here is some of my redhat server's log
Please post the line immediately before these ones.
that time,my server is very slow,I think it is under SYN(for port80)attact, how can i do?
First of all don't think but make sure. With the above you should get some leads on what it actually is.
If you're sure it's a bombardment of requests, make sure SYN cookies are enabled in the kernel, your server has a max requests that's appropriate for your system and use the firewall to rate-limit requests. All of this will not stop a D0S tho, so if this is a (remote) (production) server you (rely on, you) may wish to consult your hosting company or department for additional measures like firewalling or dropping traffic at the router. If it's an attack, file a complaint and let them do the tracebacks and handle contacting upstream providers.