<html>
<head>
<title>Kernel configuration for FreeS/WAN</title>
<meta name="keywords" content="Linux, IPsec, VPN, security, FreeSWAN, kernel">
<!--
Written by Sandy Harris for the Linux FreeS/WAN project
Freely distributable under the GNU General Public License
More information at www.freeswan.org
Feedback to users@lists.freeswan.org
CVS information:
RCS ID: $Id: kernel.html,v 1.14 2001/12/29 17:06:09 sandy Exp $
Last changed: $Date: 2001/12/29 17:06:09 $
Revision number: $Revision: 1.14 $
CVS revision numbers do not correspond to FreeS/WAN release numbers.
-->
</head>
<body>
<h1><a name="kernelconfig">Kernel configuration for FreeS/WAN</a></h1>
<p>
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.</p>
<h2><a name="notall">Not everyone needs to worry about kernel configuration</a></h2>
<p>Note that in many cases you do not need to mess with these.</p>
<p>
You may have a Linux distribution which comes with FreeS/WAN installed
(see this <a href="intro.html#products">list</a>).
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 <a href="http://www.linuxdoc.org/HOWTO/Kernel-HOWTO.html">Kernel HowTo</a>.</p>
<p>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:</p>
<ul>
<li>Configure, build and test a kernel for your system before adding FreeS/WAN. See the <a
href="http://www.linuxdoc.org/HOWTO/Kernel-HOWTO.html">Kernel HowTo</a> for details. <strong>This step cannot be
skipped</strong>. FreeS/WAN needs the results of your configuration.</li>
<li>Then use FreeS/WAN's <var>make oldgo</var> command. This sets
everything FreeS/WAN needs and retains your values everywhere else.</li>
</ul>
<p>
This document is for those who choose to configure their FreeS/WAN kernel
themselves.</p>
<h2><a name="assume">Assumptions and notation</a></h2>
<p>
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 <a href="http://www.linuxdoc.org/HOWTO/Kernel-HOWTO.html">Kernel HowTo</a>, as
necessary. This document covers only the FreeS/WAN-specific aspects of the
problem.</p>
<p>
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.</p>
<p>
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.</p>
<p>
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."</p>
<h3><a name="labels">Labels used</a></h3>
<p>
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:</p>
<dl>
<dt>[required]</dt>
<dd>essential for FreeS/WAN operation.</dd>
<dt>[incompatible]</dt>
<dd>incompatible with FreeS/WAN.</dd>
</dl>
<p>those must be set correctly or FreeS/WAN will not work</p>
<p>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 <em>these labels are only suggestions</em>.</p>
<dl>
<dt>[recommended]</dt>
<dd>useful on most FreeS/WAN gateways</dd>
<dt>[disable]</dt>
<dd>an unwelcome complication on a FreeS/WAN gateway.</dd>
<dt>[optional]</dt>
<dd>Your choice. We outline issues you might consider.</dd>
<dt>[anything]</dt>
<dd>This option has no direct effect on FreeS/WAN and related tools, so
you should be able to set it as you please.</dd>
</dl>
<p>
Of course complexity is an enemy in any effort to build secure systems.
<strong>For maximum security, any feature that can reasonably be turned off
should be</strong>. "If in doubt, leave it out."</p>
<h2><a name="kernelopt">Kernel options for FreeS/WAN</a></h2>
<p>
Indentation is based on the nesting shown by 'make menuconfig' with a
2.2.16 kernel for the i386 architecture.</p>
<dl>
<dt><a name="maturity">Code maturity and level options</a></dt>
<dd>
<dl>
<dt><a name="devel">Prompt for development ...
code/drivers</a></dt>
<dd>[optional] If this is <var>no</var>, experimental drivers are
not shown in later menus.
<p>For most FreeS/WAN work, <var>no</var> is the preferred
setting. Using new or untested components is too risky for a
security gateway.</p>
<p>However, for some hardware (such as the author's network
cards) the only drivers available are marked
<var>new/experimental</var>. 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.</p>
</dd>
<dt>Processor type and features</dt>
<dd>[anything]</dd>
<dt>Loadable module support</dt>
<dd>
<dl>
<dt>Enable loadable module support</dt>
<dd>[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
<a href="http://www.sans.org/newlook/resources/IDFAQ/knark.htm">knark</a>
which is basically a rootkit packaged as a kernel module.
<p>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.
<p>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.</p>
<p>
</dd>
<dt>Set version information ....</dt>
<dd>[optional] This provides a check to prevent loading modules
compiled for a different kernel.</dd>
<dt>Kernel module loader</dt>
<dd>[disable] It gives little benefit on a typical FreeS/WAN gate
and entails some risk.</dd>
</dl>
</dd>
<dt>General setup</dt>
<dd>We list here only the options that matter for FreeS/WAN.
<dl>
<dt>Networking support</dt>
<dd>[required]</dd>
<dt>Sysctl interface</dt>
<dd>[optional] If this option is turned on and the
<var>/proc</var> filesystem installed, then you can control
various system behaviours by writing to files under
<var>/proc/sys</var>. For example:
<pre> echo 1 > /proc/sys/net/ipv4/ipforward</pre>
turns IP forwarding on.
<p>Disabling this option breaks many firewall scripts. A true
paranoid would disable it anyway since it might conceivably be
of use to an attacker.</p>
</dd>
</dl>
</dd>
<dt>Plug and Play support</dt>
<dd>[anything]</dd>
<dt>Block devices</dt>
<dd>[anything]</dd>
<dt>Networking options</dt>
<dd>
<dl>
<dt>Packet socket</dt>
<dd>[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:
<ul>
<li>such tools can be very useful to the firewall admin,
especially during initial testing</li>
<li>should an evildoer breach your firewall, such tools could
give him or her a great deal of information about the rest
of your network</li>
</ul>
We recommend disabling this option on production gateways.</dd>
<dt><a name="netlink">Kernel/User netlink socket</a></dt>
<dd>[optional] Required if you want to use <a href="#adv">advanced
router</a> features.</dd>
<dt>Routing messages</dt>
<dd>[optional]</dd>
<dt>Netlink device emulation</dt>
<dd>[optional]</dd>
<dt>Network firewalls</dt>
<dd>[recommended] You need this if the IPsec gateway also
functions as a firewall.
<p>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.</p>
</dd>
<dt>Socket filtering</dt>
<dd>[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.</dd>
<dt>Unix domain sockets</dt>
<dd>[required] These sockets are used for communication between the
<a href="manpage.d/ipsec.8.html">ipsec(8)</a>
commands and the <a href="manpage.d/ipsec_pluto.8.html">ipsec_pluto(8)</a>
daemon.</dd>
<dt>TCP/IP networking</dt>
<dd>[required]
<dl>
<dt>IP: multicasting</dt>
<dd>[anything]</dd>
<dt><a name="adv">IP: advanced router</a></dt>
<dd>[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 <a
href="#netlink">netlink</a> interface between kernel code
and the iproute2(8) command.</dd>
<dt>IP: kernel level autoconfiguration</dt>
<dd>[disable] It gives little benefit on a typical FreeS/WAN
gate and entails some risk.</dd>
<dt>IP: firewall packet netlink device</dt>
<dd>[disable]</dd>
<dt>IP: transparent proxy support</dt>
<dd>[optional] This is required in some firewall configurations,
but should be disabled unless you have a definite need for it.
</dd>
<dt>IP: masquerading</dt>
<dd>[optional] Required if you want to use
<a href="glossary.html#non-routable">non-routable</a> private
IP addresses for your local network.</dd>
<dt>IP: Optimize as router not host</dt>
<dd>[recommended]</dd>
<dt>IP: tunneling</dt>
<dd>[required]</dd>
<dt>IP: GRE tunnels over IP</dt>
<dd>[anything]</dd>
<dt>IP: aliasing support</dt>
<dd>[anything]</dd>
<dt>IP: ARP daemon support (EXPERIMENTAL)</dt>
<dd>Not required on most systems, but might prove useful on
heavily-loaded gateways.</dd>
<dt>IP: TCP syncookie support</dt>
<dd>[recommended] It provides a defense against a <a
href="glossary.html#DOS">denial of
service attack</a> which uses bogus TCP connection
requests to waste resources on the victim machine.</dd>
<dt>IP: Reverse ARP</dt>
<dd></dd>
<dt>IP: large window support</dt>
<dd>[recommended] unless you have less than 16 meg RAM</dd>
</dl>
</dd>
<dt>IPv6</dt>
<dd>[optional] FreeS/WAN does not currently support IPv6, though work on
integrating FreeS/WAN with the Linux IPv6 stack has begun.
<a href="compat.html#ipv6">Details</a>.
<p>
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
<a href="mail.html">mailing list</a>.
<p>
We do not recommend using IPv6 on production FreeS/WAN gateways until
more testing has been done.
</dd>
<dt>Novell IPX</dt>
<dd>[disable]</dd>
<dt>Appletalk</dt>
<dd>[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.
<p>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.</p>
</dd>
</dl>
</dd>
<dt>Telephony support</dt>
<dd>[anything]</dd>
<dt>SCSI support</dt>
<dd>[anything]</dd>
<dt>I2O device support</dt>
<dd>[anything]</dd>
<dt>Network device support</dt>
<dd>[anything] should work, but there are some points to note.
<p>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.</p>
<p>If you disabled experimental drivers in the <a
href="#maturity">Code maturity</a> section above, then those drivers
will not be shown here. Check that option before going off to hunt for
missing drivers.</p>
<p>If you want Linux to automatically find more than one ethernet
interface at boot time, you need to:</p>
<ul>
<li>compile the appropriate driver(s) into your kernel. Modules will
not work for this</li>
<li>add a line such as
<pre>
append="ether=0,0,eth0 ether=0,0,eth1"
</pre>
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.</li>
<li>run lilo(8)</li>
</ul>
Having Linux find the cards this way is not necessary, but is usually more
convenient than loading modules in your boot scripts.</dd>
<dt>Amateur radio support</dt>
<dd>[anything]</dd>
<dt>IrDA (infrared) support</dt>
<dd>[anything]</dd>
<dt>ISDN subsystem</dt>
<dd>[anything]</dd>
<dt>Old CDROM drivers</dt>
<dd>[anything]</dd>
<dt>Character devices</dt>
<dd>The only required character device is:
<dl>
<dt>random(4)</dt>
<dd>[required] This is a source of <a href="glossary.html#random">random</a>
numbers which are required for many cryptographic protocols,
including several used in IPsec.
<p>If you are comfortable with C source code, it is likely a
good idea to go in and adjust the <var>#define</var> lines in
<var>/usr/src/linux/drivers/char/random.c</var> 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).</p>
</dd>
</dl>
<dt>Filesystems</dt>
<dd>[anything] should work, but we suggest limiting a gateway
machine to the standard Linux ext2 filesystem in most
cases.</dd>
<dt>Network filesystems</dt>
<dd>[disable] These systems are an unnecessary risk on an IPsec
gateway.</dd>
<dt>Console drivers</dt>
<dd>[anything]</dd>
<dt>Sound</dt>
<dd>[anything] should work, but we suggest enabling sound only if
you plan to use audible alarms for firewall problems.</dd>
<dt>Kernel hacking</dt>
<dd>[disable] This might be enabled on test machines, but should
not be on production gateways.</dd>
</dl>
</dl>
</body>
</html>
distrib
>
Mandriva
>
2010.0
>
i586
>
media
>
contrib-release
>
by-pkgid
>
cb04c52ccedb52ab907eaca84d718eba
>
files
>
98
