Forum Discussion

thebenhanson's avatar
thebenhanson
Tuning in
3 months ago
Solved

DHCP broadcasts being blocked, when using your own DHCP server?

TL;DR does the 5x allow DHCP broadcasts from WiFi devices to reach a DHCP server attached via the 5x's switch ports?

I have an old router with (only) dnsmasq running on it, for DHCP. I prefer this because I need more flexibility than most ISP routers provide (e.g. giving out a specific set of DNS server addresses for one specific device on my network). I connect this old router from one of its built-in switch ports --> one of the ISP router's built-in switch ports, and it has always worked perfectly, with my previous ISP. 

I just switched to Virgin full fibre and have the 5x router. DHCP on the 5x is disabled. I have hooked up the device into one of the switch ports on the 5x as before but my devices connected over WiFi to the 5x never get an IP address. It seems like the 5x does not permit DHCP broadcasts from my WiFi devices to reach my switch-attached DHCP server.

I know the old router is working, and routable...connected to the 5x via WiFi and a static IP, I can ping the old router's IP, and I can use it for DNS too, when I set it as my static DNS server. It just seems to be the DHCP broadcast that never reaches it.

But searching the forum, several people said they were using an external device to provide DHCP/DNS, same as me, so it seems like it should work???

-Ben

  • thebenhanson's avatar
    thebenhanson
    3 months ago

    So just to document the issue for future people, I have figured this out. The problem is that even if you disable DHCP on the 5x, it does not seem to fully turn it off. My old router running dnsmasq, when I dug into the logs, I could see a log entry that it was detecting another active DHCP server on the network, and was therefore killing the dnsmasq service, to avoid a conflict. I'm not sure exactly how it is detecting DHCP on the 5x, since the 5x should not be advertising itself as a DHCP server at all with the option disabled.

    I enabled the "force" option in dnsmasq, which forces the service on anyway, and now I am able to get DHCP addresses from my old router successfully. So, just remember: disabling DHCP on the 5x stops it from giving out IP addresses, but there is still something about it that another DHCP server on the network can detect, and that server might shut itself down.

    So the good news about this is, you absolutely can run your own DNS and DHCP on a separate device, without using modem mode or setting up a DMZ on the 5x.

8 Replies

  • Adduxi's avatar
    Adduxi
    Very Insightful Person
    3 months ago

    Are the wifi devices set to get DHCP/DNS from your server?

    • thebenhanson's avatar
      thebenhanson
      Tuning in
      3 months ago

      Hi, they are just set to get DHCP-assigned IP addresses, so will broadcast for any DHCP server to respond with an address. But they never get an answer, even though I know the DHCP server is attached and listening. Hence my assumption that the 5x is not allowing those broadcasts to reach the switch-attached DHCP server. If I enable DHCP on the 5x then of course that works fine, but it doesn't have the features I need. 

  • legacy1's avatar
    legacy1
    Alessandro Volta
    3 months ago

    Just another reason we need modem mode.

    Your just going to have to double NAT and not use VM wifi  

    • thebenhanson's avatar
      thebenhanson
      Tuning in
      3 months ago

      But you can enable modem mode, through the hidden menu? This was my follow-up question...for people using something like pi-hole for DHCP and DNS, does it require you to be in modem mode? Because people seem to be able to make pi-hole work, yet my old router is connected to the 5x in exactly the same way you'd have to hook up a Raspberry Pi device.

      • legacy1's avatar
        legacy1
        Alessandro Volta
        3 months ago

        Yes some say it don't work and only on the 1Gb port not 10Gb port

  • thebenhanson's avatar
    thebenhanson
    Tuning in
    3 months ago

    So just to document the issue for future people, I have figured this out. The problem is that even if you disable DHCP on the 5x, it does not seem to fully turn it off. My old router running dnsmasq, when I dug into the logs, I could see a log entry that it was detecting another active DHCP server on the network, and was therefore killing the dnsmasq service, to avoid a conflict. I'm not sure exactly how it is detecting DHCP on the 5x, since the 5x should not be advertising itself as a DHCP server at all with the option disabled.

    I enabled the "force" option in dnsmasq, which forces the service on anyway, and now I am able to get DHCP addresses from my old router successfully. So, just remember: disabling DHCP on the 5x stops it from giving out IP addresses, but there is still something about it that another DHCP server on the network can detect, and that server might shut itself down.

    So the good news about this is, you absolutely can run your own DNS and DHCP on a separate device, without using modem mode or setting up a DMZ on the 5x.