freebsd-skq/sbin/natd/natd.8

.\" manual page [] for natd 1.4
.\" $FreeBSD$
.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 dynamic
.Op Fl i Ar inport
.Op Fl o Ar outport
.Op Fl p Ar port
.Op Fl a Ar address
.Op Fl t Ar address
.Op Fl n Ar interface
.Op Fl f Ar configfile

.Nm
.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 target_address Ar address
.Op Fl interface Ar interface
.Op Fl config Ar configfile
.Op Fl redirect_port Ar linkspec
.Op Fl redirect_proto Ar linkspec
.Op Fl redirect_address Ar linkspec
.Op Fl reverse
.Op Fl proxy_only
.Op Fl proxy_rule Ar proxyspec

.Sh DESCRIPTION
This program provides a Network Address Translation facility for use
with
.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 -nat switch to
.Xr ppp 8 .

.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 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
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 proto targetIP:targetPORT[-targetPORT] [aliasIP:]aliasPORT[-aliasPORT] [remoteIP[:remotePORT[-remotePORT]]]
Redirect incoming connections arriving to given port(s) to another host
and port(s).
.Ar Proto
is either tcp or udp,
.Ar targetIP
is the desired target IP number,
.Ar targetPORT
is the desired target PORT number or range,
.Ar aliasPORT
is the requested PORT number or range,
and
.Ar aliasIP
is the aliasing address.
.Ar RemoteIP
and
.Ar remotePORT
can be used to specify the connection more accurately if necessary.
The
.Ar targetPORT
range and
.Ar aliasPORT
range need not be the same numerically,
but must have the same size.
If
.Ar remotePORT
is not specified,
it is assumed to be all ports.
If
.Ar remotePORT
is specified,
it must match the size of targetPORT,
or be 0
(all ports).
For example,
the argument
.Pp
.Dl Ar tcp inside1:telnet 6666
.Pp
means that incoming tcp packets destined for port 6666 on this machine will
be sent to the telnet port on the inside1 machine.
.Pp
.Dl Ar tcp inside2:2300-2399 3300-3399
.Pp
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.
.It Fl redirect_proto Ar proto localIP Xo
.Op Ar publicIP Op Ar remoteIP
.Xc
Redirect incoming IP packets of protocol
.Ar proto
.Pq see Xr protocols 5
destined for
.Ar publicIP
address to a
.Ar localIP
address and vice versa.
.Pp
If
.Ar publicIP
is not specified,
then the default aliasing address is used.
If
.Ar remoteIP
is specified,
then only packets coming from/to
.Ar remoteIP
will match the rule.
.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:
.Pp
.Dl Ar redirect_address 10.0.0.8 0.0.0.0
.Pp
The above command would redirect all incoming traffic to machine 10.0.0.8.
.Pp
If several address aliases specify the same public address as follows
.Pp
.Dl Ar redirect_address 192.168.0.2 public_addr
.Dl Ar redirect_address 192.168.0.3 public_addr
.Dl Ar redirect_address 192.168.0.4 public_addr
.Pp
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 redirect_port Ar proto Xo
.Ar targetIP Ns : Ns Xo
.Ar targetPORT Ns Oo , Ns
.Ar targetIP Ns : Ns Xo
.Ar targetPORT Ns Oo , Ns
.Ar ...
.Oc Oc
.Xc
.Xc
.Op Ar aliasIP Ns : Ns Xo
.Ar aliasPORT
.Xc
.Oo Ar remoteIP Ns
.Op : Ns Ar remotePORT
.Oc
.Xc
.It Fl redirect_address Xo
.Ar localIP Ns Oo , Ns
.Ar localIP Ns Oo , Ns
.Ar ...
.Oc Oc
.Ar publicIP
.Xc
These forms of
.Fl redirect_port
and
.Fl 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
.Em LSNAT
(RFC 2391).
For example,
the argument
.Pp
.Dl Ar tcp www1:http,www2:http,www3:http www:http
.Pp
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.
.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:
.Pp
natd   8668/divert  # Network Address Translation socket
.Pp
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 t | target_address Ar 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
.Ar address .
.Pp
The target address may be set to
.Dq 255.255.255.255 ,
in which case all new incoming packets go to the alias address set by
.Fl alias_address
or
.Fl interface .
.Pp
If this option is not used,
or called with the argument
.Dq 0.0.0.0 ,
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.
.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
.Pp
alias_address 158.152.17.1
.Pp
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
.Pp
log yes
.Pp
is synonomous with
.Fl log .
.Pp
Trailing spaces and empty lines are ignored.
A
.Ql \&#
sign will mark the rest of the line as a comment.
.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.
.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:
.Pp
options IPFIREWALL
options IPDIVERT
.Pp
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
.Pp
gateway_enable=YES
.Pp
in
.Pa /etc/rc.conf ,
or using the command
.Pp
.Nm sysctl Fl w Ar 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 :
.Pp
natd          8668/divert  # Network Address Translation socket
.Pp
This gives a default for the
.Fl p
or
.Fl port
flag.
.El
.Pp
Running
.Nm
is fairly straight forward.  The line
.Pp
.Nm natd Fl interface Ar ed0
.Pp
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:
.Pp
/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
.Pp
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.
.Pp
You should be aware of the fact,
that with these firewall settings everyone on your local network
can fake his source-address using your box as gateway.
If there are other machines on your local network,
it is highly recommended to create firewall-rules that only allow traffic
from and to your own machines.
.Pp
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.
.Pp
After translation by
.Nm 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).
.It
Enable your firewall by setting
.Pp
firewall_enable=YES
.Pp
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 socket 2 ,
.Xr getservbyname 3 ,
.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@scientech.com
(packet aliasing)
.An Eivind Eklund Aq perhaps@yes.no
(IRC support & misc additions)
.An Ari Suutari Aq suutari@iki.fi
(natd)
.An Dru Nelson Aq dnelson@redwoodsoft.com
(early PPTP support)
.An Brian Somers Aq brian@awfulhak.org
(glue)