NATD(8) BSD System Manager's Manual NATD(8)
NAME
natd -- Network Address Translation daemon
SYNOPSIS
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]
DESCRIPTION
This program provides a Network Address Translation facility for use with divert(4) sockets under
FreeBSD.
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 table.
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 table.
-log_denied
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 connection. This
option uses more system resources, but guarantees successful connections when port numbers
conflict.
-same_ports | -m
Try to keep the same port number when altering outgoing packets. With this option, proto-cols protocols
cols 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.
-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 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.
-redirect_port proto targetIP:targetPORT[-targetPORT] [aliasIP:]aliasPORT[-aliasPORT]
[remoteIP[:remotePORT[-remotePORT]]]
Redirect incoming connections arriving to given port(s) to another host and port(s). Argu-ment Argument
ment 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 numerically, 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 rule.
-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:
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 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
[remoteIP[:remotePORT]]
-redirect_address localIP[,localIP[,...]] publicIP
These forms of -redirect_port and -redirect_address are used to transparently offload net-work network
work load on a single server and distribute the load across a pool of servers. This func-tion function
tion 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 alter-ations alterations
ations to the interface passed. If the interface's IP number is changed, natd will dynami-cally dynamically
cally alter its concept of the alias address.
-in_port | -i port
Read from and write to divert(4) port port, treating all packets as ``incoming''.
-out_port | -o port
Read from and write to divert(4) port port, treating all packets as ``outgoing''.
-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 outgoing 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 specified, 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 address.
The target address may be set to 255.255.255.255, 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 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.
-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 interface.
-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
alias_address 158.152.17.1
would specify an alias address of 158.152.17.1. 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 redi-rected redirected
rected to the local machine and natd is running on the internal interface (it usually runs
on the external interface).
-proxy_only
Force natd to perform transparent proxying only. Normal address translation is not per-formed. performed.
formed.
-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 origi-nal original
nal target address can be encoded into the packet. Use encode_ip_hdr to put this informa-tion information
tion 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 connection 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 startup.
-clamp_mss
This option enables MSS clamping. The MSS value is derived from the MTU of the interface
specified in the -interface option.
RUNNING NATD
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 line
gateway_enable=YES
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 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 `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
firewall_enable=YES
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 dis-aster. disaster.
aster.
SEE ALSO
divert(4), protocols(5), rc.conf(5), services(5), syslog.conf(5), ipfw(8), ppp(8)
AUTHORS
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
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