Keywords: Linux, IPsec, VPN, security, FreeSWAN, kernel

# Kernel configuration for FreeS/WAN

This section lists many of the options available when configuring a Linux kernel, and explains how they should be set on a FreeS/WAN IPsec gateway.

## Not everyone needs to worry about kernel configuration

Note that in many cases you do not need to mess with these.

You may have a Linux distribution which comes with FreeS/WAN installed (see this list). In that case, you need not do a FreeS/WAN installation or a kernel configuration. Of course, you might still want to configure and rebuild your kernel to improve performance or security. This can be done with standard tools described in the Kernel HowTo.

If you need to install FreeS/WAN, then you do need to configure a kernel. However, you may choose to do that using the simplest procedure:

- Configure, build and test a kernel for your system before adding FreeS/WAN. See the Kernel HowTo for details. This step cannot be skipped. FreeS/WAN needs the results of your configuration.
- Then use FreeS/WAN's make menugo command, and simply save and exit. This sets everything FreeS/WAN needs and retains your values everywhere else.

This document is for those who choose to configure their FreeS/WAN kernel themselves.

## Assumptions and notation

Help text for most kernel options is included with the kernel files, and is accessible from within the configuration utilities. We assume you will refer to that, and to the Kernel HowTo, as necessary. This document covers only the FreeS/WAN-specific aspects of the problem.

To avoid duplication, this document section does not cover settings for the additional IPsec-related kernel options which become available after you have patched your kernel with FreeS/WAN patches. There is help text for those available from within the configuration utility.

We assume a common configuration in which the FreeS/WAN IPsec gateway is also doing ipchains(8) firewalling for a local network, and possibly masquerading as well.

Some suggestions below are labelled as appropriate for "a true paranoid". By this we mean they may cause inconvenience and it is not entirely clear they are necessary, but they appear to be the safest choice. Not using them might entail some risk. Of course one suggested mantra for security administrators is: "I know I'm paranoid. I wonder if I'm paranoid enough."

### Labels used

Six labels are used to indicate how options should be set. We mark the labels with [square brackets]. For two of these labels, you have no choice:

[required]
  essential for FreeS/WAN operation.
[incompatible]
  incompatible with FreeS/WAN.

those must be set correctly or FreeS/WAN will not work

FreeS/WAN should work with any settings of the others, though of course not all combinations have been tested. We do label these in various ways, but these labels are only suggestions.

[recommended]
  useful on most FreeS/WAN gateways
[disable]
  an unwelcome complication on a FreeS/WAN gateway.
[optional]
  Your choice. We outline issues you might consider.
[anything]
  This option has no direct effect on FreeS/WAN and related tools, so you should be able to set it as you please.

Of course complexity is an enemy in any effort to build secure systems. For maximum security, any feature that can reasonably be turned off should be. "If in doubt, leave it out."

## Kernel options for FreeS/WAN

Indentation is based on the nesting shown by 'make menuconfig' with a 2.2.16 kernel for the i386 architecture.

Code maturity and level options
  Prompt for development ... code/drivers
    [optional] If this is no, experimental drivers are not shown in later menus.

    For most FreeS/WAN work, no is the preferred setting. Using new or untested components is too risky for a security gateway.

    However, for some hardware (such as the author's network cards) the only drivers available are marked new/experimental. In such cases, you must enable this option or your cards will not appear under "network device support". A true paranoid would leave this option off and replace the cards.

  Processor type and features
    [anything]
  Loadable module support
    Enable loadable module support
      [optional] A true paranoid would disable this. An attacker who has root access to your machine can fairly easily install a bogus module that does awful things, provided modules are enabled. A common tool for attackers is a "rootkit", a set of tools the attacker uses once he or she has become root on your system. The kit introduces assorted additional compromises so that the attacker will continue to "own" your system despite most things you might do to recovery the situation. For Linux, there is a tool called knark which is basically a rootkit packaged as a kernel module.

      With modules disabled, an attacker cannot install a bogus module. The only way he can achieve the same effects is to install a new kernel and reboot. This is considerably more likely to be noticed.

      Many FreeS/WAN gateways run with modules enabled. This simplifies some administrative tasks and some ipchains features are available only as modules. Once an enemy has root on your machine your security is nil, so arguably defenses which come into play only in that situation are pointless.

    Set version information ....
      [optional] This provides a check to prevent loading modules compiled for a different kernel.
    Kernel module loader
      [disable] It gives little benefit on a typical FreeS/WAN gate and entails some risk.
  General setup
    We list here only the options that matter for FreeS/WAN.
    Networking support
      [required]
    Sysctl interface
      [optional] If this option is turned on and the /proc filesystem installed, then you can control various system behaviours by writing to files under /proc/sys. For example:

                  echo 1 > /proc/sys/net/ipv4/ipforward

      turns IP forwarding on.

      Disabling this option breaks many firewall scripts. A true paranoid would disable it anyway since it might conceivably be of use to an attacker.

  Plug and Play support
    [anything]
  Block devices
    [anything]
  Networking options
    Packet socket
      [optional] This kernel feature supports tools such as tcpdump(8) which communicate directly with network hardware, bypassing kernel protocols. This is very much a two-edged sword:
      - such tools can be very useful to the firewall admin, especially during initial testing
      - should an evildoer breach your firewall, such tools could give him or her a great deal of information about the rest of your network
      We recommend disabling this option on production gateways.
    Kernel/User netlink socket
      [optional] Required if you want to use advanced router features.
    Routing messages
      [optional]
    Netlink device emulation
      [optional]
    Network firewalls
      [recommended] You need this if the IPsec gateway also functions as a firewall.

      Even if the IPsec gateway is not your primary firewall, we suggest setting this so that you can protect the gateway with at least basic local packet filters.

    Socket filtering
      [disable] This enables an older filtering interface. We suggest using ipchains(8) instead. To do that, set the "Network firewalls" option just above, and not this one.
    Unix domain sockets
      [required] These sockets are used for communication between the ipsec(8) commands and the ipsec_pluto(8) daemon.
    TCP/IP networking
      [required]
      IP: multicasting
        [anything]
      IP: advanced router
        [optional] This gives you policy routing, which some people have used to good advantage in their scripts for FreeS/WAN gateway management. It is not used in our distributed scripts, so not required unless you want it for custom scripts. It requires the netlink interface between kernel code and the iproute2(8) command.
      IP: kernel level autoconfiguration
        [disable] It gives little benefit on a typical FreeS/WAN gate and entails some risk.
      IP: firewall packet netlink device
        [disable]
      IP: transparent proxy support
        [optional] This is required in some firewall configurations, but should be disabled unless you have a definite need for it.
      IP: masquerading
        [optional] Required if you want to use non-routable private IP addresses for your local network.
      IP: Optimize as router not host
        [recommended]
      IP: tunneling
        [required]
      IP: GRE tunnels over IP
        [anything]
      IP: aliasing support
        [anything]
      IP: ARP daemon support (EXPERIMENTAL)
        Not required on most systems, but might prove useful on heavily-loaded gateways.
      IP: TCP syncookie support
        [recommended] It provides a defense against a denial of service attack which uses bogus TCP connection requests to waste resources on the victim machine.
      IP: Reverse ARP
      IP: large window support
        [recommended] unless you have less than 16 meg RAM
    IPv6
      [optional] FreeS/WAN does not currently support IPv6, though work on integrating FreeS/WAN with the Linux IPv6 stack has begun. Details.

      It should be possible to use IPv4 FreeS/WAN on a machine which also does IPv6. This combination is not yet well tested. We would be quite interested in hearing results from anyone expermenting with it, via the mailing list.

      We do not recommend using IPv6 on production FreeS/WAN gateways until more testing has been done.

    Novell IPX
      [disable]
    Appletalk
      [disable] Quite a few Linux installations use IP but also have some other protocol, such as Appletalk or IPX, for communication with local desktop machines. In theory it should be possible to configure IPsec for the IP side of things without interfering with the second protocol.

      We do not recommend this. Keep the software on your gateway as simple as possible. If you need a Linux-based Appletalk or IPX server, use a separate machine.

  Telephony support
    [anything]
  SCSI support
    [anything]
  I2O device support
    [anything]
  Network device support
    [anything] should work, but there are some points to note.

    The development team test almost entirely on 10 or 100 megabit Ethernet and modems. In principle, any device that can do IP should be just fine for IPsec, but in the real world any device that has not been well-tested is somewhat risky. By all means try it, but don't bet your project on it until you have solid test results.

    If you disabled experimental drivers in the Code maturity section above, then those drivers will not be shown here. Check that option before going off to hunt for missing drivers.

    If you want Linux to automatically find more than one ethernet interface at boot time, you need to:

    - compile the appropriate driver(s) into your kernel. Modules will not work for this
    - add a line such as

             append="ether=0,0,eth0 ether=0,0,eth1"

      to your /etc/lilo.conf file. In some cases you may need to specify parameters such as IRQ or base address. The example uses "0,0" for these, which tells the system to search. If the search does not succeed on your hardware, then you should retry with explicit parameters. See the lilo.conf(5) man page for details.
    - run lilo(8)
    Having Linux find the cards this way is not necessary, but is usually more convenient than loading modules in your boot scripts.
  Amateur radio support
    [anything]
  IrDA (infrared) support
    [anything]
  ISDN subsystem
    [anything]
  Old CDROM drivers
    [anything]
  Character devices
    The only required character device is:
    random(4)
      [required] This is a source of random numbers which are required for many cryptographic protocols, including several used in IPsec.

      If you are comfortable with C source code, it is likely a good idea to go in and adjust the #define lines in /usr/src/linux/drivers/char/random.c to ensure that all sources of randomness are enabled. Relying solely on keyboard and mouse randomness is dubious procedure for a gateway machine. You could also increase the randomness pool size from the default 512 bytes (128 32-bit words).

  Filesystems
    [anything] should work, but we suggest limiting a gateway machine to the standard Linux ext2 filesystem in most cases.
  Network filesystems
    [disable] These systems are an unnecessary risk on an IPsec gateway.
  Console drivers
    [anything]
  Sound
    [anything] should work, but we suggest enabling sound only if you plan to use audible alarms for firewall problems.
  Kernel hacking
    [disable] This might be enabled on test machines, but should not be on production gateways.