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NATD(8)                 FreeBSD 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] [-log_denied]
          [-log_facility facility_name] [-punch_fw firewall_range]
          [-skinny_port port] [-log_ipfw_denied] [-pid_file | -P pidfile]

     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)

     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 stream.

     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 inter-
                 nal 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 con-

     -same_ports | -m
                 Try to keep the same port number when altering outgoing pack-
                 ets.  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) data-

                 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

                       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.  Nor-
                 mally 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 trans-
                 parently 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 spec-
                 ified in divert(4).  If port is not numeric, it is searched
                 for in the services(5) database.  If this option is not spec-
                 ified, 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

                 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 assign-
                 ments 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 associ-
                 ated 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.

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

     -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 begin-
                 ning 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 con-
                 nection) 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.

     -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/

     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 automati-
     cally 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 appropri-

          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 net-
          work, 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.

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

     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)

FreeBSD 6.2                    February 28, 2003                   FreeBSD 6.2


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