NATD(8)                    BSD System Manager's Manual                   NATD(8)

     natd — Network Address Translation daemon

     natd [-unregistered_only | -u] [-log | -l] [-proxy_only] [-reverse]
          [-deny_incoming | -d] [-use_sockets | -s] [-same_ports | -m]
          [-verbose | -v] [-dynamic] [-in_port | -i port] [-out_port | -o port]
          [-port | -p port] [-alias_address | -a address]
          [-target_address | -t address] [-interface | -n interface]
          [-proxy_rule proxyspec] [-redirect_port linkspec]
          [-redirect_proto linkspec] [-redirect_address linkspec]
          [-config | -f configfile] [-instance instancename] [-globalport port]
          [-log_denied] [-log_facility facility_name] [-punch_fw firewall_range]
          [-skinny_port port] [-log_ipfw_denied] [-pid_file | -P pidfile]
          [-exit_delay | -P ms]

     The natd utility provides a Network Address Translation facility for use
     with divert(4) sockets under FreeBSD.

     (If you need NAT on a PPP link, ppp(8) provides the -nat option that gives
     most of the natd functionality, and uses the same libalias(3) library.)

     The natd utility normally runs in the background as a daemon.  It is passed
     raw IP packets as they travel into and out of the machine, and will
     possibly change these before re-injecting them back into the IP packet

     It changes all packets destined for another host so that their source IP
     address is that of the current machine.  For each packet changed in this
     manner, an internal table entry is created to record this fact.  The source
     port number is also changed to indicate the table entry applying to the
     packet.  Packets that are received with a target IP of the current host are
     checked against this internal table.  If an entry is found, it is used to
     determine the correct target IP address and port to place in the packet.

     The following command line options are available:

     -log | -l   Log various aliasing statistics and information to the file
                 /var/log/alias.log.  This file is truncated each time natd is

     -deny_incoming | -d
                 Do not pass incoming packets that have no entry in the internal
                 translation table.

                 If this option is not used, then such a packet will be altered
                 using the rules in -target_address below, and the entry will be
                 made in the internal translation table.

                 Log denied incoming packets via syslog(3) (see also

     -log_facility facility_name
                 Use specified log facility when logging information via
                 syslog(3).  Argument facility_name is one of the keywords
                 specified in syslog.conf(5).

     -use_sockets | -s
                 Allocate a socket(2) in order to establish an FTP data or IRC
                 DCC send connection.  This option uses more system resources,
                 but guarantees successful connections when port numbers

     -same_ports | -m
                 Try to keep the same port number when altering outgoing
                 packets.  With this option, protocols such as RPC will have a
                 better chance of working.  If it is not possible to maintain
                 the port number, it will be silently changed as per normal.

     -verbose | -v
                 Do not call daemon(3) on startup.  Instead, stay attached to
                 the controlling terminal and display all packet alterations to
                 the standard output.  This option should only be used for
                 debugging purposes.

     -unregistered_only | -u
                 Only alter outgoing packets with an unregistered source
                 address.  According to RFC 1918, unregistered source addresses
                 are, and

     -redirect_port proto targetIP:targetPORT[-targetPORT]
                 Redirect incoming connections arriving to given port(s) to
                 another host and port(s).  Argument proto is either tcp or udp,
                 targetIP is the desired target IP address, targetPORT is the
                 desired target port number or range, aliasPORT is the requested
                 port number or range, and aliasIP is the aliasing address.
                 Arguments remoteIP and remotePORT can be used to specify the
                 connection more accurately if necessary.  If remotePORT is not
                 specified, it is assumed to be all ports.

                 Arguments targetIP, aliasIP and remoteIP can be given as IP
                 addresses or as hostnames.  The targetPORT, aliasPORT and
                 remotePORT ranges need not be the same numerically, but must
                 have the same size.  When targetPORT, aliasPORT or remotePORT
                 specifies a singular value (not a range), it can be given as a
                 service name that is searched for in the services(5) database.

                 For example, the argument

                       tcp inside1:telnet 6666

                 means that incoming TCP packets destined for port 6666 on this
                 machine will be sent to the telnet port on the inside1 machine.

                       tcp inside2:2300-2399 3300-3399

                 will redirect incoming connections on ports 3300-3399 to host
                 inside2, ports 2300-2399.  The mapping is 1:1 meaning port 3300
                 maps to 2300, 3301 maps to 2301, etc.

     -redirect_proto proto localIP [publicIP [remoteIP]]
                 Redirect incoming IP packets of protocol proto (see
                 protocols(5)) destined for publicIP address to a localIP
                 address and vice versa.

                 If publicIP is not specified, then the default aliasing address
                 is used.  If remoteIP is specified, then only packets coming
                 from/to remoteIP will match the rule.

     -redirect_address localIP publicIP
                 Redirect traffic for public IP address to a machine on the
                 local network.  This function is known as static NAT.  Normally
                 static NAT is useful if your ISP has allocated a small block of
                 IP addresses to you, but it can even be used in the case of
                 single address:


                 The above command would redirect all incoming traffic to

                 If several address aliases specify the same public address as

                       redirect_address public_addr
                       redirect_address public_addr
                       redirect_address public_addr

                 the incoming traffic will be directed to the last translated
                 local address (, but outgoing traffic from the
                 first two addresses will still be aliased to appear from the
                 specified public_addr.

     -redirect_port proto targetIP:targetPORT[,targetIP:targetPORT[,...]]
                 [aliasIP:]aliasPORT [remoteIP[:remotePORT]]

     -redirect_address localIP[,localIP[,...]] publicIP
                 These forms of -redirect_port and -redirect_address are used to
                 transparently offload network load on a single server and
                 distribute the load across a pool of servers.  This function is
                 known as LSNAT (RFC 2391).  For example, the argument

                       tcp www1:http,www2:http,www3:http www:http

                 means that incoming HTTP requests for host www will be
                 transparently redirected to one of the www1, www2 or www3,
                 where a host is selected simply on a round-robin basis, without
                 regard to load on the net.

     -dynamic    If the -n or -interface option is used, natd will monitor the
                 routing socket for alterations to the interface passed.  If the
                 interface's IP address is changed, natd will dynamically alter
                 its concept of the alias address.

     -in_port | -i port
                 Read from and write to divert(4) port port, treating all
                 packets as “incoming”.

     -out_port | -o port
                 Read from and write to divert(4) port port, treating all
                 packets as “outgoing”.

     -port | -p port
                 Read from and write to divert(4) port port, distinguishing
                 packets as “incoming” or “outgoing” using the rules specified
                 in divert(4).  If port is not numeric, it is searched for in
                 the services(5) database.  If this option is not specified, the
                 divert port named natd will be used as a default.

     -alias_address | -a address
                 Use address as the aliasing address.  Either this or the
                 -interface option must be used (but not both), if the
                 -proxy_only option is not specified.  The specified address is
                 usually the address assigned to the “public” network interface.

                 All data passing out will be rewritten with a source address
                 equal to address.  All data coming in will be checked to see if
                 it matches any already-aliased outgoing connection.  If it
                 does, the packet is altered accordingly.  If not, all
                 -redirect_port, -redirect_proto and -redirect_address
                 assignments are checked and actioned.  If no other action can
                 be made and if -deny_incoming is not specified, the packet is
                 delivered to the local machine using the rules specified in
                 -target_address option below.

     -t | -target_address address
                 Set the target address.  When an incoming packet not associated
                 with any pre-existing link arrives at the host machine, it will
                 be sent to the specified address.

                 The target address may be set to, in which case
                 all new incoming packets go to the alias address set by
                 -alias_address or -interface.

                 If this option is not used, or called with the argument
       , then all new incoming packets go to the address
                 specified in the packet.  This allows external machines to talk
                 directly to internal machines if they can route packets to the
                 machine in question.

     -interface | -n interface
                 Use interface to determine the aliasing address.  If there is a
                 possibility that the IP address associated with interface may
                 change, the -dynamic option should also be used.  If this
                 option is not specified, the -alias_address option must be

                 The specified interface is usually the “public” (or “external”)
                 network interface.

     -config | -f file
                 Read configuration from file.  A file should contain a list of
                 options, one per line, in the same form as the long form of the
                 above command line options.  For example, the line


                 would specify an alias address of  Options that
                 do not take an argument are specified with an argument of yes
                 or no in the configuration file.  For example, the line

                       log yes

                 is synonymous with -log.

                 Options can be divided to several sections.  Each section
                 applies to own natd instance.  This ability allows the
                 configuration of one natd process for several NAT instances.
                 The first instance that always exists is a "default" instance.
                 Each another instance should begin with

                       instance instance_name

                 At the next should be placed a configuration option.  Example:

                       # default instance
                       port 8668

                       # second instance
                       instance dsl1
                       port 8888

                 Trailing spaces and empty lines are ignored.  A ‘#’ sign will
                 mark the rest of the line as a comment.

     -instance instancename
                 This option switches command line options processing to
                 configure instance instancename (creating it if necessary) till
                 the next -instance option or end of command line.  It is easier
                 to set up multiple instances in the configuration file
                 specified with the -config option rather than on a command

     -globalport port
                 Read from and write to divert(4) port port, treating all
                 packets as “outgoing”.  This option is intended to be used with
                 multiple instances: packets received on this port are checked
                 against internal translation tables of every configured
                 instance.  If an entry is found, packet is aliased according to
                 that entry.  If no entry was found in any of the instances,
                 packet is passed unchanged, and no new entry will be created.
                 See the section MULTIPLE INSTANCES for more details.

     -reverse    This option makes natd reverse the way it handles “incoming”
                 and “outgoing” packets, allowing it to operate on the
                 “internal” network interface rather than the “external” one.

                 This can be useful in some transparent proxying situations when
                 outgoing traffic is redirected to the local machine and natd is
                 running on the internal interface (it usually runs on the
                 external interface).

                 Force natd to perform transparent proxying only.  Normal
                 address translation is not performed.

     -proxy_rule [type encode_ip_hdr | encode_tcp_stream] port xxxx server
                 Enable transparent proxying.  Outgoing TCP packets with the
                 given port going through this host to any other host are
                 redirected to the given server and port.  Optionally, the
                 original target address can be encoded into the packet.  Use
                 encode_ip_hdr to put this information into the IP option field
                 or encode_tcp_stream to inject the data into the beginning of
                 the TCP stream.

     -punch_fw basenumber:count
                 This option directs natd to “punch holes” in an ipfirewall(4)
                 based firewall for FTP/IRC DCC connections.  This is done
                 dynamically by installing temporary firewall rules which allow
                 a particular connection (and only that connection) to go
                 through the firewall.  The rules are removed once the
                 corresponding connection terminates.

                 A maximum of count rules starting from the rule number
                 basenumber will be used for punching firewall holes.  The range
                 will be cleared for all rules on startup.  This option has no
                 effect when the kernel is in security level 3, see init(8) for
                 more information.

     -skinny_port port
                 This option allows you to specify the TCP port used for the
                 Skinny Station protocol.  Skinny is used by Cisco IP phones to
                 communicate with Cisco Call Managers to set up voice over IP
                 calls.  By default, Skinny aliasing is not performed.  The
                 typical port value for Skinny is 2000.

                 Log when a packet cannot be re-injected because an ipfw(8) rule
                 blocks it.  This is the default with -verbose.

     -pid_file | -P file
                 Specify an alternate file in which to store the process ID.
                 The default is /var/run/

     -exit_delay ms
                 Specify delay in ms before daemon exit after signal.  The
                 default is 10000.

     The following steps are necessary before attempting to run natd:

     1.   Build a custom kernel with the following options:

                options IPFIREWALL
                options IPDIVERT

          Refer to the handbook for detailed instructions on building a custom

     2.   Ensure that your machine is acting as a gateway.  This can be done by
          specifying the line


          in the /etc/rc.conf file or using the command

                sysctl net.inet.ip.forwarding=1

     3.   If you use the -interface option, make sure that your interface is
          already configured.  If, for example, you wish to specify ‘tun0’ as
          your interface, and you are using ppp(8) on that interface, you must
          make sure that you start ppp prior to starting natd.

     Running natd is fairly straight forward.  The line

           natd -interface ed0

     should suffice in most cases (substituting the correct interface name).
     Please check rc.conf(5) on how to configure it to be started automatically
     during boot.  Once natd is running, you must ensure that traffic is
     diverted to natd:

     1.   You will need to adjust the /etc/rc.firewall script to taste.  If you
          are not interested in having a firewall, the following lines will do:

                /sbin/ipfw -f flush
                /sbin/ipfw add divert natd all from any to any via ed0
                /sbin/ipfw add pass all from any to any

          The second line depends on your interface (change ‘ed0’ as

          You should be aware of the fact that, with these firewall settings,
          everyone on your local network can fake his source-address using your
          host as gateway.  If there are other hosts on your local network, you
          are strongly encouraged to create firewall rules that only allow
          traffic to and from trusted hosts.

          If you specify real firewall rules, it is best to specify line 2 at
          the start of the script so that natd sees all packets before they are
          dropped by the firewall.

          After translation by natd, packets re-enter the firewall at the rule
          number following the rule number that caused the diversion (not the
          next rule if there are several at the same number).

     2.   Enable your firewall by setting


          in /etc/rc.conf.  This tells the system startup scripts to run the
          /etc/rc.firewall script.  If you do not wish to reboot now, just run
          this by hand from the console.  NEVER run this from a remote session
          unless you put it into the background.  If you do, you will lock
          yourself out after the flush takes place, and execution of
          /etc/rc.firewall will stop at this point - blocking all accesses
          permanently.  Running the script in the background should be enough to
          prevent this disaster.

     It is not so uncommon to have a need of aliasing to several external IP
     addresses.  While this traditionally was achieved by running several natd
     processes with independent configurations, natd can have multiple aliasing
     instances in a single process, also allowing them to be not so independent
     of each other.  For example, let us see a common task of load balancing two
     channels to different providers on a machine with two external interfaces
     ‘sis0’ (with IP and ‘sis2’ (with IP

  ------------------ sis0
           (router)                (
                                             sis1 -------------------
  ------------------ sis2
           (router)                (

     Default route is out via ‘sis0’.

     Interior machine ( is accessible on TCP port 122 through both
     exterior IPs, and outgoing connections choose a path randomly between
     ‘sis0’ and ‘sis2’.

     The way this works is that natd.conf builds two instances of the aliasing

     In addition to these instances' private divert(4) sockets, a third socket
     called the “globalport” is created; packets sent to natd via this one will
     be matched against all instances and translated if an existing entry is
     found, and unchanged if no entry is found.  The following lines are placed
     into /etc/natd.conf:


           instance default
           interface sis0
           port 1000
           redirect_port tcp 122

           instance sis2
           interface sis2
           port 2000
           redirect_port tcp 122

           globalport 3000

     And the following ipfw(8) rules are used:

           ipfw -f flush

           ipfw add      allow ip from any to any via sis1

           ipfw add      skipto 1000 ip from any to any in via sis0
           ipfw add      skipto 2000 ip from any to any out via sis0
           ipfw add      skipto 3000 ip from any to any in via sis2
           ipfw add      skipto 4000 ip from any to any out via sis2

           ipfw add 1000 count ip from any to any

           ipfw add      divert 1000 ip from any to any
           ipfw add      allow ip from any to any

           ipfw add 2000 count ip from any to any

           ipfw add      divert 3000 ip from any to any

           ipfw add      allow ip from to any
           ipfw add      skipto 5000 ip from to any

           ipfw add      prob .5 skipto 4000 ip from any to any

           ipfw add      divert 1000 ip from any to any
           ipfw add      allow ip from any to any

           ipfw add 3000 count ip from any to any

           ipfw add      divert 2000 ip from any to any
           ipfw add      allow ip from any to any

           ipfw add 4000 count ip from any to any

           ipfw add      divert 2000 ip from any to any

           ipfw add 5000 fwd ip from to not
           ipfw add      allow ip from any to any

     Here the packet from internal network to Internet goes out via ‘sis0’ (rule
     number 2000) and gets caught by the globalport socket (3000).  After that,
     either a match is found in a translation table of one of the two instances,
     or the packet is passed to one of the two other divert(4) ports (1000 or
     2000), with equal probability.  This ensures that load balancing is done on
     a per-flow basis (i.e., packets from a single TCP connection always flow
     through the same interface).  Translated packets with source IP of a non-
     default interface (‘sis2’) are forwarded to the appropriate router on that

     libalias(3), divert(4), protocols(5), rc.conf(5), services(5),
     syslog.conf(5), init(8), ipfw(8), ppp(8)

     The natd utility appeared in FreeBSD 3.0.

     This program is the result of the efforts of many people at different

     Archie Cobbs <> (divert sockets)
     Charles Mott <> (packet aliasing)
     Eivind Eklund <> (IRC support & misc additions)
     Ari Suutari <> (natd)
     Dru Nelson <> (early PPTP support)
     Brian Somers <> (glue)
     Ruslan Ermilov <> (natd, packet aliasing, glue)
     Poul-Henning Kamp <> (multiple instances)

BSD                              October 5, 2016                             BSD