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PF: Packet Filtering
Table of Contents
Introduction
Packet filtering is the selective passing or blocking of data
packets as they pass through a network interface. The criteria that
pf(4) uses when inspecting packets are based on the Layer 3
(IPv4 and
IPv6) and Layer 4
(TCP,
UDP,
ICMP, and
ICMPv6) headers. The most often used criteria are source and
destination address, source and destination port, and protocol.
Filter rules specify the criteria that a packet must match and the
resulting action, either block or pass, that is taken when a match is
found. Filter rules are evaluated in sequential order, first to last.
Unless the packet matches a rule containing the quick keyword,
the packet will be evaluated against all filter rules before the
final action is taken. The last rule to match is the "winner" and will
dictate what action to take on the packet. There is an implicit
pass all at the beginning of a filtering ruleset meaning that if a
packet does not match any filter rule the resulting action will be
pass.
Rule Syntax
The general, highly simplified syntax for filter rules is:
action [direction] [log] [quick] [on interface]
[af] [proto protocol] \
[from src_addr [port src_port]] [to
dst_addr [port dst_port]] \
[flags tcp_flags] [state]
- action
- The action to be taken for matching packets, either pass or
block. The pass action will pass the packet back to
the kernel for further processing while the block action will
react based on the setting of the
block-policy option.
The default reaction may be overridden by specifying either block
drop or block return.
- direction
- The direction the packet is moving on an interface, either
in or out.
- log
- Specifies that the packet should be logged via
pflogd(8). If the rule specifies the keep state,
modulate state, or synproxy state option, then only the
packet which establishes the state is logged.
To log all packets regardless, use log (all).
- quick
- If a packet matches a rule specifying quick, then that rule
is considered the last matching rule and the specified
action is taken.
- interface
- The name or group of the network interface that the packet is moving
through.
An interface group is specified as the name of the interface but without
the integer appended.
For example: ppp or fxp.
This would cause the rule to match for any packet traversing any
ppp or fxp interface, respectively.
- af
- The address family of the packet, either inet for IPv4 or
inet6 for IPv6. PF is usually able to determine this parameter
based on the source and/or destination address(es).
- protocol
- The Layer 4 protocol of the packet:
- tcp
- udp
- icmp
- icmp6
- A valid protocol name from
/etc/protocols
- A protocol number between 0 and 255
- A set of protocols using a list.
- src_addr, dst_addr
- The source/destination address in the IP header. Addresses can be
specified as:
- A single IPv4 or IPv6 address.
- A CIDR
network block.
- A fully qualified domain name that will be resolved via DNS when the
ruleset is loaded. All resulting IP addresses will be substituted into
the rule.
- The name of a network interface. Any IP addresses assigned to the
interface will be substituted into the rule.
- The name of a network interface followed by
/netmask (i.e., /24). Each IP address on the
interface is combined with the netmask to form a CIDR network block
which is substituted into the rule.
- The name of a network interface in parentheses ( ). This
tells PF to update the rule if the IP address(es) on the named interface
change. This is useful on an interface that gets its IP address via DHCP
or dial-up as the ruleset doesn't have to be reloaded each time the
address changes.
- The name of a network interface followed by any one of these
modifiers:
- :network - substitues the CIDR network block (e.g.,
192.168.0.0/24)
- :broadcast - substitutes the network broadcast address
(e.g., 192.168.0.255)
- :peer - substitues the peer's IP address on a
point-to-point link
- In addition, the :0 modifier can be appended to either an
interface name or to any of the above modifiers to indicate that PF
should not include aliased IP addresses in the substituion.
These modifiers can also be used when the interface is contained in
parentheses.
Example: fxp0:network:0
- A table.
- Any of the above but negated using the ! ("not") modifier.
- A set of addresses using a list.
- The keyword any meaning all addresses
- The keyword all which is short for from any to
any.
- src_port, dst_port
- The source/destination port in the Layer 4 packet header. Ports can
be specified as:
- A number between 1 and 65535
- A valid service name from
/etc/services
- A set of ports using a list
- A range:
- != (not equal)
- < (less than)
- > (greater than)
- <= (less than or equal)
- >= (greater than or equal)
- >< (range)
- <> (inverse range)
- The last two are binary operators (they take two arguments) and
do not include the arguments in the range.
- : (inclusive range)
- The inclusive range operator is also a binary operator and does
include the arguments in the range.
- tcp_flags
- Specifies the flags that must be set in the TCP header when using
proto tcp. Flags are specified as
flags check/mask. For example: flags
S/SA - this instructs PF to only look at the S and A (SYN and ACK)
flags and to match if only the SYN flag is "on".
- state
- Specifies whether state information is kept on packets matching this
rule.
- keep state - works with TCP, UDP, and ICMP.
- modulate state - works only with TCP. PF will generate
strong Initial Sequence Numbers (ISNs) for packets matching this rule.
- synproxy state - proxies incoming TCP connections to help
protect servers from spoofed TCP SYN floods.
This option includes the functionality of keep state and
modulate state.
Default Deny
The recommended practice when setting up a firewall is to take a
"default deny" approach. That is, to deny everything and then
selectively allow certain traffic through the firewall. This approach is
recommended because it errs on the side of caution and also makes
writing a ruleset easier.
To create a default deny filter policy, the first two filter rules should
be:
block in all
block out all
This will block all traffic on all interfaces in either direction from
anywhere to anywhere.
Passing Traffic
Traffic must now be explicitly passed through the firewall or it will be
dropped by the default deny policy. This is where packet criteria such
as source/destination port, source/destination address, and protocol
come into play. Whenever traffic is permitted to pass through the
firewall the rule(s) should be written to be as restrictive as
possible. This is to ensure that the intended traffic, and only the
intended traffic, is permitted to pass.
Some examples:
# Pass traffic in on dc0 from the local network, 192.168.0.0/24,
# to the OpenBSD machine's IP address 192.168.0.1. Also, pass the
# return traffic out on dc0.
pass in on dc0 from 192.168.0.0/24 to 192.168.0.1
pass out on dc0 from 192.168.0.1 to 192.168.0.0/24
# Pass TCP traffic in on fxp0 to the web server running on the
# OpenBSD machine. The interface name, fxp0, is used as the
# destination address so that packets will only match this rule if
# they're destined for the OpenBSD machine.
pass in on fxp0 proto tcp from any to fxp0 port www
The quick Keyword
As indicated earlier, each packet is evaluated against the filter ruleset
from top to bottom. By default, the packet is marked for passage, which
can be changed by any rule, and could be changed back and forth several
times before the end of the filter rules. The last matching rule
"wins". There is an exception to this: The quick option
on a filtering rule has the effect of canceling any further rule
processing and causes the specified action to be taken. Let's look at a
couple examples:
Wrong:
block in on fxp0 proto tcp from any to any port ssh
pass in all
In this case, the block line may be evaluated, but will never
have any effect, as it is then followed by a line which will pass
everything.
Better:
block in quick on fxp0 proto tcp from any to any port ssh
pass in all
These rules are evaluated a little differently. If the block
line is matched, due to the quick option, the packet will be
blocked, and the rest of the ruleset will be ignored.
Keeping State
One of Packet Filter's important abilities is "keeping state" or
"stateful inspection". Stateful inspection refers to PF's ability to
track the state, or progress, of a network connection. By storing
information about each connection in a state table, PF is able to
quickly determine if a packet passing through the firewall belongs to an
already established connection. If it does, it is passed through
the firewall without going through ruleset evaluation.
Keeping state has many advantages including simpler rulesets and better
packet filtering performance. PF is able to match packets moving in
either direction to state table entries meaning that filter rules
which pass returning traffic don't need to be written. And, since
packets matching stateful connections don't go through ruleset
evaluation, the time PF spends processing those packets can be greatly
lessened.
When a rule has the keep state option, the first packet
matching the rule creates a "state" between the sender and receiver.
Now, not only do packets going from the sender to receiver match the
state entry and bypass ruleset evaluation, but so do the reply packets
from receiver to sender. For example:
pass out on fxp0 proto tcp from any to any keep state
This allows any outbound TCP traffic on the fxp0 interface
and also permits the reply traffic to pass back through the firewall.
While keeping state is a nice feature, its use significantly improves
the performance of your firewall as state lookups are dramatically
faster than running a packet through the filter rules.
The modulate state option works just like keep state
except that it only applies to TCP packets. With
modulate state, the Initial Sequence Number (ISN) of outgoing
connections is randomized. This is useful for protecting connections
initiated by certain operating systems that do a poor job of choosing
ISNs.
Starting with OpenBSD 3.5, the modulate state option can be
used in rules that specify protocols other than TCP.
Keep state on outgoing TCP, UDP, and ICMP packets and modulate TCP ISNs:
pass out on fxp0 proto { tcp, udp, icmp } from any \
to any modulate state
Another advantage of keeping state is that corresponding ICMP traffic
will be passed through the firewall. For example, if keep state
is specified for a TCP connection and an ICMP source-quench message
referring to this TCP connection arrives, it will be matched to the
appropriate state entry and passed through the firewall.
The scope of a state entry is controlled globally by the
state-policy
runtime option and on a per rule basis by the if-bound,
group-bound, and floating state option keywords.
These per rule keywords have the same meaning as when used with the
state-policy option. Example:
pass out on fxp0 proto { tcp, udp, icmp } from any \
to any modulate state (if-bound)
This rule would dictate that in order for packets to match the state
entry, they must be transitting the fxp0 interface.
Note that nat,
binat, and
rdr rules implicitly create state for
matching connections as long as the connection is passed by the
filter ruleset.
Keeping State for UDP
One will sometimes hear it said that, "One can not create state with
UDP as UDP is a stateless protocol!" While it is true that a UDP
communication session does not have any concept of state (an explicit
start and stop of communications), this does not have any impact on PF's
ability to create state for a UDP session. In the case of protocols
without "start" and "end" packets, PF simply keeps track of how long it
has been since a matching packet has gone through. If the timeout is
reached, the state is cleared. The timeout values can be set in the
options section of the pf.conf
file.
Stateful Tracking Options
When a filter rule creates a state table entry through the use of any of
the keep state, modulate state, or synproxy state
keywords, certain options can be specified that control the behavior of
state creation.
The following options are available:
- max number
- Limit the maximum number of state entries the rule can create to
number.
If the maximum is reached, packets that would normally create state are
dropped until the number of existing states decreases.
- source-track
- This option enables the tracking of number of states created per
source IP address.
This option has two formats:
- source-track rule - The maximum number of states
created by this rule is limited by the rule's max-src-nodes
and max-src-states options. Only state entries created by
this particular rule count toward the rule's limits.
- source-track global - The number of states created by
all rules that use this option is limited. Each rule can specify
different max-src-nodes and max-src-states
options, however state entries created by any participating rule
count towards each individual rule's limits.
The total number of source IP addresses tracked globally can be
controlled via the
src-nodes runtime option.
- max-src-nodes number
- When the source-track option is used,
max-src-nodes will limit the number of source IP addresses that
can simultaneously create state.
This option can only be used with source-track rule.
- max-src-states number
- When the source-track option is used,
max-src-states will limit the number of simultaneous state
entries that can be created per source IP address.
The scope of this limit (i.e., states created by this rule only or
states created by all rules that use source-track) is dependent
on the source-track option specified.
An example rule:
pass in on $ext_if proto tcp to $web_server \
port www flags S/SA keep state \
(max 200, source-track rule, max-src-nodes 100,
max-src-states 3)
The rule above defines the following behavior:
- Limit the absolute maximum number of states that this rule can
create to 200
- Enable source tracking; limit state creation based on states created
by this rule only
- Limit the maximum number of nodes that can simultaneously create
state to 100
- Limit the maximum number of simultaneous states per source IP to 3
A separate set of restrictions can be placed on stateful TCP connections
that have completed the 3-way handshake.
- max-src-conn number
- Limit the maximum number of simultaneous TCP connections
which have completed the 3-way handshake that a single host can make.
- max-src-conn-rate number / interval
- Limit the rate of new connections to a certain amount per time
interval.
Both of these options automatically invoke the source-track rule
option and are incompatible with source-track global.
Since these limits are only being placed on TCP connections that have
completed the 3-way handshake, more aggresive actions can be taken on
offending IP addresses.
- overload <table>
- Put an offending host's IP address into the named table.
- flush [global]
- Kill any other states that match this rule and that were created by
this source IP.
When global is specified, kill all states matching this source
IP, regardless of which rule created the state.
An example:
table <abusive_hosts> persist
block in quick from <abusive_hosts>
pass in on $ext_if proto tcp to $web_server \
port www flags S/SA keep state \
(max-src-conn 100, max-src-conn-rate 15/5,
overload <abusive_hosts> flush)
This does the following:
- Limits the maximum number of connections per source to 100
- Rate limits the number of connections to 15 in a 5 second span
- Puts the IP address of any host that breaks these limits into the
<abusive_hosts> table
- For any offending IP addresses, flush any states created by this
rule.
TCP Flags
Matching TCP packets based on flags is most often used to filter TCP
packets that are attempting to open a new connection. The TCP flags and
their meanings are listed here:
- F : FIN - Finish; end of session
- S : SYN - Synchronize; indicates request to start session
- R : RST - Reset; drop a connection
- P : PUSH - Push; packet is sent immediately
- A : ACK - Acknowledgement
- U : URG - Urgent
- E : ECE - Explicit Congestion Notification Echo
- W : CWR - Congestion Window Reduced
To have PF inspect the TCP flags during evaluation of a rule, the
flags keyword is used with the following syntax:
flags check/mask
The mask part tells PF to only inspect the specified flags
and the check part specifies which flag(s) must be
"on" in the header for a match to occur.
pass in on fxp0 proto tcp from any to any port ssh flags S/SA
The above rule passes TCP traffic with the SYN flag set while only
looking at the SYN and ACK flags. A packet with the SYN and ECE flags
would match the above rule while a packet with SYN and ACK or just ACK
would not.
Note: in previous versions of OpenBSD, the following syntax was
supported:
. . . flags S
This is no longer true. A mask must now always be specified.
Flags are often used in conjunction with keep state rules to
help control the creation of state entries:
pass out on fxp0 proto tcp all flags S/SA keep state
This would permit the creation of state on any outgoing TCP packet
with the SYN flag set out of the SYN and ACK flags.
One should be careful with using flags -- understand what you are
doing and why, and be careful with the advice people give as a lot of
it is bad. Some people have suggested creating state "only if the SYN flag
is set and no others". Such a rule would end with:
. . . flags S/FSRPAUEW bad idea!!
The theory is, create state only on the start of the TCP session, and
the session should start with a SYN flag, and no others. The problem
is some sites are starting to use the ECN flag and any site using ECN
that tries to connect to you would be rejected by such a rule. A
much better guideline is:
. . . flags S/SAFR
While this is practical and safe, it is also unnecessary to check the
FIN and RST flags if traffic is also being
scrubbed. The scrubbing process will cause PF
to drop any incoming packets with illegal TCP flag combinations (such as
SYN and RST) and to normalize potentially ambigious combinations (such
as SYN and FIN).
It's highly recommended to always scrub incoming traffic:
scrub in on fxp0
.
.
.
pass in on fxp0 proto tcp from any to any port ssh flags S/SA \
keep state
TCP SYN Proxy
Normally when a client initiates a TCP connection to a server PF will
pass the
handshake packets between the two endpoints as they arrive.
PF has the ability, however, to proxy the handshake.
With the handshake proxied, PF itself will complete the handshake with
the client, initiate a handshake with the server, and then pass packets
between the two.
The benefit of this process is that no packets are sent to the server
before the client completes the handshake.
This eliminates the threat of spoofed TCP SYN floods affecting the
server because a spoofed client connection will be unable to complete
the handshake.
The TCP SYN proxy is enabled using the synproxy state keywords
in filter rules. Example:
pass in on $ext_if proto tcp from any to $web_server port www \
flags S/SA synproxy state
Here, connections to the web server will be TCP proxied by PF.
Because of the way synproxy state works, it also includes the
same functionality as keep state and modulate state.
The SYN proxy will not work if PF is running on a
bridge(4).
Blocking Spoofed Packets
Address "spoofing" is when an malicious user fakes the source IP address
in packets they transmit in order to either hide their real address or
to impersonate another node on the network. Once the user has spoofed
their address they can launch a network attack without revealing the
true source of the attack or attempt to gain access to network services
that are restricted to certain IP addresses.
PF offers some protection against address spoofing through the
antispoof keyword:
antispoof [log] [quick] for interface [af]
- log
- Specifies that matching packets should be logged via
pflogd(8).
- quick
- If a packet matches this rule then it will be considered the
"winning" rule and ruleset evaluation will stop.
- interface
- The network interface to activate spoofing protection on. This can
also be a list of interfaces.
- af
- The address family to activate spoofing protection for, either
inet for IPv4 or inet6 for IPv6.
Example:
antispoof for fxp0 inet
When a ruleset is loaded, any occurrences of the antispoof
keyword are expanded into two filter rules. Assuming that interface
fxp0 has IP address 10.0.0.1 and a subnet mask of 255.255.255.0
(i.e., a /24), the above antispoof rule would expand to:
block in on ! fxp0 inet from 10.0.0.0/24 to any
block in inet from 10.0.0.1 to any
These rules accomplish two things:
- Blocks all traffic coming from the 10.0.0.0/24 network that does
not pass in through fxp0. Since the 10.0.0.0/24 network is on the
fxp0 interface, packets with a source address in that network block
should never be seen coming in on any other interface.
- Blocks all incoming traffic from 10.0.0.1, the IP address on
fxp0.
The host machine should never send packets to itself through an external
interface, so any incoming packets with a source address belonging to
the machine can be considered malicious.
NOTE: The filter rules that the antispoof rule
expands to will also block packets sent over the loopback interface to
local addresses.
It's best practice to skip filtering on loopback interfaces anyways, but
this becomes a necessity when using antispoof rules:
set skip on lo0
antispoof for fxp0 inet
Usage of antispoof should be restricted to interfaces that have
been assigned an IP address. Using antispoof on an interface
without an IP address will result in filter rules such as:
block drop in on ! fxp0 inet all
block drop in inet all
With these rules there is a risk of blocking all inbound traffic
on all interfaces.
Passive Operating System Fingerprinting
Passive OS Fingerprinting (OSFP) is a method for passively detecting the
operating system of a remote host based on certain characteristics
within that host's TCP SYN packets.
This information can then be used as criteria within filter rules.
PF determines the remote operating system by comparing characteristics
of a TCP SYN packet against the
fingerprints file, which by
default is
/etc/pf.os.
Once PF is enabled, the current fingerprint list can be viewed with this
command:
# pfctl -s osfp
Within a filter rule, a fingerprint may be specified by OS class,
version, or subtype/patch level.
Each of these items is listed in the output of the pfctl
command shown above. To specify a fingerprint in a filter rule, the
os keyword is used:
pass in on $ext_if from any os OpenBSD keep state
block in on $ext_if from any os "Windows 2000"
block in on $ext_if from any os "Linux 2.4 ts"
block in on $ext_if from any os unknown
The special operating system class unknown allows for matching
packets when the OS fingerprint is not known.
TAKE NOTE of the following:
- Operating system fingerprints are occasionally wrong due to
spoofed and/or crafted packets that are made to look like they
originated from a specific operating system.
- Certain revisions or patchlevels of an operating system may change
the stack's behavior and cause it to either not match what's in the
fingerprints file or to match another entry altogether.
- OSFP only works on the TCP SYN packet; it will not work on other
protocols or on already established connections.
IP Options
By default, PF blocks packets with IP options set. This can make the job
more difficult for "OS fingerprinting" utilities like nmap. If you have
an application that requires the passing of these packets, such as
multicast or IGMP, you can use the allow-opts directive:
pass in quick on fxp0 all allow-opts
Filtering Ruleset Example
Below is an example of a filtering ruleset. The machine running PF is
acting as a firewall between a small, internal network and the Internet.
Only the filter rules are shown;
queueing,
nat,
rdr,
etc., have been left out of this example.
ext_if = "fxp0"
int_if = "dc0"
lan_net = "192.168.0.0/24"
# table containing all IP addresses assigned to the firewall
table <firewall> const { self }
# don't filter on the loopback interface
set skip on lo0
# scrub incoming packets
scrub in all
# setup a default deny policy
block all
# activate spoofing protection for the internal interface.
antispoof quick for $int_if inet
# only allow ssh connections from the local network if it's from the
# trusted computer, 192.168.0.15. use "block return" so that a TCP RST is
# sent to close blocked connections right away. use "quick" so that this
# rule is not overridden by the "pass" rules below.
block return in quick on $int_if proto tcp from ! 192.168.0.15 \
to $int_if port ssh flags S/SA
# pass all traffic to and from the local network
pass in on $int_if from $lan_net to any
pass out on $int_if from any to $lan_net
# pass tcp, udp, and icmp out on the external (Internet) interface.
# keep state on udp and icmp and modulate state on tcp.
pass out on $ext_if proto tcp all modulate state flags S/SA
pass out on $ext_if proto { udp, icmp } all keep state
# allow ssh connections in on the external interface as long as they're
# NOT destined for the firewall (i.e., they're destined for a machine on
# the local network). log the initial packet so that we can later tell
# who is trying to connect. use the tcp syn proxy to proxy the connection.
pass in log on $ext_if proto tcp from any to ! <firewall> \
port ssh flags S/SA synproxy state
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