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NG_NAT(4) Device Drivers Manual NG_NAT(4)


ng_natNAT netgraph node type


#include <netgraph/ng_nat.h>


An ng_nat node performs network address translation (NAT) of IPv4 packets passing through it. A nat node uses libalias(3) engine for packet aliasing.


This node type has two hooks:

Packets received on this hook are considered outgoing and will be masqueraded to a configured address.
Packets coming on this hook are considered incoming and will be dealiased.


This node type supports the generic control messages, plus the following:

Configure aliasing address for a node. After both hooks have been connected and aliasing address was configured, a node is ready for aliasing operation.
Set node's operation mode using supplied struct ng_nat_mode.
struct ng_nat_mode {
	uint32_t	flags;
	uint32_t	mask;
/* Supported flags: */
#define NG_NAT_LOG			0x01
#define NG_NAT_DENY_INCOMING		0x02
#define NG_NAT_SAME_PORTS		0x04
#define NG_NAT_PROXY_ONLY		0x40
#define NG_NAT_REVERSE			0x80
Configure target address for a node. When an incoming packet not associated with any pre-existing aliasing link arrives at the host machine, it will be sent to the specified address.
Redirect incoming connections arriving to given port(s) to another host and port(s). The following struct ng_nat_redirect_port must be supplied as argument.
struct ng_nat_redirect_port {
	struct in_addr	local_addr;
	struct in_addr	alias_addr;
	struct in_addr	remote_addr;
	uint16_t	local_port;
	uint16_t	alias_port;
	uint16_t	remote_port;
	uint8_t		proto;
	char		description[NG_NAT_DESC_LENGTH];

Redirection is assigned an unique ID which is returned as response to this message, and information about redirection added to list of static redirects which later can be retrieved by NGM_NAT_LIST_REDIRECTS message.

Redirect traffic for public IP address to a machine on the local network. This function is known as static NAT. The following struct ng_nat_redirect_addr must be supplied as argument.
struct ng_nat_redirect_addr {
	struct in_addr	local_addr;
	struct in_addr	alias_addr;
	char		description[NG_NAT_DESC_LENGTH];

Unique ID for this redirection is returned as response to this message.

Redirect incoming IP packets of protocol proto (see protocols(5)) to a machine on the local network. The following struct ng_nat_redirect_proto must be supplied as argument.
struct ng_nat_redirect_proto {
	struct in_addr	local_addr;
	struct in_addr	alias_addr;
	struct in_addr	remote_addr;
	uint8_t		proto;
	char		description[NG_NAT_DESC_LENGTH];

Unique ID for this redirection is returned as response to this message.

Mark redirection with specified ID as dynamic, i.e., it will serve for exactly one next connection and then will be automatically deleted from internal links table. Only fully specified links can be made dynamic. The redirection with this ID is also immediately deleted from user-visible list of static redirects (available through NGM_NAT_LIST_REDIRECTS message).
Delete redirection with specified ID (currently active connections are not affected).
Add another server to a pool. This is used to transparently offload network load on a single server and distribute the load across a pool of servers, also known as (RFC 2391). The following struct ng_nat_add_server must be supplied as argument.
struct ng_nat_add_server {
	uint32_t	id;
	struct in_addr	addr;
	uint16_t	port;

First, the redirection is set up by NGM_NAT_REDIRECT_PORT or NGM_NAT_REDIRECT_ADDR. Then, ID of that redirection is used in multiple NGM_NAT_ADD_SERVER messages to add necessary number of servers. For redirections created by NGM_NAT_REDIRECT_ADDR, the port is ignored and could have any value. Original redirection's parameters local_addr and local_port are also ignored after NGM_NAT_ADD_SERVER was used (they are effectively replaced by server pool).

Return list of configured static redirects as struct ng_nat_list_redirects.
struct ng_nat_listrdrs_entry {
	uint32_t	id;		/* Anything except zero */
	struct in_addr	local_addr;
	struct in_addr	alias_addr;
	struct in_addr	remote_addr;
	uint16_t	local_port;
	uint16_t	alias_port;
	uint16_t	remote_port;
	uint16_t	proto;		/* Valid proto or NG_NAT_REDIRPROTO_ADDR */
	uint16_t	lsnat;		/* LSNAT servers count */
	char		description[NG_NAT_DESC_LENGTH];
struct ng_nat_list_redirects {
	uint32_t		total_count;
	struct ng_nat_listrdrs_entry redirects[];

Entries of the redirects array returned in the unified format for all redirect types. Ports are meaningful only if protocol is either TCP or UDP and static NAT redirection (created by NGM_NAT_REDIRECT_ADDR) is indicated by proto set to NG_NAT_REDIRPROTO_ADDR. If lsnat servers counter is greater than zero, then local_addr and local_port are also meaningless.

Specify a transparent proxying rule (string must be supplied as argument). See libalias(3) for details.
Return internal statistics of libalias(3) instance as struct ng_nat_libalias_info.
struct ng_nat_libalias_info {
	uint32_t	icmpLinkCount;
	uint32_t	udpLinkCount;
	uint32_t	tcpLinkCount;
	uint32_t	sctpLinkCount;
	uint32_t	pptpLinkCount;
	uint32_t	protoLinkCount;
	uint32_t	fragmentIdLinkCount;
	uint32_t	fragmentPtrLinkCount;
	uint32_t	sockCount;
In case of ng_nat failed to retrieve a certain counter from its libalias instance, the corresponding field is returned as UINT32_MAX.
Sets the data link type on the in and out hooks. Currently, supported types are DLT_RAW (raw IP datagrams , no offset applied, the default) and DLT_EN10MB (Ethernet). DLT_ definitions can be found in <net/bpf.h>. If you want to work on the ipfw(8) level you must use no additional offset by specifying DLT_RAW. If, however, you attach ng_nat to a network interface directly and EN10MB is specified, then the extra offset will be applied to take into account link-level header. In this mode the ng_nat would also inspect appropriate type field in the Ethernet header and pass-through any datagrams that are not IP packets.
This control message returns the current data link type of the in and out hooks.

In all redirection messages local_addr and local_port mean address and port of target machine in the internal network, respectively. If alias_addr is zero, then default aliasing address (set by NGM_NAT_SET_IPADDR) is used. Connections can also be restricted to be accepted only from specific external machines by using non-zero remote_addr and/or remote_port. Each redirection assigned an ID which can be later used for redirection manipulation on individual basis (e.g., removal). This ID guaranteed to be unique until the node shuts down (it will not be reused after deletion), and is returned to user after making each new redirection or can be found in the stored list of all redirections. The description passed to and from node unchanged, together with ID providing a way for several entities to concurrently manipulate redirections in automated way.


This node shuts down upon receipt of a NGM_SHUTDOWN control message, or when both hooks are disconnected.


In the following example, the packets are injected into a nat node using the ng_ipfw(4) node.

# Create NAT node
ngctl mkpeer ipfw: nat 60 out
ngctl name ipfw:60 nat
ngctl connect ipfw: nat: 61 in
ngctl msg nat: setaliasaddr x.y.35.8

# Divert traffic into NAT node
ipfw add 300 netgraph 61 all from any to any in via fxp0
ipfw add 400 netgraph 60 all from any to any out via fxp0

# Let packets continue with after being (de)aliased
sysctl net.inet.ip.fw.one_pass=0

The ng_nat node can be inserted right after the ng_iface(4) node in the graph. In the following example, we perform masquerading on a serial line with HDLC encapsulation.

/usr/sbin/ngctl -f- <<-SEQ
	mkpeer cp0: cisco rawdata downstream
	name cp0:rawdata hdlc
	mkpeer hdlc: nat inet in
	name hdlc:inet nat
	mkpeer nat: iface out inet
	msg nat: setaliasaddr x.y.8.35
ifconfig ng0 x.y.8.35 x.y.8.1

The ng_nat node can also be attached directly to the physical interface via ng_ether(4) node in the graph. In the following example, we perform masquerading on a Ethernet interface connected to a public network.

ifconfig igb0 inet x.y.8.35 netmask 0xfffff000
route add default x.y.0.1
/usr/sbin/ngctl -f- <<-SEQ
        mkpeer igb0: nat lower in
        name igb0:lower igb0_NAT
        connect igb0: igb0_NAT: upper out
        msg igb0_NAT: setdlt 1
        msg igb0_NAT: setaliasaddr x.y.8.35


libalias(3), ng_ipfw(4), natd(8), ngctl(8), ng_ether(8)


The ng_nat node type was implemented in FreeBSD 6.0.


Gleb Smirnoff <>

December 12, 2018 Debian