full-disclosure - Mystery DNS Changes

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Message-ID: <20031001201116.2C79EC30C@relayer.avian.org>
From: hobbit at avian.org (*Hobbit*)
Subject: Mystery DNS Changes

   ... DHCP enabled workstations have had
   their DNS reconfigured to point to two of the three addresses

User-driven trojan or not, machines running DHCP can pretty much
be told by a DHCP server that their leases are up and it's time to
renumber, and then that their new DNS servers are X Y and maybe Z.
This is part of the protocol, astoundingly enough, but spells
"attack vector" any way *I* look at it.

This would probably work on most cable-modem infrastructures, at
least where the provider hasn't done anything about the fact that
any customer [i.e. customer's box, forget the human] can become
a rogue DHCP server.  Within a soft chewy corporate net, a rogue
server probably presents an even higher risk cuz *none* of the end
user boxes would have the benefit of a somewhat protective device
[cable modem with clueful config] in between it and the rogue.

Expect it.  Script your bootup to nuke dhclient/dhcpcd/whatever
after it's gotten an address, and sanity-check what you get back.
DHCP clients, at least in the unix world, generally run OUTSIDE
your filters, as ROOT.  Windows users, you're probably just hosed,
because if you stop "DHCP client" you release your address.

_H*