DHCP for CMTS and cable modems?



  • Has anyone used a pfsense as router for a CMTS and cable modem setup over COAX?


  • Rebel Alliance Global Moderator

    Huh?  From your cable modem there would be a ethernet connection, which yes would go to pfsense.



  • I think of the pfsense as router before the CMTS and all the cable modems as clients with IP's from the pfsense.

    I have worked a bit with it the first couple of days here in the New Year

    There is some issues with CMTS and DOCSIS. There need to be a router before the CMTS for DHCP. I would like the pfsense here.
    Then there is also the need for a  TFTP server and a ToD server (Time of Day, time RFC 868) not to be confused with NTP.

    A simple TFTP can be installed on the pfsense. For testing.
    Time server (RFC 868) port 37 udp I have had trouble with. I got it running on a Debian but have not figured out how to do it on the pfsense.

    So I was just wondering if any one else did something like that and could tell me about there solution.


  • Rebel Alliance Global Moderator

    You do not put router before your cablemodem..  How many cable modems do you have?  What is the model numbers of these modems..

    "There is some issues with CMTS and DOCSIS"

    Like what that you could think pfsense wold fix???  CMTS would be at the cable ISP location, are you a cable ISP?  If so then sure pfsense could be leveraged in your network..  But it wouldn't be between the CMTS and the cable modems - this is where the coax connection is..  But sure pfsense could be on the ethernet side of the CMTS..



  • yes, I am trying out a new "mini" CMTS from Televes. Right now I have a simple setup with

    WAN connection
    pfsense
    CMTS
    (coax)
    cable modems (5 units for testing)

    My issue is that the cable modems do send an DCHP discovery and the pfsense is creating a DHCP offer. But then no more happens. The cable modems should make a DHCP request, but that does not happen.

    If have TFTP-server and also a few custom options to the DHCP offer as:
    4 - time server (RFC 868)
    67 - boot file name

    I will try with a more "CMTS" friendly router and will copy the configuration from there to the pfsense. And post the findins here if I get it up and running.


  • Rebel Alliance Global Moderator

    Well does your client connected to the dhcp modem actually see the offer?

    CMTS friendly or not has nothing to do with it from your last post..

    If pfsense is seeing the discover and sending an offer - what does that have to do with the client not sending a request?  Does the client not see the offer?  Be it a cmts or not - pfsense doesn't care - it is sending out offer over an ethernet connection.  What happens after it is on the wire has nothing to do with being cmts friendly or not ;)



  • Sorry… didn't press notify on this thread

    The cable modems do get the offer. But if the offer is not formatted the right way (that is DOCSIS standard used by CMTS and cable modems) then the cable modems will wait a short while and do another discovery.

    But I did make it work! Using the pfsense. It was not correct according the DOCSIS standard to add a tftp server as a tftp server... it had to be done under "Enable network booting".

    But don't put any more thought into it. The CMTS and DOCSIS is nothing like normal network using LAN cables and switches. My findins are, that the pfsense can only be used if all cable modems can use the same config boot file. If there is need for different config files per client or per subnet then some manual work need to be added or another DHCP server should be used on LAN.

    Alternatively a modified tftp-server that can generate a custom config file per device based on client ip (given by the pfsense)

    All this only makes sense to those working with CMTS and DOCSIS standard.