IP Filter Based Firewalls HOWTO
Brendan Conoboy
Erik Fichtner
@(#): $Id: ipf-howto.txt,v 1.33 2000/01/28 23:49:26 emf Exp $
This document is designed to demonstrate how to maximize the
effectiveness of your firewall system by using a package called IP Filter,
and to demonstrate why IP Filter may be better for you instead of the
other alternatives.
0.0 Table Of Contents
1.0 Intro.
1.1 Disclaimer
1.2 Copyright
1.3 Where to obtain relevant bits
2.0 the filter rules / config file
2.1 basic rule processing / order and precedence, "pass/block"
2.2 basic control of processing flow, the "quick" keyword
2.3 basic filtering via IP address, the "from/to" keyword
2.4 filtering by interface, the "on" keyword"
2.5 using interface and ip address together
2.6 bi-directional filtering, the "out" keyword.
2.7 basic logging, the "log" keyword
2.8 Complete bidirectional filtering by interface.
2.9 filtering by protocol, icmp example, the "proto" keyword
2.10 filtering icmp, "icmp-type" keyword, merging rules
2.11 filtering tcp/udp, "port" keyword
2.12 Rampant paranoia / default-deny stance
2.13 Implicit allow using "keep state"
2.14 Stateful guts
2.15 Fin scan detection, "flags" keyword, keep "frags" keyword
2.16 Responding to a blocked packet.
2.17 Fancy logging techniques
2.18 Putting it all together (it's not really put together yet)
2.19 improving performance with rule groups
2.20 fastroute; the keyword of stealthiness.
#2.21 Filtering on ip options
#2.22 the ipf utility
3.0 NAT and Proxies
3.1 mapping many addresses to one address; the portmap keyword
3.2 mapping many addresses to a pool of addresses.
3.3 one to one mapping; where bimap becomes handy
3.4 spoofing services; the rdr keyword.
3.5 transparent proxy support; the rdr keyword applied to the real world.
3.6 magic ipfilter internal proxies.
#3.7 ipnat utility
4.0 Monitoring and Debugging
4.1 ipfstat
4.2 ipmon
A.0 Appendix, things that don't yet fit anywhere else, but should be
mentioned.
A.1 keeping state on provided services
A.2 coping with ftp
A.3 assorted kernel variables
#A.n NAT and KeepState interactions
B.0 Appendix B, Fun with ipf!
B.1 Local Host Filtering.
B.2 What Firewall? Transparent filtering.
B.2.1 Using transparent filtering to fix network design boo-boos.
B.3 Drop-safe logging with dup-to and to.
B.3.1 The dup-to method.
B.3.2 The to method.
==============================================================================
1.0 Introduction
IP Filter is a neat 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 understanding of
firewalls, the authors reccomend reading "Building Internet Firewalls",
Chapman & Zwicky, O'Reilly and Associates; 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 document. This document is meant as
an introduction to building a firewall based on IP-Filter. If you do not
feel comfortable taking responsibility for your own actions, you should
stop reading this document and hire a qualified security professional to
install your firewall for you.
1.2 Copyright
Unless otherwise stated, HOWTO documents are copyrighted 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 relevant bits
The official IPF homepage is at:
Toomas Soome (tsoome@ut.ee) has converted this document into LaTeX
and placed it at:
Thanks!
The most up-to-date version of this document can be found at:
2.0 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.
2.1 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 Filter'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
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.
2.2 Controlling Rule Processing
If you have experience with other packet filters, you may find this
layout to be confusing, and you may be speculating 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 the next rule?
pass in all
This rule is never encountered. It could just as easily 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.
2.3 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 traffic. 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.
Lets 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.
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, lets 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 the unroutable IP space.
2.4 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 recieve comes from a network interface; every packet
you transmit goes out a network interface. 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 xl0, the first
rule will not match, the second rule will, the packet will be passed.
2.5 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 recieved 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
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, lets take care
of the rest of it:
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
pass in all
You've already seen the first 3 blocks, but not the fourth. 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.
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 illegitimate 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 destination. You can
accomplish this particularly easily with what you already know of IPF.
The new ruleset would be:
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 20.20.20.0/24 to any
pass in all
2.6 bi-directional filtering, the "out" keyword.
So far, we've been passing or blocking inbound traffic. 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. 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 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
In the narrowest viewpoint, this doesn't enhance your security. 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.
2.7 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"
keyword:
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 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 coming to
it from suspect places, but there's still more to be done. For one
thing, we're accepting packets destined anywhere. 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
network 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 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
2.8 Complete bidirectional filtering by interface.
Thusfar, only fragments of a complete ruleset have been presented.
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 count on some default behaviour, make everything as specific as
possible, interface by interface, until every base in covered.
First we'll start with the lo0 interface, which wants to run wild and
free. Since these are programs talking to other on the local system,
go ahead and keep it unrestricted:
pass out quick on lo0
pass in quick on lo0
Next, there's the xl0 interface. Later on we'll begin placing
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 10.0.0.0/8
pass out quick on tun0 from 20.20.20.0/24 to any
block out quick on tun0 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 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
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.
2.9 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 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.
2.10 Filtering ICMP with "icmp-type" keyword, merging rulesets
Of course, dropping all ICMP isn't really an ideal situation. 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 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 "quick"-ly pass an
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:
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 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 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
pass in all
Because we block spoofed traffic before the icmp rules are processed,
a spoofed packet never makes it to the icmp ruleset. It's very important
to keep such situations in mind when merging rules.
2.11 TCP and udp ports, "port" keyword
Now that we've started blocking packets based on protocol, 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 telnet are all very convenient to have, but
also hidiously 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 8
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
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
2.12 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 service?
Instead of dealing with all that hassle, lets 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. Lets 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 tcp/80 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. Perhaps this is all one needs?
2.13 Implicit allow, or the "keep state" rule
Well, I usually find myself needing more. I want to be able to telnet
out over the tun0 interface. I want convenience and security in one.
Lots of people seem to, that's why ciscos have an "established" clause
that lets established tcp sessions go through. Ipfw has established.
Ipfwadm has setup/established. They all have this feature, but the
name is very misleading.
When I first saw it, I thought it meant my 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. That's why they only support established TCP connections,
that's the only protocol that has flags which the router can
extrapolate the established state of the connection. Anybody who can
create a packet with bogus flags can get by such a firewall.
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 udp and icmp, not just tcp. The
only problem is this: the way it does it is non-intuitive. Ipf calls
it keeping state. The keyword for the ruleset is "keep state".
Keeping state's setup is weird. Normally, we know that when we want a
packet to come in, we used "pass in" and when we want a packet coming
in to be blocked we use "block in.". State is different in that, if
you want a packet to come in, you say "pass out", and if you want a
packet to go out, you say "pass in" By passing the packet with state
in one direction , a reciprocal rule is created that allows a reply to
come back in the other direction. Lets show this as an actual rule:
pass out quick proto tcp from 20.20.20.1/32 to any keep state
Though it doesn't say so, you must imagine there being an auxillery
rule that exists in the same place in line that reads:
pass in quick proto tcp With State to 20.20.20.1/32
There is no way to express the previous line, don't even try putting
it in there, it won't work. The "keep state" option in the "pass out"
rule makes it implicit.
Here's what our ruleset looks like now:
block in on tun0
pass out quick on tun0 proto tcp 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
The workings of the keep state ruleset is much like the workings of the
saying "do not speak until spoken to." It's just not permitted (except
on port 80). Actually, lets keep state on udp and icmp packets as
well:
block in on tun0
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
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.
Please note that you must keep state from the very first packet of a
potential connection. For example, if we have a set of rules that
looks like this:
pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 23 flags S
pass out quick on tun0 proto tcp from any to any keep state
At first glance, this set of rules looks pretty good. It's filtering
such that it's only allowing SYN packets to come in from the outside to
our telnet port, and we're keeping state on outbound connections.
This is pretty good. But, what happens here is that the SYN packet comes
through the firewall just fine and tickles the telnet daemon. The tcp
stack in the kernel then replies with a SYN-ACK packet, which passes
through the firewall. The firewall says "New connection! Time to keep
state!" and sets up an entry in the state table. The telnet client end's
tcp stack then replies with an ACK packet, thus completing the three-way
handshake. Only the firewall has only seen the last two packets to record
in the state table. The firewall is confused. It will default to sitting
around for 60 seconds waiting for the three-way to complete successfully
(and send the connection into what ipf calls 4/4 mode, which you can see
by using "ipfstat -s" to examine the state table). But the three-way
will never complete now. So you have exactly 60 seconds to actually do
anything in your telnet session. Well, not exactly. See, every subsequent
ACK packet that the server replies with (which will be one for every packet
transmitted. In an interactive session, this is one for every letter you
type. Usually.) So you really have 60 seconds from the time you stop
typing before the incomplete state entry is torn down. Since we're only
letting SYN packets through unmolested, and you're now in the middle of
an interactive session and only sending ACK packets, you'll find that
you can no longer type anything. Unless the server end happens to
say something to you first, and then you have another 60 seconds to play.
The moral of the story is, keep state on the first packet of a potential
connection, if you're like us and want to rely on state to keep your rules
small. Use rules like this:
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 keep state
2.14 Stateful guts
Lets take a look at what happens, rule by rule, 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 2131, destined for 198.41.0.5,
port 53. If a packet comes back from 198.41.0.5 port 53 destined
for 20.20.20.1 port 2131 within a short period of time, the reply
packet will be let through. 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.
2.15 Fin scan detection, "flags" keyword, keep "frags" keyword
Lets go back to the 3 rule set:
block in on tun0
pass out quick on tun0 proto tcp 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
As it stands, anybody can connect to port 80. That's good, that's
what it's there for in the first place. Still, we can make the system
a little tighter. When a TCP session begins, there is a certain type
of handshake that takes place between the two machines. The first
packet of the handshake has a unique set of flags that is only used
in the initialization of a TCP session. Since we're keeping state on
every reply, the only kind of TCP packet to port 80 we really need
to pass in is that first handshake packet. To do this, we use the
flags option:
pass in quick on tun0 proto tcp from any to 20.20.20.1/32 port = 80 \
flags S
Now all incoming packets must either be handshakes or have state
already. If anything else comes in, it's probably 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 "keep frags" keyword. With it, IPF will notice and
keep track of packets that are fragmented, allowing the expected
fragments to to go through. Lets rewrite the 3 rules to log forgeries
and allow fragments:
block in log on tun0
pass out quick on tun0 proto tcp 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 keep frags
The reason this works is because the first reply, even in the
handshake phase, has state kept. As such, all subsequent packets,
even though they aren't handshake packets, still make it through
because the state code has made an exception for them. In TCP there
is an ack for every packet, so the two machines may always send
packets back and forth, even out of turn. 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.
2.16 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 desirable 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 normally 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 from any to 20.20.20.0/24 proto tcp 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 somebody sends a packet
to a UDP port on your system. Whereas once you might have used:
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-unr is probably your best be. 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 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
format is:
block return-icmp-as-dest(port-unr) in log on tun0 proto udp \
from any to 20.20.20.0/24 port = 111
2.17 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 actually 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 blocked packet. We find this behavior detrimental to effective
logging, but it may suit someone's purposes.
Another useful thing you can do with the logs is to keep track of
interesting peices 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 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.
2.18 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 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
2.19 Improving performance by using 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 figure 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 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
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 21 flags S
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 20 flags S
pass out quick on xl1 proto tcp from any to 20.20.20.65/32 port = 53 flags S
pass out quick on xl1 proto udp from any to 20.20.20.65/32 port = 53
pass out quick on xl1 proto tcp from any to 20.20.20.66/32 port = 53 flags S
pass out quick on xl1 proto udp from any to 20.20.20.66/32 port = 53
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 additional 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 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.
block in quick on xl0 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 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 group 10
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 21 flags S group 10
pass out quick on xl1 proto tcp from any to 20.20.20.64/26 port = 20 flags S group 10
pass out quick on xl1 proto tcp from any to 20.20.20.65/32 port = 53 flags S group 10
pass out quick on xl1 proto udp from any to 20.20.20.65/32 port = 53 group 10
pass out quick on xl1 proto tcp from any to 20.20.20.66/32 port = 53 flags S group 10
pass out quick on xl1 proto udp from any to 20.20.20.66/32 port = 53 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.
2.20 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 applications like unix traceroute which uses udp packets with
various TTL values to map the hops between two sites. If we 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.
3.0 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
familiar 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.
3.1 Mapping Many Addresses Into One Address
The basic use of NAT accomplishes the same thing that Linux's IP
Masquerading function does, and it does it with one simple rule:
map tun0 192.168.1.0/24 -> 20.20.20.1/32
Very simple. Whenever a packet goes across the tun0 interface with a
source address matching the CIDR network mask of 192.168.1.0/24 (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 in section 2.) 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 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 disconnected and redial, 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
changed to the first available port on the NAT host. 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 we use the
NAT host for other things and we wish to limit the amount of ports that
the NAT system can consume. ipnat helps us here too 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.
3.2 Mapping Many Addresses Into a Pool of Addresses
Another use common use of NAT is to take a small statically allocated
block of addresses and map many computers into this smaller address
space. This is easy to accomplish using what you already know about
the map and portmap keywords by writing a rule like so:
map tun0 192.168.1.0/24 -> 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
3.3 One to One Mappings
Occasionally it is desirable to have a system with one IP address behind
the firewall to appear to have a completely 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.
3.4 Spoofing Services
Spoofing services? What does that have to do with anything? Plenty.
Lets 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 port 80 -> 20.20.20.5 port 8000
We can also specify the protocol here, if we wanted to redirect 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
3.5 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 accomplished 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.
This specific example of FTP proxying does lead to some complications
when used with web browsers or other automatic-login type clients that
are unaware of the requirements of communicating with the proxy. There
are patches for TIS Firewall Toolkit's ftp-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 useful 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 secretarial staff doing so.)
3.6 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 firewalls. 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 messes around with the
packet, the ftp proxy won't notice the ftp connection, and you'll
get errors.
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.
4.0 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 monitoring tools.
4.1 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 12558609 nomatch 14686 counted 0
output packets: blocked 4200 passed 12843745 nomatch 14687 counted 0
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 abnormal going on,
since we've got a lot of blocked packets outbound, even with a very
permissive pass out rule. Something here may warrant further
investigation, or it may be functioning perfectly by design. ipfstat
can't tell you if your rules are right or wrong, it can only tell you
what is happening 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 peice of really interesting information that ipfstat 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
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 connection. The
output will vary slightly from version to version, 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 second 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 represents 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 KeepState code, showing that this connection is an
inbound connection.
4.2 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 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. I don't know what that PR means, but
the eighth and ninth fields tells you the protocol and 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
earlier? Due to the often laggy nature of the Internet, sometimes
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 different 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 KeepState code has
already torn down the connection before the last packet has had a chance
to make it to your firewall. 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
This is a 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 provide ident
service for the hosts behind our NAT, since they are typically unable to
provide this service for themselves with ordinary natting.
A.0 Appendix - Things that don't fit, but should be mentioned anyway.
A.1 Keeping 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 connection, but in some instances, this isn't correct.
Namely, when mixing state tracking with filtering on flags. Lets say
that we have three rules like so:
block in all
pass in quick proto tcp from any to 20.20.20.20/32 port = 23 flags S keep state
pass out all
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:
1)
block in all
pass in quick proto tcp from any to 20.20.20.20/32 port = 23 keep state
block out all
or:
2)
block in all
pass in quick proto tcp from any to 20.20.20.20/32 port = 23 flags S keep state
pass out all keep state
Either of these sets of rules will result in a fully established state
entry for a connection to your server.
A.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. Running a server is the
least of your problems, and you can solve most everything with two
basic rules:
pass in quick proto tcp from any to 20.20.20.20/32 port = 21 flags S
pass out all keep state
These rules will allow active ftp sessions, the most common type, to your
ftp server on 20.20.20.20. However, this hoses up passive mode FTP
transfers, which other people behind firewalls like to use since it's
easier for them to filter. Passive mode servers are also difficult to
filter, since when the client sends "PASV" to server:21, the server then
allocates a listening socket greater than 1024 (typically) and informs
the client to connect to this port.
FTP clients are also hard for the administrator to deal with. Passive FTP
will work fine, assuming you're using a "pass out all keep state" or
similar rule for your outbound connections, however this will break
Active FTP, which is unfortunately the default for most ftp clients.
Traditionally, the administrator either needs to open up large holes in
the firewall to allow these active mode data connections, or install a
proxy package somewhere. As an example, lets say that we have installed
the TIS fwtk ftp-gw proxy on our firewall 20.20.20.1. In this case,
we would inform our firewall machine of the ranges of ephemeral ports
that we allow, and then we would write a rule like:
pass in quick proto tcp from any port = 20 to 20.20.20.1/32 port 29999 >< 65535 flags S
However, FTP proxies are difficult for most users to use, so ipfilter
has a way to help us out without additional software. As of version 3.3,
the internal ftp proxy is stable enough to function, and we can write a
rule with ipnat like:
map xl0 20.20.20.0/24 -> 20.20.20.1/32 proxy port ftp ftp/tcp
For more details on ipfilter's internal proxies, see section 3.6
A.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 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.portrange.last = 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 freebsd: net.inet.ip.sourceroute=0 net.inet.ip.accept_sourceroute=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
Appendix B. 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.
B.1 Localhost filtering.
A long time ago at a university far, far away, Weitse 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 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 suffered 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 services 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" nightmares 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?
B.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 arguement for
blackbox hardware solutions, under the flawed notion that inherant
obscurity of their closed system increases their security. We have
a better way.
Many network admins are familiar with the common ethernet 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
somewhat higher than a piece of cat5 would normally make it, and
that packets don't seem to make it to their final destination.
The setup for this sort of ruleset is surprisingly simple, 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 neither
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. Lets start with some
basic filtering to illistrate what's happened. Assume the
network used to look like this:
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 identical. 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 network diagrams are identical. Now
lets 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 virtual 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, especially if you're
a sysadmin and like pinging the world, to traceroute, or to resize your
MTU. Lets 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.
Thusfar, 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. 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 seems presently broken with bridging in OpenBSD. 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.
B.2.1 Using transparent filtering to fix network design boo-boos.
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 propper subnets, trust levels, filters, and so
are in both time consuming 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:
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 coherancy. 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:
| 20.20.20.2 unix server
router (20.20.20.1) ____________| 20.20.20.3 unix server
| / | 20.20.20.6 unix server
| /xl1 | 20.20.20.7 nt server
\------------/xl0 IPF Bridge <
\ xl2 | 20.20.20.4 win98 workstation
\____________| 20.20.20.8 nt workstation
| 20.20.20.9 unix workstation
| 20.20.20.10 win95 workstation
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 normally. 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
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 workstations.
Perhaps the opposite is true, maybe you want your workstations to
be able to get to the servers, but not the outside 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
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.
B.3 Drop-safe logging with dup-to and to.
Until now, we've been using the filter to drop packets. Instead of
dropping them, lets consider passing them on to another system that can
do something useful with this information 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 characteristics 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 differences 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.
B.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
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 usefulness of the log. For
this reason, we reccomend 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 network for logging and
examination.
B.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 your system to panic.
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