This is a description of the kernel configuration options that relate to security, and an explanation of what they do, and how to use them.
As the kernel controls your computer's networking, it is important that it be very secure, and not be compromised. To prevent some of the latest networking attacks, you should try to keep your kernel version current. You can find new kernels at ftp://ftp.kernel.org.
There is also a international group providing a single unified crypto patch to the mainstream linux kernel. This patch provides support for a number of cyrptographic subsystems and things that cannot be included in the mainstream kernel due to export restrictions. For more information, visit their web page at: http://www.kerneli.org
For 2.0.x kernels, the following options apply. You should see these when you are doing a 'make config' , a 'make menuconfig' or a 'make xconfig'.
This option should be enabled. Source routed frames contain the entire path to their destination inside of the packet. This means that routers through which the packet goes do not need to inspect it, and just forward it on. This could lead to data entering your system that may be a potential exploit.
This option is necessary if you are going to configure your machine as a firewall, do masquerading, or wish to protect your dial-up workstation from someone entering via your PPP dial-up interface.
If you enable IP forwarding, your Linux box essentially becomes a router. If your machine is on a network, you could be forwarding data from one network to another, and perhaps subverting a firewall that was put there to prevent this from happening. Normal dial-up users will want to disable this, and other users should concentrate on the security implications of doing this. Firewall machines will want this enabled, and used in conjunction with firewall software.
You can enable and disable IP forwarding dynamically using the following command:
root# echo 1 > /proc/sys/net/ipv4/ip_forward
root# echo 0 > /proc/sys/net/ipv4/ip_forward
/proc) will always appear to be
zero length, but in fact aren't. This is a newly-introduced kernel
feature, so be sure you're using kernel 2.0.33 or later.
This option gives you information about packets your firewall received, like sender, recipient, port, etc.
Generally this option is disabled, but if you are building a firewall or a masquerading host, you will want to enable it. When data is sent from one host to another, it does not always get sent as a single packet of data, but rather it is fragmented into several pieces. The problem with this is that the port numbers are only stored in the first fragment. This means that someone can insert information into the remaining packets that isn't supposed to be there. It could also prevent a teardrop attack against an internal host that is not yet itself patched against it.
a "SYN Attack" is a denial of service (DoS) attack that consumes all the resources on your machine, forcing you to reboot. We can't think of a reason you wouldn't normally enable this. In the 2.1 kernel series this config option mearly allows syn cookies, but does not enable them. To enable them, you have to do:
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
This is an option that is available in the 2.1 kernel series that will sign NCP packets for stronger security. Normally you can leave it off, but it is there if you do need it.
This is a really neat option that allows you to analyze the first 128 bytes of the packets in a user-space program, to determine if you would like to accept or deny the packet, based on its validity.
For 2.2.x kernels, many of the options are the same, but a few new ones have poped up.
This option should be on if you intend to run any firewalling or masquerading on your linux machine. If it's just going to be a regular client machine, it's safe to say no.
For most people, it's safe to say no to this option. This option allows you to connect a userspace filter to any socket and determine if packets should be allowed or denied. Unless you have a very specific need and are capable of programming such a filter, you should say no. Also note that as of this writing, all protocols were supported except TCP.
There are a few block and character devices available on Linux that will also help you with security.
The two devices /dev/random and /dev/urandom are provided by the
kernel to provide random data at any time.
Both /dev/random and /dev/urandom should be secure enough to use in
generating PGP keys, ssh challenges, and other applications where
secure random numbers are requisite. Attackers should be unable to
predict the next number given any initial sequence of numbers from these
sources. There has been a lot of effort put in to ensuring that the
numbers you get from these sources are random in every sense of the word.
The only difference is that /dev/random runs out of random bytes
and it makes you wait for more to be accumulated. Note that on some
systems, it can block for a long time waiting for new user-generated
entry to be entered into the system. So you have to use care before
using /dev/random. (Perhaps the best thing to do is to use it when
you're generating sensitive keying information, and you tell the user to
pound on the keyboard repeatedly until you print out "OK, enough".)
/dev/random is high quality entropy, generated from measuring the
inter-interrupt times etc. It blocks until enough bits of random data
are available.
/dev/urandom is similar, but when the store of entropy is running low,
it'll return a cryptographically strong hash of what there is. This
isn't as secure, but it's enough for most applications.
You might read from the devices using something like:
root# head -c 6 /dev/urandom | uuencode -
See /usr/src/linux/drivers/char/random.c for a description of the
algorithm.
Thanks to Theodore Y. Ts'o, Jon Lewis, and others from Linux-kernel for helping me (Dave) with this.