Configuration Files Tips and Hints Tom Eastep 2001-2011 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. This article applies to Shorewall 4.3 and later. If you are running a version of Shorewall earlier than Shorewall 4.3.5 then please see the documentation for that release. 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.
Introduction 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.
Files /etc/shorewall/shorewall.conf - used to set global 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/rules - defines rules that are exceptions to the overall policies established in /etc/shorewall/policy. /etc/shorewall/nat - defines one-to-one NAT rules. /etc/shorewall/proxyarp - defines use of Proxy ARP. /etc/shorewall/routestopped - defines hosts accessible when Shorewall is stopped. /etc/shorewall/tcrules - The file has a rather unfortunate name because it is used to define marking of packets for later use by both traffic control/shaping and policy routing. /etc/shorewall/tos - defines rules for setting the TOS field in packet headers. /etc/shorewall/tunnels - defines tunnels (VPN) with end-points on the firewall system. /etc/shorewall/blacklist - lists blacklisted IP/subnet/MAC addresses. /etc/shorewall/init - commands that you wish to execute at the beginning of a shorewall start or shorewall restart. /etc/shorewall/start - commands that you wish to execute at the completion of a shorewall start or shorewall restart /etc/shorewall/stop - commands that you wish to execute at the beginning of a shorewall stop. /etc/shorewall/stopped - commands that you wish to execute at the completion of a shorewall stop. /etc/shorewall/ecn - disable Explicit Congestion Notification (ECN - RFC 3168) to remote hosts or networks. /etc/shorewall/accounting - define IP traffic accounting rules /etc/shorewall/actions and /usr/share/shorewall/action.template allow user-defined actions. /etc/shorewall/providers - defines an alternate routing table. /etc/shorewall/route_rules - Defines routing rules to be used in conjunction with the routing tables defined in /etc/shorewall/providers. /etc/shorewall/tcdevices, /etc/shorewall/tcclasses, /etc/shorewall/tcfilters - Define complex traffic shaping. /etc/shorewall/tcrules - Mark or classify traffic for traffic shaping or multiple providers. /etc/shorewall/tcinterfaces and /etc/shorewall-tcpri - Define simple traffic shaping. /etc/shorewall/secmarks - Added in Shorewall 4.4.13. Attach an SELinux context to selected packets. /etc/shorewall/vardir - Determines the directory where Shorewall maintains its state. /usr/share/shorewall/actions.std - Actions defined by Shorewall. /usr/share/shorewall/action.* - Details of actions defined by Shorewall. /usr/share/shorewall/macro.* - Details of macros defined by Shorewall. /usr/share/shorewall/modules - directs the firewall to load kernel modules. /usr/share/modules — Specifies the kernel modules to be loaded during shorewall start/restart. /usr/share/helpers — Added in Shorewall 4.4.7. Specifies the kernel modules to be loaded during shorewall start/restart when LOAD_HELPERS_ONLY=Yes in shorewall.conf. If you need to change a file in /usr/share/shorewall/, copy it to /etc/shorewall and modify the copy
Man Pages 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
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. Comments in a Configuration File # This is a comment ACCEPT net $FW tcp www #This is an end-of-line comment
Names 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".
Attach Comment to Netfilter Rules If you kernel and iptables contain comment match support (see the output of shorewall show capabilities), then you can attach comments to Netfilter rules. This feature is available in the following files: /etc/shorewall/accounting /etc/shorewall/masq /etc/shorewall/nat /etc/shorewall/notrack /etc/shorewall/rules /etc/shorewall/secmarks /etc/shorewall/tcrules /etc/shorewall/tunnels Action definition files (/etc/shorewall/action.*) Macro definition files (/etc/shorewall/macro.*) To attach a comment to one or more rules, insert a record above the rules that begins with the word COMMENT (must be in all caps). The remainder of the line is treated as a comment -- that comment will appear delimited by "/* ... */" in the output of the shorewall[-lite] show and shorewall[-lite] dump commands. The comment will be attached to each generated rule until another COMMENT line appears. To stop attaching comments to rules, simply insert a line that contains the single word COMMENT. 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 loc2net 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 loc2net (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 DEST SOURCE RATE USER/ # PORT(S) PORT(S) LIMIT GROUP COMMENT SSH PARAM - - tcp 22 /etc/shorewall/rules:COMMENT Allow SSH from home SSH(ACCEPT) net:$MYIP $FW COMMENTThe 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.
"Blank" Columns 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
Line Continuation You may continue lines in the configuration files using the usual backslash (\) followed immediately by a new line character (Enter key). Line Continuation ACCEPT net $FW tcp \↵ smtp,www,pop3,imap #Services running on the firewall In certain cases, leading white space is ignored in continuation lines: The continued line ends with a colon (":") The continued line ends with a comma (",") Example (/etc/shorewall/rules): #ACTION SOURCE DEST PROTO DEST # PORT(S) 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.
Alternate Specification of Column Values - Shorewall 4.4.24 and Later 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: At any point, you can enter a semicolon (';') followed by one or more specifications of the following forms: column-name=value column-name=>value column-name:value The value may optionally be enclosed in double quotes. The pairs must be separated by white space, but you can add a comma adjacent to the values for readability as in: ; proto=>udp, port=1024 You can enclose the pairs in curly brackets ("{...}") rather than separating them from columns by a semicolon: { proto:udp, port:1024 } The following table shows the column names for each of the table-oriented configuration files. Column names are case-insensitive. File Column names accounting action,chain, source, dest, proto, dport, sport, user, mark, ipsec, headers blacklist networks,proto,port,options ecn interface,hosts hosts zone,hosts,options interfaces zone,interface,broadcast,options maclist disposition,interface,mac,addresses masq interface,source,address,proto,port,ipsec,mark,user nat external,interface,internal,allints,local netmap type,net1,interface,net2,net3,proto,dport,sport notrack source,dest,proto,dport,sport,user policy source,dest,policy,loglevel,limit,connlimit providers table,number,mark,duplicate,interface,gateway,options,copy proxyarp and proxyndp address,interface,external,haveroute,persistent route_rules source,dest,provider,priority routes provider,dest,gateway,device routestopped interface,hosts,options,proto,dport,sport rules action,source,dest,proto,dport,sport,origdest,rate,user,mark,connlimit,time,headers,switch secmarks secmark,chain,source,dest,proto,dport,sport,user,mark tcclasses interface,mark,rate,ceil,prio,options tcdevices interface,in_bandwidth,out_bandwidth,options,redirect tcfilters class,source,dest,proto,dport,sport,tos,length tcinterfaces interface,type,in_bandwidth,out_bandwidth tcpri band,proto,port,address,interface,helper tcrules mark,source,dest,proto,dport,sport,user,test,length,tos,connbytes,helper,headers tos source,dest,proto,dport,sport,tos,mark tunnels type,zone,gateway,gateway_zone zones zone,type,options,in_options,out_options Example (rules file): #ACTION SOURCE DEST PROTO DEST # PORT(S) 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 }
Addresses In both Shorewall and Shorewall6, there are two basic types of addresses: Host Address This address type refer to a single host. In IPv4, the format is i.j.k.l where i through l are decimal numbers between 1 and 255. In IPv6, the format is a:b:c:d:e:f:g:h where a through h consist of 1 to 4 hexidecimal digits (leading zeros may be omitted). a single series of 0 addresses may be omitted. For example 2001:227:e857:1:0:0:0:0:1 may be written 2001:227:e857:1::1. Network Address A network address refers to 1 or more hosts and consists of a host address followed by a slash ("/") and a Variable Length Subnet Mask (VLSM). This is known as Classless Internet Domain Routing (CIDR) notation. The VLSM is a decimal number. For IPv4, it is in the range 0 through 32. For IPv6, the range is 0 through 128. The number represents the number of leading bits in the address that represent the network address; the remainder of the bits are a host address and are generally given as zero. Examples: IPv4: 192.168.1.0/24 IPv6: 2001:227:e857:1:0:0:0:0:1/64 In the Shorewall documentation and manpages, we have tried to make it clear which type of address is accepted in each specific case. For more information about addressing, see the Setup Guide.
Specifying SOURCE and DEST 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 (":"): ZONE — The name of a zone declared in /etc/shorewall/zones or /etc/shorewall6/zones. This part is only available in the rules file (/etc/shorewall/rules and /etc/shorewall6/rules). INTERFACE — The name of an interface that matches an entry in /etc/shorewall/interfaces (/etc/shorewall6/interfaces). ADDRESS LIST — A list of one or more addresses (host or network) or address ranges, separated by commas. In an IPv6 configuration, this list must be includef in square or angled brackets ("[...]" or "<...>"). The list may have exclusion. Examples. All hosts in the net zone — net Subnet 192.168.1.0/29 in the loc zone — loc:192.168.1.0/29 All hosts in the net zone connecting through ppp0 — net:ppp0 All hosts interfaced by eth3 — eth3 Subnet 10.0.1.0/24 interfacing through eth2 — eth2:10.0.1.0/24 Host 2002:ce7c:92b4:1:a00:27ff:feb1:46a9 in the loc zone — loc:[2002:ce7c:92b4:1:a00:27ff:feb1:46a9] The primary IP address of eth0 in the $FW zone - $FW:&eth0 (see Run-time Address Variables below)
INCLUDE Directive 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: clear findgw init isusable refresh refreshed restore restored start started stop stopped tcclear When used in these scripts, the INCLUDEd files are copied into the compiled firewall script. Prior to Shorewall 4.4.17, if you are using Shorewall Lite , it is not advisable to use INCLUDE in the params file in an export directory if you set EXPORTPARAMS=Yes in shorewall.conf (5). If you do that, you must ensure that the included file is also present on the firewall system's /etc/shorewall-lite/ directory. If you only need the params file at compile time, you can set EXPORTPARAMS=No in shorewall.conf. That prevents the params file from being copied into the compiled script. With EXPORTPARAMS=No, it is perfectly okay to use INCLUDE in the params file. Note that with Shorewall 4.4.17 and later: The variables set at compile time are available at run-time even with EXPORTPARAMS=No. The INCLUDE directive in the params file is processed at compile time and the INCLUDEd file is copied into the compiled script. Use of INCLUDE 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    #LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE    ----- 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    #LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE    ----- end rules ----- You may include multiple files in one command using an embedded shell command. Example (include all of the files ending in ".rules" in a directory:):gateway:/etc/shorewall # ls rules.d ALL.rules DNAT.rules FW.rules NET.rules REDIRECT.rules VPN.rules gateway:/etc/shorewall # /etc/shorewall/rules:SECTION NEW SHELL cat /etc/shorewall/rules.d/*.rules If you are the sort to put such an entry in your rules file even though /etc/shorewall/rules.d might not exist or might be empty, then you probably want: SECTION NEW SHELL cat /etc/shorewall/rules.d/*.rules 2> /dev/null || true
CONFIG_PATH The CONFIG_PATH option in shorewall.conf determines where the compiler searches for 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.
Using Shell Variables 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. Any option from shorewall.conf (5) COMMAND CONFDIR DEBUG ECHO_E ECHO_N EXPORT FAST FILEMODE HOSTNAME IPT_OPTIONS NOROUTES PREVIEW PRODUCT PROFILE PURGE RECOVERING RESTOREPATH RING_BELL SHAREDIR Any name beginning with SHOREWALL_ or SW_ STOPPING TEST TIMESTAMP USE_VERBOSITY VARDIR VERBOSE VERBOSE_OFFSET VERSION Example:
    /etc/shorewall/params NET_IF=eth0 NET_BCAST=130.252.100.255 NET_OPTIONS=routefilter,routefilter     /etc/shorewall/interfaces record: net $NET_IF $NET_BCAST $NET_OPTIONS     The result will be the same as if the record had been written net eth0 130.252.100.255 routefilter,routefilter
Variables may be used anywhere in the other configuration files. If you use "$FW" on the right side of assignments in the /etc/shorewall/params file, you must also set the FW variable in that file. Example:/etc/shorewall/zones: #ZONE TYPE OPTIONS fw firewall /etc/shorewall/params: FW=fw BLARG=$FW:206.124.146.176 Because the /etc/shorewall/params file is simply sourced into the shell, you can place arbitrary shell code in the file and it will be executed each time that the file is read. Any code included should follow these guidelines: The code should not have side effects, especially on other shorewall configuration files. The code should be safe to execute multiple times without producing different results. Should not depend on where the code is called from. Should not assume anything about the state of Shorewall. The names of any functions or variables declared should begin with an upper case letter. The /etc/shorewall/params file is processed by the compiler at compile-time and by the compiled script at run-time. If you have set EXPORTPARAMS=No in shorewall.conf, then the params file is only processed by the compiler; it is not run by the compiled script. Beginning with Shorewall 4.4.17, the values of the variables set at compile time are available at run time with EXPORTPRARMS=No. If you are using Shorewall Lite and if the params script needs to set shell variables based on the configuration of the firewall system, you can use this trick: EXT_IP=$(ssh root@firewall "/sbin/shorewall-lite call find_first_interface_address eth0") The shorewall-lite call command allows you to to call interactively any Shorewall function that you can call in an extension script. Within your configuration files, only the $VAR and ${VAR} forms of variable expansion are supported. You may not use the more exotic forms supported by the shell (${VAR:=val}, ${VAR:-val}, ...) Given that shell variables are expanded at compile time, there is no way to cause such variables to be expended 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. Example: &eth0 would represent the primary IP address of eth0. Run-time address variables may be used in the SOURCE and DEST column of the following configuration files: shorewall-accounting (5) Action files shorewall-blacklist (5) Macro files shorewall-rules (5) shorewall-tcrules (5) shorewall-tos (5) They may also appear in the ORIGINAL DEST column of: shorewall-accounting (5) Macro files shorewall-rules (5) For optional interfaces, if the interface is not usable at the time that the firewall starts the all-zero address will be used (0.0.0.0 in IPv4 and :: in IPv6), resulting in no packets matching the rule. Beginning with Shorewall 4.4.27, you may also use options in shorewall.conf (5) (e.g., $BLACKLIST_LOGLEVEL). When an option is set to 'No' in shorewall.conf, the corresponding shell variable is will be empty. Options that were not set in shorewall.conf will expand to their default value.
Embedded Shell and Perl 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. Embedded scripts can be either single-line or multi-line. Single line scripts take one of the following forms: PERL <perl script> SHELL <shell script> 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"; doneACCEPT net fw tcp 22 ACCEPT loc fw tcp 22 ACCEPT dmz fw tcp 22 Perl scripts run in the context of 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 $_ fw tcp 22"; }A couple of more points should be mentioned: Compile-time extension scripts are also implicitly prefixed by "package Shorewall::User;". A compile extension script is supported. That script is run early in the compilation process and allows users to load additional modules and to define data and functions for use in subsequent embedded scripts and extension scripts. Manual Chains may be added in the compile extension script.. Multi-line scripts use one of the following forms:BEGIN SHELL <shell script> END [ SHELL ]BEGIN PERL [;] <perl script> END [ PERL ] [;] Note: The '[' and ']' above are meta-characters which indicate that what they enclose is optional and may be omitted. So you may follow PERL with a semicolon ( ';') or you may omit the semicolon.
Using DNS Names I personally recommend strongly against using DNS names in Shorewall configuration files. If you use DNS names and you are called out of bed at 2:00AM because Shorewall won't start as a result of DNS problems then don't say that you were not forewarned. Host addresses in Shorewall configuration files may be specified as either IP addresses or DNS Names. DNS names in iptables rules aren't nearly as useful as they first appear. When a DNS name appears in a rule, the iptables utility resolves the name to one or more IP addresses and inserts those addresses into the rule. So changes in the DNS->IP address relationship that occur after the firewall has started have absolutely no effect on the firewall's rule set. For some sites, using DNS names is very risky. Here's an example: teastep@ursa:~$ 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 DEST # PORT(S) POP(ACCEPT) loc net:pop.gmail.com 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 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. Valid DNS Names mail.shorewall.net shorewall.net. (note the trailing period). Invalid DNS Names mail (not fully qualified) shorewall.net (only one period) DNS names may not be used as: 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. These restrictions are imposed by Netfilter and not by Shorewall.
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: routefilter,dhcp,arpfilter Invalid: routefilter,     dhcp,     arpfilter If you use line continuation to break a comma-separated list, the comma must be the last thing on the continued line before '\' unless the continuation line has no leading white space. Entries in a comma-separated list may appear in any order.
Complementing an Address, Subnet, Protocol or Port 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 preceed 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.
Exclusion Lists 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.
IP Address Ranges 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: NAT: Available Packet Mangling: Available Multi-port Match: Available Connection Tracking Match: Available Packet Type Match: Not available Policy Match: Available Physdev Match: Available IP range Match: Available <--------------
Protocol Number/Names and Port Numbers/Service Names 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. The rules compiler translates protocol names to protocol numbers and service names to port numbers itself. Also, unless otherwise documented, a protocol number/name can be preceded by '!' to specify "All protocols except this one" (e.g., "!tcp").
ICMP and ICMP6 Types and Codes 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' => 1/2 port-unreachable' => 1/3 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 if ICMP (ICMP6) types prior to Shorewall 4.4.19.
Port Ranges 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 DEST PORTS(S) 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").
Port Lists 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.
Using MAC Addresses 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. MAC Address of an Ethernet Controller 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. It is not necessary to use the special Shorewall notation in the /etc/shorewall/maclist file.
Rate Limiting (Rate and Burst) 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): LOGRATE=10/minute LOGBURST=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. The LOGRATE and LOGBURST options are deprecated in favor of LOGLIMIT. Shorewall also supports per-IP rate limiting. Another example from shorewall.conf (5): LOGLIMIT="s:5/min: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.
Switches There are times when you would like to enable or disable one or more rules in the configuration without having to do a 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. As of this writing, Condition Match Support requires that you install xtables-addons. The SWITCH column contains the name of a switch. Each switch that is initially in the off position. You can turn on the switch named switch1 by: echo 1 > /proc/net/nf_condition/switch1 You can turn it off again by: echo 0 > /proc/net/nf_condition/switch1 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: begin with a letter and be composed of letters, digits, underscore ('_') or hyphen ('-'); and be 30 characters or less in length. Multiple rules can be controlled by the same switch. Example:
Forward port 80 to dmz host $BACKUP if switch 'primary_down' is on. #ACTION SOURCE DEST PROTO DEST SOURCE ORIGINAL RATE USER/ MARK CONNLIMIT TIME HEADERS SWITCH # PORT(S) PORT(S) DEST LIMIT GROUP DNAT net dmz:$BACKUP tcp 80 - - - - - - - - primary_down
Logical Interface Names When dealing with a complex configuration, it is often awkward to use physical interface names in the Shorewall configuration. You need to remember which interface is which. If you move the configuration to another firewall, the interface names might not be the same. Beginning with Shorewall 4.4.4, you can use logical interface names which are mapped to the actual interface using the option in shorewall-interfaces (5). Here is an example: #ZONE INTERFACE BROADCAST OPTIONS net COM_IF detect dhcp,blacklist,tcpflags,optional,upnp,routefilter=0,nosmurfs,logmartians=0,physical=eth0 net EXT_IF detect dhcp,blacklist,tcpflags,optional,routefilter=0,nosmurfs,logmartians=0,proxyarp=1,physical=eth2 loc INT_IF detect dhcp,logmartians=1,routefilter=1,tcpflags,nets=172.20.1.0/24,physical=eth1 dmz VPS_IF detect logmartians=1,routefilter=0,routeback,physical=venet0 loc TUN_IF detect 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.
Zone and Chain Names 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". The default separator is "2" but you can override that by setting ZONE_SEPARATOR="-" 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.
Optional and Required Interfaces 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 and restart and adjust its configuration accordingly. The arp_filter, arp_ignore, routefilter, logmartians, proxyarp and sourceroute options are not enforced when the interface is down, thus avoiding an error message such as:WARNING: Cannot set Martian logging on ppp0 If the interface is associated with a provider in shorewall-providers (5), start and restart will not fail if the interface is not usable. When DETECT_DNAT_IPADDRS=Yes in shorewall.conf (5), DNAT rules in shorewall-rules (5) involving the interface will be omitted when the interface does not have an IP address. If detect is specified in the ADDRESS column of an entry in shorewall-masq (5) then the firewall still start if the optional interface in the INTERFACE column does not have an IP address. 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 Configurations Shorewall allows you to have configuration directories other than /etc/shorewall. The shorewall check, 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. Shorewall requires that the file /etc/shorewall/shorewall.conf to always exist. Certain global settings are always obtained from that file. If you create alternative configuration directories, do not remove /etc/shorewall/shorewall.conf. This facility permits you to easily create a test or temporary configuration by copying the files that need modification from /etc/shorewall to a separate directory; modify those files in the separate directory; and specifying the separate directory in a shorewall start or shorewall restart command (e.g., shorewall restart /etc/testconfig )
Saved Configurations 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. The default saved configuration is used by Shorewall in a number of ways besides in the restore command; to avoid surprises, I recommend that you read the Shorewall Operations documentation section about saved configurations before creating one.