npf.conf(5) - NetBSD Manual Pages

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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 as: variables 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 parameter settings.
Variables 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 interfaces. Tables Tables are specified using a name between angle brackets `<' and `>'. The following is an example of table definition: table <blocklist> type ipset Currently, tables support three data storage types: ipset, lpm, or const. The contents of the table may be pre-loaded from the specified file. The const tables are immutable (no insertions or deletions after loading) and therefore must always be loaded from a file. The specified file should contain a list of IP addresses and/or networks in the form of or Tables of type ipset and const can only contain IP addresses (without masks). The lpm tables can contain networks and they will perform the longest prefix match on lookup. Interfaces 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. This function can also be used with map to specify the translation address, see below. 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- son. Groups 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 } Rules 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 IPv6. The from and to keywords are provided to filter by source or destination IP addresses. They can be used in conjunction with the port keyword. Negation (the exclamation mark) can be used in front of the address fil- ter criteria. 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 NPF supports stateful packet inspection which can be used to bypass unnecessary rule processing as well as to complement NAT. The connection state is uniquely identified by an n-tuple: IP version, layer 4 protocol, source and destination IP addresses and port numbers. Each state is rep- resented by two keys: one for the original flow and one for the reverse flow, so that the reverse lookup on the returning packets would succeed. The packets are matched against the connection direction respectively. Depending on the settings (see the section on state.key in the npf-params(7) manual), the connection identifier (keys) may also include the interface ID, making the states per-interface. Stateful packet inspection is enabled using the stateful or stateful-all keywords. The former matches the interface after the state lookup, while the latter avoids matching the interface (assuming the state.key.interface parameter is disabled), i.e. making the state global, and must be used with caution. 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. Map 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 NAT) 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 In the regular dynamic NAT case, it is also possible to disable port translation using the no-ports flag. The translation address can also be dynamic, based on the interface. The following would select the IPv4 address(es) currently assigned to the interface: map $ext_if dynamic -> ifaddrs($ext_if) If the dynamic NAT is configured with multiple translation addresses, then a custom selection algorithm can be chosen using the algo keyword. The currently available algorithms for the dynamic translation are: ip-hash The translation address for a new connection is selected based on a hash of the original source and destination addresses. This algorithms attempts to keep all connections of particular client associ- ated with the same translation address. This is the default algorithm. round-robin The translation address for each new connection is selected on a round-robin basis. netmap See the description below. The static NAT can also have different address translation algorithms, chosen using the algo keyword. The currently available algorithms are: netmap Network address mapping from one segment to another, leaving the host part as-is. The new address is com- puted as following: addr = net-addr | (orig-addr & ~mask) npt66 IPv6-to-IPv6 network prefix translation (NPTv6). If no algorithm is specified, then 1:1 address mapping is assumed. Cur- rently, 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 (ALGs). 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 modload(8). Procedures 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. Parameter settings NPF supports a set of dynamically tunable configuration-wide parameters. For example: set state.tcp.timeout.time_wait 0 # destroy the state immediately See npf-params(7) for the list of parameters and their details. Misc 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" ( "ipset" | "lpm" | "const" ) [ "file" path ] # Mapping for address translation. map = map-common | map-ruleset map-common = "map" interface ( "static" [ "algo" map-algo ] | "dynamic" ) [ map-flags ] [ proto ] map-seg ( "->" | "<-" | "<->" ) map-seg [ "pass" [ proto ] filt-opts ] map-ruleset = "map" "ruleset" group-opts map-algo = "ip-hash" | "round-robin" | "netmap" | "npt66" map-flags = "no-ports" 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-all" ] [ "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 <blocklist> type ipset file "/etc/npf_blocklist" table <limited> type lpm $services_tcp = { http, https, smtp, domain, 6000, 9022 } $services_udp = { domain, ntp, 6000 } $localnet = { } alg "icmp" # These NAT rules will dynamically select the interface address(es). map $ext_if dynamic -> ifaddrs($ext_if) map $ext_if dynamic proto tcp port 22 <- ifaddrs($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 <blocklist> 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), npf-params(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 10.0 May 19, 2020 NetBSD 10.0
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