Home Man
Today's Posts

Linux & Unix Commands - Search Man Pages

OpenDarwin 7.2.1 - man page for natd (opendarwin section 8)

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] [-log_denied] [-log_facility facility_name]
	  [-punch_fw firewall_range] [-clamp_mss]

     This program provides a Network Address Translation facility for use with divert(4) sockets
     under FreeBSD.  It is intended for use with NICs - if you want to do NAT on a PPP link, use
     the -nat switch to ppp(8).

     The natd 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 number 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 number 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 started.

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

		 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

		 Log denied incoming packets via syslog(3) (see also -log_facility).

     -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 connec-
		 tion.	This option uses more system resources, but guarantees successful connec-
		 tions when port numbers conflict.

     -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

     -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] [aliasIP:]aliasPORT[-aliasPORT]
		 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
		 number, 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.  The targetPORT range and aliasPORT range need not be the same numer-
		 ically, but must have the same size.  If remotePORT is not specified, it is
		 assumed to be all ports.  If remotePORT is specified, it must match the size of
		 targetPORT, or be 0 (all ports).  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

     -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 machine

		 If several address aliases specify the same public address as follows

		       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

     -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 number 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

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

     -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.  If this option is not specified, the
		 -interface option must be used.  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 out-
		 going 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 spec-
		 ified, 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

		 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 exter-
		 nal 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 number associated with interface may change, the -dynamic option should
		 also be used.	If this option is not specified, the -alias_address option must
		 be used.

		 The specified interface is usually the ``public'' (or ``external'') network

     -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 a.b.c.d:yyyy
		 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 connec-
		 tion 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

		 This option enables MSS clamping.  The MSS value is derived from the MTU of the
		 interface specified in the -interface option.

     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 kernel.

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


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

		sysctl -w 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 en0

     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 inter-
	  ested 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 'en0' as appropriate).

	  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.

     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 times:

     Archie Cobbs <archie@whistle.com> (divert sockets)
     Charles Mott <cmott@scientech.com> (packet aliasing)
     Eivind Eklund <perhaps@yes.no> (IRC support & misc additions)
     Ari Suutari <suutari@iki.fi> (natd)
     Dru Nelson <dnelson@redwoodsoft.com> (early PPTP support)
     Brian Somers <brian@awfulhak.org> (glue)
     Ruslan Ermilov <ru@FreeBSD.org> (natd, packet aliasing, glue)

Darwin					  June 27, 2000 				   Darwin

All times are GMT -4. The time now is 07:17 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
Show Password