iptables - connect from the ssh server to the remote computer using a local ip address

I have a remote Linux computer connecting on a local ssh server, creating a reverse ssh tunnel on port 5051. On the ssh server itself I run the following two commands, in order to give the remote computer a local IP address.

ip addr add 192.168.1.51/24 dev eth0
iptables -t nat -A PREROUTING -d 192.168.1.51 -p tcp --dport 22 -j REDIRECT --to-port 5051

On the ssh server I have also configured GatewayPorts yes in sshd_conf.

From a third computer on my network if I ssh on 192.168.1.51, I connect directly on the remote Linux computer.

But from the ssh server if I ssh 192.168.1.51 I connect on the ssh server again. I don't connect on the remote computer. The only way to connect on the remote computer from the ssh server is to use ssh root@localhost -p 5051

But I don't want to do that. I want to be able to ssh 192.168.1.51 from the ssh server, and connect on the remote computer.

Answer

IPTables NAT table's PREROUTING chain rules are only applied to IP packets arriving from other systems via the network adapter connected to the network.

If you want to apply a rule to locally generated packets, you need to add a rule to the OUTPUT chain.

So, you need to do this:

iptables -t nat -A OUTPUT -d 192.168.1.51 -p tcp --dport 22 -j REDIRECT --to-port 5051