DHCP Justification

Posted on 2004-11-12
Last Modified: 2007-02-13

I would in an environment where all IP addresses are static. This is causing a nightmare from an admin point of view as you can imagine.

I have been tasked with justifying DHCP as there are many security issues with such a protocol.

I was wondering if anyone knows of any resources that could help me? Things like:

Authenticating DHCP requests
DHCP Security Issues
Reasons to have DHCP etc

Thanks guys
Question by:stewatts
    LVL 18

    Expert Comment

    The reason is what you know, the administrative task for dhcp server is much easier than configuring each workstation.
    The discovered dhcp vulnerabilities (I know about the linux dhcp vulnerabilities, not windows) affect only the dhcp service, does not allow unauthorised access to data stored in networks.
    DHCP is not a major risk for windows networks. A good configured dhcp server will not respond to request from internet, only from local network.

    Author Comment

    What about rogue DHCP Servers (none microsoft) and Denial of service with Clients requesting all the leased IP addresses stopping legitimate clients from logging on?
    LVL 18

    Expert Comment

    That attack must come from the interface served by the dhcp, and cannot be stopped. You must know that any authentication protocol work after the workstation gets its address, and the request of address from dhcp server cannot be encrypted. But, if security is so important, a dhcp server can be forced by making reservation to always assign some adresses to known mac addresses. I know that a mac address can be impersonated, but it's not that easy and ip addresses conflicts wil allow tracing the attack somehow.

    I repeat: dhcp is not a major risk, but if a user can get access to the dhcp server it has access to the local network.

    Are you an Internet provider, in other words, your dhcp server will be available to people outside the local network?
    LVL 104

    Accepted Solution

    If you have control over the machines connecting to the network, and have quality networking hardware (Cisco not Netgear for example) then there are some clever things that you can do.

    For one, you can have a 1to1 DHCP assignment. This is where every IP address is assigned to a MAC address. This can either be a valid address or an invalid address (00000000, 00000001 etc). An unauthorised client machine will not get an address and will have to come to you for one.
    If you have Cisco switches then you can use their features to route unauthorised traffic to to a blackhole or a web site telling the user that they must speak to IT for permission to connect the machine to the network.

    Unauthorised DHCP servers cannot be avoided technically. They can usually be spotted when clients get invalid addresses. You may even have some without knowing about it. I have heard of groups purchasing a home wireless router. This router is given the static address information from one of the workstations, then the workstation and anything else is connected to the router. You see it as one device, but they are sharing the connection in much the same way as you share an internet connection at home.
    In cases like that I will have to go back to a saying I heard years ago: "There are seldem technical solutions to management problems". In this case management need to say that using those devices are not allowed and enforce that policy. The first bit is easy, the second is not - until something happens when an unauthorised device is used to breach the network.

    If you have control over your network, over the site and the connections to the network then DHCP isn't really a major security risk. As you haven't said what type of environment or the number of users that you have, we cannot say how much of a risk it is for your particular situation.


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