# Shorewall Version 4

Tom
Eastep
2007
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.

## Introduction

Shorewall version 4 is currently in development and is available for beta testing.

Shorewall version 4 represents a substantial shift in direction for Shorewall. Up to now

- Shorewall has been written entirely in Bourne Shell.

- Shorewall ran the `iptables` utility to add each Netfilter rule.

Shorewall version 4 offers you a choice. You can continue to use the existing shell-based implementation or you can use a new implementation of the Shorewall compiler written in the Perl programming language. The new compiler:

- has a small disk footprint

- is very fast.

- generates a firewall script that uses `iptables-restore`; so the script is very fast.

Both compilers may be installed on your system and you can use whichever one suits you in a particular case.

## Installing Shorewall Version 4

You can download the development version of Shorewall Version 4 from any of the download sites with the exception of SourceForge. It is contained in the `/pub/shorewall/development/4.0` directory.

Shorewall 4 contains four packages:

- Shorewall-shell - the old shell-based compiler and related components.

- Shorewall-perl - the new Perl-based compiler. May be installed under Shorewall 3.4.2 or later or 4.0.x.

- Shorewall - the part of Shorewall common to both compilers.

- Shorewall-lite- same as the 3.4 version of Shorewall Lite. Can run scripts generated by either Shorewall-perl or Shorewall-shell.

If you upgrade to Shorewall Version 4, you must install Shorewall-shell and/or Shorewall-perl; in fact, if you are using the tarball for your installation, you must install Shorewall-shell and/or Shorewall-perl before you upgrade Shorewall.

## Prerequisites for using the Shorewall Version 4 Perl-based Compiler

- Perl (I use Perl 5.8.8 but other versions should work fine)

- Perl Cwd Module

- Perl File::Basename Module

- Perl File::Temp Module

## Incompatibilities Introduced in the Shorewall Version 4 Perl-based Compiler

The Shorewall-perl compiler is not 100% compatible with the Shorewall-shell version.

1. The Perl-based compiler requires the following capabilities in your kernel and iptables.

   - addrtype match (may be relaxed later)

   - multiport match (will not be relaxed)

   These capabilities are in current distributions.

2. Now that Netfilter has features to deal reasonably with port lists, I see no reason to duplicate those features in Shorewall. The Shorewall-shell compiler goes to great pain (in some cases) to break very long port lists ( > 15 where port ranges in lists count as two ports) into individual rules. In the new compiler, I'm avoiding the ugliness required to do that. The new compiler just generates an error if your list is too long. It will also produce an error if you insert a port range into a port list and you don't have extended multiport support.

3. BRIDGING=Yes is not supported. The kernel code necessary to support this option was removed in Linux kernel 2.6.20.

4. The BROADCAST column in the interfaces file is essentially unused; if you enter anything in this column but '-' or 'detect', you will receive a warning. This will be relaxed if and when the addrtype match requirement is relaxed.

5. The 'refresh' command is now synonymous with 'restart'.

6. Because the compiler is now written in Perl, your compile-time extension scripts from earlier versions will no longer work. Compile-time extension scripts are executed using the Perl 'eval `cat <file>`' mechanism. Be sure that each script returns a 'true' value; otherwise, the compiler will assume that the script failed and will abort the compilation.

   When a script is invoked, the $chainref scalar variable will hold a reference to a chain table entry.

   - $chainref->{name} contains the name of the chain
   - $chainref->{table} holds the table name

   To add a rule to the chain:

   - add_rule $chainref, <the rule>

   Where

   - <the rule> is a scalar argument holding the rule text. Do not include "-A <chain name>"

   Example:

   - add_rule $chainref, '-j ACCEPT';

   To insert a rule into the chain:

   - insert_rule $chainref, <rulenum>, <the rule>

   The log_rule_limit function works like it does in the shell compiler with two exceptions:

   - You pass the chain reference rather than the name of the chain.

   - The commands are 'add' and 'insert' rather than '-A' and '-I'.

   - There is only a single "pass as-is to iptables" argument (so you must quote that part

   Example:

           log_rule_limit
                     'info' ,
                     $chainref ,
                     $chainref->{name},
                     'DROP' ,
                     '',    #Limit
                     '' ,   #Log tag
                     'add'
                     '-p tcp ';

7. The `/etc/shorewall/tos` file now has zone-independent SOURCE and DEST columns as do all other files except the rules and policy files.

   The SOURCE column may be one of the following:

   - [`all`:]<address>[,...]
   - [`all`:]<interface>[:<address>[,...]]
   - `$FW`[:<address>[,...]]

   The DEST column may be one of the following:

   - [`all`:]<address>[,...]
   - [`all`:]<interface>[:<address>[,...]]

   This is a permanent change. The old zone-based rules have never worked right and this is a good time to replace them. I've tried to make the new syntax cover the most common cases without requiring change to existing files. In particular, it will handle the tos file released with Shorewall 1.4 and earlier.

8. Currently, support for ipsets is untested. That will change with future pre-releases but one thing is certain -- Shorewall is now out of the ipset load/reload business. With scripts generated by the Perl-based Compiler, the Netfilter ruleset is never cleared. That means that there is no opportunity for Shorewall to load/reload your ipsets since that cannot be done while there are any current rules using ipsets.

   So:

   1. Your ipsets must be loaded before Shorewall starts. You are free to try to do that with the following code in `/etc/shorewall/start`:

          if [ "$COMMAND" = start ]; then
              ipset -U :all: :all:
              ipset -F
              ipset -X
              ipset -R < /etc/shorewall/ipsets
          fi

      The file `/etc/shorewall/ipsets` will normally be produced using the `ipset -S` command.

      The above will work most of the time but will fail in a `shorewall stop` - `shorewall start` sequence if you use ipsets in your routestopped file (see below).

   2. Your ipsets may not be reloaded until Shorewall is stopped or cleared.

   3. If you specify ipsets in your routestopped file then Shorewall must be cleared in order to reload your ipsets.

   As a consequence, scripts generated by the Perl-based compiler will ignore `/etc/shorewall/ipsets` and will issue a warning if you set SAVE_IPSETS=Yes in `shorewall.conf`.

9. Because the configuration files (with the exception of `/etc/shorewall/params`) are now processed by the Shorewall-perl compiler rather than by the shell, only the basic forms of Shell expansion ($variable and ${variable}) are supported. The more exotic forms such as ${variable:=default} are not supported. Both variables defined in /etc/shorewall/params and environmental variables (exported by the shell) can be used in configuration files.

10. USE_ACTIONS=No is not supported. That option is intended to minimize Shorewall's footprint in embedded applications. As a consequence, Default Macros are not supported.

11. DELAYBLACKLISTLOAD=Yes is not supported. The entire ruleset is atomically loaded with one execution of `iptables-restore`.

12. MAPOLDACTIONS=Yes is not supported. People should have converted to using macros by now.

13. The pre Shorewall-3.0 format of the zones file is not supported; neither is the `/etc/shorewall/ipsec` file.

14. BLACKLISTNEWONLY=No is not permitted with FASTACCEPT=Yes. This combination doesn't work in previous versions of Shorewall so the Perl-based compiler simply rejects it.

15. Shorewall-perl has a single rule generator that is used for all rule-oriented files. So it is important that the syntax is consistent between files.

    With shorewall-shell, there is a special syntax in the SOURCE column of /etc/shorewall/masq to designate "all traffic entering the firewall on this interface except...".

    Example:

        #INTERFACE       SOURCE                  ADDRESSES
        eth0             eth1!192.168.4.9        ...

    Shorewall-perl uses syntax that is consistent with the rest of Shorewall:

        #INTERFACE       SOURCE                  ADDRESSES
        eth0             eth1:!192.168.4.9       ...

16. The 'allowoutUPnP' built-in action is no longer supported. In kernel 2.6.14, the Netfilter team have removed support for '-m owner --owner-cmd' which that action depended on.

## Compiler Selection

If you only install one compiler, then that compiler will be used.

If you install both compilers, then the compiler actually used depends on the SHOREWALL_COMPILER setting in `shorewall.conf`.

The value of this new option can be either 'perl' or 'shell'.

If you add 'SHOREWALL_COMPILER=perl' to `/etc/shorewall/shorewall.conf` then by default, the new compiler will be used on the system. If you add it to `shorewall.conf` in a separate directory (such as a Shorewall-lite export directory) then the new compiler will only be used when you compile from that directory.

If you only install one compiler, it is suggested that you do not set SHOREWALL_COMPILER.

You can select the compiler to use on the command line using the 'C option:

- '-C shell' means use the shell compiler
- '-C perl' means use the perl compiler

The -C option overrides the setting in shorewall.conf.

Example:

    shorewall restart -C perl

Regardless of the setting of SHOREWALL_COMPILER, there is one change in Shorewall operation that is triggered simply by installing shorewall-perl. Your params file will be processed during compilation with the shell's '-a' option which causes any variables that you set or create in that file to be automatically exported. Since the params file is processed before shorewall.conf, using -a insures that the settings of your params variables are available to the new compiler should its use be specified in shorewall.conf.