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Something strange in my logs!!!

Subject: Something strange in my logs!!!
Date: Fri, 20 Jul 2007 00:10:02 +0200
situation:
DMZ linux mail server with qmail. only this service is accesible from
the net througth a dnat rule on the firewall.

yesterday i controlled the logs:
all main logs (messages wtmp btmp syslog secure ecc...) looks VERY
strange: from 3 july to 18 july absolutely no record... after and before
they are normal. even those rotated with logrotate are similar.

the mail logs, saved in a non-standard directory, are all ok even in the
period described before.

executing "w" i have that the server is up from 6 days. When i logged
through ssh (from the intranet, ssh is not accessible from outside, only
25/tcp port is open) i read that my last login was at 3july (and it
could mainly be correct).

i've downloaded chkrootkit and it says that there is nothing
strange.(but we know how much trust we can give to this program)

but where are all the logs of that 15 day has gone?
the system was surely up and running, because the mail server worked out
the mail normally (the mail logs are intact and demonstrate a normal
work during that period), from the gateway i've looked for strange
connections, but none was found.

using "last" command i can only see my login, no information about
reboot, boot or system failure. Obviusly before the 3July all is correct.

any ideas?
what can i do to discover something more?


Thanks...

Nicola

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