--- name: "ng_nat" section: "4" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/ng_nat" --- # ng\_nat(4) Network Address Translation netgraph node ## NAME **ng\_nat** – Network Address Translation netgraph node ## SYNOPSIS **#include \** ## DESCRIPTION An **ng\_nat** node performs network address translation (NAT) of IPv4 packets passing through it. A **nat** node uses [libalias(3)](/libalias/3.md) engine for packet aliasing. ## HOOKS This node type has two hooks: - *out* Packets received on this hook are considered outgoing and will be masqueraded to a configured address. - *in* Packets coming on this hook are considered incoming and will be dealiased. ## CONTROL MESSAGES This node type supports the generic control messages, plus the following: - NGM\_NAT\_SET\_IPADDR (**setaliasaddr**) Configure aliasing address for a node. After both hooks have been connected and aliasing address was configured, a node is ready for aliasing operation. - NGM\_NAT\_SET\_MODE (**setmode**) 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_UNREGISTERED_ONLY 0x10 #define NG_NAT_RESET_ON_ADDR_CHANGE 0x20 #define NG_NAT_PROXY_ONLY 0x40 #define NG_NAT_REVERSE 0x80 #define NG_NAT_UNREGISTERED_CGN 0x100 #define NG_NAT_UDP_EIM 0x200 ``` The corresponding libalias flags can be found by replacing the *NG\_NAT* prefix with *PKT\_ALIAS*. - NGM\_NAT\_SET\_TARGET (**settarget**) 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. - NGM\_NAT\_REDIRECT\_PORT (**redirectport**) 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. ``` #define NG_NAT_DESC_LENGTH 64 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. - NGM\_NAT\_REDIRECT\_ADDR (**redirectaddr**) 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. - NGM\_NAT\_REDIRECT\_PROTO (**redirectproto**) Redirect incoming IP packets of protocol *proto* (see [protocols(5)](/protocols/5.md)) 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. - NGM\_NAT\_REDIRECT\_DYNAMIC (**redirectdynamic**) 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). - NGM\_NAT\_REDIRECT\_DELETE (**redirectdelete**) Delete redirection with specified ID (currently active connections are not affected). - NGM\_NAT\_ADD\_SERVER (**addserver**) 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 *LSNAT* (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). - NGM\_NAT\_LIST\_REDIRECTS (**listredirects**) 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[]; }; #define NG_NAT_REDIRPROTO_ADDR (IPPROTO_MAX + 3) ``` 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. - NGM\_NAT\_PROXY\_RULE (**proxyrule**) Specify a transparent proxying rule (string must be supplied as argument). See [libalias(3)](/libalias/3.md) for details. - NGM\_NAT\_LIBALIAS\_INFO (**libaliasinfo**) Return internal statistics of [libalias(3)](/libalias/3.md) 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(3)](/libalias/3.md) instance, the corresponding field is returned as *UINT32\_MAX*. - NGM\_NAT\_SET\_DLT (**setdlt**) 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)](/ipfw/8.md) 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. - NGM\_NAT\_GET\_DLT (**getdlt**) 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. ## SHUTDOWN This node shuts down upon receipt of a NGM\_SHUTDOWN control message, or when both hooks are disconnected. ## EXAMPLES In the following example, the packets are injected into a **nat** node using the [ng\_ipfw(4)](/ng_ipfw/4.md) 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)](/ng_iface/4.md) 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 SEQ 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)](/ng_ether/4.md) 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 SEQ ``` ## SEE ALSO [libalias(3)](/libalias/3.md), [ng\_ipfw(4)](/ng_ipfw/4.md), [natd(8)](/natd/8.md), ng\_ether(8), [ngctl(8)](/ngctl/8.md) ## HISTORY The **ng\_nat** node type was implemented in FreeBSD 6.0. ## AUTHORS Gleb Smirnoff ⟨*glebius@FreeBSD.org*⟩