<a href="http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO.html#toc4.1">
I just want masquerading! Help!
</a>
This is what most people want. If you have a dynamically allocated IP PPP dialup (if you don't know, this is you), you simply want to tell your box that all packets coming from your internal network should be made to look like they are coming from the PPP dialup box.
# Load the NAT module (this pulls in all the others).
modprobe iptable_nat
# In the NAT table (-t nat), Append a rule (-A) after routing
# (POSTROUTING) for all packets going out ppp0 (-o ppp0) which says to
# MASQUERADE the connection (-j MASQUERADE).
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
# Turn on IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward
Note that you are not doing any packet filtering here: for that, see the Packet Filtering HOWTO: `Mixing NAT and Packet Filtering'.
<a href="http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO.html#toc6.1">
Source NAT
</a>
You want to do Source NAT; change the source address of connections to something different. This is done in the POSTROUTING chain, just before it is finally sent out; this is an important detail, since it means that anything else on the Linux box itself (routing, packet filtering) will see the packet unchanged. It also means that the `-o' (outgoing interface) option can be used.
Source NAT is specified using `-j SNAT', and the `--to-source' option specifies an IP address, a range of IP addresses, and an optional port or range of ports (for UDP and TCP protocols only).
## Change source addresses to 1.2.3.4.
# iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4
## Change source addresses to 1.2.3.4, 1.2.3.5 or 1.2.3.6
# iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4-1.2.3.6
## Change source addresses to 1.2.3.4, ports 1-1023
# iptables -t nat -A POSTROUTING -p tcp -o eth0 -j SNAT --to 1.2.3.4:1-1023
Masquerading
There is a specialized case of Source NAT called masquerading: it should only be used for dynamically-assigned IP addresses, such as standard dialups (for static IP addresses, use SNAT above).
You don't need to put in the source address explicitly with masquerading: it will use the source address of the interface the packet is going out from. But more importantly, if the link goes down, the connections (which are now lost anyway) are forgotten, meaning fewer glitches when connection comes back up with a new IP address.
## Masquerade everything out ppp0.
# iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
Вариант для распечатки
OpenNET: Виртуальная конференция (Public)
on
18-Апр-03, 13:56 (MSK)
