DHCP Snooping










DHCP snooping is a technique where we configure our switch to listen in on DHCP traffic and stop any malicious DHCP packets. This is best explained with an example so take a look at the picture below:

core distribution access dhcp hacker

In the picture above I have a DHCP server connected to the switch on the top left. At the bottom right you see a legitimate client that would like to get an IP address. What if the l33t hacker script kiddy on the left would run DHCP server software on his computer? Who do you think will respond first to the DHCP discover message? The legitimate DHCP server or the script kiddy’s DHCP server software?

On larger networks you will probably find a central DHCP server somewhere in the server farm. If an attacker runs a DHCP server in the same subnet he will probably respond faster to the DHCP discover message of the client. If this succeeds he might assign the client with its own IP address as the default gateway for a man-in-the-middle attack. Another option would be to send your own IP address as the DNS server so you can spoof websites etc.

The attacker could also send DHCP discover messages to the DHCP server and try to deplete its DHCP pool. So what can we do to stop this madness? DHCP snooping to the rescue! We can configure our switches so they track the DHCP discover and DHCP offer messages. Here’s how:

dhcp snooping discover offer packets

Interfaces that connect to clients should never be allowed to send a DHCP offer message. We can enforce this by making them untrusted. An interface that is untrusted will block DHCP offer messages. Only an interface that has been configured as trusted is allowed to forward DHCP offer messages. We can also rate-limit interfaces to they can’t send an unlimited amount of DHCP discover messages, this will prevent attacks from depleting the DHCP pool.

When a Cisco Catalyst Switch receives a DHCP Discover, it will only forward it on trusted interfaces. This prevents rogue DHCP servers on untrusted interfaces from receiving it in the first place.

Let’s see how we can configure DHCP snooping…

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Forum Replies

  1. Hi Rene,
    question, I’m working with cisco 3550 switch, version 12.1 (13) , (C3550-I5Q3L2-M), and it doesn’t have the option # ip dhcp snooping, under # ip dhcp just have this options (conflict, database, excluded-address, limited-broadcast-address, ping, pool, relay, smart-relay). what should i do, it seems to be a version issue, what version should i upgrade the switch or any other advice.

    thanks
    Ramon

  2. Hi Ramon,

    It’s probably the version since the 3550 does support DHCP snooping. It’s best to get one of the “IP services” images. This is what you should do:

    Go to Cisco.com -> Support.
    Select the 3550.
    Select Download Software.
    Select one of the EMI versions.
    Select Downloads.
    Select IOS Software.

    The latest “IP services” version is “c3550-ipservices-mz.122-44.SE6.bin” but an older version would also work.

    Here’s the direct link if you want:
    https://software.cisco.com/download/release.html?mdfid=275935148&softwareid=280805680&os=&release=12.2.44-SE6&relind=AVAI

    ... Continue reading in our forum

  3. Hi Rene, could you describe using your simply language, for what we need option 82?

  4. Hi Yevgeniy,

    DHCP Option 82 stands for “DHCP Relay Agent Information Option”. If you haven’t seen how DHCP relay works before, take a look at this lesson:

    http://networklessons.com/network-services/cisco-ios-dhcp-relay-agent/

    Option 82 was originally created for really large (WAN) networks where we have a lot of DHCP relays and central DHCP servers. The idea behind it is that the DHCP relay can add extra information which the DHCP server can use to decide which pool to use for assigning an IP address. Option 82 has a “Circuit ID” and a “Remote ID”.

    The Circuit

    ... Continue reading in our forum

  5. Hi Rene,

    If we consider configuring DHCP SNOOPING on your first diagram with 4 switches and those switches are trunking, would you configure all those trunks or port-channels as dhcp snooping trusted? or will you configure the SVI’s on the Core switches as trusted? Which is the best way to configure them.
    Please advise

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