This article offers hints about how to accomplish common tasks with Shorewall. The Introduction to Shorewall is required reading for being able to use this article effectively. For information about setting up your first Shorewall-based firewall, see the Quickstart Guides.

If you need to change a file in /usr/share/shorewall/, copy it to /etc/shorewall and modify the copy

Man pages are provided in section 5 for each of the Shorewall configuration files. The name of the page is formed by prefixing the file name with "shorewall-".

Example — To view the manual page for /etc/shorewall/interfaces:

man shorewall-interfaces

The /etc/shorewall/shorewall.conf file is an exception -- the man page for that file is 'shorewall.conf':

man shorewall.conf

Parts of this and other articles are also available as manpages:

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.

# This is a comment
ACCEPT  net     $FW      tcp     www     #This is an end-of-line comment

When you define an object in Shorewall (Zone, Logical Interface, ipsets, Actions, etc., you give it a name. Shorewall names start with a letter and consist of letters, digits or underscores ("_"). Except for Zone names, Shorewall does not impose a limit on name length.

When an ipset is referenced, the name must be preceded by a plus sign ("+").

The last character of an interface may also be a plus sign to indicate a wildcard name.

Physical interface names match names shown by 'ip link ls'; if the name includes an at sign ("@"), do not include that character or any character that follows. For example, "sit1@NONE" is referred to as simply 'sit1".

For a pair of zones, Shorewall creates two Netfilter chains; one for connections in each direction. The names of these chains are formed by separating the names of the two zones by either "2" or "-".

Example: Traffic from zone A to zone B would go through chain A2B (think "A to B") or "A-B".

In Shorewall 4.6, the default separator is "-" but you can override that by setting ZONE_SEPARATOR="2" in shorewall.conf (5).

Zones themselves have names that begin with a letter and are composed of letters, numerals, and "_". The maximum length of a name is dependent on the setting of LOGFORMAT in shorewall.conf (5). See shorewall-zones (5) for details.

Shorewall probes your system to determine the features that it supports. The result of this probing is a set of capabilities. This probing is normally done each time that the compiler is run but can also be done by executing the shorewall show capabilities command. Regardless of whether the compiler or the command does the probing, this probing may produce error messages in your system log. These log messages are to be expected and do not represent a problem; they merely indicate that capabilities that are being probed are not supported on your system.

Probing may be suppressed by using a capabilities file. A capabilities file may be generated using this command:

shorewall show -f capabilities > /etc/shorewall/capabilities

If you don't want to supply a value in a column but want to supply a value in a following column, simply enter '-' to make the column appear empty.

Example:

#INTERFACE         BROADCAST            OPTIONS
br0                -                    routeback

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

ACCEPT  net     $FW      tcp \↵
smtp,www,pop3,imap  #Services running on the firewall

In certain cases, leading white space is ignored in continuation lines:

Example (/etc/shorewall/rules):

#ACTION     SOURCE          DEST            PROTO           DPORT
ACCEPT      net:\
            206.124.146.177,\
            206.124.146.178,\
            206.124.146.180\
                            dmz             tcp             873

The leading white space on the first through third continuation lines is ignored so the SOURCE column effectively contains "net:206.124.146.177,206.124.147.178,206.124.146.180". Because the third continuation line does not end with a comma or colon, the leading white space in the last line is not ignored.

#ACTION     SOURCE          DEST            PROTO           DPORT
#ACCEPT     net:\
            206.124.146.177,\
            206.124.146.178,\
            206.124.146.180\
                            dmz             tcp             873

Some of the configuration files now have a large number of columns. That makes it awkward to specify a value for one of the right-most columns as you must have the correct number of intervening '-' columns.

This problem is addressed by allowing column values to be specified as column-name/value pairs.

There is considerable flexibility in how you specify the pairs:

In Shorewall 5.0.3, the sample configuration files and the man pages were updated to use the same column names in both the column headings and in the alternate specification format. The following table shows the column names for each of the table-oriented configuration files.

Example (rules file):

#ACTION         SOURCE            DEST            PROTO   DPORT
DNAT            net               loc:10.0.0.1    tcp     80    ; mark="88"

Here's the same line in several equivalent formats:

{ action=>DNAT, source=>net, dest=>loc:10.0.0.1, proto=>tcp, dport=>80, mark=>88 }
; action:"DNAT" source:"net"  dest:"loc:10.0.0.1" proto:"tcp" dport:"80" mark:"88"
DNAT { source=net dest=loc:10.0.0.1 proto=tcp dport=80 mark=88 }

Beginning with Shorewall 5.0.11, ip[6]table comments can be attached to individual rules using the comment keyword.

Example from the rules file:

        ACCEPT net $FW { proto=tcp, dport=22, comment="Accept \"SSH\"" }

As shown in that example, when the comment contains whitespace, it must be enclosed in double quotes and any embedded double quotes must be escaped using a backslash ("\").

Shorewall doesn't contain built-in support for all ip[6]tables targets and matches. Nevertheless, you can still use the unsupported ip[6]tables features through several Shorewall facilities.

In both Shorewall and Shorewall6, there are two basic types of addresses:

In the Shorewall documentation and manpages, we have tried to make it clear which type of address is accepted in each specific case.

Because Shorewall uses a colon (":") as a separator in many contexts, IPv6 addresses are best written using the standard convention in which the address itself is enclosed in square brackets:

For more information about addressing, see the Setup Guide.

Entries in Shorewall configuration files often deal with the source (SOURCE) and destination (DEST) of connections and Shorewall implements a uniform way for specifying them.

A SOURCE or DEST consists of one to three parts separated by colons (":"):

Examples.

Any configuration file may contain INCLUDE directives. An INCLUDE directive consists of the word INCLUDE followed by a path name and causes the contents of the named file to be logically included into the file containing the INCLUDE. Relative path names given in an INCLUDE directive are resolved using the current CONFIG_PATH setting (see shorewall.conf(5)).

INCLUDE's may be nested to a level of 3 -- further nested INCLUDE directives are ignored with a warning message.

Beginning with Shorewall 4.4.17, the INCLUDE directive may also appear in the following extension scripts:

When used in these scripts, the INCLUDEd files are copied into the compiled firewall script.

     shorewall/params.mgmt:
 
        MGMT_SERVERS=1.1.1.1,2.2.2.2,3.3.3.3
         TIME_SERVERS=4.4.4.4
         BACKUP_SERVERS=5.5.5.5
 
        ----- end params.mgmt -----
 
     shorewall/params:
 
        # Shorewall 1.3 /etc/shorewall/params
         [..]
         #######################################
  
         INCLUDE params.mgmt    
   
       # params unique to this host here
       
       ----- end params -----
 
     shorewall/rules.mgmt:
 
       ACCEPT net:$MGMT_SERVERS   $FW                  tcp    22
       ACCEPT $FW                 net:$TIME_SERVERS    udp    123
       ACCEPT $FW                 net:$BACKUP_SERVERS  tcp    22
 
      ----- end rules.mgmt -----
 
     shorewall/rules:
 
      # Shorewall version 1.3 - Rules File
       [..]
       #######################################
  
       INCLUDE rules.mgmt     
   
       # rules unique to this host here
       
 
     ----- end rules -----

gateway:/etc/shorewall # ls rules.d
ALL.rules  DNAT.rules  FW.rules  NET.rules  REDIRECT.rules  VPN.rules
gateway:/etc/shorewall # 

?SECTION NEW
SHELL cat /etc/shorewall/rules.d/*.rules

?SECTION NEW
SHELL cat /etc/shorewall/rules.d/*.rules 2> /dev/null || true

?INCLUDE common.rules

A number of configuration files support multiple formats. Prior to Shorewall 4.5.11, the format was specified by a line having 'FORMAT' as the first token. This requires each of the file processors to handle FORMAT separately.

In Shorewall 4.5.11, the ?FORMAT directive was created to centralize processing of FORMAT directives. The old entries, while still supported in Shorewall 4.5-4.6, are now deprecated. They are no longer supported in Shorewall 5.0 and later versions.

The ?FORMAT directive is as follows:

A number of files allow attaching comments to generated Netfilter rules:

Prior to Shorewall 4.5.11, comments were specified by a line having COMMENT as the first token. The remainder of the line is treated as a comment to be attached to rules.

In Shorewall 4.5.11, the ?COMMENT directive was created to centralize processing of COMMENT directives. The old entries, while still supported in Shorewall 4.5 and 4.6, are now deprecated. They are no longer supported in Shorewall 5.0 and later versions.

Use of this directive requires Comment support in your kernel and iptables - see the output of shorewall show capabilities.

The ?COMMENT directive is as follows:

Example (/etc/shorewall/rules):

?COMMENT Stop NETBIOS noise

REJECT          loc                             net                     tcp     137,445
REJECT          loc                             net                     udp     137:139

?COMMENT Stop my idiotic work laptop from sending to the net with an HP source/dest IP address

DROP            loc:!192.168.0.0/22             net

?COMMENT

Here's the corresponding output from /sbin/shorewall-lite:

gateway:~ # shorewall-lite show loc-net
Shorewall Lite 4.3.3 Chains loc2net at gateway - Mon Oct 16 15:04:52 PDT 2008

Counters reset Mon Oct 16 14:52:17 PDT 2006

Chain loc-net (1 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 LOG        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:25 LOG flags 0 level 6 prefix `FW:loc2net:REJECT:'
    0     0 reject     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:25
    0     0 LOG        udp  --  *      *       0.0.0.0/0            0.0.0.0/0           udp dpts:1025:1031 LOG flags 0 level 6 prefix `FW:loc2net:REJECT:'
    0     0 reject     udp  --  *      *       0.0.0.0/0            0.0.0.0/0           udp dpts:1025:1031
    0     0 reject     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           multiport dports 137,445 /* Stop NETBIOS noise */
    0     0 reject     udp  --  *      *       0.0.0.0/0            0.0.0.0/0           udp dpts:137:139 /* Stop NETBIOS noise */
    0     0 DROP       all  --  *      *      !192.168.0.0/22       0.0.0.0/0           /* Stop my idiotic work laptop from sending to the net with an HP source/dest IP address */
    5   316 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0
gateway:~ #

?COMMENT lines in macro files work somewhat differently from other files. ?COMMENT lines in macros are ignored if COMMENT support is not available or if there was a COMMENT in use when the top-level macro was invoked. This allows the following:

/usr/share/shorewall/macro.SSH:

#ACTION SOURCE DEST  PROTO DPORT   SPORT   RATE  USER
?COMMENT SSH
PARAM   -      -     tcp   22 

/etc/shorewall/rules:

?COMMENT Allow SSH from home
SSH(ACCEPT)    net:$MYIP      $FW
?COMMENT

The comment line in macro.SSH will not override the ?COMMENT line in the rules file and the generated rule will show /* Allow SSH from home */ when displayed through the Shorewall show and dump commands.

Beginning with Shorewall 5.0.11, the alternate input format allows attaching comments to individual rules in the files listed above.

The CONFIG_PATH option in shorewall.conf determines where the compiler searches for configuration files. The default setting is CONFIG_PATH=/etc/shorewall:/usr/share/shorewall which means that the compiler first looks in /etc/shorewall and if it doesn't find the file, it then looks in /usr/share/shorewall.

You can change this setting to have the compiler look in different places. For example, if you want to put your own versions of standard macros in /etc/shorewall/Macros, then you could set CONFIG_PATH=/etc/shorewall:/etc/shorewall/Macros:/usr/share/shorewall and the compiler will use your versions rather than the standard ones.

You may use the /etc/shorewall/params file to set shell variables that you can then use in 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

The following variable names must be avoided. Those in bold font must be avoided in all Shorewall versions; those in regular font must be avoided in versions prior to 4.4.8.

Example:

Variables may be used anywhere in the other configuration files.

/etc/shorewall/zones:

        #ZONE        TYPE          OPTIONS
        fw           firewall

/etc/shorewall/params:

        FW=fw
        BLARG=$FW:206.124.146.176

Because the /etc/shorewall/params file is simply sourced into the shell, you can place arbitrary shell code in the file and it will be executed each time that the file is read. Any code included should follow these guidelines:

Beginning with Shorewall 4.4.27, you may also use options in shorewall.conf (5) (e.g., $BLACKLIST_LOGLEVEL).

Beginning with Shorewall 4.5.2, configuration files can access variables defined in the shorewallrc file.

Beginning with Shorewall 4.5.11, variables can be altered by compiler directives.

Action variables are read-only and cannot be ?SET (although you can change their values using embedded Perl).

Beginning with Shorewall 4.5.13, Shorewall Variables may be set. When setting a Shorewall Variable, the variable must include the leading '@' and the @{...} form is not allowed.

Given that shell variables are expanded at compile time, there is no way to cause such variables to be expanded at run time. Prior to Shorewall 4.4.17, this made it difficult (to impossible) to include dynamic IP addresses in a Shorewall-lite configuration.

Version 4.4.17 implemented Run-time address variables. In configuration files, these variables are expressed as an apersand ('&') followed by the logical name of an interface defined in shorewall-interfaces (5). Wildcard interfaces (those ending in '+') are not supported and will cause a compilation error.

Beginning with Shorewall 4.5.11, you can define your own address variables by using this syntax:

where variable is a valid shell variable name. The generated script will verify that the variable contains a valid host or network address, either from the environment or from it being assigned in your init extension script, and will raise an error if it does not. In the error case, the state of the firewall will remain unchanged.

Example:

/etc/shorewall/init:

SMC_ADDR=10.1.10.11

/etc/shorewall/rules:

test:debug	net:&{SMC_ADDR}		fw

A second form is also available beginning with Shorewall 4.5.11

Unlike with the first form, this form does not require the variable to be set. If the variable is empty, the generated script will supply the all-zeros address (0.0.0.0 in IPv4 and :: in IPv6). In most cases, the compiler simply omits rules containing matches on the all-zeros address.

Example:

/etc/shorewall/init:

SMC_ADDR=10.1.10.11

/etc/shorewall/rules:

test:debug	net:%{SMC_ADDR}		fw

Run-time address variables may be used in the SOURCE and DEST column of the following configuration files:

They may also appear in the ORIGDEST column of:

They may also be used as the parameter to SNAT() in shorewall-snat(5).

For optional interfaces, if the interface is not usable at the time that the firewall starts, one of two approaches are taken, depending on the context:

Beginning with Shorewall 4.5.1, Run-time Gateway Variables in the form of a percent sign ('%') followed by a logical interface name are also supported. These are expanded at run-time to the gateway through the named interface. For optional interfaces, if the interface is not usable at the time that the firewall starts, the nil address will be used (0.0.0.0 in IPv4 and :: in IPv6), resulting in no packets matching the rule. Run-time gateway variables may be used in the SOURCE and DEST columns of the following configuration files:

If there is no gateway out of the named interface, rules containing the intefaces's run-time gateway variable are omitted.

Beginning with Shorewall 5.1.5, Run-time Port Variables are supported. These variables have the format %{variable} and may appear any place that a port number or service name may appear. Like their address-variable counterparts above, Run-time Port Variables are most useful when Shorewall[6]-lite is being used.

Example using both Run-time Address and Run-time Port Variables:

/etc/shorewall/init:

     SERVER_PORT=4126
     SERVER_ADDRESS=192.0.44.12

/etc/shorewall/rules:

     ACCEPT         net            dmz:%{SERVER_ADDRESS}           tcp          %{SERVER_PORT}

Rather than assigning a numerical literal to SERVER_PORT in the init extension script as shown above, the variable could be assigned a dynamic value based on a database lookup.

      LOW_PORT=100
      HIGH_PORT=50

      ACCEPT    net     $FW      tcp      ${LOW_PORT}:${HIGH_PORT}

Action variables were introduced in Shorewall 4.4.16 and may be accessed within the body of an action.

Shorewall Variables were introduced in Shorewall 4.5.11. To insure uniqueness, these variables start with the character @; the name of the variable must be enclosed in {...} when the following character is alphanumeric or is an underscore ("_"). With the exception of @0 (or it's alias @chain), Shorewall variables may only be used within an action body.

Prior to Shorewall 4.5.13, Shorewall variables are read-only. Beginning with Shorewall 4.5.13, their values may be altered using the ?SET directive.

The Shorewall variables are:

Beginning with Shorewall 4.5.13, the values of @chain and @disposition are used to generated the --log-prefix in logging rules. When either is empty, the historical value is used to generate the --log-prefix.

Within an action body, if a parameter is omitted in a DEFAULTS statement, then the value of the corresponding action and Shorewall variables is '-', while if the parameter is specified as '-' in the parameter list, the value of the action/Shorewall variable is '', if it is expanded before the DEFAULTS statement.

Additionally, when an expression is evaluated, the value 0 evaluates as false, so '?IF @n' and '?IF $n' fail if the nth parameter is passed with value zero. To make testing of the presense of parameters more efficient and uniform, an new function has been added in Shorewall 5.0.7 for use in ?IF and ?ELSEIF:

where <variable> is an action or Shorewall variable.

'passed(@n)' and 'passed($n)' evaluate to true if the nth parameter is not empty and its contents are other than '-'. If '!' is present, the result is inverted.

In this simple form, the expression is evaluated by the compiler without having to invoke the (expensive) Perl exec() function. The 'passed' function may also be used in more complex expressions, but exec() will be invoked to evaluate those expressions.

Beginning with Shorewall 4.5.2, lines in configuration files may be conditionally included or omitted based on the setting of Shell variables.

The general form is:

?IF $variable

<lines to be included if $variable is non-empty and non-zero>

?ELSE

<lines to be omitted if $variable is non-empty and non-zero>

?ENDIF

The compiler predefines two special variables that may only be used in ?IF lines:

Unless variable is one of these pre-defined ones, it is searched for in the following places in the order listed.

If the variable is still not found:

If it is not found in any of those places, the variable is assumed to have a value of 0 (false) in Shorewall versions prior to 4.5.11. In 4.5.11 and later, it is assumed to have the value '' (an empty string, which also evaluates to false).

The setting in /etc/shorewall/params may be overridden at runtime, provided the setting in /etc/shorewall/params is done like this:

[ -n "${variable:=0}" ]

or like this:

[ -n "${variable}" ] || variable=0

Either of those will set variable to 0 if it is not set to a non-empty value in the environment. The setting can be overridden at runtime:

variable=1 shorewall restart -c # use -c to force recompilation if AUTOMAKE=Yes in /etc/shorewall/shorewall.conf

The ?ELSE may be omitted if there are no lines to be omitted.

The test may also be inverted using '!':

?IF ! $variable

<lines to be omitted if $variable is non-empty and non-zero>

?ELSE

<lines to be included if $variable is non-empty and non-zero>

?ENDIF

Conditional entries may be nested but the number of ?IFs must match the number of ?ENDs in any give file. INCLUDE directives are ignored in omitted lines.

?IF $variable1

<lines to be included if $variable1 is non-empty and non-zero>

   ?IF $variable2

<lines to be included if $variable1 and $variable2 are non-empty and non-zero>

   ?ELSE

<lines to be omitted if $variable1 is non-empty and non-zero and if $variable2 is empty or zero>

   ?ENDIF

<lines to be included if $variable1 is non-empty and non-zero>

?ELSE

<lines to be omitted if $variable is non-empty and non-zero>

?ENDIF

Beginning with Shorewall 4.5.6, rather than a simple variable in ?IF directives, Perl-compatible expressions are allowed (after the Shorewall compiler expands all variables, the resulting expression is then evaluated by Perl). Variables in the expressions are as described above.

Example:

?IF $BLACKLIST_LOGLEVEL == 6 && ! __LOG_OPTIONS

Additionally, a ?ELSIF directive is supported.

Example:

?IF expression-1

<lines to be included if expression-1 evaluates to true (non-empty and non-zero)

?ELSIF expression1-2

<lines to be included if expression-1 evaluates to false (zero or empty) and expression-2 evaluates to true

?ELSIF expression-3

<lines to be included if expression-1 and expression-2 both evalute to false and expression-3 evalutes to true

?ELSE

<lines to be included if all three expressions evaluate to false.

?ENDIF

Beginning in Shorewall 5.0.7, an error can be raised using the ?ERROR directive:

?ERROR message

Variables in the message are evaluated and the result appears in a standard Shorewall ERROR: message.

Example from the 5.0.7 action.GlusterFS:

?if @1 !~ /^\d+/ || ! @1 || @1 > 1024
    ?error Invalid value for Bricks (@1)
?elsif @2 !~ /^[01]$/
    ?error Invalid value for IB (@2)
?endif

The above code insures that the first action paramater is a non-zero number <= 1024 and that the second parameter is either 0 or 1. If 2000 is passed for the first parameter, the following error message is generated:

   ERROR: Invalid value for Bricks (2000) /usr/share/shorewall/action.GlusterFS (line 15)
      from /etc/shorewall/rules (line 45)

In Shorewall 5.0.8, ?WARNING and ?INFO directives were added.

?WARNING message
?INFO message

?WARNING message produces a standard Shorewall WARNING: message, while ?INFO produces a similar message which is prefaced by INFO: rather than WARNING:. Both write the message to STDERR. The message is also written to the STARTUP_LOG, if any, provided that the command is start, try, restart, reload, refresh, or one of the safe-* commands.

See the VERBOSE_MESSAGES option in shorewall.conf(5) for additional information.

In Shorewall 5.1.4, the behavior of ?ERROR, ?WARNING and ?INFO was changed when they appear in an action file. Rather than reporting the action filename and line number, the generated message reports where the action was invoked. For example, the GlusterFS message above was changed to:

   ERROR: Invalid value (2000) for the GlusterFS Bricks argument /etc/shorewall/rules (line 45)

Earlier versions of Shorewall offered extension scripts to allow users to extend Shorewall's functionality. Extension scripts were designed to work under the limitations of the Bourne Shell. With the current Perl-based compiler, Embedded scripts offer a richer and more flexible extension capability.

While inline scripts may be written in either Shell or Perl, those written in Perl have a lot more power. They may be used in all configuration files except /etc/shorewall/params and /etc/shorewall/shorewall.conf.

Note:In this section, '[' and ']' are meta-characters which indicate that what they enclose is optional and may be omitted.

Single line scripts take one of the following forms:

Shell scripts run in a child shell process and their output is piped back to the compiler which processes that output as if it were embedded at the point of the script.

Example: The following entries in /etc/shorewall/rules are equivalent:

SHELL for z in net loc dmz; do echo "ACCEPT $z fw tcp 22"; done

ACCEPT net fw tcp 22
ACCEPT loc fw tcp 22
ACCEPT dmz fw tcp 22

Perl scripts run in the context of the compiler process using Perl's eval() function. Perl scripts are implicitly prefixed by the following:

package Shorewall::User;
use Shorewall::Config ( qw/shorewall/ );

To produce output that will be processed by the compiler as if it were embedded in the file at the point of the script, pass that output to the Shorewall::Config::shorewall() function. The Perl equivalent of the above SHELL script would be:

PERL for ( qw/net loc dmz/ ) { shorewall "ACCEPT $_ fw tcp 22"; }

A couple of more points should be mentioned:

Multi-line scripts use one of the following forms:

[?]BEGIN SHELL
<shell script>
[?]END [ SHELL ]

[?]BEGIN PERL [;]
<perl script>
[?]END [ PERL ] [;]

Host addresses in Shorewall configuration files may be specified as either IP addresses or 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 changes in the DNS->IP address relationship that occur after the firewall has started have absolutely no effect on the firewall's rule set.

For some sites, using DNS names is very risky. Here's an example:

teastep@ursa:~$ dig pop.gmail.com

; <<>> DiG 9.4.2-P1 <<>> pop.gmail.com
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 1774
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 7, ADDITIONAL: 0

;; QUESTION SECTION:
;pop.gmail.com.               IN A

;; ANSWER SECTION:
pop.gmail.com.          300   IN CNAME gmail-pop.l.google.com.
gmail-pop.l.google.com. 300   IN A     209.85.201.109
gmail-pop.l.google.com. 300   IN A     209.85.201.111

Note that the TTL is 300 -- 300 seconds is only 5 minutes. So five minutes later, the answer may change!

So this rule may work for five minutes then suddently stop working:

#ACTION        SOURCE               DEST              PROTO             DPORT
POP(ACCEPT)     loc                  net:pop.gmail.com

There are two options in shorewall[6].conf(5) that affect the use of DNS names in Shorewall[6] config files:

So by setting AUTOMAKE=Yes, and DEFER_DNS_RESOLUTION=No, compilation will only take place at boot time if a change had been make to the config but no restart or reload had taken place. This is clearly spelled out in the shorewall.conf manpage. So with these settings, so long as a 'reload' or 'restart' takes place after the Shorewall configuration is changes, there should be no DNS-related problems at boot time.

If your firewall rules include DNS names then, even if DEFER_DNS_RESOLUTION=No and AUTOMAKE=Yes:

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

DNS names may not be used as:

These restrictions are imposed by Netfilter and not by Shorewall.

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

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”. There must be no white space following the “!”.

Similarly, in columns that specify an IP protocol, you can precede the protocol name or number by "!". For example, !tcp means "any protocol except tcp".

This also works with port lists, providing that the list contains 15 or fewer ports (where a port range counts as two ports). For example !ssh,smtp means "any port except 22 and 25".

In Shorewall 4.4.19 and later, icmp type lists are supported but complementing an icmp type list is not supported. You may, however, complement a single icmp (icmp6) type.

Where a comma-separated list of addresses is accepted, an exclusion list may also be included. An exclusion list is a comma-separated list of addresses that begins with "!".

Example:

!192.168.1.3,192.168.1.12,192.168.1.32/27

The above list refers to "All addresses except 192.168.1.3, 192.168.1.12 and 192.168.1.32-192.168.1.63.

Exclusion lists can also be added after a network address.

Example:

192.168.1.0/24!192.168.1.3,192.168.1.12,192.168.1.32/27

The above list refers to "All addresses in 192.168.1.0-192.168.1.255 except 192.168.1.3, 192.168.1.12 and 192.168.1.32-192.168.1.63.

If you kernel and iptables have iprange match support, you may use IP address ranges in Shorewall configuration file entries; IP address ranges have the syntax <low IP address>-<high IP address>. Example: 192.168.1.5-192.168.1.12.

To see if your kernel and iptables have the required support, use the shorewall show capabilities command:

>~ shorewall show capabilities
Shorewall has detected the following iptables/netfilter capabilities:
   ACCOUNT Target (ACCOUNT_TARGET): Not available
   Address Type Match (ADDRTYPE): Available
   Amanda Helper: Available
...
   IPMARK Target (IPMARK_TARGET): Not available
   IPP2P Match (IPP2P_MATCH): Not available
   IP range Match(IPRANGE_MATCH): Available <================

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

Also, unless otherwise documented, a protocol number/name can be preceded by '!' to specify "All protocols except this one" (e.g., "!tcp").

If you need to specify a range of ports, the proper syntax is <low port number>:<high port number>. For example, if you want to forward the range of tcp ports 4000 through 4100 to local host 192.168.1.3, the entry in /etc/shorewall/rules is:

#ACTION    SOURCE     DESTINATION     PROTO     DPORT
DNAT       net        loc:192.168.1.3 tcp       4000:4100

If you omit the low port number, a value of zero is assumed; if you omit the high port number, a value of 65535 is assumed.

Also, unless otherwise documented, a port range can be preceded by '!' to specify "All ports except those in this range" (e.g., "!4000:4100").

Beginning with Shorewall 5.0.14, a hyphen ("-") may also be used to separate the two port numbers; when using service names, the colon must still be used.

#ACTION    SOURCE     DESTINATION     PROTO     DPORT
DNAT       net        loc:192.168.1.3 tcp       4000-4100

In most cases where a port or port range may appear, a comma-separated list of ports or port ranges may also be entered. Shorewall requires the Netfilter multiport match capability if ports lists are used (see the output of "shorewall show capabilities").

Also, unless otherwise documented, a port list can be preceded by '!' to specify "All ports except these" (e.g., "!80,443").

Prior to Shorewall 4.4.4, port lists appearing in the shorewall-routestopped (5) file may specify no more than 15 ports; port ranges appearing in a list count as two ports each.

When dealing with ICMP, the DEST PORT specifies the type or type and code. You may specify the numeric type, the numeric type and code separated by a slash (e.g., 3/4) or you may use a type name.

Type names for IPv4 and their corresponding type or type/code are:

echo-reply'                  => 0
destination-unreachable      => 3
    network-unreachable      => 3/0
    host-unreachable         => 3/1
protocol-unreachable         => 3/2
port-unreachable             => 3/3
fragmentation-needed         => 3/4
source-route-failed          => 3/5
network-unknown              => 3/6
host-unknown                 => 3/7
network-prohibited           => 3/9
host-prohibited              => 3/10
TOS-network-unreachable      => 3/11
TOS-host-unreachable         => 3/12
communication-prohibited     => 3/13
host-precedence-violation    => 3/14
precedence-cutoff            => 3/15
source-quench                => 4
redirect                     => 5
   network-redirect          => 5/0
   host-redirect             => 5/1
   TOS-network-redirect      => 5/2
   TOS-host-redirect         => 5/3
echo-request                 => 8
router-advertisement         => 9
router-solicitation          => 10
time-exceeded                => 11
   ttl-zero-during-transit   => 11/0
   ttl-zero-during-reassembly=> 11/1
parameter-problem            => 12
   ip-header-bad             => 12/0
   required-option-missing   => 12/1
timestamp-request            => 13
timestamp-reply              => 14
address-mask-request         => 17
address-mask-reply           => 18

Type names for IPv6 and their corresponding type or type/code are:

destination-unreachable       => 1
   no-route'                  => 1/0
   communication-prohibited   => 1/1
   address-unreachable'       => 1/3
   port-unreachable'          => 1/4
packet-too-big                =>  2
time-exceeded'                =>  3
ttl-exceeded'                 =>  3
   ttl-zero-during-transit    => 3/0
   ttl-zero-during-reassembly => 3/1
parameter-problem             =>  4
   bad-header                 => 4/0
   unknown-header-type        => 4/1
   unknown-option             => 4/2
echo-request                  => 128
echo-reply                    => 129
router-solicitation           => 133
router-advertisement          => 134
neighbour-solicitation        => 135
neighbour-advertisement       => 136
redirect                      => 137

Shorewall 4.4 does not accept lists of ICMP (ICMP6) types prior to Shorewall 4.4.19.

Media Access Control (MAC) addresses can be used to specify packet source in several of the configuration files. In order to control traffic to/from a host by its MAC address, the host must be on the same network as the firewall.

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.

     gateway:~ # ip link ls dev eth0
     4: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc htb qlen 1000
         link/ether 02:00:08:E3:FA:55 brd ff:ff:ff:ff:ff:ff
     gateway:~ #

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 supports rate limiting in a number of ways. When specifying a rate limit, both a rate and a burst value are given.

Example from shorewall.conf (5):

For each logging rule, the first time the rule is reached, the packet will be logged; in fact, since the burst is 5, the first five packets will be logged. After this, it will be 6 seconds (1 minute divided by the rate of 10) before a message will be logged from the rule, regardless of how many packets reach it. Also, every 6 seconds which passes, one of the bursts will be regained; if no packets hit the rule for 30 seconds, the burst will be fully recharged; back where we started.

Shorewall also supports per-IP rate limiting.

Another example from shorewall.conf (5):

Here, the leading "s:" indicates that logging is to be limited by source IP address ("d:" would indicate limiting by destination IP address).

"s:" is followed by the rate (5 messages per minute) and the burst (5).

The rate and limit arguments have the same meaning as in the example above.

Several of the files include a TIME colum that allows you to specify times when the rule is to be applied. Contents of this column is a list of timeelements separated by apersands (&).

Each timeelement is one of the following:

Examples:

There are times when you would like to enable or disable one or more rules in the configuration without having to do a shorewall reload or shorewall restart. This may be accomplished using the SWITCH column in shorewall-rules (5) or shorewall6-rules (5). Using this column requires that your kernel and iptables include Condition Match Support and you must be running Shorewall 4.4.24 or later. See the output of shorewall show capabilities and shorewall version to determine if you can use this feature.

The SWITCH column contains the name of a switch. Each switch is initially in the off position. You can turn on the switch named switch1 by:

You can turn it off again by:

If you simply include the switch name in the SWITCH column, then the rule is enabled only when the switch is on. If you precede the switch name with ! (e.g., !switch1), then the rule is enabled only when the switch is off. Switch settings are retained over shorewall restart.

Shorewall requires that switch names:

Multiple rules can be controlled by the same switch.

Example:

When dealing with a complex configuration, it is often awkward to use physical interface names in the Shorewall configuration.

Beginning with Shorewall 4.4.4, you can use logical interface names which are mapped to the actual interface using the physical option in shorewall-interfaces (5).

Here is an example:

#ZONE  INTERFACE  OPTIONS
net    COM_IF     dhcp,blacklist,tcpflags,optional,upnp,routefilter=0,nosmurfs,logmartians=0,physical=eth0
net    EXT_IF     dhcp,blacklist,tcpflags,optional,routefilter=0,nosmurfs,logmartians=0,proxyarp=1,physical=eth2
loc    INT_IF     dhcp,logmartians=1,routefilter=1,tcpflags,nets=172.20.1.0/24,physical=eth1
dmz    VPS_IF     logmartians=1,routefilter=0,routeback,physical=venet0
loc    TUN_IF     physical=tun+

In this example, COM_IF is a logical interface name that refers to Ethernet interface eth0, EXT_IF is a logical interface name that refers to Ethernet interface eth2, and so on.

Here are a couple of more files from the same configuration:

shorewall-masq (5):

#INTERFACE SOURCE                    ADDRESS

COMMENT Masquerade Local Network
COM_IF     0.0.0.0/0
EXT_IF     !206.124.146.0/24         206.124.146.179:persistent

shorewall-providers (5)

#NAME   NUMBER   MARK    DUPLICATE  INTERFACE  GATEWAY         OPTIONS               COPY
Avvanta 1        0x10000 main       EXT_IF     206.124.146.254 loose,fallback        INT_IF,VPS_IF,TUN_IF
Comcast 2        0x20000 main       COM_IF     detect          balance               INT_IF,VPS_IF,TUN_IF

Note in particular that Shorewall translates TUN_IF to tun* in the COPY column.

Normally, Shorewall assumes that all interfaces described in shorewall-interfaces (5) are going to be in an up and usable state when Shorewall starts or restarts. You can alter that assumption by specifying the optional option in the OPTIONS column.

When an interface is marked as optional, Shorewall will determine the interface state at start, reload and restart and adjust its configuration accordingly.

If you don't want the firewall to start unless a given interface is usable, then specify required in the OPTIONS column of shorewall-interfaces (5). If you have installed and configured the Shorewall-init package, then when the interface becomes available, an automatic attempt will be made to start the firewall.

Shorewall allows you to have configuration directories other than /etc/shorewall. The shorewall check, start, reload 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

Shorewall allows you to save the currently-running configuration in a form that permits it to be re-installed quickly. When you save the configuration using the shorewall save command, the running configuration is saved in a file in the /var/lib/shorewall directory. The default name of that file is /var/lib/shorewall/restore but you can specify a different name as part of the command. For example, the command shorewall save standard will save the running configuration in /var/lib/shorewall/standard. A saved configuration is re-installed using the shorewall restore command. Again, that command normally will restore the configuration saved in /var/lib/shorewall/restore but as with the save command, you can specify a different file name in the command. For example, shorewall restore standard will re-install the configuration saved in /var/lib/shorewall/standard. By permitting you to save different configurations under different names, Shorewall provides a means for quickly switching between these different saved configurations.

As mentioned above, the default configuration is called 'restore' but like most things in Shorewall, that default can be changed. The default name is specified using the RESTOREFILE option in /etc/shorewall/shorewall.conf.