Copyright © 2002-2009 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”.
2020/02/16
Table of Contents
Caution
This article applies to Shorewall 4.4 and later. If you are running a version of Shorewall earlier than Shorewall 4.4.0 then please see the documentation for that release.
Setting up Shorewall on a standalone Linux system is very easy if you understand the basics and follow the documentation.
This guide doesn't attempt to acquaint you with all of the features of Shorewall. It rather focuses on what is required to configure Shorewall in one of its most common configurations:
Linux system
Single external IP address
Connection through Cable Modem, DSL, ISDN, Frame Relay, dial-up... or connected to a LAN and you simply wish to protect your Linux system from other systems on that LAN.
Shorewall requires 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 read through the guide first to familiarize yourself with what's involved then go back through it again making your configuration changes.
Caution
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. 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.
If you have an ADSL Modem and you use PPTP to communicate with a server in that modem, you must make the changes recommended here in addition to those detailed below. ADSL with PPTP is most commonly found in Europe, notably in Austria.
The configuration files for Shorewall are contained in the directory
/etc/shorewall -- for simple
setups, you only need to deal with a few of these as described in this
guide. After you have installed
Shorewall, you can find the Samples as follows:
If you installed using an RPM, the samples will be in the
Samples/one-interfacesubdirectory of the Shorewall documentation directory. If you don't know where the Shorewall documentation directory is, you can find the samples using this command:~# rpm -ql shorewall | fgrep one-interface /usr/share/doc/packages/shorewall/Samples/one-interface /usr/share/doc/packages/shorewall/Samples/one-interface/interfaces /usr/share/doc/packages/shorewall/Samples/one-interface/policy /usr/share/doc/packages/shorewall/Samples/one-interface/rules /usr/share/doc/packages/shorewall/Samples/one-interface/zones ~#
If you installed using the tarball, the samples are in the
Samples/one-interfacedirectory in the tarball.
If you installed using a Shorewall 4.x .deb, the samples are in
/usr/share/doc/shorewall/examples/one-interface.. You do not need the shorewall-doc package to have access to the samples.
Warning
Note to Debian Users
You will find that your /etc/shorewall directory is empty. This is
intentional. If you need configuration files other than those found in
/usr/share/doc/shorewall/examples/one-interface,
they may be found on your system in the directory /usr/share/doc/shorewall/default-config.
Simply copy the files you need from that directory to /etc/shorewall and modify the
copies.
As each file is introduced, I suggest that you look at the actual
file on your system and that you look at the man page for that
file. For example, to look at the man page for the
/etc/shorewall/zones file, type man
shorewall-zones at a shell prompt.
Note: Beginning with Shorewall 4.4.20.1, there are versions of the sample files that are annotated with the corresponding manpage contents. These files have names ending in '.annotated'. You might choose to look at those files instead.
Shorewall views the network where it is running as being composed of a set of zones. In the one-interface sample configuration, only two zones are defined:
#ZONE TYPE OPTIONS IN OUT # OPTIONS OPTIONS fw firewall net ipv4
Shorewall zones are defined in /etc/shorewall/zones.
Note that Shorewall recognizes the firewall system as its own zone.
When the /etc/shorewall/zones file is processed, the
name of the firewall zone (“fw” in the above example) is
stored in the shell variable $FW which may be used
to refer to the firewall zone throughout the Shorewall
configuration.
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/policyfile.You define exceptions to those default policies in the
/etc/shorewall/rulesfile.
For each connection request entering the firewall, the request is
first checked against the
/etc/shorewall/rules/etc/shorewall/policy that matches the request is
applied. If there is a common
action defined for the policy in
/etc/shorewall/actions or
/usr/share/shorewall/actions.std then that action is
performed before the policy is applied. The purpose of the common action
is two-fold:
It silently drops or rejects harmless common traffic that would otherwise clutter up your log — Broadcasts for example.
If ensures that traffic critical to correct operation is allowed through the firewall — ICMP fragmentation-needed for example.
The /etc/shorewall/policy file included with
the one-interface sample has the following policies:
#SOURCE DEST POLICY LOGLEVEL LIMIT $FW net ACCEPT net all DROP info all all REJECT info
The above policy will:
allow all connection requests from the firewall to the Internet
drop (ignore) all connection requests from the Internet to your firewall
reject all other connection requests (Shorewall requires this catchall policy).
The word info in the LOG LEVEL column for the last two policies indicates that packets dropped or rejected under those policies should be logged at that level.
At this point, edit your /etc/shorewall/policy
and make any changes that you wish.
The firewall has a single network interface. Where Internet
connectivity is through a cable or DSL
“Modem”, the External Interface will be
the Ethernet adapter (eth0) that
is connected to that “Modem” 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, your external interface will be ippp0.
Caution
Be sure you know which interface is your external interface. Many hours have been spent floundering by users who have configured the wrong interface. If you are unsure, then as root type ip route ls at the command line. The device listed in the last (default) route should be your external interface.
Example:
root@lists:~# ip route ls
192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1
10.13.10.0/24 dev tun1 scope link
192.168.2.0/24 via 192.168.2.2 dev tun0
206.124.146.0/24 dev eth0 proto kernel scope link src 206.124.146.176
10.10.10.0/24 dev tun1 scope link
default via 206.124.146.254 dev eth0
root@lists:~# In that example, eth0 is
the external interface.
The Shorewall one-interface sample configuration assumes that the
external interface is eth0. If
your configuration is different, you will have to modify the sample
/etc/shorewall/interfaces file accordingly. While you
are there, you may wish to review the list of options that are specified
for the interface. Some hints:
Tip
If your external interface is ppp0 or ippp0 or if you have a static IP address,
you can remove “dhcp” from the option list.
Before going further, we should say a few words about Internet Protocol (IP) addresses. Normally, your Internet Service Provider (ISP) will assign you a single IP address. That address can be assigned statically, by the Dynamic Host Configuration Protocol (DHCP), through the establishment of your dial-up connection, or during establishment of your other type of PPP (PPPoA, PPPoE, etc.) connection.
RFC-1918 reserves several Private IP address ranges for use in private networks:
10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255
These addresses are sometimes referred to as non-routable because the Internet backbone routers will not forward a packet whose destination address is reserved by RFC-1918. In some cases though, ISPs are assigning these addresses then using Network Address Translation - NAT) to rewrite packet headers when forwarding to/from the Internet.
Shorewall does not maintain a log itself but rather relies on your system's logging configuration. The following commands rely on knowing where Netfilter messages are logged:
shorewall show log (Displays the last 20 Netfilter log messages)
shorewall logwatch (Polls the log at a settable interval
shorewall dump (Produces an extensive report for inclusion in Shorewall problem reports)
It is important that these commands work properly because when you encounter connection problems when Shorewall is running, the first thing that you should do is to look at the Netfilter log; with the help of Shorewall FAQ 17, you can usually resolve the problem quickly.
The Netfilter log location is distribution-dependent:
Debian and its derivatives log Netfilter messages to
/var/log/kern.log.Recent SuSE/OpenSuSE™ releases come preconfigured with syslog-ng and log netfilter messages to
/var/log/firewall.For other distributions, Netfilter messages are most commonly logged to
/var/log/messages.
If you are running a distribution that logs Netfilter messages to a
log other than /var/log/messages, then modify the
LOGFILE setting in /etc/shorewall/shorewall.conf to
specify the name of your log.
Important
The LOGFILE setting does not control where the Netfilter log is
maintained -- it simply tells the /sbin/shorewall
utility where to find the log.
Beginning in Shorewall 4.4.7,
/etc/shorewall/shorewall.conf contains a
LOAD_HELPERS_ONLY option which is set to Yes in the
samples. This causes Shorewall to attempt to load the modules listed in
/usr/share/shorewall/helpers. In addition, it sets
sip_direct_media=0 when loading the
nf_conntrack_sip module. That setting is somewhat less secure than
sip_direct_media=1, but it generally
makes VOIP through the firewall work much better.
The modules in /usr/share/shorewall/helpers are
those that are not autoloaded. If your kernel does not support module
autoloading and you want Shorewall to attempt to load all netfilter
modules that it might require, then set LOAD_HELPERS_ONLY=No. That will
cause Shorewall to try to load the modules listed in
/usr/share/shorewall/modules. That file does not set
sip_direct_media=0.
Important
In Shorewall 5.2.3, the LOAD_HELPERS_ONLY option was removed and the behavior is the same as if LOAD_HELPERS_ONLY=Yes.
If you need to modify either
/usr/share/shorewall/helpers or
/usr/share/shorewall/modules then copy the file to
/etc/shorewall and modify the copy.
Modify the setting of LOAD_HELPER_ONLY as necessary.
Shorewall includes a collection of macros that can be used to
quickly allow or deny services. You can find a list of the macros included
in your version of Shorewall using the command ls
/usr/share/shorewall/macro.*.
If you wish to enable connections from the Internet to your firewall
and you find an appropriate macro in
/usr/share/shorewall/macro.*, the general format of a
rule in /etc/shorewall/rules is:
#ACTION SOURCE DEST PROTO DPORT
<macro>(ACCEPT) net $FWImportant
Be sure to add your rules after the line that reads ?SECTION NEW.
Example 1. You want to run a Web Server and a IMAP Server on your firewall system:
#ACTION SOURCE DEST PROTO DPORT Web(ACCEPT) net $FW IMAP(ACCEPT)net $FW
Caution
The Shorewall-provided macros assume that the associated service is using it's standard port and will not work with services listening on a non-standard port.
You may also choose to code your rules directly without using the
pre-defined macros. This will be necessary in the event that there is not
a pre-defined macro that meets your requirements. In that case the general
format of a rule in /etc/shorewall/rules is:
#ACTION SOURCE DEST PROTO DPORT ACCEPT net $FW <protocol> <port>
Example 2. You want to run a Web Server and a IMAP Server on your firewall system:
#ACTION SOURCE DEST PROTO DPORT ACCEPT net $FW tcp 80 ACCEPT net $FW tcp 143
If you don't know what port and protocol a particular application uses, see here.
Important
I don't recommend enabling telnet to/from the Internet because it uses clear text (even for login!). If you want shell access to your firewall from the Internet, use SSH:
#ACTION SOURCE DESTINATION PROTO DPORT SSH(ACCEPT) net $FW
At this point, edit /etc/shorewall/rules to add
other connections as desired.
The installation procedure configures your system to start Shorewall at system boot but startup is disabled so that your system won't try to start Shorewall before configuration is complete. Once you have completed configuration of your firewall, you must edit /etc/shorewall/shorewall.conf and set STARTUP_ENABLED=Yes.
Important
Users of the .deb package must edit
/etc/default/shorewall and set
startup=1.
Important
You must enable startup by editing
/etc/shorewall/shorewall.conf and setting
STARTUP_ENABLED=Yes.
While you are editing shorewall.conf, it is a
good idea to check the value of the SUBSYSLOCK option. You can find a
description of this option by typing 'man shorewall.conf' at a shell
prompt and searching for SUBSYSLOCK.
The firewall is started using the “shorewall
start” command and stopped using
“shorewall stop”. When the firewall is
stopped, traffic is enabled on those hosts that have an entry in
/etc/shorewall/stoppedrules
(/etc/shorewall/routestopped
in Shorewall 4.5.7 and earlier). A running firewall may be restarted using
the “shorewall reload” command. If you
want to totally remove any trace of Shorewall from your Netfilter
configuration, use “shorewall
clear”.
Warning
If you are connected to your firewall from the Internet, do not issue a “shorewall stop” command unless you have either:
Used ADMINISABSENTMINDED=Yes in
/etc/shorewall/shorewall.conforadded an entry for the IP address that you are connected from to
/etc/shorewall/routestopped.
Also, I don't recommend using “shorewall reload”; it is better to create an alternate configuration and test it using the “shorewall try” command.
The firewall will start after your network interface has been brought up. This leaves a small window between the time that the network interface is working and when the firewall is controlling connections through that interface. If this is a concern, you can close that window by installing the Shorewall Init Package.
Re-check each of the items flagged with a red arrow above.
Check your log.
Check the Troubleshooting Guide.
Check the FAQ.
Before starting Shorewall for the first time, it's a good idea to stop your existing firewall. On older Redhat/CentOS/Fedora:
service iptables stopOn recent Fedora systems that run systemd, the command is:
systemctl stop iptables.serviceIf you are running SuSE, use Yast or Yast2 to stop SuSEFirewall.
On other systems that use a classic SysV init system:
/etc/init.d/iptables stopOnce you have Shorewall running to your satisfaction, you should totally disable your existing firewall. On older Redhat/CentOS/Fedora:
chkconfig --del iptablesOn Debian systems:
update-rc.d iptables disableOn recent Fedora system running systemd:
systemctl disable iptables.service
At this point, disable your existing firewall service.
I highly recommend that you review the Common Configuration File Features page -- it contains helpful tips about Shorewall features than make administering your firewall easier. Also, Operating Shorewall and Shorewall Lite contains a lot of useful operational hints.
