Configuration Files

Warning: If you copy or edit your configuration files on a system running Microsoft Windows, you must run them through dos2unix before you use them with Shorewall.

Files

Shorewall's configuration files are in the directory /etc/shorewall.

• /etc/shorewall/shorewall.conf - used to set several firewall parameters.
• /etc/shorewall/params - use this file to set shell variables that you will expand in other files.
• /etc/shorewall/zones - partition the firewall's view of the world into zones.
• /etc/shorewall/policy - establishes firewall high-level policy.
• /etc/shorewall/interfaces - describes the interfaces on the firewall system.
• /etc/shorewall/hosts - allows defining zones in terms of individual hosts and subnetworks.
• /etc/shorewall/masq - directs the firewall where to use many-to-one (dynamic) Network Address Translation (a.k.a. Masquerading) and Source Network Address Translation (SNAT).
• /etc/shorewall/modules - directs the firewall to load kernel modules.
• /etc/shorewall/rules - defines rules that are exceptions to the overall policies established in /etc/shorewall/policy.
• /etc/shorewall/nat - defines static NAT rules.
• /etc/shorewall/proxyarp - defines use of Proxy ARP.
• /etc/shorewall/routestopped (Shorewall 1.3.4 and later) - defines hosts accessible when Shorewall is stopped.
• /etc/shorewall/tcrules - defines marking of packets for later use by traffic control/shaping or policy routing.
• /etc/shorewall/tos - defines rules for setting the TOS field in packet headers.
• /etc/shorewall/tunnels - defines IPSEC, GRE and IPIP tunnels with end-points on the firewall system.
• /etc/shorewall/blacklist - lists blacklisted IP/subnet/MAC addresses.

Comments

You may place comments in configuration files by making the first non-whitespace character a pound sign ("#"). You may also place comments at the end of any line, again by delimiting the comment from the rest of the line with a pound sign.

Examples:

# This is a comment

ACCEPT	net	fw	tcp	www	#This is an end-of-line comment

Line Continuation

You may continue lines in the configuration files using the usual backslash ("\") followed immediately by a new line character.

Example:

ACCEPT	net	fw	tcp \
smtp,www,pop3,imap  #Services running on the firewall

Using DNS Names

WARNING: I personally recommend strongly against using DNS names in Shorewall configuration files. If you use DNS names and you are called out of bed at 2:00AM because Shorewall won't start as a result of DNS problems then don't say that you were not forewarned.

    -Tom

Beginning with Shorwall 1.3.9, Host addresses in Shorewall configuration files may be specified either as IP addresses or as DNS Names.

DNS names in iptables rules aren't nearly as useful as they first appear. When a DNS name appears in a rule, the iptables utility resolves the name to one or more IP addresses and inserts those addresses into the rule. So change in the DNS->IP address relationship that occur after the firewall has started have absolutely no effect on the firewall's ruleset.

If your firewall rules include DNS names then:

• If your /etc/resolv.conf is wrong then your firewall won't start.
• If your /etc/nsswitch.conf is wrong then your firewall won't start.
• If your Name Server(s) is(are) down then your firewall won't start.
• If your startup scripts try to start your firewall before starting your DNS server then your firewall won't start.
  
• Factors totally outside your control (your ISP's router is down for example), can prevent your firewall from starting.
• You must bring up your network interfaces prior to starting your firewall.
  

Each DNS name much be fully qualified and include a minumum of two periods (although one may be trailing). This restriction is imposed by Shorewall to insure backward compatibility with existing configuration files.

Examples of valid DNS names:

• mail.shorewall.net
• shorewall.net.

• mail (not fully qualified)
• shorewall.net (only one period)

• The server address in a DNAT rule (/etc/shorewall/rules file)
• In the ADDRESS column of an entry in /etc/shorewall/masq.
• In the /etc/shorewall/nat file.

Complementing an Address or Subnet

Where specifying an IP address, a subnet or an interface, you can precede the item with "!" to specify the complement of the item. For example, !192.168.1.4 means "any host but 192.168.1.4".

Comma-separated Lists

Comma-separated lists are allowed in a number of contexts within the configuration files. A comma separated list:

• Must not have any embedded white space.
  Valid: routestopped,dhcp,norfc1918
  Invalid: routestopped,     dhcp,     norfc1818
• If you use line continuation to break a comma-separated list, the continuation line(s) must begin in column 1 (or there would be embedded white space)
• Entries in a comma-separated list may appear in any order.

Port Numbers/Service Names

Unless otherwise specified, when giving a port number you can use either an integer or a service name from /etc/services.

Port Ranges

If you need to specify a range of ports, the proper syntax is <low port number>:<high port number>.

Using Shell Variables

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.

Using MAC Addresses

Media Access Control (MAC) addresses can be used to specify packet source in several of the configuration files. To use this feature, your kernel must have MAC Address Match support (CONFIG_IP_NF_MATCH_MAC) included.

MAC addresses are 48 bits wide and each Ethernet Controller has a unique MAC address.

In GNU/Linux, MAC addresses are usually written as a series of 6 hex numbers separated by colons. Example:

     [root@gateway root]# ifconfig eth0
     eth0 Link encap:Ethernet HWaddr 02:00:08:E3:FA:55
     inet addr:206.124.146.176 Bcast:206.124.146.255 Mask:255.255.255.0
     UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
     RX packets:2398102 errors:0 dropped:0 overruns:0 frame:0
     TX packets:3044698 errors:0 dropped:0 overruns:0 carrier:0
     collisions:30394 txqueuelen:100
     RX bytes:419871805 (400.4 Mb) TX bytes:1659782221 (1582.8 Mb)
     Interrupt:11 Base address:0x1800

Because Shorewall uses colons as a separator for address fields, Shorewall requires MAC addresses to be written in another way. In Shorewall, MAC addresses begin with a tilde ("~") and consist of 6 hex numbers separated by hyphens. In Shorewall, the MAC address in the example above would be written "~02-00-08-E3-FA-55".

Shorewall Configurations

Shorewall allows you to have configuration directories other than /etc/shorewall. The shorewall start and restart commands allow you to specify an alternate configuration directory and Shorewall will use the files in the alternate directory rather than the corresponding files in /etc/shorewall. The alternate directory need not contain a complete configuration; those files not in the alternate directory will be read from /etc/shorewall.

This facility permits you to easily create a test or temporary configuration by:

1. copying the files that need modification from /etc/shorewall to a separate directory;
2. modify those files in the separate directory; and
3. specifying the separate directory in a shorewall start or shorewall restart command (e.g., shorewall -c /etc/testconfig restart ).

Updated 9/24/2002 - Tom Eastep

Copyright © 2001, 2002 Thomas M. Eastep.