pfctl -t mytable -T replace -f pf.mytable
This guide is written for the person very new to firewalling. Please realize that the sample firewall we build should not be considered appropriate for actual use. I just try to cover a few basics, that took me awhile to grasp from the better known (and more detailed) documentation referenced below
It's my hope that this guide will not only get you started, but give you enough of a grasp of using pf so that you will then be able to go to those more advanced guides and perfect your firewalling skills.
The pf packet filter was developed for OpenBSD but is now included in FreeBSD, which is where I've used it. Having it run at boot and the like is covered in the various documents, however I'll quickly run through the steps for FreeBSD.
First one adds to /etc/rc.conf
pf_enable="YES" pf_rules="/etc/pf.conf" pflog_enable="YES" pflog_logfile="/var/log/pflog" |
This will start the pf filter at boot. We'll cover starting it while running after we put in some rules.
sleep 120; pfctl -d |
which will disable pf after 2 minutes. Another, slightly more involved solution is to add a quick cronjob to turn off pf, so that if you do make this mistake, you can get back into the box shortly. Don't do this now, wait till we've made some rules, but let's make a few preparations. Make a quick change to /usr/local/etc/sudoers (in FreeBSD, it may be elsewhere on other operating systems.)
visudo -f /usr/local/etc/sudoers |
If your user name is john, then add a line (below any other lines giving you less privileges--sudo processes the sudoers file from top to bottom)
john ALL= NOPASSWD: /sbin/pfctl |
Now we edit the crontab.
crontab -e |
Then, in your crontab add
*/2 * * * * /usr/local/bin/sudo /sbin/pfctl -d |
I dislike the NOPASSWD option, as it makes it easier to make serious mistakes--however, when I'm testing a bunch of pf rules, I usually put it in there as I'm constantly using pfctl. When done, and the ruleset works, I remove the NOPASSWD part.
Ok, now that we've ensured we'll be able to get back in the box, let's create an /etc/pf.conf file. In FreeBSD, the file already exists. What I usually do is start in my home directory, create the rules, and then test them, loading them with sudo. Then, when it's working the way I want it to work, I backup the default pf.conf and copy it over.
mv /etc/pf.conf /etc/pf.conf.bak cp pf.conf /etc |
(As we haven't yet created the file, don't do it yet) :).
tcp_pass = "{ 80 22 }"
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We've created a macro. Later, when we make rules, rather than having to make one rule for port 80 and a second for port 22 we can simply do something like
pass out proto tcp to port $tcp_pass |
(Note that you need the $ in front of the macro's name.)
Then, if we decide we want to add port 123, the Network Time Protocol, rather than write a new rule, we can simply add 123 to our macro. We can also realize we forgot to allow it to get email so we'll add all three, pop3 to get email, smtp to send it and ntp to connect to time servers.
tcp_pass= "{ 80 22 25 110 123 }"
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The pf filter will read the rules we create from top to bottom. So, we start with a rule to block everything
block all |
This keeps the box pretty secure, but we won't be able to do anything. So, in order to access web pages, allow the Network Time Protocol to connect to a time server and to ssh to an outside machine, we can use the macro we defined. Our pf.conf looks like this.
tcp_pass = "{ 80 22 25 110 123 }"
block all
pass out on fxp0 proto tcp to any port $tcp_pass keep state
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Let's look at what we've done so far. First, we defined a macro. We used port numbers. While one can mix and match if they want, for clarity's sake, we try to use the same pattern for all ports. We can also put in commas if we want or the protocol name if it's defined in /etc/services. So, that macro could actually read
tcp_pass = "{ 80 ssh, ntp smtp 110}"
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and still work. However, one should strive for consistancy.
My personal habit is to have one white space between left bracket and first port and another space between the last port listed and the white bracket. (However, that white space, as shown in the example above, is also optional--that is one can do { 80 or {80 and they will both work. Also, keeping in them in numerical order helps you remember what you're doing. So, let's clean up the above example a little bit.
tcp_pass= "{ 22 25 80 110 123 }"
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You can only use a name if the port is defined in /etc/services. For instance, if you decide to set ssh to only accept connections on port 1222, you would have to put 1222 in that macro, NOT ssh. Port 1222 is defined in /etc/services as nerv, the SNI R&D network, so if you check your rules with pfctl, it'll show that you have a rule to pass out to nerv. To avoid confusion, if you're going to use a non-standard port, use something that isn't listed in /etc/services.
You can also specify a range of ports with a colon. For instance, if I wanted to add samba, which uses ports 137, 138 and 139, I could have added 137:139.
Next, we block everything as our default. Now, we have to add rules to let things through. Remember, pf processes rules from top to bottom.
So, we are allowing out web traffic, ssh connections, sending mail, getting mail with pop3, and we're able to contact time servers.
So, we start with the action, pass. We're passing, not blocking. The order of syntax is important. I'm just giving the basic options here, again, this article can be considered a prep for more advanced tutorials.
pass out means we're allowing things out, not necessarily in. The next part, on fxp0 refers to the interface--in this case, a network card that in FreeBSD parlance is called fxp0. (An Intel card).
The to any part means that we're allowing it to go anywhere. Next, we specify the ports we're talking about--here we use our macro, tcp_pass, meaning we're allowing to the ports mentioned above.
The keep state part can be important. It means that once we've established the connection, pf is going to keep track of it, so the answer from, for example, a web page, doesn't have to go through checking each rule, but can just be opened. The same with pop3. One uses pop3 to contact a pop3 server (hrrm, obviously), and the server answers. As we have the keep state keywords, the server's answer can go right through once the connection is established.
Many of these are optional. For example, if we write pass on fxp0 rather than pass out on fxp0, then traffic will be allowed in both directions, in and out.
If you look through /etc/services, you'll see that some things, such as ipp, port 631 used with CUPS, use both tcp and udp. To deal with such things, we could insert another macro
udp_pass = "{ 631 }"
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We might find that sending email isn't working properly. Checking /etc/services we find that smtp can use udp.
grep smtp /etc/services smtp 25/tcp mail #Simple Mail Transfer smtp 25/udp mail #Simple Mail Transfer smtps 465/tcp #smtp protocol over TLS/SSL (was ssmtp) smtps 465/udp #smtp protocol over TLS/SSL (was ssmtp) |
However, with our new macro, this is simple--we simply add it to udp_pass
udp_pass = "{ 110 631 }"
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Now, let's add the udp_pass macro to our ruleset
tcp_pass = "{ 22 25 80 110 123 }"
udp_pass = "{ 110 631 }"
block all
pass out on fxp0 proto tcp to any port $tcp_pass keep state
pass out on fxp0 proto udp to any port $udp_pass keep state
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However, we find CUPS isn't working. A quick grep of ipp in /etc/services shows that it also uses tcp. So, we add 631 to our list of ports in the tcp_pass macro. Now we have
tcp_pass = "{ 22 25 80 110 123 631 }"
udp_pass = "{ 110 631 }"
block all
pass out on fxp0 proto tcp to any port $tcp_pass keep state
pass out on fxp0 proto udp to any port $udp_pass keep state
|
The same holds true for any other ports that you've forgotten. For instance, these rules don't allow DNS, port 53. We grep domain in /etc/services, and see that it uses both tcp and udp so we add 53 to both macros
tcp_pass = "{ 22 25 53 80 110 123 631 }"
udp_pass = "{ 53 110 631 }"
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In such cases, you can use the quick keyword. If a packet matches something in that line, it stops going through the rules and processes the packet. For example, let's say that you have a web server on your LAN behind a firewall, and you are sure that all requests for port 80 are coming from your internal network, so you want to quickly pass them through. (I can't see that being true in real life, but this is for an example of using quick).
pass in quick on fxp0 proto tcp to any port 80 keep state |
Put that above the other rules, right after the macro definitions. Now, requests coming in for the LAN webserver will be passed right though (note that in this case it was pass in) without being matched against the rest of the ruleset.
table <local> { 192.168.8.0/24, 192.168.9.0/24 }
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Insert that line above your rules. Now, to allow everything from those two networks (and we'll make use of the quick keyword as well) add a rule. Since we're using quick we want this rule towards the top. If it was at the end, there's no point in using the quick keyword, for the packets would have already been matched against every rule above this one
pass in quick from <local> to any keep state |
pfctl -t local -T show |
will show you the contents of your table. Note that if you edit your pf.conf, adding a table, and then simply pfctl -d and -e, to disable and re-enable pf, the table rules may not be applied. The way to do it is
pfctl -t local -Tl -f /etc/pf.conf |
The -t is for the table name, in this case, local. The -T is used to give various commands. In this case, we are using l for load. That is a lower case letter L, not the numeral one. The -f is for file, in this case, /etc/pf.conf where we have added the table.
We can use pfctl for various useful table commands. We've already mentioned the show command.
pfctl -t mytable -T show |
will show you the current working contents of a table called mytable. If you keep mytable in a static file, for example, /etc/pf.mytable, and edit the file, the way to get pf to use it is
pfctl -t mytable -T replace -f pf.mytable |
When using a text file, create a line in pf.conf like
table <mytable> persist file "/etc/pf.mytable" |
In this case, it is replacing whatever the former contents of mytable were with the latest contents of mytable.
Tables can be added and deleted on the fly, see the links at the end of this article for more details. To add or delete an address to a table on the fly, one can use, to add the address 192.168.1.115 to a table called mytable
pfctl -t mytable -T add 192.168.1.115 |
To remove the address
pfctl -t mytable -T delete 192.168.1.115 |
This all takes effect immediately. As mentioned above, you can confirm that the desired address has been added or deleted with
pfctl -t mytable -T show |
Another popular use of tables is to block bruteforce attacks. This can go directly into pf.conf
table <bruteforce> block in quick from <bruteforce> pass in inet proto tcp from any to any port ssh \ flags S/SA keep state \ (max-src-conn 10 max-src-conn-rate 10/30, \ overload <bruteforce> flush global) |
This will block any IP trying to connect more than 10 times in 30 seconds. You may want much higher numbers, depending upon circumstances. In the Peter Hansteen article that showed me this, he has a max-src-conn at 100 and max-src-conn-rate at 15/5.
For example, I have an anchor to allow me to use ftp. I create a file that reads
pass out proto tcp from any to port 21 keep state pass out proto tcp from any to port > 1023 keep state |
anchor ftpanchor |
Now, I want to grab something from ftp.FreeBSD.org. I load the anchor ruleset.
pfctl -a ftpanchor -f /etc/ftp-anchor |
I can check that the rules are loaded.
pfctl -a ftpanchor -s rules |
Now I do whatever I had to do. I want to remove those rules.
pfctl -a ftpanchor -F rules |
One can edit the anchor ruleset and reload and unload it at will without having to restart pf or reload the entire pf.conf ruleset. It's handy for testing various rules. For example, perhaps I also need to allow out udp to do what I want. Rather than adding a line to pf.conf and reloading the entire ruleset, I can edit /etc/ftp-anchor, add a line to allow out udp, then reload the anchor with pfctl -a -f /etc/ftp-anchor again. Once again, when finished, I can flush the anchor rules and my pf ruleset is back to normal. (One doesn't need udp for ftp, but this is for example.)
If you think you will usually want to have the anchor in place, and only unload it on rare occasions, you would add, below the anchor line
load anchor ftpanchor from "/etc/ftp-anchor" |
Anchors are quite flexible. The above gives very simple examples. The more detailed guides listed at the end of this article go into greater detail.
sudo pfctl -d |
pfctl -n -v -f /etc/pf.conf |
The -v is for verbose and the -f for the file.
To put the packet filter we've created into effect, assuming you are user john and you've created it in your home directory, as john
sudo pfctl -ef pf.conf |
The -f stands for file. Say we've checked it out and we don't like it so we want to go back to our default rules, in /etc/
sudo pfctl -f /etc/pf.conf |
There are a variety of uses for pfctl. Doing pfctl -s info gives you a quick look at what's going on. Doing pfctl -vs rules shows you your rules and what's happening, for example
pass out proto tcp from any to any port = http keep state [ Evaluations: 96 Packets: 906 Bytes: 496407 States: 0 ] pass out proto tcp from any to any port = pop3 keep state [ Evaluations: 96 Packets: 514 Bytes: 71260 States: 0 ] |
The man page gives a complete list, but other useful ones are pfctl -sr to show rules, -sn to show nat, and -sa to show all.
Now it's time to test this. It's early morning, and I'm not thinking that clearly. Hopefully, however, I've remembered to add myself to sudoers as being able to run pfctl without a password and remember to quickly set up the cronjob so that if I make a mistake, pf will be disabled shortly. (VERY necessary if you're testing this on a machine to which you don't have physical access). So, I put these rules which I've saved into a file pf.conf in my home directory into operation.
sudo pfctl -ef pf.conf |
I find that I've locked myself out of the box. Oops. I forgot to allow ssh connections in, and the connection that I was using has just been blocked.
So, I wait a few minutes for the crontab to disable pf and log back into the remote machine. This time, I remember to add a rule
pass in proto tcp to port 22 keep state |
Now, these rules seem to be working. So, I copy them over to /etc/pf.conf.
sudo cp pf.conf /etc/ sudo pftcl -f /etc/pf.conf sudo pftcl -s rules |
Listing the rules will show me that it's doing what I want to do.
Now, of course, I remove the cronjob. If you're going to be experimenting with your rules frequently, and will be needing it again, just do crontab -e and comment the line out with a #.
Most of what is covered in the more advanced guides seems to be aimed at having an ftp server. To simply use an ftp client from your workstation, rather than use the rdr (for redirect) rules, you might simply use an anchor to allow out proto tcp while using ftp. I gave an example in the section about anchors.
There is also the scrub keyword. Again, while this is more fully explained in the references below in a nutshell it normalizes fragmented packets. The usual line is
scrub in all |
This rule must also go above the filtering rules (and above the rdr to redirect ftp if you use it.) It is the "normalization" referred to in that error message. At the top of the ruleset you define your macros, tables and the like. Then would come the scrub rule, then the rdr rule, then your filtering rules (the group that in our example begins with block all). Using the scrub keyword can help protect against certain kinds of attacks, and it's a good line to have.
In FreeBSD, one of the older versions, (5.something or perhaps 6.0) you have to add the pf module to your kernel. In later versions, it's not necessary, it will load the module if listed in /etc/rc.conf. Check /usr/src/sys/conf/NOTES to see the various options, but I just had
device pf device pflog device pfsync |
I also have
options ALTQ |
in my kernel. Although I don't make use of queuing, otherwise, one gets a warning every time they reload their pf rules that ALTQ isn't enabled in the kernel.
(I'm not sure if this is still needed or not, all the boxes I have still have it in their custom kernels and I've never removed it.)
If you use the module, it assumes the presence of device bpf, INET and INET6 in the kernel. (See the handbook page on pf.)
pflog_enable="YES" pflog_logfile="/var/log/pf.log" |
The log keyword comes after pass or block in or out. For example, we have a rule
pass out proto tcp from any to port 21 keep state |
To log it, we would change that line to read
pass out log proto tcp from any to port 21 keep state |
You may have to run the command touch /var/log/pf.log, or it may be automatically created. After this is done, restart pf
pfctl -f /etc/pf.rules |
It isn't in readable form, you have to use tcpdump to read it.
tcpdump -n -e -ttt -r /var/log/pf.log |
To read it in realtime
tcpdump -n -e -ttt -i pflog0 |
cloned_interfaces=lo1" ipv4_addrs_lo1="192.168.1.1-9/29" |
(That is a lower case L, lowercase letter O, and the numeral one, as in loopback one.) That gives a range from 192.168.1.1 to 192.168.1.9. To do this on the fly, the commands are
ifconfig lo1 create ifconfig lo1 inet 192.168.1.1/29 |
I haven't needed this often enough to figure out the syntax of creating multiple lo1 addresses with ifconfig. If I were doing it on the fly, I would set up rc.conf as mentioned, but use an alias on the command line, e.g.
ifconfig lo1 alias 192.168.1.2/29. |
(You could also use aliases in /etc/rc.conf instead of the syntax I have--as the saying goes, in Unix there's always more than one way to do something with a corollary that someone thinks your way is stupid, but I digress.)
We'll assume you have a public address of 5.6.7.8 and you will redirect inquiries to your webserver running on cloned lo1 interface 192.168.1.1. Our public network interface will be bce0. We'll assume the reader has built a jail to host their web server. I have a page on jails for readers looking to get started with them.
Now we edit /etc/pf.conf. At the top, we define macros.
IP_PUB="5.6.7.8"
NETJAIL="192.168.1.1"
WEBPORT="{ 80, 443 }"
scrub in all
nat pass on bce0 proto tcp from $NETJAIL to any -> $IP_PUB
nat pass on bce0 proto udp from $NETJAIL to any -> $IP_PUB
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These two lines allow things from the jail to go out through the
public interface, in case that wasn't clear. It may not even be
necessary. In some cases, you may need tcp but not udp. The user can adapt
it to their needs.
These should all be added above rules, otherwise, one gets an error that rules must be in order; options, normalization, queueing, translation, filtering.
We can add other jails. For example, if we did give lo1 a range of addresses, we might have 192.168.1.2 be a jail running MySQL. Create a couple of new macros.
SQLJAIL="192.168.1.2"
SQLPORT="{ 3306 }"
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Then add
rdr pass on bce0 proto tcp from any to $IP_PUB port $SQLPORT -> $SQLJAIL |
(Or, as we expect 3306 to be the only port used, don't bother making a macro for it, just have the rule read from any to $IP_PUB port 3306. As long as the reader understands the syntax, they can do adapt it to their needs.)
This page is only meant to be an introduction. However, if you've understood these simple rulesets, you're probably ready to look at the more sophisticated tutorials. The two that I use most frequently are the OpenBSD PF User's Guide and Peter N.M. Hansteen's tutorial Feel free to email me if you detect any glaring errors. I am no expert on pf, but hope that this brief introduction will make it easier for the novice to grasp its basic concepts.
There have been various, sometimes significant, divergence in OpenBSD's and FreeBSD's pf syntax, so the more recent OpenBSD guides may not work as expected in FreeBSD. To get the older OpenBSD faqs, you can use cvs. For the 4.1 version use
cvs -d anoncvs@anoncvs1.ca.openbsd.org:/cvs get -D "May 2, 2007" www/faq/pf |
To get the 4.5 version
cvs -d anoncvs@anoncvs1.ca.openbsd.org:/cvs get -D "May 2, 2009" www/faq/pf |
(Thanks to daemonforums user jggimi for that tip.)