Re: [cobalt-users] SMTP Instability

[Ted Behling <TBehling@xxxxxxxxxxxxx>]

At 02:49 PM 9/13/01, Joseph R. Brennskag wrote:
> I was wondering if anyone has noticed some instability in SMTP or sendmail
> recently.  I just recently applied all the patches to my Raq3i and now users
> have been calling me to tell me that they are not able to send mail,
> receiving mail is fine.  I have verified this and have shut down email
> service through the GUI and then restarted it through the GUI and all is
> well again.  Any ideas?

On the RaQ3, Sendmail is set to create no more than 12 child 
processes.  Children are used for incoming and outgoing connections, and 
other tasks.  To get around this arbitrarily-low limit, edit your 
/etc/sendmail.cf file and look for the following line:

        O MaxDaemonChildren=12

If you find it, I suggest you comment it out and and add or edit these lines:

        # load average at which we just queue messages
        O QueueLA=8

        # load average at which we refuse connections
        O RefuseLA=15

        # maximum number of new connections per second
        O ConnectionRateThrottle=10

These lines will configure Sendmail to limit the number of incoming 
connections per second to 10 and queue messages or refuse connections 
entirely only when the system's load average (LA) is unusually high.  This 
is useful to keep people from maliciously connecting to your Sendmail 
daemon fast enough to cause a DoS.  Cobalt's default MaxDaemonChildren 
setting is designed to protect the server from DoSes, but ironically, it 
sets up a Sendmail DoS in so doing.  By the way, O'Reilly's "bat book" 
explicitly says not to use the MaxDaemonChildren option in the Sendmail 
config file, and advocates using ConnectionRateThrottle in its place.

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