SUMMARY: denial of service attacks

Date: Fri Sep 26 1997 - 12:08:50 CDT

My original inquiry was:
>Up to a few patches before the latest set for 2.5.1, the following
>/usr/sbin/ndd -set /dev/tcp tcp_conn_req_max 1024
>in the file /etc/init.d/inetinit
>worked in preventing denial of service attacks. Now, ndd doesn't take the
>tcp_conn_req_max anymore and complains about it. Does anyone know what
>the parameter is to increase the TCP listening queue?

Thanks to the people who responded:
David Schiffrin <>
Shaun Welch <>
Casper Dik <casper@holland.Sun.COM>

It's patch 103582-11 that causes this.

The best response was from Casper Dik:

The old paremeter setting didn't really prevent denial of service attacks, it
made them more difficult.

After the new patch, the denial of service fix is in the TCP code; the
two new tunables are tcp_conn_req_max_q and tcp_conn_req_max_q0;
it's a split of the old one.

The first one is a queue of established connection (which successfully completed
the 3way handshake; only if your server is slow this queue can overflow).

The second queue is a queue that handles connections of which only the initial
SYN was received; if it overflows, old entries in the queue are dropped,
so possibly valid connections will still get through.

The default for q0 is 1024, and you dont' really need to tune it anymore.


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