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IPF(5)                    FreeBSD File Formats Manual                   IPF(5)

NAME
       ipf, ipf.conf - IPFilter firewall rules file format

DESCRIPTION
       The ipf.conf file is used to specify rules for the firewall, packet
       authentication and packet accounting components of IPFilter. To load
       rules specified in the ipf.conf file, the ipf(8) program is used.

       For use as a firewall, there are two important rule types: those that
       block and drop packets (block rules) and those that allow packets
       through (pass rules.) Accompanying the decision to apply is a
       collection of statements that specify under what conditions the result
       is to be applied and how.

       The simplest rules that can be used in ipf.conf are expressed like
       this:

       block in all
       pass out all

       Each rule must contain at least the following three components

              *      a decision keyword (pass, block, etc.)

              *      the direction of the packet (in or out)

              *      address patterns or "all" to match any address
                     information

   Long lines
       For rules lines that are particularly long, it is possible to split
       them over multiple lines implicity like this:

       pass in on bgeo proto tcp from 1.1.1.1 port > 1000
           to 2.2.2.2 port < 5000 flags S keep state

       or explicitly using the backslash ('\') character:

       pass in on bgeo proto tcp from 1.1.1.1 port > 1000 \
           to 2.2.2.2 port < 5000 flags S keep state

   Comments
       Comments in the ipf.conf file are indicated by the use of the '#'
       character.  This can either be at the start of the line, like this:

       # Allow all ICMP packets in
       pass in proto icmp from any to any

       Or at the end of a like, like this:

       pass in proto icmp from any to any # Allow all ICMP packets in

Firewall rules
       This section goes into detail on how to construct firewall rules that
       are placed in the ipf.conf file.

       It is beyond the scope of this document to describe what makes a good
       firewall rule set or which packets should be blocked or allowed in.
       Some suggestions will be provided but further reading is expected to
       fully understand what is safe and unsafe to allow in/out.

   Filter rule keywords
       The first word found in any filter rule describes what the eventual
       outcome of a packet that matches it will be. Descriptions of the many
       and various sections that can be used to match on the contents of
       packet headers will follow on below.

       The complete list of keywords, along with what they do is as follows:

              pass rules that match a packet indicate to ipfilter that it
                     should be allowed to continue on in the direction it is
                     flowing.

              block rules are used when it is desirable to prevent a packet
                     from going any further. Packets that are blocked on the
                     "in" side are never seen by TCP/IP and those that are
                     blocked going "out" are never seen on the wire.

              log when IPFilter successfully matches a packet against a log
                     rule a log record is generated and made available for
                     ipmon(8) to read. These rules have no impact on whether
                     or not a packet is allowed through or not.  So if a
                     packet first matched a block rule and then matched a log
                     rule, the status of the packet after the log rule is that
                     it will still be blocked.

              count rules provide the administrator with the ability to count
                     packets and bytes that match the criteria laid out in the
                     configuration file.  The count rules are applied after
                     NAT and filter rules on the inbound path. For outbound
                     packets, count rules are applied before NAT and before
                     the packet is dropped. Thus the count rule cannot be used
                     as a true indicator of link layer

              auth rules cause the matching packet to be queued up for
                     processing by a user space program. The user space
                     program is responsible for making an ioctl system call to
                     collect the information about the queued packet and
                     another ioctl system call to return the verdict (block,
                     pass, etc) on what to do with the packet. In the event
                     that the queue becomes full, the packets will end up
                     being dropped.

              call provides access to functions built into IPFilter that allow
                     for more complex actions to be taken as part of the
                     decision making that goes with the rule.

              decapsulate rules instruct ipfilter to remove any other headers
                     (IP, UDP, AH) and then process what is inside as a new
                     packet.  For non-UDP packets, there are builtin checks
                     that are applied in addition to whatever is specified in
                     the rule, to only allow decapsulation of recognised
                     protocols. After decapsulating the inner packet, any
                     filtering result that is applied to the inner packet is
                     also applied to the other packet.

              The default way in which filter rules are applied is for the
              last matching rule to be used as the decision maker. So even if
              the first rule to match a packet is a pass, if there is a later
              matching rule that is a block and no further rules match the
              packet, then it will be blocked.

   Matching Network Interfaces
       On systems with more than one network interface, it is necessary to be
       able to specify different filter rules for each of them.  In the first
       instance, this is because different networks will send us packets via
       each network interface but it is also because of the hosts, the role
       and the resulting security policy that we need to be able to
       distinguish which network interface a packet is on.

       To accomodate systems where the presence of a network interface is
       dynamic, it is not necessary for the network interface named in a
       filter rule to be present in the system when the rule is loaded.  This
       can lead to silent errors being introduced and unexpected behaviour
       with the simplest of keyboard mistakes - for example, typing in hem0
       instead of hme0 or hme2 instead of hme3.

       On Solaris systems prior to Solaris 10 Update 4, it is not possible to
       filter packets on the loopback interface (lo0) so filter rules that
       specify it will have no impact on the corresponding flow of packets.
       See below for Solaris specific tips on how to enable this.

       Some examples of including the network interface in filter rules are:

       block in on bge0 all
       pass out on bge0 all

   Address matching (basic)
       The first and most basic part of matching for filtering rules is to
       specify IP addresses and TCP/UDP port numbers. The source address
       information is matched by the "from" information in a filter rule and
       the destination address information is matched with the "to"
       information in a filter rule.

       The typical format used for IP addresses is CIDR notation, where an IP
       address (or network) is followed by a '/' and a number representing the
       size of the netmask in bits. This notation is used for specifying
       address matching in both IPv4 and IPv6. If the '/' and bitmask size are
       excluded from the matching string, it is assumed that the address
       specified is a host address and that the netmask applied should be all
       1's.

       Some examples of this are:

       pass in from 10.1.0.0/24 to any
       block out from any to 10.1.1.1

       It is not possible to specify a range of addresses that does not have a
       boundary that can be defined by a standard subnet mask.

              Names instead of addresses

              Hostnames, resolved either via DNS or /etc/hosts, or network
              names, resolved via /etc/networks, may be used in place of
              actual addresses in the filter rules. WARNING: if a hostname
              expands to more than one address, only the *first* is used in
              building the filter rule.

              Caution should be exercised when relying on DNS for filter rules
              in case the sending and receiving of DNS packets is blocked when
              ipf(8) is processing that part of the configuration file,
              leading to long delays, if not errors, in loading the filter
              rules.

   Protocol Matching
       To match packets based on TCP/UDP port information, it is first
       necessary to indicate which protocol the packet must be. This is done
       using the "proto" keyword, followed by either the protocol number or a
       name which is mapped to the protocol number, usually through the
       /etc/protocols file.

       pass in proto tcp from 10.1.0.0/24 to any
       block out proto udp from any to 10.1.1.1
       pass in proto icmp from any to 192.168.0.0/16

   Sending back error packets
       When a packet is just discarded using a block rule, there is no
       feedback given to the host that sent the packet. This is both good and
       bad. If this is the desired behaviour and it is not desirable to send
       any feedback about packets that are to be denied. The catch is that
       often a host trying to connect to a TCP port or with a UDP based
       application will send more than one packet because it assumes that just
       one packet may be discarded so a retry is required. The end result
       being logs can become cluttered with duplicate entries due to the
       retries.

       To address this problem, a block rule can be qualified in two ways.
       The first of these is specific to TCP and instructs IPFilter to send
       back a reset (RST) packet. This packet indicates to the remote system
       that the packet it sent has been rejected and that it shouldn't make
       any further attempts to send packets to that port. Telling IPFilter to
       return a TCP RST packet in response to something that has been received
       is achieved with the return-rst keyword like this:

       block return-rst in proto tcp from 10.0.0.0/8 to any

       When sending back a TCP RST packet, IPFilter must construct a new
       packet that has the source address of the intended target, not the
       source address of the system it is running on (if they are different.)

       For all of the other protocols handled by the IP protocol suite, to
       send back an error indicating that the received packet was dropped
       requires sending back an ICMP error packet. Whilst these can also be
       used for TCP, the sending host may not treat the received ICMP error as
       a hard error in the same way as it does the TCP RST packet. To return
       an ICMP error it is necessary to place return-icmp after the block
       keyword like this:

       block return-icmp in proto udp from any to 192.168.0.1/24

       When electing to return an ICMP error packet, it is also possible to
       select what type of ICMP error is returned. Whilst the full compliment
       of ICMP unreachable codes can be used by specifying a number instead of
       the string below, only the following should be used in conjunction with
       return-icmp. Which return code to use is a choice to be made when
       weighing up the pro's and con's. Using some of the codes may make it
       more obvious that a firewall is being used rather than just the host
       not responding.

              filter-prohib (prohibited by filter) sending packets to the
                     destination given in the received packet is prohibited
                     due to the application of a packet filter

              net-prohib (prohibited network) sending packets to the
                     destination given in the received packet is
                     administratively prohibited.

              host-unk (host unknown) the destination host address is not
                     known by the system receiving the packet and therefore
                     cannot be reached.

              host-unr (host unreachable) it is not possible to reach the host
                     as given by the destination address in the packet header.

              net-unk (network unknown) the destination network address is not
                     known by the system receiving the packet and therefore
                     cannot be reached.

              net-unr (network unreachable) it is not possible to forward the
                     packet on to its final destination as given by the
                     destination address

              port-unr (port unreachable) there is no application using the
                     given destination port and therefore it is not possible
                     to reach that port.

              proto-unr (protocol unreachable) the IP protocol specified in
                     the packet is not available to receive packets.

              An example that shows how to send back a port unreachable packet
              for UDP packets to 192.168.1.0/24 is as follows:

              block return-icmp(port-unr) in proto udp from any to 192.168.1.0/24

              In the above examples, when sending the ICMP packet, IPFilter
              will construct a new ICMP packet with a source address of the
              network interface used to send the packet back to the original
              source. This can give away that there is an intermediate system
              blocking packets. To have IPFilter send back ICMP packets where
              the source address is the original destination, regardless of
              whether or not it is on the local host, return-icmp-as-dest is
              used like this:

              block return-icmp-as-dest(port-unr) in proto udp \
                  from any to 192.168.1.0/24

   TCP/UDP Port Matching
       Having specified which protocol is being matched, it is then possible
       to indicate which port numbers a packet must have in order to match the
       rule.  Due to port numbers being used differently to addresses, it is
       therefore possible to match on them in different ways. IPFilter allows
       you to use the following logical operations:

       < x    is true if the port number is greater than or equal to x and
              less than or equal to y is true if the port number in the packet
              is less than x

       <= x   is true if the port number in the packet is less than or equal
              to x

       > x    is true if the port number in the packet is greater than x

       >= x   is true if the port number in the packet is greater or equal to
              x

       = x    is true if the port number in the packet is equal to x

       != x   is true if the port number in the packet is not equal to x

       Additionally, there are a number of ways to specify a range of ports:

       x <> y is true if the port number is less than a and greater than y

       x >< y is true if the port number is greater than x and less than y

       x:y    is true if the port number is greater than or equal to x and
              less than or equal to y

       Some examples of this are:

       block in proto tcp from any port >= 1024 to any port < 1024
       pass in proto tcp from 10.1.0.0/24 to any port = 22
       block out proto udp from any to 10.1.1.1 port = 135
       pass in proto udp from 1.1.1.1 port = 123 to 10.1.1.1 port = 123
       pass in proto tcp from 127.0.0.0/8 to any port = 6000:6009

       If there is no desire to mention any specific source or destintion
       information in a filter rule then the word "all" can be used to
       indicate that all addresses are considered to match the rule.

   IPv4 or IPv6
       If a filter rule is constructed without any addresses then IPFilter
       will attempt to match both IPv4 and IPv6 packets with it. In the next
       list of rules, each one can be applied to either network protocol
       because there is no address specified from which IPFilter can derive
       with network protocol to expect.

       pass in proto udp from any to any port = 53
       block in proto tcp from any port < 1024 to any

       To explicitly match a particular network address family with a specific
       rule, the family must be added to the rule. For IPv4 it is necessary to
       add family inet and for IPv6, family inet6. Thus the next rule will
       block all packets (both IPv4 and IPv6:

       block in all

       but in the following example, we block all IPv4 packets and only allow
       in IPv6 packets:

       block in family inet all
       pass in family inet6 all

       To continue on from the example where we allowed either IPv4 or IPv6
       packets to port 53 in, to change that such that only IPv6 packets to
       port 53 need to allowed blocked then it is possible to add in a
       protocol family qualifier:

       pass in family inet6 proto udp from any to any port = 53

   First match vs last match
       To change the default behaviour from being the last matched rule
       decides the outcome to being the first matched rule, the word "quick"
       is inserted to the rule.

Extended Packet Matching
   Beyond using plain addresses
       On firewalls that are working with large numbers of hosts and networks
       or simply trying to filter discretely against various hosts, it can be
       an easier administration task to define a pool of addresses and have a
       filter rule reference that address pool rather than have a rule for
       each address.

       In addition to being able to use address pools, it is possible to use
       the interface name(s) in the from/to address fields of a rule. If the
       name being used in the address section can be matched to any of the
       interface names mentioned in the rule's "on" or "via" fields then it
       can be used with one of the following keywords for extended effect:

       broadcast use the primary broadcast address of the network interface
              for matching packets with this filter rule;

              pass in on fxp0 proto udp from any to fxp0/broadcast port = 123

       peer use the peer address on point to point network interfaces for
              matching packets with this filter rule. This option typically
              only has meaningful use with link protocols such as SLIP and
              PPP.  For example, this rule allows ICMP packets from the remote
              peer of ppp0 to be received if they're destined for the address
              assigned to the link at the firewall end.

              pass in on ppp0 proto icmp from ppp0/peer to ppp0/32

       netmasked use the primary network address, with its netmask, of the
              network interface for matching packets with this filter rule. If
              a network interface had an IP address of 192.168.1.1 and its
              netmask was 255.255.255.0 (/24), then using the word "netmasked"
              after the interface name would match any addresses that would
              match 192.168.1.0/24. If we assume that bge0 has this IP address
              and netmask then the following two rules both serve to produce
              the same effect:

              pass in on bge0 proto icmp from any to 192.168.1.0/24
              pass in on bge0 proto icmp from any to bge0/netmasked

       network using the primary network address, and its netmask, of the
              network interface, construct an address for exact matching. If a
              network interface has an address of 192.168.1.1 and its netmask
              is 255.255.255.0, using this option would only match packets to
              192.168.1.0.

              pass in on bge0 proto icmp from any to bge0/network

       Another way to use the name of a network interface to get the address
       is to wrap the name in ()'s. In the above method, IPFilter looks at the
       interface names in use and to decide whether or not the name given is a
       hostname or network interface name. With the use of ()'s, it is
       possible to tell IPFilter that the name should be treated as a network
       interface name even though it doesn't appear in the list of network
       interface that the rule ias associated with.

              pass in proto icmp from any to (bge0)/32

   Using address pools
       Rather than list out multiple rules that either allow or deny specific
       addresses, it is possible to create a single object, call an address
       pool, that contains all of those addresses and reference that in the
       filter rule. For documentation on how to write the configuration file
       for those pools and load them, see ippool.conf(5) and ippool(8).  There
       are two types of address pools that can be defined in ippool.conf(5):
       trees and hash tables. To refer to a tree defined in ippool.conf(5),
       use this syntax:

       pass in from pool/trusted to any

       Either a name or number can be used after the '/', just so long as it
       matches up with something that has already been defined in
       ipool.conf(5) and loaded in with ippool(8). Loading a filter rule that
       references a pool that does not exist will result in an error.

       If hash tables have been used in ippool.conf(5) to store the addresses
       in instead of a tree, then replace the word pool with hash:

              pass in from any to hash/webservers

       There are different operational characteristics with each, so there may
       be some situations where a pool works better than hash and vice versa.

   Matching TCP flags
       The TCP header contains a field of flags that is used to decide if the
       packet is a connection request, connection termination, data, etc.  By
       matching on the flags in conjunction with port numbers, it is possible
       to restrict the way in which IPFilter allows connections to be created.
       A quick overview of the TCP flags is below. Each is listed with the
       letter used in IPFilter rules, followed by its three or four letter
       pneumonic.

       S SYN - this bit is set when a host is setting up a connection.  The
              initiator typically sends a packet with the SYN bit and the
              responder sends back SYN plus ACK.

       A ACK - this bit is set when the sender wishes to acknowledge the
              receipt of a packet from another host

       P PUSH - this bit is set when a sending host has send some data that is
              yet to be acknowledged and a reply is sought

       F FIN - this bit is set when one end of a connection starts to close
              the connection down

       U URG - this bit is set to indicate that the packet contains urgent
              data

       R RST - this bit is set only in packets that are a reply to another
              that has been received but is not targetted at any open port

       C CWN

       E ECN

       When matching TCP flags, it is normal to just list the flag that you
       wish to be set. By default the set of flags it is compared against is
       "FSRPAU". Rules that say "flags S" will be displayed by ipfstat(8) as
       having "flags S/FSRPAU". This is normal.  The last two flags, "C" and
       "E", are optional - they may or may not be used by an end host and have
       no bearing on either the acceptance of data nor control of the
       connection. Masking them out with "flags S/FSRPAUCE" may cause problems
       for remote hosts making a successful connection.

       pass in quick proto tcp from any to any port = 22 flags S/SAFR
       pass out quick proto tcp from any port = 22 to any flags SA

       By itself, filtering based on the TCP flags becomes more work but when
       combined with stateful filtering (see below), the situation changes.

   Matching on ICMP header information
       The TCP and UDP are not the only protocols for which filtering beyond
       just the IP header is possible, extended matching on ICMP packets is
       also available. The list of valid ICMP types is different for IPv4 vs
       IPv6.

       As a practical example, to allow the ping command to work against a
       specific target requires allowing two different types of ICMP packets,
       like this:

       pass in proto icmp from any to webserver icmp-type echo
       pass out proto icmp from webserver to any icmp-type echorep

       The ICMP header has two fields that are of interest for filtering: the
       ICMP type and code. Filter rules can accept either a name or number for
       both the type and code. The list of names supported for ICMP types is
       listed below, however only ICMP unreachable errors have named codes
       (see above.)

       The list of ICMP types that are available for matching an IPv4 packet
       are as follows:

       echo (echo request), echorep (echo reply), inforeq (information
       request), inforep (information reply), maskreq (mask request), maskrep
       (mask reply), paramprob (parameter problem), redir (redirect), routerad
       (router advertisement), routersol (router solicit), squence (source
       quence), timest (timestamp), timestreq (timestamp reply), timex (time
       exceeded), unreach (unreachable).

       The list of ICMP types that are available for matching an IPv6 packet
       are as follows:

       echo (echo request), echorep (echo reply), fqdnquery (FQDN query),
       fqdnreply (FQDN reply), inforeq (information request), inforep
       (information reply), listendone (MLD listener done), listendqry (MLD
       listener query), listendrep (MLD listener reply), neighadvert
       (neighbour advert), neighborsol (neighbour solicit), paramprob
       (parameter problem), redir (redirect), renumber (router renumbering),
       routerad (router advertisement), routersol (router solicit), timex
       (time exceeded), toobig (packet too big), unreach (unreachable, whoreq
       (WRU request), whorep (WRU reply).

Stateful Packet Filtering
       Stateful packet filtering is where IPFilter remembers some information
       from one or more packets that it has seen and is able to apply it to
       future packets that it receives from the network.

       What this means for each transport layer protocol is different.  For
       TCP it means that if IPFilter sees the very first packet of an attempt
       to make a connection, it has enough information to allow all other
       subsequent packets without there needing to be any explicit rules to
       match them. IPFilter uses the TCP port numbers, TCP flags, window size
       and sequence numbers to determine which packets should be matched. For
       UDP, only the UDP port numbers are available.  For ICMP, the ICMP types
       can be combined with the ICMP id field to determine which reply packets
       match a request/query that has already been seen. For all other
       protocols, only matching on IP address and protocol number is available
       for determining if a packet received is a mate to one that has already
       been let through.

       The difference this makes is a reduction in the number of rules from 2
       or 4 to 1. For example, these 4 rules:

       pass in on bge0 proto tcp from any to any port = 22
       pass out on bge1 proto tcp from any to any port = 22
       pass in on bge1 proto tcp from any port = 22 to any
       pass out on bge0 proto tcp from any port = 22 to any

       can be replaced with this single rule:

       pass in on bge0 proto tcp from any to any port = 22 flags S keep state

       Similar rules for UDP and ICMP might be:

       pass in on bge0 proto udp from any to any port = 53 keep state
       pass in on bge0 proto icmp all icmp-type echo keep state

       When using stateful filtering with TCP it is best to add "flags S" to
       the rule to ensure that state is only created when a packet is seen
       that is an indication of a new connection. Although IPFilter can gather
       some information from packets in the middle of a TCP connection to do
       stateful filtering, there are some options that are only sent at the
       start of the connection which alter the valid window of what TCP
       accepts. The end result of trying to pickup TCP state in mid connection
       is that some later packets that are part of the connection may not
       match the known state information and be dropped or blocked, causing
       problems. If a TCP packet matches IP addresses and port numbers but
       does not fit into the recognised window, it will not be automatically
       allowed and will be flagged inside of IPFitler as "out of window"
       (oow). See below, "Extra packet attributes", for how to match on this
       attribute.

       Once a TCP connection has reached the established state, the default
       timeout allows for it to be idle for 5 days before it is removed from
       the state table. The timeouts for the other TCP connection states vary
       from 240 seconds to 30 seconds.  Both UDP and ICMP state entries have
       asymetric timeouts where the timeout set upon seeing packets in the
       forward direction is much larger than for the reverse direction. For
       UDP the default timeouts are 120 and 12 seconds, for ICMP 60 and 6
       seconds. This is a reflection of the use of these protocols being more
       for query-response than for ongoing connections.  For all other
       protocols the timeout is 60 seconds in both directions.

   Stateful filtering options
       The following options can be used with stateful filtering:

       limit limit the number of state table entries that this rule can create
              to the number given after limit. A rule that has a limit
              specified is always permitted that many state table entries,
              even if creating an additional entry would cause the table to
              have more entries than the otherwise global limit.

              pass ... keep state(limit 100)

       age sets the timeout for the state entry when it sees packets going
              through it. Additionally it is possible to set the tieout for
              the reply packets that come back through the firewall to a
              different value than for the forward path. allowing a short
              timeout to be set after the reply has been seen and the state no
              longer required.

              pass in quick proto icmp all icmp-type echo \
                  keep state (age 3)
              pass in quick proto udp from any \
                  to any port = 53 keep state (age 30/1)

       strict only has an impact when used with TCP. It forces all packets
              that are allowed through the firewall to be sequential: no out
              of order delivery of packets is allowed. This can cause
              significant slowdown for some connections and may stall others.
              Use with caution.

              pass in proto tcp ... keep state(strict)

       noicmperr prevents ICMP error packets from being able to match state
              table entries created with this flag using the contents of the
              original packet included.

              pass ... keep state(noicmperr)

       sync indicates to IPFilter that it needs to provide information to the
              user land daemons responsible for syncing other machines state
              tables up with this one.

              pass ... keep state(sync)

       nolog do not generate any log records for the creation or deletion of
              state table entries.

              pass ... keep state(nolog)

       icmp-head rather than just precent ICMP error packets from being able
              to match state table entries, allow an ACL to be processed that
              can filter in or out ICMP error packets based as you would with
              normal firewall rules.  The icmp-head option requires a filter
              rule group number or name to be present, just as you would use
              with head.

              pass in quick proto tcp ... keep state(icmp-head 101)
              block in proto icmp from 10.0.0.0/8 to any group 101

       max-srcs allows the number of distinct hosts that can create a state
              entry to be defined.

              pass ... keep state(max-srcs 100)
              pass ... keep state(limit 1000, max-srcs 100)

       max-per-src whilst max-srcs limits the number of individual hosts that
              may cause the creation of a state table entry, each one of those
              hosts is still table to fill up the state table with new entries
              until the global maximum is reached. This option allows the
              number of state table entries per address to be limited.

              pass ... keep state(max-srcs 100, max-per-src 1)
              pass ... keep state(limit 100, max-srcs 100, max-per-src 1)

              Whilst these two rules might seem identical, in that they both
              ultimately limit the number of hosts and state table entries
              created from the rule to 100, there is a subtle difference: the
              second will always allow up to 100 state table entries to be
              created whereas the first may not if the state table fills up
              from other rules.

              Further, it is possible to specify a netmask size after the per-
              host limit that enables the per-host limit to become a per-
              subnet or per-network limit.

              pass ... keep state(max-srcs 100, max-per-src 1/24)

              If there is no IP protocol implied by addresses or other
              features of the rule, IPFilter will assume that no netmask is an
              all ones netmask for both IPv4 and IPv6.

   Tieing down a connection
       For any connection that transits a firewall, each packet will be seen
       twice: once going in and once going out. Thus a connection has 4 flows
       of packets:

       forward inbound packets

       forward outbound packets

       reverse inbound packets

       reverse outbound packets

       IPFilter allows you to define the network interface to be used at all
       four points in the flow of packets. For rules that match inbound
       packets, out-via is used to specify which interfaces the packets go
       out, For rules that match outbound packets, in-via is used to match the
       inbound packets.  In each case, the syntax generalises to this:

       pass ... in on forward-in,reverse-in \
              out-via forward-out,reverse-out ...

       pass ... out on forward-out,reverse-out \
                in-via forward-in,reverse-in ...

       An example that pins down all 4 network interfaces used by an ssh
       connection might look something like this:

       pass in on bge0,bge1 out-via bge1,bge0 proto tcp \
           from any to any port = 22 flags S keep state

   Working with packet fragments
       Fragmented packets result in 1 packet containing all of the layer 3 and
       4 header information whilst the data is split across a number of other
       packets.

       To enforce access control on fragmented packets, one of two approaches
       can be taken. The first is to allow through all of the data fragments
       (those that made up the body of the original packet) and rely on
       matching the header information in the "first" fragment, when it is
       seen. The reception of body fragments without the first will result in
       the receiving host being unable to completely reassemble the packet and
       discarding all of the fragments. The following three rules deny all
       fragmented packets from being received except those that are UDP and
       even then only allows those destined for port 2049 to be completed.

       block in all with frags
       pass in proto udp from any to any with frag-body
       pass in proto udp from any to any port = 2049 with frags

       Another mechanism that is available is to track "fragment state".  This
       relies on the first fragment of a packet that arrives to be the
       fragment that contains all of the layer 3 and layer 4 header
       information. With the receipt of that fragment before any other, it is
       possible to determine which other fragments are to be allowed through
       without needing to explicitly allow all fragment body packets.  An
       example of how this is done is as follows:

       pass in proto udp from any prot = 2049 to any with frags keep fags

Building a tree of rules
       Writing your filter rules as one long list of rules can be both
       inefficient in terms of processing the rules and difficult to
       understand. To make the construction of filter rules easier, it is
       possible to place them in groups.  A rule can be both a member of a
       group and the head of a new group.

       Using filter groups requires at least two rules: one to be in the group
       one one to send matchign packets to the group. If a packet matches a
       filtre rule that is a group head but does not match any of the rules in
       that group, then the packet is considered to have matched the head
       rule.

       Rules that are a member of a group contain the word group followed by
       either a name or number that defines which group they're in. Rules that
       form the branch point or starting point for the group must use the word
       head, followed by either a group name or number. If rules are loaded in
       that define a group but there is no matching head then they will
       effectively be orphaned rules. It is possible to have more than one
       head rule point to the same group, allowing groups to be used like
       subroutines to implement specific common policies.

       A common use of filter groups is to define head rules that exist in the
       filter "main line" for each direction with the interfaces in use. For
       example:

       block in quick on bge0 all head 100
       block out quick on bge0 all head 101
       block in quick on fxp0 all head internal-in
       block out quick on fxp0 all head internal-out
       pass in quick proto icmp all icmp-type echo group 100

       In the above set of rules, there are four groups defined but only one
       of them has a member rule. The only packets that would be allowed
       through the above ruleset would be ICMP echo packets that are received
       on bge0.

       Rules can be both a member of a group and the head of a new group,
       allowing groups to specialise.

       block in quick on bge0 all head 100
       block in quick proto tcp all head 1006 group 100

       Another use of filter rule groups is to provide a place for rules to be
       dynamically added without needing to worry about their specific
       ordering amongst the entire ruleset. For example, if I was using this
       simple ruleset:

       block in quick all with bad
       block in proto tcp from any to any port = smtp head spammers
       pass in quick proto tcp from any to any port = smtp flags S keep state

       and I was getting lots of connections to my email server from 10.1.1.1
       to deliver spam, I could load the following rule to complement the
       above:

              block in quick from 10.1.1.1 to any group spammers

   Decapsulation
       Rule groups also form a different but vital role for decapsulation
       rules.  With the following simple rule, if IPFilter receives an IP
       packet that has an AH header as its layer 4 payload, IPFilter would
       adjust its view of the packet internally and then jump to group 1001
       using the data beyond the AH header as the new transport header.

       decapsulate in proto ah all head 1001

       For protocols that are recognised as being used with tunnelling or
       otherwise encapsulating IP protocols, IPFilter is able to decide what
       it has on the inside without any assistance. Some tunnelling protocols
       use UDP as the transport mechanism. In this case, it is necessary to
       instruct IPFilter as to what protocol is inside UDP.

       decapsulate l5-as(ip) in proto udp from any \
           to any port = 1520 head 1001

       Currently IPFilter only supports finding IPv4 and IPv6 headers directly
       after the UDP header.

       If a packet matches a decapsulate rule but fails to match any of the
       rules that are within the specified group, processing of the packet
       continues to the next rule after the decapsulate and IPFilter's
       internal view of the packet is returned to what it was prior to the
       decapsulate rule.

       It is possible to construct a decapsulate rule without the group head
       at the end that ipf(8) will accept but such rules will not result in
       anything happening.

   Policy Based Routing
       With firewalls being in the position they often are, at the boundary of
       different networks connecting together and multiple connections that
       have different properties, it is often desirable to have packets flow
       in a direction different to what the routing table instructs the
       kernel.  These decisions can often be extended to changing the route
       based on both source and destination address or even port numbers.

       To support this kind of configuration, IPFilter allows the next hop
       destination to be specified with a filter rule. The next hop is given
       with the interface name to use for output. The syntax for this is
       interface:ip.address. It is expected that the address given as the next
       hop is directly connected to the network to which the interface is.

       pass in on bge0 to bge1:1.1.1.1 proto tcp \
           from 1.1.2.3 to any port = 80 flags S keep state

       When this feature is combined with stateful filtering, it becomes
       possible to influence the network interface used to transmit packets in
       both directions because we now have a sense for what its reverse flow
       of packets is.

       pass in on bge0 to bge1:1.1.1.1 reply-to hme1:2.1.1.2 \
           proto tcp from 1.1.2.3 to any port = 80 flags S keep state

       If the actions of the routing table are perfectly acceptable, but you
       would like to mask the presence of the firewall by not changing the TTL
       in IP packets as they transit it, IPFilter can be instructed to do a
       "fastroute" action like this:

       pass in on bge0 fastroute proto icmp all

       This should be used with caution as it can lead to endless packet
       loops. Additionally, policy based routing does not change the IP
       header's TTL value.

       A variation on this type of rule supports a duplicate of the original
       packet being created and sent out a different network. This can be
       useful for monitoring traffic and other purposes.

       pass in on bge0 to bge1:1.1.1.1 reply-to hme1:2.1.1.2 \
           dup-to fxp0:10.0.0.1 proto tcp from 1.1.2.3 \
           to any port = 80 flags S keep state

   Matching IPv4 options
       The design for IPv4 allows for the header to be upto 64 bytes long,
       however most traffic only uses the basic header which is 20 bytes long.
       The other 44 bytes can be uesd to store IP options. These options are
       generally not necessary for proper interaction and function on the
       Internet today. For most people it is sufficient to block and drop all
       packets that have any options set. This can be achieved with this rule:

       block in quick all with ipopts

       This rule is usually placed towards the top of the configuration so
       that all incoming packets are blocked.

       If you wanted to allow in a specific IP option type, the syntax changes
       slightly:

       pass in quick proto igmp all with opt rtralrt

       The following is a list of IP options that most people encounter and
       what their use/threat is.

       lsrr (loose source route) the sender of the packet includes a list of
              addresses that they wish the packet to be routed through to on
              the way to the destination. Because replies to such packets are
              expected to use the list of addresses in reverse, hackers are
              able to very effectively use this header option in address
              spoofing attacks.

       rr (record route) the sender allocates some buffer space for recording
              the IP address of each router that the packet goes through. This
              is most often used with ping, where the ping response contains a
              copy of all addresses from the original packet, telling the
              sender what route the packet took to get there. Due to
              performance and security issues with IP header options, this is
              almost no longer used.

       rtralrt (router alert) this option is often used in IGMP messages as a
              flag to routers that the packet needs to be handled differently.
              It is unlikely to ever be received from an unknown sender. It
              may be found on LANs or otherwise controlled networks where the
              RSVP protocol and multicast traffic is in heavy use.

       ssrr (strict source route) the sender of the packet includes a list of
              addresses that they wish the packet to be routed through to on
              the way to the destination. Where the lsrr option allows the
              sender to specify only some of the nodes the packet must go
              through, with the ssrr option, every next hop router must be
              specified.

       The complete list of IPv4 options that can be matched on is: addext
       (Address Extention), cipso (Classical IP Security Option), dps (Dynamic
       Packet State), e-sec (Extended Security), eip (Extended Internet
       Protocol), encode (ENCODE), finn (Experimental Flow Control), imitd
       (IMI Traffic Descriptor), lsrr (Loose Source Route), mtup (MTU Probe -
       obsolete), mtur (MTU response - obsolete), nop (No Operation), nsapa
       (NSAP Address), rr (Record Route), rtralrt (Router Alert), satid
       (Stream Identifier), sdb (Selective Directed Broadcast), sec
       (Security), ssrr (Strict Source Route), tr (Tracerote), ts (Timestamp),
       ump (Upstream Multicast Packet), visa (Experimental Access Control) and
       zsu (Experimental Measurement).

   Security with CIPSO and IPSO
       IPFilter supports filtering on IPv4 packets using security attributes
       embedded in the IP options part of the packet. These options are
       usually only used on networks and systems that are using lablled
       security. Unless you know that you are using labelled security and your
       networking is also labelled, it is highly unlikely that this section
       will be relevant to you.

       With the traditional IP Security Options (IPSO), packets can be tagged
       with a security level. The following keywords are recognised and match
       with the relevant RFC with respect to the bit patterns matched: confid
       (confidential), rserve-1 (1st reserved value), rserve-2 (2nd reserved
       value), rserve-3 (3rd reserved value), rserve-4 (4th reserved value),
       secret (secret), topsecret (top secret), unclass (unclassified).

       block in quick all with opt sec-class unclass
       pass in all with opt sec-class secret

   Matching IPv6 extension headers
       Just as it is possible to filter on the various IPv4 header options, so
       too it is possible to filter on the IPv6 extension headers that are
       placed between the IPv6 header and the transport protocol header.

       dstopts (destination options), esp (encrypted, secure, payload), frag
       (fragment), hopopts (hop-by-hop options), ipv6 (IPv6 header), mobility
       (IP mobility), none, routing.

   Logging
       There are two ways in which packets can be logged with IPFilter. The
       first is with a rule that specifically says log these types of packets
       and the second is a qualifier to one of the other keywords. Thus it is
       possible to both log and allow or deny a packet with a single rule.

       pass in log quick proto tcp from any to any port = 22

       When using stateful filtering, the log action becomes part of the
       result that is remembered about a packet. Thus if the above rule was
       qualified with keep state, every packet in the connection would be
       logged. To only log the first packet from every packet flow tracked
       with keep state, it is necessary to indicate to IPFilter that you only
       wish to log the first packet.

       pass in log first quick proto tcp from any to any port = 22 \
           flags S keep state

       If it is a requirement that the logging provide an accurate
       representation of which connections are allowed, the log action can be
       qualified with the option or-block. This allows the administrator to
       instruct IPFilter to block the packet if the attempt to record the
       packet in IPFilter's kernel log records (which have an upper bound on
       size) failed. Unless the system shuts down or reboots, once a log
       record is written into the kernel buffer, it is there until ipmon(8)
       reads it.

       block in log proto tcp from any to any port = smtp
       pass in log or-block first quick proto tcp from any \
           to any port = 22 flags S keep state

       By default, IPFilter will only log the header portion of a packet
       received on the network. A portion of the body of a packet, upto 128
       bytes, can also be logged with the body keyword. ipmon(8) will display
       the contents of the portion of the body logged in hex.

       block in log body proto icmp all

       When logging packets from ipmon(8) to syslog, by default ipmon(8) will
       control what syslog facility and priority a packet will be logged with.
       This can be tuned on a per rule basis like this:

       block in quick log level err all with bad
       pass in log level local1.info proto tcp \
           from any to any port = 22 flags S keep state

       ipfstat(8) reports how many packets have been successfully logged and
       how many failed attempts to log a packet there were.

   Filter rule comments
       If there is a desire to associate a text string, be it an
       administrative comment or otherwise, with an IPFilter rule, this can be
       achieved by giving the filter rule a comment.  The comment is loaded
       with the rule into the kernel and can be seen when the rules are listed
       with ipfstat.

       pass in quick proto tcp from any \
           to port = 80 comment "all web server traffic is ok"
       pass out quick proto tcp from any port = 80 \
           to any comment "all web server traffic is ok"

   Tags
       To enable filtering and NAT to correctly match up packets with rules,
       tags can be added at with NAT (for inbound packets) and filtering (for
       outbound packets.) This allows a filter to be correctly mated with its
       NAT rule in the event that the NAT rule changed the packet in a way
       that would mean it is not obvious what it was.

       For inbound packets, IPFilter can match the tag used in the filter
       rules with that set by NAT. For outbound rules, it is the reverse, the
       filter sets the tag and the NAT rule matches up with it.

       pass in ... match-tag(nat=proxy)
       pass out ... set-tag(nat=proxy)

       Another use of tags is to supply a number that is only used with
       logging.  When packets match these rules, the log tag is carried over
       into the log file records generated by ipmon(8). With the correct use
       of tools such as grep, extracting log records of interest is
       simplified.

       block in quick log ... set-tag(log=33)

Filter Rule Expiration
       IPFilter allows rules to be added into the kernel that it will remove
       after a specific period of time by specifying rule-ttl at the end of a
       rule.  When listing rules in the kernel using ipfstat(8), rules that
       are going to expire will NOT display "rule-ttl" with the timeout,
       rather what will be seen is a comment with how many ipfilter ticks left
       the rule has to live.

       The time to live is specified in seconds.

       pass in on fxp0 proto tcp from any \
           to port = 22 flags S keep state rule-ttl 30

Internal packet attributes
       In addition to being able to filter on very specific network and
       transport header fields, it is possible to filter on other attributes
       that IPFilter attaches to a packet. These attributes are placed in a
       rule after the keyword "with", as can be seen with frags and frag-body
       above. The following is a list of the other attributes available:

       oow the packet's IP addresses and TCP ports match an existing entry in
              the state table but the sequence numbers indicate that it is
              outside of the accepted window.

              block return-rst in quick proto tcp from any to any with not oow

       bcast this is set by IPFilter when it receives notification that the
              link layer packet was a broadcast packet. No checking of the IP
              addresses is performned to determine if it is a broadcast
              destination or not.

              block in quick proto udp all with bcast

       mcast this is set by IPFilter when it receives notification that the
              link layer packet was a multicast packet. No checking of the IP
              addresses is performned to determine if it is a multicast
              destination or not.

              pass in quick proto udp from any to any port = dns with mcast

       mbcast can be used to match a packet that is either a multicast or
              broadcast packet at the link layer, as indicated by the
              operating system.

              pass in quick proto udp from any to any port = ntp with mbcast

       nat the packet positively matched a NAT table entry.

       bad sanity checking of the packet failed. This could indicate that the
              layer 3/4 headers are not properly formed.

       bad-src when reverse path verification is enabled, this flag will be
              set when the interface the packet is received on does not match
              that which would be used to send a packet out of to the source
              address in the received packet.

       bad-nat an attempt to perform NAT on the packet failed.

       not each one of the attributes matched using the "with" keyword can
              also be looked for to not be present. For example, to only allow
              in good packets, I can do this:

       block in all
       pass in all with not bad

Tuning IPFilter
       The ipf.conf file can also be used to tune the behaviour of IPFilter,
       allowing, for example, timeouts for the NAT/state table(s) to be set
       along with their sizes. The presence and names of tunables may change
       from one release of IPFilter to the next. The tunables that can be
       changed via ipf.conf is the same as those that can be seen and modified
       using the -T command line option to ipf(8).

       NOTE: When parsing ipf.conf, ipf(8) will apply the settings before
       loading any rules. Thus if your settings are at the top, these may be
       applied whilst the rules not applied if there is an error further down
       in the configuration file.

       To set one of the values below, the syntax is simple: "set", followed
       by the name of the tuneable to set and then the value to set it to.

       set state_max 9999;
       set state_size 10101;

       A list of the currently available variables inside IPFilter that may be
       tuned from ipf.conf are as follows:

       active set through -s command line switch of ipf(8). See ipf(8) for
              detals.

       chksrc when set, enables reverse path verification on source addresses
              and for filters to match packets with bad-src attribute.

       control_forwarding when set turns off kernel forwarding when IPFilter
              is disabled or unloaded.

       default_pass the default policy - whether packets are blocked or
              passed, etc - is represented by the value of this variable. It
              is a bit field and the bits that can be set are found in
              <netinet/ip_fil.h>. It is not recommended to tune this value
              directly.

       ftp_debug set the debugging level of the in-kernel FTP proxy.  Debug
              messages will be printed to the system console.

       ftp_forcepasv when set the FTP proxy must see a PASV/EPSV command
              before creating the state/NAT entries for the 227 response.

       ftp_insecure when set the FTP proxy will not wait for a user to login
              before allowing data connections to be created.

       ftp_pasvonly when set the proxy will not create state/NAT entries for
              when it sees either the PORT or EPRT command.

       ftp_pasvrdr when enabled causes the FTP proxy to create very insecure
              NAT/state entries that will allow any connection between the
              client and server hosts when a 227 reply is seen.  Use with
              extreme caution.

       ftp_single_xfer when set the FTP proxy will only allow one data
              connection at a time.

       hostmap_size sets the size of the hostmap table used by NAT to store
              address mappings for use with sticky rules.

       icmp_ack_timeout default timeout used for ICMP NAT/state when a reply
              packet is seen for an ICMP state that already exists

       icmp_minfragmtu sets the minimum MTU that is considered acceptable in
              an ICMP error before deciding it is a bad packet.

       icmp_timeout default timeout used for ICMP NAT/state when the packet
              matches the rule

       ip_timeout default timeout used for NAT/state entries that are not
              TCP/UDP/ICMP.

       ipf_flags

       ips_proxy_debug this sets the debugging level for the proxy support
              code.  When enabled, debugging messages will be printed to the
              system console.

       log_all when set it changes the behaviour of "log body" to log the
              entire packet rather than just the first 128 bytes.

       log_size sets the size of the in-kernel log buffer in bytes.

       log_suppress when set, IPFilter will check to see if the packet it is
              logging is similar to the one it previously logged and if so,
              increases the occurance count for that packet. The previously
              logged packet must not have yet been read by ipmon(8).

       min_ttl is used to set the TTL value that packets below will be marked
              with the low-ttl attribute.

       nat_doflush if set it will cause the NAT code to do a more aggressive
              flush of the NAT table at the next opportunity. Once the flush
              has been done, the value is reset to 0.

       nat_lock this should only be changed using ipfs(8)

       nat_logging when set, NAT will create log records that can be read from
              /dev/ipnat.

       nat_maxbucket maximum number of entries allowed to exist in each NAT
              hash bucket.  This prevents an attacker trying to load up the
              hash table with entries in a single bucket, reducing
              performance.

       nat_rules_size size of the hash table to store map rules.

       nat_table_max maximum number of entries allowed into the NAT table

       nat_table_size size of the hash table used for NAT

       nat_table_wm_high when the fill percentage of the NAT table exceeds
              this mark, more aggressive flushing is enabled.

       nat_table_wm_low this sets the percentage at which the NAT table's
              agressive flushing will turn itself off at.

       rdr_rules_size size of the hash table to store rdr rules.

       state_lock this should only be changed using ipfs(8)

       state_logging when set, the stateful filtering will create log records
              that can be read from /dev/ipstate.

       state_max maximum number of entries allowed into the state table

       state_maxbucket maximum number of entries allowed to exist in each
              state hash bucket.  This prevents an attacker trying to load up
              the hash table with entries in a single bucket, reducing
              performance.

       state_size size of the hash table used for stateful filtering

       state_wm_freq this controls how often the agressive flushing should be
              run once the state table exceeds state_wm_high in percentage
              full.

       state_wm_high when the fill percentage of the state table exceeds this
              mark, more aggressive flushing is enabled.

       state_wm_low this sets the percentage at which the state table's
              agressive flushing will turn itself off at.

       tcp_close_wait timeout used when a TCP state entry reaches the
              FIN_WAIT_2 state.

       tcp_closed timeout used when a TCP state entry is ready to be removed
              after either a RST packet is seen.

       tcp_half_closed timeout used when a TCP state entry reaches the
              CLOSE_WAIT state.

       tcp_idle_timeout timeout used when a TCP state entry reaches the
              ESTABLISHED state.

       tcp_last_ack timeout used when a TCP NAT/state entry reaches the
              LAST_ACK state.

       tcp_syn_received timeout applied to a TCP NAT/state entry after SYN-ACK
              packet has been seen.

       tcp_syn_sent timeout applied to a TCP NAT/state entry after SYN packet
              has been seen.

       tcp_time_wait timeout used when a TCP NAT/state entry reaches the
              TIME_WAIT state.

       tcp_timeout timeout used when a TCP NAT/state entry reaches either the
              half established state (one ack is seen after a SYN-ACK) or one
              side is in FIN_WAIT_1.

       udp_ack_timeout default timeout used for UDP NAT/state when a reply
              packet is seen for a UDP state that already exists

       udp_timeout default timeout used for UDP NAT/state when the packet
              matches the rule

       update_ipid when set, turns on changing the IP id field in NAT'd
              packets to a random number.

   Table of visible variables
       A list of all of the tunables, their minimum, maximum and current
       values is as follows.

       Name                Min  Max  Current
       active              0    0    0
       chksrc              0    1    0
       control_forwarding  0    1    0
       default_pass        0    MAXUINT   134217730
       ftp_debug           0    10   0
       ftp_forcepasv       0    1    1
       ftp_insecure        0    1    0
       ftp_pasvonly        0    1    0
       ftp_pasvrdr         0    1    0
       ftp_single_xfer     0    1    0
       hostmap_size        1    MAXINT    2047
       icmp_ack_timeout    1    MAXINT    12
       icmp_minfragmtu     0    1    68
       icmp_timeout        1    MAXINT    120
       ip_timeout          1    MAXINT    120
       ipf_flags           0    MAXUINT   0
       ips_proxy_debug     0    10   0
       log_all             0    1    0
       log_size            0    524288    32768
       log_suppress        0    1    1
       min_ttl             0    1    4
       nat_doflush         0    1    0
       nat_lock            0    1    0
       nat_logging         0    1    1
       nat_maxbucket       1    MAXINT    22
       nat_rules_size      1    MAXINT    127
       nat_table_max       1    MAXINT    30000
       nat_table_size      1    MAXINT    2047
       nat_table_wm_high   2    100  99
       nat_table_wm_low    1    99   90
       rdr_rules_size      1    MAXINT    127
       state_lock          0    1    0
       state_logging       0    1    1
       state_max           1    MAXINT    4013
       state_maxbucket     1    MAXINT    26
       state_size          1    MAXINT    5737
       state_wm_freq       2    999999    20
       state_wm_high       2    100  99
       state_wm_low        1    99   90
       tcp_close_wait      1    MAXINT    480
       tcp_closed          1    MAXINT    60
       tcp_half_closed     1    MAXINT    14400
       tcp_idle_timeout    1    MAXINT    864000
       tcp_last_ack        1    MAXINT    60
       tcp_syn_received    1    MAXINT    480
       tcp_syn_sent        1    MAXINT    480
       tcp_time_wait       1    MAXINT    480
       tcp_timeout         1    MAXINT    480
       udp_ack_timeout     1    MAXINT    24
       udp_timeout         1    MAXINT    240
       update_ipid         0    1    0

Calling out to internal functions
       IPFilter provides a pair of functions that can be called from a rule
       that allow for a single rule to jump out to a group rather than walk
       through a list of rules to find the group. If you've got multiple
       networks, each with its own group of rules, this feature may help
       provide better filtering performance.

       The lookup to find which rule group to jump to is done on either the
       source address or the destination address but not both.

       In this example below, we are blocking all packets by default but then
       doing a lookup on the source address from group 1010. The two rules in
       the ipf.conf section are lone members of their group. For an incoming
       packet that is from 1.1.1.1, it will go through three rules: (1) the
       block rule, (2) the call rule and (3) the pass rule for group 1020.
       For a packet that is from 3.3.2.2, it will also go through three rules:
       (1) the block rule, (2) the call rule and (3) the pass rule for group
       1030. Should a packet from 3.1.1.1 arrive, it will be blocked as it
       does not match any of the entries in group 1010, leaving it to only
       match the first rule.

       from ipf.conf
       -------------
       block in all
       call now srcgrpmap/1010 in all
       pass in proto tcp from any to any port = 80 group 1020
       pass in proto icmp all icmp-type echo group 1030

       from ippool.conf
       ----------------
       group-map in role=ipf number=1010
           { 1.1.1.1 group = 1020, 3.3.0.0/16 group = 1030; };

   IPFilter matching expressions
       An experimental feature that has been added to filter rules is to use
       the same expression matching that is available with various commands to
       flush and list state/NAT table entries. The use of such an expression
       precludes the filter rule from using the normal IP header matching.

       pass in exp { "tcp.sport 23 or tcp.sport 50" } keep state

   Filter rules with BPF
       On platforms that have the BPF built into the kernel, IPFilter can be
       built to allow BPF expressions in filter rules. This allows for packet
       matching to be on arbitrary data in the packt. The use of a BPF
       expression replaces all of the other protocol header matching done by
       IPFilter.

       pass in bpf-v4 { "tcp and (src port 23 or src port 50)" } \
           keep state

       These rules tend to be write-only because the act of compiling the
       filter expression into the BPF instructions loaded into the kernel can
       make it difficut to accurately reconstruct the original text filter.
       The end result is that while ipf.conf() can be easy to read,
       understanding the output from ipfstat might not be.

VARIABLES
       This configuration file, like all others used with IPFilter, supports
       the use of variable substitution throughout the text.

       nif="ppp0";
       pass in on $nif from any to any

       would become

       pass in on ppp0 from any to any

       Variables can be used recursively, such as 'foo="$bar baz";', so long
       as $bar exists when the parser reaches the assignment for foo.

       See ipf(8) for instructions on how to define variables to be used from
       a shell environment.

FILES
       /dev/ipf /etc/ipf.conf
       /usr/share/examples/ipf  Directory with examples.

SEE ALSO
       ipf(8), ipfstat(8), ippool.conf(5), ippool(8)

                                                                        IPF(5)

NAME | DESCRIPTION | Firewall rules | Extended Packet Matching | Stateful Packet Filtering | Building a tree of rules | Filter Rule Expiration | Internal packet attributes | Tuning IPFilter | Calling out to internal functions | VARIABLES | FILES | SEE ALSO

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