Linux iptables NAT of an entire /24 subnetwork

Question

My purpose is to NAT (translate) all IP addresses that reach eth0 on all ports and protocols this way: 192.168.55.x -> 192.168.42.x (I have ip forwarding enabled and 192.168.42.0/24 subnetwork is on eth1). I can do this for one host per pair of lines like this, for example with x=20:

iptables -t nat -A PREROUTING -i eth0 -d 192.168.55.20 -j DNAT --to 192.168.42.20
iptables -t nat -A POSTROUTING -o eth0 -s 192.168.42.20 -j SNAT --to 192.168.55.20

But I cannot find a way to translate all 253 possible hosts. I tried this:

iptables -t nat -A PREROUTING -i eth0 -d 192.168.55.0/24 -j DNAT --to 192.168.42.0-192.168.42.255
iptables -t nat -A POSTROUTING -o eth0 -s 192.168.42.20/24 -j SNAT --to 192.168.55.0-192-168.55.255

This results in a random host translation. Useful for load balancing. But how to get a plain one-to-one translation?

Thank you.

Answer 1

From man iptables-extensions:

NETMAP
This target allows you to statically map a whole network of addresses onto another network of addresses. It can only be used from rules in the nat table.

So one rule only is needed:

iptables -t nat -A PREROUTING -d 192.168.55.0/24 -i eth0 -j NETMAP --to 192.168.42.0/24

If you also want connections initially started from the "42" net to be seen correctly, complete with this second rule:

iptables -t nat -A POSTROUTING -s 192.168.42.0/24 -o eth0 -j NETMAP --to 192.168.55.0/24