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PF.CONF(5) File Formats Manual PF.CONF(5) NAME pf.conf -- packet filter configuration file DESCRIPTION The pf(4) packet filter modifies, drops or passes packets according to rules or definitions specified in pf.conf. STATEMENT ORDER There are seven types of statements in pf.conf: Macros User-defined variables may be defined and used later, simplifying the configuration file. Macros must be defined before they are referenced in pf.conf. Tables Tables provide a mechanism for increasing the performance and flexibility of rules with large numbers of source or destination addresses. Options Options tune the behaviour of the packet filtering engine. Traffic Normalization (e.g. scrub) Traffic normalization protects internal machines against incon- sistencies in Internet protocols and implementations. Queueing Queueing provides rule-based bandwidth control. Translation (Various forms of NAT) Translation rules specify how addresses are to be mapped or redi- rected to other addresses. Packet Filtering Packet filtering provides rule-based blocking or passing of pack- ets. With 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 key- word, for example: include "/etc/pf/sub.filter.conf" MACROS Macros can be defined that will later be expanded in context. Macro names must start with a letter, and may contain letters, digits and un- derscores. 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 Tables are named structures which can hold a collection of addresses and networks. Lookups against tables in pf(4) 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 automati- cally 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 ad- dress 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. manually Persistent tables can be manually created with the add or replace option of pfctl(8), before or after the ruleset has been loaded. pf.conf Table definitions can be placed directly in this file, and loaded at the same time as other rules are loaded, atomi- cally. Table definitions inside pf.conf use the table state- ment, and are especially useful to define non-persistent ta- bles. 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 attributes: persist The persist flag forces the kernel to keep the table even when no rules refer to it. If the flag is not set, the kernel will automatically remove the table when the last rule referring to it is flushed. const The const flag prevents the user from altering the contents of the table once it has been created. Without that flag, pfctl(8) can be used to add or remove addresses from the table at any time, even when running with securelevel(7) = 2. counters The counters flag enables per-address packet and byte counters which can be displayed with pfctl(8). Note that this feature carries significant memory overhead for large tables. 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 fil- ter 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 refer- ence 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 ig- nored. 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 speci- fying a valid interface name, a valid interface group or the self key- word, in which case all addresses assigned to the interface(s) will be added to the table. OPTIONS pf(4) may be tuned for various situations using the set command. set timeout interval Interval between purging expired states and fragments. frag Seconds before an unassembled fragment is expired. src.track Length of time to retain a source tracking entry after the last state expires. 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. tcp.first The state after the first packet. tcp.opening The state before the destination host ever sends a packet. tcp.established The fully established state. tcp.closing The state after the first FIN has been sent. tcp.finwait The state after both FINs have been exchanged and the con- nection is closed. Some hosts (notably web servers on So- laris) send TCP packets even after closing the connection. Increasing tcp.finwait (and possibly tcp.closing) can pre- vent blocking of such packets. tcp.closed The state after one endpoint sends an RST. ICMP and UDP are handled in a fashion similar to TCP, but with a much more limited set of states: udp.first The state after the first packet. udp.single The state if the source host sends more than one packet but the destination host has never sent one back. udp.multiple The state if both hosts have sent packets. icmp.first The state after the first packet. icmp.error The state after an ICMP error came back in response to an ICMP packet. Other protocols are handled similarly to UDP: other.first other.single other.multiple Timeout values can be reduced adaptively as the number of state table entries grows. adaptive.start When the number of state entries exceeds this value, adap- tive scaling begins. All timeout values are scaled lin- early with factor (adaptive.end - number of states) / (adaptive.end - adaptive.start). adaptive.end When reaching this number of state entries, all timeout values become zero, effectively purging all state entries immediately. This value is used to define the scale fac- tor, it should not actually be reached (set a lower state limit, see below). 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 set- ting 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). set loginterface Enable collection of packet and byte count statistics for the given interface or interface group. These statistics can be viewed using # pfctl -s info In this example pf(4) collects statistics on the interface named dc0: set loginterface dc0 One can disable the loginterface using: set loginterface none set limit Sets hard limits on the memory pools used by the packet filter. See zone(9) for an explanation of memory pools. 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 } set ruleset-optimization none Disable the ruleset optimizer. basic Enable basic ruleset optimization. This is the default behaviour. Basic ruleset optimization does four things to improve the performance of ruleset evaluations: 1. remove duplicate rules 2. remove rules that are a subset of another rule 3. combine multiple rules into a table when advanta- geous 4. re-order the rules to improve evaluation perfor- mance profile Uses the currently loaded ruleset as a feedback profile to tailor the ordering of quick rules to actual network traffic. 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 impor- tant for billing purposes or whatnot, either the ruleset opti- mizer 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. set optimization Optimize state timeouts for one of the following network environ- ments: normal A normal network environment. Suitable for almost all net- works. high-latency A high-latency environment (such as a satellite connec- tion). satellite Alias for high-latency. aggressive Aggressively expire connections. This can greatly reduce the memory usage of the firewall at the cost of dropping idle connections early. conservative Extremely conservative settings. Avoid dropping legitimate connections at the expense of greater memory utilization (possibly much greater on a busy network) and slightly in- creased processor utilization. For example: set optimization aggressive set block-policy The block-policy option sets the default behaviour for the packet block action: drop Packet is silently dropped. return A TCP RST is returned for blocked TCP packets, an ICMP UNREACHABLE is returned for blocked UDP packets, and all other packets are silently dropped. For example: set block-policy return set fail-policy The fail-policy option sets the behaviour of rules which should pass a packet but were unable to do so. This might happen when a nat or route-to rule uses an empty table as list of targets or if a rule fails to create state or source node. The following block actions are possible: drop Incoming packet is silently dropped. return Incoming packet is dropped and TCP RST is returned for TCP packets, an ICMP UNREACHABLE is returned for UDP packets, and no response is sent for other packets. For example: set fail-policy return set state-policy The state-policy option sets the default behaviour for states: if-bound States are bound to interface. floating States can match packets on any interfaces (the de- fault). For example: set state-policy if-bound set syncookies never | always | adaptive When syncookies are active, pf will answer each incoming TCP SYN with a syncookie SYNACK, without allocating any resources. Upon reception of the client's ACK in response to the syncookie SYNACK, pf will evaluate the ruleset and create state if the ruleset permits it, complete the three way handshake with the target host and continue the connection with synproxy in place. This allows pf to be resilient against large synflood attacks which would run the state table against its limits otherwise. Due to the blind answers to every incoming SYN syncookies share the caveats of synproxy, namely seemingly accepting connections that will be dropped later on. never pf will never send syncookie SYNACKs (the default). always pf will always send syncookie SYNACKs. adaptive pf will enable syncookie mode when a given percentage of the state table is used up by half-open TCP connec- tions, as in, those that saw the initial SYN but didn't finish the three way handshake. The thresholds for en- tering and leaving syncookie mode can be specified us- ing set syncookies adaptive (start 25%, end 12%) set state-defaults The state-defaults option sets the state options for states cre- ated from rules without an explicit keep state. For example: set state-defaults no-sync set hostid The 32-bit hostid identifies this firewall's state table entries to other firewalls in a pfsync(4) failover cluster. By default the hostid is set to a pseudo-random value, however it may be de- sirable to manually configure it, for example to more easily identify the source of state table entries. set hostid 1 The hostid may be specified in either decimal or hexadecimal. set require-order By default pfctl(8) enforces an ordering of the statement types in the ruleset to: options, normalization, queueing, translation, filtering. Setting this option to no disables this enforcement. There may be non-trivial and non-obvious implications to an out of order ruleset. Consider carefully before disabling the order enforcement. set fingerprints Load fingerprints of known operating systems from the given file- name. By default fingerprints of known operating systems are au- tomatically loaded from pf.os(5) in /etc but can be overridden via this option. Setting this option may leave a small period of time where the fingerprints referenced by the currently active ruleset are inconsistent until the new ruleset finishes loading. For example: set fingerprints "/etc/pf.os.devel" set skip on <ifspec> List interfaces for which packets should not be filtered. Pack- ets passing in or out on such interfaces are passed as if pf was disabled, i.e. pf does not process them in any way. This can be useful on loopback and other virtual interfaces, when packet fil- tering is not desired and can have unexpected effects. For exam- ple: set skip on lo0 set debug Set the debug level to one of the following: none Don't generate debug messages. urgent Generate debug messages only for serious errors. misc Generate debug messages for various errors. loud Generate debug messages for common conditions. set keepcounters Preserve rule counters across rule updates. Usually rule coun- ters are reset to zero on every update of the ruleset. With keepcounters set pf will attempt to find matching rules between old and new rulesets and preserve the rule counters. TRAFFIC NORMALIZATION Traffic 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: no-df Clears the dont-fragment bit from a matching IP packet. Some op- erating systems are known to generate fragmented packets with the dont-fragment bit set. This is particularly true with NFS. Scrub will drop such fragmented dont-fragment packets unless no-df is specified. 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 recom- mended in combination with the no-df modifier to ensure unique IP identifiers. min-ttl <number> Enforces a minimum TTL for matching IP packets. max-mss <number> Enforces a maximum MSS for matching TCP packets. set-tos <string> | <number> Enforces a TOS for matching IP packets. TOS may be given as one of critical, inetcontrol, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, va, af11 ... af43, cs0 ... cs7; or as either hex or decimal. random-id Replaces the IP identification field with random values to com- pensate for predictable values generated by many hosts. This op- tion only applies to packets that are not fragmented after the optional fragment reassembly. fragment reassemble Using scrub rules, fragments can be reassembled by normalization. In this case, fragments are buffered until they form a complete packet, and only the completed packet is passed on to the filter. The advantage is that filter rules have to deal only with com- plete packets, and can ignore fragments. The drawback of caching fragments is the additional memory cost. reassemble tcp Statefully normalizes TCP connections. scrub reassemble tcp rules may not have the direction (in/out) specified. reassemble tcp performs the following normalizations: ttl Neither side of the connection is allowed to reduce their IP TTL. An attacker may send a packet such that it reaches the firewall, affects the firewall state, and expires before reaching the destination host. reassemble tcp will raise the TTL of all packets back up to the highest value seen on the connection. timestamp modulation Modern TCP stacks will send a timestamp on every TCP packet and echo the other endpoint's timestamp back to them. Many operating systems will merely start the timestamp at zero when first booted, and increment it several times a second. The uptime of the host can be deduced by reading the timestamp and multiplying by a constant. Also observing several different timestamps can be used to count hosts behind a NAT device. And spoofing TCP packets into a connection requires knowing or guessing valid timestamps. Timestamps merely need to be monotonically increasing and not derived off a guess- able base time. reassemble tcp will cause scrub to mod- ulate the TCP timestamps with a random number. extended PAWS checks There is a problem with TCP on long fat pipes, in that a packet might get delayed for longer than it takes the connection to wrap its 32-bit sequence space. In such an occurrence, the old packet would be indistinguishable from a new packet and would be accepted as such. The solution to this is called PAWS: Protection Against Wrapped Sequence numbers. It protects against it by making sure the timestamp on each packet does not go backwards. reassemble tcp also makes sure the timestamp on the packet does not go forward more than the RFC al- lows. By doing this, pf(4) artificially extends the se- curity of TCP sequence numbers by 10 to 18 bits when the host uses appropriately randomized timestamps, since a blind attacker would have to guess the timestamp as well. For example, scrub in on $ext_if all fragment reassemble The no option prefixed to a scrub rule causes matching packets to re- main 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. QUEUEING The ALTQ system is currently not available in the GENERIC kernel nor as loadable modules. In order to use the herein after called queueing op- tions one has to use a custom built kernel. Please refer to altq(4) to learn about the related kernel options. 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. cbq Class Based Queueing. Queues attached to an interface build a tree, thus each queue can have further child queues. Each queue can have a priority and a bandwidth assigned. Priority mainly controls the time packets take to get sent out, while bandwidth has primarily effects on throughput. cbq achieves both parti- tioning and sharing of link bandwidth by hierarchically struc- tured classes. Each class has its own queue and is assigned its share of bandwidth. A child class can borrow bandwidth from its parent class as long as excess bandwidth is available (see the option borrow, below). priq Priority Queueing. Queues are flat attached to the interface, thus, queues cannot have further child queues. Each queue has a unique priority assigned, ranging from 0 to 15. Packets in the queue with the highest priority are processed first. hfsc Hierarchical Fair Service Curve. Queues attached to an interface build a tree, thus each queue can have further child queues. Each queue can have a priority and a bandwidth assigned. Priority mainly controls the time packets take to get sent out, while bandwidth primarily affects throughput. hfsc supports both link-sharing and guaranteed real-time services. It employs a service curve based QoS model, and its unique feature is an abil- ity to decouple delay and bandwidth allocation. The interfaces on which queueing should be activated are declared using the altq on declaration. altq on has the following keywords: <interface> Queueing is enabled on the named interface. <scheduler> Specifies which queueing scheduler to use. Currently supported values are cbq for Class Based Queueing, priq for Priority Queue- ing and hfsc for the Hierarchical Fair Service Curve scheduler. bandwidth <bw> The maximum bitrate for all queues on an interface may be speci- fied using the bandwidth keyword. The value can be specified as an absolute value or as a percentage of the interface bandwidth. When using an absolute value, the suffixes b, Kb, Mb, and Gb are used to represent bits, kilobits, megabits, and gigabits per sec- ond, respectively. The value must not exceed the interface band- width. If bandwidth is not specified, the interface bandwidth is used (but take note that some interfaces do not know their band- width, or can adapt their bandwidth rates). qlimit <limit> The maximum number of packets held in the queue. The default is 50. tbrsize <size> Adjusts the size, in bytes, of the token bucket regulator. If not specified, heuristics based on the interface bandwidth are used to determine the size. queue <list> Defines a list of subqueues to create on an interface. In the following example, the interface dc0 should queue up to 5Mbps 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: on <interface> Specifies the interface the queue operates on. If not given, it operates on all matching interfaces. bandwidth <bw> Specifies the maximum bitrate to be processed by the queue. This value must not exceed the value of the parent queue and can be specified as an absolute value or a percentage of the parent queue's bandwidth. If not specified, defaults to 100% of the parent queue's bandwidth. The priq scheduler does not support bandwidth specification. priority <level> Between queues a priority level can be set. For cbq and hfsc, the range is 0 to 7 and for priq, the range is 0 to 15. The de- fault for all is 1. Priq queues with a higher priority are al- ways served first. Cbq and Hfsc queues with a higher priority are preferred in the case of overload. qlimit <limit> The maximum number of packets held in the queue. The default is 50. The scheduler can get additional parameters with <scheduler> (<parameters>). Parameters are as follows: default Packets not matched by another queue are assigned to this one. Exactly one default queue is required. red Enable RED (Random Early Detection) on this queue. RED drops packets with a probability proportional to the aver- age queue length. rio Enables RIO on this queue. RIO is RED with IN/OUT, thus running RED two times more than RIO would achieve the same effect. RIO is currently not supported in the GENERIC ker- nel. ecn Enables ECN (Explicit Congestion Notification) on this queue. ECN implies RED. The cbq scheduler supports an additional option: borrow The queue can borrow bandwidth from the parent. The hfsc scheduler supports some additional options: realtime <sc> The minimum required bandwidth for the queue. upperlimit <sc> The maximum allowed bandwidth for the queue. linkshare <sc> The bandwidth share of a backlogged queue. <sc> is an acronym for service curve. The format for service curve specifications is (m1, d, m2). m2 con- trols 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 Translation rules modify either the source or destination address of the packets associated with a stateful connection. A stateful connec- tion 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 trans- lated. Filter rules will therefore have to filter based on the trans- lated 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 pf(4) to keep track of the original ad- dress for traffic associated with that state and correctly direct re- turn traffic for that connection. Various types of translation are possible with pf: binat A binat rule specifies a bidirectional mapping between an exter- nal IP netblock and an internal IP netblock. nat A nat rule specifies that IP addresses are to be changed as the packet traverses the given interface. This technique allows one or more IP addresses on the translating host to support network traffic for a larger range of machines on an "inside" network. Although in theory any IP address can be used on the inside, it is strongly recommended that one of the address ranges defined by RFC 1918 be used. These netblocks are: 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) rdr The packet is redirected to another destination and possibly a different port. rdr rules can optionally specify port ranges in- stead of single ports. rdr ... port 2000:2999 -> ... port 4000 redirects ports 2000 to 2999 (inclusive) to port 4000. rdr ... port 2000:2999 -> ... port 4000:* redirects port 2000 to 4000, 2001 to 4001, ..., 2999 to 4999. In addition to modifying the address, some translation rules may modify source or destination ports for tcp(4) or udp(4) connections; implic- itly 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 ex- ternal interface to an internal web server will only work for connec- tions 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. Redirec- tions 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 ad- dress, 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 de- sired, any of the local non-loopback addresses should be used as redi- rection target instead, which allows external connections only to dae- mons bound to this address or not bound to any address. See "TRANSLATION EXAMPLES" below. PACKET FILTERING pf(4) has the ability to block , pass and match packets based on at- tributes 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. For block and pass , the last matching rule decides what action is taken. For match , rules are evaulated every time they match; the pass/block state of a packet remains unchanged. If no rule matches the packet, the default action is to pass the packet. The following actions can be used in the filter: block The packet is blocked. There are a number of ways in which a block rule can behave when blocking a packet. The default behav- iour is to drop packets silently, however this can be overridden or made explicit either globally, by setting the block-policy op- tion, or on a per-rule basis with one of the following options: drop The packet is silently dropped. return-rst This applies only to tcp(4) packets, and issues a TCP RST which closes the connection. return-icmp return-icmp6 This causes ICMP messages to be returned for packets which match the rule. By default this is an ICMP UNREACHABLE message, however this can be overridden by specifying a message as a code or number. return This causes a TCP RST to be returned for tcp(4) packets and an ICMP UNREACHABLE for UDP and other packets. Options returning ICMP packets currently have no effect if pf(4) operates on a if_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 match The packet is matched. This mechanism is used to provide fine grained filtering without altering the block/pass state of a packet. match rules differ from block and pass rules in that pa- rameters are set every time a packet matches the rule, not only on the last matching rule. For the following parameters, this means that the parameter effectively becomes "sticky" until ex- plicitly overridden: queue pass The packet is passed; state is created unless the no state option is specified. By default pf(4) 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 con- nection is closed or times out, the state entry is automatically re- moved. This has several advantages. For TCP connections, comparing a packet to a state involves checking its sequence numbers, as well as TCP time- stamps 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 at- tacker sends packets with a fake source address/port but does not know the connection's sequence numbers. Similarly, pf(4) 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. pf(4) 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 mes- sage 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 transla- tion on returning packets. pf(4) 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 pf(4) handles state tracking. See "STATEFUL TRACKING OPTIONS" below for further details. PARAMETERS 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. in or out This rule applies to incoming or outgoing packets. If neither in nor out are specified, the rule will match packets in both direc- tions. log In addition to the action specified, a log message is generated. Only the packet that establishes the state is logged, unless the no state option is specified. The logged packets are sent to a pflog(4) interface, by default pflog0. This interface is moni- tored by the pflogd(8) logging daemon, which dumps the logged packets to the file /var/log/pflog in pcap(3) binary format. log (all) Used to force logging of all packets for a connection. This is not necessary when no state is explicitly specified. As with log, packets are logged to pflog(4). log (user) Logs the Unix user ID of the user that owns the socket and the PID of the process that has the socket open where the packet is sourced from or destined to (depending on which socket is local). This is in addition to the normal information logged. Only the first packet logged via log (all, user) will have the user credentials logged when using stateful matching. log (to <interface>) Send logs to the specified pflog(4) interface instead of pflog0. quick If a packet matches a rule which has the quick option set, this rule is considered the last matching rule, and evaluation of sub- sequent rules is skipped. on <interface> This rule applies only to packets coming in on, or going out through, this particular interface or interface group. For more information on interface groups, see the group keyword in ifconfig(8). <af> This rule applies only to packets of this address family. Sup- ported values are inet and inet6. proto <protocol> This rule applies only to packets of this protocol. Common pro- tocols are icmp(4), icmp6(4), tcp(4), and udp(4). For a list of all the protocol name to number mappings used by pfctl(8), see the file /etc/protocols. from <source> port <source> os <source> to <dest> port <dest> This rule applies only to packets with the specified source and destination addresses and ports. 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: any Any address. no-route Any address which is not currently routable. urpf-failed Any source address that fails a unicast reverse path forwarding (URPF) check, i.e. packets coming in on an interface other than that which holds the route back to the packet's source address. <table> Any address that matches the given table. 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 ap- pended: :network Translates to the network(s) attached to the inter- face. :broadcast Translates to the interface's broadcast ad- dress(es). :peer Translates to the point-to-point interface's peer address(es). :0 Do not include interface aliases. Host names may also have the :0 option appended to restrict the name resolution to the first of each v4 and non-link-local v6 ad- dress 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 paren- theses changes this behaviour. When the interface name is sur- rounded by parentheses, the rule is automatically updated when- ever 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 argu- ments). For instance: port 2000:2004 means `all ports >= 2000 and <= 2004', hence ports 2000, 2001, 2002, 2003 and 2004. port 2000 >< 2004 means `all ports > 2000 and < 2004', hence ports 2001, 2002 and 2003. port 2000 <> 2004 means `all ports < 2000 or > 2004', hence ports 1-1999 and 2005-65535. 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 fol- lowing 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" all This is equivalent to "from any to any". group <group> Similar to user, this rule only applies to packets of sockets owned by the specified group. user <user> This rule only applies to packets of sockets owned by the speci- fied user. For outgoing connections initiated from the firewall, this is the user that opened the connection. For incoming con- nections to the firewall itself, this is the user that listens on the destination port. For forwarded connections, where the fire- wall is not a connection endpoint, the user and group are unknown. All packets, both outgoing and incoming, of one connection are associated with the same user and group. Only TCP and UDP pack- ets can be associated with users; for other protocols these para- meters 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 se- lected users to open outgoing connections: block out proto { tcp, udp } all pass out proto { tcp, udp } all user { < 1000, dhartmei } flags <a> /<b> | /<b> | any This rule only applies to TCP packets that have the flags <a> set out of set <b>. Flags not specified in <b> are ignored. For stateful connections, the default is flags S/SA. To indicate that flags should not be checked at all, specify flags any. The flags are: (F)IN, (S)YN, (R)ST, (P)USH, (A)CK, (U)RG, (E)CE, and C(W)R. flags S/S Flag SYN is set. The other flags are ignored. flags S/SA This is the default setting for stateful connections. Out of SYN and ACK, exactly SYN may be set. SYN, SYN+PSH and SYN+RST match, but SYN+ACK, ACK and ACK+RST do not. This is more restrictive than the previous example. flags /SFRA If the first set is not specified, it defaults to none. All of SYN, FIN, RST and ACK must be unset. 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 pf(4) to syn- chronize 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. icmp-type <type> code <code> icmp6-type <type> code <code> This rule only applies to ICMP or ICMPv6 packets with the speci- fied type and code. Text names for ICMP types and codes are listed in icmp(4) and icmp6(4). This parameter is only valid for rules that cover protocols ICMP or ICMP6. The protocol and the ICMP type indicator (icmp-type or icmp6-type) must match. tos <string> | <number> This rule applies to packets with the specified TOS bits set. TOS may be given as one of critical, inetcontrol, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, va, af11 ... af43, cs0 ... cs7; or as either hex or decimal. For example, the following rules are identical: pass all tos lowdelay pass all tos 0x10 pass all tos 16 allow-opts By default, IPv4 packets with IP options or IPv6 packets with routing extension headers are blocked. When allow-opts is speci- fied for a pass rule, packets that pass the filter based on that rule (last matching) do so even if they contain IP options or routing extension headers. For packets that match state, the rule that initially created the state is used. The implicit pass rule that is used when a packet does not match any rules does not allow IP options. label <string> Adds a label (name) to the rule, which can be used to identify the rule. For instance, pfctl -s labels shows per-rule statis- tics for rules that have labels. The following macros can be used in labels: $if The interface. $srcaddr The source IP address. $dstaddr The destination IP address. $srcport The source port specification. $dstport The destination port specification. $proto The protocol name. $nr The rule number. 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 con- figuration file parse time, not during runtime. ridentifier <number> Add an identifier (number) to the rule, which can be used to cor- relate the rule to pflog entries, even after ruleset updates. queue <queue> | (<queue>, <queue>) Packets matching this rule will be assigned to the specified queue. If two queues are given, packets which have a TOS of lowdelay and TCP ACKs with no data payload will be assigned to the second one. See "QUEUEING" for setup details. For example: pass in proto tcp to port 25 queue mail pass in proto tcp to port 22 queue(ssh_bulk, ssh_prio) set prio priority | (priority, priority) Packets matching this rule will be assigned a specific queueing priority. Priorities are assigned as integers 0 through 7. If the packet is transmitted on a vlan(4) interface, the queueing priority will be written as the priority code point in the 802.1Q VLAN header. If two priorities are given, packets which have a TOS of lowdelay and TCP ACKs with no data payload will be as- signed to the second one. For example: pass in proto tcp to port 25 set prio 2 pass in proto tcp to port 22 set prio (2, 5) tag <string> Packets matching this rule will be tagged with the specified string. The tag acts as an internal marker that can be used to identify these packets later on. This can be used, for example, to provide trust between interfaces and to determine if packets have been processed by translation rules. Tags are "sticky", meaning that the packet will be tagged even if the rule is not the last matching rule. Further matching rules can replace the tag with a new one but will not remove a previously applied tag. A packet is only ever assigned one tag at a time. Packet tagging can be done during nat, rdr, or binat rules in addition to filter rules. Tags take the same macros as labels (see above). tagged <string> Used with filter, translation or scrub rules to specify that packets must already be tagged with the given tag in order to match the rule. Inverse tag matching can also be done by speci- fying the ! operator before the tagged keyword. rtable <number> Used to select an alternate routing table for the routing lookup. Only effective before the route lookup happened, i.e. when fil- tering inbound. divert-to <host> port <port> Used to redirect packets to a local socket bound to host and port. The packets will not be modified, so getsockname(2) on the socket will return the original destination address of the packet. divert-reply Used to receive replies for sockets that are bound to addresses which are not local to the machine. See setsockopt(2) for infor- mation on how to bind these sockets. probability <number> A probability attribute can be attached to a rule, with a value set between 0 and 1, bounds not included. In that case, the rule will be honoured using the given probability value only. For ex- ample, the following rule will drop 20% of incoming ICMP packets: block in proto icmp probability 20% prio <number> Only match packets which have the given queueing priority as- signed. ROUTING 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. route-to The route-to option routes the packet to the specified interface with an optional address for the next hop. When a route-to rule creates state, only packets that pass in the same direction as the filter rule specifies will be routed in this way. Packets passing in the opposite direction (replies) are not affected and are routed normally. reply-to The reply-to option is similar to route-to, but routes packets that pass in the opposite direction (replies) to the specified interface. Opposite direction is only defined in the context of a state entry, and reply-to is useful only in rules that create state. It can be used on systems with multiple external connec- tions to route all outgoing packets of a connection through the interface the incoming connection arrived through (symmetric routing enforcement). dup-to The dup-to option creates a duplicate of the packet and routes it like route-to. The original packet gets routed as it normally would. POOL OPTIONS 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: bitmask The bitmask option applies the network portion of the redirection address to the address to be modified (source with nat, destina- tion with rdr). random The random option selects an address at random within the defined block of addresses. source-hash The source-hash option uses a hash of the source address to de- termine the redirection address, ensuring that the redirection address is always the same for a given source. An optional key can be specified after this keyword either in hex or as a string; by default pfctl(8) randomly generates a key for source-hash every time the ruleset is reloaded. round-robin The round-robin option loops through the redirection address(es). When more than one redirection address is specified, round-robin is the only permitted pool type. static-port With nat rules, the static-port option prevents pf(4) from modi- fying the source port on TCP and UDP packets. map-e-portset <psid-offset> / <psid-len> / <psid> With nat rules, the map-e-portset option enables the source port translation of MAP-E (RFC 7597) Customer Edge. In order to make the host act as a MAP-E Customer Edge, setting up a tunneling in- terface and pass rules for encapsulated packets are required in addition to the map-e-portset nat rule. For example: nat on $gif_mape_if from $int_if:network to any \ -> $ipv4_mape_src map-e-portset 6/8/0x34 sets PSID offset 6, PSID length 8, PSID 0x34. 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, in- crease the global options with set timeout src.track. See "STATEFUL TRACKING OPTIONS" for more ways to control the source tracking. STATE MODULATION Much of the security derived from TCP is attributable to how well the initial sequence numbers (ISNs) are chosen. Some popular stack imple- mentations choose very poor ISNs and thus are normally susceptible to ISN prediction exploits. By applying a modulate state rule to a TCP connection, pf(4) 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...). pf(4) will not be able to infer a connection again after the state table flushes the connection's modulator. When the state is lost, the con- nection may be left dangling until the respective endpoints time out the connection. It is possible on a fast local network for the end- points 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. Note that alternative methods are available to prevent loss of the state table and allow for firewall failover. See carp(4) and pfsync(4) for further information. SYN PROXY By default, pf(4) passes packets that are part of a tcp(4) handshake between the endpoints. The synproxy state option can be used to cause pf(4) itself to complete the handshake with the active endpoint, per- form 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 con- nection from/to the other endpoint. pf(4) chooses random initial se- quence 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 pf(4) operates on a bridge(4). Example: pass in proto tcp from any to any port www synproxy state STATEFUL TRACKING OPTIONS 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 op- tions to a rule. max <number> Limits the number of concurrent states the rule may create. When this limit is reached, further packets that would create state will not match this rule until existing states time out. no-sync Prevent state changes for states created by this rule from ap- pearing on the pfsync(4) interface. <timeout> <seconds> Changes the timeout values used for states created by this rule. For a list of all valid timeout names, see "OPTIONS" above. sloppy Uses a sloppy TCP connection tracker that does not check sequence numbers at all, which makes insertion and ICMP teardown attacks way easier. This is intended to be used in situations where one does not see all packets of a connection, e.g. in asymmetric routing situations. Cannot be used with modulate or synproxy state. 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. 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 par- ticipating rule count towards each individual rule's limits. The following limits can be set: max-src-nodes <number> Limits the maximum number of source addresses which can simulta- neously have state table entries. max-src-states <number> Limits the maximum number of simultaneous state entries that a single source address can create with this rule. For stateful TCP connections, limits on established connections (con- nections which have completed the TCP 3-way handshake) can also be en- forced per source IP. max-src-conn <number> Limits 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> / <seconds> Limit the rate of new connections over a time interval. The con- nection rate is an approximation calculated as a moving average. Because the 3-way handshake ensures that the source address is not be- ing 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) OPERATING SYSTEM FINGERPRINTING Passive OS Fingerprinting is a mechanism to inspect nuances of a TCP connection's initial SYN packet and guess at the host's operating sys- tem. 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 ver- sion, or by subtype/patchlevel. The class of an operating system is typically the vendor or genre and would be OpenBSD for the pf(4) fire- wall 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 pf(4) is running, a complete list of known operating system fingerprints may be listed by running: # pfctl -so Filter rules can enforce policy at any level of operating system speci- fication 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 ap- pear 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. BLOCKING SPOOFED TRAFFIC "Spoofing" is the faking of IP addresses, typically for malicious pur- poses. 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 incom- ing 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. FRAGMENT HANDLING 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 pf(4) 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 subpro- tocol 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 infor- mation is not available for each fragment. This also means that frag- ments 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 mem- ory 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. When forwarding reassembled IPv6 packets, pf refragments them with the original maximum fragment size. This allows the sender to determine the optimal fragment size by path MTU discovery. ANCHORS Besides the main ruleset, pfctl(8) can load rulesets into anchor at- tachment 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 attach- ing 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 fol- lowing kinds of rules: nat-anchor <name> Evaluates the nat rules in the specified anchor. rdr-anchor <name> Evaluates the rdr rules in the specified anchor. binat-anchor <name> Evaluates the binat rules in the specified anchor. anchor <name> Evaluates the filter rules in the specified anchor. load anchor <name> from <file> Loads the rules from the specified file into the anchor name. When evaluation of the main ruleset reaches an anchor rule, pf(4) 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 alphabet- ical 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 resolu- tion, 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 $ext_if { 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 refer- ence to an anchor name containing `/' characters will require double quote (`"') characters around the anchor name. TRANSLATION EXAMPLES 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 ma- chine 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 ad- dresses using a pool of public addresses (192.0.2.16/28) and to redi- rect 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 FILTER EXAMPLES # 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 GRAMMAR 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" ) ] [ "state-defaults" state-opts ] [ "require-order" ( "yes" | "no" ) ] [ "fingerprints" filename ] | [ "skip on" ifspec ] | [ "debug" ( "none" | "urgent" | "misc" | "loud" ) ] [ "keepcounters" ] ) pf-rule = action [ ( "in" | "out" ) ] [ "log" [ "(" logopts ")"] ] [ "quick" ] [ "on" ifspec ] [ 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" tos | ( "no" | "keep" | "modulate" | "synproxy" ) "state" [ "(" state-opts ")" ] | "fragment" | "no-df" | "min-ttl" number | "set-tos" tos | "max-mss" number | "random-id" | "reassemble tcp" | fragmentation | "allow-opts" | "label" string | "tag" string | [ ! ] "tagged" string | "set prio" ( number | "(" number [ [ "," ] number ] ")" ) | "queue" ( string | "(" string [ [ "," ] string ] ")" ) | "rtable" number | "probability" number"%" | "prio" number | "ridentifier" number nat-rule = [ "no" ] "nat" [ "pass" [ "log" [ "(" logopts ")" ] ] ] [ "on" ifspec ] [ af ] [ protospec ] hosts [ "tag" string ] [ "tagged" string ] [ "->" ( redirhost | "{" redirhost-list "}" ) [ portspec ] [ pooltype ] [ "static-port" ] [ "map-e-portset" number "/" number "/" number ] ] 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 ] [ "ridentifier" number ] table-rule = "table" "<" string ">" [ tableopts-list ] tableopts-list = tableopts-list tableopts | tableopts tableopts = "persist" | "const" | "counters" | "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 "}" ) [ port ] [ os ] "to" ( "any" | "no-route" | "self" | host | "{" host-list "}" ) [ 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 = ( "lowdelay" | "throughput" | "reliability" | [ "0x" ] number ) state-opts = state-opt [ [ "," ] state-opts ] state-opt = ( "max" number | "no-sync" | timeout | "sloppy" | "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" ] 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 ")" ) include = "include" filename FILES /etc/hosts Host name database. /etc/pf.conf Default location of the ruleset file. The file has to be created manually as it is not installed with a stan- dard installation. /etc/pf.os Default location of OS fingerprints. /etc/protocols Protocol name database. /etc/services Service name database. SEE ALSO altq(4), carp(4), icmp(4), icmp6(4), ip(4), ip6(4), pf(4), pfsync(4), tcp(4), udp(4), hosts(5), pf.os(5), protocols(5), services(5), ftp-proxy(8), pfctl(8), pflogd(8) HISTORY The pf.conf file format first appeared in OpenBSD 3.0. FreeBSD 13.2 September 25, 2021 PF.CONF(5)
NAME | DESCRIPTION | STATEMENT ORDER | MACROS | TABLES | OPTIONS | TRAFFIC NORMALIZATION | QUEUEING | TRANSLATION | PACKET FILTERING | PARAMETERS | ROUTING | POOL OPTIONS | STATE MODULATION | SYN PROXY | STATEFUL TRACKING OPTIONS | OPERATING SYSTEM FINGERPRINTING | BLOCKING SPOOFED TRAFFIC | FRAGMENT HANDLING | ANCHORS | TRANSLATION EXAMPLES | FILTER EXAMPLES | GRAMMAR | FILES | SEE ALSO | HISTORY
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