5. Firewalls

If passwords provide a 'door' to cover the 'doorway' into your 'house', then
firewalls provide 'shutters' to cover the 'windows'. Bear with me, we're
extending the metaphor further than we probably should.

Your network has a lot of windows. These aren't just casual windows that let
you see out, the metaphor is closer if you think of them as service windows,
like at a drive-through of them have people (programs) at them to provide
service, some of them are empty.

A firewall provides shutters to close the empty service windows.

A firewall does absolutely nothing to protect the windows you leave open -
that's the job of the programs which provide the services at those windows.
But if you don't have a firewall, there's all those empty windows that an
intruder can use to break in through.

The firewall is ideally a separate computer which is between your network and
the internet. It can be a purpose-built device - there are some available
which are small black boxes which look like network hubs. Or it can be your
brother's old 486, with a highly secure operating system that provides an
inbuilt firewall. Whatever you choose, ensure that your local computer expert
approves of it, and do your best to ensure that he knows how to make sure it
really is secure.

None of your computers should be able to access the internet or be accessed
from the internet without going through the firewall.

Note The technical term for the windows is 'ports'.
-----------------------------------------------------------------------------

6. Other security measures

6.1. Unused programs

At each 'service window' that your firewall leaves open (technical term:
'open port'), you should have a computer program. This program should be
providing some sort of service to your users.

Any program which isn't being used, but which has a connection outside your
network, should be shut down and the 'service window' (port) closed at the
firewall. Every port which isn't specifically in use should be shut down.
Admittedly, this is a 'paranoia' position - the rationale for shutting them
down being that a closed port is safer than an open one, regardless of how
good the program is.
-----------------------------------------------------------------------------

6.2. Bugs & patches

Programs which you are using need to stay operational, and their ports
'open'. However, occasionally programs are vulnerable to clever attackers.

Vulnerabilities are reported to organisations on the Internet which make a
point of informing the companies or groups who write those programs, and
distributing the modifications that these companies or groups produce to
patch the vulnerabilities.

Every so often someone in your company should go to those sites, read their
reports for your programs, and install the patches. Once a month is common,
but you need to determine your own balance between security and convenience.
-----------------------------------------------------------------------------

6.3. Monitoring

How do you know if someone has broken into your system? The only way to know
for sure is to monitor it.

Some common types of monitoring tools are:

??*?The tripwire: On a read-only medium (like a write-protected floppy),
store a program and a small database. The program checks every file in
the database to find out when it was last changed, and sends the user the
list of all the files which have changed since it first ran. To prevent
false reporting, the database should only include files which should
never be changed.

If any of the files have been changed, you may have been broken into. (Or
your system administrator installed a new version of the operating system
and forgot to warn whoever does the monitoring!)

?? *?The sniffer: This tool checks all the traffic which goes through the
network, looking for suspicious activity. It's usually installed on the
firewall, or on a special box just to one side or the other of the
firewall - though it would be more useful on the outside.

It doesn't attempt to block any activity, only to report it when it finds
it.

?? *?The honeypot: One for special circumstances - this system has most of the
useful programs (like directory listers or file removers or editors)
removed and replaced with special programs that shut the computer down as
soon as they're run. The shutdown prevents the intruder from further
intrusion, and also from modifying the honeypot's logs.

These aren't very useful as working computers - they're simply traps.

?? *?Log analysis: This is difficult - most intruders will be careful to wipe
traces of their activity out of the logs. I don't recommend its use by
laymen, and include it here only because it is an important tool for more
experienced administrators.

Most operating systems keep a set of logs of their network activity. This
usually consists of things like 'opened this port', 'sent mail to this
person', 'closed the port'. The content of the mail is not kept, but the
fact of its being sent is. This sort of information is a useful tool for
intrusion analysis (and for checking whether the system is running
correctly).

Log analysis involves whoever does the monitoring going through the logs
and looking for strange occurrences. Logs look something like this:

May 13 09:57:03 gondwanah dhclient-2.2.x: DHCPDISCOVER on lo to 255.255.255.255 port 67 interval 2
May 13 09:57:05 gondwanah dhclient-2.2.x: No DHCPOFFERS received.
May 13 09:57:05 gondwanah dhclient-2.2.x: No working leases in persistent database - sleeping.
May 13 09:57:05 gondwanah dhclient-2.2.x: No DHCPOFFERS received.
May 13 09:57:05 gondwanah dhclient-2.2.x: No working leases in persistent database - sleeping.
May 13 10:00:21 gondwanah dhclient-2.2.x: DHCPREQUEST on eth0 to 10.0.3.1 port 67
May 13 10:00:21 gondwanah dhclient-2.2.x: DHCPACK from 10.0.3.1
May 13 10:00:21 gondwanah dhclient-2.2.x: bound to 10.0.1.1 -- renewal in 3500 seconds.



You're not expected to understand what this is! It's an attempt by my
computer to get an IP address (a number address) from the master computer
on our home network. Log analysis involves reading a lot of stuff like
this, knowing what's normal and what isn't, and dealing with the
abnormalities.

Which is why I don't recommend it for laymen.

-----------------------------------------------------------------------------
6.4. What do I do if I think I've been broken into?

If it was a physical break-in, call the police.

If it was a network break-in, either call the police or:

??*?Shut your computer down.

?? *?Call your trusted computer-expert friend, or hire specialists in computer
security.

?? *?Consider calling the police. Consider preserving the evidence.

?? *?Let the experts take your computer off the network, reboot it, and take a
look at the logs. They will hopefully be able to figure out what type of
attack it was.

?? *?If you chose to preserve the evidence, make sure your computer experts
know this before they change anything.

?? *?Let the experts check your files for damage. They may recommend
reinstalling the operating system, they may recommend restoring your data
from your latest backup. Ask them for the pros and cons of each option
they offer, and each recommendation they make. It's your data, but you
hired them for their knowledge. So lean towards their advice, but you
make the decision.

?? *?Get their advice on further securing your system. Listen to it.

-----------------------------------------------------------------------------
6.5. Final words

Your security system is only as strong as its weakest part. A determined
intruder will keep looking until they find a vulnerability.

Security through obscurity is weak. A hidden thing is more secure than a
highly visible one, but don't trust hiding on its own to protect your data. A
hidden safe is more secure than a sock under the floorboards.
-----------------------------------------------------------------------------

7. Links and further information

??*?[http://www.w3.org/Security/Faq/www-security-faq.html] WWW Security FAQ

?? *?[http://www.cert.org/] CERT, one of the major centres for vulnerability
reporting and patch coordination

?? *?[http://netsecurity.about.com/] About.com's Security page

?? *?[http://security.oreilly.com/] O'Reilly security books

?? *?[http://www.securityfocus.com] Security Focus, another centre for
security news

Notes

[1] Note that once someone has physical access to your computer, there are a
number of ways that they can access your information. Most systems have
some sort of emergency feature that allows someone with physical access
to get in and change the superuser password, or access the data. Even if
your system doesn't have that, or it's disabled, they can always just
pick up the computer or remove the hard drive and carry it out. More on
this in the physical security article.