RE: Dhcp security
Date: 01/28/05

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    Date: Fri, 28 Jan 2005 10:04:19 +0100
    To: <>

    Hello Paul,

    This might be overkill in your environment but:

    Another idea is to leverage your AD infrastructure to authenticate users
    using the 802.1x protocol.

    The 802.1x protocol only allows authenticated users to connect to a
    switch port, or can grant limited connectivity to unauthenticated users.

    The following Microsoft link shows how to set this up for a wireless

    Setting up a 802.1x wired network requires:
    - a 802.1x client on the users workstations/laptops
    - a 802.1x compatible switch (supported by most Cisco switches)
    - a RADIUS server (I believe W2K Server includes a RADIUS service, which
    then proxies the authentication to the AD domain server)

    It provides the advantage of scaling to large deployments, compared to
    manual MAC address/switch port configuration.

    Regarding controling virus spreading from uncontrolled devices, some
    vendors, including Cisco, provide solutions to ensure that only properly
    configured/patched/AV updated devices can connect to the network:
    "Cisco Trust Agent-Software that resides on an endpoint system. The
    trust agent collects security state information from multiple security
    software clients, such as anti-virus clients, and then communicates this
    information to Cisco network access devices, which enforce admission
    control. Cisco has licensed trust agent technology to its anti-virus
    co-sponsors so that it can be integrated with their security software
    client products. The trust agent will also be integrated with the Cisco
    Security Agent to enforce access privileges based on an endpoint's
    operating system patch level. Cisco Security Agent, a day-zero host
    protection software solution, will assess the operating system version,
    patch, and hot fix information and will communicate this information to
    the Cisco Trust Agent. Hosts that are not running the proper patches may
    be given limited access or denied network access."

    I hope this helps.

    Best Regards,

    Skander Ben Mansour, CISA CISSP

    -----Original Message-----
    From: JJ Cummings []
    Sent: vendredi 21 janvier 2005 04:51
    To: Paul Aviles;
    Subject: RE: Dhcp security
    One way "depending on how many clients you are servicing" would be to
    create MAC (layer 2) based reservations, and only allow that exact
    number of addresses in the available scope (again, each with a specific
    MAC reservation).  This does not, however, prevent static IP addressing
    of unauthorized clients.  For this you would need some hardware ACL
    stuff, either on a switch capable of MAC filtering or route the traffic
    through a security device (layer 2 again) before allowing network
    access.  All of this would have to be layer 2 at this point.
    AND / OR...
    Another option that could also be used in conjunction with the
    aforementioned would be VLAN membership rubbish.  By this I mean
    configure a specific VLAN to have DHCP services on it; you then setup
    the NIC on the client to be a member of this specific VLAN (most new
    decent NICs allow for this) and configure the switchport/switch to allow
    only traffic from this specific VLAN.  I say use this in conjunction
    with the first, because someone could figure out the VLAN ID and simply
    set it, much like a use both for a multi-layer approach
    (always a good idea "defense and depth").
    I will think about this some more and give more specific info if you
    like, I am fairly fried from sleep depravation right now so my brain
    functions may not be functioning as they should :-P
    ``The lyf so short, the craft so long to lerne.'' - Chaucer
    -----Original Message-----
    From: Paul Aviles []
    Sent: Wednesday, January 19, 2005 3:30 PM
    Subject: Dhcp security
    I have a weird question maybe. Is there a way to prevent our DHCP from
    giving leases to computers not in our domain? I don't want anyone that
    walks in to just connect and have the possibility of a network viruses
    getting loose. Is this possible?
    My setup is a typical AD 2K environment, simple domain no empty root.
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