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README

                       -= Arno's iptables firewall =-
         Single- & multi-homed firewall script with DSL/ADSL support

                      ~ In memory of my dear father ~

(C) Copyright 2001-2017 by Arno van Amersfoort & Lonnie Abelbeck
Homepage   : http://rocky.eld.leidenuniv.nl/
Email      : a r n o v a AT r o c k y DOT e l d DOT l e i d e n u n i v DOT n l
             (note: you must remove all spaces and substitute the @ and the .
             at the proper locations!)
-------------------------------------------------------------------------------
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
version 2 as published by the Free Software Foundation.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
-------------------------------------------------------------------------------

A MESSAGE FROM THE AUTHOR:
--------------------------
Almost *all* my work is distributed under the terms of the GNU GPL License,
which means it's free (open-source) software. If you like my work or you want
me to implement a certain feature, you are encouraged to donate money. You can
(preferably) donate directly to me through my bank account (mail me for my IBAN
number (International Bank Account Number). My favourite charity organisations are:
- foundations for cancer research (in The Netherlands: "KWF Kanker Bestrijding");
- foundations for brain desease research (in The Netherlands: "De Hersenstichting");
- foundations for the welfare of animals ("IFAW" or in the Netherlands: "De Dierenbescherming")

Note that *ALL* donations I receive go to one of the above foundations.

IMPORTANT NOTE:
---------------
I don't provide enduser support on my email address. Any problems & questions
directly related to the use/implementation of my firewall should go to the
mailinglist, for which you can sign up on my website. Also consult the FAQs
before reporting a problem/question. Please use this way as I'm simply too busy
to help everybody out with every (trivial) issue. Furthermore read the
information in the troubleshooting section below!

An explanation of the files in the package:
-------------------------------------------
/bin/arno-iptables-firewall :
        The actual firewall script, core of Arno's iptables firewall. You
        should put this file in eg. /usr/local/sbin/ . You should make sure
        it's executable (use "chmod 700 or chmod +x).

/bin/arno-fwfilter :
        A pipe filter script to make the firewall-log better readable. It can
        be used for example in conjuction with a tail to log your firewall to
        local tty10 (-12). It can be used for both /var/log/messages and
        /var/log/firewall (or whatever name you configured syslogd), depending
        on the log-level specified in the configuration file. An example on how
        to use it can be found in the beginning of the fwfilter script. Any
        options for fwfilter can be configured within the script itself. You 
        should put this file in eg. /usr/local/bin/.

/etc/arno-iptables-firewall/firewall.conf :
        The configuration file used for Arno's iptables firewall script.
        Normally you should put it in /etc/arno-iptables-firewall/. Make sure
        root is owner/group (with "chown 0:0").

/etc/arno-iptables-firewall/plugins/ :
        Put any plugin config files (.conf files) for my firewall in this
        directory.

/etc/arno-iptables-firewall/conf.d/ :
        Put any (override) configuration files in the directory. Any files here
        with a .conf-extension(!) will be sourced AFTER the main firewall.conf
        file has been read.

/etc/arno-iptables-firewall/custom-rules :
        Put any (iptables) custom rules in this file. This file
        should be put in /etc/arno-iptables-firewall/ . Make sure root
        is owner/group (with "chown 0:0").

/etc/init.d/arno-iptables-firewall :
        The init.d script to start/stop the script at boot etc. On some (older)
        distributions you may need to put it in /etc/rc.d/ (instead of
        /etc/init.d).  You should make sure it's executable (use "chmod 700 or
        chmod +x). Inside this script you can also enable VERBOSE(=1) logging
        for eg. debugging purposes.

/share/arno-iptables-firewall/environment :
        This is the environment-file required by the firewall and some plugins.
        It contains several global functions. It should normally be put in
        /usr/local/share/ on your local system.

/share/arno-iptables-firewall/plugins/ :
        Put any plugin binaries (.plugin files) for my firewall in this
        directory. It should normally be put in /usr/local/share/ on your local
        system.

/share/man/man8/arno-iptables-firewall.8 :
        A man page for the arno-iptables-firewall script.

/share/man/man1/arno-fwfilter.1 :
        A man page for the arno-fwfilter script.

/CHANGELOG :
        The version changelog of my firewall.

/README :
        "This" file.

/configure.sh :
        Script to setup a basic configuration.

/install.sh :
        Install script to deploy my firewall on your system.

/uninstall.sh :
        Uninstall script to remove my firewall from your system.

/contrib/ :
        Directory contains any misc. (user contributed) files (scripts etc.) It
        also contains examples on how to modify your syslogger to log your
        firewall stuff into a separate file.

------------------------------------------
| Some IMPORTANT (security) information: |
------------------------------------------
1) If possible try to start the firewall before you enable your (ADSL) internet
   connection. For an ppp-interface that doesn't exist yet
   you can use the wildcard device called "ppp+" (but you can only use
   ppp+ if there aren't any other ppp interfaces!).

2) Don't change any (security) settings ('EXPERT SETTINGS') if you don't
   really understand what they mean. Changing them anyway could have a big
   impact on the security of your machine.

3) I get a lot of emails from people complaining that their webserver etc.
   stopped working after installing my firewall. This is the CORRECT
   behaviour for a firewall: BLOCKING ALL incoming traffic by default!
   Configure your eg. OPEN_TCP accordingly!


-----------------
| General hints |
-----------------
1) For IPv4 addresses you can use IP ranges in all variables by specifying it as
   eg. "192.168.1.10-50" (which would make the range start with 192.168.1.10
   and end at 192.168.1.50). Note that this only works for Class-C(/24) ranges,
   so specifying eg. 192.168.1.1-192.168.2.1 does NOT work!

2) My firewall has mixed IPv4/IPv6 support. You can switch from IPv4-only to
   IPv4/IPv6 support by simply setting "IPV6_SUPPORT=1" in the config file.

3) You can use the $ANYPORT and $ANYHOST macros to specify "ALL ports" or
   "ALL hosts" in the configuration variables/rules.

4) The default separator for configuration-variables/rules is ~. For source
   target definitions the separator to separate source and target is >. All of
   this is also explained (with examples) in the configuration file.

5) For configuration-variables/rules which are related to the external
   (internet) interface one can restrict the interface(s) for which it is
   applied to by adding either "{interface1,interface2,...}#" or
   "{interface_ip1,interface_ip2}#" at the beginning of the rule. The latter
   is especially handy for aliased interfaces.
   Example 1: OPEN_TCP="eth0#22", would only open TCP port 22 (SSH) for
              interface eth0
   Example 2: OPEN_TCP="1.2.3.4#22", would only open TCP port 22 (SSH) for
              interface which has the IP 1.2.3.4

   This feature can also be used to enable NAT port forwarding for certain
   (external) interfaces. Examples:
   Example 1: NAT_FORWARD_TCP="eth0#0/0~22>{internal_host}" means:
              - Forwards TCP port 22;
              - Forward is available for the whole world (0/0);
              - Forward is applied to eth0 only;
              - {internal_host} is the host the port should be forwarded to.
   Example 2: NAT_FORWARD_TCP="1.2.3.4#0/0~80>{internal_host}" means:
              - Forwards TCP port 80;
              - Forward is available for the whole world (0/0);
              - Forward is applied to the (external) (aliased) interface with
                IP 1.2.3.4;
              - {internal_host} is the host the port should be forwarded to.

6) Port ranges should be written as port_start:port_end, eg. "137:139" would
   select ports 137,138 and 139.

-----------------------------------------
| Debian package repository information |
-----------------------------------------
When you are using Debian (or a Debian-flavor: Ubuntu, Kubuntu etc.), you can
either use the package that comes with the distribution or you can use
Michael Hanke's package repository. Simply add:
        deb http://apsy.gse.uni-magdeburg.de/debian sarge main

to your /etc/apt/sources.list and installing the firewall is then as easy as:
        apt-get install arno-iptables-firewall

That way one can automatically profit from any package update. Even if no
binary package is available one could build it by adding:
        deb-src http://apsy.gse.uni-magdeburg.de/debian/source ./

to /etc/apt/sources.list and execute:
        apt-src install arno-iptables-firewall
        apt-src build arno-iptables-firewall

NOTE: Currently there is no package available yet for the 1.9-branch, only for
      version 1.8. If you like to run 1.9, which is recommended, use the
      .tgz-version from my website.

---------------
| Quick setup |
---------------
If you want to have it running ASAP or are a novice user, than this is the part
that's important. Remember that my firewall has a lot of other useful features
which will NOT be used in this way. On the other hand, various security
features are enabled by default to protect you from hostile attacks.

1) First we've to check whether your Linux setup is OK in order to make the
   script work correctly:
        - It needs iptables and iproute(2) to be installed (probably come 
          as packages with your distro).
        - It requires a POSIX compliant /bin/sh (should live on any UNIX system
          by default)
        - My scripts need the following binaries (in your path): 
          iptables (obviously), ip (from the iproute package), sysctl, modprobe,
          logger, uname, date, awk, tr, grep, sed, cut, head, tail, wc, which,
          & cat.
        - If you plan to use DNS resolving (eg. for certain plugins) then the
          binary 'dig' (from the dnsutils package) or as a fall-back 'nslookup'
          should also be available.

2) Now we need to determine whether you have a single- or dual-homed machine.
   Single means you ONLY have one network-interface, which is the one connected
   to the outside "evil" world (internet). Dual-homed also have a local subnet
   connected to an additional network interface.

3) Run the install script and follow the instructions: ./install.sh

   a) Configure your external network interfaces, EXT_IF. In case of a
      dual(multi)-homed it's the interface which is connected to the internet, in
      case of a LAN it's the one connected to your network. When you have an
      (dynamically) IP assigned to you (by your ISP) via DHCP, you should set
      "EXT_IF_DHCP_IP=1" else leave it off (0, default). If you have multiple
      (non-aliased) external interfaces, you should ALL specify them here (space
      separated). Note that for aliased interfaces you should only specify the
      "parent"-interface in EXT_IF. So if you have eth0, eth0:1 and eth0:2, you
      should make EXT_IF="eth0" (only).

   b) When your public IP is assigned to you by your ISP (through DHCP) then you
      should enable support for an DHCP external assigned IP.

   c) Now we configure what ports should be open for the outside world. If you
      eg. are running an HTTP-server(port 80), an SSH-server(port 22), and/or
      an FTP-server (port 21) which should be accessible from the internet you
      should configure the OPEN_TCP / OPEN_UDP variables like this:
      OPEN_TCP="21 22 80"
      OPEN_UDP=""

   d) For dual-homed machines you should also configure INT_IF, the interface
      used for the local network and you should set your local subnet range in
      "INTERNAL_NET=". If you want your internal network to be able to access
      the internet (aka. internet-sharing), you should also enable NAT
      (masquerading) by setting "NAT=1"). For single-homed machines (part of a
      LAN), you shouldn't touch INT_IF (leave it disabled) and just stick to
      using EXT_IF.


4) Now your firewall is ready but I'd suggest to review this additional info:

   a) In case you use an (A)DSL modem (which works with a PPtP connection to
      your machine) you should enable the dsl-ppp-modem plugin (You can verify
      this with 'ifconfig', if a ppp device with your public IP exists you
      need this).

      We must enable/configure the dsl-ppp-modem plugin via
      /etc/arno-iptables-firewall/dsl-ppp-modem.conf by setting ENABLED=1.
      Now we must configure the network interface(ethX) to which your modem is
      physically connected (=MODEM_IF, which is commented(#) out by default),
      and this is NOT ppp+, ppp0 etc.! Here are some examples on how to do it
      for some providers (it's assumed that the modem is connected to eth0):

      PPPoE connection with a static public IP (eg. MxStream in the Netherlands)
      (setup with the ADSL4Linux package from http://www.adsl4linux.nl):
      - MODEM_IF="eth0"
      - MODEM_IF_IP="10.0.0.150"
      - MODEM_IP="10.0.0.138"              # Make sure this IP corresponds to
                                             the one used by your modem!

      T-DSL (Germany) with a dynamic public IP:
      - MODEM_IF="eth0"
      - MODEM_IF_IP="192.168.99.1"
      - MODEM_IP=""

      PPPoA connection with a dynamic public IP:
      - MODEM_IF="eth0"
      - MODEM_IF_IP=""                     # This MUST be unset("") (default)
      - MODEM_IP="10.0.0.138"              # Make sure this IP corresponds to
                                             the one used by your modem!

      NOTE 1: For extra security you *can* set the IP of your modem (MODEM_IP),
              but it's not neccessary (anymore). If you don't know its IP or
              believe it doesn't have an IP, you can leave MODEM_IP="".
              The same applies for the IP of the modem network interface
              (MODEM_IF_IP).

      NOTE 2: If both your modem AND your network interface don't have an IP
              you probably don't have to configure your modem settings (at all).

      NOTE 3: In case of a PPPoA (PPP-over-ATM) you MUST leave MODEM_IF_IP
              empty(="")!

      NOTE 4: Don't forget to set EXT_IF_DHCP_IP=1 in firewall.conf too, in
              case your ISP uses DHCP.

   b) In case your on a corporate network which uses public IPs I'd suggest to add
      your local subnet (range) to "FULL_ACCESS_HOSTS".

   c) Some people mentioned that protocols like IRC or some (older)
      FTP/POP3/SMTP servers don't work (properly) if port 113(Identd) is
      filtered (firewalled). I really hate the fact that these type of
      protocols still depend on the "not-so-secure" IDENT-protocol. But if you
      really need it, you can do 2 things to make them work properly:
      1) If you don't want to run an IDENT-daemon, simply add port 113 to the
         REJECT_TCP-variable (Recommended).
      2) Or if you really want to run an IDENT-daemon, you should add port 113
         to the OPEN_TCP-variable. (Not recommended)

9)  You're now ready to start the firewall by issueing:
    "/etc/init.d/arno-iptables-firewall start"
    Everything should be working OK now, if it doesn't, carefully review all
    steps and your configuration. For troubleshouting you can first consult 
    the FAQs on my webpage.

    NOTE 1: Make sure that when you use NAT, you should properly configure the
            client's "default gateway" and the (public) DNS server(s) it should
            use! Note that you don't have to setup any proxy settings in eg.
            your client's browser.

    NOTE 2: Additional (more advanced) options are (also) explained in the
            configuration-file comments and in the QA's on my webpage (eg.
            IPSec VPN support).


Troubleshooting: What if it doesn't work?:
------------------------------------------------------------------------------
1)  Check your settings (.conf) at least 10 times. It's quite common for a
    human being to make mistakes.
    TIPS / Common errors:
    - Make sure that EXT_IF, MODEM_IF and/or INT_IF are not the same. If they
      are, YOU made a mistake, as they can never EVER be the same!
    - Another error I once saw was someone that used something like
      "127.0.0.0/24" for his local subnet. "127.0.0.0" is the address of the
      local loopback and therefor should never ever appear in the configuration
      file!
2)  Obtain the latest version of your (distribution) kernel & iptables.
3)  Make sure your (self-built) kernel supports all required options.
4)  Carefully inspect the output generated when issueing
    "arno-iptables-firewall start"
5)  Read the README file at least 3 times
6)  Download the latest (beta) version of my script and check whether this
    fixes your problem.
7)  Read the README file one more time and review your .conf-file also one more
    time, just in case ;-)
8)  Do NOT send enduser requests to my personal email address, instead post
    your question/problem on the firewall mailing list. Provide us with:
    - your (firewall) *.conf files
    - the screen output of "/usr/local/sbin/arno-iptables-firewall start" 
      (or whatever it is located)
    - the output of 'ifconfig'
    - (firewall) logs
    - the version of my script you're using (or date if you use the development
      script)
    - detailed explanation of your setup
    - and anything else that might help
    Remember that people that don't obey these rules, get a low, very low
    priority, or won't get any reaction at all!


Plugin support
------------------------------------------------------------
As of version 1.8.7-RC2 my firewall also supports plugins -> little scripts
that implement specific stuff which doesn't exist in the main script (yet).
They latest versions of all supported plugins can be found here:
   http://rocky.eld.leidenuniv.nl/iptables-firewall/plugins/

Notes on plugins:
-----------------
1) Plugins should be put in /etc/arno-iptables-firewall/plugins/
2) The priority/loading order of the plugins can be adjusted by the changing
   the number in front of the plugin-name (50=default). Increasing the number
   gives lower priority, decreasing the number gives higher priority.
3) All plugins have an option called "ENABLED" (in their config file) which is
   set to 0 by default, meaning it is disabled. So if you actually want to use
   a plugin, you have to make ENABLED=1
4) Plugins can have their own additional set of configuration variables, don't
   forget to set/review those too.

Everyone is invited to write their own plugins to implement other things, and
to submit them.

Notes on writing your own plugins
---------------------------------
1)  When you write your own plugins, make sure you know what you're doing. You
    can severely compromise security or break things with buggy plugins.
2)  Submit plugins to me, if you think they can be of use to others, but note
    that I always reserve the right to decline the plugin (because it was eg.
    poorly written). Submitted plugins must be (at least) compatible with the
    GPLv2 license.
3)  The plugin should have/use these variables:
    - PLUGIN_NAME (Plugin name/description)
    - PLUGIN_VERSION (Plugin version)
    - PLUGIN_CONF_FILE (Location of the plugin config file)

    Furthermore it should honour the ENABLED variable from the config-file to
    enable/disable the plugin.

    Use one of my plugins as a template(skeleton) for writing your own plugins
    (I recommend to have a look at the "SSH Brute-Force protection"-plugin),
    in this way it's easier to understand it for me and for others.
4)  Plugins should have a separate config file (.conf) with all user
    variables(settings). It should at least contain the "ENABLED="-variable
    to enable/disable the plugin.
5)  Plugins should also have a separate file with their CHANGELOG (.changelog)
6)  Plugins should be preferably POSIX shell compatible (eg. work with "Dash")
7)  Plugins can use all variables/functions/chains from the main-script and
    main configuration file. Plugin specific configuration variables should be
    put inside the plugin's configuration file (.conf).
8)  Make sure that when you create new iptables-chains, they don't conflict
    with the main script or other plugins. The same goes for the iptables
    MARK-module, make sure that you use an unique MARK-number that doesn't
    conflict with other plugins.
9)  Plugins should in principle always cleanup up their own chains (and
    possibly other stuff) they created at start when stopping.
10) A list of available chains created by AIF's main script can be found below.
    Note that I strongly recommend NOT to directly use any builtin iptables
    chains like INPUT/OUTPUT/FORWARD/PREROUTING/POSTROUTING. Only do this when
    it's absolutely necessary!


Available iptables chains created by Arno's Iptables Firewall
-------------------------------------------------------------
BASE_INPUT_CHAIN            - Base input chain. For internal use by AIF only!
BASE_OUTPUT_CHAIN           - Base output chain. For internal use by AIF only!
BASE_FORWARD_CHAIN          - Base forward chain. For internal use by AIF only!
INPUT_CHAIN                 - AIF's main INPUT chain. Use this if you want to
                              insert rules in the INPUT chain
OUTPUT_CHAIN                - AIF's main OUTPUT chain. Use this if you want to
                              insert rules in the OUTPUT chain
FORWARD_CHAIN               - AIF's main FORWARD chain. Use this if you want to
                              insert rules in the FORWARD chain
EXT_INPUT_CHAIN             - External-net INPUT chain
EXT_OUTPUT_CHAIN            - External-net OUTPUT chain
EXT_BROADCAST_CHAIN         - External-net chain for broadcast traffic
EXT_MULTICAST_CHAIN         - External-net chain for multicast traffic
EXT_FORWARD_IN_CHAIN        - External-net FORWARD chain for INcoming traffic
EXT_FORWARD_OUT_CHAIN       - External-net FORWARD chain for OUTgoing traffic
EXT_ICMP_FLOOD_CHAIN        - External-net chain where ICMP packets go which
                              are considered a "flood"
DMZ_FORWARD_IN_CHAIN        - DMZ FORWARD chain for INcoming traffic
DMZ_FORWARD_OUT_CHAIN       - DMZ FORWARD chain for OUTgoing traffic
DMZ_INET_FORWARD_CHAIN      - DMZ to internet/external-net forward chain
DMZ_INPUT_CHAIN             - DMZ INPUT chain
DMZ_LAN_FORWARD_CHAIN       - DMZ to LAN/internal-net forward chain
DMZ_OUTPUT_CHAIN            - DMZ output chain
INET_DMZ_FORWARD_CHAIN      - External-net(internet) to DMZ forward chain
HOST_BLOCK_SRC              - Chain containing the list of inbound blocked hosts
HOST_BLOCK_DST              - Chain containing the list of outbound blocked hosts
HOST_BLOCK_SRC_DROP         - Chain where packets from dropped inbound blocked hosts go
HOST_BLOCK_DST_DROP         - Chain where packets from dropped outbound blocked hosts go
INT_INPUT_CHAIN             - Internal-net INPUT chain
INT_OUTPUT_CHAIN            - Internal-net OUTPUT chain
LAN_LAN_FORWARD_CHAIN       - LAN to LAN (Inter-LAN) forward chain (AIF private use only)
LAN_INET_FORWARD_CHAIN      - LAN to internet (external net) forward chain
POST_INPUT_CHAIN            - This chain is always processed last(post) in the
                              INPUT chain
POST_OUTPUT_CHAIN           - This chain is always processed last(post) in the
                              OUTPUT chain
POST_FORWARD_CHAIN          - This chain is always processed last(post in the
                              FORWARD chain
POST_INPUT_DROP_CHAIN       - Packets dropped at the end of the INPUT chain end
                              up in this chain (Used for eg. IDS)
RESERVED_NET_CHK            - This chain holds the list of reserved nets to
                              check against
SPOOF_CHK                   - This chain contains rules for spoof checking
VALID_CHK                   - This chain contains rules for checking whether a
                              packet is valid
NAT_POSTROUTING_CHAIN       - NAT (-t nat) POSTROUTING chain
NAT_PREROUTING_CHAIN        - NAT (-t nat) PREROUTING chain
POST_NAT_POSTROUTING_CHAIN  - This chain is always processed last(post) in the
                              NAT (-t nat) POSTROUTING chain
POST_NAT_PREROUTING_CHAIN   - This chain is always processed last(post) in the
                              NAT (-t nat) PREROUTING chain


Loadbalancing/multirouting (with multiroute masquerade/SNAT)
------------------------------------------------------------
My firewall also supports multirouting (loadbalancing), optionally in
conjunction with NAT. Although this works with both conventional masquerading
and SNAT, it's strongly recommended to use SNAT. This is because the latter
is known to have a much lower chance of causing problems. Also note that I
haven't found a way (yet) to make this work with dynamic external(internet)
IP's, meaning you need static IP's from your ISP.

First of all, if you want to use multirouting, make sure that your
(vanilla)-kernel has the following network features enabled (eg. when
building from source):
- CONFIG_IP_ADVANCED_ROUTER=y
- CONFIG_IP_MULTIPLE_TABLES=y
- CONFIG_IP_ROUTE_MULTIPATH=y

Second, you should configure/enable the multiroute-plugin. And last but not
least, you should setup the firewall: adding (all) the used external
interfaces to EXT_IF. And when SNAT is used, add the corresponding
external IPs to NAT_STATIC_IP. That's it!

NOTE: Redundant connections are (currently) not supported! This is limitation
of the (current) Linux kernel (not of my firewall).


Info when building your own kernel (2.4 & 2.6) through "make menuconfig":
-------------------------------------------------------------------------------
For the firewall to work properly you need the following options enabled (as
modules or compiled in your kernel):
- "Loadable module support"
        - "Enable loadable module support" (If you want to build iptables as
           modules)
        - "Automatic kernel module loading" (Strongly recommended if you build
           iptables as modules) (Only available in newer 2.6 kernels)
- "Networking", "Networking Support", "Networking Options" :
        - "Packet socket" (If you want to use dhcp client and/or server)
        - "TCP/IP networking"
                - "IP: Multicasting"
                - "IP: advanced router"
                        - "IP: policy routing" (If you want to use load
                           balancing, eg. multiroute masquerading)
                        - "IP: equal cost multipath" (If you want to use load
                           balancing, eg. multiroute masquerading)
                - "IP: TCP syncookie support"
        - ("Network packet filtering")
                - "Core Netfilter Configuration" (For kernel =>2.6.16)
                        - "Netfilter Xtables support (Required for ip_tables)"
                                - "MARK" target support (Only required for
                                   special purposes like eg. traffic shaping
                                   & kernel 2.6 VPN support)
                                - "conntrack" connection tracking match support
                                - "limit" match support
                                - "mac" address match support (If you want to
                                   use MAC filtering)
                                - "state" match support
                                - "tcpmss" match support (If you want to use
                                   tcpmss clamping)
                - "IP: Netfilter Configuration":
                        - "Connection tracking"
                                - "Connection tracking flow accounting" (If you
                                   want to do accounting on your network traffic.
                                   (kernel 2.6 only)
                                - "FTP protocol support"
                        - "IP tables support" (NOTE: The order of sub-options can
                           differ between kernel versions):
                                - "Multiple port match support"
                                - "TOS match support" (If you want to use TOS
                                   mangling)
                                - "recent match support" (required for IDS
                                   & SSH brute-force protection)
                                - "TTL match support" (If you want to use TTL
                                   manipulation)
                                - "limit match support" (kernel <2.6.16)
                                - "MAC address match support" (If you want to
                                   use MAC filtering) (kernel <2.6.16)
                                - "Multiple port match support" (kernel <2.6.16)
                                - "tcpmss match support" (If you use tcpmss
                                   clamping) (kernel <2.6.16)
                                - "Connection state match support"
                                   (Kernel <2.6.16)
                                - "Packet filtering" (kernel <2.6.16)
                                        - "REJECT target support"
                                - "LOG target support"
                                - "TCPMSS target support (If you want to use
                                   tcpmss-clamping)
                                - "Full NAT" (If you use NAT/masquerading aka
                                   internet-sharing or transparent proxies)
                                        - "MASQUERADE target" (If you want to
                                           use masquerading)
                                        - "REDIRECT target support" (If you
                                           want to use port- forwarding,
                                           -redirection or transparent proxies)
                                - "Packet mangling"
                                        - "TOS target support" (If you want to
                                           use TOS mangling)
                                        - "MARK target support" (Only required
                                           for special purposes like eg. traffic
                                           shaping & kernel 2.6 VPN support)
                                           (kernel <2.6.16)
                                        - "TTL target support" (if you want to
                                           use TTL manipulation


Kernel configuration - Special issues
-------------------------------------
Some kernel versions, or series of versions, may have unique issues, below are
topics related to the scope of this firewall script.

1) Starting with kernel version 2.6.27, CONFIG_NF_CT_ACCT is deprecated, the
   result when the "nf_conntrack" module is loaded and the kernel has
   CONFIG_NF_CT_ACCT=y set, the following message is displayed:

     "CONFIG_NF_CT_ACCT is deprecated and will be removed soon. Please use
     nf_conntrack.acct=1 kernel paramater, acct=1 nf_conntrack module option or
     sysctl net.netfilter.nf_conntrack_acct=1 to enable it."

   The message is harmless, and can be safely ignored. The main script also
   sets net.netfilter.nf_conntrack_acct=1 in the case CONFIG_NF_CT_ACCT is not
   set.

   Though, if you find this message annoying, it can be silenced via
   "make menuconfig":

       -- Core Netfilter Configuration --
     <M> Netfilter connection tracking support
     [ ]   Connection tracking flow accounting

   if "Connection tracking flow accounting" can't be disabled, then disabling

     < >   "connbytes" per-connection counter match support

   may be required because of dependencies.  The resulting configuration
   settings are:

     CONFIG_NF_CONNTRACK=m
     # CONFIG_NF_CT_ACCT is not set
     # CONFIG_NETFILTER_XT_MATCH_CONNBYTES is not set

   CONFIG_NF_CT_ACCT was scheduled to be removed in 2.6.29, but has not yet
   been removed, as of writing.