Linux IP Masquerade HOWTO
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# 3.2. Checking your existing kernel for MASQ functionality

Almost ALL modern Linux distributions come MASQ-Ready these days but its always good to check your system before you try to set things up. Follow these few steps for your kernel to see if your kernel is MASQ ready.

To see which kernel your system is running, run the following command:

    uname -a

- Just for clarity: 2.4.x kernels run IPTABLES :: 2.2.x kernels run IPCHAINS :: 2.0.x kernels run IPFWADM

- In general, you must have kernel support for:

  - IP forwarding

  - IP masquerading

  - IP Firewalling

  - etc.

You will also need to have most MASQ-related modules compiled (most modular kernels will already have all you need already done. Then you will NOT need to re-compile the kernel. If you AREN'T SURE if your Linux distribution is MASQ ready, do the following:

- 2.4.x kernels (look for most of the following entries out of the much longer list):

  - Run the command "`ls /proc/sys/net/ipv4`" while logged into the Linux box. These items are required and should be present regardless if your kernel built IPMASQ as modules or statically.

    - `ip_dynaddr`

      `ip_forward`

  - To check if IPMASQ was compiled statically into the kernel, run the command "`/sbin/lsmod`" and see if and modules like the ones shown below for the MODULE section are loaded. No? Ok, now run the command "`ls /proc/net/`" and see if you see additional /proc files such as:

    - `ip_masquerade`

      `ip_conntrack`

      `ip_tables_names`

    If you see these /proc entries and there WEREN'T any kernel modules loaded (shown via the "lsmod" command mentioned above), then your kernel has the IPTABLES subsystem statically compiled into it and is ready to go to use IPMASQ on this system.

  - If your kernel uses IPTABLES via modules, most of the stuff listed above should have been missing (because the modules probably aren't loaded). Run the command "`ls /lib/modules/`uname -r`/kernel/net/ipv4/netfilter/`" where you should see files like:

    - `ip_conntrack.o, ip_conntrack_ftp.o, ip_conntrack_irc.o, ip_nat_ftp.o, ip_nat_irc.o`

      `ip_tables.o, ipt_MASQUERADE.o, iptable_nat.o, iptable_mangle.o, iptable_filter.o`

      And some optional ones like: `ipchains.o, ipt_REJECT.o, and ipt_tcpmss.o`

    If you see those kernel files, IPTABLES was compiled using modules and things look ready to go to use IPMASQ on this system.

- 2.2.x kernels (look for most of the following entries out of the much longer list): list):

  - Run the command "`ls /proc/sys/net/ipv4`" while logged into the Linux box. These items are required and should be present regardless if your kernel built IPMASQ as modules or statically.

    - `ip_always_defrag`

      `ip_dynaddr`

      `ip_forward`

      `ip_masq_debug`

      `ip_masq_udp_dloose` (some distros don't support this -- ignore it for now

      Other 2.2.x options can be checked by running "ls /proc/net/"

      - `ip_fwchains`

        `ip_fwnames`

        `ip_masquerade`

      Even more 2.2.x options can be checked by running "ls /proc/net/"

      - `app`

        `icmp`

        `icq`

        `mfw`

        `portfw`

        `tcp`

        `udp/`

- 2.0.x kernels (look for most of the following entries out of the much longer list):

  - Run the command "`ls /proc/sys/net/ipv4`" while logged into the Linux box. These items are required and should be present regardless if your kernel built IPMASQ as modules or statically.

    - `ip_dynaddr`

      `ip_forward`

      running "ls /proc/net"

      - `ip_forward`

        `ip_masq_app`

        `ip_masquerade`

        `ip_portfw`

Ultimately, it comes down to the fact if you see /proc files such as "i`ip_forward`", "`ip_masq_debug`", "`ip_masq_udp_dloose`"(optional), and "`ip_always_defrag`" (optional) exist.

So. Do most of the above /proc entries or kernel modules show up for your respective kernel? If so, thats good! If you cannot find any of the above entries or if you aren't sure if your distribution supports IP Masquerading by default, ASSUME IT DOESN'T SUPPORT MASQ. You can do one last check by looking at the Section 7.1 section and see if your Linux Distribution is listed. Still not there? Sounds like you'll need to compile a kernel but don't worry.. it isn't hard.

Regardless if your current kernel has MASQ support or not, reading the remainder of this section is still highly recommended as it contains other useful information.

## 3.2.1. Compiling Linux 2.4.x Kernels

- First, you'll need to get some 2.4.x kernel sources (preferably the latest kernel version - NEWER *IS* BETTER IN LINUX LAND)

  - NOTE #1: As both the 2.4.x kernel train and the iptables program development progresses, the compile configurion options will change over time. As of this version of the IPMASQ howto, this section reflects the settings for IPTABLES 1.2.7a and the 2.4.20 kernel. If you are compiling against a newer or previous kernel or IPTABLES version, the dialogs and even commands might look different. It is recommended that you update to the newest versions of both the kernel and IPTABLES for added capability, performance, and stability of the kernel.

- Next, depending on the version of the Linux kernel and IPTABLES archive you downloaded, you might want to apply some IPTABLES "patch-o-matic" patches against the kernel. These OPTIONAL patches might fix some known problems, add additional functionality you might need (H.323 protocol, specific issues with network games), etc. It should be noted that the Patch-O-Matic patches used to come with the IPTABLES archive. This is no longer the case and you have to download them (if any) seperately. You can find the the various URLs for downloading IPTABLES, the Patch-o-matic system, etc. Section 2.6.

- If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered in several URLs found in Section 2.6. Please note that the instructions included here is just one way to do build a kernel. Please see the Kernel HOWTO for full details.

  NOTE: Please notice that it IS NOT recommended to put the new kernel sources into the /usr/src/linux directory. You should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For more details on this topic, please read the "README" file in the top level directory of the kernel sources.

- For this HOWTO example, create a directory called `/usr/src/kernel`. Next, "cd" into this directory and download the newest 2.4.x kernel sources into it. Once downloaded, issue the following command (if the file ends in a .tar.gz): `tar xvzf linux-2.4.x.tar.gz` or (if the file ends in a .tar.bzip2): `tar xyvf linux-2.4.x.tar.bz2`. Please substitute the "x" in the 2.4.x filename with the Linux 2.4 kernel version you downloaded.

  BZ2 Note: Some Linux distributions use the "I" option instead of the "y" option to decompress bzip2 archives.

  Once uncompressed, I recommend that you rename the directory from the stock "linux" name to "linux-2.4.x" (replace the "x" with the specific version of your newly installed kernel) for clarity. To do this, run the command "`mv linux linux-2.4.x`". Next, make sure there is a directory or symbolic link pointing to "`/usr/src/kernel/linux`" ie. run the command:

      ln -s /usr/src/kernel/linux-2.4.x /usr/src/kernel/linux

  again subsituting the "x" for your proper kernel version.

- As mentioned above, you might consider applying any appropriate or optional patches to the kernel's MASQ code BEFORE you compile the final kernel. The IP MASQ code found in the stock kernels is already very useful and does not require any specific patching in order for the system to work for NAT-friendly network applications. Many of these patches are only to fix possible known bugs, add new features (some are /very/ cool), etc. Please refer to Section 2.6 for URLs and the IP Masquerade Resources for up-to-date information and patch URLs.

- Applying IPTABLES and Patch-o-Matic kernel patches

  Download the iptables package and optional Patch-O-matics from the Section 2.6 and put it into a directory, say "`/usr/src/archive/netfilter`". Next, go into this new netfilter directory and uncompress the iptables archive with the command:

      tar xyvf iptables-x.y.z.tar.bz2
      tar xyvf patch-o-matic-x.tar.bz2

  Now, go into the new iptables-x.y.x directory (/usr/src/archive/netfilter/iptables-x.y.z) and run the command

       #For iptables v1.2.7a:
       make KERNEL_DIR=/usr/src/kernel/linux

       #For iptables v1.2.4 (when Patch-o-matic was built-in):
       make pending-patches KERNEL_DIR=/usr/src/kernel/linux

  NOTE: this assumes that your 2.4.x kernel sources are in the `/usr/src/kernel/linux` directory.

  NOTE #2: If you append a "/" to the end of the above command line, you will get an error stating:

      "make: *** [/usr/src/kernel/linux/include/asm/socket.h] Error 1".

  Remove the trailing "/" and try again.

  Here is an example of compiling IPTABLES v1.2.7a. Your output might look different depending on what version you are trying to use.

-     # make KERNEL_DIR=/usr/src/kernel/linux

      Extensions found:

      cc -O2 -Wall -Wunused -I/usr/src/kernel/linux/include -Iinclude/
      -DIPTABLES_VERSION=\"1.2.7a\"  -fPIC -o extensions/libipt_ah_sh.o -c
      extensions/libipt_ah.c
      ld -shared -o extensions/libipt_ah.so extensions/libipt_ah_sh.o
      cc -O2 -Wall -Wunused -I/usr/src/kernel/linux/include -Iinclude/
      -DIPTABLES_VERSION=\"1.2.7a\"  -fPIC -o extensions/libipt_conntrack_sh.o -c
      extensions/libipt_conntrack.c
      ld -shared -o extensions/libipt_conntrack.so extensions/libipt_conntrack_sh.o
      cc -O2 -Wall -Wunused -I/usr/src/kernel/linux/include -Iinclude/
      -DIPTABLES_VERSION=\"1.2.7a\"  -fPIC -o extensions/libipt_dscp_sh.o -c
      extensions/libipt_dscp.c
      extensions/libipt_dscp_helper.c:69: warning: `dscp_to_name' defined but not
      used
      ld -shared -o extensions/libipt_dscp.so extensions/libipt_dscp_sh.o
      .
      .
      .
      cc -O2 -Wall -Wunused -I/usr/src/kernel/linux/include -Iinclude/
      -DIPTABLES_VERSION=\"1.2.7a\"    -c -o libipulog/libipulog.o
      libipulog/libipulog.c
      ar rv libipulog/libipulog.a libipulog/libipulog.o
      a - libipulog/libipulog.o
      rm libiptc/libip6tc.o libiptc/libip4tc.o libipulog/libipulog.o libipq/libipq.o

- Ok, hopefully the IPTABLES program compiled up for you. Now, you need to install it. To do this, directory and run the command

       make install KERNEL_DIR=/usr/src/kernel/linux

- Here is an example of installing IPTABLES v1.2.7a. Your output might look different depending on what version you are trying to use.

-     # make install KERNEL_DIR=/usr/src/kernel/linux

      cp iptables /usr/local/sbin/iptables
      cp iptables-save /usr/local/sbin/iptables-save
      cp iptables-restore /usr/local/sbin/iptables-restore
      cp ip6tables /usr/local/sbin/ip6tables
      cp extensions/libipt_ah.so /usr/local/lib/iptables/libipt_ah.so
      cp extensions/libipt_conntrack.so /usr/local/lib/iptables/libipt_conntrack.so
      cp extensions/libipt_dscp.so /usr/local/lib/iptables/libipt_dscp.so
      cp extensions/libipt_ecn.so /usr/local/lib/iptables/libipt_ecn.so
      cp extensions/libipt_esp.so /usr/local/lib/iptables/libipt_esp.so
      cp extensions/libipt_helper.so /usr/local/lib/iptables/libipt_helper.so
      .
      .
      .
      cp extensions/libip6t_udp.so /usr/local/lib/iptables/libip6t_udp.so
      cp extensions/libip6t_LOG.so /usr/local/lib/iptables/libip6t_LOG.so
      cp extensions/libip6t_MARK.so /usr/local/lib/iptables/libip6t_MARK.so

Next, if you are interested in applying a Patch-O-Matic patch set, go into the `patch-o-matic-X` directory (/usr/src/archive/netfilter/patch-o-matic-X) and run the command

-      #For Patch-O-Matic later than the release of iptables v1.2.7a:
       KERNEL_DIR=/usr/src/kernel/linux
       ./runme pending

  NOTE #1: The use of the "pending" batch is the most common for IPMASQ functionality but there are several others. See below.

  NOTE #2: this assumes that your 2.4.x kernel sources are in the `/usr/src/kernel/linux` directory.

  NOTE #3: If you append a "/" to the end of the command line, you will get an error stating:

      "make: *** [/usr/src/kernel/linux/include/asm/socket.h] Error 1".
      Remove the trailing "/" and try again.

  Here is an example of the Patch-O-Matic prompts you might receive for a 2.4.20 kernel with the "20030107" Patch-O-Matic set. You can also run the "runme" program in a batch mode to speed things up, add experimental patches, etc. if you'd like. To better understand your options, simply run the "`./runme`" command by itself. Please note that these prompts WILL CHANGE over time.

-     Welcome to Rusty's Patch-o-matic!

      Each patch is a new feature: many have minimal impact, some do not.
      Almost every one has bugs, so I don't recommend applying them all!
      -------------------------------------------------------
      Already applied: submitted/01_2.4.19
                       submitted/02_2.4.20
                       submitted/ipt_ULOG-mac_len-fix
                       submitted/ipt_multiport-invfix
                       pending/01_ip_conntrack_proto_tcp-lockfix
                       pending/02_newnat-udp-helper
                       pending/03_REJECT-fwspotting-phrack60-fix
                       pending/04_ftp-conntrack-msg-fix

      Testing... 05_ECN-tcpchecksum-littleendian-fix.patch NOT APPLIED (1 rejects out
      of 1 hunks)
      The pending/05_ECN-tcpchecksum-littleendian-fix patch:
         Author: Patrick McHardy
         Status: Pending for kernel inclusion

         The 2.4.20 kernel included the new iptables 'ECN' target, enabling a
      selective
         ECN disable mechanism.   Unfortunately there was a bug in the incremental
      TCP
         checksum update, resulting in broken TCP checksums on little endian
      machines.

         This patch fixes the Bug.

      Testing patch pending/05_ECN-tcpchecksum-littleendian-fix.patch...
         Patch pending/05_ECN-tcpchecksum-littleendian-fix.patch applied cleanly.
      Applying patch pending/05_ECN-tcpchecksum-littleendian-fix.patch...
         Patch pending/05_ECN-tcpchecksum-littleendian-fix.patch applied cleanly.

      Excellent! Kernel is now ready for compilation.

- If everything patches fine, you should see something like the text

      Excellent! Kernel is now ready for compilation.

  towards the bottom of the screen. Beyond that, you don't have to install anything at this point. The next step is to compile the new PATCHED kernel.

- Ok, now the new kernel is ready to be compiled but you should make sure that you also have the proper matching `iptables` program on your machine too (just to make sure). Run the command:

  -     whereis iptables

  and make sure its installed on the machine (the default place is in `/usr/local/sbin/iptables`. If you cannot find it or patched up your kernel sources as shown above, I recommend you just re-compile it up as shown above.

Now that the kernel sources are patched up, you need to configure it to know what kinds of features you need (HD support, Networking support, MASQ support, etc.). Here are the MINIMUM kernel configuration options required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just ONE way to configure and compile a kernel (modules vs static). The main difference from this example vs. an example given by a different MASQ guide is that some people might wish to compile kernel components either as modules OR monolithically into the kernel. Basically, compiling things as modules gives you added flexibility to what is or isn't installed into the kernel (reduces unneeded memory use for things you aren't / won't use and modules also allow for drop-in software upgrades [usually no need to reboot the machine]). On the flip side, kernel modules add more complexity to your configuration and sometimes the kernel auto-loader might make mistakes (not that I've ever seen this happen). Compiling things directly into the kernel makes things simpler BUT you loose a huge level of flexibility. The following kernel configuration example is a mixture of both a selection of kernel modules and building them in monolithically (you probably will ALWAYS need MASQ functionality ready to go).

- Side Note: It is assumed that you will also configure the kernel to use your other installed hardware such as USB printers, Ethernet network interfaces, SCSI and IDE HD controllers, etc. as well. Please refer to the Linux Kernel HOWTO and the kernel source's "`README`" file and "`Documentation/`" directory for detailed help on compiling a kernel.

You will need to answer either YES, NO, or MODULE to the following program. Not all options will be available without the proper kernel patches described later in this HOWTO. This shouldn't be an issue as most 3rd party patches are only needed for a very select group of users.

Run the following commands to configure your kernel:

- `cd /usr/src/kernel/linux`

- `make menuconfig`

Please note the following kernel prompts reflect a 2.4.14 kernel (with some of the optional Patch-O-Matic additions. Please read the following carefully for recommendations:

    [ Code maturity level options ]

      * Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]
        - YES: though not required for IP MASQ, this option allows the kernel to create
               the MASQ modules and enable the option for port forwarding

      * Enable loadable module support (CONFIG_MODULES) [Y/n/?]
        - YES: allows you to load kernel IP MASQ modules

      * Set version information on all module symbols (CONFIG_MODVERSIONS) [Y/n/?]
        - YES: allows newer kernels to load older modules if possible

      * Kernel module loader (CONFIG_KMOD) [Y/n/?]
        - OPTIONAL: Recommended : allows the kernel to load various kernel modules as it needs them

      == Non-MASQ options skipped
      ==   (CPU type, memory, SMP, FPU, specific stuff)

    [ General setup ]

      * Networking support (CONFIG_NET) [Y/n/?]
        - YES: Enables the network subsystem

      == Non-MASQ options skipped
      ==   (specific hardware, PCI, kernel binaries, PCMCIA, etc.)

      * Sysctl support (CONFIG_SYSCTL) [Y/n/?]
        - YES:  Enables the ability to enable disable options such as forwarding,
          dynamic IPs, etc. via the /proc interface

    [ Block devices ]

      == Non-MASQ options skipped
      ==   (kernel binaries, power management, PnP, RAID, etc.)

        == Don't forget to compile in support for hardware that you might need:
        ==   IDE controllers, HDs, CDROMs, etc.

    [ Networking options ]

      * Packet socket (CONFIG_PACKET) [Y/m/n/?]
        - YES: Though this is OPTIONAL, this recommended feature will allow you
               to use TCPDUMP to debug any problems with IP MASQ

      * Packet socket: mmapped IO (CONFIG_PACKET_MMAP) [N/y/?] y
        - YES: Speed up the packet protocol

      * Kernel/User netlink socket (CONFIG_NETLINK) [Y/n/?]
        - OPTIONAL:  Recommended : this feature will allow the logging of
               advanced firewall issues such as routing messages, etc

      * Routing messages (CONFIG_RTNETLINK) [N/y/?] (NEW) y
        - OPTIONAL: Allows for support of advanced kernel routing messages
                    if you enabled the CONFIG_NETLINK option

      * Netlink device emulation (CONFIG_NETLINK_DEV) [N/y/m/?] (NEW)
        - NO:  This option does not have anything to do with packet firewall
               logging

      * Network packet filtering (replaces ipchains) (CONFIG_NETFILTER) [N/y/?] y
        - YES: Enable this option to let IPTABLES configure the TCP/IP subsection
               of the kernel.  By enabling this, then you can turn on advanced
               routing mechanisms like IP Masq, packet filtering, etc.

      * Network packet filtering debugging (CONFIG_NETFILTER_DEBUG) [N/y/?] (NEW) n
        - NO: Not required for Masquerading functionality though it may help
              for troubleshooting.  There might be a performance penalty when
              enabling this.

      * Socket Filtering (CONFIG_FILTER) [Y/n/?]
        - OPTIONAL:  Recommended : Though this doesn't have anything do with IPMASQ,
          if you plan on implimenting a DHCP server on the internal network, you WILL
          need to enable this option.

      * Unix domain sockets (CONFIG_UNIX) [Y/m/n/?]
        - YES:  This enables the UNIX TCP/IP sockets mechanisms

      * TCP/IP networking (CONFIG_INET) [Y/n/?]
        - YES: Enables the TCP/IP protocol

      * IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?]
        - OPTIONAL:  You can enable this if you want to be able to receive
                     Multicast traffic.  Please note that your ISP must
                     support Multicast as well for this all to work at all

      * IP: advanced router (CONFIG_IP_ADVANCED_ROUTER) [Y/n/?]
        - OPTIONAL:  Though there is nothing in this section mandatory for
                     Masquerade, some specific options might be useful

        == Non-MASQ options skipped
        ==   ( autoconf, tunneling )

      * IP: multicast routing (CONFIG_IP_MROUTE) [N/y/?] n
        - OPTIONAL:  Though not needed for IPMASQ, enabling this feature will
                     let you route multicast traffic through your Linux box.
                     Please note that this requires that your ISP be multicast
                     enabled as well.

        == Non-MASQ options skipped
        ==   (ARPd)

      * IP: TCP Explicit Congestion Notification support (CONFIG_INET_ECN) [N/y/?] n
        - NO: Though enabling this option would be great, there are many Internet
              sites out there that will block this.  Hit the "?" when configuring
              the kernel to learn more about it but it is recommended to say NO for
              now.

      * IP: TCP syncookie support (disabled per default) (CONFIG_SYN_COOKIES) [Y/n/?]
        - YES: Recommended : for basic TCP/IP network security

    [ Networking options --> IP: Netfilter Configuration ]

      * Connection tracking (required for masq/NAT) (CONFIG_IP_NF_CONNTRACK) [N/y/m/?] (NEW) m
        - YES: (Module) This enables the kernel to track various network connections.
               This option is required for Masquerading support as well as to enable
               Stateful tracking for various filewall mechanisms.  Please note that
               if you compile this directly into the kernel, you cannot enable
               the legacy IPCHAINS or IPFWADM compatibility modules.

      * FTP protocol support (CONFIG_IP_NF_FTP) [M/n/?] (NEW) m
        - YES: (Module) This enables the proper Masquerading of FTP connections if
               CONFIG_IP_NF_CONNTRACK was enabled above

      * IRC protocol support (CONFIG_IP_NF_IRC) [M/n/?] (NEW) m
        - YES: (Module) This enables the proper Masquerading of IRC connections if
               CONFIG_IP_NF_CONNTRACK was enabled above

      * Userspace queueing via NETLINK (EXPERIMENTAL) (CONFIG_IP_NF_QUEUE) [N/y/m/?] (NEW) m
        - OPTIONAL: Though this is OPTIONAL, this feature will allow IPTABLES to
                    copy specific packets to UserSpace tools for additional checks

      * IP tables support (required for filtering/masq/NAT) (CONFIG_IP_NF_IPTABLES) [N/y/m/?] (NEW) m
        - YES: (Module) Enables IPTABLES support

      * limit match support (CONFIG_IP_NF_MATCH_LIMIT) [N/y/m/?] (NEW) y
        - OPTIONAL:  (Module) Recommended : Though not required, this option can used to
                     enable rate limiting of both traffic and loggin messages help slow down denial
                     of service (DoS) attacks.

      * MAC address match support (CONFIG_IP_NF_MATCH_MAC) [N/y/m/?] (NEW) m
        - OPTIONAL:  Though not required, the option can allow you to
                     filter traffic based upon the SOURCE Ethernet MAC address.

      * netfilter MARK match support (CONFIG_IP_NF_MATCH_MARK) [N/y/m/?] (NEW) y
        - YES: (Module) Recommended : This enables IPTABLES to take action upon marked packets.
               This mechanism can allow for PORTFW functionality, TOS marking, etc.

      * Multiple port match support (CONFIG_IP_NF_MATCH_MULTIPORT) [N/y/m/?] (NEW) y
        - YES: (Module) Recommended : This enables IPTABLES to accept mutliple SRC/DST port
               ranges (non-contiguous) instead of one port range per IPTABLES
               statement.

      * TOS match support (CONFIG_IP_NF_MATCH_TOS) [Y/m/n/?] n
        - OPTIONAL:  This allows IPTABLES to match packets based upon their
                     DIFFSERV settings.

      * LENGTH match support (CONFIG_IP_NF_MATCH_LENGTH) [N/m/?] (NEW) n
        - OPTIONAL:  This allows IPTABLES to match packets based upon their
                     packet length.

      * TTL match support (CONFIG_IP_NF_MATCH_TTL) [N/m/?] (NEW) ? n
        - OPTIONAL:  This allows IPTABLES to match packets based upon their
                     TTL settings.

      * tcpmss match support (CONFIG_IP_NF_MATCH_TCPMSS) [N/y/m/?] m
        - OPTIONAL: (Module) Recommended :  This option allows users to examine the MSS value in
                     TCP SYN packets.  This is an advanced knob but can be very valuable in
                     troubleshooting MTU problems.

      * Connection state match support (CONFIG_IP_NF_MATCH_STATE) [M/n/?]  m
        - YES: (Module) Recommended : This option allows for Stateful tracking of network
                connections.

      * Unclean match support (EXPERIMENTAL) (CONFIG_IP_NF_MATCH_UNCLEAN) [N/y/m/?] y
        - YES: (Module) Recommended :  This option allows for connection tracking on odd packets.
               It cal also help in the detection of possibly malicious packets.
                This can be a valuable tool in tracking hostile people on the network.

      * Owner match support (EXPERIMENTAL) (CONFIG_IP_NF_MATCH_OWNER) [N/y/m/?] n
        - OPTIONAL:  This option allows IPTABLES to match traffic based upon the
                     user login, group, etc. who created the traffic.

      * Packet filtering (CONFIG_IP_NF_FILTER) [N/y/m/?] ? y
        - YES: (Module) This option allows for the kernel to be able filter traffic at
                the INPUT, FORWARDING, and OUTPUT traffic points.

        * REJECT target support (CONFIG_IP_NF_TARGET_REJECT) [N/y/m/?] (NEW) y
          - YES: (Module) With this option, a packet firewall can send an ICMP Reject packet
                back to the originator when a packet is blocked.

      * MIRROR target support (EXPERIMENTAL) (CONFIG_IP_NF_TARGET_MIRROR) [N/y/m/?] (NEW) n
        - OPTIONAL: This option allows the packet firewall to mirror the exact same
                    network packet back to the originator when it is supposed to be
                    blocked.  This is similar to the REJECT option above but it actually
                    sends the original packet back to the originator.  i.e. a
                    hostile user could actually portscan themselves.

      * Full NAT (CONFIG_IP_NF_NAT) [M/n/?] m
        - YES: (Module) This option enables the future menus to enable Masquerading,
               PORTFWing, Full (1:1) NAT, etc.

      * MASQUERADE target support (CONFIG_IP_NF_TARGET_MASQUERADE) [M/n/?] (NEW) m
        - YES: (Module) This option specifically enables Masquerade into the
               kernel

      * REDIRECT target support (CONFIG_IP_NF_TARGET_REDIRECT) [N/y/m/?] n
        - OPTIONAL: Not needed for normal MASQ functionality though people who
                    want to do transparent proxy via Squid will want this.

      * Basic SNMP-ALG support (EXPERIMENTAL) (CONFIG_IP_NF_NAT_SNMP_BASIC) [N/m/?] n
        - OPTIONAL: This enables IPTABLES to properly NAT internal SNMP packets so
                    that machines with duplicate addressing ranges can be properly
                    managed.

      * Packet mangling (CONFIG_IP_NF_MANGLE) [N/y/m/?] y
        - YES: (Module) This option allows for advanced IPTABLES packet manipulation
               options.

      * TOS target support (CONFIG_IP_NF_TARGET_TOS) [N/y/m/?] (NEW) n
        - OPTIONAL: Enables the kernel to modify the TOS field in a packet
               before routing it on

      * MARK target support (CONFIG_IP_NF_TARGET_MARK) [N/y/m/?] (NEW) m
        - OPTIONAL: (Module) Recommended : This enables the kernel to manipulate
                    packets based upon the MARK field.  This can be used for PORTFW
                    as well as many other things.

      * LOG target support (CONFIG_IP_NF_TARGET_LOG) [N/y/m/?]  m
        - YES: (Module)  This allows for the logging of packets before they are accepted,
               denied, rejected, etc.

      * TCPMSS target support (CONFIG_IP_NF_TARGET_TCPMSS) [N/y/m/?] ? m
        - YES: (Module) This option help some people with MTU problems.  Typically,
               most users have to set their Internet connection's MTU to
               1500 as well as ALL internal machines to 1500.  With this
               option, this whole MTU issue might be finally solved.

      * ipchains (2.2-style) support (CONFIG_IP_NF_COMPAT_IPCHAINS) [N/y/m/?] m
        - OPTIONAL: (Module) Recommended : If you have an existing IPCHAINS ruleset
               (2.2.x kernels) and enable this option, you can continue to use the
               IPCHAINS program and the majority of your old ruleset except for the
               use of any 2.2.x kernel-specific modules.  Please note that if this
               IPCHAINS module is loaded, ALL IPTABLES modules will be non-
               operational.  This is an either/or deal only intended for legacy
               rulesets.

      * ipfwadm (2.0-style) support (CONFIG_IP_NF_COMPAT_IPFWADM) [N/y/m/?] n
        - OPTIONAL: If you have an existing IPFWADM ruleset (2.0.x kernels) and
               enable this option, you can continue to use the IPFWADM program and
               the majority of your old ruleset except for the use of any 2.0.x
               kernel-specific modules.   Please note that if this IPFWADM module
               is loaded, ALL IPTABLES modules will be non operational.  This is
               an either/or deal only intended to support legacy rulesets.

        == Non-MASQ options skipped
        ==   (IPv6, khttpd, ATM, IPX, AppleTalk, etc.) --

      * Fast switching (read help!) (CONFIG_NET_FASTROUTE) [N/y/?] n
        - NO: This performance optimization is NOT compatible with IP MASQ and/or
              packet filtering

        == Non-MASQ options skipped
        == (QoS, Telephony, IDE, SCSI, 1394FW, I2O, etc)

          == Don't forget to compile in support for hardware that you might need:
          ==   IDE:    HDs, CDROMs, etc.
          ==   SCSI:   HDs, CDROMs, etc.

    [ Network device support ]

      * Network device support (CONFIG_NETDEVICES) [Y/n/?]
        - YES: Enables the Linux Network device sublayer

        == Non-MASQ options skipped
        ==   (Arcnet)

      * Dummy net driver support (CONFIG_DUMMY) [M/n/y/?]
        - YES:  Though OPTIONAL, this option can help when debugging problems

        == Non-MASQ options skipped
        == (EQL, etc..)

        == Don't forget to compile in support for hardware that you might need:
        ==   NICs:   eth, tr, etc.
        ==   MODEMs: ppp (ppp async) and/or slip
        ==   WANs:   T1, T3, ISDN, etc.
        ==   ISDN:   for internal ISDN modems

        == Non-MASQ options skipped
        ==   (Amateur Radio, IrDA, ISDN, USB, etc.)

    [ Character devices ]

        == Don't forget to compile in serial port support if you are a modem user
        == Don't forget to compile in mouse support

        == Non-MASQ options skipped
        ==   (I2C, Watchdog cards, Ftape, Video for Linux, etc. )

    [ File systems ]

        == Non-MASQ options skipped
        ==   (Quota, ISO9660, NTFS, etc )

      * /proc filesystem support (CONFIG_PROC_FS) [Y/n/?]
        - YES:  Required to dynamically configure the Linux forwarding
                and NATing systems

        == Non-MASQ options skipped
        ==   (Console drivers, Sound, USB, Kernel Hacking)

So go ahead and select "exit" and you should be prompted to save your config.

NOTE: These are just the kernel components you need for IP Masquerade networking support. You will need to select whatever other options needed for your specific setup. If you want more information on what each one of these kernel modules does, please see the FAQ section of this HOWTO for details.

- Now compile the kernel (make dep; make clean; make bzImage; make modules; make modules_install) , etc. Again, it is beyond the scope of this HOWTO if you have problems compiling your kernel. Please see Section 2.6 for URLs to the KERNEL howto, etc.

- You will then have move over the kernel binary, update your bootloader (LILO, Grub, etc.), and reboot. If you have questions about kernel compiling, I highly recommend to consult some of the URLs mentioned above in this section.

## 3.2.2. Compiling Linux 2.2.x Kernels

Please see Section 2.7 for any required software, patches, etc.

- First of all, you need the kernel source for 2.2.x (preferably the latest kernel version)

  - NOTE #1: --- UPDATE YOUR KERNEL --- Linux 2.2.x kernels less than version 2.2.20 contain several different security vulnerabilities (some were MASQ specific). Kernels less than 2.2.20 have a few local vulnerabilities. Kernel versions less than 2.2.16 have a TCP root exploit vulnerability and versions less than 2.2.11 have a IPCHAINS fragmentation bug. Because of these issues, users running a firewall with strong IPCHAINS rulesets are open to possible instrusion. Please upgrade your kernel to a fixed version.

  - NOTE #2: As the 2.2.x train progressed, the compile-time options keep on changing. As of this version, this section reflects the settings for a 2.2.20 kernel.

    If you are running either a newer or older kernel version, the dialogs will look different. It is recommended that you update to the newest kernel for added capability and stability of the system.

- If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered in several URLs found in Section 2.7. Please note that the instructions included here is just one way to do build a kernel. Please see the Kernel HOWTO for full details.

  NOTE: Please notice that it isn't recommended to put the new kernel sources into /usr/src/linux. You should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For more details on this topic, please read the "README" file in the top level directory of your kernel sources.

- For this HOWTO example, create a directory called `/usr/src/kernel`. Next, "cd" into this directory and download the newest 2.2.x kernel sources into it. Once downloaded, issue the following command (if the file ends in a .tar.gz): `tar xvzf linux-2.2.x.tar.gz` or (if the file ends in a .tar.bzip2): `tar xyvf linux-2.2.x.tar.bz2`. Please substitute the "x" in the 2.2.x filename with the Linux 2.2 kernel version you downloaded.

  NOTE: Some Linux distributions use the "I" option instead of the "y" option to decompress bzip2 archives.

  Once uncompressed, I recommend that you rename the directory from "linux" to "linux-2.2.x" for clarity. To do this, run the command `mv linux linux-2.2.x`. Next, make sure there is a directory or symbolic link pointing to `/usr/src/kernel/linux` ie. run the command: `ln -s /usr/src/kernel/linux-2.2.x /usr/src/kernel/linux`o again subsituting the "x" for your proper kernel version.

- Apply any appropriate or optional patches to the kernel source code. By default, stock Linux kernels do not require any specific patching in order for the system to work. Features like PPTP/IPSEC masqurading are already built-in in the newest kernels but other tools like Xwindows forwarders are optional. Please refer to Section 2.7 for URLs and the IP Masquerade Resources for up-to-date information and patch URLs.

- Now that the kernel is patched up (if required), here are the MINIMUM kernel configuration options required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just ONE way to compile a kernel. The main difference from this method vs. a different one is some people wish to compile things either as modules OR monolithically right into the kernel. Basically, compiling things as modules gives you added flexibility to what is or isn't installed into the kernel (reduces unneeded memory use and allow for drop-in upgrades [no need to reboot]) BUT they add more complexity to your configuration. On the flip side, compiling things directly into the kernel makes things simpler BUT you loose a level of flexibility. The following example is a mixture of both built-in AND modules.

  Side Note: It is assumed that you will also configure the kernel to use your other installed hardware such as network interfaces, optional SCSI controllers, etc. as well. Please refer to the Linux Kernel HOWTO and the kernel source's README file and Documentation/ directory for detailed help on compiling a kernel.

Please note the YES or NO ANSWERS to the following. Not all options will be available without the proper kernel patches described later in this HOWTO.

Run the following commands to configure your kernel:

- `cd /usr/src/kernel/linux`

- `make menuconfig`

The following kernel prompts reflect a 2.2.20 kernel:

    [ Code maturity level options ]

      * Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]
        - YES: though not entirely required for IP MASQ, this option allows the kernel
               to create possible additional MASQ modules such as PORTFW, etc.

      == Non-MASQ options skipped
      ==   (CPU, memory, MTRR, SMP, etc.)

    [ Loadable module support ]

      * Enable loadable module support (CONFIG_MODULES) [Y/n/?] y
        - YES: allows you to load kernel IP MASQ modules

      * Set version information on all symbols for modules (CONFIG_MODVERSIONS) [N/y/?] y
        - YES: allows newer kernels to load older modules if possible

      * Kernel module loader (CONFIG_KMOD) [Y/n/?] y
        - OPTIONAL: Recommended : allows the kernel to load various kernel modules as
             it needs them

    [ General setup ]

      * Networking support (CONFIG_NET) [Y/n/?]
        - YES: This enables the network subsystem

      == Non-MASQ options skipped
      ==   (PCI, kernel binaries, specific hardware options, etc.)

      * Sysctl support (CONFIG_SYSCTL) [Y/n/?]
        - YES:  Enables the ability to enable disable options such as forwarding,
          dynamic IPs, etc. via the /proc interface

    [ Block devices ]

      == Non-MASQ options skipped
      ==   (kernel binaries, power management, PnP, IDE, SCSI, etc.)

        == Don't forget to compile in support for hardware that you might need:
        ==   IDE controllers, HDs, CDROMs, etc.

    [ Networking options ]

      * Packet socket (CONFIG_PACKET) [Y/m/n/?] y
        - YES: Though this is OPTIONAL, this recommended feature will allow you
               to use TCPDUMP to debug any problems with IP MASQ

      * Kernel/User netlink socket (CONFIG_NETLINK) [Y/n/?] y
        - OPTIONAL: Recommended :  This feature will allow the logging of
               advanced firewall issues such as routing messages, etc

      * Routing messages (CONFIG_RTNETLINK) [Y/n/?] y
        - OPTIONAL: If you enabled the CONFIG_NETLINK option above, this option
               will send routing messages and other information to SYSLOG.

      * Netlink device emulation (CONFIG_NETLINK_DEV) [N/y/m/?] (NEW) n
        - NO:  This option does not have anything to do with packet firewall
               logging

      * Network firewalls (CONFIG_FIREWALL) [Y/n/?] y
        - YES: Enables the kernel to be comfigured by the IPCHAINS firewall tool

      * Socket Filtering (CONFIG_FILTER) [Y/n/?] y
        - OPTIONAL:  Though this doesn't have anything do with IPMASQ, if you
             plan on implimenting a DHCP server on the internal network, you
             WILL need this option.

      * Unix domain sockets (CONFIG_UNIX) [Y/m/n/?] y
        - YES:  This enables the UNIX TCP/IP sockets mechanisms

      * TCP/IP networking (CONFIG_INET) [Y/n/?] y
        - YES: Enables the TCP/IP protocol

      * IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?] y
        - OPTIONAL:  You can enable this if you want to be able to receive
                     Multicast traffic.  Please note that your ISP must
                     support Multicast as well for this all to work

      * IP: advanced router (CONFIG_IP_ADVANCED_ROUTER) [Y/n/?] n
        - OPTIONAL:  Though there is nothing in this section mandatory for
                     Masquerade, some specific options might be useful

      * IP: kernel level autoconfiguration (CONFIG_IP_PNP) [N/y/?] ?
        - NO:  Not needed for normal MASQ functionality

      * IP: firewalling (CONFIG_IP_FIREWALL) [Y/n/?] y
        - YES: This enables the kernel to support packet filtering, NAT, etc.

      * IP: firewall packet netlink device (CONFIG_IP_FIREWALL_NETLINK) [Y/n/?] n
        - OPTIONAL: Though this is OPTIONAL, this feature will allow IPCHAINS to
                    copy some packets to UserSpace tools for additional checks

      * IP: transparent proxy support (CONFIG_IP_TRANSPARENT_PROXY) [N/y/?] n
        - OPTIONAL:  Not needed for normal MASQ functionality though people who
               want to do transparent proxy via Squid will want this.  Please note
               that there is a PERFORMANCE PENALTY enabling this feature.

      * IP: masquerading (CONFIG_IP_MASQUERADE) [Y/n/?] y
        - YES: Enable IP Masquerade to re-address specific internal to external
               TCP/IP packets

      * IP: ICMP masquerading (CONFIG_IP_MASQUERADE_ICMP) [Y/n/?] y
        - YES: Enable support for masquerading ICMP ping packets (ICMP error
               codes will be MASQed regardless).  This is an important feature
               for troubleshooting connections.

      * IP: masquerading special modules support (CONFIG_IP_MASQUERADE_MOD) [Y/n/?] y
        - YES: Though OPTIONAL, this enables the option to later enable other
               modules like the PORTFW to give external computers a directly
               connection to specified internal MASQed machines.

      * IP: ipautofw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPAUTOFW) [N/y/m/?] n
        - NO:  NOT recommended : IPautofw is a legacy method of port forwarding.  It
               is mainly old code and has been found to have some issues.

      * IP: ipportfw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPPORTFW) [Y/m/n/?] y
        - OPTIONAL: Recommended : This enables PORTFW which allows external computers
               on the Internet to directly communicate to specified internal MASQed
               machines.  This feature is typically used to allow access to internal
               SMTP, TELNET, and WWW servers.  Please note that FTP port forwarding
               needs an additional patch, as described in the FAQ section of the MASQ
               HOWTO.  Please see the this FAQ section in the HOWTO for additional
               information.

      * IP: ip fwmark masq-forwarding support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_MFW) [Y/m/n/?] y
        - OPTIONAL:  This is a NEW method of performing PORTFW-like functionality which is
               similar to how the new 2.4.x kernels do things.  With this option, IPCHAINS
               can mark packets that should have additional work done upon it.  Using a
               UserSpace tool, much like IPMASQADM or IPPORFW, IPCHAINS would then
               do things like re-address the packets, change their TOS value, etc.
               Currently, this code is less tested than PORTFW but it looks promising.
               For now, this HOWTO recommends to use IPMASQADM and IPPORTFW.  If you
               have specific thoughts or comments on MFW, please email dranch.

      * IP: optimize as a router not host (CONFIG_IP_ROUTER) [Y/n/?] y
        - YES:  This optimizes the kernel for the network subsystem, though it
                isn't well known if this makes a siginificant performance difference
                or not.

      == Non-MASQ options skipped
      ==   ( autoconf, tunneling, GRE )

      * IP: multicast routing (CONFIG_IP_MROUTE) [N/y/?] n
        - OPTIONAL:  Though not needed for IPMASQ, enabling this feature will
                     let you route multicast traffic through your Linux box.
                     Please note that this requires that your ISP be multicast
                     enabled as well.

        == Non-MASQ options skipped
        ==  (Aliasing, ARPd)

      * IP: TCP syncookie support (disabled per default) (CONFIG_SYN_COOKIES) [Y/n/?]
        - YES: Recommended : for basic TCP/IP network security

      * IP: GRE tunnels over IP (CONFIG_NET_IPGRE) [N/y/m/?]
        - NO:   This OPTIONAL selection is to enable PPTP and GRE tunnels through
                the IP MASQ box

        == Non-MASQ options skipped
        ==   (aliasing, ARPd)

      * IP: TCP syncookie support (not enabled per default) (CONFIG_SYN_COOKIES) [Y/n/?]
        - YES: HIGHLY recommended for basic TCP/IP network security

        == Non-MASQ options skipped
        ==  (RARP)

      * IP: Allow large windows (not recommended if <16Mb of memory) * (CONFIG_SKB_LARGE) [Y/n/?]
        - YES:  This is recommended to optimize Linux's TCP window

        == Non-MASQ options skipped
        ==   (IPv6, IPX, WAN router, etc.)

      * Fast switching (read help!) (CONFIG_NET_FASTROUTE) [N/y/?] n
        - NO: This performance optimization is NOT compatible with IP MASQ and/or
              packet filtering

      == Non-MASQ options skipped
      == (Slow CPU, Telephony, SCSI, I2O, etc. )

        == Don't forget to compile in support for hardware that you might need:
        ==   SCSI:   HDs, CDROMs, etc.

    [ Network device support ]

      * Network device support (CONFIG_NETDEVICES) [Y/n/?]
        - YES: Enables the Linux Network device sublayer

      == Non-MASQ options skipped
      ==   (Arcnet)

      * Dummy net driver support (CONFIG_DUMMY) [M/n/y/?]
        - YES:  Though OPTIONAL, this option can help when debugging problems

      == Non-MASQ options skipped
      == (EQL, NICs, Wireless, IrDA, ISDN, etc..)

        == Don't forget to compile in support for hardware that you might need:
        ==   NICs:   eth, tr, etc.
        ==   MODEMs: ppp and/or slip
        ==   WANs:   T1, T3, ISDN, etc.
        ==   ISDN:   for internal ISDN modems

     [ Character devices ]

      == Don't forget to compile in serial port support for modem users
      == Don't forget to compile in mouse support

      == Non-MASQ options skipped
      ==   (I2C, Watchdog cards, Ftape, Video for Linux, USB, etc. )

    [ File systems ]

      == Non-MASQ options skipped
      ==   (Quota, ISO9660, NTFS, etc )

      * /proc filesystem support (CONFIG_PROC_FS) [Y/n/?]
        - YES:  Required to dynamically configure the Linux forwarding
                and NATing systems

      == Non-MASQ options skipped
      ==   (network fs, NLS, video section, sound, kernel hacking)

So go ahead and "exit" and you should be prompted to save your config.

NOTE: These are just the components you need for IP Masquerade. You will need to select whatever other options needed for your specific setup.

- Now compile the kernel (make dep; make clean; make bzImage; make modules; make modules_install) , etc. Again, it is beyond the scope of this HOWTO if you have problems compiling your kernel. Please see Section 2.7 for URLs to the KERNEL howto, etc.

- You will then have move over the kernel binary, update your bootloader (LILO, Grub, etc.), and reboot. If you have questions about kernel compiling, I highly recommend to consult some of the URLs above in this section.

## 3.2.3. Compiling Linux 2.0.x Kernels

Please see Section 2.8 for any required software, patches, etc.

- First of all, you need the kernel source for 2.0.x (preferably the latest kernel version)

  - As the 2.0.x train progress, the compile-time options keep on changing. As of this version, this section reflects the settings for a 2.0.39 kernel.

- If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered in several URLs found in Section 2.8. Please note that the instructions included here is just one way to do build a kernel. Please see the Kernel HOWTO for full details.

  NOTE: Please notice that it isn't recommended to put the new kernel sources into /usr/src/linux. You should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For more details on this topic, please read the "README" file in the top level directory of your kernel sources.

- For this HOWTO example, create a directory called `/usr/src/kernel`. Next, "cd" into this directory and download the newest 2.0.x kernel sources into it. Once downloaded, issue the following command: `tar xvzf linux-2.0.x.tar.gz` . Please substitute the "x" in the 2.0.x filename with the Linux 2.0 kernel version you downloaded.

  Once uncompressed, I recommend that you rename the directory from "linux" to "linux-2.0.x" for clarity. To do this, run the command `mv linux linux-2.0.x`. Next, make sure there is a directory or symbolic link pointing to `/usr/src/kernel/linux` ie. run the command: `ln -s /usr/src/kernel/linux-2.0.x /usr/src/kernel/linux`o again subsituting the "x" for your proper kernel version.

- Apply any appropriate or optional patches to the kernel source code. By default, stock Linux kernels do not require any specific patching in order for the system to work. Features like IPPORTFW, PPTP, and Xwindows forwarders are optional but very useful. Please refer to Section 2.8 for URLs and the IP Masquerade Resources for up-to-date information and patch URLs.

- Now that the kernel is patched up (if required), here are the MINIMUM kernel configuration options required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just ONE way to compile a kernel. The main difference from this method vs. a different one is some people wish to compile things either as modules OR monolithically right into the kernel. Basically, compiling things as modules gives you added flexibility to what is or isn't installed into the kernel (reduces unneeded memory use and allow for drop-in upgrades [no need to reboot]) BUT they add more complexity to your configuration. On the flip side, compiling things directly into the kernel makes things simpler BUT you loose a level of flexibility. The following example is a mixture of both built-in AND modules.

  Side Note: It is assumed that you will also configure the kernel to use your other installed hardware such as network interfaces, optional SCSI controllers, etc. as well. Please refer to the Linux Kernel HOWTO and the kernel source's "`README`" file and "`Documentation/`" directory for detailed help on compiling a kernel.

Please note the YES or NO ANSWERS to the following options. Not all options will be available without the proper kernel patches described later in this HOWTO:

Run the following commands to configure your kernel:

- `cd /usr/src/kernel/linux`

- `make menuconfig`

The following kernel prompts reflect a 2.0.39 kernel:

    [ Code maturity level options ]

      * Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]
        - YES: this will allow you to later select the IP Masquerade feature code

    [ Loadable module support ]

      * Enable loadable module support (CONFIG_MODULES) [Y/n/?] y
        - YES: allows you to load kernel IP MASQ modules

      * Set version information on all module symbols (CONFIG_MODVERSIONS) [N/y/?] y
        - YES: allows newer kernels to load older modules if possible

      * Kernel daemon support (e.g. autoload of modules) (CONFIG_KERNELD) [N/y/?] y
        - OPTIONAL: Recommended : allows the kernel to load various kernel modules as
             it needs them

    [ General setup ]

      == Non-MASQ options skipped
      ==   (FPU, memory)

      * Networking support (CONFIG_NET) [Y/n/?] y
        - YES: Enables the network subsystem

      == Non-MASQ options skipped
      ==   (memory, PCI, binary format, APM, etc.)

        == Don't forget to compile in support for hardware that you might need:
        ==   IDE controllers, HDs, CDROMs, etc.

    [ Networking options ]

      * Network firewalls (CONFIG_FIREWALL) [Y/n/?] y
        - YES: Enables the IPFWADM firewall tool

      == Non-MASQ options skipped
      ==   (Aliasing)

      * TCP/IP networking (CONFIG_INET) [Y/n/?] y
        - YES: Enables the TCP/IP protocol

      * IP: forwarding/gatewaying (CONFIG_IP_FORWARD) [N/y/?] y
        - YES: Enables Linux network packet forwarding and routing
               - Controlled by IPFWADM

      * IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?] y
        - OPTIONAL:  You can enable this if you want to be able to receive
                     Multicast traffic.  Please note that your ISP must
                     support Multicast as well for this all to work

      * IP: syn cookies (CONFIG_SYN_COOKIES) [Y/n/?] y
        - YES: HIGHLY recommended for basic network security

      * IP: firewalling (CONFIG_IP_FIREWALL) [Y/n/?] y
        - YES: Enable the packet firewall features

      * IP: firewall packet logging (CONFIG_IP_FIREWALL_VERBOSE) [Y/n/?] y
        - YES: Allows the kernel to report back on various packets traversing
               the firewall.

      * IP: masquerading (CONFIG_IP_MASQUERADE [Y/n/?] y
        - YES: Enable the kernel to perform IP MASQ NAT functionality

      * IP: ipautofw masquerade support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPAUTOFW) [Y/n/?] n
        - NO:  NOT Recommended : IPautofw is a legacy method of TCP/IP port forwarding.
               Though IPautofw works, IPPORTFW is a better choice.

      * IP: ipportfw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPPORTFW) [Y/n/?] y
        - YES: This option is ONLY AVAILABLE VIA A PATCH for the 2.0.x kernels.
               With this option, external computers on the Internet can directly
               communicate to specified internal MASQed machines.  This feature is
               typically used to access internal SMTP, TELNET, and WWW servers.
               FTP port forwarding sometimes might require an additional patch as
               described in the FAQ section.  Additional information on port
               forwarding is available in the Forwards section of this HOWTO.

      * IP: MS PPTP masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_PPTP) [N/y/?] (NEW) n
        - OPTIONAL: Enabling this feature will allow internal MASQ clients to
              properly connect to PPTP servers on the Internet.

      * IP: MS PPTP Call ID masq support (CONFIG_IP_MASQUERADE_PPTP_MULTICLIENT) [N/y/?] (NEW) n
        - OPTIONAL:  If you enabled the CONFIG_IP_MASQUERADE_PPTP above, this
              option will allow for multiple internal PPTP clients behind the MASQ
              server to communicate to the same PPTP server.

      * IP: MS PPTP masq debugging (DEBUG_IP_MASQUERADE_PPTP) [N/y/?] n
        - OPTIONAL:  NOT recommended : This is not required for IP MASQ or MASQing PPTP
               connections unless you need additional troubleshooting help.  If enabled,
               this can fill up your logs quickly.

      * IP: MS PPTP masq verbose debugging (DEBUG_IP_MASQUERADE_PPTP_VERBOSE) [N/y/?] (NEW) n
        - OPTIONAL: NOT Recommended : If you enabled the DEBUG_IP_MASQUERADE_PPTP
               option above, this will make the logging even more verbose.

      * IP: IPSEC ESP & ISAKMP masq support (EXPERIMENTAL) * (CONFIG_IP_MASQUERADE_IPSEC) [N/y/?] m
        - OPTIONAL: This option allows for some forms of IPSEC tunnels to be
               masquraded

      * IP: IPSEC masq table lifetime (minutes) (CONFIG_IP_MASQUERADE_IPSEC_EXPIRE) * [30] (NEW)
        - OPTIONAL: This feature allows to change the MASQ table timeouts so that
          idle IPSEC tunnels won't be prematurely disconnected.

      * IP: Disable inbound ESP destination guessing * (CONFIG_IP_MASQUERADE_IPSEC_NOGUESS) [N/y/?] n
        - OPTIONAL: This feature allows the kernel to guess where the fully encrypted IPSEC VPN
               might be going and add it to the MASQ table.

      * IP: IPSEC masq debugging (DEBUG_IP_MASQUERADE_IPSEC) [N/y/?] ? n
        - OPTIONAL:  NOT recommended : This is not required for IP MASQ or MASQing IPSEC
               connections unless you need additional troubleshooting help.  If enabled,
               this can fill up your logs quickly.

      * IP: IPSEC masq verbose debugging (DEBUG_IP_MASQUERADE_IPSEC_VERBOSE) [N/y/?] (NEW) n
        - OPTIONAL: NOT Recommended : If you enabled the DEBUG_IP_MASQUERADE_IPSEC
               option above, this will make the logging even more verbose.

      * IP: ICMP masquerading (CONFIG_IP_MASQUERADE_ICMP) [Y/n/?]
        - YES: Enable support for masquerading ICMP packets. Though thought of as
               optional, many programs will NOT function properly with out ICMP
               support.

      * IP: transparent proxy support (EXPERIMENTAL) (CONFIG_IP_TRANSPARENT_PROXY) [N/y/?] n
        - OPTIONAL:  Not needed for normal MASQ functionality though people who
               want to do transparent proxy via Squid will want this.  Please note
               that there is a PERFORMANCE PENALTY enabling this feature.

      * IP: loose UDP port managing (EXPERIMENTAL) (CONFIG_IP_MASQ_LOOSE_UDP) [Y/n/?]
        - YES: This option is ONLY AVAILABLE VIA A PATCH for the 2.0.x kernels.
               With this option, internally masqueraded computers can play
               NAT-friendly games over the Internet.  Explicit details are given
               in the FAQ section of this HOWTO.

      * IP: always defragment (CONFIG_IP_ALWAYS_DEFRAG) [Y/n/?]
        - YES:  This feature optimizes IP MASQ connections

      == Non-MASQ options skipped
      ==   (Accounting)

      * IP: optimize as router not host (CONFIG_IP_ROUTER) [Y/n/?]
        - YES:  This optimizes the kernel for the network subsystem

      == Non-MASQ options skipped
      ==   (Tunneling, Mcast routing, RARP, PMTU, etc.)

      * IP: Drop source routed frames (CONFIG_IP_NOSR) [Y/n/?]
        - YES: HIGHLY recommended for basic network security

      == Non-MASQ options skipped
      ==   (IPX, Bridging, SCSI, etc.)

        == Don't forget to compile in support for hardware that you might need:
        ==   SCSI controllers, HDs, CDROMs, etc.

    [ Network device support ]

      * Network device support (CONFIG_NETDEVICES) [Y/n/?]
        - YES: Enables the Linux Network device sublayer

      == Non-MASQ options skipped
      ==   (Dummy, EQL, PPP, SLIP, NICs, Wireless, etc.)

        == Don't forget to compile in support for hardware that you might need:
        ==   NICs:   eth, tr, etc.
        ==   MODEMs: ppp and/or slip
        ==   WANs:   T1, T3, ISDN, etc.
        ==   ISDN:   for internal ISDN modems

    [ File systems ]

      == Non-MASQ options skipped
      ==   (Quota, ISO9660, Codepages, NTFS, etc )

      * /proc filesystem support (CONFIG_PROC_FS) [Y/n/?]
        - YES:  Required to dynamically configure the Linux forwarding
                and NATing systems

     [ Character devices ]

      == Non-MASQ options skipped
      ==   (multi-port serial, parallel, mice, Ftape, Sound, etc. )

        == Don't forget to compile in serial port support for modem users
        == Don't forget to compile in mouse support

So go ahead and "exit" and you should be prompted to save your config.

NOTE: These are only components for IP Masquerade functionality. You may need to also select additional options to match your specific network and hardware setup.

- Now compile the kernel (make dep; make clean; make bzImage; make modules; make modules_install) , etc. Again, it is beyond the scope of this HOWTO if you have problems compiling your kernel. Please see Section 2.8 for URLs to the KERNEL howto, etc.

- You will then have move over the kernel binary, update your bootloader (LILO, Grub, etc.), and reboot. If you have questions about kernel compiling, I highly recommend to consult some of the URLs above in this section.

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