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  1.  Home
  2. WEB400
  3. September 2005

Re: RE: Socket Accept() error occurred in QHTTP (classic server)



Is it really though Bob?  If I look at my logs that is the period of time
when I get most DOS and other similar hack attacks.  Is it possible that
some attempt at a buffer over-run exploit or something is triggering it?


Sounds more like a SYN flood than a buffer overrun!
To establish a TCP connection, a client computer sends a SYN (Synchronize) packet to the server. This tells the server that a new connection is coming, so it sends back a SYN/ACK (Synch Acknowledge) packet and the accept() API creates a new socket descriptor for the connection.
A SYN flood is where someone writes a program that generates lots of SYN packets from fake IP addresses and sends them as fast as possible to a server. This causes the server to think that lots of simultaneous connection attempts are coming in, and it'll open up descriptors and send back SYN/ACK packets.
Eventually, they time out and the descriptors close, but if you send the SYN packets fast enough, the server won't be able to recover and will run out of descriptors. That's what the "Too many files open" message means -- that you've run out of descriptors.
IBM could fix the problem by modifying the TCP/IP stack to be more careful in uses as few resources as possible until the complete session is set up, or they could drop SYN packets when they come in faster than a certain rate, there's lots of different strategies. 

More info on SYN flooding can be found here:
http://www.cert.org/advisories/CA-1996-21.html
I don't know for sure that this is what's happening, it's just a guess on my part.

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