Before we start

This lecture[1] will be about firewalls and related functions, starting from a little theory along with a number of examples of filtering and other network traffic directing. As in any number of other endeavors, the things I discuss can be done in more than one way. Under any circumstances I will urge you to interrupt me when you need to. That is, if you will permit me to use what I learn from your comments later, either in revised versions of this lecture or in practice at a later time. But first,

This is not a HOWTO

This document is not intended as a pre-cooked recipe for cutting and pasting. Just to hammer this in, please repeat after me The Pledge of the Network Admin This is my network. It is mine or technically my employer's, it is my responsibility and I care for it with all my heart there are many other networks a lot like mine, but none are just like it. I solemnly swear that I will not mindlessly paste from HOWTOs. The point is, while the rules and configurations I show you do work, I have tested them and they are in some way related to what has been put into production, they may very well be overly simplistic and are almost certain to be at least a little off and possibly quite wrong for your network. Please keep in mind that this document is intended to show you a few useful things and inspire you to achieve good things. Please strive to understand your network and what you need to do to make it better. Please do not paste blindly from this document or any other.

Now, with that out of the way, we can go on to the meat of the matter.

PF?

First, a few words about the software we are about to discuss, OpenBSD's PF.

PF was written during the summer and autumn of 2001 by Daniel Hartmeier and a number of OpenBSD developers, and was launched as a default part of the OpenBSD 3.0 base system in December of 2001.

The need for a piece of new firewall software for OpenBSD arose when Darren Reed announced to the world that IPFilter, which at that point had been rather intimately integrated in OpenBSD, was not after all BSD licensed. In fact quite to the contrary. The license itself was almost a word by word copy of the BSD license, omitting only the right to make changes to the code. The OpenBSD version of IPFilter contained quite a number of changes and customizations, which it turned out were not allowed according to the license. IPFilter was removed from the OpenBSD source tree on May 29th, 2001, and for a few weeks OpenBSD-current did not contain any firewalling software.

Fortunately, in Switzerland Daniel Hartmeier was already doing some limited experiments involving kernel hacking in the networking code.

His starting point was hooking a small function of his own into the networking stack, making packets pass through it, and after a while he had started thinking about filtering. Then the license crisis happened.

IPFilter was pruned from the source tree on May 29th. The first commit of the PF code happened Sunday, June 24 2001 at 19:48:58 UTC.

A few months of rather intense activity followed, and the version of PF to be released with OpenBSD 3.0 contained a rather complete implementation of packet filtering, including network address translation.

From the looks of it, Daniel Hartmeier and the other PF developers made good use of their experience with the IPFilter code. Under any circumstances Daniel presented a USENIX 2002 paper with performance tests which show that the OpenBSD 3.1 PF performed equally well or better under stress than IPFilter on the same platform or iptables on Linux.

In addition, some tests were run on the original PF from OpenBSD 3.0. These tests showed mainly that the code had gained in efficiency from version 3.0 to version 3.1. The article which provides the details is available from Daniel Hartmeier's web, see http://www.benzedrine.cx/pf-paper.html.

I have not seen comparable tests performed recently, but in my own experience and that of others, the PF filtering overhead is pretty much negligible. As one data point, the machine which gateways between Datadok's network and the world is a Pentium III 450MHz with 384MB of RAM. When I've remembered to check, I've never seen the machine at less than 96 percent 'idle' according to top.

Packet filter? Firewall?

By now I have already used some terms and concepts before I've bothered to explain them, and I'll correct that oversight shortly. PF is a packet filter, that is, code which inspects network packets at the protocol and port level, and decides what to do with them. In PF's case this code for the most part operates in kernel space, inside the network code.

PF operates in a world which consists of packets, protocols, connections and ports.

Based on where a packet is coming from or where it's going, which protocol, connection of port it is designated for, PF is able to determine where to lead the packet, or decide if it is to be let through at all.

It's equally possible to direct network traffic based on packet contents, usually referred to as application level filtering, but this is not the kind of thing PF does. We will come back later to some cases where PF will hand off these kinds of tasks to other software, but first let us deal with some basics.

We've already mentioned the firewall concept. One important feature of PF and similar software, perhaps the most important feature, is that it is able to identify and block traffic which you do not want to let into your local network or let out to the world outside. At some point the term 'firewall' was coined.

NAT?

One other thing we will be talking about quit a lot are 'inner' and 'outer' addresses, 'routable' and 'non-routable' adresses. This is, at the heart of things, not directly related to firewalls or packet filtering, but due to the way the world works today, we will need to touch on it. It all comes from the time in the early 1990s when somebody started calculating the number of computers which would be connected to the Internet if the commercialization continued and the great unwashed masses of consumers were to connect at the same time.

At the time the Internet protocols were formulated, computers were usually big, expensive things which would normally serve a large number of simultaneous users, each at their own more or less dumb terminal. Under any circumstances, the only ones allowed to connect were universities and a number of companies with Pentagon contracts. Essentially 32 bit addresses of 4 octets would go an extremely long way. It would accommodate literally millions of machines, even.

Then Internet commercialization happened, and all of a sudden there were actually millions of small, inexpensive machines wanting to connect at the same time. This kind of development showed every sign of continuing and even accelerating. This meant that the smart people who had made the net work, needed to do another few pieces of work. They did a few things more or less at the same time. For one, they started working on a solution based on a larger address space - this has been dubbed IP version 6, or IPv6 for short - which uses 128 bit addresses. This has been designated as the long term solution. I thought I'd mention at this point that IPv6 support is built into OpenBSD by default, and PF has as far as I know always contained IPv6 support.

In addition, some sort of temporary solution was needed. Making the world move to addresses four times the size would take considerable time. The process is as far as we can see still pretty much in an early stage. They found a temporary solution, which consists of two parts. One part was a mechanism to offer the rest of the world 'white lies' by letting the network gateways rewrite packet addresses, the other was offered by designating some address ranges which had not been assigned earlier for use only in networks which would not communicate directly with the Internet at large. This would mean that several different machines at separate locations could have the same local IP address. But this would not matter because the address would be translated before the traffic was let out to the net at large.

If traffic with such "non routable" addresses were to hit the Internet at large, routers seeing the traffic would have a valid reason to refuse the packets to pass any further.

This is what is called "Network Address Translation", sometimes referred to as "IP masquerade" or similar. The two RFCs which define the whats and hows of this are dated 1994 and 1996 respectively [2].

There may be a number of reasons to use the so called "RFC 1918 addresses", but traditionally and historically the main reason has been that official addresses are either not available or practical.

PF today

At this point, we have covered a bit of background. Some years have passed since 2001, and PF in its present OpenBSD 3.8 form is a packet filter which is capable of doing quite a few things, if you want it to.

For one thing, PF classifies packets based on protocol, port, packet type, source or destination address. With a reasonable degree of certainty it is also able to classify packets based on source operating system.

And even if NAT is not a necessary part of a packet filter, for practical reasons it's nice that the address rewriting logic is handled somewhere nearby. Consequently, PF contains NAT logic as well.

PF is able - based on various combinations of protocol, port and other data - to direct traffic to other destinations than those designated by the sender, for example to a different machine or for further processing by a program such as a daemon listening at a port, locally or on a different machine.

Before PF was written, OpenBSD already contained the altq code to handle load balancing and traffic shaping. After a while, altq was integrated with PF. Mainly for practical reasons.

As a result of this, all those features are available to you via one single, essentially human readable configuration file, which is usually called pf.conf, stored in the /etc/ directory.

This is now available as a part of the base system on OpenBSD, on FreeBSD where PF from version 5.3 is one of three firewalling systems to be loaded at will, in NetBSD and DragonFlyBSD. The last two systems I have not had the resources to play much with myself. Something about having both hardware and time available at the same time. Anyway all indications are that only very minor details vary between these systems as far as PF is involved.

BSD vs Linux - Configuration

I assume some in the audience today are more familiar with configuring Linux or other systems than with BSD, so I'll briefly mention a few points about BSD configuration.

BSD network interfaces are not labeled eth0 and so on. The interfaces are assigned names which equal the driver name plus a sequence number, making 3Com cards using the xl driver appear as xl0, xl1, and so on, while Intel cards are likely to end up as em0, em1, SMC cards as sn0, and so on.

In general, the BSDs are organized to read the configuration from /etc/rc.conf, which is read by the /etc/rc script at startup. OpenBSD recommends using /etc/rc.conf.local for local customizations, since rc.conf contains the default values, while FreeBSD uses /etc/defaults/rc.conf to store the default settings, making /etc/rc.conf the correct place to make changes.

PF is configured by editing the /etc/pf.conf file and by using the pfctl command line tool. The pfctl application has a large number of options. We will take a closer look at some of them today.

In case you are wondering, there are web interfaces available for admin tasks, but they are not parts of the base system. The PF developers are not hostile to these things, but rather have not seen any graphical interface to PF configuration which without a doubt is preferable to pf.conf in a text editor, backed up with pfctl invocations and a few unix tricks.

Simplest possible setup (OpenBSD)

This brings us, finally, to the practical point of actually configuring PF in the simplest possible setup. We'll deal with a single machine which will communicate with a network which may very well be the Internet.

In order to start PF, as previously mentioned, you need to tell rc that you want the service to start. On OpenBSD, this is done in /etc/rc.conf.local, with the magical line

pf=YES                # enable PF

quite simply. In addition, you may if you like specify the file where PF will find its rules.

pf_rules=/etc/pf.conf # specify which file contains your rules

At the next startup, PF will be enabled. You can verify this by looking for the message PF enabled on the console. The /etc/pf.conf which comes out of a normal install of OpenBSD, FreeBSD or NetBSD contains a number of useful suggestions, but they're all commented out.

Then you really do not need to restart your machine in order to enable PF. You can do this just as easily by using pfctl. We really do not want to reboot for no good reason, so we type the command

peter@skapet:~$ sudo pfctl -e ; sudo pfctl -f /etc/pf.conf

which enables PF[3][4]. At this point we do not have a rule set, which means that PF does not actually do anything.

It is probably worth noting that if you reboot your machine at this point, the rc script on OpenBSD at least will enable a default rule set, which is in fact loaded before any of the network interfaces are enabled.

This default rule set is designed as a safety measure in case your gateway boots with an invalid configuration. It lets you log in and clean up whichever syntax error caused your rule set not to load. The default rule set allows a basic set of services: ssh from anyhere, basic name resolution and NFS mounts.

Some early versions of PF ports elsewhere appear to have neglected to bring the default rules with them.

Simplest possible setup (FreeBSD)

On FreeBSD you need a little more magic in your /etc/rc.conf, specifically according to the FreeBSD Handbook[5]

pf_enable="YES"                 # Enable PF (load module if required)
pf_rules="/etc/pf.conf"         # rules definition file for PF
pf_flags=""                     # additional flags for pfctl startup
pflog_enable="YES"              # start pflogd(8)
pflog_logfile="/var/log/pflog"  # where pflogd should store the logfile
pflog_flags=""                  # additional flags for pflogd startup

On FreeBSD, PF by default is compiled as a kernel loadable module. This means that you should be able to get started with $ sudo kldload pf, followed by $ sudo pfctl -e to enable PF. Assuming you have put these lines in your /etc/rc.conf, you can use the PF rc script to start PF:

$ sudo /etc/rc.d/pf start

Simplest possible setup (NetBSD)

On NetBSD 2.0 and newer PF is available as a loadable kernel module, installed via packages as pkgsrc/security/pflkm or compiled into a static kernel configuration. In NetBSD 3.0 onwards, PF is part of the base system.

If you want to enable PF in your kernel configuration (rather than loading the kernel module), you add these lines to your kernel configuration:

pseudo-device  pf		# PF packet filter
pseudo-device  pflog		# PF log interface

In /etc/rc.conf you need the lines

pf=YES 
pflogd=YES

to enable PF and the PF log interface, respectively.

If you installed the module, you load it with NetBSD$ sudo modload /usr/lkm/pf.o, followed by NetBSD$ sudo pfctl -e to enable PF. Alternatively, you can run the rc scripts, NetBSD$ sudo /etc/rc.d/pf start to enable PF and NetBSD$ sudo /etc/rc.d/pflogd start to enable the logging.

To load the module automatically at startup, add the following line to /etc/lkm.conf:

/usr/lkm/pf.o - - - - AFTERMOUNT

If it's still all correct at this point, you are ready to create your first PF rule set.

First rule set - single machine

This is the simplest possible setup, for a single machine which will not run any services, and which will talk to one network which may be the Internet. For now, we will use a /etc/pf.conf which looks like this:

block in all
pass out all keep state

that is, deny any incoming traffic, allow traffic we make ourselves, and retain state information on our connections. Keeping state information allows return traffic for all connections we have initiated to pass back to us. This is something you do if this is a machine you know you can trust. If you are ready to use the rule set, you load it with

$ sudo pfctl -e ; sudo pfctl -f /etc/pf.conf

Slightly stricter

For a slightly more structured and complete setup, we start by denying everything and then allowing only those things we know that we need[6]. This gives us the opportunity to introduce two of the features which make PF such a wonderful tool - lists and macros.

We'll make some changes to /etc/pf.conf, starting with

block all

Then we back up a little. Macros need to be defined before use:

tcp_services = "{ ssh, smtp, domain, www, pop3, auth, pop3s }"
udp_services = "{ domain }"

Now we've demonstrated several things at once - what macros look like, we've shown that macros may be lists, and that PF understands rules using port names equally well as it does port numbers. The names are the ones listed in /etc/services. This gives us something to put in our rules, which we edit slightly to look like this:

block all
pass out proto tcp to any port $tcp_services keep state
pass proto udp to any port $udp_services keep state

At this point some of us will point out that UDP is stateless, but PF actually manages to maintain state information despite this. When you ask a name server about a domain name, you probably want to receive its answer.

Since we've made changes to our pf.conf, we load the new rules:

peter@skapet:~$ sudo pfctl -f /etc/pf.conf

and the new rules apply. If there are no syntax errors, pfctl will not output any messages during the rule load. The -v flag will produce more verbose pfctl output.

If you have made extensive changes to your rule set, you may want to check the rules before attempting to load them. The command to do this is, pfctl -nf /etc/pf.conf. The -n option causes the rules to be interpreted only without loading the rules. This gives you an opportunity to correct any errors. Under any circumstances the last valid rule set loaded will be in force until you either disable PF or load a new rule set.

Statistics from pfctl

You may want to check that PF is actually running, and perhaps at the same time look at some statistics. pfctl offers a number of different types of information if you use pfctl -s, adding the type of information you want to display. The following example is taken from my home gateway while I was preparing the this lecture:

peter@skapet:~$ sudo pfctl -s info
Status: Enabled for 17 days 00:24:58          Debug: Urgent

Interface Stats for xl0               IPv4             IPv6
  Bytes In                      9257508558                0
  Bytes Out                      551145119              352
  Packets In
    Passed                         7004355                0
    Blocked                          18975                0
  Packets Out
    Passed                         5222502                3
    Blocked                             65                2

State Table                          Total             Rate
  current entries                       15               
  searches                        19620603           13.3/s
  inserts                           173104            0.1/s
  removals                          173089            0.1/s
Counters
  match                             196723            0.1/s
  bad-offset                             0            0.0/s
  fragment                              22            0.0/s
  short                                  0            0.0/s
  normalize                              0            0.0/s
  memory                                 0            0.0/s
  bad-timestamp                          0            0.0/s
  congestion                             0            0.0/s
  ip-option                             28            0.0/s
  proto-cksum                          325            0.0/s
  state-mismatch                       983            0.0/s
  state-insert                           0            0.0/s
  state-limit                            0            0.0/s
  src-limit                             26            0.0/s
  synproxy                               0            0.0/s

The first line here indicates that PF is enabled and has been running for for a little more than two weeks, which is equal to the time since I upgraded to what was then the latest 3.9-beta snapshot. pfctl -s all provides highly detailed information. Try it and have a look, and while there, look into some of the other pfctl options. man 8 pfctl gives you full information.

At this point you have a single machine which should be able to communicate rather well and in a reasonably secure fashion with other internet connected machines.

A few things are still missing. For example, you probably want to let at least some ICMP and UDP traffic through, if nothing else for your own troubleshooting needs.

And even though more modern and more secure options are available, you will probably be required to handle the ftp service.

We will return to these items shortly.

Simple gateway with NAT

At this point we finally move on to the more realistic or at least more common setups, where the machine with a firewall configured also acts as a gateway for at least one other machine. The other machines on the inside may of course also run firewall software, but even if they do, it does not affect what we are interested in here to any significant degree.

pass in on $int_if from $int_if:network to $ext_if:network \
     port $ports keep state

pass out on $ext_if from $int_if:network to $ext_if:network \
     port $ports keep state

pass from $int_if:network to any port $ports keep state

# sysctl net.inet.ip.forwarding=1

# sysctl net.inet6.ip6.forwarding=1

net.inet.ip.forwarding=1
net.inet6.ip6.forwarding=1

gateway_enable="YES" #for ipv4
ipv6_gateway_enable="YES" #for ipv6

ext_if = "xl0" # macro for external interface - use tun0 for PPPoE
int_if = "xl1"  # macro for internal interface
# ext_if IP address could be dynamic, hence ($ext_if)
nat on $ext_if from $int_if:network to any -> ($ext_if) 
block all
pass from { lo0, $int_if:network } to any keep state

client_out = "{ ftp-data, ftp, ssh, domain, pop3, auth, nntp, \
                https, 446, cvspserver, 2628, cvsup, 8000, 8080 }"

pass inet proto tcp from $int_if:network to any port $client_out \
     flags S/SA keep state

pass in inet proto tcp from any to any port ssh

pass in inet proto tcp from any to $ext_if port ssh

udp_services = "{ domain, ntp }"

pass quick inet proto { tcp, udp } to any port $udp_services keep state

That sad old FTP thing

The short list of real life TCP ports we looked at a few moments back contained, among other things, FTP. FTP is a sad old thing and a problem child, emphatically so for anyone trying to combine FTP and firewalls. FTP is an old and weird protocol, with a lot to not like. The most common points against it, are

• Passwords are transferred in the clear

• The protocol demands the use of at least two TCP connections (control and data) on separate ports

• When a session is established, data is communicated via ports selected at random

All of these points make for challenges security-wise, even before considering any potential weaknesses in client or server software which may lead to security issues. These things have tended to happen.

Under any circumstances, other more modern and more secure options for file transfer exist, such as sftp or scp, which feature both authentication and data transfer via encrypted connections. Competent IT professionals should have a preference for some other form of file transfer than FTP.

Regardless of our professionalism and preferences, we are all too aware that at times we will need to handle things we would prefer not to. In the case of FTP through firewalls, the main part of our handling consists of redirecting the traffic to a small program which is written specifically for this purpose.

---

FTP through NAT: ftp-proxy

	OpenBSD 3.8 or earlier only
	In November 2005, the old ftp-proxy (/usr/libexec/ftp-proxy) was replaced in OpenBSD-current with the new ftp-proxy, which lives in /usr/sbin. This is the software which is included in OpenBSD 3.9 onwards. See the Section called ftp-proxy, new style for details.

ftp-proxy is a part of the base system on OpenBSD and other systems which offer PF, and is usually called via the inetd "super server" via an appropriate /etc/inetd.conf entry.

The line quoted here specifies that ftp-proxy runs in NAT mode on the loopback interface, lo0:

127.0.0.1:8021 stream tcp nowait root /usr/libexec/ftp-proxy \
  ftp-proxy -n

This line is by default in your inetd.conf, commented out with a # character at the beginning of the line. To enable your change, you restart inetd.

On FreeBSD, NetBSD and other rcng based BSDs you do this with the command

FreeBSD$ sudo /etc/rc.d/inetd restart

or equivalent. Consult man 8 inetd if you are unsure.

The OpenBSD rc system is a tad more traditional, and the command you need is

OpenBSD$ sudo kill -HUP `cat /var/run/inetd.pid` 

At this point inetd is running, with your new settings.

Now for the actual redirection. Redirection rules and NAT rules fall into the same rule class. These rules may be referenced directly by other rules, and filtering rules may depend on these rules. Logically, rdr and nat rules need to be defined before the filtering rules.

We insert our rdr rule immediately after the nat rule in our /etc/pf.conf

rdr on $int_if proto tcp from any to any port ftp -> 127.0.0.1 \
         port 8021

In addition, the redirected traffic must be allowed to pass. We achive this with

pass in on $ext_if inet proto tcp from port ftp-data to ($ext_if) \
    user proxy flags S/SA keep state

Save pf.conf, then load the new rules with

$ sudo pfctl -f /etc/pf.conf

At this point you will probably have users noticing that FTP works before you get around to telling them what you've done.

This example assumes you are using NAT on a gateway with non routable addresses on the inside.

---

ftp-proxy, new style

	For OpenBSD 3.9 and newer
	If you are upgrading to OpenBSD 3.9 or working from a fresh 3.9 or newer install, this is the ftp-proxy version to use.

Just like its predecessor, the pftpx successor ftp-proxy configuration is mainly a matter of cut and paste from the man page.

If you are upgrading to the new ftp-proxy from an earlier version, you need remove the ftp-proxy line from your inetd.conf file and restart inetd.

Next, enable ftp-proxy by adding the following line to your /etc/rc.conf.local or /etc/rc.conf

ftpproxy_flags=""

You can start the proxy manually by running /usr/sbin/ftp-proxy if you like.

Moving on to the pf.conf file, you need two anchor definitions in the NAT section:

nat-anchor "ftp-proxy/*"
rdr-anchor "ftp-proxy/*"

Both are needed, even if your setup does not use NAT. If you are migrating from a previous version, your rule set probably contains the appropriate redirection already. If not, you add it:

rdr pass on $int_if proto tcp from any to any port ftp -> 127.0.0.1 \
         port 8021

Moving on down to the filtering rules, you add an anchor for the proxy to fill in,

anchor "ftp-proxy/*"

and finally a pass rule to let the packets pass from the proxy to the rest of the world

pass out proto tcp from $proxy to any port 21 keep state

where $proxy expands to the address the proxy daemon is bound to.

This example covers the simple setup with clients who need to contact FTP servers elsewhere. For other variations and more complicated setups, see the ftp-proxy man page.

Making your network troubleshooting friendly

Making your network troubleshooting friendly is a potentially large subject. At most times, the debugging or troubleshooting friendliness of your TCP/IP network depends on how you treat the Internet protocol which was designed specifically with debugging in mind, the Internet Control Message Protocol, or ICMP as it is usually abbreviated.

ICMP is the protocol for sending and receiving control messages between hosts and gateways, mainly to provide feedback to a sender about any unusual or difficult conditions en route to the target host.

There is a lot of ICMP traffic which usually just happens in the background while you are surfing the web, reading mail or transferring files. Routers (you are aware that you are building one, right?) use ICMP to negotiate packet sizes and other transmission parameters in a process often referred to as path MTU discovery.

You may have heard admins referring to ICMP as either 'just evil', or, if their understanding runs a little deeper, 'a necessary evil'. The reason for this attitude is purely historical. The reason can be found a few years back when it was discovered that several operating systems contained code in their networking stack which could make a machine running one of the affected systems crash and fall over, or in some cases just do really strange things, with a sufficiently large ICMP request.

One of the companies which was hit hard by this was Microsoft, and you can find rather a lot of material on the 'ping of death' bug by using your favourite search engine. This all happened in the second half of the 1990s, and all modern operating systems, at least the ones we can read, have thoroughly sanitized their network code since then. At least that's what we are lead to believe.

One of the early workarounds was to simply block either all ICMP traffic or at least ICMP ECHO, which is what ping uses. Now these rule sets have been around for roughly ten years, and the people who put them there are still scared.

pass inet proto icmp from any to any

pass inet proto icmp icmp-type $icmp_types from $int_if:network \
          to any keep state
pass inet proto icmp icmp-type $icmp_types from any to $ext_if \
          keep state

icmp_types = "echoreq"

pass inet proto icmp all icmp-type $icmp_types keep state

# allow out the default range for traceroute(8):
# "base+nhops*nqueries-1" (33434+64*3-1)
pass out on $ext_if inet proto udp from any to any \
             port 33433 >< 33626 keep state

icmp_types = "{ echoreq, unreach }"

pass inet proto icmp all icmp-type $icmp_types keep state

Hygiene: block-policy, scrub and antispoof

Our gateway does not feel quite complete without a few more items in the configuration which will make it behave a bit more sanely towards hosts on the wide net and our local network.

block-policy is an option which can be set in the options part of the ruleset, which precedes the redirection and filtering rules. This option determines which feedback, if any, PF will give to hosts which try to create connections which are subsequently blocked. The option has two possible values, drop which drops blocked packets with no feedback, and return which returns with status codes such as Connection refused or similar.

The correct strategy for block policies has been the subject of rather a lot of discussion. We choose to play nicely and instruct our firewall to issue returns:

set block-policy return

scrub is a keyword which enables network packet normalization, causing fragmented packets to be assembled and removing ambiguity. Enabling scrub provides a measure of protection against certain kinds of attacks based on incorrect handling of packet fragments. A number of supplementing options are available, but we choose the simplest form which is suitable for most configurations.

scrub in all

Some services, such as NFS, require some specific fragment handling options. This is extensively documented in the PF user guide and man pages provide all the information you could need.

antispoof is a common special case of filtering and blocking. This mechanism protects against activity from spoofed or forged IP addresses, mainly by blocking packets appearing on interfaces and in directions which are logically not possible. We specify that we want to weed out spoofed traffic coming in from the rest of the world and any spoofed packets which, however unlikely, were to originate in our own network:

antispoof for $ext_if
antispoof for $int_if

This completes our simple NATing firewall for a small local network.

A web server and a mail server on the inside

Time passes, and needs change. Rather frequently, a need to run externally accessible services develops. This quite frequently becomes just a little harder because externally visible addresses are either not available or too expensive, and running several other services on a machine which is primarily a firewall is not a desirable option.

The redirection mechanisms in PF makes it relatively easy to keep servers on the inside. If we assume that we need to run a web server which serves up data in clear text (http) and encrypted (https) and in addition we want a mail server which sends and receives e-mail while letting clients inside and outside the local network use a number of well known submission and retrieval protocols, the following lines may be all that's needed in addition to the rule set we developed earlier:

webserver = "192.168.2.7"
webports = "{ http, https }"
emailserver = "192.168.2.5"
email = "{ smtp, pop3, imap, imap3, imaps, pop3s }"

rdr on $ext_if proto tcp from any to $ext_if port \
       $webports -> $webserver
rdr on $ext_if proto tcp from any to $ext_if port \
       $email -> $emailserver

pass proto tcp from any to $webserver port $webports \
   flags S/SA synproxy state
pass proto tcp from any to $emailserver port $email \
   flags S/SA synproxy state
pass proto tcp from $emailserver to any port smtp \
   flags S/SA synproxy state

Notice the flag 'synproxy' in the new rules. This means that PF will handle the connection setup (three way handshake) on behalf of your server or client before handing the connection over to the application. This provides a certain amount of protection against certain types of attacks.

Rule sets for configurations with DMZ networks isolated behind separate network interfaces and in some cases services running on alternative ports will not necessarily be much different from this one.

rdr on $int_if proto tcp from $int_if:network to $ext_if \
       port $webports -> $webserver
rdr on $int_if proto tcp from $int_if:network to $ext_if \
       port $email -> $emailserver
no nat on $int_if proto tcp from $int_if:network to $int_if:network
nat on $int_if proto tcp from $int_if:network to $webserver \
       port $webports -> $int_if 
nat on $int_if proto tcp from $int_if:network to $emailserver \
       port $email -> $int_if 

Tables make your life easier

By this time you may be thinking that this gets awfully static and rigid. There will after all be some kinds of data which are relevant to filtering and redirection at a given time, but do not deserve to be put into a configuration file! Quite right, and PF offers mechanisms for handling these situations as well. Tables are one such feature, mainly useful as lists which can be manipulated without needing to reload the entire rule set, and where fast lookups are desirable. Table names are always enclosed in < >, like this:

table <clients> { 192.168.2.0/24, !192.168.2.5 }

here, the network 192.168.2.0/24 is part of the table, except the address 192.168.2.5, which is excluded using the ! operator (logical NOT). It is also possible to load tables from files where each item is on a separate line, such as the file /etc/clients

192.168.2.0/24
!192.168.2.5

which in turn is used to initialize the table in /etc/pf.conf:

table <clients> persist file /etc/clients

Then, for example, you can change one of our earlier rules to read

pass inet proto tcp from <clients> to any port $client_out \
         flags S/SA keep state

to manage outgoing traffic from your client computers. With this in hand, you can manipulate the table's contents live, such as

$ sudo pfctl -t clients -T add 192.168.1/16

Note that this changes the in-memory copy of the table only, meaning that the change will not survive a power failure or other reboot unless you arrange to store your changes.

You might opt to maintain the on-disk copy of the table using a cron job which dumps the table content to disk at regular intervals, using a command such as pfctl -t clients -T show >/etc/clients. Alternatively, you could edit the /etc/clients file and replace the in-memory table contents with the file data:

$ sudo pfctl -t clients -T replace -f /etc/clients

For operations you will be performing frequently, you will sooner or later end up writing shell scripts for tasks such as inserting or removing items or replacing table contents. The only real limitations lie in your own needs and your creativity.

We will be returning to other handy uses of tables shortly.

Logging

Up to now we have not mentioned much about logging. PF provides the opportunity to log exactly what you want by adding the 'log' keyword to the rules you want logged. You may want to limit the amount of data a bit by specifying one interface where the logging is to be done. You do this by adding

set loginterface $ext_if

and then editing the rules you want to log, such as

pass out log from <client> to any port $email \
         label client-email keep state

This causes the traffic to be logged in a binary format which is really only intended to be used as tcpdump input.

The label part creates a new set of counters for various statistics for the rule. This can be quite convenient if you are invoicing others for bandwidth use, for example.

peter@skapet:~$ sudo tcpdump -n -e -ttt -i pflog0
tcpdump: WARNING: pflog0: no IPv4 address assigned
tcpdump: listening on pflog0, link-type PFLOG
Feb 16 16:43:20.152187 rule 0/(match) block in on xl0: 194.54.59.189.2559 > 
194.54.107.19.139: [|tcp] (DF)
Feb 16 16:48:26.073244 rule 27/(match) pass in on xl0: 61.213.167.236 > 
194.54.107.19: icmp: echo request
Feb 16 16:49:09.563448 rule 0/(match) block in on xl0: 61.152.249.148.80 > 
194.54.107.19.55609: [|tcp]
Feb 16 16:49:14.601022 rule 0/(match) block in on xl0: 194.54.59.189.3056 > 
194.54.107.19.139: [|tcp] (DF)
Feb 16 16:53:10.110110 rule 0/(match) block in on xl0: 68.194.177.173 > 
194.54.107.19: [|icmp]
Feb 16 16:55:54.818549 rule 27/(match) pass in on xl0: 61.213.167.237 > 
194.54.107.19: icmp: echo request
Feb 16 16:57:55.577782 rule 27/(match) pass in on xl0: 202.43.202.16 > 
194.54.107.19: icmp: echo request
Feb 16 17:01:27.108404 rule 0/(match) block in on xl0: 194.54.59.189.4520 > 
194.54.107.19.139: [|tcp] (DF)
Feb 16 17:11:02.137310 rule 0/(match) block in on xl0: 222.73.4.154.80 > 
194.54.107.19.55609: [|tcp]
Feb 16 17:14:05.739403 rule 0/(match) block in on xl0: 194.54.174.246.3970 > 
194.54.107.19.135: [|tcp] (DF)
Feb 16 17:14:08.715163 rule 0/(match) block in on xl0: 194.54.174.246.3970 > 
194.54.107.19.135: [|tcp] (DF)
Feb 16 17:14:09.308355 rule 0/(match) block in on xl0: 194.54.174.246.3970 > 
194.54.107.19.135: [|tcp] (DF)
Feb 16 17:19:01.853730 rule 27/(match) pass in on xl0: 203.84.214.5 > 
194.54.107.19: icmp: echo request

block log all

Keeping an eye on things with pftop

If you are interested in keeping an eye on what passes in to and out of your network, Can Erkin Acar's pftop is a very useful tool. The name is a strong hint at what it does - pftop shows a running snapshot of your traffic in a format which is strongly inspired by top(1):

pfTop: Up State 1-21/67, View: default, Order: none, Cache: 10000       19:52:28

PR   DIR SRC                  DEST                 STATE   AGE   EXP  PKTS BYTES
tcp  Out 194.54.103.89:3847   216.193.211.2:25      9:9     28    67    29  3608
tcp  In  207.182.140.5:44870  127.0.0.1:8025        4:4     15 86400    30  1594
tcp  In  207.182.140.5:36469  127.0.0.1:8025       10:10   418    75   810 44675
tcp  In  194.54.107.19:51593  194.54.103.65:22      4:4    146 86395   158 37326
tcp  In  194.54.107.19:64926  194.54.103.65:22      4:4    193 86243   131 21186
tcp  In  194.54.103.76:3010   64.136.25.171:80      9:9    154    59    11  1570
tcp  In  194.54.103.76:3013   64.136.25.171:80      4:4      4 86397     6  1370
tcp  In  194.54.103.66:3847   216.193.211.2:25      9:9     28    67    29  3608
tcp  Out 194.54.103.76:3009   64.136.25.171:80      9:9    214     0     9  1490
tcp  Out 194.54.103.76:3010   64.136.25.171:80      4:4     64 86337     7  1410
udp  Out 194.54.107.18:41423  194.54.96.9:53        2:1     36     0     2   235
udp  In  194.54.107.19:58732  194.54.103.66:53      1:2     36     0     2   219
udp  In  194.54.107.19:54402  194.54.103.66:53      1:2     36     0     2   255
udp  In  194.54.107.19:54681  194.54.103.66:53      1:2     36     0     2   271

Your connections can be shown sorted by a number of different criteria, among others by PF rule, volume, age and so on.

This program is not in the base system itself, but is in ports on OpenBSD and FreeBSD both as /usr/ports/sysutils/pftop, on NetBSD via pkgsrc as sysutils/pftop.

Invisible gateway - bridge

A bridge in our context is a machine with two or more network interfaces, located in between the Internet and one or more internal networks, and the network interfaces are not assigned IP addresses. If the machine in question runs OpenBSD or a similarly capable operating system, it is still able to filter and redirect traffic. The advantage of such a setup is that attacking the firewall itself is more difficult. The disadvantage is that all admin tasks must be performed at the firewall's console, unless you configure a network interface which is reachable via a secured network of some kind, or even a serial console.

The exact method for configuring bridges differs in some details between the operating systems. Below is a short recipe for use on OpenBSD, which for good measure blocks all non-Internet protocol traffic. Setting up a bridge with two interfaces:

/etc/hostname.xl0

up

/etc/hostname.xl1

up

/etc/bridgename.bridge0

  add xl0 add xl1 blocknonip xl0 blocknonip xl1 up

/etc/pf.conf

ext_if = xl0
int_if  = xl1
interesting-traffic = { ... }
block all
pass quick on $ext_if all
pass log on $int_if from $int_if to any port $interesting-traffic \
     keep state

Significantly more complicated setups are possible. Experienced bridgers recommend picking one of the interfaces to perform all filtering and redirection. All packets pass through PF's view twice, making for potentially extremely complicated rules.

In addition, the OpenBSD brconfig command offers its own set of filtering options in addition to other configuration options. The bridge(4) and brconfig(8) man pages offer further information.

FreeBSD uses a slightly different set of commands to configure bridges, while the NetBSD PF implementation does not support bridging.

Directing traffic with ALTQ

ALTQ - short for ALTernate Queueing - is a very flexible mechanism for directing network traffic which lived a life of its own before getting integrated into PF. Altq was another one of those things which were integrated into PF because of the additional convenience it offered when integrated.

Altq uses the term queue about the main traffic control mechanisms. Queues are defined with a defined amount of bandwidth or a specific part of available bandwidth, where a queue can be assigned subqueues of various types.

To complete the picture, you write filtering rules which assign packets to specified queues or a selection of subqueues where packets pass according to specified criteria.

Queues are created with one of several queue disciplines. The default queue discipline without ALTQ is FIFO (first, in first out). A slightly more interesting discipline is the class based discipline (CBQ), which in practical terms means you define the queue's bandwidth as a set amount of data per second, as a percentage or in units of kilobits, megabits and so on, with an additional priority as an option, or priority based (priq), where you assign priority only. Priorities can be set at 0 to 7 for cbq queues, 0 to 15 for priq queues, with a higher value assigning a higher priority and preferential treatment. In addtion, the hierarcical queue algoritm "Hierarchical Fair Service Curve" or HFSC is available. Briefly, a simplified syntax is

altq on interface type [options ... ] main_queue { sub_q1, sub_q2 ..}
  queue sub_q1 [ options ... ]
  queue sub_q2 [ options ... ]
[...]
pass [ ... ] queue sub_q1
pass [ ... ] queue sub_q2

If you will be using these features in you own rule sets, you should under any circumstances read the pf.conf man page and the PF user guide. These documents offer a very detailed and reasonably well laid out explanation of the syntax and options.[10] [11]

altq on $ext_if cbq bandwidth 640Kb queue { def, ftp, udp, http, \
         ssh, icmp }
queue def bandwidth 18% cbq(default borrow red)
queue ftp bandwidth 10% cbq(borrow red)
queue udp bandwidth 30% cbq(borrow red)
queue http bandwidth 20% cbq(borrow red)
queue ssh bandwidth 20% cbq(borrow red) { ssh_interactive, ssh_bulk }
queue ssh_interactive priority 7
queue ssh_bulk priority 0
queue icmp bandwidth 2% cbq

pass log quick on $ext_if proto tcp from any to any port 22 flags S/SA \ 
    keep state queue (ssh_bulk, ssh_interactive)
pass in quick on $ext_if proto tcp from any to any port 20 flags S/SA  \
    keep state queue ftp
pass in quick on $ext_if proto tcp from any to any port 80 flags S/SA \
    keep state queue http
pass out on $ext_if proto udp all keep state queue udp
pass out on $ext_if proto icmp all keep state queue icmp

ext_if="kue0"

altq on $ext_if priq bandwidth 100Kb queue { q_pri, q_def }
queue q_pri priority 7
queue q_def priority 1 priq(default)

pass out on $ext_if proto tcp from $ext_if to any flags S/SA \
        keep state queue (q_def, q_pri)

pass in  on $ext_if proto tcp from any to $ext_if flags S/SA \
        keep state queue (q_def, q_pri)

altq on $ext_if cbq queue { q_default q_web q_mail }

   queue q_default cbq(default)
   queue q_web (...) 

    ## all mail limited to 1Mb/sec
    queue q_mail bandwidth 1Mb { q_mail_windows }
    ## windows mail limited to 56Kb/sec
    queue q_mail_windows bandwidth 56Kb

    pass in quick proto tcp from any os "Windows" to $ext_if port 25 \
            keep state queue q_mail_windows
    pass in quick proto tcp from any to $ext_if port 25 label "smtp" \
            keep state queue q_mail

" I can't believe I didn't see this earlier. Oh, how sweet. ... 
  Already a huge difference in my load. Bwa ha ha. "

CARP and pfsync

CARP and pfsync were two of the main new items in OpenBSD 3.5. CARP is short for Common Address Redundancy Protocol. The protocol was developed as a non patent encumbered alternative to VRRP (Virtual Router Redundancy Protocol, RFC 2281, RFC 3768), which was quite far along the track to becoming an IETF sanctioned standard, even though possible patent issued has not been resolved. The patents involved are held by Cisco, IBM and Nokia.

Both protocols are intended to ensure redundancy for essential network features, with automatic failover.

CARP is based on setting up a group of machines as one 'master' and one of more redundant 'slaves', all of which are equipped to handle a common IP address. If the master goes down, one of the slaves will inherit the IP address, and if the synchronization has been properly handled, active connections will be handed over. The handover may be authenticated using cryptographic keys.

One of the main purposes of CARP is to ensure that the network will keep functioning as usual even when a firewall or other service goes down due to errors or planned maintenance activities such as upgrades.

In the case of PF firewalls, pfsync can handle the synchronization. pfsync is a type of virtual network interface specially designed to synchronize state information between PF firewalls. pfsync interfaces are assigned to physical interfaces with ifconfig. On networks where uptime requirements are strict enough to dictate automatic failover, the number of simultaneous network connections is likely to be large enough that it will make sense to assign the pfsync network its own physical network.

I hope to explore these exciting and advanced features in a future advanced section of the tutorial. At this point, the best pfsync and CARP references are the OpenBSD FAQ, the man pages and Ryan McBride's overview article at http://www.countersiege.com/doc/pfsync-carp/.

Wireless networks made simple

It rather tempting to say that on BSD, and on OpenBSD in particular, there's no need to 'make wireless networking simple', because it already is. Getting a wireless network running is basically not very different from getting a wired one up and running, but then of course there are some issues which turn up simply because we are dealing with radio waves and not wires. We will look briefly at some of the issues before moving on to the practical setup issues.

ath0 at pci1 dev 4 function 0 "Atheros AR5212" rev 0x01: irq 11
ath0: AR5212 5.6 phy 4.1 rf5111 1.7 rf2111 2.3, ETSI1W, address 
00:0d:88:c8:a7:c4

up media autoselect mediaopt hostap mode 11b chan 6 nwid unwiredbsd \
 nwkey 0x1deadbeef9
inet 10.168.103.1

ifconfig_ath0="up media autoselect mediaopt hostap mode 11b chan 6 \
  nwid unwiredbsd nwkey 0x1deadbeef9"
ifconfig_ath0="inet 10.168.103.1"

up media autoselect mode 11b chan 6 nwid unwiredbsd nwkey 0x1deadbeef9
dhcp

ifconfig_ath0="up media autoselect mode 11b chan 6 nwid unwiredbsd \
  nwkey 0x1deadbeef9"
ifconfig_ath0="DHCP"

air_if = "ath0"

nat on $ext_if from $air_if:network to any -> ($ext_if) static-port

An open, yet tightly guarded wireless network with authpf

As always, there are other ways to configure the security of your wireless network than the one we have just seen. What little protection WEP encryption offers, security professionals tend to agree is barely enough to signal to an attacker that you do not intend to let all and sundry use your network resources.

A different approach appeared one day in my mail as a message from my friend Vegard Engen, who told me he had been setting up authpf. authpf is a user shell which lets you load PF rules on a per user basis, effectively deciding which user gets to do what. With a reasonable setup, only traffic originated by authenticated users will be let through. Vegard's annotated config follows below. His wireless network is configured without WEP encryption, preferring to handle the security side of things via PF and authpf:

Start with creating an empty /etc/authpf/authpf.conf. It needs to be there for authpf to work, but doesn't actually need any content.

The other relevant bits of /etc/pf.conf follow. First, interface macros:

int_if="sis1"
ext_if="sis0"
wi_if = "wi0"

The use of this address will become apparent later:

auth_web="192.168.27.20"

The traditional authpf table

table <authpf_users> persist 

We could put the NAT part in authpf.rules, but keeping it in the main pf.conf doesn't hurt:

nat on $ext_if from $wi_if:network to any -> ($ext_if)

Redirects to let traffic reach servers on the internal net. These could be put in authpf.rules too, but since they do not actually provide access without pass rules, keeping them here won't hurt anything.

rdr on $wi_if proto tcp from any to $myaddr port $tcp_in -> $server
rdr on $wi_if proto udp from any to $myaddr port $udp_in -> $server

The next redirect sends all web traffic from non authenticated users to port 80 on $auth_web. In Vegard's setup, this is a web server which displays contact info for people who stumble onto the wireless net. In a commercial setting, this would be where you would put something which could handle credit cards and create users.

rdr on $wi_if proto tcp from ! <authpf_users> to any \
 port 80 -> $auth_web

To activate nat, binat or redirects in authpf

nat-anchor "authpf/*"
binat-anchor "authpf/*"
rdr-anchor "authpf/*"

On to the filtering rules, we start with a sensible default

block all

Other global, user independent rules would go here. Next for the authpf anchor, we make sure non-authenticated users connecting to the wireless interface get redirected to $auth_web

anchor "authpf/*" in on wi0

pass in on $wi_if inet proto tcp from any to $auth_web \
 port 80 keep state

There are three things we want anyway on the wireless interface: Name service (DNS), DHCP and SSH in to the gateway. Three rules do the trick

pass in on $wi_if inet proto udp from any port 53 keep state

pass in on $wi_if inet proto udp from any to $wi_if port 67

pass in on $wi_if inet proto tcp from any to $wi_if \
 port 22 keep state

Next up, the we define the rules which get loaded for all users who log in with their shell set to /usr/sbin/authpf. These rules go in /etc/authpf/authpf.rules,

int_if = "sis1"
ext_if = "sis0"
wi_if = "wi0"
server = "192.168.27.15"
myaddr = "213.187.n.m"

# Services which live on the internal network 
# and need to be accessible
tcp_services = "{ 22, 25, 53, 80, 110, 113, 995 }"
udp_services = "{ 53 }"
tcp_in = " { 22, 25, 53, 80, 993, 2317, pop3}"
udp_in = "{ 53 }"

# Pass traffic to elsewhere, that is the outside world
pass in on $wi_if inet from <authpf_users> to ! $int_if:network \
   keep state

# Let authenticated users use services on 
# the internal network.

pass in on $wi_if inet proto tcp from <authpf_users> to $server \
  port $tcp_in keep state
pass in on $wi_if inet proto udp from <authpf_users> to $server \
  port $udp_in keep state

# Also pass to external address. This means you can access 
# internal services on external addesses.

pass in on $wi_if inet proto tcp from <authpf_users> to $myaddr \
    port $tcp_in keep state
pass in on $wi_if inet proto udp from <authpf_users> to $myaddr \
    port $udp_in keep state

At this point we have an open net where anybody can connect and get an IP address from DHCP. All HTTP requests get redirected to port 80 on 192.168.27.20, which is a server on the internal net where all requests are answered with the same page, which displays contact info in case you want to be registered and be allowed to use the net.

You are allowed to ssh in to the gateway. Users with valid user IDs and passwords get rule sets with appropriate pass rules loaded for their assigned IP address.

We can fine tune this even more by making user specific rules in /etc/authpf/users/$user/authpf.rules. Per user rules can use the $user_ip macro for the user's IP address. For example, if I want to give myself unlimited access, create the following /etc/authpf/users/vegard/authpf.rules:

wi_if="wi0"
pass in on $wi_if from $user_ip to any keep state

Turning away the brutes

If you run a Secure Shell login service anywhere which is accessible from the Internet, I'm sure you've seen things like these in your authentication logs:

Sep 26 03:12:34 skapet sshd[25771]: Failed password for root from 
200.72.41.31 port 40992 ssh2
Sep 26 03:12:34 skapet sshd[5279]: Failed password for root from 
200.72.41.31 port 40992 ssh2
Sep 26 03:12:35 skapet sshd[5279]: Received disconnect from 
200.72.41.31: 11: Bye Bye
Sep 26 03:12:44 skapet sshd[29635]: Invalid user admin from 
200.72.41.31
Sep 26 03:12:44 skapet sshd[24703]: input_userauth_request: 
invalid user admin
Sep 26 03:12:44 skapet sshd[24703]: Failed password for invalid user 
admin from 200.72.41.31 port 41484 ssh2
Sep 26 03:12:44 skapet sshd[29635]: Failed password for invalid user 
admin from 200.72.41.31 port 41484 ssh2
Sep 26 03:12:45 skapet sshd[24703]: Connection closed by 200.72.41.31
Sep 26 03:13:10 skapet sshd[11459]: Failed password for root from 
200.72.41.31 port 43344 ssh2
Sep 26 03:13:10 skapet sshd[7635]: Failed password for root from 
200.72.41.31 port 43344 ssh2
Sep 26 03:13:10 skapet sshd[11459]: Received disconnect from 
200.72.41.31: 11: Bye Bye
Sep 26 03:13:15 skapet sshd[31357]: Invalid user admin from 200.72.41.31
Sep 26 03:13:15 skapet sshd[10543]: input_userauth_request: invalid 
user admin
Sep 26 03:13:15 skapet sshd[10543]: Failed password for invalid user 
admin from 200.72.41.31 port 43811 ssh2
Sep 26 03:13:15 skapet sshd[31357]: Failed password for invalid user 
admin from 200.72.41.31 port 43811 ssh2
Sep 26 03:13:15 skapet sshd[10543]: Received disconnect from 
200.72.41.31: 11: Bye Bye
Sep 26 03:13:25 skapet sshd[6526]: Connection closed by 200.72.41.31

It gets repetetive after that. This is what a brute force attack looks like. Essentially somebody, or more likely, a cracked computer somewhere, is trying by brute force to find a combination of user name and password which will let them into your system.

The simplest response would be to write a pf.conf rule which blocks all access. This leads to another class of problems, including what you do in order to let people with legitimate business on your system access it anyway. You might consider moving the service to some other port, but then again, the ones flooding you on port 22 would probably be able to scan their way to port 22222 for a repeat performance.

Since OpenBSD 3.7, PF has offered a slightly more elegant solution. You can write your pass rules so they maintain certain limits on what connecting hosts can do. For good measure, you can banish violators to a table of addresses which which you deny some or all access. You can even choose to drop all existing connections from machines which overreach your limits, if you like. Here's how it's done:

Now first set up the table. In your tables section, add

table <bruteforce> persist

Then somewhere fairly early in your rule set you set up to block from the bruteforcers

block quick from <bruteforce>

And finally, your pass rule.

pass inet proto tcp from any to $int_if:network port $tcp_services \
        flags S/SA keep state \
	(max-src-conn 100, max-src-conn-rate 15/5, \
         overload <bruteforce> flush global)

This is rather similar to what we've seen before, isn't it? In fact, the first part is identical to the one we constructed earlier. The part in brackets is the new stuff which will ease your network load even further.

max-src-conn is the number of simultaneous connections you allow from one host. In this example, I've set it at 100, in your setup you may want a slightly higher or lower value.

max-src-conn-rate is the rate of new connections allowed from any single host, here 15 connections per 5 seconds. Again, you are the one to judge what suits your setup.

overload <bruteforce> means that any host which exceeds these limits gets its address added to the table bruteforce. Our rule set blocks all traffic from addresses in the bruteforce table.

finally, flush global says that when a host reaches the limit, that host's connections will be terminated (flushed). The global part says that for good measure, this applies to connections which match other pass rules too.

The effect is dramatic. My bruteforcers more often than not end up with "Fatal: timeout before authentication" messages, which is exactly what we want.

Once again, please keep in mind that this example rule is intended mainly as an illustration. It is not unlikely that your network's needs are better served by rather different rules or combinations of rules.

If, for example, you want to allow a generous number of connections in general, but would like to be a little more tight fisted when it comes to ssh, you could supplement the rule above with something like one early on in your rule set:

pass quick proto { tcp, udp } from any to any port ssh \
        flags S/SA keep state \
        (max-src-conn 15, max-src-conn-rate 5/3, \
        overload <bruteforce> flush global)

You should be able to find the set of parameters which is just right for your situation by reading the relevant man pages and the PF User Guide, and perhaps a bit of experimentation.

Giving spammers a hard time

At this point we've covered quite some ground, and I'm more than happy to present something really useful: PF as a means to make spammers' lives harder. Based on our recent exposure to PF rulesets, understanding the following /etc/pf.conf parts should be straighforward:

table <spamd> persist
table <spamd-white> persist file "/var/mail/whitelist.txt"
rdr pass on $ext_if inet proto tcp from <spamd> to \
         { $ext_if, $int_if:network } port smtp -> 127.0.0.1 port 8025
rdr pass on $ext_if inet proto tcp from !<spamd-white> to \
         { $ext_if, $int_if:network } port smtp -> 127.0.0.1 port 8025

We have two tables, and one of them gets filled up from a file somewhere in the file system. SMTP traffic from the addresses in the first table plus the ones which are not in the other table are redirected to a daemon listening at port 8025.

Oct  2 19:55:05 delilah spamd[26905]: (GREY) 83.23.213.115: 
<gilbert@keyholes.net> -> <wkitp98zpu.fsf@datadok.no>
Oct  2 19:55:05 delilah spamd[26905]: 83.23.213.115: disconnected after 
0 seconds.
Oct  2 19:55:05 delilah spamd[26905]: 83.23.213.115: connected (2/1)
Oct  2 19:55:06 delilah spamd[26905]: (GREY) 83.23.213.115: 
<gilbert@keyholes.net> -> <wkitp98zpu.fsf@datadok.no>
Oct  2 19:55:06 delilah spamd[26905]: 83.23.213.115: disconnected after 
1 seconds.
Oct  2 19:57:07 delilah spamd[26905]: (BLACK) 65.210.185.131:
 <bounce-3C7E40A4B3@branch15.summer-bargainz.com> -> <adm@dataped.no>
Oct  2 19:58:50 delilah spamd[26905]: 65.210.185.131: From: Auto 
lnsurance Savings <noreply@branch15.summer-bargainz.com>
Oct  2 19:58:50 delilah spamd[26905]: 65.210.185.131: Subject: Start 
SAVlNG M0NEY on Auto lnsurance
Oct  2 19:58:50 delilah spamd[26905]: 65.210.185.131: To: adm@dataped.no
Oct  2 20:00:05 delilah spamd[26905]: 65.210.185.131: disconnected after 
404 seconds. lists: spews1
Oct  2 20:03:48 delilah spamd[26905]: 222.240.6.118: connected (1/0)
Oct  2 20:03:48 delilah spamd[26905]: 222.240.6.118: disconnected after 
0 seconds.
Oct  2 20:06:51 delilah spamd[26905]: 24.71.110.10: connected (1/1), 
lists: spews1
Oct  2 20:07:00 delilah spamd[26905]: 221.196.37.249: connected (2/1)
Oct  2 20:07:00 delilah spamd[26905]: 221.196.37.249: disconnected after 
0 seconds.
Oct  2 20:07:12 delilah spamd[26905]: 24.71.110.10: disconnected after 
21 seconds. lists: spews1

Dec 11 23:57:24 delilah spamd[32048]: 69.6.40.26: connected (1/1), 
lists: spamhaus spews1 spews2
Dec 12 00:30:08 delilah spamd[32048]: 69.6.40.26: disconnected 
after 1964 seconds. lists: spamhaus spews1 spews2

peter@delilah:~$ grep disconnected /var/log/spamd | awk '{print $9}' \
| sort -rn | uniq -c | head
   1 36099
   1 14714
   1 5495
   1 2193
   1 1964
   1 1872
   1 1718
   1 1677
   1 1617
   1 1594

Aug 26 10:10:17 delilah spamd[26905]: 146.151.48.74: connected (1/0)
Aug 26 20:11:56 delilah spamd[26905]: 146.151.48.74: disconnected 
after 36099 seconds.

PF - Haiku

Finally, an indication of the level of feeling inspired by PF in its users is in order. On the PF mailing list, a message with the subject of "Things pf can't do?" appeared in May 2004. The message had been written by someone who did not have a lot of firewalls experience, and who consequently found it hard to get the setup he or she wanted.

This, of course, lead to some discussion, with several participants saying that if PF was hard on a newbie, the alternatives were certainly not a bit better. The thread ended in the following haiku of praise from Jason Dixon, which is given intact as it came, along with Jason's comments:

Compared to working with iptables, PF is like this haiku:

A breath of fresh air,
floating on white rose petals,
eating strawberries.

Now I'm getting carried away:

Hartmeier codes now,
Henning knows not why it fails,
fails only for n00b.

Tables load my lists,
tarpit for the asshole spammer,
death to his mail store.

CARP due to Cisco,
redundant blessed packets,
licensed free for me.

Jason Dixon, on the PF email list, May 20th, 2004 (http://www.benzedrine.cx/pf/msg04702.html)

References

OpenBSDs web http://www.openbsd.org/

OpenBSDs FAQ, http://www.openbsd.org/faq/index.html

PF User Guide http://www.openbsd.org/faq/pf/index.html

Daniel Hartmeier's PF pages, http://www.benzedrine.cx/pf.html

Daniel Hartmeier: Design and Performance of the OpenBSD Stateful Packet Filter (pf), http://www.benzedrine.cx/pf-paper.html (presented at Usenix 2002)

Nate Underwood: HOWTO: Transparent Packet Filtering with OpenBSD, http://ezine.daemonnews.org/200207/transpfobsd.html

Randal L. Schwartz: Monitoring Net Traffic with OpenBSD's Packet Filter, http://www.samag.com/documents/s=9053/sam0403j/0403j.htm

Unix.se: Brandvägg med OpenBSD, http://unix.se/Brandv%E4gg_med_OpenBSD

Randal L. Schwartz: Blog for Thu, Jan 29, 2004, http://use.perl.org/~merlyn/journal/17094

RFC 1631, "The IP Network Address Translator (NAT)", May 1994 http://www.ietf.org/rfc/rfc1631.txt?number=1631

RFC 1918, "Address Allocation for Private Internets", February 1996 http://www.ietf.org/rfc/rfc1918.txt?number=1918

The FreeBSD PF home page, http://pf4freebsd.love2party.net/

Peter Postma's PF on NetBSD pages, http://nedbsd.nl/~ppostma/pf/

Marcus Ranum: The Six Dumbest Ideas in Computer Security, September 1, 2005

Kjell Jørgen Hole WiFi courseware, http://www.kjhole.com/Standards/WiFi/WiFiDownloads.html, also see wifinetnews.com; also The Unofficial 802.11 Security Web Page comes higly recommended.

Greylisting.org greylisting.org

Evan Harris: The Next Step in the Spam Control War: Greylisting (the original greylisting paper)

Where to find the tutorial on the web

Work in progress edition:

Several formats http://www.bgnett.no/~peter/pf/

This is where updated versions will appear more or less in the natural course of tinkering, in between events. Please let me know if you want to be told of future updates.