The value of firewalls
You might find this surprising, but there is at least one person in the world who thinks firewalls are pointless. You can find a thread where this is discussed at Groups.googe.com, but I don't recommend reading it: it is very long, and very repetitive.
If your immediate reaction is "That person must be an idiot", you are wrong. He's far from it, and his argument has some value. Let's look at that part first, and to keep things simple, we'll pretend you have one and only one service open to the web, and that service is ssh. All other ports are closed. If this represents our configuration, a firewall adds no value to it. How can it? All ports but ssh are closed; the firewall can't make them "more closed". The ssh port is open, so the firewall again can do nothing but pass it on through. In this narrow circumstance, the firewall is a useless appendage.
Well, that's not entirely true. It's true if our firewall only
has the capability of passing or rejecting packets based on header
data (source and destination parts, ip addresses, etc.). As that
is the limit (or close to it) of many firewalls, hardware or software,
the argument holds true.
If, however, we have a device capable
of doing packet inspection, the firewall may have value because it
can close the door against packets carrying certain data. This
becomes possible because certain attacks have known payloads and
their signature can be recognized. These are unusual situations,
but not so unusual that this type of filtering isn't done; see Intrusion Prevention
and Active Response for a good overview of this subject.
But even without that, this is a pretty simplistic scenario and probably
doesn't reflect the real world. I have servers with ssh running
on them, but I certainly don't want just anyone logging in. There
are a number of configuration changes you can make to ssh to
restrict logins (and many articles here
on that subject). Let's specifically take the case of a branch
office in some other state. If we need to allow ssh from there, an
obvious security addition might be to use public key authentication. That could lock down ssh to only
allow access if the public key generated by our remote office is used.
Before we go into this more, let's remind ourselves of what this doesn't
protect us from. It doesn't protect us from someone breaking into the
branch office physically or electronically and accessing our server. We can
assume that we have some degree of physical security, and can assume that
we don't have any deliberate electronic access to the remote office (they
can log in to us over the internet, but no one is supposed to log in to them,
or only we are allowed - using our public keys). But we never have 100%
security no matter what we do. Keep that in mind always. All we are
ever attempting is to increase our security - there will always be
circumstances we cannot control.
But that's what we have done here. Our security is increased, because
now at least joe from randomplace.com can't guess passwords and log in. He
can't even login if he knows a password that would otherwise work; he needs
the keys from our branch office. We have increased our security. Would
a firewall (the non-packet-inspecting kind) add anything? No, of course
not. The firewall is pointless. Or is it? The branch office is at a
known ip - let's have the firewall only allow packets from that ip pass to
our ssh daemon.
This is where the argument starts at the long thread referenced above.
Our anti-firewall person still insists that the firewall adds no value. I'm
going to list some circumstances where it will, but it's again important
to remember that there still obviously are conditions where it will
not. If someone breaks into our branch office, and logs in to the machine
account with the public keys, he obviously has immediate access to our server.
That truth has no bearing on the other conditions I'm about to state: we
are talking about adding value, not about perfect protection.
Remember also that although we've used ssh in this example, the
conditions could apply to any daemon.
There are at least three circumstances where our added ip filtering
can help us. These are:
Some daemons have incredibly complex configuration, and it can be very easy
to make a mistake. It's also sometimes easy to misunderstand the needed
configuration; Apache's position dependent "Order Allow,Deny" is a good
example of that.
It's also easy to make editing mistakes. Just yesterday I accidentally
deleted an important line from a shell script. I noticed it and fixed
it immediately, but I might not have. We're all human, mistakes happen.
If we do accidentally enable ordinary passwords in our ssh setup,
we've lost our public key protection. Now anyone with a password can
log in, and password guessing is a game played for hours on end at every ssh
server. However, if we have filtered to only allow our branch office ip
to see ssh, the firewall has protected us.
Software bug or exploit
Although we hope no such things exist in ssh, it's always possible
that some method might exist that will bypass our configuration directive
and revert to base logins. Again, remember that although we are using
ssh and public key authentication as an example here, we could just as
well be talking about any protocol and any authentication scheme. If
a bug or exploit causes this error, our firewall will once again protect
Theft of keys
It's possible for someone to obtain the key pair that we generated on our
branch office machine. It might come from a stolen backup, a usb memory
stick, or a careless email. If we don't have a firewall restricting this
to the ip we expect to be using those keys, they can be used from anywhere
in the world. Once again, the firewall protects us.
We should also note that the theft of keys from a backup, etc. isn't
necessarily unlikely. It is if we are considering random attacks, but if
we have something valuable to protect, and someone who wants to get to
it, stealing backups isn't as far fetched as it otherwise would be.
Our anti-firewall person doesn't stop here. "OK", he says, "It's
possible that a one-legged nun might attack you with a machine gun -
do you worry about that?" (I'm not making precise quotes, by the way,
this is just to give the flavor of what was discussed). "It's pointless
to protect against things that have a negligible likelihood of ever
Well, that's not quite true, is it? In fact, we often protect
ourselves from things we think have very little chance of happening. I
insure my house against fire and other sorts of damage that are
extremely unlikely to affect me. Why do I waste my money? Because
although the odds are long, if I did have a fire, the cost to me would
be extreme. I'm willing to pay the insurance cost because a fire
would be disastrous - it would cost me too much. That's just the
case with our server: while the theft of a backup or the existence of
a bug or an accidental misconfiguration may be unlikely, the cost to
me is very high if any of these things do happen. The cost of deploying
the firewall is very low, so it makes sense to do it. And, as we noted,
some daemons are more difficult to configure, which increases the chance of
error, and if someone is actively targetting us, the use of exploits or
stolen information is much more likely. The firewall adds protection.
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