So now we know how to select the packets we want to mangle. To complete our rule, we need to tell the kernel exactly what we want it to do to the packets.
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
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
This is done in the PREROUTING chain, just as the packet comes in; this means that anything else on the Linux box itself (routing, packet filtering) will see the packet going to its `real' destination. It also means that the `-i' (incoming interface) option can be used.
To alter the destination of locally-generated packets, the OUTPUT chain can be used, but this is more unusual.
Destination NAT is specified using `-j DNAT', and the `--to-destination' 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 destination addresses to 5.6.7.8
# iptables -t nat -A PREROUTING -i eth1 -j DNAT --to 5.6.7.8
## Change destination addresses to 5.6.7.8, 5.6.7.9 or 5.6.7.10.
# iptables -t nat -A PREROUTING -i eth1 -j DNAT --to 5.6.7.8-5.6.7.10
## Change destination addresses of web traffic to 5.6.7.8, port 8080.
# iptables -t nat -A PREROUTING -p tcp --dport 80 -i eth1 \
-j DNAT --to 5.6.7.8:8080
## Redirect local packets to 1.2.3.4 to loopback.
# iptables -t nat -A OUTPUT -d 1.2.3.4 -j DNAT --to 127.0.0.1
There is a specialized case of Destination NAT called redirection: it is a simple convenience which is exactly equivalent to doing DNAT to the address of the incoming interface.
## Send incoming port-80 web traffic to our squid (transparent) proxy
# iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 \
-j REDIRECT --to-port 3128