natd.8   [plain text]

.\" manual page [] for natd 1.4
.\"	$Id: natd.8,v 2000/01/11 01:48:51 wsanchez Exp $
.Dd 15 April 1997
.Os FreeBSD
.Dt NATD 8
.Nm natd
Network Address Translation Daemon
.Op Fl ldsmvu
.Op Fl dynamic
.Op Fl i Ar inport
.Op Fl o Ar outport
.Op Fl p Ar port
.Op Fl a Ar address
.Op Fl n Ar interface
.Op Fl f Ar configfile

.Op Fl log
.Op Fl deny_incoming
.Op Fl log_denied
.Op Fl use_sockets
.Op Fl same_ports
.Op Fl verbose
.Op Fl log_facility Ar facility_name
.Op Fl unregistered_only
.Op Fl dynamic
.Op Fl inport Ar inport
.Op Fl outport Ar outport
.Op Fl port Ar port
.Op Fl alias_address Ar address
.Op Fl interface Ar interface
.Op Fl config Ar configfile
.Op Fl redirect_port Ar linkspec
.Op Fl redirect_address Ar localIP publicIP
.Op Fl reverse
.Op Fl proxy_only
.Op Fl proxy_rule Ar proxyspec
.Op Fl pptpalias Ar localIP

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

.Nm 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.

.Nm Natd
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.
.Bl -tag -width Fl

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

.It Fl deny_incoming | d
Reject packets destined for the current IP number that have no entry
in the internal translation table.

.It Fl log_denied
Log denied incoming packets via syslog (see also log_facility)

.It Fl log_facility Ar facility_name
Use specified log facility when logging information via syslog.
Facility names are as in
.Xr syslog.conf 5

.It Fl use_sockets | s
Allocate a
.Xr 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 conflict.

.It Fl 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.

.It Fl verbose | v
Don't call
.Xr fork 2
.Xr daemon 3
on startup.  Instead, stay attached to the controling terminal and
display all packet alterations to the standard output.  This option
should only be used for debugging purposes.

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

.It Fl redirect_port Ar proto targetIP:targetPORT [aliasIP:]aliasPORT [remoteIP[:remotePORT]]
Redirect incoming connections arriving to given port to another host and port.
Proto is either tcp or udp, targetIP is the desired target IP
number, targetPORT is the desired target PORT number, aliasPORT
is the requested PORT number and aliasIP is the aliasing address.
RemoteIP and remotePORT can be used to specify the connection
more accurately if necessary.
For example, the argument

.Ar tcp inside1:telnet 6666

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

.It Fl redirect_address Ar 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 to the first two addresses will still be aliased
to specified public address.

.It Fl dynamic
If the
.Fl n
.Fl interface
option is used,
will monitor the routing socket for alterations to the
.Ar interface
passed.  If the interfaces IP number is changed,
will dynamically alter its concept of the alias address.

.It Fl i | inport Ar inport
Read from and write to
.Ar inport ,
treating all packets as packets coming into the machine.

.It Fl o | outport Ar outport
Read from and write to
.Ar outport ,
treating all packets as packets going out of the machine.

.It Fl p | port Ar port
Read from and write to
.Ar port ,
distinguishing packets as incoming our outgoing using the rules specified in
.Xr divert 4 .
.Ar port
is not numeric, it is searched for in the
.Pa /etc/services
database using the
.Xr getservbyname 3
function.  If this flag is not specified, the divert port named natd will
be used as a default.  An example entry in the
.Pa /etc/services
database would be:

  natd   8668/divert  # Network Address Translation socket

Refer to
.Xr services 5
for further details.

.It Fl a | alias_address Ar address
.Ar address
as the alias address.  If this option is not specified, the
.Fl n
.Fl interface
option must be used.  The specified address should be the address assigned
to the public network interface.
All data passing out through this addresses interface will be rewritten
with a source address equal to
.Ar address .
All data arriving at the interface from outside will be checked to
see if it matches any already-aliased outgoing connection.  If it does,
the packet is altered accordingly.  If not, all
.Fl redirect_port
.Fl redirect_address
assignments are checked and actioned.  If no other action can be made,
and if
.Fl deny_incoming
is not specified, the packet is delivered to the local machine and port
as specified in the packet.

.It Fl n | interface Ar interface
.Ar interface
to determine the alias address.  If there is a possibility that the
IP number associated with
.Ar interface
may change, the
.Fl dynamic
flag should also be used.  If this option is not specified, the
.Fl a
.Fl alias_address
flag must be used.
The specified
.Ar interface
must be the public network interface.
.It Fl f | config Ar configfile
Read configuration from
.Ar configfile .
.Ar Configfile
contains a list of options, one per line in the same form as the
long form of the above command line flags.  For example, the line


would specify an alias address of  Options that don't
take an argument are specified with an option of
.Ar yes
.Ar no
in the configuration file.  For example, the line

  log yes

is synonomous with
.Fl log .
Empty lines and lines beginning with '#' are ignored.

.It Fl reverse
Reverse operation of natd. This can be useful in some 
transparent proxying situations when outgoing traffic
is redirected to the local machine and natd is running on the
incoming interface (it usually runs on the outgoing interface).

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

.It Fl proxy_rule Ar [type encode_ip_hdr|encode_tcp_stream] port xxxx server a.b.c.d:yyyy
Enable transparent proxying. 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 
.Dq encode_ip_hdr
to put this information into the IP option field or
.Dq encode_tcp_stream
to inject the data into the beginning of the TCP stream.

.It Fl pptpalias Ar localIP
Allow PPTP packets to go to the defined localIP address. PPTP is a VPN or secure
IP tunneling technology being developed primarily by Microsoft. For its encrypted traffic,
it uses an old IP encapsulation protocol called GRE (47). This
natd option will translate any traffic of this protocol to a
single, specified IP address. This would allow either one client or one server 
to be serviced with natd. If you are setting up a server, don't forget to allow the TCP traffic
for the PPTP setup. For a client or server, you must allow GRE (protocol 47) if you have firewall lists active.


The following steps are necessary before attempting to run
.Nm natd :

.Bl -enum
Get FreeBSD version 2.2 or higher.  Versions before this do not support
.Xr divert 4

Build a custom kernel with the following options:

  options IPFIREWALL
  options IPDIVERT

Refer to the handbook for detailed instructions on building a custom

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


.Pa /etc/rc.conf ,
or using the command

  sysctl -w net.inet.ip.forwarding=1

If you wish to use the
.Fl n
.Fl interface
flags, make sure that your interface is already configured.  If, for
example, you wish to specify tun0 as your
.Ar interface ,
and you're using
.Xr ppp 8
on that interface, you must make sure that you start
.Nm ppp
prior to starting
.Nm natd .

Create an entry in
.Pa /etc/services :

  natd          8668/divert  # Network Address Translation socket

This gives a default for the
.Fl p
.Fl port

is fairly straight forward.  The line

  natd -interface ed0

should suffice in most cases (substituting the correct interface name).  Once
is running, you must ensure that traffic is diverted to natd:

.Bl -enum
You will need to adjust the
.Pa /etc/rc.firewall
script to taste.  If you're 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 appropriate)
and assumes that you've updated
.Pa /etc/services
with the natd entry as above.  If you specify real firewall rules, it's
best to specify line 2 at the start of the script so that
sees all packets before they are dropped by the firewall.  The firewall
rules will be run again on each packet after translation by
.Nm natd ,
minus any divert rules.

Enable your firewall by setting


.Pa /etc/rc.conf .
This tells the system startup scripts to run the
.Pa /etc/rc.firewall
script.  If you don't wish to reboot now, just run this by hand from the
console.  NEVER run this from a virtual session unless you put it into
the background.  If you do, you'll lock yourself out after the flush
takes place, and execution of
.Pa /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.


.Xr getservbyname 2 ,
.Xr socket 2 ,
.Xr divert 4 ,
.Xr services 5 ,
.Xr ipfw 8

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

.An Archie Cobbs Aq
(divert sockets)
.An Charles Mott Aq
(packet aliasing)
.An Eivind Eklund Aq
(IRC support & misc additions)
.An Ari Suutari Aq
.An Dru Nelson Aq
(PPTP support)
.An Brian Somers Aq