In testing, we have to often simulate errors. How can we simulate a mysql connection error on a typical network that can be found in a corporate environment? Most times, the servers themselves are some place we have no physical access to and, what is more of a problem, shared by others. So we can't shut down the server, or unplug the cable.
There is a way we can simulate a mysql connection error using the swiss army knife of the hacker - netcat. netcat allows us to forward traffic coming to a particular port, to another one of our choosing. When we set up netcat this way, it starts listening on a port, and forwards all traffic received on this port to the other port.
Here is an example :
ncat -l 10.2.2.47 8080 --sh-exec "ncat 10.2.2.47 3306"
This command will have netcat start listening on port 8080, you don't have to use port 8080, you can use any unused port on the machine. Then it forwards all traffic to port 8080 to port 3306. Now port 3306 is where mysql listens on. On the machine I ran this, the idea was to route all traffic to port 8080 to mysql on the same box. Now all you have to do is to change the connection string of your application being tested, to speak to port 8080 (instead of the default mysql port, 3306). Once you do that, port forwarding will still make the mysql calls work seamlessly. But now, when you want to simulate a mysql connection error, simply kill the ncat that is listening on port 8080. When you want to bring mysql back up, simply issue the ncat command again.
Here is how you'd use the mysql client to go via 8080:
mysql -uuser -ppass -h 10.2.2.47 --port 8080
(Make sure the host is specified as the IP. Specifying localhost will ignore the port and start using a socket file for communication.)
Voila, now you have a fairly painless way of simulating mysql errors. Of course the same technique can be used to simulate any errors generated by network calls where a server listens on a known port, which is all socket based applications.