forward all mysql traffic to a ssh tunnel

Question

I have a local application that needs to connect to a remote mysql server at 40.40.40.40:3306

The main firewall blocks all connections but ssh, I can set up a ssh tunnel and connect to the server without problems

ssh [email protected] -L 3306:127.0.0.1:3306 -N  

(in another terminal)

$ mysql -udb_user -h127.0.0.1 -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
[...]

My intent is to configure iptables to forwards the connection destinated to 40.40.40.40:3306 to my tunnel at 127.0.0.1:3306

# iptables -t nat -A PREROUTING -d 40.40.40.40 -p tcp --dport 3306 -j DNAT --to-destination 127.0.0.1:3306

# iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         
DNAT       tcp  --  anywhere             40.40.40.40       tcp dpt:mysql to:127.0.0.1:3306

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination 

#cat /proc/sys/net/ipv4/ip_forward
1

With this setup my applications still can't connect to the database, if I change the connection settings to 127.0.0.1 I have no problems, so I assume that the application works perfectly.

Answer 1

You need to use the OUTPUT chain to redirect an outbound connection to a local port.
This rule will work as you need:

iptables -t nat -A OUTPUT -p tcp -d 40.40.40.40 --dport 3306 -j REDIRECT --to-port 3306