| PF.CONF(5) | File Formats Manual | PF.CONF(5) |
pf.conf — packet
filter configuration file
The packet filter modifies, drops or passes packets according to
rules or definitions specified in pf.conf.
There are seven types of statements in
pf.conf:
Macrospf.conf.TablesOptionsTraffic
Normalization (e.g.
scrub)QueueingTranslation
(Various forms of NAT)Packet
FilteringWith the exception of macros and
tables, the types of statements should be grouped
and appear in pf.conf in the order shown above, as
this matches the operation of the underlying packet filtering engine. By
default pfctl(8) enforces this order (see
set require-order below).
Comments can be put anywhere in the file using a hash mark (‘#’), and extend to the end of the current line.
Additional configuration files can be included with the
include keyword, for example:
include "/etc/pf/sub.filter.conf"
Macros can be defined that will later be expanded in context. Macro names must start with a letter, and may contain letters, digits and underscores. Macro names may not be reserved words (for example pass, in, out). Macros are not expanded inside quotes.
For example,
ext_if = "kue0"
all_ifs = "{" $ext_if lo0 "}"
pass out on $ext_if from any to any
pass in on $ext_if proto tcp from any to any port 25
Tables are named structures which can hold a collection of addresses and networks. Lookups against tables in packet filter are relatively fast, making a single rule with tables much more efficient, in terms of processor usage and memory consumption, than a large number of rules which differ only in IP address (either created explicitly or automatically by rule expansion).
Tables can be used as the source or destination of filter rules, scrub rules or translation rules such as nat or rdr (see below for details on the various rule types). Tables can also be used for the redirect address of nat and rdr rules and in the routing options of filter rules, but only for round-robin pools.
Tables can be defined with any of the following pfctl(8) mechanisms. As with macros, reserved words may not be used as table names.
pf.conf use the table
statement, and are especially useful to define non-persistent tables. The
contents of a pre-existing table defined without a list of addresses to
initialize it is not altered when pf.conf is
loaded. A table initialized with the empty list, {
}, will be cleared on load.Tables may be defined with the following two attributes:
For example,
table <private> const { 10/8, 172.16/12, 192.168/16 }
table <badhosts> persist
block on fxp0 from { <private>, <badhosts> } to any
creates a table called private, to hold RFC 1918 private network blocks, and a table called badhosts, which is initially empty. A filter rule is set up to block all traffic coming from addresses listed in either table. The private table cannot have its contents changed and the badhosts table will exist even when no active filter rules reference it. Addresses may later be added to the badhosts table, so that traffic from these hosts can be blocked by using
# pfctl -t badhosts -Tadd 204.92.77.111
A table can also be initialized with an address list specified in one or more external files, using the following syntax:
table <spam> persist file "/etc/spammers" file "/etc/openrelays" block on fxp0 from <spam> to any
The files /etc/spammers and /etc/openrelays list IP addresses, one per line. Any lines beginning with a # are treated as comments and ignored. In addition to being specified by IP address, hosts may also be specified by their hostname. When the resolver is called to add a hostname to a table, all resulting IPv4 and IPv6 addresses are placed into the table. IP addresses can also be entered in a table by specifying a valid interface name, a valid interface group or the self keyword, in which case all addresses assigned to the interface(s) will be added to the table.
Packet filter may be tuned for various situations using the set command.
When a packet matches a stateful connection, the seconds to live for the connection will be updated to that of the proto.modifier which corresponds to the connection state. Each packet which matches this state will reset the TTL. Tuning these values may improve the performance of the firewall at the risk of dropping valid idle connections.
ICMP and UDP are handled in a fashion similar to TCP, but with a much more limited set of states:
Other protocols are handled similarly to UDP:
Timeout values can be reduced adaptively as the number of state table entries grows.
Adaptive timeouts are enabled by default, with an adaptive.start value equal to 60% of the state limit, and an adaptive.end value equal to 120% of the state limit. They can be disabled by setting both adaptive.start and adaptive.end to 0.
The adaptive timeout values can be defined both globally and for each rule. When used on a per-rule basis, the values relate to the number of states created by the rule, otherwise to the total number of states.
For example:
set timeout tcp.first 120
set timeout tcp.established 86400
set timeout { adaptive.start 6000, adaptive.end 12000 }
set limit states 10000
With 9000 state table entries, the timeout values are scaled to 50% (tcp.first 60, tcp.established 43200).
# pfctl -s info
In this example packet filter collects statistics on the interface named dc0:
set loginterface dc0
One can disable the loginterface using:
set loginterface none
For example,
set limit states 20000
sets the maximum number of entries in the memory pool used by state table entries (generated by pass rules which do not specify no state) to 20000. Using
set limit frags 20000
sets the maximum number of entries in the memory pool used for fragment reassembly (generated by scrub rules) to 20000. Using
set limit src-nodes 2000
sets the maximum number of entries in the memory pool used for tracking source IP addresses (generated by the sticky-address and src.track options) to 2000. Using
set limit tables 1000 set limit table-entries 100000
sets limits on the memory pools used by tables. The first limits the number of tables that can exist to 1000. The second limits the overall number of addresses that can be stored in tables to 100000.
Various limits can be combined on a single line:
set limit { states 20000, frags 20000, src-nodes 2000 }
It is important to note that the ruleset optimizer will modify the ruleset to improve performance. A side effect of the ruleset modification is that per-rule accounting statistics will have different meanings than before. If per-rule accounting is important for billing purposes or whatnot, either the ruleset optimizer should not be used or a label field should be added to all of the accounting rules to act as optimization barriers.
Optimization can also be set as a command-line argument to
pfctl(8), overriding the settings in
pf.conf.
For example:
set optimization aggressive
For example:
set block-policy return
For example:
set state-policy if-bound
The hostid may be specified in either decimal or hexadecimal.
For example:
set fingerprints
"/etc/pf.os.devel"set skip on lo0Traffic normalization is used to sanitize packet content in such a way that there are no ambiguities in packet interpretation on the receiving side. The normalizer does IP fragment reassembly to prevent attacks that confuse intrusion detection systems by sending overlapping IP fragments. Packet normalization is invoked with the scrub directive.
scrub has the following options:
Unfortunately some operating systems also generate their dont-fragment packets with a zero IP identification field. Clearing the dont-fragment bit on packets with a zero IP ID may cause deleterious results if an upstream router later fragments the packet. Using the random-id modifier (see below) is recommended in combination with the no-df modifier to ensure unique IP identifiers.
For example,
scrub in on $ext_if all fragment reassemble
The no option prefixed to a scrub rule causes matching packets to remain unscrubbed, much in the same way as drop quick works in the packet filter (see below). This mechanism should be used when it is necessary to exclude specific packets from broader scrub rules.
Packets can be assigned to queues for the purpose of bandwidth
control. At least two declarations are required to configure queues, and
later any packet filtering rule can reference the defined queues by name.
During the filtering component of pf.conf, the last
referenced queue name is where any packets from
pass rules will be queued, while for
block rules it specifies where any resulting ICMP or
TCP RST packets should be queued. The scheduler
defines the algorithm used to decide which packets get delayed, dropped, or
sent out immediately. There are three schedulers
currently supported.
The interfaces on which queueing should be activated are declared using the altq on declaration. altq on has the following keywords:
In the following example, the interface dc0 should queue up to 5 Mbit/s in four second-level queues using Class Based Queueing. Those four queues will be shown in a later example.
altq on dc0 cbq bandwidth 5Mb queue { std, http, mail, ssh }
Once interfaces are activated for queueing using the altq directive, a sequence of queue directives may be defined. The name associated with a queue must match a queue defined in the altq directive (e.g. mail), or, except for the priq scheduler, in a parent queue declaration. The following keywords can be used:
The scheduler can get additional parameters with ⟨scheduler⟩ (⟨parameters⟩). Parameters are as follows:
The cbq scheduler supports an additional option:
The hfsc scheduler supports some additional options:
⟨sc⟩ is an acronym for service curve.
The format for service curve specifications is (m1, d, m2). m2 controls the bandwidth assigned to the queue. m1 and d are optional and can be used to control the initial bandwidth assignment. For the first d milliseconds the queue gets the bandwidth given as m1, afterwards the value given in m2.
Furthermore, with cbq and hfsc, child queues can be specified as in an altq declaration, thus building a tree of queues using a part of their parent's bandwidth.
Packets can be assigned to queues based on filter rules by using the queue keyword. Normally only one queue is specified; when a second one is specified it will instead be used for packets which have a TOS of lowdelay and for TCP ACKs with no data payload.
To continue the previous example, the examples below would specify the four referenced queues, plus a few child queues. Interactive ssh(1) sessions get priority over bulk transfers like scp(1) and sftp(1). The queues may then be referenced by filtering rules (see PACKET FILTERING below).
queue std bandwidth 10% cbq(default)
queue http bandwidth 60% priority 2 cbq(borrow red) \
{ employees, developers }
queue developers bandwidth 75% cbq(borrow)
queue employees bandwidth 15%
queue mail bandwidth 10% priority 0 cbq(borrow ecn)
queue ssh bandwidth 20% cbq(borrow) { ssh_interactive, ssh_bulk }
queue ssh_interactive bandwidth 50% priority 7 cbq(borrow)
queue ssh_bulk bandwidth 50% priority 0 cbq(borrow)
block return out on dc0 inet all queue std
pass out on dc0 inet proto tcp from $developerhosts to any port 80 \
queue developers
pass out on dc0 inet proto tcp from $employeehosts to any port 80 \
queue employees
pass out on dc0 inet proto tcp from any to any port 22 \
queue(ssh_bulk, ssh_interactive)
pass out on dc0 inet proto tcp from any to any port 25 \
queue mail
Translation rules modify either the source or destination address
of the packets associated with a stateful connection. A stateful connection
is automatically created to track packets matching such a rule as long as
they are not blocked by the filtering section of
pf.conf. The translation engine modifies the
specified address and/or port in the packet, recalculates IP, TCP and UDP
checksums as necessary, and passes it to the packet filter for
evaluation.
Since translation occurs before filtering the filter engine will see packets as they look after any addresses and ports have been translated. Filter rules will therefore have to filter based on the translated address and port number. Packets that match a translation rule are only automatically passed if the pass modifier is given, otherwise they are still subject to block and pass rules.
The state entry created permits packet filter to keep track of the original address for traffic associated with that state and correctly direct return traffic for that connection.
Various types of translation are possible with pf:
10.0.0.0 - 10.255.255.255 (all of net 10, i.e., 10/8) 172.16.0.0 - 172.31.255.255 (i.e., 172.16/12) 192.168.0.0 - 192.168.255.255 (i.e., 192.168/16)
In addition to modifying the address, some translation rules may modify source or destination ports for tcp(4) or udp(4) connections; implicitly in the case of nat rules and explicitly in the case of rdr rules. Port numbers are never translated with a binat rule.
Evaluation order of the translation rules is dependent on the type of the translation rules and of the direction of a packet. binat rules are always evaluated first. Then either the rdr rules are evaluated on an inbound packet or the nat rules on an outbound packet. Rules of the same type are evaluated in the same order in which they appear in the ruleset. The first matching rule decides what action is taken.
The no option prefixed to a translation rule causes packets to remain untranslated, much in the same way as drop quick works in the packet filter (see below). If no rule matches the packet it is passed to the filter engine unmodified.
Translation rules apply only to packets that pass through the specified interface, and if no interface is specified, translation is applied to packets on all interfaces. For instance, redirecting port 80 on an external interface to an internal web server will only work for connections originating from the outside. Connections to the address of the external interface from local hosts will not be redirected, since such packets do not actually pass through the external interface. Redirections cannot reflect packets back through the interface they arrive on, they can only be redirected to hosts connected to different interfaces or to the firewall itself.
Note that redirecting external incoming connections to the loopback address, as in
rdr on ne3 inet proto tcp to port smtp -> 127.0.0.1 port spamd
will effectively allow an external host to connect to daemons bound solely to the loopback address, circumventing the traditional blocking of such connections on a real interface. Unless this effect is desired, any of the local non-loopback addresses should be used as redirection target instead, which allows external connections only to daemons bound to this address or not bound to any address.
See TRANSLATION EXAMPLES below.
Packet filter has the ability to block and pass packets based on attributes of their layer 3 (see ip(4) and ip6(4)) and layer 4 (see icmp(4), icmp6(4), tcp(4), udp(4)) headers. In addition, packets may also be assigned to queues for the purpose of bandwidth control.
For each packet processed by the packet filter, the filter rules are evaluated in sequential order, from first to last. The last matching rule decides what action is taken. If no rule matches the packet, the default action is to pass the packet.
The following actions can be used in the filter:
Options returning ICMP packets currently have no effect if packet filter operates on a bridge(4), as the code to support this feature has not yet been implemented.
The simplest mechanism to block everything by default and only pass packets that match explicit rules is specify a first filter rule of:
block all
By default packet filter filters packets statefully; the first time a packet matches a pass rule, a state entry is created; for subsequent packets the filter checks whether the packet matches any state. If it does, the packet is passed without evaluation of any rules. After the connection is closed or times out, the state entry is automatically removed.
This has several advantages. For TCP connections, comparing a packet to a state involves checking its sequence numbers, as well as TCP timestamps if a scrub reassemble tcp rule applies to the connection. If these values are outside the narrow windows of expected values, the packet is dropped. This prevents spoofing attacks, such as when an attacker sends packets with a fake source address/port but does not know the connection's sequence numbers. Similarly, packet filter knows how to match ICMP replies to states. For example,
pass out inet proto icmp all icmp-type echoreq
allows echo requests (such as those created by ping(8)) out statefully, and matches incoming echo replies correctly to states.
Also, looking up states is usually faster than evaluating rules. If there are 50 rules, all of them are evaluated sequentially in O(n). Even with 50000 states, only 16 comparisons are needed to match a state, since states are stored in a binary search tree that allows searches in O(log2 n).
Furthermore, correct handling of ICMP error messages is critical to many protocols, particularly TCP. Packet filter matches ICMP error messages to the correct connection, checks them against connection parameters, and passes them if appropriate. For example if an ICMP source quench message referring to a stateful TCP connection arrives, it will be matched to the state and get passed.
Finally, state tracking is required for nat, binat and rdr rules, in order to track address and port translations and reverse the translation on returning packets.
Packet filter will also create state for other protocols which are effectively stateless by nature. UDP packets are matched to states using only host addresses and ports, and other protocols are matched to states using only the host addresses.
If stateless filtering of individual packets is desired, the no state keyword can be used to specify that state will not be created if this is the last matching rule. A number of parameters can also be set to affect how packet filter handles state tracking. See STATEFUL TRACKING OPTIONS below for further details.
The rule parameters specify the packets to which a rule applies. A packet always comes in on, or goes out through, one interface. Most parameters are optional. If a parameter is specified, the rule only applies to packets with matching attributes. Certain parameters can be expressed as lists, in which case pfctl(8) generates all needed rule combinations.
group keyword in
ifconfig(8).Addresses can be specified in CIDR notation (matching netblocks), as symbolic host names, interface names or interface group names, or as any of the following keywords:
Ranges of addresses are specified by using the ‘-’ operator. For instance: “10.1.1.10 - 10.1.1.12” means all addresses from 10.1.1.10 to 10.1.1.12, hence addresses 10.1.1.10, 10.1.1.11, and 10.1.1.12.
Interface names and interface group names can have modifiers appended:
Host names may also have the :0 option appended to restrict the name resolution to the first of each v4 and v6 address found.
Host name resolution and interface to address translation are done at ruleset load-time. When the address of an interface (or host name) changes (under DHCP or PPP, for instance), the ruleset must be reloaded for the change to be reflected in the kernel. Surrounding the interface name (and optional modifiers) in parentheses changes this behaviour. When the interface name is surrounded by parentheses, the rule is automatically updated whenever the interface changes its address. The ruleset does not need to be reloaded. This is especially useful with nat.
Ports can be specified either by number or by name. For example, port 80 can be specified as www. For a list of all port name to number mappings used by pfctl(8), see the file /etc/services.
Ports and ranges of ports are specified by using these operators:
= (equal) != (unequal) < (less than) ≤ (less than or equal) > (greater than) ≥ (greater than or equal) : (range including boundaries) >< (range excluding boundaries) <> (except range)
‘><’, ‘<>’ and ‘:’ are binary operators (they take two arguments). For instance:
The operating system of the source host can be specified in the case of TCP rules with the OS modifier. See the OPERATING SYSTEM FINGERPRINTING section for more information.
The host, port and OS specifications are optional, as in the following examples:
pass in all
pass in from any to any
pass in proto tcp from any port ≤ 1024 to any
pass in proto tcp from any to any port 25
pass in proto tcp from 10.0.0.0/8 port > 1024 \
to ! 10.1.2.3 port != ssh
pass in proto tcp from any os "OpenBSD"
pass in proto tcp from route "DTAG"
All packets, both outgoing and incoming, of one connection are associated with the same user and group. Only TCP and UDP packets can be associated with users; for other protocols these parameters are ignored.
User and group refer to the effective (as opposed to the real) IDs, in case the socket is created by a setuid/setgid process. User and group IDs are stored when a socket is created; when a process creates a listening socket as root (for instance, by binding to a privileged port) and subsequently changes to another user ID (to drop privileges), the credentials will remain root.
User and group IDs can be specified as either numbers or
names. The syntax is similar to the one for ports. The value
unknown matches packets of forwarded connections.
unknown can only be used with the operators
= and !=. Other
constructs like user ≥ unknown are
invalid. Forwarded packets with unknown user and group ID match only
rules that explicitly compare against unknown with the
operators = or !=. For
instance user ≥ 0 does not match
forwarded packets. The following example allows only selected users to
open outgoing connections:
block out proto { tcp, udp } all
pass out proto { tcp, udp } all user { < 1000, dhartmei }
Because flags S/SA is applied by default (unless no state is specified), only the initial SYN packet of a TCP handshake will create a state for a TCP connection. It is possible to be less restrictive, and allow state creation from intermediate (non-SYN) packets, by specifying flags any. This will cause packet filter to synchronize to existing connections, for instance if one flushes the state table. However, states created from such intermediate packets may be missing connection details such as the TCP window scaling factor. States which modify the packet flow, such as those affected by nat, binat or rdr rules, modulate or synproxy state options, or scrubbed with reassemble tcp will also not be recoverable from intermediate packets. Such connections will stall and time out.
For example, the following rules are identical:
pass all tos lowdelay pass all tos 0x10 pass all tos 16
The following macros can be used in labels:
For example:
ips = "{ 1.2.3.4, 1.2.3.5 }"
pass in proto tcp from any to $ips \
port > 1023 label "$dstaddr:$dstport"
expands to
pass in inet proto tcp from any to 1.2.3.4 \
port > 1023 label "1.2.3.4:>1023"
pass in inet proto tcp from any to 1.2.3.5 \
port > 1023 label "1.2.3.5:>1023"
The macro expansion for the label directive occurs only at configuration file parse time, not during runtime.
For example:
pass in proto tcp to port 25 queue mail pass in proto tcp to port 22 queue(ssh_bulk, ssh_prio)
!
operator before the tagged keyword.block in proto icmp probability 20%
If a packet matches a rule with a route option set, the packet filter will route the packet according to the type of route option. When such a rule creates state, the route option is also applied to all packets matching the same connection.
For nat and rdr rules, (as well as for the route-to, reply-to and dup-to rule options) for which there is a single redirection address which has a subnet mask smaller than 32 for IPv4 or 128 for IPv6 (more than one IP address), a variety of different methods for assigning this address can be used:
When more than one redirection address is specified, round-robin is the only permitted pool type.
Additionally, the sticky-address option can be specified to help ensure that multiple connections from the same source are mapped to the same redirection address. This option can be used with the random and round-robin pool options. Note that by default these associations are destroyed as soon as there are no longer states which refer to them; in order to make the mappings last beyond the lifetime of the states, increase the global options with set timeout src.track. See STATEFUL TRACKING OPTIONS for more ways to control the source tracking.
Much of the security derived from TCP is attributable to how well the initial sequence numbers (ISNs) are chosen. Some popular stack implementations choose very poor ISNs and thus are normally susceptible to ISN prediction exploits. By applying a modulate state rule to a TCP connection, packet filter will create a high quality random sequence number for each connection endpoint.
The modulate state directive implicitly keeps state on the rule and is only applicable to TCP connections.
For instance:
block all pass out proto tcp from any to any modulate state pass in proto tcp from any to any port 25 flags S/SFRA modulate state
Note that modulated connections will not recover when the state table is lost (firewall reboot, flushing the state table, etc...). Packet filter will not be able to infer a connection again after the state table flushes the connection's modulator. When the state is lost, the connection may be left dangling until the respective endpoints time out the connection. It is possible on a fast local network for the endpoints to start an ACK storm while trying to resynchronize after the loss of the modulator. The default flags settings (or a more strict equivalent) should be used on modulate state rules to prevent ACK storms.
By default, packet filter passes packets that are part of a tcp(4) handshake between the endpoints. The synproxy state option can be used to cause packet filter itself to complete the handshake with the active endpoint, perform a handshake with the passive endpoint, and then forward packets between the endpoints.
No packets are sent to the passive endpoint before the active endpoint has completed the handshake, hence so-called SYN floods with spoofed source addresses will not reach the passive endpoint, as the sender can't complete the handshake.
The proxy is transparent to both endpoints, they each see a single connection from/to the other endpoint. Packet filter chooses random initial sequence numbers for both handshakes. Once the handshakes are completed, the sequence number modulators (see previous section) are used to translate further packets of the connection. synproxy state includes modulate state.
Rules with synproxy will not work if packet filter operates on a bridge(4).
Example:
pass in proto tcp from any to any port www synproxy state
A number of options related to stateful tracking can be applied on a per-rule basis. keep state, modulate state and synproxy state support these options, and keep state must be specified explicitly to apply options to a rule.
Multiple options can be specified, separated by commas:
pass in proto tcp from any to any \
port www keep state \
(max 100, source-track rule, max-src-nodes 75, \
max-src-states 3, tcp.established 60, tcp.closing 5)
When the source-track keyword is specified, the number of states per source IP is tracked.
The following limits can be set:
For stateful TCP connections, limits on established connections (connections which have completed the TCP 3-way handshake) can also be enforced per source IP.
Because the 3-way handshake ensures that the source address is not being spoofed, more aggressive action can be taken based on these limits. With the overload ⟨table⟩ state option, source IP addresses which hit either of the limits on established connections will be added to the named table. This table can be used in the ruleset to block further activity from the offending host, redirect it to a tarpit process, or restrict its bandwidth.
The optional flush keyword kills all states created by the matching rule which originate from the host which exceeds these limits. The global modifier to the flush command kills all states originating from the offending host, regardless of which rule created the state.
For example, the following rules will protect the webserver against hosts making more than 100 connections in 10 seconds. Any host which connects faster than this rate will have its address added to the ⟨bad_hosts⟩ table and have all states originating from it flushed. Any new packets arriving from this host will be dropped unconditionally by the block rule.
block quick from <bad_hosts> pass in on $ext_if proto tcp to $webserver port www keep state \ (max-src-conn-rate 100/10, overload <bad_hosts> flush global)
Passive OS Fingerprinting is a mechanism to inspect nuances of a TCP connection's initial SYN packet and guess at the host's operating system. Unfortunately these nuances are easily spoofed by an attacker so the fingerprint is not useful in making security decisions. But the fingerprint is typically accurate enough to make policy decisions upon.
The fingerprints may be specified by operating system class, by version, or by subtype/patchlevel. The class of an operating system is typically the vendor or genre and would be OpenBSD for the packet filter firewall itself. The version of the oldest available OpenBSD release on the main FTP site would be 2.6 and the fingerprint would be written
"OpenBSD 2.6"The subtype of an operating system is typically used to describe the patchlevel if that patch led to changes in the TCP stack behavior. In the case of OpenBSD, the only subtype is for a fingerprint that was normalized by the no-df scrub option and would be specified as
"OpenBSD 3.3
no-df"Fingerprints for most popular operating systems are provided by pf.os(5). Once packet filter is running, a complete list of known operating system fingerprints may be listed by running:
# pfctl -soFilter rules can enforce policy at any level of operating system specification assuming a fingerprint is present. Policy could limit traffic to approved operating systems or even ban traffic from hosts that aren't at the latest service pack.
The unknown class can also be used as the fingerprint which will match packets for which no operating system fingerprint is known.
Examples:
pass out proto tcp from any os OpenBSD block out proto tcp from any os Doors block out proto tcp from any os "Doors PT" block out proto tcp from any os "Doors PT SP3" block out from any os "unknown" pass on lo0 proto tcp from any os "OpenBSD 3.3 lo0"
Operating system fingerprinting is limited only to the TCP SYN packet. This means that it will not work on other protocols and will not match a currently established connection.
Caveat: operating system fingerprints are occasionally wrong. There are three problems: an attacker can trivially craft his packets to appear as any operating system he chooses; an operating system patch could change the stack behavior and no fingerprints will match it until the database is updated; and multiple operating systems may have the same fingerprint.
"Spoofing" is the faking of IP addresses, typically for malicious purposes. The antispoof directive expands to a set of filter rules which will block all traffic with a source IP from the network(s) directly connected to the specified interface(s) from entering the system through any other interface.
For example, the line
antispoof for lo0
expands to
block drop in on ! lo0 inet from 127.0.0.1/8 to any block drop in on ! lo0 inet6 from ::1 to any
For non-loopback interfaces, there are additional rules to block incoming packets with a source IP address identical to the interface's IP(s). For example, assuming the interface wi0 had an IP address of 10.0.0.1 and a netmask of 255.255.255.0, the line
antispoof for wi0 inet
expands to
block drop in on ! wi0 inet from 10.0.0.0/24 to any block drop in inet from 10.0.0.1 to any
Caveat: Rules created by the antispoof directive interfere with packets sent over loopback interfaces to local addresses. One should pass these explicitly.
The size of IP datagrams (packets) can be significantly larger than the maximum transmission unit (MTU) of the network. In cases when it is necessary or more efficient to send such large packets, the large packet will be fragmented into many smaller packets that will each fit onto the wire. Unfortunately for a firewalling device, only the first logical fragment will contain the necessary header information for the subprotocol that allows packet filter to filter on things such as TCP ports or to perform NAT.
Besides the use of scrub rules as described in TRAFFIC NORMALIZATION above, there are three options for handling fragments in the packet filter.
One alternative is to filter individual fragments with filter rules. If no scrub rule applies to a fragment, it is passed to the filter. Filter rules with matching IP header parameters decide whether the fragment is passed or blocked, in the same way as complete packets are filtered. Without reassembly, fragments can only be filtered based on IP header fields (source/destination address, protocol), since subprotocol header fields are not available (TCP/UDP port numbers, ICMP code/type). The fragment option can be used to restrict filter rules to apply only to fragments, but not complete packets. Filter rules without the fragment option still apply to fragments, if they only specify IP header fields. For instance, the rule
pass in proto tcp from any to any port 80
never applies to a fragment, even if the fragment is part of a TCP packet with destination port 80, because without reassembly this information is not available for each fragment. This also means that fragments cannot create new or match existing state table entries, which makes stateful filtering and address translation (NAT, redirection) for fragments impossible.
It's also possible to reassemble only certain fragments by specifying source or destination addresses or protocols as parameters in scrub rules.
In most cases, the benefits of reassembly outweigh the additional memory cost, and it's recommended to use scrub rules to reassemble all fragments via the fragment reassemble modifier.
The memory allocated for fragment caching can be limited using pfctl(8). Once this limit is reached, fragments that would have to be cached are dropped until other entries time out. The timeout value can also be adjusted.
Currently, only IPv4 fragments are supported and IPv6 fragments are blocked unconditionally.
Besides the main ruleset, pfctl(8) can load rulesets into anchor attachment points. An anchor is a container that can hold rules, address tables, and other anchors.
An anchor has a name which specifies the path where pfctl(8) can be used to access the anchor to perform operations on it, such as attaching child anchors to it or loading rules into it. Anchors may be nested, with components separated by ‘/’ characters, similar to how file system hierarchies are laid out. The main ruleset is actually the default anchor, so filter and translation rules, for example, may also be contained in any anchor.
An anchor can reference another anchor attachment point using the following kinds of rules:
When evaluation of the main ruleset reaches an anchor rule, packet filter will proceed to evaluate all rules specified in that anchor.
Matching filter and translation rules marked with the quick option are final and abort the evaluation of the rules in other anchors and the main ruleset. If the anchor itself is marked with the quick option, ruleset evaluation will terminate when the anchor is exited if the packet is matched by any rule within the anchor.
anchor rules are evaluated relative to the anchor in which they are contained. For example, all anchor rules specified in the main ruleset will reference anchor attachment points underneath the main ruleset, and anchor rules specified in a file loaded from a load anchor rule will be attached under that anchor point.
Rules may be contained in anchor attachment points which do not contain any rules when the main ruleset is loaded, and later such anchors can be manipulated through pfctl(8) without reloading the main ruleset or other anchors. For example,
ext_if = "kue0"
block on $ext_if all
anchor spam
pass out on $ext_if all
pass in on $ext_if proto tcp from any \
to $ext_if port smtp
blocks all packets on the external interface by default, then evaluates all rules in the anchor named "spam", and finally passes all outgoing connections and incoming connections to port 25.
# echo "block in quick from 1.2.3.4 to any" | \
pfctl -a spam -f -
This loads a single rule into the anchor, which blocks all packets from a specific address.
The anchor can also be populated by adding a load anchor rule after the anchor rule:
anchor spam load anchor spam from "/etc/pf-spam.conf"
When pfctl(8) loads
pf.conf, it will also load all the rules from the
file /etc/pf-spam.conf into the anchor.
Optionally, anchor rules can specify packet filtering parameters using the same syntax as filter rules. When parameters are used, the anchor rule is only evaluated for matching packets. This allows conditional evaluation of anchors, like:
block on $ext_if all anchor spam proto tcp from any to any port smtp pass out on $ext_if all pass in on $ext_if proto tcp from any to $ext_if port smtp
The rules inside anchor spam are only evaluated for tcp packets with destination port 25. Hence,
# echo "block in quick from 1.2.3.4 to any" | \
pfctl -a spam -f -
will only block connections from 1.2.3.4 to port 25.
Anchors may end with the asterisk (‘*’) character, which signifies that all anchors attached at that point should be evaluated in the alphabetical ordering of their anchor name. For example,
anchor "spam/*"
will evaluate each rule in each anchor attached to the
spam anchor. Note that it will only evaluate anchors
that are directly attached to the spam anchor, and
will not descend to evaluate anchors recursively.
Since anchors are evaluated relative to the anchor in which they are contained, there is a mechanism for accessing the parent and ancestor anchors of a given anchor. Similar to file system path name resolution, if the sequence “..” appears as an anchor path component, the parent anchor of the current anchor in the path evaluation at that point will become the new current anchor. As an example, consider the following:
# echo ' anchor "spam/allowed" ' | pfctl -f -
# echo -e ' anchor "../banned" \n pass' | \
pfctl -a spam/allowed -f -
Evaluation of the main ruleset will lead into the
spam/allowed anchor, which will evaluate the rules
in the spam/banned anchor, if any, before finally
evaluating the pass rule.
Filter rule anchors can also be loaded inline in the ruleset within a brace ('{' '}') delimited block. Brace delimited blocks may contain rules or other brace-delimited blocks. When anchors are loaded this way the anchor name becomes optional.
anchor "external" on egress {
block
anchor out {
pass proto tcp from any to port { 25, 80, 443 }
}
pass in proto tcp to any port 22
}
Since the parser specification for anchor names is a string, any reference to an anchor name containing ‘/’ characters will require double quote (‘"’) characters around the anchor name.
This example maps incoming requests on port 80 to port 8080, on which a daemon is running (because, for example, it is not run as root, and therefore lacks permission to bind to port 80).
# use a macro for the interface name, so it can be changed easily ext_if = "ne3" # map daemon on 8080 to appear to be on 80 rdr on $ext_if proto tcp from any to any port 80 -> 127.0.0.1 port 8080
If the pass modifier is given, packets matching the translation rule are passed without inspecting the filter rules:
rdr pass on $ext_if proto tcp from any to any port 80 -> 127.0.0.1 \
port 8080
In the example below, vlan12 is configured as 192.168.168.1; the machine translates all packets coming from 192.168.168.0/24 to 204.92.77.111 when they are going out any interface except vlan12. This has the net effect of making traffic from the 192.168.168.0/24 network appear as though it is the Internet routable address 204.92.77.111 to nodes behind any interface on the router except for the nodes on vlan12. (Thus, 192.168.168.1 can talk to the 192.168.168.0/24 nodes.)
nat on ! vlan12 from 192.168.168.0/24 to any -> 204.92.77.111
In the example below, the machine sits between a fake internal 144.19.74.* network, and a routable external IP of 204.92.77.100. The no nat rule excludes protocol AH from being translated.
# NO NAT no nat on $ext_if proto ah from 144.19.74.0/24 to any nat on $ext_if from 144.19.74.0/24 to any -> 204.92.77.100
In the example below, packets bound for one specific server, as well as those generated by the sysadmins are not proxied; all other connections are.
# NO RDR
no rdr on $int_if proto { tcp, udp } from any to $server port 80
no rdr on $int_if proto { tcp, udp } from $sysadmins to any port 80
rdr on $int_if proto { tcp, udp } from any to any port 80 -> 127.0.0.1 \
port 80
This longer example uses both a NAT and a redirection. The external interface has the address 157.161.48.183. On localhost, we are running ftp-proxy(8), waiting for FTP sessions to be redirected to it. The three mandatory anchors for ftp-proxy(8) are omitted from this example; see the ftp-proxy(8) manpage.
# NAT
# Translate outgoing packets' source addresses (any protocol).
# In this case, any address but the gateway's external address is mapped.
nat on $ext_if inet from ! ($ext_if) to any -> ($ext_if)
# NAT PROXYING
# Map outgoing packets' source port to an assigned proxy port instead of
# an arbitrary port.
# In this case, proxy outgoing isakmp with port 500 on the gateway.
nat on $ext_if inet proto udp from any port = isakmp to any -> ($ext_if) \
port 500
# BINAT
# Translate outgoing packets' source address (any protocol).
# Translate incoming packets' destination address to an internal machine
# (bidirectional).
binat on $ext_if from 10.1.2.150 to any -> $ext_if
# RDR
# Translate incoming packets' destination addresses.
# As an example, redirect a TCP and UDP port to an internal machine.
rdr on $ext_if inet proto tcp from any to ($ext_if) port 8080 \
-> 10.1.2.151 port 22
rdr on $ext_if inet proto udp from any to ($ext_if) port 8080 \
-> 10.1.2.151 port 53
# RDR
# Translate outgoing ftp control connections to send them to localhost
# for proxying with ftp-proxy(8) running on port 8021.
rdr on $int_if proto tcp from any to any port 21 -> 127.0.0.1 port 8021
In this example, a NAT gateway is set up to translate internal addresses using a pool of public addresses (192.0.2.16/28) and to redirect incoming web server connections to a group of web servers on the internal network.
# NAT LOAD BALANCE
# Translate outgoing packets' source addresses using an address pool.
# A given source address is always translated to the same pool address by
# using the source-hash keyword.
nat on $ext_if inet from any to any -> 192.0.2.16/28 source-hash
# RDR ROUND ROBIN
# Translate incoming web server connections to a group of web servers on
# the internal network.
rdr on $ext_if proto tcp from any to any port 80 \
-> { 10.1.2.155, 10.1.2.160, 10.1.2.161 } round-robin
# The external interface is kue0
# (157.161.48.183, the only routable address)
# and the private network is 10.0.0.0/8, for which we are doing NAT.
# use a macro for the interface name, so it can be changed easily
ext_if = "kue0"
# normalize all incoming traffic
scrub in on $ext_if all fragment reassemble
# block and log everything by default
block return log on $ext_if all
# block anything coming from source we have no back routes for
block in from no-route to any
# block packets whose ingress interface does not match the one in
# the route back to their source address
block in from urpf-failed to any
# block and log outgoing packets that do not have our address as source,
# they are either spoofed or something is misconfigured (NAT disabled,
# for instance), we want to be nice and do not send out garbage.
block out log quick on $ext_if from ! 157.161.48.183 to any
# silently drop broadcasts (cable modem noise)
block in quick on $ext_if from any to 255.255.255.255
# block and log incoming packets from reserved address space and invalid
# addresses, they are either spoofed or misconfigured, we cannot reply to
# them anyway (hence, no return-rst).
block in log quick on $ext_if from { 10.0.0.0/8, 172.16.0.0/12, \
192.168.0.0/16, 255.255.255.255/32 } to any
# ICMP
# pass out/in certain ICMP queries and keep state (ping)
# state matching is done on host addresses and ICMP id (not type/code),
# so replies (like 0/0 for 8/0) will match queries
# ICMP error messages (which always refer to a TCP/UDP packet) are
# handled by the TCP/UDP states
pass on $ext_if inet proto icmp all icmp-type 8 code 0
# UDP
# pass out all UDP connections and keep state
pass out on $ext_if proto udp all
# pass in certain UDP connections and keep state (DNS)
pass in on $ext_if proto udp from any to any port domain
# TCP
# pass out all TCP connections and modulate state
pass out on $ext_if proto tcp all modulate state
# pass in certain TCP connections and keep state (SSH, SMTP, DNS, IDENT)
pass in on $ext_if proto tcp from any to any port { ssh, smtp, domain, \
auth }
# Do not allow Windows 9x SMTP connections since they are typically
# a viral worm. Alternately we could limit these OSes to 1 connection each.
block in on $ext_if proto tcp from any os {"Windows 95", "Windows 98"} \
to any port smtp
# IPv6
# pass in/out all IPv6 traffic: note that we have to enable this in two
# different ways, on both our physical interface and our tunnel
pass quick on gif0 inet6
pass quick on $ext_if proto ipv6
# Packet Tagging
# three interfaces: $int_if, $ext_if, and $wifi_if (wireless). NAT is
# being done on $ext_if for all outgoing packets. tag packets in on
# $int_if and pass those tagged packets out on $ext_if. all other
# outgoing packets (i.e., packets from the wireless network) are only
# permitted to access port 80.
pass in on $int_if from any to any tag INTNET
pass in on $wifi_if from any to any
block out on $ext_if from any to any
pass out quick on $ext_if tagged INTNET
pass out on $ext_if proto tcp from any to any port 80
# tag incoming packets as they are redirected to spamd(8). use the tag
# to pass those packets through the packet filter.
rdr on $ext_if inet proto tcp from <spammers> to port smtp \
tag SPAMD -> 127.0.0.1 port spamd
block in on $ext_if
pass in on $ext_if inet proto tcp tagged SPAMD
Syntax for pf.conf in BNF:
line = ( option | pf-rule | nat-rule | binat-rule | rdr-rule |
antispoof-rule | altq-rule | queue-rule | trans-anchors |
anchor-rule | anchor-close | load-anchor | table-rule |
include )
option = "set" ( [ "timeout" ( timeout | "{" timeout-list "}" ) ] |
[ "ruleset-optimization" [ "none" | "basic" | "profile" ]] |
[ "optimization" [ "default" | "normal" |
"high-latency" | "satellite" |
"aggressive" | "conservative" ] ]
[ "limit" ( limit-item | "{" limit-list "}" ) ] |
[ "loginterface" ( interface-name | "none" ) ] |
[ "block-policy" ( "drop" | "return" ) ] |
[ "state-policy" ( "if-bound" | "floating" ) ]
[ "require-order" ( "yes" | "no" ) ]
[ "fingerprints" filename ] |
[ "skip on" ifspec ] |
[ "debug" ( "none" | "urgent" | "misc" | "loud" ) ] )
pf-rule = action [ ( "in" | "out" ) ]
[ "log" [ "(" logopts ")"] ] [ "quick" ]
[ "on" ifspec ] [ "fastroute" | route ] [ af ] [ protospec ]
hosts [ filteropt-list ]
logopts = logopt [ "," logopts ]
logopt = "all" | "user" | "to" interface-name
filteropt-list = filteropt-list filteropt | filteropt
filteropt = user | group | flags | icmp-type | icmp6-type | tos |
( "no" | "keep" | "modulate" | "synproxy" ) "state"
[ "(" state-opts ")" ] |
"fragment" | "no-df" | "min-ttl" number |
"max-mss" number | "random-id" | "reassemble tcp" |
fragmentation | "allow-opts" |
"label" string | "tag" string | [ ! ] "tagged" string |
"queue" ( string | "(" string [ [ "," ] string ] ")" ) |
"rtable" number | "probability" number"%"
nat-rule = [ "no" ] "nat" [ "pass" [ "log" [ "(" logopts ")" ] ] ]
[ "on" ifspec ] [ af ]
[ protospec ] hosts [ "tag" string ] [ "tagged" string ]
[ "->" ( redirhost | "{" redirhost-list "}" )
[ portspec ] [ pooltype ] [ "static-port" ] ]
binat-rule = [ "no" ] "binat" [ "pass" [ "log" [ "(" logopts ")" ] ] ]
[ "on" interface-name ] [ af ]
[ "proto" ( proto-name | proto-number ) ]
"from" address [ "/" mask-bits ] "to" ipspec
[ "tag" string ] [ "tagged" string ]
[ "->" address [ "/" mask-bits ] ]
rdr-rule = [ "no" ] "rdr" [ "pass" [ "log" [ "(" logopts ")" ] ] ]
[ "on" ifspec ] [ af ]
[ protospec ] hosts [ "tag" string ] [ "tagged" string ]
[ "->" ( redirhost | "{" redirhost-list "}" )
[ portspec ] [ pooltype ] ]
antispoof-rule = "antispoof" [ "log" ] [ "quick" ]
"for" ifspec [ af ] [ "label" string ]
table-rule = "table" "<" string ">" [ tableopts-list ]
tableopts-list = tableopts-list tableopts | tableopts
tableopts = "persist" | "const" | "file" string |
"{" [ tableaddr-list ] "}"
tableaddr-list = tableaddr-list [ "," ] tableaddr-spec | tableaddr-spec
tableaddr-spec = [ "!" ] tableaddr [ "/" mask-bits ]
tableaddr = hostname | ifspec | "self" |
ipv4-dotted-quad | ipv6-coloned-hex
altq-rule = "altq on" interface-name queueopts-list
"queue" subqueue
queue-rule = "queue" string [ "on" interface-name ] queueopts-list
subqueue
anchor-rule = "anchor" [ string ] [ ( "in" | "out" ) ] [ "on" ifspec ]
[ af ] [ protospec ] [ hosts ] [ filteropt-list ] [ "{" ]
anchor-close = "}"
trans-anchors = ( "nat-anchor" | "rdr-anchor" | "binat-anchor" ) string
[ "on" ifspec ] [ af ] [ "proto" ] [ protospec ] [ hosts ]
load-anchor = "load anchor" string "from" filename
queueopts-list = queueopts-list queueopts | queueopts
queueopts = [ "bandwidth" bandwidth-spec ] |
[ "qlimit" number ] | [ "tbrsize" number ] |
[ "priority" number ] | [ schedulers ]
schedulers = ( cbq-def | priq-def | hfsc-def )
bandwidth-spec = "number" ( "b" | "Kb" | "Mb" | "Gb" | "%" )
action = "pass" | "block" [ return ] | [ "no" ] "scrub"
return = "drop" | "return" | "return-rst" [ "( ttl" number ")" ] |
"return-icmp" [ "(" icmpcode [ [ "," ] icmp6code ] ")" ] |
"return-icmp6" [ "(" icmp6code ")" ]
icmpcode = ( icmp-code-name | icmp-code-number )
icmp6code = ( icmp6-code-name | icmp6-code-number )
ifspec = ( [ "!" ] ( interface-name | interface-group ) ) |
"{" interface-list "}"
interface-list = [ "!" ] ( interface-name | interface-group )
[ [ "," ] interface-list ]
route = ( "route-to" | "reply-to" | "dup-to" )
( routehost | "{" routehost-list "}" )
[ pooltype ]
af = "inet" | "inet6"
protospec = "proto" ( proto-name | proto-number |
"{" proto-list "}" )
proto-list = ( proto-name | proto-number ) [ [ "," ] proto-list ]
hosts = "all" |
"from" ( "any" | "no-route" | "urpf-failed" | "self" | host |
"{" host-list "}" | "route" string ) [ port ] [ os ]
"to" ( "any" | "no-route" | "self" | host |
"{" host-list "}" | "route" string ) [ port ]
ipspec = "any" | host | "{" host-list "}"
host = [ "!" ] ( address [ "/" mask-bits ] | "<" string ">" )
redirhost = address [ "/" mask-bits ]
routehost = "(" interface-name [ address [ "/" mask-bits ] ] ")"
address = ( interface-name | interface-group |
"(" ( interface-name | interface-group ) ")" |
hostname | ipv4-dotted-quad | ipv6-coloned-hex )
host-list = host [ [ "," ] host-list ]
redirhost-list = redirhost [ [ "," ] redirhost-list ]
routehost-list = routehost [ [ "," ] routehost-list ]
port = "port" ( unary-op | binary-op | "{" op-list "}" )
portspec = "port" ( number | name ) [ ":" ( "*" | number | name ) ]
os = "os" ( os-name | "{" os-list "}" )
user = "user" ( unary-op | binary-op | "{" op-list "}" )
group = "group" ( unary-op | binary-op | "{" op-list "}" )
unary-op = [ "=" | "!=" | "<" | "≤" | ">" | "≥" ]
( name | number )
binary-op = number ( "<>" | "><" | ":" ) number
op-list = ( unary-op | binary-op ) [ [ "," ] op-list ]
os-name = operating-system-name
os-list = os-name [ [ "," ] os-list ]
flags = "flags" ( [ flag-set ] "/" flag-set | "any" )
flag-set = [ "F" ] [ "S" ] [ "R" ] [ "P" ] [ "A" ] [ "U" ] [ "E" ]
[ "W" ]
icmp-type = "icmp-type" ( icmp-type-code | "{" icmp-list "}" )
icmp6-type = "icmp6-type" ( icmp-type-code | "{" icmp-list "}" )
icmp-type-code = ( icmp-type-name | icmp-type-number )
[ "code" ( icmp-code-name | icmp-code-number ) ]
icmp-list = icmp-type-code [ [ "," ] icmp-list ]
tos = "tos" ( "lowdelay" | "throughput" | "reliability" |
[ "0x" ] number )
state-opts = state-opt [ [ "," ] state-opts ]
state-opt = ( "max" number | "no-sync" | timeout |
"source-track" [ ( "rule" | "global" ) ] |
"max-src-nodes" number | "max-src-states" number |
"max-src-conn" number |
"max-src-conn-rate" number "/" number |
"overload" "<" string ">" [ "flush" ] |
"if-bound" | "floating" )
fragmentation = [ "fragment reassemble" | "fragment crop" |
"fragment drop-ovl" ]
timeout-list = timeout [ [ "," ] timeout-list ]
timeout = ( "tcp.first" | "tcp.opening" | "tcp.established" |
"tcp.closing" | "tcp.finwait" | "tcp.closed" |
"udp.first" | "udp.single" | "udp.multiple" |
"icmp.first" | "icmp.error" |
"other.first" | "other.single" | "other.multiple" |
"frag" | "interval" | "src.track" |
"adaptive.start" | "adaptive.end" ) number
limit-list = limit-item [ [ "," ] limit-list ]
limit-item = ( "states" | "frags" | "src-nodes" ) number
pooltype = ( "bitmask" | "random" |
"source-hash" [ ( hex-key | string-key ) ] |
"round-robin" ) [ sticky-address ]
subqueue = string | "{" queue-list "}"
queue-list = string [ [ "," ] string ]
cbq-def = "cbq" [ "(" cbq-opt [ [ "," ] cbq-opt ] ")" ]
priq-def = "priq" [ "(" priq-opt [ [ "," ] priq-opt ] ")" ]
hfsc-def = "hfsc" [ "(" hfsc-opt [ [ "," ] hfsc-opt ] ")" ]
cbq-opt = ( "default" | "borrow" | "red" | "ecn" | "rio" )
priq-opt = ( "default" | "red" | "ecn" | "rio" )
hfsc-opt = ( "default" | "red" | "ecn" | "rio" |
linkshare-sc | realtime-sc | upperlimit-sc )
linkshare-sc = "linkshare" sc-spec
realtime-sc = "realtime" sc-spec
upperlimit-sc = "upperlimit" sc-spec
sc-spec = ( bandwidth-spec |
"(" bandwidth-spec number bandwidth-spec ")" )
icmp(4), icmp6(4), ip(4), ip6(4), route(4), tcp(4), udp(4), hosts(5), pf.os(5), protocols(5), services(5), ftp-proxy(8), pfctl(8), route(8)
The pf.conf file format first appeared in
OpenBSD 3.0.
| February 11, 2008 | Mac OS X 12 |