Shorewall on RPM-based Servers

Alteeve Wiki :: How To :: Shorewall on RPM-based Servers

This covers setup and maintenance of Shorewall 4.5 on Red Hat Enterprise Linux 5.x, 6.x, RHEL derivatives and several recent Fedora releases.

Install

Note: Updated on 2015-05-05 for Shorewall release 4.6.8.

Install is trivial, we just need to install dependencies and and the latest RPMs.

yum install perl perl-Digest-SHA perl-Digest-SHA1

You can check for the latest version here.

rpm -Uvh http://canada.shorewall.net/pub/shorewall/4.6/shorewall-4.6.8/shorewall-core-4.6.8-0base.noarch.rpm \
         http://canada.shorewall.net/pub/shorewall/4.6/shorewall-4.6.8/shorewall-4.6.8-0base.noarch.rpm

Done!

Setup

You need to decide which interface will have your internet connection on it and which will connect to your internal network. For this tutorial;

• eth0; Faces the internal network, has the IP 10.255.255.254/16 and provides DHCP services to the LAN.
• eth1; Faces the Internet.

Configuring Shorewall

All configuration files are in the /etc/shorewall directory, unless explicitly defined. The main Shorewall configuration file, which we will edit last is /etc/shorewall/shorewall.conf.

The files to edit are listed in the order we will edit them in the following subsections.

zones

This controls the main "zones" used by Shorewall. The fw is special in that it defines the firewall itself. The net zone is the Internet-facing network (eth1 in this tutorial). The loc is the local network, the internal network of machines the firewall is protecting, which is eth0 in this tutorial. Both eth0 and eth1 are ipv4 networks.

Append two new lines telling shorewall that we have two new ipv4 networks that it will use:

vim /etc/shorewall/zones

net     ipv4
loc     ipv4

So that the zones file looks like:

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

interfaces

Just above, we told shorewall that we had two new ipv4 networks. In the interfaces configuration file, we link these networks to physical interfaces.

Note: If you plan to setup a DHCP server on your firewall, you will need to specify the dhcp option, as shown here. You can see a full list of options and their uses on Shorewall's interfaces page.

To link the new networks to the physical interfaces, append the following entries;

vim /etc/shorewall/interfaces

loc             eth0                    dhcp
net             eth1

So that the interfaces file looks like:

###############################################################################
?FORMAT 2
###############################################################################
#ZONE           INTERFACE               OPTIONS
loc             eth0                    dhcp
net             eth1

policy

Here you tell shorewall what the default policy is for each network when receiving new connection requests. You don't need to worry about ESTABLISHED and RELATED connections as shorewall handles these rules. The choices are:

• ACCEPT; Accept the connection.
• DROP; Ignore the connection request.
• REJECT; Return an appropriate error to the connection request.

You can also set the log level for connection requests that fall off the chain and hit these policies. It's a good idea to log info level so you can see twits trying to do "bad things(tm)". The one downside to using info is that it pushes a lot of data into the log files, which might make debugging other issues on the firewall. It's really up to you in the end.

Append the following default policies;

vim /etc/shorewall/policy

# Let everything from the firewall machine out onto the net.
fw      net     ACCEPT

# Likewise, allow everything from the firewall out onto the local network.
fw      loc     ACCEPT

# Don't allow incoming connections from the web into the fireall *or* into the
# local network. Add 'info' here if you want to log failed connection attempts.
net     all     DROP            info

# Don't allow incoming connections from the local network into the firewall.
loc     fw      DROP

# Let machines on the local network out onto the web
loc     net     ACCEPT

So that the policy file looks like:

###############################################################################
#SOURCE DEST    POLICY          LOG     LIMIT:          CONNLIMIT:
#                               LEVEL   BURST           MASK

# Let everything from the firewall machine out onto the net.
fw      net     ACCEPT

# Likewise, allow everything from the firewall out onto the local network.
fw      loc     ACCEPT

# Don't allow incoming connections from the web into the fireall *or* into the
# local network. Add 'info' here if you want to log failed connection attempts.
net     all     DROP            info

# Don't allow incoming connections from the local network into the firewall.
loc     fw      DROP

# Let machines on the local network out onto the web
loc     net     ACCEPT

rules

This is really the heart of the firewall.

Here you tell shorewall what the exceptions there are to the default policies. The first rule to match is used.

The example below shows a setup where remote access in to the firewall itself is allowed only on port 22000 (modified SSH port). Then two Microsoft Windows servers are setup. Both servers are internally set to listen for RDP connections on the same default port (3389). To allow for this with just one external IP address, the firewall is told to route incoming connections on port 3390 to the internal machine at IP 10.255.0.11 on port 3393. Likewise, incoming connections on port 3389 will be forwarded to directly to 10.255.0.10:3389.

We'll also add a couple special rules that tells shorewall to respond to ICMP ping requests. Some people don't like this as ping sweeps are a quick way for malicious people to find servers on the net. Personally, I find the usefulness of being able to ping my firewall more beneficial.

Append rules so that the it file looks like.

vim /etc/shorewall/rules

### Rules for data going into the firewall. Consult /etc/services or your local
### search engine for ports and protocols used by your favourite programs.
# Answer ICMP queries
Ping(ACCEPT)	net		fw
Ping(ACCEPT)	loc		fw

# Allow incoming SSH connections to the firewall itself from the web on port
# 22000. Allow incoming SSH connections to the firewall on port 22 and 22000
# from the local network.
ACCEPT          net             fw                      tcp     22000
ACCEPT          loc             fw                      tcp     22
ACCEPT          loc             fw                      tcp     22000

# Allow incoming connections from the internet to two windows servers listening
# for RDP connections on the same port. This will be handled using different
# external ports using destination network address translation.
ACCEPT          net             loc:10.255.0.10:3389    tcp     3389
ACCEPT          net             loc:10.255.0.11:3389    tcp     3390

This is a spartan example of what you can do. It's meant to show how you can do matching and non-matching TCP port forwards. With this simple format, you should be able to create all the rules you need to setup your network.

masq

This is the file that handles MASQerading the machines on the local LAN (the loc zone). This is how shorewall provides internet access to an entire subnet of machines on a given network.

So to enable Internet access from your machines, you need to add a line with the Internet facing interface followed by the subnet of the local network that you will be masquerading.

vim /etc/shorewall/masq

eth1                    10.255.0.0/16

So that the masq file looks like:

################################################################################################################
#INTERFACE:DEST         SOURCE          ADDRESS         PROTO   PORT(S) IPSEC   MARK    USER/   SWITCH  ORIGINAL
#                                                                                       GROUP           DEST
eth1                    10.255.0.0/16

shorewall.conf

Once you have the above files in place, you need to enable the firewall.

Edit /etc/shorewall/shorewall.conf and change the following lines:

vim /etc/shorewall/shorewall.conf

STARTUP_ENABLED=No

To:

STARTUP_ENABLED=Yes

Starting the Firewall

To start the firewall, simply run:

/etc/init.d/shorewall restart

The firewall should now be running. To see the new rules, simply run:

iptables-save

This will print out the actual firewall rules. You will need some experience with iptables to understand all their meaning, but the general flow should be understandable.

Lastly, make sure the firewall starts on boot by running:

chkconfig shorewall on

Fixing SELinux Problems

If you get an error like:

/etc/init.d/shorewall start

Compiling...
Can't exec "/usr/lib/shorewall/getparams": Permission denied at /usr/share/perl5/Shorewall/Config.pm line 5041.
   ERROR: Processing of /etc/shorewall/params failed

If we check /var/log/audit/audit.log, we see:

type=AVC msg=audit(1403851868.309:165): avc:  denied  { execute_no_trans } for  pid=11114 comm="perl" path="/usr/lib/shorewall/getparams" dev=sda3 ino=1705335 scontext=unconfined_u:system_r:shorewall_t:s0 tcontext=system_u:object_r:lib_t:s0 tclass=file

To fix this, run:

semanage fcontext -a -t bin_t /usr/lib/shorewall/getparams
restorecon -vF /usr/lib/shorewall/getparams

Now shorewall should start properly.

/etc/init.d/shorewall start

Compiling...
Shorewall configuration compiled to /var/lib/shorewall/.start
Starting Shorewall....
done.

 

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