The software firewalls in the server already protect against all the more common attacks. Hardware firewall help to mitigate any cpu/memory type of load put on a server from having to run the firewall software to defend itself by offloading this to the firewall. Not sure what you intend to accomplish by simulating the attacks all they will show is you should use a hardware firewall.
You need to decide which of the many attacks you intend to try and there are programs to generate those.
If you are just looking at a brute force attack that send a lot of data you can use a simple tool like iperf and tell it to send at whatever rate you like. Still why even bother there is no way to fix a attack whose goal is use up all you...