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     ng_nat — NAT 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:

     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.


     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

     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)) to a machine on the
          local network.  The following struct ng_nat_redirect_proto must be supplied as

          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

     NGM_NAT_PROXY_RULE (proxyrule)
          Specify a transparent proxying rule (string must be supplied as argument).  See
          libalias(3) for details.

     NGM_NAT_LIBALIAS_INFO (libaliasinfo)
          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.

     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) 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.


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


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

           # 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 <>