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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:

A variable may also be defined as a set:

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:

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 storage type of “cdb”.

The specified file should contain a list of IP addresses and/or networks in the form of 10.1.1.1 or 10.0.0.0/24

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 consider 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 chosen list type and 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 configured 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 reason.

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 depending 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:

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:

Further packet specification at present is limited to TCP and UDP understanding source and destination ports, and ICMP and IPv6-ICMP understanding 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:

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

Stateful

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 interface 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 behavior; however, it can be overridden with the aforementioned flags keyword.

Map

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

The following would translate the source (10.1.1.0/24) to the IP address specified by $pub_ip for the packets on the interface $ext_if.

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 10.1.1.0/24 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 following provides “port forwarding”, redirecting the public port 9022 to the port 22 of an internal host:

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

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 performed by packet filter extensions called Application Level Gateways (ALGs).

NPF supports the following ALGs:

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:

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:

The available normalization options are:

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.

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.