Problems with IIS

["Marcelo Lećo Caffaro" <leao () employer com br>]

Hi, i'm a security analyst of a big website, this website work with average
1000 access simultaneous, and my problem is:

My server is a IIS5.0 running in Microsoft Windows 2000 Advanced Server....,
with 2gb of ram

The website work add new curriculum vitae (totally free), search for new
jobs oportunities, free, or
it the user pay the month plan, the user can see total description of job
oportunities. (name of employer, address, etc).

The more recent job oportunities are send to vip user .....


I see in the last 2 days anormally of number visits of site, after check the
log i see one dificult method of attack, this attack working
with simultaneous connections, if i check the website database, can i see 30
or 50 querys to website database (ms-sql) , but in log in one second i have
more than
30 ips, the log not contain know attack string, unicode, or another iis bug,
the log have the url only....

My dll host stay with 950 mb and i have dllhost error, after reboot, in one
or 2 seconds after network restart, the process cpu is 100%, i think this
attack is about many
bot making numerous querys in database to decrease the web performance....

My question is, how the best way to stop this type of attack?, if a make one
session with IP, cookies and reverse dns can i stop this?

Anyone can help-me?