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


[править] NAME

    natd — Network Address Translation daemon

[править] SYNOPSIS

    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)
    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 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
                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.
                Options can be divided to several sections.  Each section
                applies to own natd instance.  This ability allows to config‐
                ure 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.  Exam‐
                      # 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 con‐
                figure instance instancename (creating it if necessary) till
                the next -instance option or end of command line.  It is eas‐
                ier 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 config‐
                ured 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
    -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 connection) to
                go through the firewall.  The rules are removed once the cor‐
                responding 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.

[править] RUNNING NATD

    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.


    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 alias‐
    ing instances in a single process, also allowing them to be not so inde‐
    pendent 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 catched 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 interface.

[править] SEE ALSO

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

[править] AUTHORS

    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)

FreeBSD 9.0 June 23, 2008 FreeBSD 9.0

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