NPF.CONF(5)               NetBSD File Formats Manual               NPF.CONF(5)

     npf.conf -- NPF packet filter configuration file

     npf.conf is the default configuration file for the NPF packet filter.

     This manual page serves as a reference for editing npf.conf.  Please
     refer to the official NPF documentation website for comprehensive and in-
     depth information.

     There are multiple structural elements that npf.conf may contain, such

              table definitions (with or without content)
              abstraction groups
              packet filtering rules
              map rules for address translation
              application level gateways
              procedure definitions to call on filtered packets.

     Variables are specified using the dollar ($) sign, which is used for both
     definition and referencing of a variable.  Variables are defined by
     assigning a value to them as follows:

           $var1 =

     A variable may also be defined as a set:

           $var2 = {, }

     Common variable definitions are for IP addresses, networks, ports, and

     Tables are specified using a name between angle brackets `<' and `>'.
     The following is an example of table definition:

           table <black> type hash dynamic

     Currently, tables support three data storage types: hash, tree, or cdb.
     Tables can also be set as containing dynamic data or static file filename
     data loaded from a specified file.  Tables of type ``hash'' and ``cdb''
     can only contain IP addresses.  Only static data can be used with a stor-
     age type of ``cdb''.

     The specified file should contain a list of IP addresses and/or networks
     in the form of or

     In NPF, an interface can be referenced directly by using its name, or can
     be passed to an extraction function which will return a list of IP
     addresses configured on the actual associated interface.

     It is legal to pass an extracted list from an interface in keywords where
     NPF would expect instead a direct reference to said interface.  In this
     case, NPF infers a direct reference to the interface, and does not con-
     sider the list.

     There are two types of IP address lists.  With a static list, NPF will
     capture the interface addresses on configuration load, whereas with a
     dynamic list NPF will capture the runtime list of addresses, reflecting
     any changes to the interface, including the attach and detach.  Note that
     with a dynamic list, bringing the interface down has no effect, all
     addresses will remain present.

     Three functions exist, to extract addresses from an interface with a cho-
     sen list type and IP address type:

           inet4(interface)      Static list.  IPv4 addresses.

           inet6(interface)      Static list.  IPv6 addresses.

           ifaddrs(interface)    Dynamic list.  Both IPv4 and IPv6.  The
                                 family keyword of a filtering rule can be
                                 used in combination to explicitly select an
                                 IP address type.

     Example of configuration:

           $var1 = inet4(wm0)
           $var2 = ifaddrs(wm0)
           group default {
                   block in on wm0 all               # rule 1
                   block in on $var1 all             # rule 2
                   block in on inet4(wm0) all        # rule 3
                   pass in on inet6(wm0) from $var2  # rule 4
                   pass in on wm0 from ifaddrs(wm0)  # rule 5

     In the above example, $var1 is the static list of IPv4 addresses config-
     ured on wm0, and $var2 is the dynamic list of all the IPv4 and IPv6
     addresses configured on wm0.  The first three rules are equivalent,
     because with the block ... on <interface> syntax, NPF expects a direct
     reference to an interface, and therefore does not consider the extraction
     functions.  The fourth and fifth rules are equivalent, for the same rea-

     NPF requires that all rules be defined within groups.  Groups can be
     thought of as higher level rules which can contain subrules.  Groups may
     have the following options: name, interface, and direction.  Packets
     matching group criteria are passed to the ruleset of that group.  If a
     packet does not match any group, it is passed to the default group.  The
     default group must always be defined.

     Example of configuration:

           group "my-name" in on wm0 {
                   # List of rules, for packets received on wm0
           group default {
                   # List of rules, for the other packets

     With a rule statement NPF is instructed to pass or block a packet depend-
     ing on packet header information, transit direction and the interface it
     arrived on, either immediately upon match or using the last match.

     If a packet matches a rule which has the final option set, this rule is
     considered the last matching rule, and evaluation of subsequent rules is
     skipped.  Otherwise, the last matching rule is used.

     The proto keyword can be used to filter packets by layer 4 protocol (TCP,
     UDP, ICMP or other).  Its parameter should be a protocol number or its
     symbolic name, as specified in the /etc/protocols file.  This keyword can
     additionally have protocol-specific options, such as flags.

     The flags keyword can be used to match the packets against specific TCP
     flags, according to the following syntax:

           proto tcp flags match[/mask]

     Where match is the set of TCP flags to be matched, out of the mask set,
     both sets being represented as a string combination of: `S' (SYN), `A'
     (ACK), `F' (FIN), and `R' (RST).  The flags that are not present in mask
     are ignored.

     To notify the sender of a blocking decision, three return options can be
     used in conjunction with a block rule:

           return          Behaves as return-rst or return-icmp, depending on
                           whether the packet being blocked is TCP or UDP.

           return-rst      Return a TCP RST message, when the packet being
                           blocked is a TCP packet.  Applies to IPv4 and IPv6.

           return-icmp     Return an ICMP UNREACHABLE message, when the packet
                           being blocked is a UDP packet.  Applies to IPv4 and

     Further packet specification at present is limited to TCP and UDP under-
     standing source and destination ports, and ICMP and IPv6-ICMP understand-
     ing icmp-type.

     A rule can also instruct NPF to create an entry in the state table when
     passing the packet or to apply a procedure to the packet (e.g. "log").

     A ``fully-featured'' rule would for example be:

           pass stateful in final family inet4 proto tcp flags S/SA \
                   from $source port $sport to $dest port $dport    \
                   apply "someproc"

     Alternatively, NPF supports pcap-filter(7) syntax, for example:

           block out final pcap-filter "tcp and dst"

     Fragments are not selectable since NPF always reassembles packets before
     further processing.

     Stateful packet inspection is enabled using the stateful or stateful-ends
     keywords.  The former creates a state which is uniquely identified by a
     5-tuple (source and destination IP addresses, port numbers and an inter-
     face identifier).  The latter excludes the interface identifier and must
     be used with precaution.  In both cases, a full TCP state tracking is
     performed for TCP connections and a limited tracking for message-based
     protocols (UDP and ICMP).

     By default, a stateful rule implies SYN-only flag check (``flags
     S/SAFR'') for the TCP packets.  It is not advisable to change this behav-
     ior; however, it can be overridden with the aforementioned flags keyword.

     Network Address Translation (NAT) is expressed in a form of segment map-
     ping.  The translation may be dynamic (stateful) or static (stateless).
     The following mapping types are available:

           ->      outbound NAT (translation of the source)
           <-      inbound NAT (translation of the destination)
           <->     bi-directional NAT (combination of inbound and outbound

     The following would translate the source ( to the IP address
     specified by $pub_ip for the packets on the interface $ext_if.

           map $ext_if dynamic -> $pub_ip

     Translations are implicitly filtered by limiting the operation to the
     network segments specified, that is, translation would be performed only
     on packets originating from the network.  Explicit filter
     criteria can be specified using pass criteria ... as an additional option
     of the mapping.

     The dynamic NAT implies network address and port translation (NAPT).  The
     port translation can be controlled explicitly.  For example, the follow-
     ing provides ``port forwarding'', redirecting the public port 9022 to the
     port 22 of an internal host:

           map $ext_if dynamic proto tcp port 22 <- $ext_if port 9022

     The static NAT can have different address translation algorithms, which
     can be chosen using the algo keyword.  The currently available algorithms

           npt66     IPv6-to-IPv6 network prefix translation (NPTv6).

     Currently, the static NAT algorithms do not perform port translation.

   Application Level Gateways
     Certain application layer protocols are not compatible with NAT and
     require translation outside layers 3 and 4.  Such translation is per-
     formed by packet filter extensions called Application Level Gateways

     NPF supports the following ALGs:

           icmp     ICMP ALG.  Applies to IPv4 and IPv6.  Allows to find an
                    active connection by looking at the ICMP payload, and to
                    perform NAT translation of the ICMP payload.  Generally,
                    this ALG is necessary to support traceroute(8) behind the
                    NAT, when using the UDP or TCP probes.

     The ALGs are built-in.  If NPF is used as kernel module, then they come
     as kernel modules too.  In such case, the ALG kernel modules can be
     autoloaded through the configuration, using the alg keyword.

     For example:

           alg "icmp"

     Alternatively, the ALG kernel modules can be loaded manually, using

     A rule procedure is defined as a collection of extension calls (it may
     have none).  Every extension call has a name and a list of options in the
     form of key-value pairs.  Depending on the call, the key might represent
     the argument and the value might be optional.  Available options:

           log: interface        Log events.  This requires the npf_ext_log
                                 kernel module, which would normally get auto-
                                 loaded by NPF.  The specified npflog inter-
                                 face would also be auto-created once the con-
                                 figuration is loaded.  The log packets can be
                                 written to a file using the npfd(8) daemon.

           normalize: option1[, option2 ...]
                                 Modify packets according to the specified
                                 normalization options.  This requires the
                                 npf_ext_normalize kernel module, which would
                                 normally get auto-loaded by NPF.

     The available normalization options are:

           "max-mss" value        Enforce a maximum value for the Maximum Seg-
                                  ment Size (MSS) TCP option.  Typically, for
                                  ``MSS clamping''.

           "min-ttl" value        Enforce a minimum value for the IPv4 Time To
                                  Live (TTL) parameter.

           "no-df"                Remove the Don't Fragment (DF) flag from
                                  IPv4 packets.

           "random-id"            Randomize the IPv4 ID parameter.

     For example:

           procedure "someproc" {
                   log: npflog0
                   normalize: "random-id", "min-ttl" 64, "max-mss" 1432

     In this case, the procedure calls the logging and normalization modules.

     Text after a hash (`#') character is considered a comment.  The backslash
     (`\') character at the end of a line marks a continuation line, i.e., the
     next line is considered an extension of the present line.

     The following is a non-formal BNF-like definition of the grammar.  The
     definition is simplified and is intended to be human readable, therefore
     it does not strictly represent the formal grammar.

     # Syntax of a single line.  Lines can be separated by LF (\n) or
     # a semicolon.  Comments start with a hash (#) character.

     syntax          = var-def | set-param | alg | table-def |
                       map | group | proc | comment

     # Variable definition.  Names can be alpha-numeric, including "_"
     # character.

     var-name        = "$" . string
     interface       = interface-name | var-name
     var-def         = var "=" ( var-value | "{" value *[ "," value ] "}" )

     # Parameter setting.
     set-param       = "set" param-value

     # Application level gateway.  The name should be in double quotes.

     alg             = "alg" alg-name
     alg-name        = "icmp"

     # Table definition.  Table ID shall be numeric.  Path is in the
     # double quotes.

     table-id        = <table-name>
     table-def       = "table" table-id "type" ( "hash" | "tree" | "cdb" )
                       ( "dynamic" | "file" path )

     # Mapping for address translation.

     map             = "map" interface
                       ( "static" [ "algo" map-algo ] | "dynamic" )
                       [ proto ]
                       map-seg ( "->" | "<-" | "<->" ) map-seg
                       [ "pass" [ proto ] filt-opts ]

     map-algo        = "npt66"
     map-seg         = ( addr-mask | interface ) [ port-opts ]

     # Rule procedure definition.  The name should be in the double quotes.
     # Each call can have its own options in a form of key-value pairs.
     # Both key and values may be strings (either in double quotes or not)
     # and numbers, depending on the extension.

     proc            = "procedure" proc-name "{" *( proc-call [ new-line ] ) "}"
     proc-opts       = key [ " " val ] [ "," proc-opts ]
     proc-call       = call-name ":" proc-opts new-line

     # Group definition and the rule list.

     group           = "group" ( "default" | group-opts ) "{" rule-list "}"
     group-opts      = name-string [ "in" | "out" ] [ "on" interface ]
     rule-list       = [ rule new-line ] rule-list

     npf-filter      = [ "family" family-opt ] [ proto ] ( "all" | filt-opts )
     static-rule     = ( "block" [ block-opts ] | "pass" )
                       [ "stateful" | "stateful-ends" ]
                       [ "in" | "out" ] [ "final" ] [ "on" interface ]
                       ( npf-filter | "pcap-filter" pcap-filter-expr )
                       [ "apply" proc-name ]

     dynamic-ruleset = "ruleset" group-opts
     rule            = static-rule | dynamic-ruleset

     tcp-flag-mask   = tcp-flags
     tcp-flags       = [ "S" ] [ "A" ] [ "F" ] [ "R" ]
     block-opts      = "return-rst" | "return-icmp" | "return"

     family-opt      = "inet4" | "inet6"
     proto-opts      = "flags" tcp-flags [ "/" tcp-flag-mask ] |
                       "icmp-type" type [ "code" icmp-code ]
     proto           = "proto" protocol [ proto-opts ]

     filt-opts       = "from" filt-addr [ port-opts ] "to" filt-addr
                       [ port-opts ]
     filt-addr       = [ "!" ] [ interface | addr-mask | table-id | "any" ]

     port-opts       = "port" ( port-num | port-from "-" port-to | var-name )
     addr-mask       = addr [ "/" mask ]

     /dev/npf                 control device
     /etc/npf.conf            default configuration file
     /usr/share/examples/npf  directory containing further examples

     $ext_if = { inet4(wm0) }
     $int_if = { inet4(wm1) }

     table <blacklist> type hash file "/etc/npf_blacklist"
     table <limited> type tree dynamic

     $services_tcp = { http, https, smtp, domain, 6000, 9022 }
     $services_udp = { domain, ntp, 6000 }
     $localnet = { }

     alg "icmp"

     # Note: if $ext_if has multiple IP address (e.g. IPv6 as well),
     # then the translation address has to be specified explicitly.
     map $ext_if dynamic -> $ext_if
     map $ext_if dynamic proto tcp port 22 <- $ext_if port 9022

     procedure "log" {
             # The logging facility can be used together with npfd(8).
             log: npflog0

     group "external" on $ext_if {
             pass stateful out final all

             block in final from <blacklist>
             pass stateful in final family inet4 proto tcp to $ext_if \
                     port ssh apply "log"
             pass stateful in final proto tcp to $ext_if \
                     port $services_tcp
             pass stateful in final proto udp to $ext_if \
                     port $services_udp
             pass stateful in final proto tcp to $ext_if \
                     port 49151-65535  # passive FTP
             pass stateful in final proto udp to $ext_if \
                     port 33434-33600  # traceroute

     group "internal" on $int_if {
             block in all
             block in final from <limited>

             # Ingress filtering as per BCP 38 / RFC 2827.
             pass in final from $localnet
             pass out final all

     group default {
             pass final on lo0 all
             block all

     bpf(4), npf(7), pcap-filter(7), npfctl(8), npfd(8)

     NPF documentation website:

     NPF first appeared in NetBSD 6.0.

     NPF was designed and implemented by Mindaugas Rasiukevicius.

NetBSD 8.1                    September 21, 2018                    NetBSD 8.1

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