Quanta & DTest explained what the command does. Everyone will tell you that a few hundred connections does not a DoS make (talk to me when you have at least 5-10 thousand), and I'll expand on that by saying that in order for it to be a DDos you'd be seeing a lot more entries (probably with a lot more connections each) than what you're showing above.
When you have a problem with a server DO NOT jump to the exotic causes (DDoS, Cosmic Rays, Z0MG H4X0R3D!, etc.) -- Chances are you have a far more boring and mundane problem.
You say "it crashes" -- do you mean the whole server locks up, panics or otherwise requires a hard reboot?
If so, check your RAM (MemTest86+ or similar). That's usually the issue.
If it's not a real, hard crash start looking at the normal mundane troubleshooting items:
- What is the load average? What is it when you have a problem?
- How much swap are you using? Are you using more when you have a problem? (If so, memory leak!)
- What programs are trying to get on the CPU?
- Run your operating system's disk I/O information tools (Not a Debian guy, maybe someone can list 'em?)
- Are you disk bound? (is the disk constantly using 100% of its bandwidth?)
- Look at your network statistics
- Are you hitting a bandwidth cap from your ISP?
- Look at your ancillary programs, if applicable
- Database Connections
- Shared File Systems
- Any other resource that may be locked/blocking when you need it