freebsd/sbin/natd/natd.8

.\" manual page [] for natd 1.4
.Dd 15 April 1997
.Os FreeBSD
.Dt NATD 8
.Sh NAME
.Nm natd
.Nd
Network Address Translation Daemon
.Sh SYNOPSIS
.Nm
.Op Fl ldsmvu
.Op Fl permanent_link
.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

.Nm
.Op Fl log
.Op Fl deny_incoming
.Op Fl use_sockets
.Op Fl same_ports
.Op Fl verbose
.Op Fl unregistered_only
.Op Fl permanent_link
.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

.Sh DESCRIPTION
This program provides a Network Address Translation facility for use
with
.Xr divert 4
sockets under FreeBSD.  Most of the command line options are available
in a single character short form or in a long form.  Use of the long
form is encouraged as it makes things clearer to the casual observer.

.Pp
.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.

.Pp
.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.

.Pp
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 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
or
.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 10.0.0.0/8,
172.16.0.0/12 and 192.168.0.0/16.

.It Fl redirect_port Ar linkspec
Redirect incoming connections arriving to given port to another host and port.
Linkspec is of the form

  proto targetIP:targetPORT [aliasIP:]aliasPORT [remoteIP[:remotePORT]]

where 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:

  redirect_address 10.0.0.8 0.0.0.0

The above command would redirect all incoming traffic
to machine 10.0.0.8.

If several address aliases specify the same public address
as follows

  redirect_address 192.168.0.2 public_addr
  redirect_address 192.168.0.3 public_addr
  redirect_address 192.168.0.4 public_addr
  
the incoming traffic will be directed to the last
translated local address (192.168.0.4), but outgoing
traffic to the first two addresses will still be aliased
to specified public address.

.It Fl permanent_link Ar linkspec
Create a permanent entry in the internal alias table. Linkspec is
of the form

  proto targetIP:targetPORT sourceIP:sourcePORT aliasPORT

where proto is either tcp or udp, targetIP is the desired target IP
number, targetPORT is the desired target PORT number, sourceIP and
sourcePORT match the incoming packet, and aliasPORT is the requested
PORT number.  Values of zero are considered as wildcards.  For example,
the argument

.Ar tcp inside1:telnet outside1:0 6666

means that tcp packets destined for port 6666 on this machine from the
outside1 machine (any port) will be sent to the telnet port on the
inside1 machine.

New installations are encouraged to use redirect_port instead.

.It Fl dynamic
If the
.Fl n
or
.Fl interface
option is used,
.Nm
will monitor the routing socket for alterations to the
.Ar interface
passed.  If the interfaces IP number is changed,
.Nm
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 .
If
.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   6668/divert  # Network Address Translation socket

Refer to
.Xr services 5
for further details.

.It Fl a | alias_address Ar address
Use
.Ar address
as the alias address.  If this option is not specified, the
.Fl n
or
.Fl interface
option must be used.  The specified address should be the address assigned
to the public network interface.
.Pp
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
and
.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
Use
.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
or
.Fl alias_address
flag must be used.
.Pp
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

  alias_address 158.152.17.1

would specify an alias address of 158.152.17.1.  Options that don't
take an argument are specified with an option of
.Ar yes
or
.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.

.El

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

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

.It
Build a custom kernel with the following options:

  options IPFIREWALL
  options IPDIVERT

Refer to the handbook for detailed instructions on building a custom
kernel.

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

  gateway_enable=YES

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

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

.It
If you wish to use the
.Fl n
or
.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 .

.It
Create an entry in
.Pa /etc/services :

  natd          6668/divert  # Network Address Translation socket

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

.El
.Pp
Running
.Nm
is fairly straight forward.  The line

  natd -interface ed0

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

.Bl -enum
.It
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
.Nm
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.

.It
Enable your firewall by setting

  firewall_enable=YES

in
.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.

.El

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

.Sh AUTHORS
This program is the result of the efforts of many people at different
times:

.An Archie Cobbs Aq archie@whistle.com
(divert sockets)
.An Charles Mott Aq cmott@srv.net
(packet aliasing)
.An Eivind Eklund Aq perhaps@yes.no
(IRC support & misc additions)
.An Ari Suutari Aq suutari@iki.fi
(natd)
.An Brian Somers Aq brian@awfulhak.org
(glue)