sbin/natd/natd.8

.\" $FreeBSD$
.Dd June 27, 2000
.Dt NATD 8
.Os FreeBSD
.Sh NAME
.Nm natd
.Nd Network Address Translation Daemon
.Sh SYNOPSIS
.Nm natd
.Op Fl unregistered_only | u
.Op Fl log | l
.Op Fl proxy_only
.Op Fl reverse
.Op Fl deny_incoming | d
.Op Fl use_sockets | s
.Op Fl same_ports | m
.Op Fl verbose | v
.Op Fl dynamic
.Op Fl in_port | i Ar port
.Op Fl out_port | o Ar port
.Op Fl port | p Ar port
.Op Fl alias_address | a Ar address
.Op Fl target_address | t Ar address
.Op Fl interface | n Ar interface
.Op Fl proxy_rule Ar proxyspec
.Op Fl redirect_port Ar linkspec
.Op Fl redirect_proto Ar linkspec
.Op Fl redirect_address Ar linkspec
.Op Fl config | f Ar configfile
.Op Fl log_denied
.Op Fl log_facility Ar facility_name
.Op Fl punch_fw Ar firewall_range
.Sh DESCRIPTION
This program provides a Network Address Translation facility for use
with
.Xr divert 4
sockets under
.Fx .
It is intended for use with NICs - if you want to do NAT on a PPP link,
use the
.Fl nat
switch to
.Xr ppp 8 .
.Pp
The
.Nm
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
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.
.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
.Nm
is started.
.It Fl deny_incoming | d
Do not pass 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
.Xr syslog 3
.Po
see also
.Fl log_facility
.Pc .
.It Fl log_facility Ar facility_name
Use specified log facility when logging information via
.Xr syslog 3 .
Argument
.Ar facility_name
is one of the keywords specified 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
Do not call
.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
.Em 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 Xo
.Ar targetIP Ns : Ns Xo
.Ar targetPORT Ns Op - Ns Ar targetPORT Xc
.Op Ar aliasIP Ns : Ns Xo
.Ar aliasPORT Ns Op - Ns Ar aliasPORT Xc
.Oo Ar remoteIP Ns Oo : Ns
.Ar remotePORT Ns Op - Ns Ar remotePORT
.Oc Oc
.Xc
Redirect incoming connections arriving to given port(s) to another host
and port(s).
Argument
.Ar proto
is either
.Ar tcp
or
.Ar 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.
Arguments
.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
.Ar 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 Oo
.Ar publicIP Op Ar remoteIP
.Oc
Redirect incoming IP packets of protocol
.Ar proto
.Po see Xr protocols 5
.Pc
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
.Em 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
.Bd -literal -offset indent
.Ar redirect_address 192.168.0.2 public_addr
.Ar redirect_address 192.168.0.3 public_addr
.Ar redirect_address 192.168.0.4 public_addr
.Ed
.Pp
the incoming traffic will be directed to the last
translated local address (192.168.0.4), but outgoing
traffic from the first two addresses will still be aliased
to appear from the specified
.Ar public_addr .
.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 interface's IP number is changed,
.Nm
will dynamically alter its concept of the alias address.
.It Fl in_port | i Ar port
Read from and write to
.Ar port ,
treating all packets as packets coming into the machine.
.It Fl out_port | o Ar port
Read from and write to
.Ar port ,
treating all packets as packets going out of the machine.
.It Fl port | p 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
.Xr services 5
database.
If this option is not specified, the divert port named
.Em natd
will be used as a default.
.It Fl alias_address | a Ar address
Use
.Ar address
as the aliasing address.
If this option is not specified, the
.Fl interface
option must be used.
The specified address is usually the address assigned to the
public network interface.
.Pp
All data passing
.Em out
will be rewritten with a source address equal to
.Ar address .
All data coming
.Em 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
.Fl redirect_port ,
.Fl redirect_proto
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 unaltered to the local
machine and port as specified in the packet, but see the
.Fl target_address
option below.
.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
.Ar 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
.Ar 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 interface | n Ar interface
Use
.Ar interface
to determine the aliasing address.
If there is a possibility that the IP number associated with
.Ar interface
may change, the
.Fl dynamic
option should also be used.
If this option is not specified, the
.Fl alias_address
option must be used.
.Pp
The specified
.Ar interface
is usually the public network interface.
.It Fl config | f Ar file
Read configuration from
.Ar file .
A
.Ar 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
.Pp
.Dl alias_address 158.152.17.1
.Pp
would specify an alias address of 158.152.17.1.
Options that do not take an argument are specified with an option of
.Ar yes
or
.Ar no
in the configuration file.
For example, the line
.Pp
.Dl log yes
.Pp
is synonymous 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
This option makes
.Nm
reverse the way it handles incoming and outgoing packets,
allowing it to operate on the internal interface rather than
the external one.
.Pp
This can be useful in some transparent proxying situations
when outgoing traffic is redirected to the local machine
and
.Nm
is running on the internal interface (it usually runs on the
external interface).
.It Fl proxy_only
Force
.Nm
to perform transparent proxying only.
Normal address translation is not performed.
.It Fl proxy_rule Xo
.Op Ar type encode_ip_hdr | encode_tcp_stream
.Ar port xxxx
.Ar server a.b.c.d:yyyy
.Xc
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
.Ar encode_ip_hdr
to put this information into the IP option field or
.Ar encode_tcp_stream
to inject the data into the beginning of the TCP stream.
.It Fl punch_fw Xo
.Ar basenumber Ns : Ns Ar count
.Xc
This option directs
.Nm
to
.Dq punch holes
in an
.Xr 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 connection terminates.
.Pp
A maximum of
.Ar count
rules starting from the rule number
.Ar basenumber
will be used for punching firewall holes.
The range will be cleared for all rules on startup.
.El
.Sh RUNNING NATD
The following steps are necessary before attempting to run
.Nm natd :
.Bl -enum
.It
Build a custom kernel with the following options:
.Bd -literal -offset indent
options IPFIREWALL
options IPDIVERT
.Ed
.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
.Dl gateway_enable=YES
.Pp
in the
.Pa /etc/rc.conf
file or using the command
.Pp
.Dl sysctl -w net.inet.ip.forwarding=1
.Pp
.It
If you use the
.Fl interface
option, make sure that your interface is already configured.
If, for example, you wish to specify tun0 as your
.Ar interface ,
and you are using
.Xr ppp 8
on that interface, you must make sure that you start
.Nm ppp
prior to starting
.Nm natd .
.El
.Pp
Running
.Nm
is fairly straight forward.
The line
.Pp
.Dl natd -interface ed0
.Pp
should suffice in most cases (substituting the correct interface name).
Please check
.Xr rc.conf 5
on how to configure it to be started automatically during boot.
Once
.Nm
is running, you must ensure that traffic is diverted to
.Nm natd :
.Bl -enum
.It
You will need to adjust the
.Pa /etc/rc.firewall
script to taste.
If you are not interested in having a firewall, the
following lines will do:
.Bd -literal -offset indent
/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
.Ed
.Pp
The second line depends on your interface (change ed0 as appropriate).
.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
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.
.Pp
If you specify real firewall rules, it is 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
.Dl 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 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
.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 divert 4 ,
.Xr protocols 5 ,
.Xr rc.conf 5 ,
.Xr services 5 ,
.Xr syslog.conf 5 ,
.Xr ipfw 8 ,
.Xr ppp 8 .
.Sh AUTHORS
This program is the result of the efforts of many people at different
times:
.Pp
.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)