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IP Filter Based Firewalls HOWTO

Brendan Conoboy <synk@swcp.com>
Erik Fichtner <emf@obfuscation.org>
$FreeBSD: release/9.1.0/share/examples/ipfilter/ipf-howto.txt 151702 2005-10-26 17:39:35Z jhb $

Fri Apr 20 09:31:14 EDT 2001

Abstract: This document is intended to introduce a new
user to the IP Filter firewalling package and, at the
same time, teach the user some basic fundamentals of
good firewall design.

1. Introduction

IP Filter is a great little firewall package. It does
just about everything other free firewalls (ipfwadm,
ipchains, ipfw) do, but it's also portable and does neat
stuff the others don't. This document is intended to make
some cohesive sense of the sparse documentation presently
available for ipfilter. Some prior familiarity with packet
filtering will be useful, however too much familiarity may
make this document a waste of your time. For greater under-
standing of firewalls, the authors recommend reading Build-
ing Internet Firewalls, Chapman & Zwicky, O'Reilly and Asso-
ciates; and TCP/IP Illustrated, Volume 1, Stevens, Addison-
Wesley.

1.1. Disclaimer

The authors of this document are not responsible for
any damages incurred due to actions taken based on this doc-
ument. This document is meant as an introduction to building
a firewall based on IP-Filter. If you do not feel

-2-

comfortable taking responsibility for your own actions, you
should stop reading this document and hire a qualified secu-
rity professional to install your firewall for you.

1.2. Copyright

Unless otherwise stated, HOWTO documents are copy-
righted by their respective authors. HOWTO documents may be
reproduced and distributed in whole or in part, in any
medium physical or electronic, as long as this copyright
notice is retained on all copies. Commercial redistribution
is allowed and encouraged; however, the authors would like
to be notified of any such distributions.

All translations, derivative works, or aggregate works
incorporating any HOWTO documents must be covered under this
copyright notice. That is, you may not produce a derivative
work from a HOWTO and impose additional restrictions on its
distribution. Exceptions to these rules may be granted under
certain conditions; please contact the HOWTO coordinator.

In short, we wish to promote dissemination of this
information through as many channels as possible. However,
we do wish to retain copyright on the HOWTO documents, and
would like to be notified of any plans to redistribute the
HOWTOs.

1.3. Where to obtain the important pieces

The official IPF homepage is at:
<http://coombs.anu.edu.au/~avalon/ip-filter.html>

The most up-to-date version of this document can be
found at: <http://www.obfuscation.org/ipf/>

2. Basic Firewalling

This section is designed to familiarize you with ipfil-
ter's syntax, and firewall theory in general. The features
discussed here are features you'll find in any good firewall
package. This section will give you a good foundation to
make reading and understanding the advanced section very
easy. It must be emphasized that this section alone is not
enough to build a good firewall, and that the advanced sec-
tion really is required reading for anybody who wants to
build an effective security system.

-3-

2.1. Config File Dynamics, Order and Precedence

IPF (IP Filter) has a config file (as opposed to say,
running some command again and again for each new rule).
The config file drips with Unix: There's one rule per line,
the "#" mark denotes a comment, and you can have a rule and
a comment on the same line. Extraneous whitespace is
allowed, and is encouraged to keep the rules readable.

2.2. Basic Rule Processing

The rules are processed from top to bottom, each one
appended after another. This quite simply means that if the
entirety of your config file is:

block in all
pass in all

The computer sees it as:

block in all
pass in all

Which is to say that when a packet comes in, the first thing
IPF applies is:

block in all

Should IPF deem it necessary to move on to the next rule, it
would then apply the second rule:

pass in all

At this point, you might want to ask yourself "would
IPF move on to the second rule?" If you're familiar with
ipfwadm or ipfw, you probably won't ask yourself this.
Shortly after, you will become bewildered at the weird way
packets are always getting denied or passed when they
shouldn't. Many packet filters stop comparing packets to
rulesets the moment the first match is made; IPF is not one
of them.

Unlike the other packet filters, IPF keeps a flag on
whether or not it's going to pass the packet. Unless you
interrupt the flow, IPF will go through the entire ruleset,
making its decision on whether or not to pass or drop the
packet based on the last matching rule. The scene: IP Fil-
ter's on duty. It's been been scheduled a slice of CPU
time. It has a checkpoint clipboard that reads:

block in all
pass in all

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A packet comes in the interface and it's time to go to work.
It takes a look at the packet, it takes a look at the first
rule:

block in all

"So far I think I will block this packet" says IPF. It
takes a look at the second rule:

pass in all

"So far I think I will pass this packet" says IPF. It takes
a look at a third rule. There is no third rule, so it goes
with what its last motivation was, to pass the packet
onward.

It's a good time to point out that even if the ruleset had
been

block in all
block in all
block in all
block in all
pass in all

that the packet would still have gone through. There is no
cumulative effect. The last matching rule always takes
precedence.

2.3. Controlling Rule Processing

If you have experience with other packet filters, you
may find this layout to be confusing, and you may be specu-
lating that there are problems with portability with other
filters and speed of rule matching. Imagine if you had 100
rules and most of the applicable ones were the first 10.
There would be a terrible overhead for every packet coming
in to go through 100 rules every time. Fortunately, there
is a simple keyword you can add to any rule that makes it
take action at that match. That keyword is quick.

Here's a modified copy of the original ruleset using the
quick keyword:

block in quick all
pass in all

In this case, IPF looks at the first rule:

block in quick all

The packet matches and the search is over. The packet is
expunged without a peep. There are no notices, no logs, no
memorial service. Cake will not be served. So what about

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the next rule?

pass in all

This rule is never encountered. It could just as eas-
ily not be in the config file at all. The sweeping match of
all and the terminal keyword quick from the previous rule
make certain that no rules are followed afterward.

Having half a config file laid to waste is rarely a
desirable state. On the other hand, IPF is here to block
packets and as configured, it's doing a very good job.
Nonetheless, IPF is also here to let some packets through,
so a change to the ruleset to make this possible is called
for.

2.4. Basic filtering by IP address

IPF will match packets on many criteria. The one that
we most commonly think of is the IP address. There are some
blocks of address space from which we should never get traf-
fic. One such block is from the unroutable networks,
192.168.0.0/16 (/16 is the CIDR notation for a netmask. You
may be more familiar with the dotted decimal format,
255.255.0.0. IPF accepts both). If you wanted to block
192.168.0.0/16, this is one way to do it:

block in quick from 192.168.0.0/16 to any
pass in all

Now we have a less stringent ruleset that actually does
something for us. Let's imagine a packet comes in from
1.2.3.4. The first rule is applied:

block in quick from 192.168.0.0/16 to any

The packet is from 1.2.3.4, not 192.168.*.*, so there is no
match. The second rule is applied:

pass in all

The packet from 1.2.3.4 is definitely a part of all, so the
packet is sent to whatever it's destination happened to be.

On the other hand, suppose we have a packet that comes
in from 192.168.1.2. The first rule is applied:

block in quick from 192.168.0.0/16 to any

There's a match, the packet is dropped, and that's the end.
Again, it doesn't move to the second rule because the first
rule matches and contains the quick keyword.

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At this point you can build a fairly extensive set of
definitive addresses which are passed or blocked. Since
we've already started blocking private address space from
entering our firewall, let's take care of the rest of it:

block in quick from 192.168.0.0/16 to any
block in quick from 172.16.0.0/12 to any
block in quick from 10.0.0.0/8 to any
pass in all

The first three address blocks are some of the private IP
space.

2.5. Controlling Your Interfaces

It seems very frequent that companies have internal
networks before they want a link to the outside world. In
fact, it's probably reasonable to say that's the main reason
people consider firewalls in the first place. The machine
that bridges the outside world to the inside world and vice
versa is the router. What separates the router from any
other machine is simple: It has more than one interface.

Every packet you receive comes from a network inter-
face; every packet you transmit goes out a network inter-
face. Say your machine has 3 interfaces, lo0 (loopback),
xl0 (3com ethernet), and tun0 (FreeBSD's generic tunnel
interface that PPP uses), but you don't want packets coming
in on the tun0 interface?

block in quick on tun0 all
pass in all

In this case, the on keyword means that that data is coming
in on the named interface. If a packet comes in on tun0,
the first rule will block it. If a packet comes in on lo0
or in on xl0, the first rule will not match, the second rule
will, the packet will be passed.

2.6. Using IP Address and Interface Together

It's an odd state of affairs when one decides it best
to have the tun0 interface up, but not allow any data to be
received from it. The more criteria the firewall matches
against, the tighter (or looser) the firewall can become.
Maybe you want data from tun0, but not from 192.168.0.0/16?
This is the start of a powerful firewall.

block in quick on tun0 from 192.168.0.0/16 to any
-----------
See rfc1918 at
<http://www.faqs.org/rfcs/rfc1918.html> and
<http://www.ietf.org/internet-drafts/draft-man-
ning-dsua-06.txt>

-7-

pass in all

Compare this to our previous rule:

block in quick from 192.168.0.0/16 to any
pass in all

The old way, all traffic from 192.168.0.0/16, regardless of
interface, was completely blocked. The new way, using on
tun0 means that it's only blocked if it comes in on the tun0
interface. If a packet arrived on the xl0 interface from
192.168.0.0/16, it would be passed.

At this point you can build a fairly extensive set of
definitive addresses which are passed or blocked. Since
we've already started blocking private address space from
entering tun0, let's take care of the rest of it:

You've already seen the first three blocks, but not the
rest. The fourth is a largely wasted class-A network used
for loopback. Much software communicates with itself on
127.0.0.1 so blocking it from an external source is a good
idea. The fifth, 0.0.0.0/8, should never be seen on the
internet. Most IP stacks treat "0.0.0.0/32" as the default
gateway, and the rest of the 0.*.*.* network gets handled
strangely by various systems as a byproduct of how routing
decisions are made. You should treat 0.0.0.0/8 just like
127.0.0.0/8. 169.254.0.0/16 has been assigned by the IANA
for use in auto-configuration when systems have not yet been
able to obtain an IP address via DHCP or the like. Most
notably, Microsoft Windows will use addresses in this range
if they are set to DHCP and cannot find a DHCP server.
192.0.2.0/24 has also been reserved for use as an example IP
netblock for documentation authors. We specifically do not
use this range as it would cause confusion when we tell you
to block it, and thus all our examples come from
20.20.20.0/24. 204.152.64.0/23 is an odd netblock reserved
by Sun Microsystems for private cluster interconnects, and
blocking this is up to your own judgement. Lastly,
224.0.0.0/3 wipes out the "Class D and E" networks which is
used mostly for multicast traffic, although further defini-
tion of "Class E" space can be found in RFC 1166.

-8-

There's a very important principle in packet filtering
which has only been alluded to with the private network
blocking and that is this: When you know there's certain
types of data that only comes from certain places, you setup
the system to only allow that kind of data from those
places. In the case of the unroutable addresses, you know
that nothing from 10.0.0.0/8 should be arriving on tun0
because you have no way to reply to it. It's an illegiti-
mate packet. The same goes for the other unroutables as
well as 127.0.0.0/8.

Many pieces of software do all their authentication
based upon the packet's originating IP address. When you
have an internal network, say 20.20.20.0/24, you know that
the only traffic for that internal network is going to come
off the local ethernet. Should a packet from 20.20.20.0/24
arrive over a PPP dialup, it's perfectly reasonable to drop
it on the floor, or put it in a dark room for interrogation.
It should by no means be allowed to get to its final desti-
nation. You can accomplish this particularly easily with
what you already know of IPF. The new ruleset would be:

2.7. Bi-Directional Filtering; The "out" Keyword

Up until now, we've been passing or blocking inbound
traffic. To clarify, inbound traffic is all traffic that
enters the firewall on any interface. Conversely, outbound
traffic is all traffic that leaves on any interface (whether
locally generated or simply passing through). This means
that all packets coming in are not only filtered as they
enter the firewall, they're also filtered as they exit.
Thusfar there's been an implied pass out all that may or may
not be desirable. Just as you may pass and block incoming
traffic, you may do the same with outgoing traffic.

Now that we know there's a way to filter outbound pack-
ets just like inbound, it's up to us to find a conceivable
use for such a thing. One possible use of this idea is to
keep spoofed packets from exiting your own network. Instead
of passing any traffic out the router, you could instead
limit permitted traffic to packets originating at

-9-

20.20.20.0/24. You might do it like this:

pass out quick on tun0 from 20.20.20.0/24 to any
block out quick on tun0 from any to any

If a packet comes from 20.20.20.1/32, it gets sent out by
the first rule. If a packet comes from 1.2.3.4/32 it gets
blocked by the second.

You can also make similar rules for the unroutable
addresses. If some machine tries to route a packet through
IPF with a destination in 192.168.0.0/16, why not drop it?
The worst that can happen is that you'll spare yourself some
bandwidth:

block out quick on tun0 from any to 192.168.0.0/16
block out quick on tun0 from any to 172.16.0.0/12
block out quick on tun0 from any to 10.0.0.0/8
block out quick on tun0 from any to 0.0.0.0/8
block out quick on tun0 from any to 127.0.0.0/8
block out quick on tun0 from any to 169.254.0.0/16
block out quick on tun0 from any to 192.0.2.0/24
block out quick on tun0 from any to 204.152.64.0/23
block out quick on tun0 from any to 224.0.0.0/3
block out quick on tun0 from !20.20.20.0/24 to any

In the narrowest viewpoint, this doesn't enhance your secu-
rity. It enhances everybody else's security, and that's a
nice thing to do. As another viewpoint, one might suppose
that because nobody can send spoofed packets from your site,
that your site has less value as a relay for crackers, and
as such is less of a target.

You'll likely find a number of uses for blocking out-
bound packets. One thing to always keep in mind is that in
and out directions are in reference to your firewall, never
any other machine.

2.8. Logging What Happens; The "log" Keyword

Up to this point, all blocked and passed packets have
been silently blocked and silently passed. Usually you want
to know if you're being attacked rather than wonder if that
firewall is really buying you any added benefits. While I
wouldn't want to log every passed packet, and in some cases
every blocked packet, I would want to know about the blocked
packets from 20.20.20.0/24. To do this, we add the log key-
word:

block in quick on tun0 from 192.168.0.0/16 to any
-----------
This can, of course, be changed by using -DIPFIL-
TER_DEFAULT_BLOCK when compiling ipfilter on your
system.

-10-

block in quick on tun0 from 172.16.0.0/12 to any
block in quick on tun0 from 10.0.0.0/8 to any
block in quick on tun0 from 127.0.0.0/8 to any
block in quick on tun0 from 0.0.0.0/8 to any
block in quick on tun0 from 169.254.0.0/16 to any
block in quick on tun0 from 192.0.2.0/24 to any
block in quick on tun0 from 204.152.64.0/23 to any
block in quick on tun0 from 224.0.0.0/3 to any
block in log quick on tun0 from 20.20.20.0/24 to any
pass in all

So far, our firewall is pretty good at blocking packets com-
ing to it from suspect places, but there's still more to be
done. For one thing, we're accepting packets destined any-
where. One thing we ought to do is make sure packets to
20.20.20.0/32 and 20.20.20.255/32 get dropped on the floor.
To do otherwise opens the internal network for a smurf
attack. These two lines would prevent our hypothetical net-
work from being used as a smurf relay:

block in log quick on tun0 from any to 20.20.20.0/32
block in log quick on tun0 from any to 20.20.20.255/32

This brings our total ruleset to look something like this:

block in quick on tun0 from 192.168.0.0/16 to any
block in quick on tun0 from 172.16.0.0/12 to any
block in quick on tun0 from 10.0.0.0/8 to any
block in quick on tun0 from 127.0.0.0/8 to any
block in quick on tun0 from 0.0.0.0/8 to any
block in quick on tun0 from 169.254.0.0/16 to any
block in quick on tun0 from 192.0.2.0/24 to any
block in quick on tun0 from 204.152.64.0/23 to any
block in quick on tun0 from 224.0.0.0/3 to any
block in log quick on tun0 from 20.20.20.0/24 to any
block in log quick on tun0 from any to 20.20.20.0/32
block in log quick on tun0 from any to 20.20.20.255/32
pass in all

2.9. Complete Bi-Directional Filtering By Interface

So far we have only presented fragments of a complete
ruleset. When you're actually creating your ruleset, you
should setup rules for every direction and every interface.
The default state of ipfilter is to pass packets. It is as
though there were an invisible rule at the beginning which
states pass in all and pass out all. Rather than rely on
some default behaviour, make everything as specific as pos-
sible, interface by interface, until every base is covered.

First we'll start with the lo0 interface, which wants
to run wild and free. Since these are programs talking to
others on the local system, go ahead and keep it unre-
stricted:

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pass out quick on lo0
pass in quick on lo0

Next, there's the xl0 interface. Later on we'll begin plac-
ing restrictions on the xl0 interface, but to start with,
we'll act as though everything on our local network is
trustworthy and give it much the same treatment as lo0:

pass out quick on xl0
pass in quick on xl0

Finally, there's the tun0 interface, which we've been half-
filtering with up until now:

block out quick on tun0 from any to 192.168.0.0/16
block out quick on tun0 from any to 172.16.0.0/12
block out quick on tun0 from any to 127.0.0.0/8
block out quick on tun0 from any to 10.0.0.0/8
block out quick on tun0 from any to 0.0.0.0/8
block out quick on tun0 from any to 169.254.0.0/16
block out quick on tun0 from any to 192.0.2.0/24
block out quick on tun0 from any to 204.152.64.0/23
block out quick on tun0 from any to 224.0.0.0/3
pass out quick on tun0 from 20.20.20.0/24 to any
block out quick on tun0 from any to any

This is a pretty significant amount of filtering already,
protecting 20.20.20.0/24 from being spoofed or being used
for spoofing. Future examples will continue to show one-
sideness, but keep in mind that it's for brevity's sake, and
when setting up your own ruleset, adding rules for every
direction and every interface is necessary.

2.10. Controlling Specific Protocols; The "proto" Keyword

Denial of Service attacks are as rampant as buffer
overflow exploits. Many denial of service attacks rely on
glitches in the OS's TCP/IP stack. Frequently, this has
come in the form of ICMP packets. Why not block them

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entirely?

block in log quick on tun0 proto icmp from any to any

Now any ICMP traffic coming in from tun0 will be logged and
discarded.

2.11. Filtering ICMP with the "icmp-type" Keyword; Merging
Rulesets

Of course, dropping all ICMP isn't really an ideal sit-
uation. Why not drop all ICMP? Well, because it's useful
to have partially enabled. So maybe you want to keep some
types of ICMP traffic and drop other kinds. If you want
ping and traceroute to work, you need to let in ICMP types 0
and 11. Strictly speaking, this might not be a good idea,
but if you need to weigh security against convenience, IPF
lets you do it.

pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 0
pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 11

Remember that ruleset order is important. Since we're doing
everything quick we must have our passes before our blocks,
so we really want the last three rules in this order:

pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 0
pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 11
block in log quick on tun0 proto icmp from any to any

Adding these 3 rules to the anti-spoofing rules is a bit
tricky. One error might be to put the new ICMP rules at the
beginning:

pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 0
pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 11
block in log quick on tun0 proto icmp from any to any
block in quick on tun0 from 192.168.0.0/16 to any
block in quick on tun0 from 172.16.0.0/12 to any
block in quick on tun0 from 10.0.0.0/8 to any
block in quick on tun0 from 127.0.0.0/8 to any
block in quick on tun0 from 0.0.0.0/8 to any
block in quick on tun0 from 169.254.0.0/16 to any
block in quick on tun0 from 192.0.2.0/24 to any
block in quick on tun0 from 204.152.64.0/23 to any
block in quick on tun0 from 224.0.0.0/3 to any
block in log quick on tun0 from 20.20.20.0/24 to any
block in log quick on tun0 from any to 20.20.20.0/32
block in log quick on tun0 from any to 20.20.20.255/32
pass in all

The problem with this is that an ICMP type 0 packet from
192.168.0.0/16 will get passed by the first rule, and never
blocked by the fourth rule. Also, since we quickly pass an

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ICMP ECHO_REPLY (type 0) to 20.20.20.0/24, we've just opened
ourselves back up to a nasty smurf attack and nullified
those last two block rules. Oops. To avoid this, we place
the ICMP rules after the anti-spoofing rules:

Because we block spoofed traffic before the ICMP rules are
processed, a spoofed packet never makes it to the ICMP rule-
set. It's very important to keep such situations in mind
when merging rules.

2.12. TCP and UDP Ports; The "port" Keyword

Now that we've started blocking packets based on proto-
col, we can start blocking packets based on specific aspects
of each protocol. The most frequently used of these aspects
is the port number. Services such as rsh, rlogin, and tel-
net are all very convenient to have, but also hideously
insecure against network sniffing and spoofing. One great
compromise is to only allow the services to run internally,
then block them externally. This is easy to do because
rlogin, rsh, and telnet use specific TCP ports (513, 514,
and 23 respectively). As such, creating rules to block them
is easy:

block in log quick on tun0 proto tcp from any to 20.20.20.0/24 port = 513
block in log quick on tun0 proto tcp from any to 20.20.20.0/24 port = 514
block in log quick on tun0 proto tcp from any to 20.20.20.0/24 port = 23

Make sure all 3 are before the pass in all and they'll be
closed off from the outside (leaving out spoofing for
brevity's sake):

pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 0
pass in quick on tun0 proto icmp from any to 20.20.20.0/24 icmp-type 11
block in log quick on tun0 proto icmp from any to any
block in log quick on tun0 proto tcp from any to 20.20.20.0/24 port = 513
block in log quick on tun0 proto tcp from any to 20.20.20.0/24 port = 514

-14-

block in log quick on tun0 proto tcp from any to 20.20.20.0/24 port = 23
pass in all

You might also want to block 514/udp (syslog), 111/tcp &
111/udp (portmap), 515/tcp (lpd), 2049/tcp and 2049/udp
(NFS), 6000/tcp (X11) and so on and so forth. You can get a
complete listing of the ports being listened to by using
netstat -a (or lsof -i, if you have it installed).

Blocking UDP instead of TCP only requires replacing
proto tcp with proto udp. The rule for syslog would be:

block in log quick on tun0 proto udp from any to 20.20.20.0/24 port = 514

IPF also has a shorthand way to write rules that apply to
both proto tcp and proto udp at the same time, such as
portmap or NFS. The rule for portmap would be:

block in log quick on tun0 proto tcp/udp from any to 20.20.20.0/24 port = 111

3. Advanced Firewalling Introduction

This section is designed as an immediate followup to
the basic section. Contained below are both concepts for
advanced firewall design, and advanced features contained
only within ipfilter. Once you are comfortable with this
section, you should be able to build a very strong firewall.

3.1. Rampant Paranoia; or The Default-Deny Stance

There's a big problem with blocking services by the
port: sometimes they move. RPC based programs are terrible
about this, lockd, statd, even nfsd listens places other
than 2049. It's awfully hard to predict, and even worse to
automate adjusting all the time. What if you miss a ser-
vice? Instead of dealing with all that hassle, let's start
over with a clean slate. The current ruleset looks like
this:

Yes, we really are starting over. The first rule we're
going to use is this:

block in all

No network traffic gets through. None. Not a peep. You're
rather secure with this setup. Not terribly useful, but
quite secure. The great thing is that it doesn't take much
more to make your box rather secure, yet useful too. Let's

-15-

say the machine this is running on is a web server, nothing
more, nothing less. It doesn't even do DNS lookups. It
just wants to take connections on 80/tcp and that's it. We
can do that. We can do that with a second rule, and you
already know how:

block in on tun0 all
pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 80

This machine will pass in port 80 traffic for 20.20.20.1,
and deny everything else. For basic firewalling, this is
all one needs.

3.2. Implicit Allow; The "keep state" Rule

The job of your firewall is to prevent unwanted traffic
getting to point B from point A. We have general rules
which say "as long as this packet is to port 23, it's okay."
We have general rules which say "as long as this packet has
its FIN flag set, it's okay." Our firewalls don't know the
beginning, middle, or end of any TCP/UDP/ICMP session. They
merely have vague rules that are applied to all packets.
We're left to hope that the packet with its FIN flag set
isn't really a FIN scan, mapping our services. We hope that
the packet to port 23 isn't an attempted hijack of our tel-
net session. What if there was a way to identify and autho-
rize individual TCP/UDP/ICMP sessions and distinguish them
from port scanners and DoS attacks? There is a way, it's
called keeping state.

We want convenience and security in one. Lots of peo-
ple do, that's why Ciscos have an "established" clause that
lets established tcp sessions go through. Ipfw has estab-
lished. Ipfwadm has setup/established. They all have this
feature, but the name is very misleading. When we first saw
it, we thought it meant our packet filter was keeping track
of what was going on, that it knew if a connection was
really established or not. The fact is, they're all taking
the packet's word for it from a part of the packet anybody
can lie about. They read the TCP packet's flags section and
there's the reason UDP/ICMP don't work with it, they have no
such thing. Anybody who can create a packet with bogus
flags can get by a firewall with this setup.

Where does IPF come in to play here, you ask? Well,
unlike the other firewalls, IPF really can keep track of
whether or not a connection is established. And it'll do it
with TCP, UDP and ICMP, not just TCP. Ipf calls it keeping
state. The keyword for the ruleset is keep state.

Up until now, we've told you that packets come in, then
the ruleset gets checked; packets go out, then the ruleset
gets checked. Actually, what happens is packets come in,
the state table gets checked, then *maybe* the inbound

-16-

ruleset gets checked; packets go out, the state table gets
checked, then *maybe* the outbound ruleset gets checked.
The state table is a list of TCP/UDP/ICMP sessions that are
unquestionadely passed through the firewall, circumventing
the entire ruleset. Sound like a serious security hole?
Hang on, it's the best thing that ever happened to your
firewall.

All TCP/IP sessions have a start, a middle, and an end
(even though they're sometimes all in the same packet). You
can't have an end without a middle and you can't have a mid-
dle without a start. This means that all you really need to
filter on is the beginning of a TCP/UDP/ICMP session. If
the beginning of the session is allowed by your firewall
rules, you really want the middle and end to be allowed too
(lest your IP stack should overflow and your machines become
useless). Keeping state allows you to ignore the middle and
end and simply focus on blocking/passing new sessions. If
the new session is passed, all its subsequent packets will
be allowed through. If it's blocked, none of its subsequent
packets will be allowed through. Here's an example for run-
ning an ssh server (and nothing but an ssh server):

block out quick on tun0 all
pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 22 keep state

The first thing you might notice is that there's no "pass
out" provision. In fact, there's only an all-inclusive
"block out" rule. Despite this, the ruleset is complete.
This is because by keeping state, the entire ruleset is cir-
cumvented. Once the first SYN packet hits the ssh server,
state is created and the remainder of the ssh session is
allowed to take place without interference from the fire-
wall. Here's another example:

block in quick on tun0 all
pass out quick on tun0 proto tcp from 20.20.20.1/32 to any keep state

In this case, the server is running no services. Infact,
it's not a server, it's a client. And this client doesn't
want unauthorized packets entering its IP stack at all.
However, the client wants full access to the internet and
the reply packets that such privledge entails. This simple
ruleset creates state entries for every new outgoing TCP
session. Again, since a state entry is created, these new
TCP sessions are free to talk back and forth as they please
without the hinderance or inspection of the firewall rule-
set. We mentioned that this also works for UDP and ICMP:

block in quick on tun0 all
pass out quick on tun0 proto tcp from 20.20.20.1/32 to any keep state
pass out quick on tun0 proto udp from 20.20.20.1/32 to any keep state
pass out quick on tun0 proto icmp from 20.20.20.1/32 to any keep state

-17-

Yes Virginia, we can ping. Now we're keeping state on TCP,
UDP, ICMP. Now we can make outgoing connections as though
there's no firewall at all, yet would-be attackers can't get
back in. This is very handy because there's no need to
track down what ports we're listening to, only the ports we
want people to be able to get to.

State is pretty handy, but it's also a bit tricky. You
can shoot yourself in the foot in strange and mysterious
ways. Consider the following ruleset:

pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 23
pass out quick on tun0 proto tcp from any to any keep state
block in quick all
block out quick all

At first glance, this seems to be a good setup. We allow
incoming sessions to port 23, and outgoing sessions any-
where. Naturally packets going to port 23 will have reply
packets, but the ruleset is setup in such a way that the
pass out rule will generate a state entry and everything
will work perfectly. At least, you'd think so.

The unfortunate truth is that after 60 seconds of idle
time the state entry will be closed (as opposed to the nor-
mal 5 days). This is because the state tracker never saw
the original SYN packet destined to port 23, it only saw the
SYN ACK. IPF is very good about following TCP sessions from
start to finish, but it's not very good about coming into
the middle of a connection, so rewrite the rule to look like
this:

pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 23 keep state
pass out quick on tun0 proto tcp from any to any keep state
block in quick all
block out quick all

The additional of this rule will enter the very first packet
into the state table and everything will work as expected.
Once the 3-way handshake has been witness by the state
engine, it is marked in 4/4 mode, which means it's setup for
long-term data exchange until such time as the connection is
torn down (wherein the mode changes again. You can see the
current modes of your state table with ipfstat -s.

3.3. Stateful UDP

UDP is stateless so naturally it's a bit harder to do a
reliable job of keeping state on it. Nonetheless, ipf does
a pretty good job. When machine A sends a UDP packet to
machine B with source port X and destination port Y, ipf
will allow a reply from machine B to machine A with source
port Y and destination port X. This is a short term state
entry, a mere 60 seconds.

-18-

Here's an example of what happens if we use nslookup to
get the IP address of www.3com.com:

$ nslookup www.3com.com

A DNS packet is generated:

17:54:25.499852 20.20.20.1.2111 > 198.41.0.5.53: 51979+

The packet is from 20.20.20.1, port 2111, destined for
198.41.0.5, port 53. A 60 second state entry is created.
If a packet comes back from 198.41.0.5 port 53 destined for
20.20.20.1 port 2111 within that period of time, the reply
packet will be let through. As you can see, milliseconds
later:

17:54:25.501209 198.41.0.5.53 > 20.20.20.1.2111: 51979 q: www.3com.com

The reply packet matches the state criteria and is let
through. At that same moment that packet is let through,
the state gateway is closed and no new incoming packets will
be allowed in, even if they claim to be from the same place.

3.4. Stateful ICMP

IPFilter handles ICMP states in the manner that one
would expect from understanding how ICMP is used with TCP
and UDP, and with your understanding of how keep state
works. There are two general types of ICMP messages;
requests and replies. When you write a rule such as:

pass out on tun0 proto icmp from any to any icmp-type 8 keep state

to allow outbound echo requests (a typical ping), the resul-
tant icmp-type 0 packet that comes back will be allowed in.
This state entry has a default timeout of an incomplete 0/0
state of 60 seconds. Thus, if you are keeping state on any
outbound icmp message that will elicit an icmp message in
reply, you need a proto icmp [...] keep state rule.

However, the majority of ICMP messages are status mes-
sages generated by some failure in UDP (and sometimes TCP),
and in 3.4.x and greater IPFilters, any ICMP error status
message (say icmp-type 3 code 3 port unreachable, or icmp-
type 11 time exceeded) that matches an active state table
entry that could have generated that message, the ICMP
packet is let in. For example, in older IPFilters, if you
wanted traceroute to work, you needed to use:

pass out on tun0 proto udp from any to any port 33434><33690 keep state
pass in on tun0 proto icmp from any to any icmp-type timex

whereas now you can do the right thing and just keep state
on udp with:

-19-

pass out on tun0 proto udp from any to any port 33434><33690 keep state

To provide some protection against a third-party sneaking
ICMP messages through your firewall when an active connec-
tion is known to be in your state table, the incoming ICMP
packet is checked not only for matching source and destina-
tion addresses (and ports, when applicable) but a tiny part
of the payload of the packet that the ICMP message is claim-
ing it was generated by.

3.5. FIN Scan Detection; "flags" Keyword, "keep frags" Key-
word

Let's go back to the 4 rule set from the previous section:

pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 23 keep state
pass out quick on tun0 proto tcp from any to any keep state
block in quick all
block out quick all

This is almost, but not quite, satisfactory. The problem is
that it's not just SYN packets that're allowed to go to port
23, any old packet can get through. We can change this by
using the flags option:

pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 23 flags S keep state
pass out quick on tun0 proto tcp from any to any flags S keep state
block in quick all
block out quick all

Now only TCP packets, destined for 20.20.20.1, at port 23,
with a lone SYN flag will be allowed in and entered into the
state table. A lone SYN flag is only present as the very
first packet in a TCP session (called the TCP handshake) and
that's really what we wanted all along. There's at least
two advantages to this: No arbitrary packets can come in
and make a mess of your state table. Also, FIN and XMAS
scans will fail since they set flags other than the SYN
flag. Now all incoming packets must either be handshakes or
have state already. If anything else comes in, it's proba-
bly a port scan or a forged packet. There's one exception
to that, which is when a packet comes in that's fragmented
from its journey. IPF has provisions for this as well, the
-----------
Some examples use flags S/SA instead of flags S.
flags S actually equates to flags S/AUPRFS and
matches against only the SYN packet out of all six
possible flags, while flags S/SA will allow pack-
ets that may or may not have the URG, PSH, FIN, or
RST flags set. Some protocols demand the URG or
PSH flags, and S/SAFR would be a better choice for
these, however we feel that it is less secure to
blindly use S/SA when it isn't required. But it's
your firewall.

-20-

keep frags keyword. With it, IPF will notice and keep track
of packets that are fragmented, allowing the expected frag-
ments to to go through. Let's rewrite the 3 rules to log
forgeries and allow fragments:

pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 23 flags S keep state keep frags
pass out quick on tun0 proto tcp from any to any keep state flags S keep frags
block in log quick all
block out log quick all

This works because every packet that should be allowed
through makes it into the state table before the blocking
rules are reached. The only scan this won't detect is a SYN
scan itself. If you're truely worried about that, you might
even want to log all initial SYN packets.

3.6. Responding To a Blocked Packet

So far, all of our blocked packets have been dumped on
the floor, logged or not, we've never sent anything back to
the originating host. Sometimes this isn't the most desir-
able of responses because in doing so, we actually tell the
attacker that a packet filter is present. It seems a far
better thing to misguide the attacker into believing that,
while there's no packet filter running, there's likewise no
services to break into. This is where fancier blocking
comes into play.

When a service isn't running on a Unix system, it nor-
mally lets the remote host know with some sort of return
packet. In TCP, this is done with an RST (Reset) packet.
When blocking a TCP packet, IPF can actually return an RST
to the origin by using the return-rst keyword.

Where once we did:

block in log on tun0 proto tcp from any to 20.20.20.0/24 port = 23
pass in all

We might now do:

block return-rst in log proto tcp from any to 20.20.20.0/24 port = 23
block in log quick on tun0
pass in all

We need two block statements since return-rst only works
with TCP, and we still want to block protocols such as UDP,
ICMP, and others. Now that this is done, the remote side
will get "connection refused" instead of "connection timed
out".

It's also possible to send an error message when some-
body sends a packet to a UDP port on your system. Whereas
once you might have used:

-21-

block in log quick on tun0 proto udp from any to 20.20.20.0/24 port = 111

You could instead use the return-icmp keyword to send a
reply:

block return-icmp(port-unr) in log quick on tun0 proto udp from any to 20.20.20.0/24 port = 111

According to TCP/IP Illustrated, port-unreachable is the
correct ICMP type to return when no service is listening on
the port in question. You can use any ICMP type you like,
but port-unreachable is probably your best bet. It's also
the default ICMP type for return-icmp.

However, when using return-icmp, you'll notice that
it's not very stealthy, and it returns the ICMP packet with
the IP address of the firewall, not the original destination
of the packet. This was fixed in ipfilter 3.3, and a new
keyword; return-icmp-as-dest, has been added. The new for-
mat is:

block return-icmp-as-dest(port-unr) in log on tun0 proto udp from any to 20.20.20.0/24 port = 111

3.7. Fancy Logging Techniques

It is important to note that the presence of the log
keyword only ensures that the packet will be available to
the ipfilter logging device; /dev/ipl. In order to actu-
ally see this log information, one must be running the ipmon
utility (or some other utility that reads from /dev/ipl).
The typical usage of log is coupled with ipmon -s to log the
information to syslog. As of ipfilter 3.3, one can now even
control the logging behavior of syslog by using log level
keywords, as in rules such as this:

block in log level auth.info quick on tun0 from 20.20.20.0/24 to any
block in log level auth.alert quick on tun0 proto tcp from any to 20.20.20.0/24 port = 21

In addition to this, you can tailor what information is
being logged. For example, you may not be interested that
someone attempted to probe your telnet port 500 times, but
you are interested that they probed you once. You can use
the log first keyword to only log the first example of a
packet. Of course, the notion of "first-ness" only applies
to packets in a specific session, and for the typical
blocked packet, you will be hard pressed to encounter situa-
tions where this does what you expect. However, if used in
conjunction with pass and keep state, this can be a valuable
keyword for keeping tabs on traffic.

Another useful thing you can do with the logs is to
keep track of interesting pieces of the packet in addition
to the header information normally being logged. Ipfilter
will give you the first 128 bytes of the packet if you use
the log body keyword. You should limit the use of body

-22-

logging, as it makes your logs very verbose, but for certain
applications, it is often handy to be able to go back and
take a look at the packet, or to send this data to another
application that can examine it further.

3.8. Putting It All Together

So now we have a pretty tight firewall, but it can
still be tighter. Some of the original ruleset we wiped
clean is actually very useful. I'd suggest bringing back
all the anti-spoofing stuff. This leaves us with:

block in on tun0
block in quick on tun0 from 192.168.0.0/16 to any
block in quick on tun0 from 172.16.0.0/12 to any
block in quick on tun0 from 10.0.0.0/8 to any
block in quick on tun0 from 127.0.0.0/8 to any
block in quick on tun0 from 0.0.0.0/8 to any
block in quick on tun0 from 169.254.0.0/16 to any
block in quick on tun0 from 192.0.2.0/24 to any
block in quick on tun0 from 204.152.64.0/23 to any
block in quick on tun0 from 224.0.0.0/3 to any
block in log quick on tun0 from 20.20.20.0/24 to any
block in log quick on tun0 from any to 20.20.20.0/32
block in log quick on tun0 from any to 20.20.20.255/32
pass out quick on tun0 proto tcp/udp from 20.20.20.1/32 to any keep state
pass out quick on tun0 proto icmp from 20.20.20.1/32 to any keep state
pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 80 flags S keep state

3.9. Improving Performance With Rule Groups

Let's extend our use of our firewall by creating a much
more complicated, and we hope more applicable to the real
world, example configuration For this example, we're going
to change the interface names, and network numbers. Let's
assume that we have three interfaces in our firewall with
interfaces xl0, xl1, and xl2.

xl0 is connected to our external network 20.20.20.0/26
xl1 is connected to our "DMZ" network 20.20.20.64/26
xl2 is connected to our protected network 20.20.20.128/25

We'll define the entire ruleset in one swoop, since we fig-
ure that you can read these rules by now:

block in quick on xl0 from 192.168.0.0/16 to any
block in quick on xl0 from 172.16.0.0/12 to any
block in quick on xl0 from 10.0.0.0/8 to any
block in quick on xl0 from 127.0.0.0/8 to any
block in quick on xl0 from 0.0.0.0/8 to any
block in quick on xl0 from 169.254.0.0/16 to any
block in quick on xl0 from 192.0.2.0/24 to any
block in quick on xl0 from 204.152.64.0/23 to any
block in quick on xl0 from 224.0.0.0/3 to any

-23-

block in log quick on xl0 from 20.20.20.0/24 to any
block in log quick on xl0 from any to 20.20.20.0/32
block in log quick on xl0 from any to 20.20.20.63/32
block in log quick on xl0 from any to 20.20.20.64/32
block in log quick on xl0 from any to 20.20.20.127/32
block in log quick on xl0 from any to 20.20.20.128/32
block in log quick on xl0 from any to 20.20.20.255/32
pass out on xl0 all

pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 80 flags S keep state
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 21 flags S keep state
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 20 flags S keep state
pass out quick on xl1 proto tcp from any to 20.20.20.65/32 port = 53 flags S keep state
pass out quick on xl1 proto udp from any to 20.20.20.65/32 port = 53 keep state
pass out quick on xl1 proto tcp from any to 20.20.20.66/32 port = 53 flags S keep state
pass out quick on xl1 proto udp from any to 20.20.20.66/32 port = 53 keep state
block out on xl1 all
pass in quick on xl1 proto tcp/udp from 20.20.20.64/26 to any keep state

block out on xl2 all
pass in quick on xl2 proto tcp/udp from 20.20.20.128/25 to any keep state

From this arbitarary example, we can already see that our
ruleset is becoming unwieldy. To make matters worse, as we
add more specific rules to our DMZ network, we add addi-
tional tests that must be parsed for every packet, which
affects the performance of the xl0 <-> xl2 connections. If
you set up a firewall with a ruleset like this, and you have
lots of bandwidth and a moderate amount of cpu, everyone
that has a workstation on the xl2 network is going to come
looking for your head to place on a platter. So, to keep
your head <-> torso network intact, you can speed things
along by creating rule groups. Rule groups allow you to
write your ruleset in a tree fashion, instead of as a linear
list, so that if your packet has nothing to do with the set
of tests (say, all those xl1 rules) those rules will never
be consulted. It's somewhat like having multiple firewalls
all on the same machine.

Here's a simple example to get us started:

block out quick on xl1 all head 10
pass out quick proto tcp from any to 20.20.20.64/26 port = 80 flags S keep state group 10
block out on xl2 all

In this simplistic example, we can see a small hint of the
power of the rule group. If the packet is not destined for
xl1, the head of rule group 10 will not match, and we will
go on with our tests. If the packet does match for xl1, the
quick keyword will short-circuit all further processing at
the root level (rule group 0), and focus the testing on
rules which belong to group 10; namely, the SYN check for
80/tcp. In this way, we can re-write the above rules so
that we can maximize performance of our firewall.

-24-

block in quick on xl0 all head 1
block in quick on xl0 from 192.168.0.0/16 to any group 1
block in quick on xl0 from 172.16.0.0/12 to any group 1
block in quick on xl0 from 10.0.0.0/8 to any group 1
block in quick on xl0 from 127.0.0.0/8 to any group 1
block in quick on xl0 from 0.0.0.0/8 to any group 1
block in quick on xl0 from 169.254.0.0/16 to any group 1
block in quick on xl0 from 192.0.2.0/24 to any group 1
block in quick on xl0 from 204.152.64.0/23 to any group 1
block in quick on xl0 from 224.0.0.0/3 to any group 1
block in log quick on xl0 from 20.20.20.0/24 to any group 1
block in log quick on xl0 from any to 20.20.20.0/32 group 1
block in log quick on xl0 from any to 20.20.20.63/32 group 1
block in log quick on xl0 from any to 20.20.20.64/32 group 1
block in log quick on xl0 from any to 20.20.20.127/32 group 1
block in log quick on xl0 from any to 20.20.20.128/32 group 1
block in log quick on xl0 from any to 20.20.20.255/32 group 1
pass in on xl0 all group 1

pass out on xl0 all

block out quick on xl1 all head 10
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 80 flags S keep state group 10
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 21 flags S keep state group 10
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 20 flags S keep state group 10
pass out quick on xl1 proto tcp from any to 20.20.20.65/32 port = 53 flags S keep state group 10
pass out quick on xl1 proto udp from any to 20.20.20.65/32 port = 53 keep state group 10
pass out quick on xl1 proto tcp from any to 20.20.20.66/32 port = 53 flags S keep state
pass out quick on xl1 proto udp from any to 20.20.20.66/32 port = 53 keep state group 10

pass in quick on xl1 proto tcp/udp from 20.20.20.64/26 to any keep state

block out on xl2 all

pass in quick on xl2 proto tcp/udp from 20.20.20.128/25 to any keep state

Now you can see the rule groups in action. For a host on
the xl2 network, we can completely bypass all the checks in
group 10 when we're not communicating with hosts on that
network.

Depending on your situation, it may be prudent to group
your rules by protocol, or various machines, or netblocks,
or whatever makes it flow smoothly.

3.10. "Fastroute"; The Keyword of Stealthiness

Even though we're forwarding some packets, and blocking
other packets, we're typically behaving like a well behaved
router should by decrementing the TTL on the packet and
acknowledging to the entire world that yes, there is a hop
here. But we can hide our presence from inquisitive appli-
cations like unix traceroute which uses UDP packets with
various TTL values to map the hops between two sites. If we

-25-

want incoming traceroutes to work, but we do not want to
announce the presence of our firewall as a hop, we can do so
with a rule like this:

block in quick on xl0 fastroute proto udp from any to any port 33434 >< 33465

The presence of the fastroute keyword will signal ipfilter
to not pass the packet into the Unix IP stack for routing
which results in a TTL decrement. The packet will be placed
gently on the output interface by ipfilter itself and no
such decrement will happen. Ipfilter will of course use the
system's routing table to figure out what the appropriate
output interface really is, but it will take care of the
actual task of routing itself.

There's a reason we used block quick in our example,
too. If we had used pass, and if we had IP Forwarding
enabled in our kernel, we would end up having two paths for
a packet to come out of, and we would probably panic our
kernel.

It should be noted, however, that most Unix kernels
(and certainly the ones underlying the systems that ipfilter
usually runs on) have far more efficient routing code than
what exists in ipfilter, and this keyword should not be
thought of as a way to improve the operating speed of your
firewall, and should only be used in places where stealth is
an issue.

4. NAT and Proxies

Outside of the corporate environment, one of the
biggest enticements of firewall technology to the end user
is the ability to connect several computers through a common
external interface, often without the approval, knowledge,
or even consent of their service provider. To those famil-
iar with Linux, this concept is called IP Masquerading, but
to the rest of the world it is known by the more obscure
name of Network Address Translation, or NAT for short.

4.1. Mapping Many Addresses Into One Address

The basic use of NAT accomplishes much the same thing
that Linux's IP Masquerading function does, and it does it
-----------
To be pedantic, what IPFilter provides is really
called NPAT, for Network and Port Address Transla-
tion, which means we can change any of the source
and destination IP Addresses and their source and
destination ports. True NAT only allows one to
change the addresses.

-26-

with one simple rule:

map tun0 192.168.1.0/24 -> 20.20.20.1/32

Very simple. Whenever a packet goes out the tun0 interface
with a source address matching the CIDR network mask of
192.168.1.0/24 this packet will be rewritten within the IP
stack such that its source address is 20.20.20.1, and it
will be sent on to its original destination. The system
also keeps a list of what translated connections are in
progress so that it can perform the reverse and remap the
response (which will be directed to 20.20.20.1) to the
internal host that really generated the packet.

There is a drawback to the rule we have just written,
though. In a large number of cases, we do not happen to
know what the IP address of our outside link is (if we're
using tun0 or ppp0 and a typical ISP) so it makes setting up
our NAT tables a chore. Luckily, NAT is smart enough to
accept an address of 0/32 as a signal that it needs to go
look at what the address of that interface really is and we
can rewrite our rule as follows:

map tun0 192.168.1.0/24 -> 0/32

Now we can load our ipnat rules with impunity and connect to
the outside world without having to edit anything. You do
have to run ipf -y to refresh the address if you get discon-
nected and redial or if your DHCP lease changes, though.

Some of you may be wondering what happens to the source
port when the mapping happens. With our current rule, the
packet's source port is unchanged from the original source
port. There can be instances where we do not desire this
behavior; maybe we have another firewall further upstream we
have to pass through, or perhaps many hosts are trying to
use the same source port, causing a collision where the rule
doesn't match and the packet is passed untranslated. ipnat
helps us here with the portmap keyword:

map tun0 192.168.1.0/24 -> 0/32 portmap tcp/udp 20000:30000

Our rule now shoehorns all the translated connections (which
can be tcp, udp, or tcp/udp) into the port range of 20000 to
30000.

-----------
This is a typical internal address space, since
it's non-routable on the Real Internet it is often
used for internal networks. You should still
block these packets coming in from the outside
world as discussed earlier.

-27-

4.2. Mapping Many Addresses Into a Pool of Addresses

Another use common use of NAT is to take a small stati-
cally allocated block of addresses and map many computers
into this smaller address space. This is easy to accom-
plish using what you already know about the map and portmap
keywords by writing a rule like so:

map tun0 192.168.0.0/16 -> 20.20.20.0/24 portmap tcp/udp 20000:60000

Also, there may be instances where a remote application
requires that multiple connections all come from the same IP
address. We can help with these situations by telling NAT
to statically map sessions from a host into the pool of
addresses and work some magic to choose a port. This uses a
the keyword map-block as follows:

map-block tun0 192.168.1.0/24 -> 20.20.20.0/24

4.3. One to One Mappings

Occasionally it is desirable to have a system with one
IP address behind the firewall to appear to have a com-
pletely different IP address. One example of how this would
work would be a lab of computers which are then attached to
various networks that are to be put under some kind of test.
In this example, you would not want to have to reconfigure
the entire lab when you could place a NAT system in front
and change the addresses in one simple place. We can do
that with the bimap keyword, for bidirectional mapping.
Bimap has some additional protections on it to ensure a
known state for the connection, whereas the map keyword is
designed to allocate an address and a source port and
rewrite the packet and go on with life.

bimap tun0 192.168.1.1/32 -> 20.20.20.1/32

will accomplish the mapping for one host.

4.4. Spoofing Services

Spoofing services? What does that have to do with any-
thing? Plenty. Let's pretend that we have a web server
running on 20.20.20.5, and since we've gotten increasingly
suspicious of our network security, we desire to not run
this server on port 80 since that requires a brief lifespan
as the root user. But how do we run it on a less
privledged port of 8000 in this world of "anything dot com"?
How will anyone find our server? We can use the redirection
facilities of NAT to solve this problem by instructing it to
remap any connections destined for 20.20.20.5:80 to really
point to 20.20.20.5:8000. This uses the rdr keyword:

rdr tun0 20.20.20.5/32 port 80 -> 192.168.0.5 port 8000

-28-

We can also specify the protocol here, if we wanted to redi-
rect a UDP service, instead of a TCP service (which is the
default). For example, if we had a honeypot on our firewall
to impersonate the popular Back Orifice for Windows, we
could shovel our entire network into this one place with a
simple rule:

rdr tun0 20.20.20.0/24 port 31337 -> 127.0.0.1 port 31337 udp

An extremely important point must be made about rdr: You
cannot easily use this feature as a "reflector". E.g:

rdr tun0 20.20.20.5/32 port 80 -> 20.20.20.6 port 80 tcp

will not work in the situation where .5 and .6 are on the
same LAN segment. The rdr function is applied to packets
that enter the firewall on the specified interface. When a
packet comes in that matches a rdr rule, its destination
address is then rewritten, it is pushed into ipf for filter-
ing, and should it successfully run the gauntlet of filter
rules, it is then sent to the unix routing code. Since this
packet is still inbound on the same interface that it will
need to leave the system on to reach a host, the system gets
confused. Reflectors don't work. Neither does specifying
the address of the interface the packet just came in on.
Always remember that rdr destinations must exit out of the
firewall host on a different interface.

4.5. Transparent Proxy Support; Redirection Made Useful

Since you're installing a firewall, you may have
decided that it is prudent to use a proxy for many of your
outgoing connections so that you can further tighten your
filter rules protecting your internal network, or you may
have run into a situation that the NAT mapping process does
not currently handle properly. This can also be accom-
plished with a redirection statement:

rdr xl0 0.0.0.0/0 port 21 -> 127.0.0.1 port 21

This statement says that any packet coming in on the xl0
interface destined for any address (0.0.0.0/0) on the ftp
port should be rewritten to connect it with a proxy that is
running on the NAT system on port 21.

-----------
Yes. There is a way to do this. It's so convo-
luted that I refuse to use it, though. Smart peo-
ple who require this functionality will transpar-
ently redirect into something like TIS plug-gw on
127.0.0.1. Stupid people will set up a dummy loop
interface pair and double rewrite.
This includes 127.0.0.1, by the way. That's on
lo0. Neat, huh?

-29-

This specific example of FTP proxying does lead to some
complications when used with web browsers or other auto-
matic-login type clients that are unaware of the require-
ments of communicating with the proxy. There are patches
for TIS Firewall Toolkit'sftp-gw to mate it with the nat
process so that it can determine where you were trying to go
and automatically send you there. Many proxy packages now
work in a transparent proxy environment (Squid for example,
located at http://squid.nlanr.net, works fine.)

This application of the rdr keyword is often more use-
ful when you wish to force users to authenticate themselves
with the proxy. (For example, you desire your engineers to
be able to surf the web, but you would rather not have your
call-center staff doing so.)

4.6. Magic Hidden Within NAT; Application Proxies

Since ipnat provides a method to rewrite packets as
they traverse the firewall, it becomes a convenient place to
build in some application level proxies to make up for well
known deficiencies of that application and typical fire-
walls. For example; FTP. We can make our firewall pay
attention to the packets going across it and when it notices
that it's dealing with an Active FTP session, it can write
itself some temporary rules, much like what happens with
keep state, so that the FTP data connection works. To do
this, we use a rule like so:

map tun0 192.168.1.0/24 -> 20.20.20.1/32 proxy port ftp ftp/tcp

You must always remember to place this proxy rule before any
portmap rules, otherwise when portmap comes along and
matches the packet and rewrites it before the proxy gets a
chance to work on it. Remember that ipnat rules are first-
match.

There also exist proxies for "rcmd" (which we suspect
is berkeley r-* commands which should be forbidden anyway,
thus we haven't looked at what this proxy does) and "raudio"
for Real Audio PNM streams. Likewise, both of these rules
should be put before any portmap rules, if you're doing NAT.

5. Loading and Manipulating Filter Rules; The ipf Utility

IP Filter rules are loaded by using the ipf utility.
The filter rules can be stored in any file on the system,
but typically these rules are stored in /etc/ipf.rules,
/usr/local/etc/ipf.rules, or /etc/opt/ipf/ipf.rules.

IP Filter has two sets of rules, the active set and the
inactive set. By default, all operations are performed on

-30-

the active set. You can manipulate the inactive set by
adding -I to the ipf command line. The two sets can be
toggled by using the -s command line option. This is very
useful for testing new rule sets without wiping out the old
rule set.

Rules can also be removed from the list instead of
added by using the -r command line option, but it is gener-
ally a safer idea to flush the rule set that you're working
on with -F and completely reload it when making changes.

In summary, the easiest way to load a rule set is ipf
-Fa -f /etc/ipf.rules. For more complicated manipulations
of the rule set, please see the ipf(1) man page.

6. Loading and Manipulating NAT Rules; The ipnat Utility

NAT rules are loaded by using the ipnat utility. The
NAT rules can be stored in any file on the system, but typi-
cally these rules are stored in /etc/ipnat.rules,
/usr/local/etc/ipnat.rules, or /etc/opt/ipf/ipnat.rules.

Rules can also be removed from the list instead of
added by using the -r command line option, but it is gener-
ally a safer idea to flush the rule set that you're working
on with -C and completely reload it when making changes.
Any active mappings are not affected by -C, and can be
removed with -F.

NAT rules and active mappings can be examined with the
-l command line option.

In summary, the easiest way to load a NAT rule set is
ipnat -CF -f /etc/ipnat.rules.

7. Monitoring and Debugging

There will come a time when you are interested in what
your firewall is actually doing, and ipfilter would be
incomplete if it didn't have a full suite of status monitor-
ing tools.

7.1. The ipfstat utility

In its simplest form, ipfstat displays a table of
interesting data about how your firewall is performing, such
as how many packets have been passed or blocked, if they
were logged or not, how many state entries have been made,
and so on. Here's an example of something you might see
from running the tool:

# ipfstat
input packets: blocked 99286 passed 1255609 nomatch 14686 counted 0
output packets: blocked 4200 passed 1284345 nomatch 14687 counted 0

-31-

input packets logged: blocked 99286 passed 0
output packets logged: blocked 0 passed 0
packets logged: input 0 output 0
log failures: input 3898 output 0
fragment state(in): kept 0 lost 0
fragment state(out): kept 0 lost 0
packet state(in): kept 169364 lost 0
packet state(out): kept 431395 lost 0
ICMP replies: 0 TCP RSTs sent: 0
Result cache hits(in): 1215208 (out): 1098963
IN Pullups succeeded: 2 failed: 0
OUT Pullups succeeded: 0 failed: 0
Fastroute successes: 0 failures: 0
TCP cksum fails(in): 0 (out): 0
Packet log flags set: (0)
none

ipfstat is also capable of showing you your current rule
list. Using the -i or the -o flag will show the currently
loaded rules for in or out, respectively. Adding a -h to
this will provide more useful information at the same time
by showing you a "hit count" on each rule. For example:

# ipfstat -ho
2451423 pass out on xl0 from any to any
354727 block out on ppp0 from any to any
430918 pass out quick on ppp0 proto tcp/udp from 20.20.20.0/24 to any keep state keep frags

From this, we can see that perhaps there's something abnor-
mal going on, since we've got a lot of blocked packets out-
bound, even with a very permissive pass out rule. Something
here may warrant further investigation, or it may be func-
tioning perfectly by design. ipfstat can't tell you if your
rules are right or wrong, it can only tell you what is hap-
pening because of your rules.

To further debug your rules, you may want to use the -n
flag, which will show the rule number next to each rule.

# ipfstat -on
@1 pass out on xl0 from any to any
@2 block out on ppp0 from any to any
@3 pass out quick on ppp0 proto tcp/udp from 20.20.20.0/24 to any keep state keep frags

The final piece of really interesting information that ipfs-
tat can provide us is a dump of the state table. This is
done with the -s flag:

# ipfstat -s
281458 TCP
319349 UDP
0 ICMP
19780145 hits
5723648 misses

-32-

0 maximum
0 no memory
1 active
319349 expired
281419 closed
100.100.100.1 -> 20.20.20.1 ttl 864000 pass 20490 pr 6 state 4/4
pkts 196 bytes 17394 987 -> 22 585538471:2213225493 16592:16500
pass in log quick keep state
pkt_flags & b = 2, pkt_options & ffffffff = 0
pkt_security & ffff = 0, pkt_auth & ffff = 0

Here we see that we have one state entry for a TCP connec-
tion. The output will vary slightly from version to ver-
sion, but the basic information is the same. We can see in
this connection that we have a fully established connection
(represented by the 4/4 state. Other states are incomplete
and will be documented fully later.) We can see that the
state entry has a time to live of 240 hours, which is an
absurdly long time, but is the default for an established
TCP connection. This TTL counter is decremented every sec-
ond that the state entry is not used, and will finally
result in the connection being purged if it has been left
idle. The TTL is also reset to 864000 whenever the state
IS used, ensuring that the entry will not time out while it
is being actively used. We can also see that we have passed
196 packets consisting of about 17kB worth of data over this
connection. We can see the ports for both endpoints, in
this case 987 and 22; which means that this state entry rep-
resents a connection from 100.100.100.1 port 987 to
20.20.20.1 port 22. The really big numbers in the second
line are the TCP sequence numbers for this connection, which
helps to ensure that someone isn't easily able to inject a
forged packet into your session. The TCP window is also
shown. The third line is a synopsis of the implicit rule
that was generated by the keep state code, showing that this
connection is an inbound connection.

7.2. The ipmon utility

ipfstat is great for collecting snapshots of what's
going on on the system, but it's often handy to have some
kind of log to look at and watch events as they happen in
time. ipmon is this tool. ipmon is capable of watching
the packet log (as created with the log keyword in your
rules), the state log, or the nat log, or any combination of
the three. This tool can either be run in the foreground,
or as a daemon which logs to syslog or a file. If we wanted
to watch the state table in action, ipmon -o S would show
this:

# ipmon -o S
01/08/1999 15:58:57.836053 STATE:NEW 100.100.100.1,53 -> 20.20.20.15,53 PR udp
01/08/1999 15:58:58.030815 STATE:NEW 20.20.20.15,123 -> 128.167.1.69,123 PR udp
01/08/1999 15:59:18.032174 STATE:NEW 20.20.20.15,123 -> 128.173.14.71,123 PR udp

-33-

01/08/1999 15:59:24.570107 STATE:EXPIRE 100.100.100.1,53 -> 20.20.20.15,53 PR udp Pkts 4 Bytes 356
01/08/1999 16:03:51.754867 STATE:NEW 20.20.20.13,1019 -> 100.100.100.10,22 PR tcp
01/08/1999 16:04:03.070127 STATE:EXPIRE 20.20.20.13,1019 -> 100.100.100.10,22 PR tcp Pkts 63 Bytes 4604

Here we see a state entry for an external dns request off
our nameserver, two xntp pings to well-known time servers,
and a very short lived outbound ssh connection.

ipmon is also capable of showing us what packets have
been logged. For example, when using state, you'll often
run into packets like this:

# ipmon -o I
15:57:33.803147 ppp0 @0:2 b 100.100.100.103,443 -> 20.20.20.10,4923 PR tcp len 20 1488 -A

What does this mean? The first field is obvious, it's a
timestamp. The second field is also pretty obvious, it's
the interface that this event happened on. The third field
@0:2 is something most people miss. This is the rule that
caused the event to happen. Remember ipfstat -in? If you
wanted to know where this came from, you could look there
for rule 2 in rule group 0. The fourth field, the little
"b" says that this packet was blocked, and you'll generally
ignore this unless you're logging passed packets as well,
which would be a little "p" instead. The fifth and sixth
fields are pretty self-explanatory, they say where this
packet came from and where it was going. The seventh ("PR")
and eighth fields tell you the protocol and the ninth field
tells you the size of the packet. The last part, the "-A"
in this case, tells you the flags that were on the packet;
This one was an ACK packet. Why did I mention state ear-
lier? Due to the often laggy nature of the Internet, some-
times packets will be regenerated. Sometimes, you'll get
two copies of the same packet, and your state rule which
keeps track of sequence numbers will have already seen this
packet, so it will assume that the packet is part of a dif-
ferent connection. Eventually this packet will run into a
real rule and have to be dealt with. You'll often see the
last packet of a session being closed get logged because the
keep state code has already torn down the connection before
the last packet has had a chance to make it to your fire-
wall. This is normal, do not be alarmed. Another example
packet that might be logged:

12:46:12.470951 xl0 @0:1 S 20.20.20.254 -> 255.255.255.255 PR icmp len 20 9216 icmp 9/0

-----------
For a technical presentation of the IP Filter
stateful inspection engine, please see the white
paper Real Stateful TCP Packet Filtering in IP
Filter, by Guido van Rooij. This paper may be
found at
<http://www.iae.nl/users/guido/papers/tcp_filter-
ing.ps.gz>

-34-

This is an ICMP router discovery broadcast. We can tell by
the ICMP type 9/0.

Finally, ipmon also lets us look at the NAT table in action.

# ipmon -o N
01/08/1999 05:30:02.466114 @2 NAT:RDR 20.20.20.253,113 <- -> 20.20.20.253,113 [100.100.100.13,45816]
01/08/1999 05:30:31.990037 @2 NAT:EXPIRE 20.20.20.253,113 <- -> 20.20.20.253,113 [100.100.100.13,45816] Pkts 10 Bytes 455

This would be a redirection to an identd that lies to pro-
vide ident service for the hosts behind our NAT, since they
are typically unable to provide this service for themselves
with ordinary natting.

8. Specific Applications of IP Filter - Things that don't
fit, but should be mentioned anyway.

8.1. Keep State With Servers and Flags.

Keeping state is a good thing, but it's quite easy to
make a mistake in the direction that you want to keep state
in. Generally, you want to have a keep state keyword on
the first rule that interacts with a packet for the connec-
tion. One common mistake that is made when mixing state
tracking with filtering on flags is this:

block in all
pass in quick proto tcp from any to 20.20.20.20/32 port = 23 flags S
pass out all keep state

That certainly appears to allow a connection to be created
to the telnet server on 20.20.20.20, and the replies to go
back. If you try using this rule, you'll see that it does
work--Momentarily. Since we're filtering for the SYN flag,
the state entry never fully gets completed, and the default
time to live for an incomplete state is 60 seconds.

We can solve this by rewriting the rules in one of two ways:

block in all
pass in quick proto tcp from any to 20.20.20.20/32 port = 23 keep state
block out all

or:

block in all
pass in quick proto tcp from any to 20.20.20.20/32 port = 23 flags S keep state

-35-

pass out all keep state

Either of these sets of rules will result in a fully estab-
lished state entry for a connection to your server.

8.2. Coping With FTP

FTP is one of those protocols that you just have to sit
back and ask "What the heck were they thinking?" FTP has
many problems that the firewall administrator needs to deal
with. What's worse, the problems the administrator must
face are different between making ftp clients work and mak-
ing ftp servers work.

Within the FTP protocol, there are two forms of data
transfer, called active and passive. Active transfers are
those where the server connects to an open port on the
client to send data. Conversely, passive transfers are
those where the client connects to the server to receive
data.

8.2.1. Running an FTP Server

In running an FTP server, handling Active FTP sessions
is easy to setup. At the same time, handling Passive FTP
sessions is a big problem. First we'll cover how to handle
Active FTP, then move on to Passive. Generally, we can han-
dle Active FTP sessions like we would an incoming HTTP or
SMTP connection; just open the ftp port and let keep state
do the rest:

pass in quick proto tcp from any to 20.20.20.20/32 port = 21 flags S keep state
pass out proto tcp all keep state

These rules will allow Active FTP sessions, the most common
type, to your ftp server on 20.20.20.20.

The next challenge becomes handling Passive FTP connec-
tions. Web browsers default to this mode, so it's becoming
quite popular and as such it should be supported. The prob-
lem with passive connections are that for every passive con-
nection, the server starts listening on a new port (usually
above 1023). This is essentially like creating a new
unknown service on the server. Assuming we have a good
firewall with a default-deny policy, that new service will
be blocked, and thus Active FTP sessions are broken. Don't
despair! There's hope yet to be had.

A person's first inclination to solving this problem
might be to just open up all ports above 1023. In truth,
this will work:

pass in quick proto tcp from any to 20.20.20.20/32 port > 1023 flags S keep state
pass out proto tcp all keep state

-36-

This is somewhat unsatisfactory, though. By letting every-
thing above 1023 in, we actually open ourselves up for a
number of potential problems. While 1-1023 is the desig-
nated area for server services to run, numerous programs
decided to use numbers higher than 1023, such as nfsd and X.

The good news is that your FTP server gets to decide
which ports get assigned to passive sessions. This means
that instead of opening all ports above 1023, you can allo-
cate ports 15001-19999 as ftp ports and only open that range
of your firewall up. In wu-ftpd, this is done with the pas-
sive ports option in ftpaccess. Please see the man page on
ftpaccess for details in wu-ftpd configuration. On the
ipfilter side, all we need do is setup corresponding rules:

pass in quick proto tcp from any to 20.20.20.20/32 port 15000 >< 20000 flags S keep state
pass out proto tcp all keep state

If even this solution doesn't satisfy you, you can always
hack IPF support into your FTP server, or FTP server support
into IPF.

8.2.2. Running an FTP Client

While FTP server support is still less than perfect in
IPF, FTP client support has been working well since 3.3.3.
As with FTP servers, there are two types of ftp client
transfers: passive and active.

The simplest type of client transfer from the fire-
wall's standpoint is the passive transfer. Assuming you're
keeping state on all outbound tcp sessions, passive trans-
fers will work already. If you're not doing this already,
please consider the following:

pass out proto tcp all keep state

The second type of client transfer, active, is a bit more
troublesome, but nonetheless a solved problem. Active
transfers cause the server to open up a second connection
back to the client for data to flow through. This is nor-
mally a problem when there's a firewall in the middle, stop-
ping outside connections from coming back in. To solve
this, ipfilter includes an ipnat proxy which temporarily
opens up a hole in the firewall just for the FTP server to
get back to the client. Even if you're not using ipnat to
do nat, the proxy is still effective. The following rules
is the bare minimum to add to the ipnat configuration file
(ep0 should be the interface name of the outbound network
connection):

map ep0 0/0 -> 0/32 proxy port 21 ftp/tcp

-37-

For more details on ipfilter's internal proxies, see section
3.6

8.3. Assorted Kernel Variables

There are some useful kernel tunes that either need to
be set for ipf to function, or are just generally handy to
know about for building firewalls. The first major one you
must set is to enable IP Forwarding, otherwise ipf will do
very little, as the underlying ip stack won't actually route
packets.

IP Forwarding:

openbsd:
net.inet.ip.forwarding=1

freebsd:
net.inet.ip.forwarding=1

netbsd:
net.inet.ip.forwarding=1

solaris:
ndd -set /dev/ip ip_forwarding 1

Ephemeral Port Adjustment:

openbsd:
net.inet.ip.portfirst = 25000

freebsd:
net.inet.ip.portrange.first = 25000 net.inet.ip.por-
trange.last = 49151

netbsd:
net.inet.ip.anonportmin = 25000 net.inet.ip.anonportmax
= 49151

solaris:
ndd -set /dev/tcp tcp_smallest_anon_port 25000
ndd -set /dev/tcp tcp_largest_anon_port 65535

Other Useful Values:

openbsd:
net.inet.ip.sourceroute = 0
net.inet.ip.directed-broadcast = 0

-38-

freebsd:
net.inet.ip.sourceroute=0
net.ip.accept_sourceroute=0

netbsd:
net.inet.ip.allowsrcrt=0
net.inet.ip.forwsrcrt=0
net.inet.ip.directed-broadcast=0
net.inet.ip.redirect=0

solaris:
ndd -set /dev/ip ip_forward_directed_broadcasts 0
ndd -set /dev/ip ip_forward_src_routed 0
ndd -set /dev/ip ip_respond_to_echo_broadcast 0

In addition, freebsd has some ipf specific sysctl variables.

net.inet.ipf.fr_flags: 0
net.inet.ipf.fr_pass: 514
net.inet.ipf.fr_active: 0
net.inet.ipf.fr_tcpidletimeout: 864000
net.inet.ipf.fr_tcpclosewait: 60
net.inet.ipf.fr_tcplastack: 20
net.inet.ipf.fr_tcptimeout: 120
net.inet.ipf.fr_tcpclosed: 1
net.inet.ipf.fr_udptimeout: 120
net.inet.ipf.fr_icmptimeout: 120
net.inet.ipf.fr_defnatage: 1200
net.inet.ipf.fr_ipfrttl: 120
net.inet.ipf.ipl_unreach: 13
net.inet.ipf.ipl_inited: 1
net.inet.ipf.fr_authsize: 32
net.inet.ipf.fr_authused: 0
net.inet.ipf.fr_defaultauthage: 600

9. Fun with ipf!

This section doesn't necessarily teach you anything new
about ipf, but it may raise an issue or two that you haven't
yet thought up on your own, or tickle your brain in a way
that you invent something interesting that we haven't
thought of.

9.1. Localhost Filtering

A long time ago at a university far, far away, Wietse
Venema created the tcp-wrapper package, and ever since, it's
been used to add a layer of protection to network services
all over the world. This is good. But, tcp-wrappers have

-39-

flaws. For starters, they only protect TCP services, as the
name suggests. Also, unless you run your service from
inetd, or you have specifically compiled it with libwrap and
the appropriate hooks, your service isn't protected. This
leaves gigantic holes in your host security. We can plug
these up by using ipf on the local host. For example, my
laptop often gets plugged into or dialed into networks that
I don't specifically trust, and so, I use the following rule
set:

pass in quick on lo0 all
pass out quick on lo0 all

block in log all
block out all

pass in quick proto tcp from any to any port = 113 flags S keep state
pass in quick proto tcp from any to any port = 22 flags S keep state
pass in quick proto tcp from any port = 20 to any port 39999 >< 45000 flags S keep state

pass out quick proto icmp from any to any keep state
pass out quick proto tcp/udp from any to any keep state keep frags

It's been like that for quite a while, and I haven't suf-
fered any pain or anguish as a result of having ipf loaded
up all the time. If I wanted to tighten it up more, I could
switch to using the NAT ftp proxy and I could add in some
rules to prevent spoofing. But even as it stands now, this
box is far more restrictive about what it presents to the
local network and beyond than the typical host does. This
is a good thing if you happen to run a machine that allows a
lot of users on it, and you want to make sure one of them
doesn't happen to start up a service they wern't supposed
to. It won't stop a malicious hacker with root access from
adjusting your ipf rules and starting a service anyway, but
it will keep the "honest" folks honest, and your weird ser-
vices safe, cozy and warm even on a malicious LAN. A big
win, in my opinion. Using local host filtering in addition
to a somewhat less-restrictive "main firewall" machine can
solve many performance issues as well as political night-
mares like "Why doesn't ICQ work?" and "Why can't I put a
web server on my own workstation! It's MY WORKSTATION!!"
Another very big win. Who says you can't have security and
convienence at the same time?

9.2. What Firewall? Transparent filtering.

One major concern in setting up a firewall is the
integrity of the firewall itself. Can somebody break into
your firewall, thereby subverting its ruleset? This is a
common problem administrators must face, particularly when
they're using firewall solutions on top of their Unix/NT
machines. Some use it as an argument for blackbox hardware
solutions, under the flawed notion that inherent obscurity

-40-

of their closed system increases their security. We have a
better way.

Many network admins are familiar with the common ether-
net bridge. This is a device that connects two separate
ethernet segments to make them one. An ethernet bridge is
typically used to connect separate buildings, switch network
speeds, and extend maximum wire lengths. Hubs and switches
are common bridges, sometimes they're just 2 ported devices
called repeaters. Recent versions of Linux, OpenBSD,
NetBSD, and FreeBSD include code to convert $1000 PCs into
$10 bridges, too! What all bridges tend to have in common
is that though they sit in the middle of a connection
between two machines, the two machines don't know the bridge
is there. Enter ipfilter and OpenBSD.

Ethernet bridging takes place at Layer2 on the ISO
stack. IP takes place on Layer3. IP Filter in primarily
concerned with Layer3, but dabbles in Layer2 by working with
interfaces. By mixing IP filter with OpenBSD's bridge
device, we can create a firewall that is both invisible and
unreachable. The system needs no IP address, it doesn't
even need to reveal its ethernet address. The only telltale
sign that the filter might be there is that latency is some-
what higher than a piece of cat5 would normally make it, and
that packets don't seem to make it to their final destina-
tion.

The setup for this sort of ruleset is surprisingly sim-
ple, too. In OpenBSD, the first bridge device is named
bridge0. Say we have two ethernet cards in our machine as
well, xl0 and xl1. To turn this machine into a bridge, all
one need do is enter the following three commands:

brconfig bridge0 add xl0 add xl1 up
ifconfig xl0 up
ifconfig xl1 up

At ths point, all traffic ariving on xl0 is sent out xl1 and
all traffic on xl1 is sent out xl0. You'll note that nei-
ther interface has been assigned an IP address, nor do we
need assign one. All things considered, it's likely best we
not add one at all.

Rulesets behave essentially the as the always have.
Though there is a bridge0 interface, we don't filter based
on it. Rules continue to be based upon the particular
interface we're using, making it important which network
cable is plugged into which network card in the back of the
machine. Let's start with some basic filtering to illis-
trate what's happened. Assume the network used to look like
this:

-41-

20.20.20.1 <---------------------------------> 20.20.20.0/24 network hub

That is, we have a router at 20.20.20.1 connected to the
20.20.20.0/24 network. All packets from the 20.20.20.0/24
network go through 20.20.20.1 to get to the outside world
and vice versa. Now we add the Ipf Bridge:

20.20.20.1 <-------/xl0 IpfBridge xl1/-------> 20.20.20.0/24 network hub

We also have the following ruleset loaded on the IpfBridge
host:

pass in quick all
pass out quick all

With this ruleset loaded, the network is functionally iden-
tical. As far as the 20.20.20.1 router is concerned, and as
far as the 20.20.20.0/24 hosts are concerned, the two net-
work diagrams are identical. Now let's change the ruleset
some:

block in quick on xl0 proto icmp
pass in quick all
pass out quick all

Still, 20.20.20.1 and 20.20.20.0/24 think the network is
identical, but if 20.20.20.1 attempts to ping 20.20.20.2, it
will never get a reply. What's more, 20.20.20.2 won't even
get the packet in the first place. IPfilter will intercept
the packet before it even gets to the other end of the vir-
tual wire. We can put a bridged filter anywhere. Using
this method we can shrink the network trust circle down an
individual host level (given enough ethernet cards:-)

Blocking icmp from the world seems kind of silly, espe-
cially if you're a sysadmin and like pinging the world, to
traceroute, or to resize your MTU. Let's construct a better
ruleset and take advantage of the original key feature of
ipf: stateful inspection.

pass in quick on xl1 proto tcp keep state
pass in quick on xl1 proto udp keep state
pass in quick on xl1 proto icmp keep state
block in quick on xl0

In this situation, the 20.20.20.0/24 network (perhaps more
aptly called the xl1 network) can now reach the outside
world, but the outside world can't reach it, and it can't
figure out why, either. The router is accessible, the hosts
are active, but the outside world just can't get in. Even
if the router itself were compromised, the firewall would
still be active and successful.

-42-

So far, we've been filtering by interface and protocol
only. Even though bridging is concerned layer2, we can
still discriminate based on IP address. Normally we have a
few services running, so our ruleset may look like this:

pass in quick on xl1 proto tcp keep state
pass in quick on xl1 proto udp keep state
pass in quick on xl1 proto icmp keep state
block in quick on xl1 # nuh-uh, we're only passing tcp/udp/icmp sir.
pass in quick on xl0 proto udp from any to 20.20.20.2/32 port=53 keep state
pass in quick on xl0 proto tcp from any to 20.20.20.2/32 port=53 flags S keep state
pass in quick on xl0 proto tcp from any to 20.20.20.3/32 port=25 flags S keep state
pass in quick on xl0 proto tcp from any to 20.20.20.7/32 port=80 flags S keep state
block in quick on xl0

Now we have a network where 20.20.20.2 is a zone serving
name server, 20.20.20.3 is an incoming mail server, and
20.20.20.7 is a web server.

Bridged IP Filter is not yet perfect, we must confess.

First, You'll note that all the rules are setup using
the in direction instead of a combination of in and out.
This is because the out direction is presently unimplemented
with bridging in OpenBSD. This was originally done to pre-
vent vast performance drops using multiple interfaces. Work
has been done in speeding it up, but it remains unimple-
mented. If you really want this feature, you might try your
hand at working on the code or asking the OpenBSD people how
you can help.

Second, using IP Filter with bridging makes the use of
IPF's NAT features inadvisable, if not downright dangerous.
The first problem is that it would give away that there's a
filtering bridge. The second problem would be that the
bridge has no IP address to masquerade with, which will most
assuredly lead to confusion and perhaps a kernel panic to
boot. You can, of course, put an IP address on the outbound
interface to make NAT work, but part of the glee of bridging
is thus diminished.

9.2.1. Using Transparent Filtering to Fix Network Design
Mistakes

Many organizations started using IP well before they
thought a firewall or a subnet would be a good idea. Now
they have class-C sized networks or larger that include all
their servers, their workstations, their routers, coffee
makers, everything. The horror! Renumbering with proper
subnets, trust levels, filters, and so are in both time con-
suming and expensive. The expense in hardware and man hours
alone is enough to make most organizations unwilling to
really solve the problem, not to mention the downtime
involved. The typical problem network looks like this:

-43-

20.20.20.1 router 20.20.20.6 unix server
20.20.20.2 unix server 20.20.20.7 nt workstation
20.20.20.3 unix server 20.20.20.8 nt server
20.20.20.4 win98 workstation 20.20.20.9 unix workstation
20.20.20.5 intelligent switch 20.20.20.10 win95 workstation

Only it's about 20 times larger and messier and frequently
undocumented. Ideally, you'd have all the trusting servers
in one subnet, all the work- stations in another, and the
network switches in a third. Then the router would filter
packets between the subnets, giving the workstations limited
access to the servers, nothing access to the switches, and
only the sysadmin's workstation access to the coffee pot.
I've never seen a class-C sized network with such coherence.
IP Filter can help.

To start with, we're going to separate the router, the
workstations, and the servers. To do this we're going to
need 2 hubs (or switches) which we probably already have,
and an IPF machine with 3 ethernet cards. We're going to
put all the servers on one hub and all the workstations on
the other. Normally we'd then connect the hubs to each
other, then to the router. Instead, we're going to plug the
router into IPF's xl0 interface, the servers into IPF's xl1
interface, and the workstations into IPF's xl2 interface.
Our network diagram looks something like this:

Where once there was nothing but interconnecting wires, now
there's a filtering bridge that not a single host needs to
be modified to take advantage of. Presumably we've already
enabled bridging so the network is behaving perfectly nor-
mally. Further, we're starting off with a ruleset much like
our last ruleset:

pass in quick on xl0 proto udp from any to 20.20.20.2/32 port=53 keep state
pass in quick on xl0 proto tcp from any to 20.20.20.2/32 port=53 flags S keep state
pass in quick on xl0 proto tcp from any to 20.20.20.3/32 port=25 flags S keep state
pass in quick on xl0 proto tcp from any to 20.20.20.7/32 port=80 flags S keep state
block in quick on xl0
pass in quick on xl1 proto tcp keep state
pass in quick on xl1 proto udp keep state
pass in quick on xl1 proto icmp keep state
block in quick on xl1 # nuh-uh, we're only passing tcp/udp/icmp sir.
pass in quick on xl2 proto tcp keep state

-44-

pass in quick on xl2 proto udp keep state
pass in quick on xl2 proto icmp keep state
block in quick on xl2 # nuh-uh, we're only passing tcp/udp/icmp sir.

Once again, traffic coming from the router is restricted to
DNS, SMTP, and HTTP. At the moment, the servers and the
workstations can exchange traffic freely. Depending on what
kind of organization you are, there might be something about
this network dynamic you don't like. Perhaps you don't want
your workstations getting access to your servers at all?
Take the xl2 ruleset of:

pass in quick on xl2 proto tcp keep state
pass in quick on xl2 proto udp keep state
pass in quick on xl2 proto icmp keep state
block in quick on xl2 # nuh-uh, we're only passing tcp/udp/icmp sir.

And change it to:

block in quick on xl2 from any to 20.20.20.0/24
pass in quick on xl2 proto tcp keep state
pass in quick on xl2 proto udp keep state
pass in quick on xl2 proto icmp keep state
block in quick on xl2 # nuh-uh, we're only passing tcp/udp/icmp sir.

Perhaps you want them to just get to the servers to get and
send their mail with IMAP? Easily done:

pass in quick on xl2 proto tcp from any to 20.20.20.3/32 port=25
pass in quick on xl2 proto tcp from any to 20.20.20.3/32 port=143
block in quick on xl2 from any to 20.20.20.0/24
pass in quick on xl2 proto tcp keep state
pass in quick on xl2 proto udp keep state
pass in quick on xl2 proto icmp keep state
block in quick on xl2 # nuh-uh, we're only passing tcp/udp/icmp sir.

Now your workstations and servers are protected from the
outside world, and the servers are protected from your work-
stations.

Perhaps the opposite is true, maybe you want your work-
stations to be able to get to the servers, but not the out-
side world. After all, the next generation of exploits is
breaking the clients, not the servers. In this case, you'd
change the xl2 rules to look more like this:

pass in quick on xl2 from any to 20.20.20.0/24
block in quick on xl2

Now the servers have free reign, but the clients can only
connect to the servers. We might want to batten down the
hatches on the servers, too:

pass in quick on xl1 from any to 20.20.20.0/24

-45-

block in quick on xl1

With the combination of these two, the clients and servers
can talk to each other, but neither can access the outside
world (though the outside world can get to the few services
from earlier). The whole ruleset would look something like
this:

pass in quick on xl0 proto udp from any to 20.20.20.2/32 port=53 keep state
pass in quick on xl0 proto tcp from any to 20.20.20.2/32 port=53 flags S keep state
pass in quick on xl0 proto tcp from any to 20.20.20.3/32 port=25 flags S keep state
pass in quick on xl0 proto tcp from any to 20.20.20.7/32 port=80 flags S keep state
block in quick on xl0
pass in quick on xl1 from any to 20.20.20.0/24
block in quick on xl1
pass in quick on xl2 from any to 20.20.20.0/24
block in quick on xl2

So remember, when your network is a mess of twisty IP
addresses and machine classes, transparent filtered bridges
can solve a problem that would otherwise be lived with and
perhaps someday exploited.

9.3. Drop-Safe Logging With dup-to and to.

Until now, we've been using the filter to drop packets.
Instead of dropping them, let's consider passing them on to
another system that can do something useful with this infor-
mation beyond the logging we can perform with ipmon. Our
firewall system, be it a bridge or a router, can have as
many interfaces as we can cram into the system. We can use
this information to create a "drop-safe" for our packets. A
good example of a use for this would be to implement an
intrusion detection network. For starters, it might be
desirable to hide the presence of our intrusion detection
systems from our real network so that we can keep them from
being detected.

Before we get started, there are some operational char-
acteristics that we need to make note of. If we are only
going to deal with blocked packets, we can use either the to
keyword or the fastroute keyword. (We'll cover the differ-
ences between these two later) If we're going to pass the
packets like we normally would, we need to make a copy of
the packet for our drop-safe log with the dup-to keyword.

9.3.1. The dup-to Method

If, for example, we wanted to send a copy of everything
going out the xl3 interface off to our drop-safe network on
ed0, we would use this rule in our filter list:

pass out on xl3 dup-to ed0 from any to any

-46-

You might also have a need to send the packet directly to a
specific IP address on your drop-safe network instead of
just making a copy of the packet out there and hoping for
the best. To do this, we modify our rule slightly:

pass out on xl3 dup-to ed0:192.168.254.2 from any to any

But be warned that this method will alter the copied
packet's destination address, and may thus destroy the use-
fulness of the log. For this reason, we recommend only
using the known address method of logging when you can be
certain that the address that you're logging to corresponds
in some way to what you're logging for (e.g.: don't use
"192.168.254.2" for logging for both your web server and
your mail server, since you'll have a hard time later trying
to figure out which system was the target of a specific set
of packets.)

This technique can be used quite effectively if you
treat an IP Address on your drop-safe network in much the
same way that you would treat a Multicast Group on the real
internet. (e.g.: "192.168.254.2" could be the channel for
your http traffic analysis system, "23.23.23.23" could be
your channel for telnet sessions, and so on.) You don't
even need to actually have this address set as an address or
alias on any of your analysis systems. Normally, your
ipfilter machine would need to ARP for the new destination
address (using dup-to ed0:192.168.254.2 style, of course)
but we can avoid that issue by creating a static arp entry
for this "channel" on our ipfilter system.

In general, though, dup-to ed0 is all that is required
to get a new copy of the packet over to our drop-safe net-
work for logging and examination.

9.3.2. The to Method

The dup-to method does have an immediate drawback,
though. Since it has to make a copy of the packet and
optionally modify it for its new destination, it's going to
take a while to complete all this work and be ready to deal
with the next packet coming in to the ipfilter system.

If we don't care about passing the packet to its normal
destination and we were going to block it anyway, we can
just use the to keyword to push this packet past the normal
routing table and force it to go out a different interface
than it would normally go out.

block in quick on xl0 to ed0 proto tcp from any to any port < 1024

we use block quick for to interface routing, because like
fastroute, the to interface code will generate two packet
paths through ipfilter when used with pass, and likely cause

-47-

your system to panic.

10. Bogus Network Filtering, the ultimate in current anti-
spoofing technology.

We've spent a little bit of time tracking down the cur-
rent vast tracts of IP address space that have been reserved
by the IANA for various reasons, or are otherwise not cur-
rently in use at the time this document was written. Since
none of these address ranges should be in use currently,
there should be no legitimate reason to ever see them as a
source address, or to send them traffic as a destination
address, right? Right!

So without further ado, the complete list of bogus net-
works:

#
# s/OUTSIDE/outside-interface (eg: fxp0)
# s/MYNET/network-cidr-address (eg: 1.2.3.0/24)
#
block in on OUTSIDE all
block in quick on OUTSIDE from 0.0.0.0/7 to any
block in quick on OUTSIDE from 2.0.0.0/8 to any
block in quick on OUTSIDE from 5.0.0.0/8 to any
block in quick on OUTSIDE from 10.0.0.0/8 to any
block in quick on OUTSIDE from 23.0.0.0/8 to any
block in quick on OUTSIDE from 27.0.0.0/8 to any
block in quick on OUTSIDE from 31.0.0.0/8 to any
block in quick on OUTSIDE from 67.0.0.0/8 to any
block in quick on OUTSIDE from 68.0.0.0/6 to any
block in quick on OUTSIDE from 72.0.0.0/5 to any
block in quick on OUTSIDE from 80.0.0.0/4 to any
block in quick on OUTSIDE from 96.0.0.0/3 to any
block in quick on OUTSIDE from 127.0.0.0/8 to any
block in quick on OUTSIDE from 128.0.0.0/16 to any
block in quick on OUTSIDE from 128.66.0.0/16 to any
block in quick on OUTSIDE from 169.254.0.0/16 to any
block in quick on OUTSIDE from 172.16.0.0/12 to any
block in quick on OUTSIDE from 191.255.0.0/16 to any
block in quick on OUTSIDE from 192.0.0.0/16 to any
block in quick on OUTSIDE from 192.168.0.0/16 to any
block in quick on OUTSIDE from 197.0.0.0/8 to any
block in quick on OUTSIDE from 201.0.0.0/8 to any
block in quick on OUTSIDE from 204.152.64.0/23 to any
block in quick on OUTSIDE from 224.0.0.0/3 to any
block in quick on OUTSIDE from MYNET to any
# Your pass rules come here...

block out on OUTSIDE all
block out quick on OUTSIDE from !MYNET to any
block out quick on OUTSIDE from MYNET to 0.0.0.0/7

-48-

block out quick on OUTSIDE from MYNET to 2.0.0.0/8
block out quick on OUTSIDE from MYNET to 5.0.0.0/8
block out quick on OUTSIDE from MYNET to 10.0.0.0/8
block out quick on OUTSIDE from MYNET to 23.0.0.0/8
block out quick on OUTSIDE from MYNET to 27.0.0.0/8
block out quick on OUTSIDE from MYNET to 31.0.0.0/8
block out quick on OUTSIDE from MYNET to 67.0.0.0/8
block out quick on OUTSIDE from MYNET to 68.0.0.0/6
block out quick on OUTSIDE from MYNET to 72.0.0.0/5
block out quick on OUTSIDE from MYNET to 80.0.0.0/4
block out quick on OUTSIDE from MYNET to 96.0.0.0/3
block out quick on OUTSIDE from MYNET to 127.0.0.0/8
block out quick on OUTSIDE from MYNET to 128.0.0.0/16
block out quick on OUTSIDE from MYNET to 128.66.0.0/16
block out quick on OUTSIDE from MYNET to 169.254.0.0/16
block out quick on OUTSIDE from MYNET to 172.16.0.0/12
block out quick on OUTSIDE from MYNET to 191.255.0.0/16
block out quick on OUTSIDE from MYNET to 192.0.0.0/16
block out quick on OUTSIDE from MYNET to 192.168.0.0/16
block out quick on OUTSIDE from MYNET to 197.0.0.0/8
block out quick on OUTSIDE from MYNET to 201.0.0.0/8
block out quick on OUTSIDE from MYNET to 204.152.64.0/23
block out quick on OUTSIDE from MYNET to 224.0.0.0/3
# Your pass rules come here...

If you're going to use these, we suggest that you become
familiar with whois.arin.net and keep an occasional eye on
these, as the IANA isn't going to notify you when they allo-
cate one of these to a new corporation or something. You
have been warned.

We'd also like to thank Frank DiGennaro <fsd@server-
vault.com> for greatly contributing to this filter list.

Man Man