Significance of all-zero checksum counts

Vernon Schryver vjs@calcite.rhyolite.com
Sat Jan 3 04:24:47 UTC 2004


> > From: Gary Mills <mills@cc.UManitoba.CA>

> > I've had a few reports from concerned users that they've received
> > spam that had all of the DCC checksum counts set to zero.  What does

> ...
> That probably means that the DCC client queried the DCC server about
> the message instead of reporting it, and DCC server's answer was that
> it had never heard of the message.  See -Q in the dccm man page.
>
> Note that if the currently selected DCC server fails to answer any of
> the 4 or 5 retransmitted requests sent by a DCC client, the client
> will switch once to the next best server.  To avoid double-counting,   
> the client also switches to querying instead of reporting the checksums
> of the message. 
> ...


I've encountered an example of this syndrome in my logs.  However, it
did not involve a failure by a DCC server to answer, but a switch in
DCC servers between an initial transmission and greylist embargo and
the final transmission.   The DCC client reports a message to the
current DCC server when it is first seen and greylist embargoed.  Each
time the client sees it again, the client only does a query and not a
report.  If the client switches to another server before the final
greylist retransmission, and if the message is not known as bulk among
the DCC servers, then it is likely that the second DCC server will say
it has never heard of the message.  In other words, the final DCC
counts will be zero.

The best way to avoid such zero counts is to make DCC clients
prefer a particular server with RTT adjustments in /var/dcc/map.


Vernon Schryver    vjs@rhyolite.com



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