ip and tc utilities, Shorewall configures Netfilter and the Linux networking subsystem to match your requirements. Shorewall can be used on a dedicated firewall system, a multi-function gateway/router/server or on a standalone GNU/Linux system. Shorewall does not use Netfilter's ipchains compatibility mode and can thus take advantage of Netfilter's connection state tracking capabilities.

Shorewall is not a daemon. Once Shorewall has configured the Linux networking subsystem, its job is complete and there is no “Shorewall process” left running in your system. The /sbin/shorewall program can be used at any time to monitor the Netfilter firewall.

Shorewall is not the easiest to use of the available iptables configuration tools but I believe that it is the most flexible and powerful. So if you are looking for a simple point-and-click set-and-forget Linux firewall solution that requires a minimum of networking knowledge, I would encourage you to check out the following alternatives:

• kmyfirewall
• firestarter

If you are looking for a Linux firewall solution that can handle complex and fast changing network environments then Shorewall is a logical choice.

Shorewall Concepts

The configuration files for Shorewall are contained in the directory /etc/shorewall -- for simple setups, you will only need to deal with a few of them.

Shorewall views the network where it is running as being composed of a set of zones. In the three-interface sample configuration for example, the following zone names are used:

#NAME                  DESCRIPTION
fw                     The firewall itself
net                    The Internet
loc                    Your Local Network
dmz                    Demilitarized Zone

Zones are declared and given a type in the /etc/shorewall/ zones file.Here is the /etc/shorewall/ zones file from the three-interface sample:

#ZONE   TYPE    OPTIONS                 IN                      OUT
#                                       OPTIONS                 OPTIONS
fw      firewall
net     ipv4
loc     ipv4
dmz     ipv4

Note that Shorewall recognizes the firewall system as its own zone. The name of the zone designating the firewall itself (usually 'fw' as shown in the above file) is stored in the shell variable $ FW which may be used throughout the Shorewall configuration to refer to the firewall zone.

The simplest way to define the hosts in a zone is to associate the zone with a network interface using the /etc/shorewall/interfaces file. In the three-interface sample, the three zones are defined using that file as follows:

#ZONE      INTERFACE     BROADCAST     OPTIONS
net        eth0          detect        dhcp,routefilter
loc        eth1          detect
dmz        eth2          detect

The above file defines the net zone as all IPv4 hosts interfacing to the firewall through eth0, the loc zone as all IPv4 hosts interfacing through eth1 and the dmz as all IPv4 hosts interfacing through eth2. It is important to note that the composition of a zone is defined in terms of a combination of addresses and interfaces. When using the /etc/shorewall/interfaces file to define a zone, all addresses are included; when you want to define a zone that contains a limited subset of the IPv4 address space, you use the /etc/shorewall/hosts file or you may use the nets= option in /etc/shorewall/interfaces:

#ZONE      INTERFACE     BROADCAST     OPTIONS
net        eth0          detect        dhcp,routefilter,nets=(!192.168.0.0/23)
loc        eth1          detect        nets=(192.168.0.0/24)
dmz        eth2          detect        nets=(192.168.1.0/24)

The above file defines the net zone as all IPv4 hosts interfacing to the firewall through eth0 except for 192.168.0.0/23, the loc zone as IPv4 hosts 192.168.0.0/24 interfacing through eth1 and the dmz as IPv4 hosts 192.168.1.0/24 interfacing through eth2 (Note that 192.168.0.0/24 together with 192.168.1.0/24 comprises 192.168.0.0/23).

Rules about what traffic to allow and what traffic to deny are expressed in terms of zones.

• You express your default policy for connections from one zone to another zone in the /etc/shorewall/ policy file. The basic choices for policy are:
  • ACCEPT - Accept the connection.
  • DROP - Ignore the connection request.
  • REJECT - Return an appropriate error to the connection request.
  Connection request logging may be specified as part of a policy and it is conventional (and highly recommended) to log DROP and REJECT policies.
• You define exceptions to these default policies in the /etc/shorewall/ rules file.
• You only need concern yourself with connection requests. You don't need to define rules for handling traffic that is part of an established connection and in most cases you don't have to worry about how related connections are handled (ICMP error packets and related TCP connection requests such as used by FTP).

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 there is a default action defined for the policy in /etc/shorewall/shorewall.conf then that action is invoked before the policy is enforced. In the standard Shorewall distribution, the DROP policy has a default action called Drop and the REJECT policy has a default action called Reject. Default actions are used primarily to discard certain packets silently so that they don't clutter up your log.

The /etc/shorewall/ policy file included with the three-interface sample has the following policies:

#SOURCE    DEST        POLICY      LOG LEVEL    LIMIT:BURST
loc        net         ACCEPT
net        all         DROP        info
all        all         REJECT      info

In the three-interface sample, the line below is included but commented out. If you want your firewall system to have full access to servers on the Internet, uncomment that line.

#SOURCE    DEST        POLICY      LOG LEVEL    LIMIT:BURST
$FW        net         ACCEPT

The above policies will:

• Allow all connection requests from your local network to the Internet
• Drop (ignore) all connection requests from the Internet to your firewall or local networks; these ignored connection requests will be logged using the info syslog priority (log level).
• Optionally accept all connection requests from the firewall to the Internet (if you uncomment the additional policy)
• reject all other connection requests; these rejected connection requests will be logged using the info syslog priority (log level).

A word about Shorewall logging is in order. Shorewall does not have direct control over where its messages are logged; that is determined by the configuration of the logging daemon (syslog, rsyslog, syslog-ng, ulogd, etc.). The LOGFILE setting in /etc/shorewall/shorewall.conf tells Shorewall where to find the log; it doesn't determine where messages are logged. See the Shorewall logging article for more information.

To illustrate how rules provide exceptions to policies, suppose that you have the polices listed above but you want to be able to connect to your firewall from the Internet using Secure Shell (SSH). Recall that SSH connects uses TCP port 22. You would add the following rule to /etc/shorewall/ rules:

#ACTION    SOURCE        DEST      PROTO      DEST
#                                             PORT(S)
ACCEPT     net           $FW       tcp        22

So although you have a policy of ignoring all connection attempts from the net zone (from the Internet), the above exception to that policy allows you to connect to the SSH server running on your firewall.

Because Shorewall makes no assumptions about what traffic you want accepted, there are certain rules (exceptions) that need to be added to almost any configuration.

• The QuickStart guides point to pre-populated files for use in common setups and the Shorewall Setup Guide shows you examples for use with other more complex setups.
• Again, to keep your firewall log from filling up with useless noise, Shorewall provides common actions that silently discard or reject such noise before it can be logged. As with everything in Shorewall, you can alter the behavior of these common actions (or do away with them entirely) as you see fit.

Compile then Execute

Shorewall uses a "compile" then "execute" approach. The Shorewall configuration compiler reads the configuration files and generates a shell script. Errors in the compilation step cause the script to be discarded and the command to be aborted. If the compilation step doesn't find any errors then the shell script is executed.

The 'compiled' scripts are placed in the directory /var/lib/shorewall and are named to correspond to the command being executed. For example, the command /sbin/shorewall start will generate a script named /var/lib/shorewall/.start and, if the compilation is error free, that script will then be executed. If the script executes successfully, it then copies itself to /var/lib/shorewall/firewall. When an /sbin/shorewall stop or /sbin/shorewall clear command is subsequently executed, /var/lib/shorewall/firewall is run to perform the requested operation.

Shorewall Packages

Shorewall 4.5 and later consists of six packages.

1. Shorewall-core. All of the other packages depend on this one.
2. Shorewall. This package must be installed on at least one system in your network. It contains everything needed to create an IPv4 firewall.
3. Shorewall6. This package requires the Shorewall package and adds those components needed to create an IPv6 fireawall.
4. Shorewall-lite. Shorewall allows for central administration of multiple IPv4 firewalls through use of Shorewall lite. The full Shorewall product is installed on a central administrative system where compiled Shorewall scripts are generated. These scripts are copied to the firewall systems where they run under the control of Shorewall-lite.
5. Shorewall6-lite. Shorewall allows for central administration of multiple IPv6 firewalls through use of Shorewall6 lite. The full Shorewall and Shorewall6 products are installed on a central administrative system where compiled Shorewall scripts are generated. These scripts are copied to the firewall systems where they run under the control of Shorewall6-lite.
6. Shorewall-init. May be installed with any of the other firewall packages. Allows the firewall to be close prior to bringing up network interfaces. It can also react to interface up/down events.